-. - 3 L
 
 COMMENTARY 
 
 NEWTON'S PRINCIPIA, 
 
 A SUPPLEMENTARY VOLUME. 
 
 DESIGNED FOR THE USE OF STUDENTS AT THE UNIVERSITIES. 
 
 BY 
 
 J. M. F. WRIGHT, A. B. 
 
 . V 
 LATE SCHOLAR OF TIUXITr COLLEGE, CAMBRIDGE, AUTHOR OF SOLUTIONS 
 
 OF THE CAMBRIDGE PROBLEMS, &c. &C. 
 
 IN TWO VOLUMES. 
 
 VOL. I. 
 
 LONDON: 
 
 PRINTED FOR T. T. & J. TEGG, 73, CHEAPSIDE; 
 AND RICHARD GRIFFIN & CO., GLASGOW. 
 
 MDCCCXXXIII.
 
 GLASGOW : 
 
 GEORGE EROOK.MAN, PRINTER, VILLAPJKLD.
 
 TO THE TUTORS 
 
 OF THE SEVERAL COLLEGES AT CAMBRIDGE, 
 THESE PAGES, 
 
 * 
 
 WHICH WERE COMPOSED WITH THE VIEW 
 
 OF PROMOTING THE STUDIES 
 
 OVER WHICH THEY SO ABLY PRESIDE, 
 
 ARE RESPECTFULLY INSCRIBED 
 
 BY THEIR DEVOTED SERVANT, 
 
 THE AUTHOR.
 
 PREFACE. 
 
 THE flattering manner in which the Glasgow Edition of Newton's Prin- 
 cipia has been received, a second impression being already on the verge 
 of* publication, has induced the projectors and edito? of that work, to 
 render, as they humbly conceive, their labours still more acceptable, by 
 presenting these additional vofnnres to the public. From amongst the 
 several testimonies of the esteem in which their former endeavours have 
 been held, it may suffice, to avoid the charge of self-eulogy, to select the 
 following, which, coming from the high authority of French mathematical 
 criticism, must be considered at once as the more decisive and impartial. 
 It nas been said by one of the first geometers of France, that " L'edilion 
 de Glasgow fait honneur aux presses de cette ville irtckretricuse. On peut 
 affirmer que jamais Tart typographique ne rendit un plus bel honrmage 
 a la memoire de Newton. Le merite de I'impression, quoique tres-remar- 
 quable, n'est pas ce que les editeurs ont recherche avec le plus de soin, 
 pour tant le materiel de leur travail, \\s pouvaient s'en rapporter a Thabi- 
 lite de leur artistes : mais le choix des meilleures editions, la revision la 
 plus scrupuleuse du texte et des epreuves, la recherche attentive des fautes 
 qui pourraient e"chapper meme au lecteur studieux, et passer inaper^ues 
 ce travail consciencieux de 1'intelligence et du savoir, voila ce qui eleve 
 cette edition au-dessus de toutes celles qui 1'ont prece'tle'e. 
 
 " Les editeurs de Glasgow ne s'etaient charges que d'un travail de re- 
 vision. S'iTs avaient con?u le projet cTameliorer et completer fceuvre des 
 
 a 3
 
 V! PREFACE. 
 
 commentatcurs, Us auraient sans doute employe) comme eux t les tranaux des 
 successeurs de Newton sur les questions traitees dans le livre des Principes. 
 
 " Les descendans de Newton sont nombreux, et leur genealogie est 
 prouvee par des litres incontestibles; ceux qui vivent aujourd'hui verraient 
 sans doute avec satisfaction que Ton format un tableau de leur famille, en 
 reunissant les productions les plus remarquables dont Fouvrage de Newton 
 a fourni le germe: que ce livre immortel soil entoure de tout ce Fon peut 
 regarder comme ses developpemens : voila son meilleur commentaire. 
 U edition de Glasgow pourrait done etre continuee, et prodigieusement 
 enricliie" 
 
 The same philosopher takes occasion again to remark, that " Le plus 
 beau monument que Fon puisse clever a la gloire de Newton, c'est une 
 bonne edition de ses ouvrages : et il est etonnant que les Anglais en aient 
 laisse ce soin aux nations etrangeres. Les presses de Glasgow viennent 
 de reparer, en partie, le tort de la nation Anglaise : la nouvelle edition 
 des Principes est effectivement la plus belle, la plus correcte et la plus com- 
 mode qui ait parujusqu'ici. La collation des anciennes editions, la revi- 
 sion des calculs, &c. ont ete confiees a un habile mathematicien et rien 
 n'a ete neglige pour eviter toutes les erreurs et toutes les omissions. 
 
 " II faut esperer que les editeurs continueront leur belle entreprise, et 
 qtfils y seront assez encourages pour nous donner, non seulement tons les 
 ouvrages de Newton, mats ceux des savans qui ont complete ses travaux." 
 
 The encouragement here anticipated has not been withheld, nor has 
 the idea of improving and completing the comments of " The Jesuits", 
 contained in the Glasgow Newton, escaped us, inasmuch as long before 
 these hints were promulgated, had the following work, which is composed 
 principally as a succedaneum to the former, been planned, and partly writ- 
 ten. It is at least, however, a pleasing confirmation of the justness of our 
 own conceptions, to have encountered even at any time with these after- 
 suggestions. The plan of the work is, nevertheless, in several respects, 
 a deviation from that here so forcibly recommended. 
 
 The object of the first volume is, to make the text of the Principia, by
 
 PREFACE. Vll 
 
 supplying numerous steps in the very concise demonstrations of the pro- 
 positions, and illustrating them by every conceivable device, as easy as 
 can be desired by students even of but moderate capacities. It is univer- 
 sally known, that Newton composed this wonderful work in a very hasty 
 manner, merely selecting from a huge mass of papers such discoveries as 
 would succeed each other as the connecting links of one vast chain, but 
 without giving himself the trouble of explaining to the world the mode of 
 fabricating those links. His comprehensive mind could, by the feeblest 
 exertion of its powers, condense into one view many syllogisms of a pro- 
 position even heretofore uncontemplated. What difficulties, then, to him 
 would seem his own discoveries ? Surely none ; and the modesty for 
 which he is proverbially remarkable, gave him in his own estimation so 
 little the advantage of the rest of created beings,, that he deemed these 
 difficulties as easy to others as to himself: the lamentable consequence of 
 which humility has been, that he himself is scarcely comprehended at this 
 day a century from the birth of the Principia. 
 
 We have had, in the first place, the Lectures of Winston, who des- 
 cants not even respectably in his lectures delivered at Cambridge, upon 
 the discoveries of his master. Then there follow even lower and less 
 competent interpreters of this great prophet of science for such Newton 
 must have been held in those dark days of knowledge whom it would be 
 time mis-spent to dwell upon. But the first, it would seem, who properly 
 estimated the Principia, was Glairaut. After a lapse of nearly half a cen- 
 tury, this distinguished geometer not only acknowledged the truths of the 
 Principia, but even extended the domain of Newton and of Mathematical 
 Science. But even Clairaut did not condescend to explain his views and 
 perceptions to the rest of mankind, farther than by publishing his own 
 discoveries. For these we owe a vast debt of gratitude, but should have 
 been still more highly benefited, had he bestowed upon us a sort of run- 
 ning Commentary on the Principia. It is generally supposed, indeed, 
 that the greater portion of the Commentary called Madame Chastellet's, 
 was due to Clairaut. The best things, however, of that work are alto- 
 
 i
 
 Vlll PREFACE 
 
 gether unworthy of so great a master ; at the most, showing the perform- 
 ance was not one of his own seeking. At any rate, this work does not 
 deserve tjie name of a Commentary on the Principia. The same may 
 safely be affirmed of many other productions intended to facilitate New- 
 ton. Pemberton's View, although a bulky tome, is little more than 
 a eulogy. Maclaurin's speculations also do but little, elucidate the 
 dark passages of the Principia, although written more immediately for 
 that purpose. This is also a heavy unreadabje performance, and not 
 worthy a place on the same shelf with the other works of that great 
 geometer. Another great mathematician, scarcely inferior to Maclaurin, 
 has also laboured unprofitably in the same field. Emerson's Comment? 
 is a book as small in value as it is in bulk, affording no helps worth the 
 perusal to the student. Thorpe's notes to the First Book of the Princi- 
 pia, however, are of a higher character, and in many instances do really 
 facilitate the reading of Newton. Jebb's notes upon certain sections deserve 
 the same commendation ; and praise ought not to be withheld from several 
 other commentators, who have more or less succeeded in making small 
 portions of the Principia more accessible to the student such as the Rev. 
 Mr. Newton's work, Mr. Carr's, Mr. Wilkinson's, Mr. Lardner's, &c, 
 It must be confessed, however, that all these fall far short in value of the 
 very learned labours, 'contained in the Glasgow Newton, of the Jesuits 
 Le Seur and Jacquier, and their great coadjutor. Much remained, how- 
 ever, to be added even to this erudite production, and subsequently to its 
 first appearance much has been excogitated, principally by the mathema- 
 ticians of Cambridge, that focus of science, and native land of the Princi- 
 pia, of which, in the composition of the following pages, the author has 
 liberally availed himself. The most valuable matter thus afforded are the 
 Tutorial MSS. in circulation at Cambridge. Of these, which are used iti 
 explaining Newton to the students by the Private Tutors there, the author 
 confesses to have had abundance, and also to have used them so far as seem- 
 ed auxiliary to his own resources. But at the same time it must be remark- 
 ed, that little has been the assistance hence derived, or, indeed, from all
 
 PUEFACE. IX 
 
 other known sources, which from the first have been constantly at com- 
 mand. 
 
 The plan of the work being to make those parts of Newton easy which 
 are required to be read at Cambridge and Dublin, that portion of the 
 Principia which is better read in the elementary works on Mechanics, 
 viz. the preliminary Definitions, Laws of Motion, and their Corollaries, 
 has been disregarded. For like reasons the fourth and fifth sections have 
 been but little dwelt upon. The eleventh section and third book have 
 not met with the attention their importance and intricacy would seem to 
 demand, partly from the circumstance of an excellent Treatise on Physics, 
 by Mr. Airey, having superseded the necessity of such labours; and 
 partly because in the second volume the reader will find the same subjects 
 treated after the easier and more comprehensive methods of Laplace. 
 
 The first section of the first book has been explained at great length, 
 and it is presumed that, for the first time, the true principles of what has 
 been so long a subject of contention in the scientific world, have there 
 been fully established. It is humbly thought (for in these intricate specu- 
 lations it is folly to be proudly confident), that what has been considered 
 in so many lights and so variously denominated Fluxions, Ultimate Ratios, 
 Differential Calculus, Calculus of Derivations, &c. &c. is here laid down 
 on a basis too firm to be shaken by future controversy. It is also hoped 
 that the text of this section, hitherto held almost impenetrably obscure, is 
 now laid open to the view of most students. The same merit it is with some 
 confidence anticipated will be awarded to the illustrations of the 2nd, 3rd, 
 6th, 7th, 8th, and 9th sections, which, although not so recondite, require 
 much explanation, and many of the steps to be supplied in the demon- 
 stration of almost every proposition. Many of the things in the first 
 volume are new to the author, but very probably not original in reality 
 so vast and various are the results of science already accumulated. Suffice 
 it to observe, that if they prove useful in unlocking the treasures of the 
 Principia, the author will rest satisfied with the meed of approbation, 
 which he will to that extent have earned from a discriminating and im- 
 partial public.
 
 X PREFACE. 
 
 The second volume is designed to form a sort of Appendix or Supple- 
 ment to the Principia. It gives the principal discoveries of Laplace, and, 
 indeed, will be found of great service, as an introduction to the entire 
 perusal of the immortal work of that author the Mecanique Celeste. 
 This volume is prefaced by much useful matter relative to the Integra- 
 tion of Partial Differences and other difficult branches of Abstract Ma- 
 thematics, those powerful auxiliaries in the higher departments of Physical 
 Astronomy, and which appear in almost every page of the Mecanique 
 Celeste. These and other preparations, designed to facilitate the com- 
 prehension of the Newton of these days, will, it is presumed, be found 
 fully acceptable to the more advanced readers, who may be prosecuting 
 researches even in the remotest and most hidden receptacles of science ; 
 and, indeed, the author trusts he is by no means unreasonably exorbitant 
 in his expectations, when he predicates of himself that throughout the 
 undertaking he has proved himself a labourer not unworthy of reward. 
 
 THE AUTHOR.
 
 A COMMENTARY 
 
 ON 
 
 NEWTON'S PRINCIPIA, 
 
 SECTION I. BOOK I. 
 
 1. THIS section is introductory to the succeeding part of the work. It 
 comprehends the substance of the method of Exhaustions of the Ancients, 
 and also of the Modern Theories, variously denominated Fluxions, Dif- 
 ferential Calculus, Calculus of Derivations, Functions, &c. &c. Like 
 them it treats of the relations which Indefinite quantities bear to one ano- 
 ther, and conducts in general by a nearer route to precisely the same 
 results. 
 
 2. In what precedes this section, Jinite quantities only are considered, 
 such as the spaces described by bodies moving uniformly in Jinite times 
 with Jinite velocities ; or at most, those described by bodies whose mo- 
 tions are uniformly accelerated. But what follows relates to the motions 
 of bodies accelerated according to various hypotheses, and requires the 
 consideration of quantities indefinitely small or great, or of such whose 
 Ratios, by their decrease or increase, continually approximate to certain 
 Limiting Values, but which they cannot reach be the quantities ever so 
 much diminished or augmented. These Limiting Ratios are called by 
 Newton, " Prune and Ultimate Ratios," Prime Ratio meaning the Limit 
 from which the Ratio of two quantities diverges, and Ultimate Ratio that 
 towards which the Ratio converges. To prevent ambiguity, the term Li- 
 miting Ratio will subsequently be used throughout this Commentary.
 
 A COMMENTARY ON [SECT. I. 
 
 LEMMA I. 
 
 3. QUANTITIES AND THE RATIOS OF QUANTITIES.] Hereby Newton 
 would infer the truth of the Lemma not only for quantities mensurable 
 by Integers, but also for such as may be denoted by Vulgar Fractions. 
 The necessity or use of the distinction is none ; there being just as much 
 reason for specifying all other sorts of quantities. The truth of the LEMMA 
 does not depend upon the species of quantities, but upon their confor- 
 mity with the following conditions, viz. 
 
 4. That they tend continually to equality, and approach nearer to each 
 other than by any given difference. They must tend continually to equa- 
 lity, that is, every Ratio of their successive corresponding values must be 
 nearer and nearer a Ratio of Equality, the number of these convergen- 
 cies being without end. By given difference is merely meant any that can 
 be assigned or proposed. 
 
 5. FINITE TIME.] Newton obviously introduces the idea of time in this 
 enunciation, to show illustratively that he supposes the quantities to con- 
 verge continually to equality, without ever actually reaching or passing that 
 state ; and since to fix such an idea, he says, " before the end of that 
 time," it was moreover necessary to consider the time Finite. Hence 
 our author would avoid the charge of " Fallacia Suppositionis" or of 
 " shifting the hypothesis" For it is contended that if you frame certain 
 relations between actual quantities, and afterwards deduce conclusions 
 from such relations on the supposition of the quantities having vanished, 
 such conclusions are illogically deduced, and ought no more to subsist 
 than the quantities themselves. 
 
 In the Scholium at the end of this Section he is more explicit. He 
 says, The ultimate Ratios, in 'which quantities vanish, are not in reality the 
 Ratios of Ultimate quantities , but the Limits to which the Ratios of quan- 
 tities continually decreasing always approach ; which they never can pass 
 beyond or arrive at, unless the quantities are continually and indefinitely 
 diminished. After all, however, neither our Author himself nor any of 
 his Commentators, though much has been advanced upon the subject, has 
 obviated this objection. Bishop Berkeley's ingenious criticisms in the 
 Analyst remain to this day unanswered. He therein facetiously denomi- 
 nates the results, obtained from the supposition that the quantities, before
 
 BOOK I.] NEWTON'S PRINCIPIA. 3 
 
 considered finite and real, have vanished, the " Ghosts of Departed 
 Quantities " and it must be admitted there is reason as well as wit in the 
 appellation. The fact is, Newton himself, if we may judge from his own 
 words in the above cited Scholium, where he says, " If two quantities, 
 whose DIFFERENCE is GIVEN are augmented continually, their Ultimate 
 Ratio will be a Ratio of Equality," had no knowledge of the true nature 
 of his Method of Prime and Ultimate Ratios. If there be meaning in 
 words, he plainly supposes in this passage, a mere Approximation to be 
 the same with an Ultimate Ratio. He loses sight of the condition ex- 
 pressed in Lemma I. namely, that the quantities tend to equality nearer 
 than by any assignable difference, by supposing the difference of the quan- 
 tities continually augmented to be given, or always the same. In this 
 sense the whole Earth, compared with the whole Earth minus a grain of 
 sand, would constitute an Ultimate Ratio of equality ; whereas so long as 
 any, the minutest difference exists between two quantities, they cannot be 
 said to be more than nearly equal. But it is now to be shown, that 
 
 6. If two quantities tend continually to equality, and approach to one 
 another nearer than by any assignable difference, their Ratio is ULTIMATE- 
 LY a Ratio of ABSOLUTE equality. This may be demonstrated as fol- 
 lows, even without supposing the quantities ultimately evanescent. 
 
 It is acknowledged by all writers on Algebra, and indeed self-evident, that 
 if in any equation put = 0, there be quantities absolutely different in kind, 
 the aggregate of each species is separately equal to 0. For example, if 
 A + a + B V~2 + b V~2 + C V~^l = 0, 
 
 since A + a is rational, (B + b) V*2 surd and C V 1 imaginary, 
 they cannot in any way destroy one another by the opposition of signs, 
 and therefore 
 
 A + a = 0, B + b = 0, C = 0. 
 
 In the same manner, if logarithms, exponentials, or any other quantities 
 differing essentially from one another constitute an equation like the above, 
 they must separately be equal to 0. This being premised, let L, I/ de- 
 note the Limits, whatever they are, towards which the quantities L + 1, 
 L' + 1' continually converge, and suppose then- difference, in any state of 
 the convergence, to be D. Then 
 
 L + 1_L' 1' = D, 
 or L L' + 1 1' D = 0, 
 
 and since L, L' are fixed and definite, and 1, 1', D always variable, the 
 former are independent of the latter, and we have 
 
 A2
 
 4 A COMMENTARY ON [SECT. I. 
 
 L 
 
 L L' = 0, or jy = 1, accurately. Q. e. d. 
 
 This way of considering the question, it is presumed, will be deemed 
 free from every objection. The principle upon which it rests depending 
 upon the nature of the variable quantities, and not upon their evanescence, 
 (as it is equally true even for constant quantities provided they be of dif- 
 ferent natures), it is hoped we have at length hit upon the true and lo- 
 gical method of expounding the doctrine of Prime and Ultimate Ratios, 
 or of Fluxions, or of the Differential Calculus, &c. 
 
 It may be here remarked, in passing, that the Method of Indeterminate 
 Coefficients, which is at bottom the same as that of Prime and Ultimate 
 Ratios, is treated illogically in most books of Algebra. Instead of 
 " shifting the hypothesis," as is done in Wood, Bonnycastle and others, 
 by making x = 0, in the equation 
 
 a + bx + cx s +dx 3 + = 0, 
 
 it is sufficient to know that each term x being indefinitely variable, is he- 
 terogeneous compared with the rest, and consequently that each term 
 must equal 0. 
 
 7. Having established the truth of LEMMA I. on incontestable princi- 
 ples, we proceed to make such applications as may produce results useful 
 to our subsequent comments. As these applications relate to the Limits 
 of the Ratios of the Differences of Quantities, we shall term, after Leib- 
 nitz, the Method of Prime and Ultimate Ratios, 
 
 THE DIFFERENTIAL CALCULUS. 
 
 8. According to the established notation, let a, b, c, &c , denote con- 
 stant quantities, and z, y, x, &c., variable ones. Also let A z, A y, A x, 
 &c., represent the difference between any two values of z, y, x, &c., re- 
 spectively. 
 
 9. Required the Limiting or Ultimate Ratio of A (a x) and A x, i. e. 
 the Limit of the Difference of a Rectangle having one side (a) constant, and 
 the other (x) variable, and of the Difference of the variable side. 
 
 Let L be the Limit sought, and L + 1 any value whatever of the va- 
 rying Ratio. Then 
 
 A (a x) a (x -j- A x) ax 
 
 L+I= -^r,M,- -njf- - = " *> 7 
 
 L = a.
 
 BOOK I.] NEWTON'S PRINCIPIA. 5 
 
 In this instance the Ratio is the same for all values of x. But if in the 
 Limit we change the characteristic A into d, we have 
 d(ax) 
 
 d x ~~ a 
 or 
 
 d (a x) = a d x- 
 d (a x), d x being called the Differentials of a x and x respectively. 
 
 
 10. Required the Limit of ^ K 
 
 Let L be the Limit required, and L + 1 the value of the Ratio gene- 
 rally. Then 
 
 A (x 8 ) _ (x + A x) 2 x 2 
 
 L + 1 = AX = " Ax 
 
 2 X A X -f A X 2 
 
 /. L 2 x + 1 Ax=0 
 and since L 2 x and 1 A x are heterogeneous 
 L 2 x = 0, 
 or 
 
 L = 2x. 
 and .*. 
 
 d (x a ) 
 
 or 
 
 d(x 2 ) = 2xdx (c) 
 
 A (x n ) 
 
 1 1. Generally, required the Limit of A x . 
 
 Let L and L + 1 be the Limit of the Ratio and the Ratio itself re- 
 spectively. Then 
 
 A (X n ) __ (X + A X) n X n 
 
 ""AX" AX 
 
 n. (n 1) 
 = nx 11 - 1 + % . x n ~ 2 Ax + &c. 
 
 and L nx n ~ 1 being essentially different from the other terms of 
 the series and from 1, we have 
 
 d(x n ) 
 
 -JY~ = L = nx n -' ord(x n ) = nx"-dx (d) 
 
 or in words, 
 
 A s
 
 6 A COMMENTARY ON [SECT. I. 
 
 The Differential of any power or root of a variable quantify is equal to 
 the product of the Differential of the quantity itself, the same power or 
 root MINUS one of the quantity, and the index of the power or root. 
 
 We have here supposed the Binomial Theorem as fully established by 
 Algebra. It may, however, easily be demonstrated by the general prin- 
 ciple explained in (7). 
 
 12. From 9 and 11 we get 
 d(ax n )=nax n - 1 dx ...... (e) 
 
 A(a + bx n + cx m + exP + & c .) 
 
 13. Required the Limit of - A 
 
 Let L be the Limit sought, and L + 1 the variable Ratio of the finite 
 differences; then 
 
 A(a + bx u + cx m + &c.) 
 L + * - AX 
 
 + &c. a bx a cx m &c. 
 
 _ 
 
 Ax 
 
 = nbx 11 - 1 +mcx m - 1 +&c. + PAx + Q(Ax) 2 + &c. 
 P, Q, &c. being the coefficients of A x, A x 2 + &c. And equating the 
 homogeneous determinate quantities, we have 
 d(a + bx n +cx 
 
 A(a + bx n + cx m + &c.) 
 14. Required the Limit of - -j- 
 
 By 1 1 we have 
 
 d. (a + bx n + cx m + &c.) r 
 
 
 and by 13 
 
 d(a+bx n + cx m + &c.) = (nbx 11 - 1 + mcx 1 " 1 - 1 + &c.) d x 
 
 d(a + bx n + cx m + &c.) r 
 - 
 
 the Limiting Ratio of the Finite Differences A(a-fbx n +cx m + &c.), 
 A x, that is the Ratio of the Differentials ofa + bx n -fcx ln + &c., 
 and x. 
 
 A+Bx u +Cx m + &c. 
 15. Required the Ratio of the Differentials ^ a o.bx' + Cx^ + &c 
 
 and x, or the Limiting Ratio of their Finite Differences. 
 
 Let L be the Limit required, and L + 1 the varying Ratio. Then 
 
 A + B (x + A x ) p + C (x + A x) m + &c. A + B x + &c. 
 L + l = a + b (x + A x)' + c (x + A x)^ + &c. ~ a + b x' + &c.
 
 BOOK I.] NEWTON'S PRINCIPIA. 7 
 
 which being expanded by the Binomial Theorem, and properly reduced 
 
 gives 
 
 L X ( a + b x' + &c.) 2 + L X [P. AX + Q (A x) 2 +&c. + 1 X a+bx + &c. 
 + P. A x + Q (A x) 2 + &c.] = (a+bx' + cx^ + &c.) X (nBx n - J 
 + m C x m ~ 1 + &c.) (A + Bx n -fCx m + &c.) X (v b x - l 
 + p c x A*- 1 + &c.) + F. A x + Q 7 (A x) 2 + &c. 
 
 P, Q, F, Q' &c. being coefficients of A' x, (A x) * &c. and independent of 
 
 them. 
 
 Now equating those homogeneous terms which are independent of the 
 
 powers of A x, we get 
 
 L(a + b x + &c.) 2 = (a + b X- + &c.) (n Ex " 1 +mC x M - ' + &c.) 
 
 (A + Bx" + Cx m + &c.) (nbx'- 1 + /ticx^- 1 + &c.) 
 
 and putting u = a ~^Tb x ' + c x * -f- &cT we nave *"!*% 
 
 d u d u 
 
 j"^ = L, and therefore ^-^ = 
 
 (a+bx'+&c.)(nBx+ u - 1 mCx m - 1 +&c.)-(A+Bx"+&c.)(>bx'- 1 +/Acx^- 1 +&c.) 
 
 the Ratio required. 
 
 16. Hence and from 1 1 we have the Ratio of the Differentials of 
 
 (A + Bx n +Cx m + &c.) P 
 / i i_ v i _ i Qrf* \ Q ^nci x 9 Jinci in snort^ iront \vnst nss si 
 
 ready been delivered it is easy to obtain the Ratio of the Differentials of 
 any Algebraic Function whatever of one variable and of that variable. 
 
 N. B. By Function of a variable is meant a quantity anyhow involving 
 that variable. The term was first used to denote the Powers of a quan- 
 tity, as x 2 , x 3 , &c. But it is now used in the general sense. 
 
 The quantities next to Algebraical ones, in point of simplicity, are Ex- 
 ponential Functions; and we therefore proceed to the investigation of 
 their Differentials. 
 
 17. Required the Ratio of the Differentials of a* and x ; or the Limit- 
 ing Ratio of their Differences. 
 
 Let L be the required Limit and L + 1 the varying Ratio ; then 
 A (a *) a x + Ai a* 
 
 "*" ~ A X ~ AX 
 
 a Ax 1 
 
 o X v 
 
 ~ a X AX
 
 8 A COMMENTARY ON [SECT. 1. 
 
 But since 
 
 a y = (l+a i)y 
 
 = 1 + y (a 1) + ^" 1) . (a I) 2 + 
 
 2.3 ~(a l) 3 + &c., 
 
 it is easily seen that the coefficient of y in the expansion is 
 
 (a- i) 2 . (^JO 3 
 
 a 1 g -f- o ixc. 
 
 Hence 
 
 a* (a I) 2 (a^ I) 3 
 
 L + l = A~! *(* 1 2 + ^-8 & c ) A x + P (Ax) 2 + &c.| 
 
 and equating homogeneous quantities, we have 
 
 d. (a 1 ) (a I) 2 (a I) 3 
 
 - = L = fa- 1 - L - + ( - -^--&c. a* 
 
 = A a x ........ (h) 
 
 or Me ^a^'o of the Differentials of any Exponential and its exponent is 
 equal to the product of the Exponential and a constant Quantity. 
 
 Hence and from the preceding articles, the Ratio of the Differentials of 
 any Algebraic Function of Exponentials having the same variable index, 
 may be found. The Student may find abundance of practice in the Col- 
 lection of Examples of the Differential and Integral Calculus, by Messrs. 
 Peacock, Herschel and Babbage. 
 
 Before we proceed farther in Differentiation of quantities, let us inves- 
 tigate the nature of the constant A which enters the equation (h). 
 
 For that purpose, let (the two first terms have been already found) 
 
 a x = l+Ax + Px 2 + Qx 3 + &c. 
 Then, by 13, 
 
 d (a x ) 
 d x =A + 2Px + 3Qx 2 + 4Rx 3 + &c. 
 
 But by equation (h) 
 
 d (a x ) 
 
 ~~ also = A a * 
 
 and equating homogeneous quantities^ we get 
 
 2 P = A 2 , 3 Q = A P, 4 R = A Q, &c. = &c.
 
 BOOK I.] NEWTON'S PRINCIPIA. 
 
 whence 
 
 A 2 AP A 3 AQ A 4 
 
 P = T> Q = ~3~ = 273' R = ~ = 27374 &c ' 
 Therefore, 
 
 A 
 
 a * = 1 + Ax + -g-x a + j-gX 3 + 2. 3.4 x 4 + &c. 
 Again, put A x = 1, then 
 
 i ill 
 
 a = 1 + 1 + - + -3 + 3-3-4 + & c . 
 
 = 2.718281828459 as is easily calculated 
 = e 
 
 by supposition. Hence 
 lo. a 
 
 (a I) 2 (a 1) 3 log, a 
 
 /. a - -3- + -3- - &c. = log e = I- a 
 
 for the system whose base is e, 1 being the characteristic of that system. 
 This system being that which gives 
 
 (e I) 2 (e I) 3 
 e 1 2 - + -- 3 -- & c * = 1 
 
 is called Natural from being the most simple. 
 Hence the equation (h) becomes 
 
 (1) 
 
 17 a. Required the Ratio of the Differentials of 1 (x) and x. 
 
 Let 1 x = u. Then e u = x 
 /. d x = d (e n ) = 1 e X e u d u = e u d u, by 16 
 d(lx) 1 1 
 
 * inr = ^ = x ....'...-. (>) 
 
 Ix 
 
 In any other system whose base is a, we have log. (x) = y^. 
 
 d lo. x 1 1 
 
 = U x x 
 
 We are now prepared to differentiate any Algebraic, or Exponential 
 Functions of Logarithmic Functions, provided there be involved but 
 one variable. 
 
 Before we differentiate circular functions, viz. the sines, cosines, tan- 
 gents, &c., of circular arcs, we shall proceed with our comments on the 
 text as far as LEMMA VIII.
 
 10 A COMMENTARY ON [SECT. I. 
 
 LEMMA II. 
 
 18. In No. 6, calling L and L' Limits of the circumscribed and inscribed 
 rectilinear figures, and L + 1, L' + 1' an y other values of them, whose 
 variable difference is D, the absolute equality of L and L 7 is clearly de- 
 monstrated, without the supposition of the bases A B, B C, C D, D E, 
 being infinitely diminished in number and augmented in magnitude. In 
 the view there taken of the subject, it is necessary merely to suppose them 
 variable. 
 
 LEMMA III. 
 
 19. This LEMMA is also demonstrable by the same process in No. 6, 
 as LEMMA II. 
 
 Cor. 1. The rectilinear figures cannot possibly coincide with the curvi- 
 linear figure, because the rectilinear boundaries albmcndoE, 
 aKbLcMdDE cut the curve a b E in the points a, b, c, d, E in 
 finite angles. The learned Jesuits, Jacquier and Le Seur, in endeavour- 
 ing to remove this difficulty, suppose the four points a, 1, b, K to coincide, 
 and thus to form a small element of the curve. But this is the language 
 of Indivisibles, and quite inadmissible. It is plain that no straight line, 
 or combination of straight lines, can form a curve line, so long as we un- 
 derstand by a straight line " that which lies evenly between its extreme 
 points," and by a curve line, " that which does not lie evenly between its 
 extreme points ;" for otherwise it would be possible for a line to be 
 straight and not straight at the same time. The truth is manifestly this. 
 The Limiting Ratio of the inscribed and circumscribed figures is that of 
 equality, because they continually tend to a fixed area, viz, that of the 
 given intermediate curve. But although this intermediate curvilinear 
 area, is the Limit towards which the rectilinear areas continually tend and 
 approach nearer than by any difference ; yet it does not follow that the 
 rectilinear boundaries also tend to the curvilinear one as a limit. The 
 rectilinear boundaries are, in fact, entirely heterogeneous with the interme- 
 diate one, and consequently cannot be equal to it, nor coincide therewith. 
 We will now clear up the above, and at the same time introduce a strik- 
 ing illustration of the necessity there exists, of taking into consideration 
 the nature of quantities, rather than their evanescence or infinitesimality.
 
 BOOK I.] 
 
 NEWTON'S PRINCIPIA. 
 
 11 
 
 m 
 
 Take the simplest example of LEMMA II., in the case of the right- 
 angled triangle a E A, having its two legs A a, A E equal. 
 
 The figure being constructed as in the text of LEMMA II, it fol- 
 lows from that Lemma, that the Ultimate Ratio of the inscribed and cir- 
 cumscribed figures is a ratio of equality ; and moreover it would also 
 follow from Cor. 1. that either of these 
 coincided ultimately with the triangle a 
 a E A. Hence then the exterior boundary 
 albmcndoE coincides exactly with 
 a E ultimately, and they are consequently 
 equal in the Limit. As we have only 
 straight lines to deal with in this example, 
 let us try to ascertain the exact ratio of 
 a E to the exterior boundary. 
 
 If n be the indefinite number of equal 
 bases A B, B C, &c., it is evident, since 
 A a = A E, that the whole length of 
 albmcndoE = 2nxAB. Also since 
 
 M. 
 
 D 
 
 K 
 
 a b = b c 
 2. A B, we have a E = n V 2. A B. 
 
 = &c. 
 
 = V"al 2 + bl 2 = 
 
 Consequently, 
 
 albmcndoE:aE::2: V 2 : : V~2 : 1. 
 
 Hence it is plain the exterior boundary cannot possibly coincide with 
 a E. Other examples might be adduced, but it must now be sufficiently 
 clear, that Newton confounded the ultimate equality of the inscribed and 
 circumscribed figures, to the intermediate one, with their actual coinci- 
 dence, merely from deducing their Ratios on principles of approximation 
 or rather of Exhaustion, instead of those, as explained in No. 6 ; which 
 relate to the homogeneity of the quantities. In the above example the 
 boundaries being heterogeneous inasmuch as they are incommensurable^ 
 cannot be compared as to magnitude, and unless lines are absolutely equal, 
 it is not easy to believe in their coincidence. 
 
 Profound as our veneration is, and ought to be, for the Great Father 
 of Mathematical Science, we must occasionally perhaps find fault with 
 his obscurities. But it shall be done with great caution, and only with 
 the view of removing them, in order to render accessible to students in 
 general, the comprehension of " This greatest monument of human ge- 
 nius." 
 
 20. Cor. 2. 3. and 4. will be explained under LEMMA VII, which re- 
 lates to the Limits of the Ratios of the chord, tangent and the arc.
 
 12 
 
 A COMMENTARY ON 
 LEMMA IV. 
 
 [SECT. I. 
 
 21. Let the areas of the parallelograms inscribed in the two figures be 
 denoted by 
 
 P, Q, R, &c. 
 p, q, r, &c. 
 respectively ; and let them be such that 
 
 P : p : : Q : q : : R : r, &c. : : m : n. 
 Then by compounding these equal ratios, we get 
 
 But P -f Q + R . . . . and p -f q + r + . . . . have with the curvili- 
 near areas an ultimate ratio of equality. Consequently these curvilinear 
 areas are in the given ratio of m : n. 
 
 Hence may be found the areas of certain curves, by comparing their 
 incremental rectangles with those of a known area. 
 
 Ex. 1. Required the area of the common Apollonian parabola comprised 
 between its vertex and a given ordinate. 
 
 Let a c E be the parabola, 
 whose vertex is E, axis E A and 
 Lotus-Rectum = a. Then A A' 
 being its circumscribing rectan- 
 gle, let any number of rectan- 
 gles vertically opposite to one 
 
 another be inscribed in the areas I ^* 1 D' 
 
 a E A, a E A', viz. A b, b A' ; 
 B c, c B', &c. 
 
 K 
 
 A' 
 B' 
 
 C' 
 
 And since 
 
 A b = A K. A B 
 
 A'b = A' LA' B' = 
 from the equation to the parabola. 
 
 A b a. A B 
 
 *' A' b = A K. A' B' 
 
 B 
 
 D 
 
 E 
 
 B' 
 
 Also 
 
 or 
 
 (A a) 2 Bb 2 = a X AE <zX 
 (A a + B b) X A' B' = a X A B
 
 BOOK I.] 
 
 NEWTON'S PRINCIPLE 
 
 13 
 
 a X ATB 
 .-. A/B , = A a + B b 
 
 A b _ Aa + Bb 2 Bb + Ka K_a 
 
 ' A' b = ~Fb~ B b : 2 + B b 
 
 A b 
 
 Hence, since in the Limit TTV becomes fixed or of the same nature with 
 
 the first term, we have 
 
 A b 
 
 A'b 
 
 = 2 
 
 ultimately. 
 
 And the same may be shown of all other corresponding pairs of rec- 
 tangles ; consequently by LEMMA IV. 
 
 a E A : a E A' : : 2 : 1 
 .-. a E A : rectangle A A' : : 2 : 3. 
 
 or the area of a parabola is equal to trvo thirds of its circumscribing rec- 
 tangle. 
 
 Ex. 2. To compare the area of a semiellipse with that of a semicircle 
 described on the same diameter. 
 
 3J 
 
 Taking any two corresponding inscribed rectangles P N, P' N ; we 
 have 
 
 P N : P N : : P M : F M : : a : b 
 
 a and b being the semiaxes major and minor of the ellipse ; and all other 
 corresponding pairs of inscribed rectangles have the same constant ratio ; 
 consequently by LEMMA IV, the semicircle has to the semiellipse the ratio 
 of the major to the minor axis. 
 
 As another example, the student may compare the area of a cycloid 
 with that of its circumscribing rectangle, in a manner very similar to 
 Ex. 1. 
 
 This method of squaring curves is very limited in its application. In 
 the progress of our remarks upon this section, we shall have to exhibit a 
 general way of attaining that object.
 
 14 A COMMENTARY ON [SECT. I. 
 
 LEMMA V. 
 
 22. For the definition of similar rectilinear figures, and the truth of this 
 LEMMA as it applies to them, see Euclid's Elements B. VI, Prop. 4, 19 
 and 20. 
 
 The farther consideration of this LEMMA must be deferred to the ex- 
 planation of LEMMA VII. 
 
 LEMMA VI. 
 
 23. In the demonstration of this LEMMA, " Continued Curvature" at 
 any point, is tacitly defined to be such, that the arc does not make with the 
 tangent at that point, an angle equal to ajinite rectilinear angle. 
 
 In a Commentary on this LEMMA if the demonstration be admitted, 
 any other definition than this is plainly inadmissible, and yet several of 
 the Annotators have stretched their ingenuity to substitute notions of 
 continued curvature, wholly inconsistent with the above. The fact is, 
 this LEMMA is so exceedingly obscure, that it is difficult to make any 
 thing of it. In the enunciation, Newton speaks of the angle between the 
 chord and tangent ultimately vanishing, and in the demonstration, it is 
 the angle between the arc and tangent that must vanish ultimately. So 
 that in the Limit, it would seem, the arc and chord actually coincide. 
 This has not yet been established. In LEMMA III, Cor. 2, the coinci- 
 dence ultimately of a chord and its arc is implied; but this conclusion by 
 no means follows from the LEMMA itself, as may easily be gathered from 
 No. 19. The very thing to be proved by aid of this LEMMA is, that the 
 Ultimate Ratio of the chord to the arc is a ratio of equality, it being 
 merely subsidiary to LEMMA VII. But if it be already considered that 
 they coincide, of course they are equal, and LEMMA VII becomes nothing 
 less than " argumentum in circulo" 
 
 Newton introduces the idea of curves of " continued curvature" or 
 such as make no angle with the tangent, to intimate that this LEMMA does 
 not apply to curves of non-continued curvature, or to such as do make a 
 Jinite angle with the tangent. At least this is the plain meaning of his 
 words. But it may be asked, are there any curves whose tangents are 
 inclined to them ? The question can only be resolved, by again admitting
 
 BOOK I.] 
 
 NEWTON'S PRINCIPIA. 
 
 15 
 
 the arc to be ultimately coincident with the chord ; and by then showing, 
 that curves may be imagined whose chord and tangent ultimately shall be 
 inclined at a finite angle. The Ellipse, for instance, whose minor axis 
 is indefinitely less than its major axis, is a curve of that kind ; for taking 
 the tangent at the vertex, and putting a, b, for the semiaxes, and y, x, for 
 the ordinate and abscissa, we have 
 b 2 
 
 y 2 = -7 2 X (Sax x 2 ) 
 
 and 
 
 2 a 
 
 X 1 = 
 
 , 
 = V 2 a x 
 
 .. since b is indefinitely smaller than a V x, x is indefinitely greater than 
 y, and supposing y to be the tangent cut off by the secant x parallel to 
 the axis, x and y are sides of a right angled A, whose hypothenuse is the 
 chord. Hence it is plain the L. opposite x is ultimately indefinitely 
 greater than the L. opposite to y. But they are together equal to a right 
 angle. Consequently the angle opposite x, or that between the chord and 
 tangent, is ultimately finite. Other cases might be adduced, but enough 
 has been said upon what it appears impossible to explain and establish as 
 logical and direct demonstration. We confess our inability to do this, 
 and feel pretty confident the critics will not accomplish it. 
 
 24. Having exposed the fallacy of Newton's reasoning in the proof of 
 this LEMMA, we shall now attempt something by way of substitute. 
 
 Let A D be the tangent to the curve at the 
 point A, and A B its chord. Then if B be 
 supposed to move indefinitely near to A, the 
 angle BAD shall indefinitely decrease, pro- 
 vided the curvature be not indefinitely great. 
 
 Draw R D passing through B at right an- 
 gles to AB, and meeting the tangent AD and 
 normal A R in the points D and R respective- 
 ly. Then since the angle BAD equals the 
 angle A R B, if A R B decrease indefinitely 
 when B approaches A ; that is, if A R be- 
 come indefinitely greater than A B; or 
 
 which is the same thing, if the curvature at A, be not indefinitely great; 
 the angle BAD also decreases indefinitely. Q. e. d. 
 
 We have already explained, by an example in the last article, what is 
 
 D 
 
 R 
 
 B
 
 16 A COMMENTARY ON [SECT. I. 
 
 meant by curvature indefinitely great. It is the same with Newton's ex- 
 pression " continued curvature." The subject will be discussed at length 
 under LEMMA XI. 
 
 As vanishing quantities are objectionable on account of their nothing- 
 ness as it has already been hinted, and it being sufficient to consider va- 
 riable quantities, to get their limiting ratios, as capable of indefinite diminu- 
 tion, the above enunciation has been somewhat modified to suit those 
 views. 
 
 LEMMA VII. 
 
 25. This LEMMA, supposing the two preceding ones to have been fully esta- 
 blished, would have been a masterpiece of ingenuity and elegance. By 
 the aid of the proportionality of the homologous sides of similar curves, 
 our author has exhibited quantities evanescent by others of any finite 
 magnitude whatever, apparently a most ingenious device, and calculated 
 to obviate all objections. But in the course of our remarks, it will be 
 shown that LEMMA V cannot be demonstrated without the aid of this 
 LEMMA. 
 
 First, by supposing A d, A b always finite, the angles at d and b and 
 therefore those at D and B which are equal to the former are virtually 
 considered finite, or R D cuts the chord and tangent at finite angles. 
 
 Hence the elaborate note upon this subject of Le Seur and Jacquier is 
 rendered valueless as a direct comment. 
 
 Secondly. In the construction of the figure in this LEMMA, the de- 
 scription of a figure similar to any given one, is taken for granted. But 
 the student would perhaps like to know how this can be effected. 
 
 LEMMA V, which is only enunciated, from being supposed to be a mere 
 corollary to LEMMA III and LEMMA IV, would afford the means immedi- 
 ately, were it thence legitimately deduced. But we have clearly shown 
 (Art. 19.) that rectilinear boundaries, consisting of lines cutting the inter- 
 mediate curve ultimately at finite angles, cannot be equal ultimately to the 
 curvilinear one, and thence we show that the boundaries formed by the 
 chords or tangents, as stated in LEMMA III, Cor. 2 and 3, are not ulti- 
 mately equal, by consequence of that LEMMA, to the curvilinear one. 
 
 Newton in Cor. 1, LEMMA III, asserts the ultimate coincidence, and 
 therefore equality of the rectilinear boundary whose component lines cut 
 the curve at finite angles, and thence would establish the succeeding cor-
 
 BOOK 1.1 NEWTON'S PRINCIPIA. 17 
 
 ollaries a fortiori. But the truth is that the curvilinear boundary is the 
 limit, as to magnitude, or length, of the tangential and chordal bounda- 
 ries ; although in the other case, it is a limit merely in respect of area. 
 Yet, we repeat it, that LEMMA V cannot be made to follow from the 
 LEMMAS preceding it. According to Newton's implied definition of simi- 
 lar curves, as explained in the note of Le Seur and Jacquier, they are the 
 curvilinear limits of similar rectilinear figures. So they might be consi- 
 dered, if it were already demonstrated that the limiting ratio of the chord 
 and arc is a ratio of equality ; but this belongs to LEMMA VII. Newton 
 himself and all the commentators whom we have perused, have thus 
 committed a solecism. Even the best Cambridge MSS. and we have 
 seen many belonging to the most celebrated private as well as college tu- 
 tors in that learned university, have the same error. Nay most of them 
 are still more inconsistent. They give definitions of similar curves wholly 
 different from Newton's notion of them, and yet endeavour to prove 
 LEMMA V, by aid of LEMMA VII. For the verification of these asser- 
 tions, which may else appear presumptuously gratuitous, let the Cantabs 
 peruse their MSS. The origin of all this may be traced to the falsely 
 deduced ultimate coincidence of the curvilinear and rectilinear boundaries, 
 in the corollaries of LEMMA III. See Art. 19. 
 
 We now give a demonstration of the LEMMA without the assistance of 
 similar curves, and yet independently of quantities actually evanescent. * 
 
 By hypothesis the secant R D cuts the chord and tangent at finite an- 
 gles. Hence, since 
 
 A + B + D = 180 
 .-. B -f D = 180 A 
 
 or L + 1+L'+1' = 180 A 
 
 L and L' being the limits of B and D and 1, 1' their variable parts as in 
 Art. 6 ; and since by LEMMA VI, or rather by Art. 24, A is indefinitely 
 diminutive, we have, by collecting homogeneous quantities 
 
 L + L' = 180 
 
 But A B, A D being ultimately not indefinitely great, it might easily 
 be shown from Euclid that L = L', and .-. A B = A D ultimately, (see 
 Art. 6 ) and the intermediate arc is equal to either of them.
 
 IB 
 
 A COMMENTARY ON 
 
 [SECT. I. 
 
 OTHERWISE, 
 
 If we refer the curve to its axis, 
 A a, B b being ordinates, &c. as 
 in the annexed diagram. Then, 
 by Euclid, we have 
 
 AD 2 = AB 2 + BD 2 -f 
 
 A D 
 
 A 
 
 = 1 + B D. 
 
 2 B D. B d 
 B D + 2 B d 
 
 AB'*" AB 2 
 
 Now, since by Art. 24 or LEMMA VI, the L- B A D is indefinitely less 
 than either of the angles B or D, .'. B D is indefinite compared with A B 
 
 A D 
 
 or A D. Hence L being the limit of * 1> an d 1 its variable part, if we 
 
 extract the root of both sides of the equation and compare homogeneous 
 terms, we get, 
 
 L = 1 or &c. &c. 
 
 26. Having thus demonstrated that the limiting Ratio of the chord, arc 
 and tangent, is a ratio of equality, 'when the secant cuts the chord and tangent 
 at FINITE angles, we must again digress from the main object of this work, 
 to take up the subject of Article 17. By thus deriving the limits of the rati- 
 os of the finite differences of functions and their variables, directly from the 
 LEMMAS of this Section, and giving to such limits a convenient algorithm 
 or notation, we shall not only clear up the doctrine of limits by nume- 
 rous examples, but also prepare the way for understanding the abstruser 
 parts of the Principia. This has been before observed. 
 
 Required to Jind the Limit of the Finite Differences of the sine of a cir- 
 cular arc and of the arc itself, *or the Ratio of their Differentials. 
 
 Let x be the arc, and A x its finite variable increment. Then L being 
 the limit required and L + 1 the variable ratio, we have 
 
 T , __ A sin. x sin. (x + A x) sin. x 
 JL -f- 1 
 
 AX AX 
 
 sin, x. cos. (A x) -f- cos, x. sin. (A x) sin, x 
 
 A X 
 
 sin. (A x) sin.x. cos. AX sin. x 
 
 = COS. X. -I 
 
 A X AX A X 
 
 Now by LEMMA VII, as demonstrated in the preceding Article, the li- 
 
 . f sin. A x . 
 nut or is 
 
 A X 
 
 , cos. (A x) sin. x , . ,, ., r ., 
 
 1, and s -, have no definite limits. 
 
 A X 
 
 A X
 
 BOOK I.] NEWTON'S PRINCIPIA. 19 
 
 Consequently putting 
 
 sin. (A x) 
 cos. x. s = cos. x + 1', 
 
 A X 
 
 we have 
 
 sin. x. cos. A x sin. x 
 
 L + 1 = cos. x + I' + 
 
 AX AX 
 
 and equating homogeneous terms 
 
 L = cos. x 
 or adopting the differential symbols 
 
 d. sin. x ~\ 
 
 j = cos x I 
 
 > (a) 
 
 or I 
 
 d sin. x = d x. cos. x ) 
 
 27. Hence and from the rules for the differentiation of algebraic, expo- 
 nential, &c. functions, we can differentiate all other circular functions of 
 one variable, viz. cosines, tangents, cotangents, secants, &c. 
 
 Thus, 
 
 dsin. (I-*) 
 
 = COS. I _ X ) = Sill. X 
 
 J f* \ 2 ' 
 
 d -(i- x ) 
 
 or 
 
 d. cos. x 
 
 j = sin. x 
 
 or 
 
 d. cos. x ~\ 
 
 dx v . ^ ; v r. (b) 
 
 or I 
 
 d. cos. x = d x. sin. x J 
 Again, since for radius 1, which is generally used as being the most simple, 
 
 1 + tan. 2 x = sec. z x = ; 
 
 cos. 2 x 
 
 , 1 2 cos. x. d. cos. x 
 
 .'. 2 tan. x. d. tan. x = d. = r 
 
 cos, x cos. x 
 
 See 12 (d). Hence and from (b) immediately above, we have 
 
 d x. sin. x 
 
 tan. x. d. tan. x = , 
 
 cos. 3 x 
 
 .'. d. tan. x = d x. 
 
 cos. * x 
 Again, 
 
 cot. x = 
 
 tan. x 
 
 B2
 
 20 A COMMENTARY ON [SECT. I. 
 
 Therefore, 
 
 1 d. tan. x 
 
 d. COt. X = d. _ x = -JJJTJ- (lid) 
 
 tan. 2 x. cos. 2 x sin. 2 x 
 Again, 
 
 1 
 
 (d) 
 
 sec. x = 
 
 cos. x 
 
 ~ d * X 
 
 .-. d. sec. x = d. -i = ~ u 7*' A (12. d) 
 cos. x cos. 2 x 
 
 d x. sin. x v 
 
 COS. 2 X 
 
 and lastly since cosec. x = sec. ( 
 
 we have 
 
 d. ( x ) sin. ( x j 
 
 i , /w \ \2 / ^2 / 
 
 d. cosec. x = d. sec. f xj = 
 
 d x. cos. x 
 
 (0 
 
 sin. x 
 
 Any function of sines, cosines, &c. may hence be differentiated. 
 
 28. In articles 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 26 and 27, are to 
 be found forms for the differentiation of any function of one variable, 
 whether it be algebraic, exponential, logarithmic, or circular. 
 
 In those Articles we have found in short, the limit of the ratio of the 
 first difference of a function, and of the first difference of its variable. 
 Now suppose in this first difference of the function, the variable x should 
 be increased again by A x, then taking the difference between the first 
 difference and what it becomes when x is thus increased, we have the dif- 
 ference of the first difference of a function, or the second difference of a 
 function, and so on through all the orders of differences, making A x al- 
 ways the same, merely for the sake of simplicity. Thus, 
 A (x 3 ) = (x + A x) 3 x 3 
 
 + AX 3 3x 2 Ax 
 
 - 3XAX 2 - AX 3 
 
 3. 2xAX 5 3Ax 3
 
 BOOK L] NEWTON'S PRINCIPIA. 21 
 
 denoting by A 2 the second difference. 
 Hence, 
 
 (-P = 3. 2. x + 3 A x 
 
 and if the limiting ratio of A 2 (x s ) and Ax 2 , or the ratio of the second 
 differential of x 3 , and the square of the differential of its variable x, be 
 required, we should have 
 
 L + 1 = 3. 2. x + 3 A x 
 
 and equating homogeneous terms 
 
 d 2 f x 3 } 
 
 VV = L = 3. 2. x 
 
 d x 2 
 
 In a word, without considering the difference, we may obtain the se- 
 cond, third, &c. differentials d 2 u, d 3 u, &c. of any function u of x im- 
 
 mediately, if we observe that -, is always a function itself of x, and 
 
 make d x constant. For example, let 
 
 u = ax n + bx m + &c. 
 Then, from Art. 13. we have 
 
 -r^ = nax n - 1 + mbx m - 1 + &c. 
 
 , /d u\ 
 
 Vd x/ d(du) d 2 u., 
 j - = r - = j (by notation) 
 d x d x 8 d x 2 v J 
 
 = n. (n l)ax n ~ 2 + m (m l)bx m ~ 2 + &c.- 
 Similarly, 
 
 j^p = n. (n 1). (n 2)ax n - 3 + &c. 
 
 &c. = &c. 
 
 Having thus explained the method of ascertaining the limits of the ra- 
 tios of all orders of finite differences of a function, and the corresponding 
 powers of the invariable first difference of the variable, or the ratios of the 
 differentials of all orders of a function, and of the corresponding power 
 of the first differential of its variable, we proceed to explain the use of 
 these limiting ratios, or ratios of differentials, by the following 
 
 B3
 
 A COMMENTARY ON 
 
 [SECT. I. 
 
 APPLICATIONS 
 
 OF THE 
 
 DIFFERENTIAL CALCULUS. 
 
 29. Let it be required to draw a tangent to a given curve at any given 
 point of it. 
 
 Let P be the given point, and A M' 
 being the axis of the curve, let P M 
 = y, A M = x,be the ordinate and 
 abscissa. Also let P' be any other 
 point; draw P N meeting the ordi- 
 nate P' M' in N, and join P P'. Now 
 let T P R meeting M' P' and M A in 
 R and T be the tangent required. 
 
 Then since by similar triangles 
 
 P 7 N : P N : : P M : M T' 
 
 .-. M T = M T + T T' = y. " 
 
 Now y being supposed, as it always is in curves, a function of x, we have 
 seen that whether that function be algebraic, exponential, &c. 
 
 in the limit, or -T - is always a definite function of x. Hence putting 
 
 A x 
 
 *7 
 
 dx 
 dy 
 
 1 
 
 we have 
 
 M T + T T = y (^ + 1) 
 and equating homogeneous terms, 
 
 dy 
 
 (e) 
 
 which being found from the equation to the curve, the point T will be 
 known, and therefore the position of the tangent P T. M T is called 
 the subtangent. 
 
 Ex. 1. In the common parabola, 
 
 y 2 = a x
 
 BOOK L] NEWTON'S PRJNCIPIA. 23 
 
 Therefore, 
 
 d x 2 y 
 d y ~ a 
 and 
 
 2 y 2 
 MT = -^- = 2x 
 
 or the subtangent M T is equal to twice the abscissa. 
 Ex. 2. In the ellipse, 
 
 b 2 
 
 ,r 2 / 2 v 2\ 
 
 y ~ a 2 ^ x / 
 
 and it will be found by differentiating, &c. that 
 
 _ (a 2 _ x 2 ) 
 MT = 
 
 A 
 
 Ex. 3. In the logarithmic curve, 
 
 y = a x 
 
 .-. ~ = 1. a X y (see 17.) 
 
 which is therefore the same for all points. 
 
 The above method of deducing the expression for the subtangent is 
 strictly logical, and obviates at once the objections of Bishop Berkeley 
 relative to the compensation of errors in the denominator. The fact is, 
 these supposed errors being different in their very essence or nature from 
 the other quantities with which they are connected, must in their aggre- 
 gate be equal to nothing, as it x has been shown in Art. 6. This ingenious 
 critic calls P' R = z ; then, says he, (see fig. above) 
 
 y. d x 
 M T ~ dy + z accurate ty J 
 
 whereas it ought to have been 
 
 MT = -^= Z 
 
 A y + z Ay z 
 
 AX "*" A X 
 
 A y 
 
 the finite differences being here considered. Now in the limit, - becomes a 
 
 d y 
 definite function of x represented by -p^* Consequently if 1 be put for 
 
 A y 
 the variable part ,of ~- t we have
 
 24 A COMMENTARY ON [SECT. I. 
 
 _ i. , i | _ 
 
 dx + l + A x . 
 and it is evident from LEMMA VII and Art. 25, that z is indefinite com- 
 
 z d y 
 
 pared with A x. .. ^ is indefinite compared with M T, -j , and y ; 
 
 and 1 is also so ; hence 
 
 gives 
 
 v. d x 
 
 MT = Ty-> 
 
 which proves generally for all curves, what Berkeley established in the 
 case of the common parabola ; and at the same time demonstrates, as had 
 been already done by using T T 7 instead of P' R, incontestably the ac- 
 curacy of the equation for the subtangent. 
 
 30. If it were required to draw a tangent to any point of a curve, re- 
 ferred to a center by a raditis-vector and the L. 6 which g describes by 
 revolving round the fixed point, instead of the rectangular coordinates 
 x, y ; then the mode of getting the subtangent will be somewhat different. 
 
 Supposing x to originate in this center, it is plain that 
 
 X = COS. d \ 
 
 y = f sin. 6 j 
 
 and substituting for x, y, d x, d y, hence derived hi the expression (29. 
 e.) we have 
 
 d f cos. 6 f d d sin. 6 
 
 r = sm ' 6 X d t sin. d + g d $ cos. t - (0 
 Ex. In the parabola 
 
 2 a 
 = 1 _ C os. 6 > 
 
 where a is the distance between the focus and vertex, or the value of g at 
 the vertex. Then substituting we get, after proper reductions 
 
 1 + cos. d 
 MT=2aX ,1^., 
 
 and the distance from the focus to the extremity of the subtangent is 
 
 / 1 + cos. 6 cos. \ 
 
 M T _ g cos. 9 = 2 a tfHtSTi ~ 1 - cos. t)
 
 BOOK L] NEWTON'S PRINCIPIA. 25 
 
 2a 
 
 "1 cos. Q *~ ?' 
 as is well known. 
 
 30. a. The expression (f) being too complicated in practice, the following 
 one may be substituted for it. 
 
 Let P T be a tangent to the R 
 
 curve, referred to the center S, 
 at the point P, meeting S T 
 drawn at right angles to S P, 
 in T ; and let P' be any other 
 point. Join P P' and produce 
 it to T'', and let T P be pro- 
 duced to meet S P' produced in 
 R, &c. Then drawing P N parallel to S T, we have 
 
 STV C T^ I T^ T^ \s C Ty 
 
 i oi-f-ii Civ x o IT 
 
 But 
 
 P N = i tan. A 0, S P' = f -f. A 
 and 
 
 Therefore, substituting and equating homogeneous terms, after having 
 applied LEMMA VII to ascertain their limits, we get 
 
 Ex. 1. In the spiral of Archimedes we have 
 
 s = a ' ; 
 ...ST = tf- : -.' \ '- 
 
 Ex. 2. In the hyperbolic spiral 
 
 .-. S T = a 
 
 31. It is sometimes useful to know the angle between the tangent and 
 axis. 
 
 PM dy 
 
 See fig. to Art 29.
 
 26 ^ A COMMENTARY ON [SECT. I. 
 
 Again, in fig. Art. 30 a. 
 
 SP dg 
 Tan - T = ST = JT* ........ < k) 
 
 32. It is frequently of great use, in the theory of curves and in many 
 other collateral subjects, to be able to expand or develope any given func- 
 tion of a variable into an infinite series, proceeding according to the 
 powers of that variable. We have already seen one use of such develop- 
 ments in Art. 17. This may be effected in a general manner by aid of 
 successive differentiations, as follows. 
 
 If u = f (x) where f (x) means any function of x, or any expression 
 involving x and constants ; then, as it has been seen, 
 
 d u = u' d x 
 
 (u' being a new function of x) 
 Similarly 
 
 d u' = u" d x 
 d u" - u'" d x 
 
 &c. = &c. 
 But 
 
 d 2 uXdx d 2 xxdu 
 
 &c. = &c. 
 
 denoting d. (d u), d. (d x) by d 2 u, d 2 x, and (d x) 2 by d x 2 , 
 according to the received notation; 
 
 Or, (to abridge these expressions) supposing dx constant, and .. d 2 x = 0, 
 
 which give the various orders of fluxions required. 
 
 Ex. 1. Let u = x n 
 Then 
 
 du 
 
 ~i ~ n x a * 
 d x ~
 
 BOOK I.] NEWTON'S PRINCIPIA. 27 
 
 ^ = n. (n 1). (n_2)x~ 3 
 
 &c. = &c. 
 
 d n u 
 
 j^ = n. (n 1). (n 2) 3. 2. 1. 
 
 Ex. 2. Let u = A + B x + C x 2 + D x 3 + E x 4 + &c. 
 Then, 
 
 ~ = B + 2Cx + 3l)x 2 + 4Ex 3 + &c. 
 
 Ci A 
 
 d 2 u 
 
 jY 2 = 2 C -f 2. 3 D x + 3. 4 E x 2 + &c. 
 
 d 3 u 
 
 3-^-3 = 2. 3 D + 2. 3. 4 E x + &c. 
 
 &c. = &c. 
 
 Hence, if u be known, and the coefficients A, B, C, D, &c. be un- 
 known, the latter may be found ; for if U, U', U", U'", &c. denote the 
 
 du d 2 u d 3 u 
 values of 11,3-,^,, g , , &c. 
 
 when x = 0, then 
 
 A = U, B = U', C = ~ U", D = ~ U'", E = -^~ U"", 
 
 &c. = &c. 
 
 and by substitution, 
 
 u = U + U' x + U" Y + U"' ^3 + &c. . '.|. ',. . . (b) 
 This method of discovering the coefficients is named (after its inventor), 
 
 MACLAURIN'S THEOREM. 
 
 The uses of this Theorem in the expansion of functions into series are 
 many and obvious. 
 
 For instance, let it be required to develope sin. x, or cos. x, or tan. x, 
 or 1. (1 + x) into series according to the powers of x. Here 
 u = sin. x, or = cos. x, or = tan. x, or = 1. (1 + x), 
 
 du 11 
 
 ' g~i = :os. x, or := sin. x, or == ^ 2 or - j- + x 
 
 d 2 u 2 sin. x 1 
 
 dx2 = sin. x, or = cos. x, or = ^73^' or = (1 + x) ,
 
 28 A COMMENTARY ON 
 
 d 3 u 2 + 4 sin. 2 x 
 
 j^s = cos. x, or = sin. x, or = CQS 4 x or = 
 
 &c. = &c. 
 
 .-. U =0, or = 1, or = 0, or = 
 
 U' =1, or = 0, or = 1, or = 1 
 
 U" =0, or = 1, or = 0, or = I 
 
 U'" = 1, or = 0, or = 2, or = 2 
 
 &c. = &c. 
 
 Hence 
 
 x 3 x 5 
 sin. x = x ir~3 + o Q A K &c. 
 
 x 2 x 
 cos. x = 1 ~ + &c> 
 
 2. 3. 4. 5 
 x 4 
 2.3.4 
 
 x 3 2 x 5 17 x 7 
 tan. x = x + -3 + 375- + 375^7 + & c - 
 
 X 2 X 3 
 
 L(l +x) = x -3- +-3- &c. 
 Hence may also be derived 
 
 TAYLOR'S THEOREM. 
 
 For let 
 
 f(x) = A + Bx + Cx 2 + Dx 3 + Ex 4 + &c. 
 Then 
 
 f (x + h) = A + B. (x + h) + C. (x + h) 2 -f- D. (x + h) 3 + &c. 
 = A + Bx + Cx 2 + Dx 3 + &c. 
 + (B + 2 Cx + 3Dx*)h 
 + (C + 3Dx + 6 Ex 2 ) h 2 
 + (D + 4 Ex + 10 Fx 2 ) h 3 
 &c. 
 
 the theorem in question, which is also of use in the expansion of series. 
 
 For the extension of these theorems to functions of two or more varia- 
 bles, and for the still more effective theorems of Lagrange and Laplace, 
 the reader is referred to the elaborate work of Lacroix. 4to. 
 
 Having shown the method of rinding the differentials of any quanti-
 
 BOOK I.] NEWTON'S PRINCTPIA. 29 
 
 ties, and moreover, entered in a small degree upon the practical applica- 
 tion of such differentials, we shall continue for a short space to explain 
 their farther utility. 
 
 33. Tojlnd the MAXIMA and MINIMA of quantities. 
 
 If a quantity increase to a certain magnitude and then decrease, the 
 state between its increase and decrease is its maximum. If it decrease 
 to a certain limit, and then increase, the intermediate state is its mi- 
 nimum. Now it is evident that in the change from increasing to decreas- 
 ing, or vice versa, which the quantity undergoes, its differential must have 
 changed signs from positive to negative, or vice versa, and therefore (since 
 moreover this change is continued) have passed through zero. Hence 
 When a quantity is a MAXIMUM or MINIMUM, its differential =r 0. . . (a) 
 
 Since a quantity may have several different maxima and minima, (as for 
 instance the ordinate of an undulating kind of curve) it is useful to have 
 some means of distinguishing between them. 
 
 34. To distinguish between MAXIMA and MINIMA. 
 
 LEMMA. To show that in Taylor's Theorem (32. c.) any one term can 
 be rendered greater than the sum of the succeeding ones, supposing the 
 coefficients of the powers of h to be finite. 
 
 Let Q h n ' be any term of the theorem, and P the greatest coefficient 
 of the succeeding terms. Then, supposing h less than unity, 
 
 P h n (1 + h + h 2 + . . . . in infra.) = P h X ]_ R 
 
 is greater than the sum ( S) of the succeeding terms. But supposing h to 
 decrease in infin. 
 
 1 
 
 P h- " i n = P h n ultimately. Hence ultimately 
 
 Ph n > S 
 Now 
 
 Qhn-l . p h n ; ; Q; ph, 
 
 and since Q and P are finite, and h infinitely small ; therefore Q is > P h, 
 Hence Qh n ~ 1 is> Ph n , and a fortiori > S. 
 
 Having established this point, let 
 
 i u = f (x) 
 
 be the function whose maxima and minima are to be determined ; also 
 when u = max. or min. let x = a. Then by Taylor's Theorem 
 
 f/ d u . d 2 u h 2 d 3 u h 3 
 
 f (a - h) = f - d- a h + d^' ~ 37*0 + &c "
 
 30 A COMMENTARY ON [SECT. I. 
 
 and 
 
 and since by the LEMMA, the sign of each term is the sign of the sum of 
 that and the subsequent terms, 
 
 .-. f (a h) = f (a) i?. M 
 
 . 
 d a 
 
 f(a + h) = 
 
 Now since f (a) = max. or min. f (a) is > or < than both f (a h) 
 and f (a + h), which cannot be unless 
 
 Hence 
 
 d'u 
 
 f(a-h) =f(a) 
 
 f (a + h) = f (a) 
 
 and f (a) is max. or min. or neither, according as f (a) is >, < or = to 
 both f (a h) and f (a + h), or according as 
 
 d 2 u . 
 
 -3 - is negative, positive, or zero 
 
 If it be zero as well as -: , we have 
 d a 
 
 f(a + h) = f (a) + -r. N" 
 
 and f (a) cannot = max. or min. unless 
 d 3 u 
 
 which being the case we have 
 
 f (a h) = f a + 15, M" 
 
 f(a + h) = fa + i 
 
 and as before,
 
 BOOK I.] NEWTON'S PRINCIPIA. 31 
 
 d 4 u . 
 f (a) is max. or min. or neither, according as -= - is negative, positive, or 
 
 zero, and so on continually. 
 
 Hence the following criterion. 
 
 If in u = f (x), -j - = 0, the resulting value of x shall give u = MAX. 
 
 d 2 u . 
 or MIN. or NEITHER, according as -= 2 zs negative, positive, or zm?. 
 
 tl X 
 
 //" -^ = 0, - = 0, and -: = 0. then the resulting value of u 
 ' d x d x 2 dx 3 
 
 d 4 u 
 shall be a MAX., MIN. or NEITHER according as -^ ^ is NEGATIVE, PO- 
 
 SITIVE, or ZERO ; and so on continually. 
 
 Ex. 1. To Jind the MAX. and MIN. of the ordinate of a common para- 
 bola. 
 
 y = V a x 
 d y 1 V a 
 " d x ~ " ' v~x 
 winch cannot = 0, unless x = a . 
 
 Hence the parabola has no maxima or minima ordinates. 
 Ex. 2. To Jind the MAXIMA and MINIMA of y in the equation 
 
 y 2 2 a x y + x 2 = b 2 . 
 Here 
 
 2 a 
 
 dy_ay xd 2 y dx 
 
 d x ~" y ax'dx*~ y ax 
 
 dy 
 
 and putting -a-*- = 0, we get 
 
 ^ab + b d 
 
 : 
 
 _ 
 
 1 a 2 )' V (1-^O J d x 2 ~ ITV~(1 a ~) 
 
 which indicate and determine both a maximum and a minimum. 
 
 Ex. 3. To divide a in such a manner that the product of the m tb power 
 of the one part, and the w th power of the other shall be a maximum. 
 Let x be one part, then a x = the other, and by the question 
 u = x m . (a x) n = max. 
 
 d u _ 
 
 .*. -j = x m - l . (a x) n - x X (ma x. m + n)
 
 32 A COMMENTARY ON [SECT. I. 
 
 and 
 
 u 
 
 dx 1 
 
 = x m -* (a x) n ~ 2 X (m + n 1. m + n. x 2 &c.) 
 
 Put -5 = ; then 
 d x 
 
 m a 
 
 x = 0, or x = a, or x = - , 
 
 m -j- n 
 
 the two former of which when m and n are even numbers give minima, 
 
 and the last the required maximum. 
 
 \_ 
 
 Ex. 4. Let u = x x . 
 Here 
 
 d u 1 1. x 
 
 j-^ = u. j 0, .'. 1. x = 1, and x = e the hyperbolic base 
 
 = 2.71828, &c. 
 
 Innumerable other examples occur in researches in the doctrine of 
 curves, optics, astronomy, and in short, every branch of both abstract and 
 applied mathematics. Enough has been said, however, fully to demon- 
 strate the general principle, when applied to functions of one independent 
 variable only. 
 
 For the MAXIMA and MINIMA of functions of two or more variables, see 
 Lacroix, 4to. 
 
 35. If in the expression (30 a. g) S T should be finite when g is infinite, 
 then the corresponding tangent is called an Asymptote to the curve, and 
 since g and this Asymptote are both infinite they are parallel. Hence 
 To Jind the Asymptotes to a curve, 
 
 dd 
 In S T = z -J , make f = a , then eachjinite value of S T gives an 
 
 Asymptote ; which may be drawn, by finding from the equation to the 
 curve the values of 6 for = a, (which will determine the positions of g), 
 then by drawing through S at right angles to , S T, S T', S T", &c. the 
 several values of the subtangent of the asymptotes, and finally through 
 T, T', T", &c. perpendiculars to S T, S T 7 , S T", &c. These perpen- 
 diculars will be the asymptotes required. 
 Ex. In the hyperbola 
 
 _ _ __ 
 ^ ~~ a ( 1 e cos. 0)' 
 
 Here = a , gives 1 e cos. 6 = 0, .'. cos. 6 = 
 
 .'. + 6 = t, whose cos. is -- 
 
 e
 
 BOOK I.] 
 Also S T = 
 
 NEWTON'S PRINCIPIA. 
 
 b* 
 
 33 
 
 =r b ; whence it will be seen that 
 
 a e sin. 6 a V e 2 1 
 the asymptotes are equally inclined (viz. by L. 6) to the axis, and pass 
 through the center. 
 
 The expression (29. e) will also lead to the discovery and construction 
 of asymptotes. 
 
 Since the tangent is the nearest straight line that can be drawn to the 
 curve at the point of contact, it affords the means of ascertaining the in- 
 clination of the curve to any line given in position ; also whether at any 
 point the curve be inflected, or from concave become convex and vice ver- 
 sa ; also whether at any point two or more branches of the curve meet, 
 i. e. whether that point be double, triple, &c. 
 
 36. To Jind the inclination of a curve at any point of it to a given line . 
 Jind that of the tangent at that given point, which will be the inclination 
 
 required. 
 
 Hence if the inclination of the tangent to the axis of a curve be zero, 
 the ordinate will then be a maximum or minimum ; for then 
 
 tan. T = i| = (31. h) 
 
 37. To Jind the points of Inflexion of a curve. 
 
 A B A B 
 
 Let y = f (x) be the equation to the curve a b ; then A. a, B b being 
 any two ordinates, and ana tangent-at the point a, if we put A a == y, 
 and A B = h, we get 
 A a = f x 
 
 d v . d 2 v h 2 
 
 + TIT. + & c ' ( 32 c ) 
 
 dx 2 1.2 
 
 But 
 
 dx 
 
 n = y. 4- T*'. Consequently B b is < or > B n 
 
 * dx 
 
 j- o-y . . ... 
 
 according as -r - t is negative or positive, i. e. the curve ts concave or coti-
 
 34 A COMMENTARY ON [SECT. I. 
 
 d 2 v 
 vex towards its axis according as -= - t is negative or positive. 
 
 Hence also, since a quantity in passing from positive to negative, and 
 vice versa, must become zero or infinity, at a point of inflexion 
 
 d 2 y 
 
 -r = or 
 
 d x 2 
 
 Ex. In the Conchoid ofNicomedes 
 
 x y = (a + y ) v (b 2 y ) 
 
 which gives, by making d y constant, 
 
 d 2 x _ 2 b *a b 2 y__ 3 b g ay * 
 d y 2 = : (b~ y 3 ITy5)~v' (b~* _ y y 
 
 and putting this = 0, and reducing, there results 
 
 y 3 + Say 2 = 2 b 2 a 
 which will give y and then x. 
 
 These points of inflexion are those which the Theory of (34) indicates 
 as belonging to neither maxima nor minima ; and pursuing this subject 
 still farther, it will be found, in like manner, that in some curves 
 
 d 4 v d 6 v 
 
 -5 *t = or a , -r i = or a , &c. = &c. 
 
 d x 4 d x 6 
 
 also determine Points of Inflexion. 
 
 38. Tojind DOUBLE, TRIPLE, fyc. points of a curve. 
 
 If the branches of the curve cut one another, there will evidently be as 
 many tangents as branches, and consequently either of the expressions, 
 
 Tan. T = i (31. h) 
 
 d x 
 
 M T = ^^ (29. e) 
 
 as derived from the equation of the curve, will have as many values as 
 there are branches, and thus the nature and position of the point will be 
 ascertained. 
 
 If the branches of the curve touch, then the tangents coincide, and the 
 method of discovering such multiple points becomes too intricate to be in- 
 troduced in a brief sketch like the present. For the entire Theory of 
 Curves the reader is referred to Cramer's express treatise on that subject, 
 or to Lacroix's Different, and Integ. Calculus, 4to. edit. 
 
 39. We once more return to the text, and resume our comments. We 
 pass by LEMMA VIII as containing no difficulty which has not been al- 
 ready explained. 
 
 As similar figures and their properties are required for the demonstra-
 
 BOOK I.] NEWTON'S PRJNCIPIA. 35 
 
 tion of LEMMA IX, we shall now use LEMMA VII in establishing LEMMA 
 V, and shall thence proceed to show what figures are similar and how to 
 construct them. 
 
 According to Newton's notion of similar curvilinear figures, we may 
 define two curvilinear ^figures to be similar when any rectilinear polygon 
 being inscribed in one of them, a rectilinear polygon similar to the former, 
 may always be inscribed in the other. 
 
 Hence, increasing the number of the sides of the polygons, and dimi- 
 nishing their lengths indefinitely, the lengths and areas of the curvilinear 
 figures will be the limits by LEMMAS VII and III, of those of the recti- 
 linear polygons, and we shall, therefore, have by Euclid these lengths 
 and areas in direct and duplicate proportions of the homologous sides 
 respectively. 
 
 40. To construct curves similar to given ones. 
 
 If y, x be the ordinate and abscissa, and x' the corresponding abscissa 
 of the required curve, we have 
 
 x : y : : x' : i X x' = y' (a) 
 
 the ordinate of the required curve, which gives that point in it which 
 corresponds to the point in the given curve whose coordinates are x, y ; 
 and in the same manner may as many other points as M*e please be de- 
 termined. 
 
 In such curves, however, as admit a practical or mechanical construc- 
 tion, it will frequently be sufficient to determine but one or two values of y 7 . 
 
 Ex. 1. In the circle let x, measured along the diameter from its extre- 
 mity, be r (the radius) ; then y = r, and we have 
 
 y' = -2- X x' = x' 
 x 
 
 where x' may be of any magnitude whatever. Hence, all semicircles, and 
 therefore circles, are similar Jigures. 
 
 Ex. 2. In a circular arc (2 a) let x be measured along the chord (2 b), 
 and suppose x = r sin. a ; then y = r . vers. a 
 
 vers. a 
 
 y = . X x 
 
 sin. a 
 
 which gives the greatest ordinate to any semichord as an abscissa, of the 
 required arc, and thence since 
 
 y' = r' V r' 2 x' * 
 
 it will be easy to find the radius r' and centre, and to describe the arc 
 required.
 
 t> A COMMENTARY ON [SECT. I. 
 
 But since 
 
 y' r' vers. a! vers. a' vers. a 
 
 x' r 1 sin. a.' sin. a' sin. a 
 
 therefore 
 1 cos. a 
 
 2 sin. 
 
 1 cos. a' 
 
 2 sin. 
 
 '"5 
 
 sin. a 
 
 a a 
 2 cos. -sin.- 
 
 sin. a' 
 
 ~ a/ 
 2 cos. - 
 
 a' 
 sin. 
 
 or 
 
 a a' 
 
 tan. = tan. , 
 
 and 
 
 .-. a = a' 
 
 which accords with Euclid, and shows that similar arcs of circles subtend 
 equal angles. 
 
 Ex. 3. Given an arc of a parabola, whose latus-rectum is p, to Jind a 
 similar one, whose latus-rectum shall be p'. 
 
 In the first place, since the arc is given, the coordinates at its extremi- 
 ties are ; whence may be determined its axis and vertex ; and by the usual 
 mode of describing the parabola it may be completed to the vertex. 
 Now, since 
 
 y 2 = p x 
 x, x' being measured along the axis, and when 
 
 --, y =^ 
 
 4 ' y 2 
 
 P P 
 
 X =r v JE- 
 
 .-. y' = -- . x' = -- . x' = 2 x' 
 x y 
 
 which shows that all semi-parabolas, and therefore parabolas, are similar 
 figures. Hence, having described upon the axis of the given parabola, 
 any other having the same vertex, the arc of this latter intercepted be- 
 tween the points whose coordinates correspond to those of the extremi- 
 ties of the given arc will be the arc required. 
 
 Ex. 4. In the ellipse whose semi-diameters are a, b, if x be measured 
 along the axis, when x = a, y = b. Hence 
 
 and x' or the semi-axis major being assumed any whatever, this value of 
 y' will give the semi~axis minor, whence the ellipse may be described. 
 This being accomplished, let (a, /3) (a', p) be the coordinates at the
 
 BOOK L] NEWTON'S PRINCIPIA. 37 
 
 extremities of any given arc of the given ellipse, then the similar one of 
 the ellipse described will be that intercepted between the points whose 
 coordinates, (x', y') (x", y") are given by 
 
 y' = V (2 a' x' x' 2 ) 
 : /3 : : x' : y' 1 ^ * a' 
 
 a : /3' : : x'' : y' f f * b" ,. 
 
 y' = V (2 a x ' x 2 ) 
 
 
 
 In like manner it may be found, that 
 
 All cycloids are similar. 
 
 Epicycloids are so, "when the radii of their "wheels or radii of the spheres. 
 
 Catenaries are similar when the bases a tensions, fyc. Sfc. 
 
 40. If it were required to describe the curve A c b (fig. to LEMMA 
 VII) not only similar to A C B, but also such that its chord should be of 
 the given length (c) ; then having found, as in the last example, the co- 
 ordinates (x', y') (x", y") in terms of the assumed value of the abscissa^ 
 (as of in Ex. 4), and (a, /3), (a', /3') the coordinates at the extremities of 
 the given arc, we have 
 
 C = V~(X' X") 2 + (y' y'^p = f (') 
 a function of a' : whence a' may be found. 
 
 Ex. In the case of a parabola whose equation is y 2 = a x, it will be 
 found that (y' 2 = a' x' being the equation of the required parabola) 
 
 whence (a') is known, or the latus-rectum of the required parabola is so 
 determined, that the arc similar to the given one shall have a chord = c. 
 
 41. It is also assumed in the construction both to LEMMA VII and 
 LEMMA IX, that, If in similar Jigures, originating in the same point, the 
 chords or axes coincide, the tangents at that origin will coincide also. 
 
 Since the chords A B, A b (fig. to LEMMA VII), the parallel secants 
 B D, b d, and the tangents A D, A d are corresponding sides, each to 
 each, to the similar figures, we have (by LEMMA V) 
 
 A B : B D : : A b : b d 
 
 and L. B = L. b. Consequently, by Euclid the /iBAD = bAd, 
 or the tangents coincide.
 
 38 
 
 A COMMENTARY ON 
 
 [SECT. I. 
 
 Mb : 
 Mb 7 : 
 
 a b : : 
 a 7 b 7 : 
 
 BC : 
 : B 7 C 7 
 
 : A a : Bb \ 
 : A 7 a 7 : B'b 7 ) 
 
 A a : B C 1 
 
 : A 7 a 7 : B 7 C 7 J 
 
 : M a : A a "1 
 
 : : Ma 7 : A 7 a 7 / 
 
 A 7 a 7 
 
 To make this still clearer. Let 
 M B, M B 7 be two similar curves, 
 and A B, A' B 7 similar parts of them. 
 Let fall from A, B, A 7 , B 7 , the or- 
 dinates A a, B b, A 7 a 7 , B 7 b 7 cut- 
 ting off the corresponding abscissae 
 M a, M b, M a 7 , M b 7 , and draw 
 the chords A B, A 7 B 7 ; also draw 
 A C, A 7 C 7 at right angles to B b, B 7 C 7 . 
 
 Then, since (by LEMMA V) 
 
 Ma : 
 
 Ma 7 
 
 .-.Ma: 
 Ma 7 
 
 .-.AC : 
 
 A 7 CX 
 But 
 
 Ma : 
 .-.AC: 
 
 and the L. C = L. C 7 
 
 .-. the triangles A B C, A 7 B' C 7 are similar, and the L. B A C = 
 z. B 7 A 7 C 7 , i. e. A B is parallel to A 7 B 7 . 
 
 Hence if B, B 7 move up to A, the chords A B, A' B' shall ultimately 
 be parallel, i. e. the tangents (see LEMMA III, Cor. 2 and 3, or LEMMA 
 VI,) at A, A 7 are parallel. 
 
 Hence, if the chords coincide, as in fig. to LEMMA VII, the tangents 
 coincide also. 
 
 The student is now prepared for the demonstration of the LEMMA. 
 He will perceive that as B approaches A, new curves, or parts of curves, 
 A c b similar to the parts A C B are supposed continually to be described, 
 the point b also approaching d, which may not only be at zjinite distance 
 from A, but absolutely fixed. It is also apparent, that as the ratio be- 
 tween A B and A b decreases, the curve A c b approaches to the straight 
 line A b as its limit 
 
 42. LEMMA XI. The construction will be better understood when 
 thus effected. 
 
 Take A e of any given magnitude and draw the ordinate e c meeting 
 A C produced in c, and upon A c describe the curve Abe (see 39) 
 
 Aa 
 BC 
 
 Ma 7 : 
 A 7 C 7
 
 BOOK L] NEWTON'S PRINCIPIA. 39 
 
 A D 
 
 similar to A B C. Take A d = A e X -r-^ and erect the ordinate d b 
 
 A iii 
 
 meeting A b c in b. Then, since A d, A e are the abscissae corre- 
 sponding to A D, A E, the ordinates d b, e c also correspond to the 
 ordinates D B, E C, and by LEMMA V we have 
 
 d b : D B : : e c : E C : : A e : A E 
 
 : : A d : A C (by construction) 
 and the L- D = L d. Hence 
 b is in the straight line A B produced, &c. &c. 
 
 43. This LEMMA may be proved, without the aid of similar curves, as 
 follows : 
 
 A B D = ^-5 . (D F + F B) 
 
 
 
 . tan. a A D. B F 
 
 = AD '- + nr 
 
 and 
 
 where a = L. D A F. 
 
 A BD _ AD 2 , tan. + A D . B F 
 ''ACE " A E 2 . tan. + A E . C G 
 
 Now by LEMMA VII, since L- B A F is indefinite compared with F or B ; 
 therefore B F, C G are indefinite compared with A D or A E. Hence 
 
 if L be the limit of , and L + 1 its varying value, we have 
 
 A v/ K-I 
 
 AD 2 , tan, a + A D . B F 
 = AE 2 .tan. a + A E. CG 
 
 and multiplying by the denominator and equating homogeneous terms 
 we get 
 
 L . A E * . tan. a = A D 2 . tan. a 
 
 ,ABD AD 2 
 
 __ __. 
 
 44. LEMMA X. " Continually increased or diminished." The word 
 " continually" is here introduced for the same reason as '' continued 
 curvature" in LEMMA VI. 
 
 If the force, moreover, be not "finite" neither will its effects be ; or 
 the velocity, space described, and time will not admit of comparison.
 
 40 A COMMENTARY ON [SECT. I. 
 
 45. Let the time A D be divided into several portions, such as D d, 
 A b B being the locus of the extremities of the ordinates which D repre- 
 sent, the velocities acquired D B, d b, 
 
 &c. Then upon these lines D d, &c. 
 as bases, there being inscribed rect- 
 angles in the figure A D B, and when 
 their number is increased and bases 
 diminished indefinitely, their ultimate 
 sum shall = the curvilinear area D d D' A 
 
 A B D (LEMMA III.) But each of these rectangles represents the space 
 described in the time denoted by its base ; for during an instant the ve- 
 locity may be considered constant, and by mechanics we have for constant 
 velocities S = T X V. Hence the area A B D represents the whole 
 space described in the time A D. 
 
 In the same manner, ACE (see fig. LEMMA X) represents the time 
 A E. But by LEMMA IX these areas are " ipso motus initio," as A D 2 
 and A E 2 Hence, in the very beginning of the motion, the spaces de- 
 scribed are also in the duplicate ratio of the times. 
 
 46. Hence may be derived the differential expressions for the space 
 described^ velocity acquired, &c. 
 
 Let the velocity B D acquired in the time t (AD) be denoted by v, 
 and the space described, by s. 
 Then, ultimately r , we have 
 
 Dd= d t, B n = d v, 
 
 and 
 
 Dnbd = ds = Ddxdb = dtXv. 
 
 Hence 
 
 d s d s . x 
 
 v = j- t , ds = v dt,dt = (a) 
 
 Again, if D d = d D', the spaces described in these successive instants, 
 are D b, D' m, and therefore ultimately the fluxion of the space repre- 
 sented by the ultimate state of D' m is b n r m or 2 b m B'. Hence 
 
 d (d s) = 2 X b m B' ultimately, 
 
 and supposing B' to move up to A, since in the limit at A, B' coincides 
 with A, and B' m with A D, and therefore b m B' or d (d s) represents 
 the space described " in the very beginning of the motion." 
 Hence by the LEMMA, 
 
 d (ds) a 2d t 2 a d t 2 
 or with the same accelerating force 
 
 d s s a dt 2 (b)
 
 BOOK I.] NEWTON'S PRINCIPIA. 41 
 
 With different accelerating forces d 2 s must be proportionably increased 
 or diminished, and .. (see Wood's Mechanics) 
 
 d 2 s a Fdt 2 
 Hence we have, after properly adjusting the units of force, &c. - 
 
 d 2 s = Fdt 2 , 
 and .*. 
 
 . 
 
 d^s f 
 
 ~ dt 2 * 
 
 F 
 
 Hence also and by means of (a) considering d t constant, 
 
 F = ^1 vdv = Fds (d) 
 
 all of which expressions will be of the utmost use in our subsequent 
 comments. 
 
 47. LEMMA X. COR. I. To make this corollary intelligible it will be 
 useful to prove the general principle, that 
 
 If a body, moving in a curve, be acted upon by any new accelerating 
 force, the distance between the points at which it would arrive WITHOUT 
 and WITH the new force in the same time, or " error," is equal to the space 
 that the new force, acting solely, would cause it to describe in that same 
 time. 
 
 Let a body move in the curve ABC, and when at B, let an additional 
 force act upon it in the direction B b. Also let B D, D E, E C ; 
 B F, F G, G b be spaces that would be described in equal times by the 
 body moving in the curve, and when moved by the sole action of the new 
 force. Then draw tangents at the points B, D, E meeting D d, E e, C c, 
 each parallel to B b, in P, Q, R. Also draw F M, G R, b d parallel to 
 B P; M S, R N, d e parallel to D Q; and S V, N T, e c parallel to 
 ER.
 
 42 A COMMENTARY ON [SECT. I. 
 
 Now since the body at B is acted upon by forces which separately 
 would cause it to move through B D, B F, or, when the number of 
 the spaces is increased and their magnitude diminished in inftnitum, 
 through B P, B F in same time, therefore by LAW III, Cor. 1, when 
 these forces act together, the body will move in that time through the 
 diagonal up to M. In the same manner it may be shown to move from 
 M to N, and from N to C in the succeeding times. Hence, if the num- 
 ber of the times be increased and their duration indefinitely diminished, 
 the body will have moved through an indefinite number of points M, N, 
 &c. up to C, describing a curve B C. Also since b d, d e, e c are each 
 parallel to the tangents at B, D, E, or ultimately to the curve B D E C ; 
 .'. b d e c ultimately assimilates itself to a curve equal and parallel to 
 B D E C ; moreover C c is parallel to B b. Hence C c is also equal 
 to Bb. 
 
 Hence, then, The Error caused by any disturbing force acting upon a 
 body moving in a curve, is equal to the space that would be described by 
 means of the sole action of that force, and moreover it is parallel to the 
 direction of that force. Wherefore, if the disturbing force be constant, it is 
 easily inferred from LEMMAS X and IX, and indeed is shown in all books 
 on Mechanics, that the errors are as the squares of the times in which they 
 are generated. Also, if the disturbing forces be nearly constant, then the 
 errors areas the squares of the times quamproxime. But these conclusions, 
 the same as those which Note 118 of the Jesuits, Le Seur and Jacquier, 
 (see Glasgow edit. 1822.) leads to, do not prove the assertion of Newton 
 in the corollary under consideration, inasmuch as they are general for all 
 curves, and apply not to similar curves in particular. 
 
 48. Now let a curve similar to the above be constructed, and completing 
 the figure, let the points corresponding to A, B, &c. be denoted by 
 A', B 7 , &c. and let the times in which the similar parts of these curves, 
 viz. B D, B' D' ; D E, D 7 E' ; E C, E' C' are described, be in the ratio 
 t : t'. Then the times in which, by the same disturbing force, the spaces 
 B F, B' F'; F G, F' G'; G b, G' b' are described, are in the ratio of 
 t : t'. Hence, " in ipso motus initio" (by LEMMA X) we have 
 B F : B'F' : : t 2 : t' 2 
 F G : F'G' : : t 2 : t'*- 
 
 &c. &c. 
 and therefore, 
 
 B F + F G + &c. : B' F 7 + F G' + &c. : : t' : t'
 
 BOOK I.] NEWTON'S PRINCIPIA. 43 
 
 But, (by 15,) 
 
 B F + F G + &c. = the error C c, 
 and 
 
 B' F' + F G' + &c. = the error C' c', 
 
 and the times in which B C, B 7 C' are described, are in the ratio t : t'. 
 Hence then 
 
 Cc : C' c' : : t 2 : t' 2 
 
 or The ERRORS arising from equal forces, applied at corresponding points, 
 disturbing the motions of bodies in similar curves, "which describe similar 
 parts of those curves in proportional times, are as the squares of the times 
 in which they are generated EXACTLY, and not " quam proxime" 
 
 Hence Newton appears to have neglected to investigate this corollary. 
 The corollary indeed did not merit any great attention, being limited by 
 several restrictions to very particular cases. 
 
 It would seem from this and the last No. that Newton's meaning in 
 the forces being " similarly applied," is merely that they are to be applied 
 at corresponding points, and do not necessarily act in directions similarly 
 situated with respect to the curves. 
 
 For explanation with regard to the other corollaries, see 46. 
 
 49. LEMMA XI. " Finite Curvature" Before we can form any precise 
 notion as to the curvature at any point of a curve's being Finite, Infinite or 
 Infinitesimal, some method of measuring curvature in general must be de- 
 vised. This measure evidently depends on the ultimate angle contained by 
 the chord and tangent (A B, AD) or on the angle of contact. Now, although 
 this angle can have no finite value when singly considered, yet when two 
 such angles are compared, their ratio may be finite, and if any known 
 curvature be assumed of a standard magnitude, we shall have, by the 
 equality between the ratios of the angles of contact and the curvatures, the 
 curvature at any point in any curve whatever. In practice, however, it 
 is more commodious to compare the subtenses of the angles of contact 
 (which may be considered circular arcs, see LEMMA VII, having radii in 
 a ratio of equality, and therefore are accurate measures of them), than the 
 angles themselves. 
 
 50. Ex. 1. Let the circumference of a circle be divided into any num- 
 ber of equal parts and the points of division being joined, let there be ? 
 tangent drawn at every such point meeting a perpendicular let fall from 
 the next point ; then it may easily be shown that these perpendiculars or 
 subtenses are all equal, and if the number of parts be increased, and their
 
 44 
 
 A COMMENTARY ON 
 
 [SECT. I.- 
 
 magnitude diminished, in infinitum, they will have a ratio of equality. 
 Hence, the CIRCLE has the same curvature at every point, or it is a curve 
 of uniform curvature. 
 
 51. Ex. 2. Let two circles touch one 
 another in the point A, having the 
 common tangent A D. Also let B D 
 be perpendicular to A D and cut the 
 circle A D in B'. Join A B, A B'. 
 Then since A B, A B' are ultimately 
 equal to A D (LEMMA VII) they are 
 equal to one another, and consequently 
 the limiting ratio of B D and B' D, is 
 that of the curvatures of the respective 
 circles A C, A D (by 17.) 
 
 But, by the nature of the circle, 
 
 AD 2 = 2 R x D B' D B' 2 = 2r x D B D B 2 
 R and r being the radii of the circles. 
 
 Therefore 
 
 T 1 PB 2 R D B' 
 ~DB'~2r DB 
 
 and equating homogeneous terms we have 
 
 L = T' 
 
 i. e. The curvatures of circles are inversely as their radii. 
 
 52. Hence, if the curvature of the circle whose radius is 1, (inch, foot, 
 or any other measure,) be denoted by C, that of any other circle whose 
 radius is r, is 
 
 53. Hence, if the radius r of a circle compared with 1, be Jinite, its 
 curvature compared with C, isjinite ,- if r be infinite the curvature is 
 infinitesimal ; if r be infinitesimal the curvature is injinite, and so on through 
 all the higher orders of iiifinites and infinitesimals. By infinites and in- 
 finitesimals are understood quantities indefinitely great or small. 
 
 The above sufficiently explains why curvature, compared with a given 
 standard (as C), can be said to bejinite or indejinite. We are yet to show 
 the reason of the restriction to curves ofjinite curvature, in the enuncia- 
 tion of the LEMMA. 
 
 54. The circles which pass through A, B, G; a, b, g, (fig. LEMMA XI)
 
 BOOK I.] NEWTON'S PRINCIPIA. 45 
 
 have the same tangent A D with the curve and the same subtenses. Hence 
 (49. and 52.) these circles ultimately have the same curvature as the curve, 
 i. e. A I is the diameter of that circle which has the same curvature as the 
 curve at A. Hence, according as A I is finite or indefinite, the curvature 
 at A is so likewise, compared with that of circles of finite radius. 
 Now A G ultimately, or 
 
 AI- AB ' 
 " BD 
 
 whether A I be finite or not. If finite, B D a A B 2 , as we also learn 
 from the text. 
 
 A B 2 
 
 55. If the curvature be infinitesimal or A I infinite ; then since -^-rr- 
 
 .tJ U 
 
 is infinite, B D must be infinitely less than A B 2 , or, A B being 
 always considered in its ultimate state an infinitesimal of the first order, 
 B D is that of the third order, i. e. B D <x A B 3 . The converse is 
 also true. 
 
 Ex. In the cubical parabola, the abscissa a as the cube of the or- 
 dinate; hence at its vertex the curvature is infinitely small. At other 
 points, however, of this curve, as we shall see hereafter, the curvature is 
 finite. 
 
 To show at once the different proportions between the subtenses of the 
 angles of contact and the conterminous arcs, corresponding to the differ- 
 ent orders of infinitesimal or infinite curvatures, and to make intelligible 
 this intricate subject, let A B ultimately considered be indefinitely small 
 
 A B 2 
 
 compared with 1 ; then since ^ = A B, A B 2 is infinitesimal com- 
 
 A B n 
 
 pared with A B ; and generally . p n _ i = A B, shows that A B n is 
 
 infinitely small compared with A B n ~~ * so that the different orders of in- 
 Jinitesimals may be correctly denoted by 
 
 AB, AB 2 , AB 3 , AB 4 , &c. 
 
 Also since 1 is infinite compared with the infinitesimal A B, and A B 
 compared with A B 2 , &c. the different orders of infinites may be repre- 
 sented by 
 
 L * _J_ 1 &c 
 
 AB' AB 2 ' AB 3 ' AB 4 ' 
 
 56. Hence if the curvature at any point of a curve be infinitesimal in 
 the second degree
 
 46 A COMMENTARY ON [SECT. 1 
 
 A B 2 1 
 
 - ^ oc , and B D at A B 4 , and conversely. 
 
 I > I ' -V I ' 
 
 And generally, if the curvature be infinitesimal in the n th degree, 
 
 A B 2 1 
 
 p ^ a , and BDot AB n + , and conversely. 
 
 1) 1 ' A. ij 
 
 Again, if the curvature be infinite in the n th degree, 
 
 A B 2 
 
 -5-r- ot A B n , and B D a A B 2 - n , and conversely. 
 D Lf * , 
 
 The parabolas of the different orders will afford examples to the above 
 conclusions. 
 
 57. The above is sufficient to explain the first case of the LEMMA. 
 Case 2. presents no difficulty ; for b d, B D being inclined at any equal 
 
 angles to A D, they will be parallel and form, with the perpendiculars let 
 fall from b, B upon A D, similar triangles, whose sides being propor- 
 tional, the ratio between B D, b d will be the same as in Case 1. 
 
 Case 3. If B D converge, i. e. pass through when produced to a given 
 point, b d will also, and ultimately when d and D move up to A, the 
 difference between the angles A d b, A D B will be less than any 
 that can be assigned, i. e. B D and b d will be ultimately parallel ; 
 which reduces this case to Case 2. (See Note 125. of PP. Le Seur and 
 Jacquier.) 
 
 Instead of passing through a given point, B D, b d may be supposed 
 to touch perpetually any given curve, as a circle for instance, and B D 
 will still a AD 2 ; for the angles D, d are ultimately equal, inasmuch as 
 from the same point A there can evidently be but one line drawn touch- 
 ing the circle or curve. 
 
 Many other laws determining B D might be devised, but the above 
 will be sufficient to illustrate Newton's expression, " or let B D be deter- 
 mined by any other law whatever." It may, however, be farther observed 
 that this law must be definite or such as vfilljix B D. For instance, the 
 LEMMA would not be true if this law were that B D should cut instead of 
 touch the given circle. 
 
 58. LEMMA XI. Con. II. It may be thus explained. Let P be 
 the given point towards which the sagittae S G, s g, bisecting the chords 
 A B, A b, converge. S G, s g shall ultimately be as the squares of 
 A B, A b, &c.
 
 BOOK I.] 
 
 NEWTON'S PRINCIPLE 
 
 47 
 
 For join P B, P b and produce 
 them, as also P G, P g, to meet the 
 tangent in D, d, T, t. Then if B 
 and b move up to A, the angles 
 T P D, t P d, or the differences be- 
 tween the angles ATP and A D P, 
 and between A t P and A d P, may 
 be diminished without limit; that is, 
 (LEMMA I), the angles at T, D and 
 at t, d are ultimately equal. Hence 
 the triangles ATS, A D B are 
 similar, as likewise are A t s, A d b. 
 Consequently 
 
 S T : D B : : A S : A B 
 and 
 
 st:db: :As:Ab 
 and 
 
 .-. S T : s t : 
 Also by LEMMA VII, 
 
 ST : st : 
 
 and by LEMMA XI, Case 3, 
 D B : d b 
 .-. S G : s g : 
 
 DB : db 
 
 S G : sg 
 
 AB 2 
 AB* 
 
 Ab 8 
 
 Ab 2 
 
 t T d' D' d 
 
 L) 
 
 B 
 
 Q. e. d. 
 
 Moreover, it hence appears, that the sagitta 'which cut the chords, in 
 ANY GIVEN RATIO WHATEVER, and tend to a given point, have ultimately 
 the same ratio as the subtenses of the angles of contact, and are as the squares 
 of the corresponding arcs, chords, or tangents. 
 
 59. LEMMA XI. COR. III. If the velocity of a body be constant or 
 "given," the space described is proportional to the time t. Hence 
 A B a t, and .-. S G <x A B 2 <x t 8 . 
 
 60. LEMMA XI. COR. IV. Supposing B D, b d at right angles to 
 A D (and they have the same proportion when inclined at a given angle 
 to A D, and also when tending to a given point, &c.) we have
 
 48 A COMMENTARY ON [SECT. I. 
 
 : : ~ X A D : A d 
 
 A D* 
 
 : ' ^-^p X A D : A d 
 
 : : AD 3 : Ad 3 . 
 Also 
 
 A d 
 
 : : (DB)* : (db)* 
 
 It may be observed here, that the tyro, on reverting to LEMMA IX, 
 usually infers from it that 
 
 A A D B a A D 2 and does not a. AD 3 , 
 
 but then he does not consider that A D, in LEMMA IX, cuts or makes a 
 jinite angle with the curve, whereas in LEMMA XI it touches the curve. 
 
 61. LEMMA XI. COR. V. Since in the common parabola the ab- 
 scissa <x square of the ordinate, and likewise B D or A C <x AD 2 or 
 CD 2 , it is evident that the curve may ultimately be considered a 
 parabola. 
 
 This being admitted, we learn from Ex. 1, No. 4, that the curvilinear 
 area A C B = f of the rectangle C D. Whence the curvilinear area 
 A B D = of C D = f of the triangle A B D, or the area A B D <x 
 triangle A B D a A D 3 , &c. (by Cor. 4.) So far B D, b d have been 
 considered at right angles to A D. Let them now be inclined to it at a 
 given angle, or let them tend to a given point, or " be determined by any 
 other law ;" then (LEMMA, Case 3, and No. 25) B D, b d will ultimately 
 be parallel. Hence, B D', b d' (fig. No. 26) being the corresponding 
 subtenses perpendicular to A D, it is plain enough that the ultimate dif- 
 ferences between the curvilinear areas A B D, A B D' and between 
 A b d, A b d 7 are the similar triangles B D D', b d d', which 
 differences are therefore as B D 2 , b d 2 , or as A B 4 , A b *, i. e. 
 BDD'a A B 4 . 
 
 But we have shown that A B D a A B 3 .
 
 BOOK I.] NEWTON'S PRINCIPIA. 49 
 
 Consequently 
 
 ABD' = ABD+BDD' = axAB 3 + bxAB 4 =AB 3 (a 
 and b X A B being indefinite compared with a, (see Art. 6,) 
 ABD' = a x A B 3 a A B 3 . 
 
 Q. e. d. 
 
 SCHOLIUM TO SECTION I. 
 
 62. What Newton asserts in the Scholium, and his commentators Le 
 Seur and Jacquier endeavour (unsuccessfully) to elucidate, with regard to 
 the different orders of the angles of contact or curvatures, may be briefly 
 explained, thus. 
 
 Let D B a. A D m . Then the diameter of curvature, which equals 
 
 ~- (see No. 22 and 24), <x A D 2 - ra . Similarly if D B ct A D n , the 
 
 diameter of curvature cc A D 2 ~ n . Hence D and D' represents these 
 diameters, we have 
 
 D a X AD 2 ~ m a . . . 
 
 D 7 = a" X AD 2 -" = "a 7 m (a and a' being finite) 
 
 and if n = 2 or D' bejinite, then D will bejinite, infinitesimal, or infinite, 
 according as m = 2, or is any number, (whole, fractional, or even transcen- 
 dental) less than 2, or any number greater than 2. Again, if m = n 
 then D compared with 1^ is finite, since D : D' : : a : a'. If m be less 
 than n in any finite degree, then n m is positive, and D is always in- 
 finitely less than D'. If m be greater than n, then 
 
 D a_ 1 
 
 D' " a' X A D m n 
 
 and m n being positive, D is always infinite compared with D 7 . 
 
 Hence then, there is no limit to the orders of diameters of curvature, 
 with regard to infinite and infinitesimal, and consequently not to the 
 curvatures. 
 
 63. In this Scholium Newton says, that " Those things which have 
 been demonstrated of curve lines and the surfaces which they comprehend 
 are easily applied to the curve surfaces and contents of solids." Let us 
 attempt this application, or rather to show, 
 
 1st, That if any number of parallelepipeds of equal bases be inscribed in 
 any solid, and the same number having the same bases be also circumscribed 
 
 VOL. I. D
 
 A COMMENTARY ON 
 
 [SECT. I. 
 
 about it ; then the number of these parallelepipeds being increased and their 
 magnitude diminished IN INFINITUM^ the ultimate ratios which the aggre- 
 gates of the inscribed and circumscribed parallelopipeds have to one another 
 and to the soliu, are ratios of equality. 
 
 Let A S T U V Z Y X W A be any portion of a solid cut off by three 
 planes A A 7 V, A A' Z and Z A' V, passing through the same point A', 
 and perpendicular to one another. Also let the intersections of these 
 planes with one another be A A', A 7 V, A' Z, arid with the surface of the 
 solid be A U V, A Y Z and Z 1 V. Moreover let A' V, A 7 Z be each 
 divided into any number of equal parts in the points B 7 , T 7 , U 7 ; D 7 , X 7 , Y 7 , 
 and through them let planes, parallel to A A 7 Z and A A 7 V respectively, 
 be supposed to pass, whose intersections with the planes A A 7 V, A A 7 Z
 
 BOOK I.] NEWTON'S PRINCIPIA. 51 
 
 shall be S B', T T', U U 7 ; W D 7 , X X', Y Y 7 , and with the plane 
 A' Z V, 1 B 7 , m T', n U' ; t D 7 , s X 7 , o Y', respectively. Again, let the 
 intersections of these planes with the curve surface be S P 1, T Q m, 
 URn; WPt,XQs, YRo respectively. Also suppose their several 
 mutual intersections to be P C 7 , F E 7 , P" x, P'" G 7 , Q F 7 , Q' H 7 , Q" K', 
 &c. ; those of these planes taken in pairs and of the plane A' Z V, being 
 the points C 7 , E 7 , x, G 7 , F', H 7 , K', I', &c. and those of these pairs of 
 planes and of the curve surface, the points P, P', P", F", Q, Q', Q", R, &c. 
 
 Now the planes, passing through B', T 7 , U 7 , being all parallel to 
 A A' Z, are parallel to one another and perpendicular to A A' V. Also 
 because the planes passing through D 7 , X 7 , Y 7 are parallel to A A 7 V, 
 they are parallel to one another, and perpendicular to A A 7 Z. Hence 
 (Euc. B. XL) S B 7 , T T 7 , U U 7 , W D 7 , X X 7 , Y Y 7 , as also P C 7 , F E 7 , 
 P 77 x, P' 77 G 7 , Q F 7 , Q 7 H 7 , Q 77 K 7 , &c. &c. are parallel to A A 7 and to 
 one another. It is also evident, for the same reasons, that B 7 1, T 7 m, U 7 n, 
 are parallel to A 7 Z and to one another, as also are D 7 1, X 7 s, Y 7 o to 
 A 7 V and to one another. Hence also it follows that A 7 B 7 C 7 D 7 , 
 B' C 7 E 7 T 7 , &c. are rectangles, which rectangles, having their sides equal, 
 are themselves equal. 
 
 Again, from the points A, P, Q, R in the curve surface, draw A B, 
 A D; P E, P G; Q H, Q K; R L, R N parallel to A 7 B 7 , A 7 D 7 ; 
 C 7 E 7 , C 7 G 7 ; F 7 H 7 , T 7 K 7 , I 7 o, I 7 n and meeting B 7 S, D 7 W; E 7 P', 
 G 7 F 77 ; H 7 Q 7 , K 7 Q 77 produced in the points B, D; E, G; H, K, re- 
 spectively. Then complete the rectangles A C, P F, Q I which, being 
 equal and parallel to A 7 C 7 , C 7 F 7 , F 7 I 7 , will evidently, when C 7 P, F 7 Q, 
 I 7 R are produced to C, F, I, complete the rectangular parallelepipeds 
 A C 7 , P F 7 , Q I 7 . Moreover, supposing F 7 I 7 the last rectangle wholly 
 within the curve Z V produce K 7 I 7 , H 7 I 7 and make I 7 L 7 , I 7 N 7 equal 
 K 7 I 7 , H 7 I 7 , and complete the rectangle I' M 7 . Also complete the 
 parallelepiped R M 7 . 
 
 Again, produce E P, G P, H Q, K Q ; L R, N R to the points d, b ; 
 g, e ; k, h, and complete the rectangles Pa, Q p, R q thereby dividing 
 the parallelepipeds A C 7 , P F 7 , Q I 7 , each into two others, viz. A P, 
 aC 7 ; PQ, pF 7 ; Q R, q I 7 . 
 
 Now the difference between the sum of the inscribed parallelepipeds 
 a C 7 , p F 7 , q I 7 , and that of the circumscribed ones A C', P F 7 , Q I 7 , R M 7 , 
 is evidently the sum of the parallel opipeds A P, P Q, Q R, R M 7 ; that 
 is, since their bases are equal and the altitudes I 7 R 7 , R I, Q F, PC 
 are together equal to A A 7 , this difference is equal to the parallelepiped 
 A C 7 . In the same manner if a series of inscribed and circumscribed 
 
 D2
 
 52 A COMMENTARY ON [SECT. I. 
 
 rectangular parallelepipeds, having the bases B' E 7 , E 7 H 7 , H' L 7 , be 
 constructed, the difference between their aggregates will equal the paral- 
 lelepiped whose base is B' E 7 and altitude S B', and so on with every 
 series that can be constructed on bases succeeding each other diagonally. 
 Hence then the difference between the sums of all the parallelepipeds 
 that can be inscribed in the curve surface A Z V and circumscribed about 
 it, is the sum of the parallelepipeds whose bases are each equal to A' C 7 
 and altitudes are A A', S B', T T 7 , U U 7 , W D 7 , X X 7 , Y Y 7 . Let 
 now the number of the parts A' B', B' T 7 , T' U', U' V, and of the parts 
 A D 7 , D' X 7 , X 7 Y', Y 7 Z be increased, and their magnitude diminished 
 in infinitum, and it is evident the aforesaid sum of the parallelepipeds, 
 which are comprised between the planes A A 7 Z, S B' 1 and between the 
 planes A A 7 V, W D 7 1, will also be diminished without limit ; that is, the 
 difference between the inscribed and circumscribed whole solid is ulti- 
 mately less than any that can be assigned, and these solids are ultimately 
 equal, and a fortiori is the intermediate curve-surfaced solid equal to either 
 of them (see LEMMA I and Art. 6.) Q. e. d. 
 
 Hitherto only such portions of solids as are bounded by three planes 
 perpendicular to one another, and passing through the same point, have 
 been considered. But since a complete curve-surfaced solid will consist of 
 four such portions, it is evident that what has been demonstrated of any 
 one portion must hold with regard to the whole. Moreover, if the solid 
 should not be curve-surfaced throughout, but have one, two, or three plane 
 faces, there will be no difficulty in modifying the above to suit any parti- 
 cular case. 
 
 2dly, If in two curve-surfaced solids there be inscribed two series ofparal- 
 lelopipeds, each of the same number ; and ultimately these parallelepipeds 
 have to each other a given ratio, the solids themselves have to one another 
 that same ratio. 
 
 This follows at once from the above and the composition of ratios. 
 
 3dly, All the corresponding edges or sides, rectilinear or curvilinear, of 
 similar solids are proportionals ; also the corresponding surfaces, plane or 
 curved, are in the duplicate ratio of the sides ; and the volumes or contents 
 are in the triplicate ratio of the sides. 
 
 When the solids have plane surfaces only, the above is shown to be 
 true by Euclid. 
 
 When, however, the solids are curve-surfaced, wholly or in part, we 
 must define them to be similar when any plane- surfaced solid whatever 
 being inscribed in any one of them, similar ones may also be inscribed in the
 
 BOOK I.] NEWTON'S PRINCIPIA. 53 
 
 others. Hence it is evident that the corresponding plane surfaces are 
 similar, and consequently, by LEMMA V, the corresponding edges are 
 proportional, and the corresponding plane surfaces are in the duplicate 
 ratio of these edges or sides. Moreover, if the same number of similar 
 parallelepipeds be inscribed in the solids, and that number be indefinitely 
 increased, it follows from 63. 1 and the composition of ratios, that the 
 curved surfaces are proportional to the corresponding plane surfaces, and 
 therefore in the duplicate ratio of the corresponding edges ; and also that 
 the contents are proportional to the corresponding inscribed parallelepi- 
 peds, or (by Euclid) in the triplicate ratio of the edges. 
 
 These three cases will enable the student of himself to pursue the ana- 
 logy as far as he may wish. We shall " leave him to his own devices," 
 after cautioning him against supposing that a curved-surface, at any point 
 of it, has a certain fixed degree of curvature or deflection from the tangent- 
 plane, and therefore that there is a sphere, touching the tangent-plane at 
 that point, whose diameter shall be the limit of the diameters of all the 
 spheres that can be made to touch the tangent-plane or curved-surface 
 analogously to A I in LEMMA XI. Every curvilinear section of a curved- 
 surface, made by a plane passing through a given point, has at that point 
 a different curvature, the curved-surface being taken in the general sense; 
 and it is a problem of Maxima and Minima To determine those sections 
 "which present the greatest and least degrees of curvature. 
 
 The other points of this Scholium require no particular remarks. If 
 the student be desirous of knowing in what consists the distinction be- 
 tween the obsolete methods of Exhaustions, Indivisibles, &c. and that of 
 PRIME AND ULTIMATE RATIOS, let him go to the original sources to the 
 works of Archimedes, Cavalerius, &c. 
 
 64. Before we close our comments upon this very important part of the 
 Principia, we may be excused, perhaps, if we enter into the detail of the 
 Principle delivered in Art. 6, which has already afforded us so much 
 illustration of the text, and, as we shall see hereafter, so many valuable 
 results. We have thence obtained a number of the ordinary rules for 
 deducing indefinite forms from given definite functions of one variable ; 
 and it will be confessed, by competent and candid judges, that these ap- 
 plications of the principle strongly confirm it. Enough has indeed been 
 already developed of the principle, to prove it clearly divested of all the 
 metaphysical obscurities and inconsistencies, which render the methods of 
 Fluxions, Differential Calculus, &c. &c. so objectionable as to their logic, 
 and which have given rise to so many theories, all tending to establish 
 
 D3
 
 54 A COMMENTARY ON [SECT. I. 
 
 the same rules. It is incredible that the great men, who successively in- 
 troduced their several theories, should have been satisfied with the 
 reasonings by which they attempted to establish them. So many con- 
 flicting opinions, as to the principles of the science, go only to show that 
 all were founded in error. Although it is generally difficult, and often 
 impossible, for even the most sharp-sighted of men, to discern truth 
 through the clouds of error in which she is usually enveloped, yet, when 
 she does break through, it is with such distinct beauty and simplicity that 
 she is instantly recognized by all. In the murkiness around her there are 
 indeed false lights innumerable, and each passing meteor is in turn, by 
 many observers, mistaken for the real presence ; but these instantly vanish 
 when exposed to the refulgent brightness of truth herself. Thus we have 
 seen the various systems of the world, as devised by Ptolemy, Tycho 
 Brahe, and Descartes, give way, by the unanimous consent of philoso- 
 phers, to the demonstrative one of Newton. It is true, the principle of 
 gravitation was received at first with caution, from its non-accordance 
 with astronomical observations ; but the moment the cause of this discre- 
 pancy, viz. the erroneous admeasurement of an arc of the meridian, was 
 removed, it was hailed universally as truth, and will doubtless be coeval 
 with time itself. The Theories relative to quantities indefinitely variable, 
 present an argument from which may be drawn conclusions directly op- 
 posite to the above. Newton himself, dissatisfied with his Fluxions, pro- 
 duces PRIME AND ULTIMATE RATIOS, and again, dissatisfied with these, he 
 introduces the idea of Moments in the second volume of the Principia. 
 He is every where constrained to apologize for his obscurities, first in his 
 Fluxions for the use of time and velocities, and then again in the Scholium, 
 at the end of Sect. I of the Principia, (and in this instance we have shown 
 how little it avails him) for reasoning upon nothings. After Newton comes 
 Leibnitz, his great though dishonest rival, (we may so designate him, such 
 being evidently the sentiments of Newton himself)* who, bent upon oblite- 
 rating all traces of his spoil, melts it down into another form, but yet falls 
 into greater errors, as to the true nature of the thing, than the discoverer 
 himself. From his Infinitesimals, considered as absolute nothings of the dif- 
 ferent orders, nothing can be logically deduced, unless by Him (we speak 
 with reverence) who made all things from nothing. Suchjiats we mortals 
 cannot issue with the same effect, nor do we therefore admit in science, finite 
 and tangible consequences deduced from the arithmetic of absolute no- 
 things, be they ever so many. Then we have a number of theories pro- 
 mulgated by D'Alembert, Euler, Simpson, Marquise L'Hopital, &c. &c.
 
 BOOK I.] NEWTON'S PRINCIPIA. 55 
 
 all more or less modifications of the others all struggling to establish 
 and illustrate what the great inventor, with all his almost supernatural 
 genius, failed to accomplish. All these diversities in the views of philo- 
 sophers make, as it has been already observed, a strong argument that 
 truth had not then unveiled herself to any of them. Newton strove most 
 of any to have a full view, but he caught only a glimpse, as we may per- 
 ceive by his remaining dissatisfied with it. Hence then it appears, to us 
 at least, that the true metaphysics of the doctrine of quantities indefinitely 
 variable, remain to this day undiscovered. But it may be asked, after 
 this sweeping conclusion, how comes it that the results and rules thence 
 obtained all agree in form, and in their application to physics produce 
 consequences exactly in conformity with experience and observation ? 
 The answer is easy. These forms and results are accurately true, al- 
 though illogically deduced, from a mere compensation of errors. This has 
 been clearly shown in the general expression for the subtangent (Art. 29), 
 and all the methods, not even Lagrange's Calcul des Fonctions excepted, 
 are liable to the paralogism. Innumerable other instances might be 
 adduced, but this one we deem amply sufficient to warrant the above 
 assertion. 
 
 After these preliminary observations upon the state of darkness and 
 error, which prevails to this day over the scientific horizon, it may per- 
 haps be expected of us to shine forth to dispel the fog. But we arrogate 
 to ourselves no such extraordinary powers. All we pretend to is self- 
 satisfaction as to the removal of the difficulties of the science. Having 
 engaged to write a Commentary upon the Principia, we naturally sought 
 to be satisfied as to the correctness of the method of Prime and Ultimate 
 Ratios. The more we endeavoured to remove objections, the more they 
 continually presented themselves ; so that after spending many months in 
 the fruitless attempt, we had nearly abandoned the work altogether; 
 when suddenly, in examining the method of Indeterminate Coefficients in 
 Dr. Wood's Algebra, it occurred that the aggregates of the coefficients of 
 the like powers of the indefinite variable, must be separately equal to zero, 
 not because the variable might be assumed equal to zero, (which it never 
 is, although it is capable of indefinite diminution,) but because of the 
 different powers being essentially different from, and forming no part of 
 one another. 
 
 From this a train of reflections followed, relative to the treatment of 
 homogeneous definite quantities in other branches of Algebra. It was 
 soon perceptible that any equation put = 0, consisting of an aggregate of 
 
 D4
 
 56 A COMMENTARY ON [SECT. I. 
 
 different quantities incapable of amalgamation by the opposition of plus 
 and minus, must give each of these quantities equal to zero. Reverting to 
 indefinites, it then appeared that their whole theory might be developed 
 on the same principles, and making trial as in Art. 6, and the subsequent 
 parts of the preceding commentary, we have satisfied ourselves most fully 
 of having thus hit upon a method of clearing up all the difficulties of 
 what we shall henceforth, contrary to the intention expressed in Art. 7, 
 entitle 
 
 THE CALCULUS 
 
 OF 
 
 INDEFINITE DIFFERENCES. 
 
 65. A constant quantity is such, that from its very nature it cannot be 
 made less or greater. 
 
 Constants, as such quantities may briefly be called, are denoted generally 
 by the first letters of the alphabet, 
 
 a, b, c, d, &c. 
 
 A definite quantity is a GIVEN VALUE of a quantity essentially variable. 
 
 Definite quantities are denoted by the last letters of the alphabet, as 
 
 /, y, x, w, &c. 
 
 An INDEFINITE quantity is a quantity essentially variable through all 
 degrees of diminution or of augmentation short of absolute NOTHINGNESS or 
 INFINITUDE. 
 
 Thus the ordinate of a curve, considered generally, is an indefinite, 
 being capable of every degree of diminution. But if any particular value, 
 as that which to a given abscissa, for instance, be fixed upon, this value is 
 definite. All abstract numbers, as 1, 2, 3, &c. and quantities absolutely 
 fixed, are constants. 
 
 66. The difference between two definite values of the same quantity (y) is 
 a definite quantity, and may be represented by 
 
 adopting the notation of the Calculus of Finite (or definite] Differences. 
 
 In the same manner the difference between two definite values of A y is 
 a definite quantity, and is denoted by
 
 (b) 
 
 BOOK I.] NEWTON'S PRINCIPIA. 57 
 
 or more simply by 
 
 and so on to 
 
 A'y. 
 
 67. The difference between a Definite value and the Indefinite value of 
 any quantity y is Indefinite, and we call it the Indefinite Difference of y, and 
 denote it, agreeably to the received algorithm, by 
 
 dy - - (c) 
 
 In the same manner 
 
 d(dy) 
 or 
 
 d 2 y 
 
 the Indefinite Difference of the Indefinite Difference of y, or the second in- 
 definite difference of y. 
 
 Proceeding thus we arrive at 
 
 dy (d) 
 
 which means the n 111 indefinite difference of y. 
 
 68. Definite and Indefinite Differences admit of being also represented 
 by lines, as follows : 
 
 t" 
 
 Let P P' = y be any fixed or definite ordinate of the curve A U, and 
 taking P' Q' = Q> R' = R' S' = &c. let ordinates be erected meeting 
 the curve in Q, R, S, T, &c. Join P Q, Q R, R S, &c. and produce 
 them to meet the ordinates produced in r, s, t, &c. Also draw r s', s t',
 
 58 A COMMENTARY ON [SECT. I. 
 
 &c. parallel to R S, S T, &c. and draw s t", &c. parallel to s t', &c. ; and 
 finally draw P m, Q n, R o, &c. perpendicular to the ordinates. 
 
 Now supposing not only P P' but also Q Q', R R', &c. fixed or defi- 
 nite; then 
 
 Qm = QQ / PP / = APP / = Ay 
 Rr =nr nR = Qm Rn = AQm 
 
 = A(AP P') = A 2 PP = A 2 y 
 s s' =Ss Ss / =Ss Rr =ARr 
 = A'y 
 
 t t" = t t' t' t" = t t 7 S S' = A S S' 
 
 = A(A 3 y) =A 4 y 
 and so on to any extent. 
 
 But if the equal parts P' Q', Q' R', &c. be arbitrary or indefinite, then 
 Q m, R r, s s', 1 1", &c. become so, and they represent the several Inde- 
 finite Differences of y, viz. 
 
 d y, d 2 y, d 3 y, d 4 y, &c. 
 
 69. The reader will henceforth know the distinction between Definite 
 and Indefinite Differences. We now proceed to establish, of Indefinite 
 Differences, the 
 
 FUNDAMENTAL PRINCIPLE. 
 
 It is evidently a truth perfectly axiomatic, that No aggregate of INDEFI- 
 NITE quantities can be a definite quantity^ or aggregate of definite quanti- 
 ties^ unless these aggregates are equal to zero. 
 
 It may be said that (a x) + ( a + x ) 2 a, in which (x) is indefinite, 
 and (a) constant or definite, is an instance to the contrary ; but then the 
 reply is, a x and a + x are not indefinites in the sense of Art. 65. 
 
 70. Hence if in any equation 
 
 A + B x + C x 2 + D x 3 + &c. = 
 
 A, B, C, &c. be definite quantities and x an indefinite quantity ; then we 
 have 
 
 A = 0, B = 0, C = 0, &c. 
 
 For B x + C x 2 + D x 8 + &c. cannot equal A unless A = 0. 
 But by transposing A to the other side of the equation, it does = A. 
 Therefore A = and consequently 
 
 Bx + Cx 2 + Dx s + &c. = 
 or 
 
 x(B + Cx + Dx 2 + &c.) =
 
 BOOK I.] NEWTON'S PRINCIPIA. 59 
 
 But x being indefinite cannot be equal to ; .. 
 
 B + Cx + Dx 2 + &c. = 
 Hence, as before, it may be shown that B = 0, and therefore 
 
 x (C + Dx + &c.) = 
 Hence C = 0, and so on throughout. 
 71. Again, if in the equation 
 
 A, B, B', C, C', C", D, &c. be definite quantities, and x, y INDEFINITES ; 
 then 
 
 A = 0-\ 
 
 B x + B' y = f-wfien y is a Junction o/*x. 
 Cx 2 + C'xy + C"y 2 = oj 
 
 &c. = 
 For, let y = z x, then substituting 
 
 A + x (B + B' z) + x 2 (C + C' z + C" z 2 ) 
 
 + x 3 (D + D' z + D" z 2 + D"'z 3 ) + &c. = 
 Hence by 70, 
 
 A = 0, B + B' z = 0, C + C' z + C" z 2 = 0, &c. 
 
 y 
 
 and substituting *- for z and reducing we get 
 
 A = 0, B x + B' y = 0, &c. 
 
 In the same manner, if we have an equation involving three or more 
 indefinites, it may be shown that the aggregates of the homogeneous terms 
 must each equal zero. 
 
 This general principle, which is that of Indeterminate Coefficients 
 legitimately established and generalized, (the ordinary proofs divide 
 
 B x + C x 2 + &c. = by x, which gives B + C x + D x "- + &c. = 
 
 and not ; x is then put = 0, and thence truly results B = , which 
 
 instead of being 0, may be any quantity whatever, as we know from alge- 
 bra ; whereas in 70, by considering the nature of x, and the absurdity of 
 making it = we avoid the paralogism) conducts us by a near route to 
 the Indefinite Differences of functions of one or MORE variables. 
 
 72. Tojind the Indefinite Difference of any function ofx. 
 
 Let u = f x denote the function. 
 
 Then d u and d x being the indefinite differences of the function and 
 
 of x itself, we have 
 
 u + du = f(x + dx) 
 Assume 
 
 f (x + d x) = A + B d x + C d x * + &c.
 
 60 A COMMENTARY ON [SECT. I. 
 
 A, B, &c. being independent of d x or definite quantities involving x and 
 
 constants ; then 
 
 u + d u = A + B d x + C d x 8 + &c. 
 
 and by 71, we have 
 
 u = A, d u = B . d x 
 Hence then this general rule, 
 
 The INDEFINITE DIFFERENCE of any function of x, f x, is the second 
 term in the developement off (x + d x) according to the increasing powers 
 qfd x. 
 
 Ex. Let u = x n . Then it may easily be shown independently of the 
 Binomial Theorem that 
 
 (x + dx) n = x n +n.x n ~ 1 dx + Pdx 2 
 
 .-. d (x n ).= n .x "- 1 d x 
 
 The student may deduce the results also of Art. 9, 1 0, &c. from this general 
 rule. 
 
 73. To find the indefinite difference of the product of two variables. 
 Let u = x y. Then 
 
 u + du = (x + dx).(y+dy)=xy+x dy + y dx + dxdy 
 .. d u = x dy+y dx + dx dy 
 and by 71, or directly from the homogeneity of the quantities, we have 
 
 du = xdy + ydx (a) 
 
 Hence 
 
 d (x y z) = * d (y z) + y z d x 
 
 and so on for any number of variables. 
 
 x 
 
 Again, required d . . 
 
 Let = u. Then 
 
 y 
 
 x = y u, and dx = u dy + y du 
 
 , x d x u , 
 .*. d =rdu= dy 
 
 y y y 
 
 _y dx x dy 
 
 y 2 
 
 Hence, and from rules already delivered, may be found the Indefinite 
 Differences of any functions whatever of two or more variables. We 
 refer the student to Peacock's Examples of the Differential Calculus for 
 practice. 
 
 The result (a) may be deduced geometrically from the fig. in Art. 21. 
 The sum of the indefinite rectangles A b, b A' makes the Indefinite 
 Difference.
 
 BOOK I.] 
 
 NEWTON'S PRINCIPIA. 
 
 61 
 
 We might, in this place, investigate the second, third, Sic, Indefinite 
 Differences, and give rules for the maxima and minima of functions of two 
 or more variables, and extend the Theorems of Maclaurin and Taylor to 
 such cases. Much might also be said upon various other applications, 
 but the complete discussion of the science we reserve for an express 
 Treatise on the subject. We shall hasten to deduce such results as we 
 shall obviously want in the course of our subsequent remarks ; beginning 
 with the research of a general expression for the radius of curvature of a 
 given curve, or for the radius of that circle whose deflection from the 
 tangent is the same as that of the curve at the point of contact. 
 
 74. Required the radius of curvature for any point of a given curve. 
 
 Let A P Q R be the given 
 curve, referred to the axis A O 
 by the ordinate and abscissa 
 P M, A M or y and x. P M 
 being fixed let Q N, O R be 
 any other ordinates taken at 
 equal indefinite intervals M N, 
 N O. Join P Q and produce 
 it to meet O R in r ; and let 
 P t be the tangent at P drawn 
 by Art. 29, meeting Q N, O R 
 in q and t respectively. Again 
 draw a circle (as in construc- 
 tion of LEMMA XI, or other- 
 wise) passing through P and Q and touching the tangent P t, and there- 
 fore touching the curve ; and let B D be its diameter parallel to A O. 
 
 Now 
 
 Qn = dy, Pn = dx, Pq = PQ (LEMMA VII) = 
 V (d x * + d y 2 ) or d s, if s = arc A P. 
 
 Moreover let 
 
 PM' = y'; 
 
 then it readily appears (see Art. 27) that d s = - 
 
 R being the ra- 
 
 dius of the circle. 
 Again 
 
 Pq* = Qq X (Qq+ 2 Q N')
 
 62 A COMMENTARY ON [SECT. I. 
 
 or 
 
 ;;;^;';: |Z ( d .) = Q q (Q q + 2 d y + 2 - n d ^-) 
 
 But since 
 
 R t : Q q : : P r 2 : P Q 2 : : 4 : 1 {LEMMA XI) 
 
 and 
 
 Q q : t r : : 1 : 2 
 
 .. R t = 2 t r, or R r = t r = 2 Q q 
 
 ' Q q = T = ^ (by Art ' 68<) 
 
 Consequently 
 
 and equating Homogeneous Indefinites 
 
 R dx d 2 y 
 
 d s 2 = - 1 - - 
 d s 
 
 . - ds 3 __ (dx 8 
 
 ~dxd*y~ dxd 2 y 
 
 (^a^) 1 CV ^ 
 
 = - 3-= - ........ .. (d) 
 
 d 2 y 
 
 d~x~ 2 
 
 the general expression for the radius of curvature. 
 Ex. 1. In the parabola y 2 = a x. 
 
 d y a 
 ' d~x ~~ 2~y 
 and since when the curve is concave to the axis d 2 y is negative, 
 
 d 2 y a d y a* 
 
 ~~ d x 2 = ~ 2y~ 2 * d~x ~ "" 4Ty~ 3 ~ 
 
 Hence at the vertex R = - , and at the extremity of the latus rectum, 
 
 SB 
 
 3 
 
 2^ 
 R = i a = a V 2. 
 
 m
 
 BOOK I.] NEWTON'S PRINCIPIA. 63 
 
 Ex. 2. If p be the parameter or the double ordinate passing through 
 the focus and 2 a the axis-major of any conic section, its equation is 
 
 Hence 
 
 2ydy = p d x + x d x 
 ~ a 
 
 and 
 
 2dy * + 2y d 2 y = + - d x * 
 
 P (l + - 
 V a 
 
 . dy 
 
 "dx ~ 2y 
 
 and 
 
 Dfi + 2LY-r-?_Pv 2 
 
 d*y _ P V ~ a>> + a y 
 dx 2 " 4y 3 
 
 .* R - 
 
 t 
 
 which reduces to 
 
 | p2+ ?_p 
 
 T? < - . 
 
 Ex. 3. In the cycloid it is easy to show that 
 
 ll = / %r y 
 dx *V y 
 
 r being the radius of the generating circle, and x, y referred to the base 
 or path of the circle. 
 
 d g y _ j^ 
 '"'dx 2= y 2 
 
 .-. R = 2 V 2 r y = 2 the normal. 
 
 Hence it is an easy problem tojind the equation to the locus of the centres 
 of curvature for the several points of a given curve. 
 
 If y and x be the coordinates of the given curve, and Y and X those of 
 the required locus, all referred to the same origin and axis, then the stu- 
 dent will easily prove that
 
 64 A COMMENTARY ON [SECT. I. 
 
 JL\ 
 
 ^dxV 
 
 and 
 
 i + ll! 
 
 Y-y + - dx ' 
 
 * ~* J 1 J 2 .. 
 
 dx 2 
 
 which will give the equation required, by substituting by means of the 
 equation to the given curve. 
 In the cycloid for instance 
 
 X = x + ^ (2ry y 2 ) 
 Y = -y 
 
 whence it easily appears that the locus required is the same cycloid, only 
 differing in position from the given one. 
 
 75. Required to express the radius of curvature in terms of the polar co- 
 ordinates of a curve, viz. in terms of the radius vector % and traced- 
 
 angle 6. 
 
 x = g cos. 
 and 
 
 = g cos. o -\ 
 = P sin. & ) 
 
 .'. taking the indefinite differences, and substituting in equation (d) of Art. 
 74, we get 
 
 lv ~ 
 
 which by means of the equation to the curve will give the radius of curva- 
 ture required. 
 
 Ex. 1. In the logarithmic spiral 
 
 e 
 S = a ; 
 
 -4 = la Xa - (Art. 17.) 
 
 ..-^! = - 
 
 . R . (g 2 + (la) e ') s * (IJhjl a)') 
 
 - ' :
 
 BOOK L] NEWTON'S PRINCIPIA. 
 
 Ex. 2. In the spiral of Archimedes 
 
 S = a ^ 
 and 
 
 R - (g 2 + **) f > 
 " 
 
 Ex. 3. jfo ^e hyperbolic spiral 
 
 . R ._ 
 
 A *> 
 
 Ex. 4. / Me Lituus 
 
 . P - J_ (4a 4 + g 4 ) 
 
 ~ 2a 2 ' 4 a 4 ? 4 
 Ex. 5. In the Epicycloid 
 
 s = (r + r') - 2 r (r -f r') cos. 4 
 
 r and r 7 being the radius of the wheel and globe respectively. 
 Here 
 
 (r + rQ (3 r 2 2 r r 7 r' * + 2 g )* 
 
 2(3r 2 2r r' r 72 ) + 3 
 
 Having already given those results of the Calculus of Indefinite Differ- 
 ences which are most useful, we proceed to the reverse of the calculus, 
 which consists in the investigation of the Indefinites themselves from their 
 indefinite differences. In the direct method we seek the Indefinite Differ- 
 ence of a given function. In the inverse method we have given the Inde- 
 finite Difference to find the function whose Indefinite Difference it is. This 
 inverse method we call 
 
 THE INTEGRAL CALCULUS 
 
 OK 
 
 INDEFINITE DIFFERENCES. 
 
 76. The integral of d x is evidently x + C, since the indefinite differ- 
 ence of x + C is d x. 
 
 77. "Required the integral o/'a d x ? 
 By Art. 9, we have 
 
 d (a x) = a d x. 
 Vol.. I. E
 
 G6 A COMMENTARY ON [SECT. I. 
 
 Hence reversely the integral of a d x is a x. This is only one of the in- 
 numerable integrals which there are of a d x. We have not only d (a x) 
 = a d x but also 
 
 d (a x + C) = a d x 
 in which C is any constant whatever. 
 
 .-. ax + C =/adx = a/dx . . . (a) (see 76) 
 generally, f being the characteristic of an integral. 
 78. Required the integral of 
 
 a x P d x. 
 By Art. 12 
 
 d(ax n -fC)=nax n - 1 dx 
 . . a x n -f- C =fn a x *d x 
 
 = n X/ax n ~ 1 dx (77) 
 
 ax n C 
 
 .-. /a x n - 1 d x = -- 1 -- . 
 n n 
 
 rj 
 
 But since C is any constant whatever may be written C. 
 
 ../ax- I dx = + C 
 n 
 
 Hence it is plain that 
 
 Or To find the integral of the product of a constant the p 11 * power of the 
 variable and the Indefinite Difference of that variable, let the index of the 
 power be increased by 1, suppress the Indefinite Difference, multiply by the 
 constant, divide by the increased index, and add an arbitrary constant. 
 
 79. Hence 
 
 /(a xPdx + bx*dx + &c.) = 
 
 p + 1 ' c7+T 
 
 80. Hence also 
 
 ._ 
 
 81. Required the integral of 
 
 a x m - ' d x (b + ex m )P. 
 Let' 
 
 u = b + e x m 
 
 .. d u = mex m ~ l dx 
 
 .. a x m - ' d x = - . d u 
 m e 
 
 .'./ax ra ~ l dx(b + ex m ) p =/* u? d u
 
 BOOK L] NEWTON'S PRINCIPIA. 67 
 
 a 
 
 m e . (p + 1) 
 
 r. UP+ 1 + C (78) 
 
 a 
 
 ~ m e (p + 1) 
 
 d x 
 
 82. Required the integral of . 
 
 By 80 it would seem that 
 
 and if when 
 
 . (b X ex m )P+ 1 + C. 
 
 But by Art. 17 a. we know that 
 
 d x 
 
 d. 1 x = 
 x 
 
 Therefore 
 
 Here it may be convenient to make the arbitrary constant of the form 1 C 
 Therefore 
 
 /i^.= lx + 1C = ICx 
 J x 
 
 Hence the integral of a fraction whose numerator is the Indefinite Differ- 
 ence of the denominator) is the hyperbolic logarithm of the denominator PLUS 
 an arbitrary constant. 
 
 83. Hence 
 
 ax m ~ I dx a f mx m ~ 1 dx 
 
 /ax m ~ 1 dx_ a /" m x m -1 d: 
 / x "^ TT 
 
 .i.V A . T1~) + C 
 
 m 
 
 b m ' 
 
 and so on for more complicated forms. 
 84. Required the integral of a* d x. 
 By Art. 17 
 
 d.a x = la.a x dx 
 
 E 2
 
 68 A COMMENTARY ON [SECT. I. 
 
 85. If y, x, t, s denote the sine, cosine, tangent, and secant of an angle 
 6 ; then we have, Art. 26, 27. 
 
 dy dx dt ds 
 
 " ~~ ~ 
 
 c = cos --' x + c 
 
 C = tan.-'t+C 
 
 /* , ds . = 6 + C = sec.- 1 s + C 
 y s V2s s 2 
 
 sin. ~ l y, cos."" 1 x, &c. being symbols for the arc whose sine is y, cosine is 
 x, &c. respectively. 
 
 86. Hence, more generally, 
 
 du _ _1_ / 
 (a-bu').- Vb/ 
 
 or = -JT X angle whose sine is u ^J to rad. 1 + C. 
 
 Also 
 
 f du 1 / b n , . 
 
 /-7-j - j -sr = -7-7- . COS. U ^ . + C . . (b) 
 
 ^ V (a bu 2 ) -/ b \ a 
 
 Again 
 
 du 
 
 / V 
 * d u _ / a 
 
 a + bu 2 :: V"ab*/ 1 + i u 
 
 and
 
 BOOK I.] NEWTON'S PRINCIPIA. 69 
 
 Moreover, if u be the versed sine of an angte 0, then the sine 
 = V (2u u 8 ) and 
 
 d u = d (1 cos. 6) = dd . sin. 6 (Art. 27.) 
 = d0. V(2u u 2 ) 
 
 du 
 
 Hence 
 
 = vers. l u + C 
 and generally 
 
 , du 
 
 du ' I a 
 
 /" 
 u - / 
 
 r=w5 = / 
 
 87. Required the integrals of 
 
 dx dx d x 
 
 -' 1 u + c ' ' 
 
 a + bx' a bx' a bx 8 * 
 
 f dx _ J_ /d. (a + bx) 
 'a + bx b * -^ a + bx 
 
 TT ' ' ' a X ' "*" ' 
 
 and 
 
 /d x _ _l^/d(a bx) 
 a bx~ b^ *a bx 
 
 " V * ' a ' 
 see Art. 17 a. 
 Hence, 
 
 1 1 V / 2 adx 
 
 ~+Tx a bxj" -J ** b z x 
 
 = -i-.].(a + bx)-l.l.(a-bx) + C 
 J_ . a + bx c
 
 70 A COMMENTARY ON [SECT. I. 
 
 Hence we easily get by analogy 
 
 /. dx _ 1 j V a + V b. x 
 
 -'a b x 2 ~~ V a b V a bx 2 
 
 2 V ab' " a Vb. x 
 88. Required the integral of 
 
 d x 
 
 a x 2 + b x -f- c" 
 In the first place 
 
 b V (b 2 4 a c) 
 
 X 
 
 b V (b z 4 a c) \ f / b 2 b 2 4 a c 
 + 2T -- ^ -- } = a i( X +2-- 
 Hence, putting 
 
 we have 
 
 d x = d u 
 and 
 
 d x d u 
 
 / b 2 4ac 
 
 ax 2 + bx + c a(u 2 -- 5 
 
 C ft 
 
 which presents the following cases. 
 
 Case 1. Let a be negative and c be positive ; then 
 
 d x d u 
 
 ax 
 
 dx V~2 I 2a 
 
 tan 
 
 I 
 
 u / 
 V 
 
 (seeArt86)= . , 2 _ -. tan.- i r x + ") / , o 2 a +C . . (i) 
 a(b 2 +4ac) * \ ^2a/-\ ; b 2 + 4ac T 
 
 Case 2. Ze^ c J^ negative and a positive , then 
 
 d x /" d u 
 
 / x /" 
 
 a x z + b x c "~ / / , 
 
 / a ( u 
 
 b 2 + 4 a 
 
 du 
 
 b 2 + 4 a c 
 
 ' u 
 
 2a 
 
 /b'+4 
 1 V 2 a 
 
 ac 
 
 N 2a(b 2 +4ac)" /b_ 2 +4ac_ __b 
 V ~^^ X 2a 
 see Art 87.
 
 BOOK I.] NEWTON'S PRINCIPIA. 71 
 
 Case 3. Let b 2 be > 4 a c <zrf a, c be both positive ,- then 
 
 f dx _ / d u 
 
 / ax 2 -fbx + c"/ / b 2 4 a 
 
 x \ o _ 
 
 1 ,' du 
 
 a/ b 2 4 a c 
 
 / 3T= u 
 
 /b 2 4ac 
 _ + x +^n 
 
 _ , ..1 U! 
 
 > 2a(b 2 4ac) b 2 4ac b_ 
 
 V 2 a ~2~^ 
 
 Case 4. ietf b 2 be < 4 a c and a, c fo both positive , 
 7Xi 
 
 /dx __ 1 /" du 
 
 ax 2 +bx+c~~a/ 4ac b 2 
 
 7 +U ~ 
 
 2a 
 2 
 
 a(4ac b 2 ) * 
 Case 5. 7)fb 2 fe>4ac and a, c both negative ; 
 Then 
 
 d x 1 r d u 
 
 + C ' ' 
 
 _ = _L /" 
 
 c a / b 2 ' 
 
 ax 2 +bx c a / b 2 4ac 
 
 Case 6. Ifb z be < 4 a c and a awrf c 60^ negative,- 
 Then 
 
 / dx _ 1 /" du 
 
 V_ax 2 +bx c~"a/ 4ac b 2 
 
 / ^ u 2 
 
 2 a 
 
 . / i ,V 
 
 ~ V 2a(4ac b 2 ) ' 
 
 2a 
 
 4ac 1 
 
 2a 
 
 2a(4ac b 2 ) 4ac b 2 
 
 " 2a 2a 
 
 89. Required the integral of any rational function "whatever of one 
 variable^ multiplied by. the indefinite difference of that variable. 
 
 Every rational function of x is comprised under the general form 
 Ax m + Bx m 7 1 + Cx m - 2 + &c. K x + L 
 
 ax n + bx n - 1 + cx n - 2 + &c. k x +1 
 E4
 
 72 A COMMENTARY ON [SECT. I, 
 
 in which A, B, C, &c. a, b, c, &c. and m, n are any constants whatever. 
 
 If 
 
 n = 0, 
 
 then we have (Art. 77) 
 
 m 
 
 (T x m 1 . J 
 
 4- - r \- &c.) h constant. 
 
 m 1 /a 
 
 Again, if m be > n the above can always be reduced by actual division 
 to the form 
 
 A " X ~ ' + B "* 
 
 > 
 
 a x n + bx"- 1 4- &c. 
 
 and if the whole be multiplied by d x its integral will consist of two parts, 
 one of which is found to be (by 77) 
 
 A' B' x ra ~ n 
 
 - Xm-*+l+- -- 1_ &c> 
 
 m n + 1 m n 
 
 and the other 
 
 A" x ~ * + B" x n ~ 2 4- &c. 
 
 ,> 
 J 
 
 
 ax n + bx"- 1 + &c. 
 Hence then it is necessary to consider only functions of the general 
 
 form 
 
 x~ l + Ax p ~ 2 + Bx n ~ 3 +&c. _ U 
 x" + ax"" 1 + b x a ~ 2 + &c. "V 
 
 in order to integrate an indefinite difference, whose definite part is any 
 rational function whatever. 
 
 Case 1. Let the denominator V consist qfn unequal real factors, x a, 
 x 0, &c. according to the theory of algebraic equations. Assume 
 
 and reducing to a common denominator we shall have 
 
 U = P . x /3 . x y ... to (n 1) terms 
 
 + Q.X .x y 
 
 + R . x ~o . x j3 
 
 = (P + Q + R + SccOx"- 1 
 
 *P.(S ) + Q.(S 0) 
 
 1. S 
 
 &c. 
 
 where S, S &c. denote the sum of a, /S, y &c. the sum of the products of 
 every two of them and so on.
 
 BOOK I.] NEWTON'S PRINCIPIA. 73 
 
 But by the theory of equations 
 
 S= a 
 
 S= b 
 
 1.2 
 
 &c. = &c. 
 .-. U = (P + Q + R + fcc^x"- 1 
 
 + { a (P + Q + R + &c.) + Pa+Q/3 + Ry+ &c.} X x m - * 
 + Jb (P + Q + R + &c.) + a(Pa + Q|8+ &c.) + 
 (Pa 2 + Q/3 2 + Ry 2 + &c.)j x n ~ 3 + &c. 
 Hence equating like quantities (6) 
 
 P + Q + R + &c. = 1 
 
 b + a (A a) + Pa 2 +Q/3 2 +Ry 2 + &c. = B 
 
 &c. = &c. 
 giving n independent equations to determine P, Q, R, &c. 
 
 Ex. 1. Let = r-^ 
 
 Here 
 
 p + Q + R = 1-v 
 
 6 + P+2Q + 3R = 6 >whence 
 11 + P+4Q + 9R = 37 
 
 P = 1, Q = 5 and R = 3 
 
 Hence 
 
 f U d x _ r d x / 5 dx f 3 dx 
 
 = C 1. (x + 1) + 5 1. (x + 2) 31. (x + 3). 
 P, Q, R, &c. may be more easily found as follows : 
 Since 
 
 x 11 - 1 + Ax n ~ 2 &c. = P (x |8). (x y). &c. 
 
 + Q ( X _ ). ( X _ y). &C. 
 
 + R (x ). (x |8). &c. 
 + &c. 
 let x = a, |8, y, &c. successively ; we shall then have 
 
 Aa n ~ 2 + &C. = P.(a j8) . (a y)&c.' 
 
 (A) 
 
 (y a ) (7 
 
 &c. = &c. 
 In the above example we have 
 
 a = 1, ]8 = 2, y = 3 and n = 3 
 A = 6 and B = 3. 
 
 ~ 2 + &C. = P. (a j8) . ( a _y)&c.-v 
 
 -e + &c . _ Q.(/3 a). (j3 y)&C. V. . . 
 
 ~ 2 + &c. = R. a . j8&cJ
 
 74 A COMMENTARY ON [SECT. I. 
 
 . p 16+3 
 1. 2. 
 
 9 6.3 + 3 
 
 2ir^i~ 
 
 as before. 
 
 Hence then the factors of V being supposed all unequal, either of the 
 above methods will give the coefficients P, Q, R, &c. and therefore 
 
 enable us to analyze the general expression ^ into the partial fractions 
 as expressed by 
 
 V = x~=^ + ^T^8 + &C> 
 and we then have 
 
 _) + Ql. (x-/3) + 8cc. + C. 
 
 Ex 
 
 2 /*' + bx ' e: f- d 4- + r dx - a + b f- 
 /a 2 x x 3 ""-' x 2 /a x 2 ^ a 
 
 dx 
 
 + 
 
 a -J- b , , x a + b , , x ^ 
 
 = a 1 x 31 l ( a x) 7- - 1 . (a + x) + C 
 
 = a 1 x (a + b) 1 V a 2 x 2 + C 
 by the nature of logarithms. 
 
 3 x 5 /-d 
 
 -,6x + 8 x 2 4 
 
 = |.l(x_4)_il.(x 2) + C. 
 
 Ex. 4./-^_4A2L__ = / P ^L. + /QllL = P 1 (x + a ) 
 / x 2 + 4ax b 2 'x + a^^x + /S 
 
 + Ql.(x + j8) + C 
 where 
 
 a = 2 a + V(4a 2 + b 2 ), |3 = 2 a V (4 a 2 + b J ) 
 
 and 
 
 p ... _ 2a+ 
 "a 13 = 2V 
 
 O = 
 
 ~ 
 
 a 8 + b g ) 2 a 
 
 /3 2 V (4 a 8 + b 2 ) 
 
 Case 2. Z> all the factors of V be real and equal, or suppose a = (3 
 = 7 = &c. 
 Then 
 
 y - xl> ~'+ A x " ~ 2 + &c. 
 V (x ap
 
 BOOK I.] NEWTON'S PRINCIPIA. 75 
 
 and since 
 
 a /3 = 0, a 7 = &c. 
 
 the forms marked (A) will not give us P, Q, R, &c. In this case we 
 must assume 
 
 P Q R 
 
 V - (x a) n (x a) n - ' (x a) 
 
 -- 
 
 to n 1 terms, and reducing to a common denominator, we get 
 
 U = P + Q . (X a) + R (X a) 2 + &C. 
 now let x = a, and we have 
 
 a n-i_j_ A a n - 2 + &c. =P. 
 
 Also 
 
 1-5 = Q + 2 R . (x a) + 3 S . (x a) 2 + &c. 
 
 U -\. 
 
 d * U = 2 R + 3 . 2 . S . (x a) + 4 . 3 . T (x a) 2 + &c. 
 dx 2 
 
 1LH = 2 . 3 . S + 4 . 3 . 2 T (x a) + &c. 
 dx 3 
 
 &c. = &c. 
 
 and if in each of these x be put = a, we have by Maclaurin's theorem 
 the values of Q, R, S, &c. 
 
 v i T * U x' 3x+ 2 
 Ex. 1. Let ^ = -. - TV; . 
 
 V (x 4) 3 
 
 Then 
 
 U = x 8 3x + 2 
 
 dx 
 d*U 
 
 ' 
 .'. P = 6 
 
 Q = 8 3 = 5 
 R = A. 2 = 1 
 
 5 d x d 
 
 '' f x r x p x f x 
 
 / ~V~ ~ / x 4 s + J x 4 2 + / T 
 
 (x 4) s (x 4) 
 
 Ex.2.
 
 76 A COMMENTARY ON [SECT. I. 
 
 Here 
 
 U = x s + x 3 
 
 dx" = 5x4 + 3x2 
 
 d 
 
 d x 
 
 ^" 
 
 d x 
 
 .-. P = 3 5 + 3 3 = 27 X 10 = 270 
 Q = 27 X 16 = 432 
 
 =120 
 5 
 
 9 X 60 + 6 
 2X3 
 
 T - - - 15 
 
 -2.3.4." 
 
 2.3.4.5 
 Hence 
 
 - 270 f dx + 432 r dx I- 279 f dx 
 
 ?0 6-t J5H 7 
 
 d x 
 
 - 108 ' ( X _ 3) - 93< (x 3) 3 J ' (x 3) 
 which admits of farther reduction. 
 z + x U 
 
 Here 
 
 U = x 2 
 
 -j- = 2x 
 dx 
 
 and 
 
 d*U 
 dx 2 ~ 2 '
 
 NEWTON'S PRINCIPIA. 77 
 
 '(x_l)4 (x I) 3 ! '(x I) 2 
 
 " 2(x I) 4 
 
 It appears from this example, and indeed is otherwise evident, that the 
 number of partial fractions into which it is necessary to split the function 
 exceeds the dimension of-s. in U, by unity. 
 
 This is the first time, unless we mistake, that Maclaurin*s Theorem 
 has been used to analyze rational fractions into partial rational fractions. 
 It produces them with less labour than any other method that has fallen 
 under our notice. 
 
 Case 3. Let the factors of the denominator V be all imaginary and un- 
 equal. 
 
 We know then if in V, which is real, there is an imaginary factor of 
 the form x + h-J-kv' 1, then there is also another of the form 
 x + h k V 1. Hence V must be of an even number of dimensions, 
 and must consist of quadratic real factors of the form arising from 
 
 (x + h + kV l)(x + h k V 1) 
 or of the form 
 
 Hence, assuming 
 
 V = (x +t)?+ f + (x + JQM.V + &c ' 
 
 and reducing to a common denominator, we have 
 
 U = (P + Qx) f (x + a') 2 + j3' 2 } [(x + a") 2 + /3" 2 J X &c. 
 + (P'+ Q'x) [(x + a) 2 + /3 2 J J(x + ") 2 + jS" 2 } X Sic. 
 + (P" + Q" x) {(x + ) 2 + /3 2 H( X + "') *+ P*} X ** 
 + &c. 
 Now for x substitute successively 
 
 & + |3 ^ 1, a! + p V 1, a" + $" V 1, &0. 
 then U will become for each partly real and partly imaginary, and we 
 have as many equations containing respectively P, Q ; P x , Q' ; P", Q" &c. 
 as there are pairs of these coefficients ; whence by equating homogeneous 
 quantities, viz. real and imaginary ones, we shall obtain P, Q ; P', Q'. &c.
 
 78 A COMMENTARY ON [SECT. I. 
 
 Ex. 1. Required the integral of 
 
 x 3 d x 
 
 x*+ 3x 2 + 2' 
 Here the quadratic factors of V are x 2 + 1, x 2 + 2 
 
 .-. a = 0, of = 0, jS = 1, and |8' = V~2 . 
 Consequently 
 
 x 3 = (P + Qx)(x 2 + 2) 
 + (P' + Q'x)(x' + l) 
 
 _ 
 Letx = V ]. Then 
 
 V 1 = (P + Q V 1) . (_ i + 2) 
 = P + Q V~=l 
 
 _' P=0> Q = l 
 Again, let x = V 2. V 1, and we have 
 
 3 
 
 __2* V 1 = (F + Q' V2. V 1)( 2+ I) 
 
 = P Qf V~2 . V~=H[ 
 .-. P 7 = 0, and Q' = 2 
 Hence 
 
 /- x 3 d x / xdx y2 xd x 
 
 Ex. 2. Required the integral of 
 
 - dx 
 
 l + X 2 
 
 To find the quadratic factors of 
 
 1 + x 
 we assume 
 
 x 2 n + 1 = 0, 
 and tlien we have 
 
 x sn = 1 =cos.(2p+ l)^r-f- V lsin.(2p+ 1)* 
 T being 180 of the circle whose diameter is 1, and p any integer what- 
 ever. 
 
 Hence by Demoivre's Theorem 
 
 2p+l . 2p+l 
 
 x = cos. fe-J ff 4. V 1 . sin. T 
 
 2 n 2 n 
 
 But since imaginary roots of an equation enter it by pairs of the form 
 A db V 1 . B, we have also
 
 BOOK I.] NEWTON'S PRINCIPLE 79 
 
 and 
 
 x 
 
 . .- 
 
 2 n 2 n 
 
 / 2p+ 1 , - - 2p 
 
 ( x _cos. -_c + V - 1 sm.-| 
 
 2 n 
 
 which is the general quadratic factor of x 2 n + 1. Hence putting 
 p = 0, 1, 2 ...... n 1 successively, 
 
 x 2n +1 = (x 2 2xcos. -+ 1 ) . (x 2 -2XCOS. ~ + 1 ) x 
 
 (x 2 2xcos. ~ + 1 ) x ---- (x 2 2xcos. 2 ~* + l) . 
 
 Hence to get the values of P and Q corresponding to the general factor, 
 assume 
 
 1 P+Qx N 
 
 i - 
 
 l+x 2n ~ , 2p+ 1 , , M 
 
 X 2 2XCOS. *1 ir+ I 
 2n 
 
 Then 
 
 But 
 
 l+X 2n 
 
 M = 
 
 2n 
 
 and becomes of the form when for x we put cos. it + V 1 X 
 
 sin. "t ; its value however may thus be found 
 & n 
 
 2 p + 1 , . 2 p + 1 
 
 Let cos. ~ v + V 1 sin. *i * = r 
 2 n 2 n 
 
 then 
 
 2 p + 1 . 2 p + 1 1 
 
 cos. ^ it V 1 . sin. if- 1 * = 
 2 n 2 n r 
 
 and 
 
 M = 1+ *' 
 
 Again let x r = y ; then 
 M = 1 + y gn +2ny 
 
 2nyl . 2 n-l + r 8iv
 
 80 A COMMENTARY ON [SECT. I. 
 
 But 
 
 r* B = cos. 2 p + 1 . * + V 1 sin. 2p + l.w = 1 
 
 y*"-'+2n y g - 2 .r + . . . . 2n r 2 "- 1 
 .-. M _ 
 
 Hence when for x we put r, y = 0, and 
 
 ., 2n r*"- 1 
 M = H 
 
 and from the above equation we have 
 
 2 n r s n l 
 ~ 
 
 or 
 
 Qr_i_i 2 p -f- 1 . 2 n 1 , 
 
 . 2p4-1.2n 1 _ ^ 7 . 1V 
 
 sin. *^ ff 2 n Q (since r 8n = 1) 
 
 *w n 
 
 .. equating homogeneous quantities we get 
 
 . 2p-fl . 2p+1.2n 
 
 sin. -: g=nP.sm. r . = 
 
 2 n 2 n 
 
 and 
 
 But 
 
 2p+1.2n 1 ^ 
 P . cos. ^ * = Q. 
 
 & n 
 
 2p+ 1 .2n 1 .ir _ 
 2n ~~ 
 
 Hence the above equations become 
 
 . 2p+ 1 2p + 1 
 
 .'. sin. ^- <s = n P sin. -~ 
 2n 2 n 
 
 2 P+ 1 ^ 
 
 P cos. *g-2- <r = Q 
 
 .pi irk 1 2p+ 1 
 
 . r = -, andQrr . cos. K . 
 
 n n 2 n 
 
 Hence the general partial integral of 
 
 dx .
 
 BOOK I.] NEWTON'S PRINCIPIA. 81 
 
 2p+ 1 
 
 -f 
 
 n / 
 
 ' x 2 
 
 /. 2 p + 1 \ i 
 
 (1 - X COS. -_ ^ d 
 
 2 x cos. " Q * + 1 
 
 11 
 
 cos. ^ /-2xdx 2 cos. ^ ir . d x 
 
 2 n / 2 n 
 
 2n ~; / 2p+ 1 
 
 x 2 2 x cos. * * + 1 
 
 2 p + 1 
 
 sin. 2 % <r / , 
 
 2 n / d x 
 
 
 n / 2 
 
 y x 2 2 x cos. 
 
 2p+ 1 
 
 cos. ^ - * 
 
 2 n 
 
 C 
 
 . 
 2 n V 2 n 
 
 x 
 
 / 
 
 2p+l 
 sin. . x cos. 
 
 . . 
 
 2 n _./ 2n 
 
 xtan ' - 
 
 . 2 P +l 
 sm - 2H- '/ 
 
 see Art 88. Case 4. 
 
 d x 
 
 Hence then the integral of -r r- 5 , which is the aggregate of the results 
 
 1 + x 
 
 obtained from the above general form by substituting for p = 0, 1, 2 ... 
 n 1, may readily be ascertained. 
 
 As a particular instance let J ,- - 6 be required. 
 
 Here 
 
 n = 3 
 and the general term is 
 
 2p+ 1 
 cos. *- it 
 
 6 
 
 . 2p + 1 2p + 1 
 
 sin. r ' * x cos. r ' * 
 
 + =2 tan.-' 
 
 o 2 p + 1 
 
 sm. - 7; w 
 
 Let p = 0, 1, 2, collect the terms, and reduce them ; and it will appear that 
 
 J X * +X V3 + 1 .^xCI-x 2 )] 
 
 l * 2 "* 4 H 
 
 By proceeding according to the above method it will be found, that the 
 general partial fractions to be integrated in the integrals of 
 
 VOL. I. F
 
 82 A COMMENTARY ON [SECT. I. 
 
 d* ind ill?. 
 
 i-zn J * x-i 
 
 are respectively 
 
 cos. 2 p.* T _ ! 
 
 1. _ B - - - -.dx 
 n x ~_ 2 xcos.llp+l 
 
 and 
 
 X - - 9 - d x . 
 
 x *_2cos.?J^x + 1 
 n 
 
 and when these partial integrals are obtained, the entire ones will be 
 found by putting p = 0, 1 ---- | or -" ~ according as p is even or 
 
 odd. 
 
 Ex. 3. Required the integral of 
 
 x r dx _ 
 x sn 2ax n + 1 
 where a is < 1. 
 
 First let us find the quadratic factors of x 2 n 2 a x n + 1. For that 
 purpose put 
 
 x zn 2ax n = 1 
 Then _ 
 
 x n = a+ Va 2 I 
 
 ~^T. V 1 a 
 since a is < 1. 
 
 Now put a = cos. 8; then 
 
 x n = cos. 6 + V 1 sin. 8 
 
 = cos. (2 p * + 3) V~l sin. (2 p + a) 
 
 .*. x = cos. 
 
 , , - , . 
 v 1 sin. 
 
 n n 
 
 and the general quadratic factor of 
 
 x 2D ~- 2 ax n -f 1 
 
 is 2 p cr + _ 
 
 x 2 2 x cos. i- - 1 + 1 
 
 where p may be any number from 0, 1, &c. to n 1. 
 
 Hence to find the general partial integral of the given indefinite differ- 
 ence, we assume 
 
 _ x^ _ P + Qx __ N 
 
 x 2ax"+ 1 1" 2p*-M, . + M 
 
 x * 2 cos. - - + 1 
 n
 
 BOOK I.] NEWTON'S PRINCIPIA. 
 
 and proceeding as in the last example, we get 
 Q = sin. f- 1 + D (2 P *+ 
 
 __ 
 
 n n sin. 8 
 
 and 
 
 P = sin. (n-').(2pr+*) x _J^ _ 
 
 n n sin. 8 
 
 whence the remainder of the process is easy. 
 
 Case 4. Let the factors of the denominator be all imaginary and equal in 
 pairs. 
 
 In this Case, we have the form 
 
 u u 
 
 and assuming as in Case 2. 
 
 U = P + Qx , P' + Q'x 
 
 v (x~+'fl| + ) + (r+^i s + /3 2 )"- 1 H 
 
 K + Lx K? + L' x 
 
 and reducing to a common denominator, 
 
 U = P + Qx + (F + Q'x) (x~+~3 2 + /3 2 ) + &c. 
 
 and substituting for x one of its imaginary values, and equating homoge- 
 neous terms, in the result we get P and Q. Deriving from hence the 
 
 values of -5 , -5 - , &c. and in each of these values substituting for x 
 d x d x 2 
 
 one of the quantities which makes" x -f- 1 2 + P 2 = 0, and equating ho- 
 mogeneous terms we shall successively obtain 
 
 P x , Q'; P", Q", &c. 
 
 This method, however, not being very commodious in practice, for the 
 present case, we shall recommend either the actual developement of the 
 alcove expression according to the powers of x, and the comparison of the 
 coefficients of the like powers (by art. 6), or the following method. 
 
 Having determined P and Q as above, make 
 U (P + Q x) 
 
 x~+^l 2 + /3 2 
 _ U'' ' 
 - 
 
 IT'" - 
 
 &c. = &c. 
 
 Then since U', U". U"', &c. have the same form as U, or have an 
 
 F2
 
 84 A COMMENTARY ON [SECT. I. 
 
 integer form, if we put for x that value which makes (x + a) 2 -f- j8 2 =r 
 0, and afterwards in the several results, equate homogeneous quantities 
 we shall obtain the several coefficients. 
 
 P', Q'; P", Q",&c. 
 
 Case 5. If the denominator V consist of one set of Factors simple and 
 unequal of the form 
 
 x ax a', &c. ; 
 of sevei'al sets of equal simple factors, as 
 
 (x e) P, (x e') % &c. 
 
 and of equal and unequal sets of quadratic factors of the forms 
 x * + a x + b, x * + a' x + b', &c. 
 (x 2 + 1 x + r) >*, (x 8 + 1' x + r') ', &c. 
 then the general assumption for obtaining the partial fractions must be 
 
 ' 
 
 _ _ 
 
 V x ax a 
 
 E F E' F' 
 
 &C * + ' + &C ' 
 
 ' (x e)P r (x e)P~ 1 ^ - (x e') q nr (x e 
 A+Sx R' + S'x 10 ,_ G + Hx , G'+H'x 
 
 i "I *" 
 
 and the several coefficients may be found by applying the foregoing rules 
 for each corresponding set. They may also be had at once by reducing 
 to a common denominator both sides of the equation, and arranging the 
 numerators according to the powers of x, and then equating homogeneous 
 quantities. 
 
 We have thus shown that every rational fraction, whose denominator 
 can be decomposed into simple or quadratic factors, may be itself analyzed 
 into as many partial fractions as there are factors, and hence it is clear 
 that the integral of the general function 
 
 Ax m + Bx m ~ 1 + &c. Kx-f L 
 
 a x n + b x n - 1 + &c. k x + I 
 
 may, under these restrictions, always be obtained. It is always reducible, 
 in short, to one or other or a combination of the forms 
 r / d x / d x 
 
 Having disposed of rational forms we next consider irrational ones. 
 Already (see Art. 86, &c.) 
 
 r j:dx ^ d x r d x 
 
 J V (a bx 2 )' ^xV(bx 2 a)' / V (ax bx 2 )
 
 BOOK I.] NEWTON'S PRINCIPIA. 85 
 
 have been found in terms of circular arcs. We now proceed to treat of 
 Irrationals generally ; and the most natural and obvious way of so doing 
 is to investigate such forms as admit of being rationalized. 
 90. Required the integral of 
 
 dxXF^x, x% x n , xP, xS&c.? 
 
 where F denotes any rational Junction of the quantities between the brackets. 
 Let 
 
 x _ u m n p q ^ &c> 
 
 Then 
 
 i 
 x lu = u npqr .... 
 
 i 
 
 x n _ u mpqr 
 
 1 
 X p = U mnqr .... 
 
 &C. = &C. 
 
 and 
 
 dx = mnpq.... X u ma P q -- 1 x du 
 
 and substituting for these quantities in the above expression, it becomes 
 rational, and consequently integrable by the preceding article. 
 
 + cx 
 Here 
 
 x = u 
 
 X 3 = U 1 
 
 and 
 
 dx = 6u 59 du. 
 
 Hence the expression is transformed to 
 
 u^H- 2au 4 + 1 
 
 - r- - , 5 
 
 b + c u I5 
 
 whose integral may be found by Art 89, Case 3, Ex. 2. 
 91. Required the integral of 
 
 dx X F Jx, (a + b x) , (a + bx) s , 
 where F, as before, means any rational function. 
 
 Put a + bx = u nmp ---- then substitute, and we get 
 
 which is rational.
 
 86 A COMMENTARY ON [SECT. I. 
 
 Examples to this general result are 
 
 x 4 dx , x 2 dx(a + bx)f 
 
 - 5 and - -x 
 
 c x 5 + (a + b x) * x + c (a + b x) f 
 which are easily resolved. 
 92. Required the integral of 
 
 _, f /a + b x\ 2 /a + b x\ E \ 
 
 d x F 1 x, (~ - ) u jf^- - ) q , &c. > 
 Vf+gx)' ^f+gx/ 
 
 Assume 
 
 and then by substituting, the expression becomes rational and mtegrable. 
 
 93. Required the integral of 
 
 A x F x, V (a + b x + c x 2 )J 
 Case 1. When c is positive, let 
 
 a + bx + cx 2 = c(x + u). 
 Then 
 
 a cu 2 ,, 2c(cu 2 b u + a) d u 
 
 x = - - - and d x = --- v .- .7. y 
 
 2cu b (2cu b)* 
 
 / , cu 2 bu+a . 
 
 v(a + bx+cx 2 )= s - jr- . v c 
 2 cu b 
 
 and substituting, the expression becomes rational. 
 
 Case 2. When c is negative, if r, r 7 be the roots of the equation 
 
 a _j_ bx cx 2 = 
 Then assume 
 
 V c(x r) (r 7 x) = (x r) c u 
 and we have 
 
 c r u s + r 7 , (r r 7 ) 2cu du 
 
 : cu 2 + 1 ' (cu 2 + I) 2 
 
 V(a + bx cx 2 )= (r/ "7^ C / 
 
 cy + 1 
 
 and by substitution, the expression becomes rational. 
 
 94. Required the integral of 
 
 d x F Jx, (a + b x) *, (a' + b'x) Q - 
 Make 
 
 a + bx = (a 1 +b'x)u 2 j 
 Then 
 
 __a a^ 2 (&'b b r a)2u du 
 
 ~b'u 2 b j (b'u 2 b) 2 
 
 .. , uV(ab' a'b) , , , ,, . 
 
 ; / /
 
 BOOK I.] NEWTON'S PRINCIPIA. 87 
 
 Hence, substituting, the above expression becomes of the form 
 
 duF'Ju, V(b'u 2 b)} 
 
 F' denoting a rational function different from that represented by F. 
 But this form may be rationalized by 93 ; whence the expression becomes 
 integrable. 
 
 95. Required the integral of 
 
 x m-i d x ( a + b x n )?. 
 
 This form may be rationalized when either , or -- }- is an. integer. 
 
 n n q 
 
 Case l.Leta+bx n = u; then(a+b x n )lf = UP, x a = U<1 ^" a , x m = 
 
 ' nb 
 
 Hence the expression becomes 
 
 q/H <J __ o 
 -T-. uP + i- 1 du (^-r 
 n b b 
 
 which is rational and integrable when is an integer. 
 
 Case 2. Let a + bx n = x n u q ; then substituting as before, we get the 
 transformed expression 
 
 which is rational and integrable when -f is an integer. 
 
 Examples are 
 
 x 2 dx x 2m dx 
 
 2m + 1 x 6 dx 
 
 96. Required the integral of 
 
 x m - 1 dx(a + bx n )q X F(x n ). 
 
 This expression becomes rational in the same cases, and by the same sub- 
 stitutions, as that of 95. To this form belongs 
 
 Y n j "4* n 1 /I v / a i Vj V n \ 
 
 and the more general one 
 
 P ,P 
 
 ^x^-'dx X a + bx) i
 
 88 A COMMENTARY ON [SECT. I. 
 
 where 
 
 P = A + Bx n + Cx 2n + &c. 
 
 and 
 
 Q = A' + B'x n +C'x 2n + &c. 
 
 97. Required the integral of 
 
 x m ~ 1 dx X F^x 10 , x n , (a + bx D )^? 
 Make a + bx n = u q ; then 
 
 qi^-'du /ii' a\Jl_i , 
 X^-'dxrr^ -- .(__) n du 
 
 n b \ b / 
 
 and in the cases where is an integer, the whole expression becomes ra- 
 
 tional and integrable. 
 
 98. Required the integral of 
 
 Xdx 
 
 X' + X" + V (a + b x + c x 2 ) 
 where X, X', X" denote any rational functions of*. 
 Multiply and divide by 
 
 X' + X" V(a + bx + x 2 ) 
 and the result is, after reduction, 
 
 XX'dx XX"dx V 
 
 _ __ 
 
 X' 2 X" 2 (a + bx + ex 2 ) X' 2 X" 2 (a + bx + ex 2 ) 
 consisting of a rational and an irrational part. The irrational part, in 
 many cases, may also be rationalized, and thus the whole made integrable. 
 
 99. Required the integral of 
 
 x m dxF Jx n , \/(a + bx n + ex 2 ")] 
 Let x n = u ; then the expression may be transformed into 
 
 1 m + 1 i 
 u n " l duF{u, V(a + bu + cu )J 
 
 I 
 
 which may be rationalized by Art. 93, when - is an integer. 
 
 100. Required the integral of 
 
 x m dxF X D , V (a + b 2 x 2u ), bx n + V (a + b s x 2n )}. 
 Let 
 
 bx n + V(a + b*x 2n ) = u; 
 then 
 
 - -+ 
 
 and the whole expression evidently becomes rational when - is an 
 integer. 
 
 Many other general expressions may be rationalized, and much might
 
 BOOK I.] NEWTON'S PRINCIPIA. 89 
 
 be said further upon the subject ; but the foregoing cases will exhibit the 
 general method of such reductions. If the reader be not satisfied let him 
 consult a paper in the Philosophical Transactions for 1816, by E. Ffrench 
 Bromhead, Esq. which is decidedly the best production upon the Integrals 
 of Irrational Functions, which has ever appeared. 
 
 Perfect as is the theory of Rational Functions, yet the like has not been 
 attained with regard to Irrational Functions. The above and similar arti- 
 fices will lead to the integration of a vast number of forms, and to that of 
 many which really occur in the resolution of philosophical and other 
 problems ; but a method universally applicable has not yet been discover- 
 ed, and probably never will be. 
 
 Hitherto the integrals of algebraic forms have been investigated. We 
 now proceed to Transcendental Functions. 
 
 101. Required the integral of 
 
 a x dx. 
 By Art 17, 
 
 d.a* =l.a X a x dx .." 
 
 Hence 
 
 -mla 
 102. Required the integral of 
 
 Xa*dx 
 where X is an algebraic Junction ofji. 
 
 By the form (see 73) 
 
 d (u v) = u d v + v d u 
 
 we have 
 
 f u d v = u v fy d u. 
 Hence 
 
 k 'l a J la 
 
 /dX a*dx _ dX a* /.a* d 2 X 
 ^ dx ' la = dx'( 1 a) 2 -'(la) 2 dx 
 y-d*X a x dx _ d g X a x / a x d^X 
 
 -'dx 2 ' (la) 2 ~dx 2 *(la) 3 " ^(la) 2 d"x 2 
 
 Sec. ~~ &c. 
 the law of continuation being manifest.
 
 90 A COMMENTARY ON [SECT. I 
 
 Hence, by substitution, 
 
 /V ^ Y flX dX ^ + d2X '^ & C 
 
 / Xa dx= x u dT-fur^d^'M" 3 " 
 
 whiclr will terminate when X is of the form 
 
 A + Bx + Cx 2 + &c. 
 a*x 3 3a*x* 3.2a*x 3.2a* 
 
 OTHERWISE 
 
 /a x Xdx = a x /Xdx /la.a x dx/Xdx 
 
 = a x X' la/a x X'dx 
 putting 
 
 X'=/Xdx. 
 
 Hence 
 
 /a x X'dx = a x X" la/a x X"dx 
 
 &c. = &c. 
 and substituting, we get 
 
 /a*Xdx = a x X' la.a x X" + (la) 2 a x X'" &c. 
 X', X", X'", &c. being equal to/X d x, /X' d x, /X 7/ d x, &c. re- 
 spectively. 
 
 which does not terminate. 
 
 By this last example we see how an Indefinite Difference may be in- 
 tegrated in an infinite series. If in that example x be supposed less 
 than 1, the terms of the integral become less and less or the series is con- 
 vergent Hence then by taking a few of the first terms we get an ap- 
 proximate value of the integral, which in the absence of an exact one, will 
 frequently suffice in practice. 
 
 The general formula for obtaining the integral in an infinite or finite 
 series, corresponding to that of Taylor in the Calculus of Indefinite 
 Differences, is the following one, ascribed to John Bernoulli, and usually 
 termed 
 
 JOHN BERNOULLI'S THEOREM. 
 
 /Xdx = Xx /xdX 
 rdX _ dX x 2 r x 2 dx d 2 X 
 
 J dx ' = dT'T~./~2~- K* 
 
 /d 2 X x 2 dxd 2 X ** /oc 3 dx d 3 X 
 
 _ 
 
 dx 2 ' 2 ~dx 1 ' 2.3 2.3 ' dx 
 
 &c. = &c.
 
 BOOK L] NEWTON'S PRINCIPIA. 91 
 
 Hence 
 
 /v j -v d A. x d .X. x Q . /-t 
 /Xdx = Xx-^ r . +-^. - -&c. + ( 
 
 the theorem in question. 
 
 + 1 &c. + C 
 
 ~ 5 
 
 m . m 1 
 
 
 \ . 
 &c.)+ 
 
 m 
 But since 
 
 l f m + l.m m+l- m ' m 1 o. \ i r 
 j(m+l HI- \-~ 2 3 &C.J+C 
 
 (1 
 
 = 1 nT+1 + m + J m &c. = 
 
 as in Art. 78. 
 
 102. Required the integral of 
 
 Xdx(lx)" 
 
 where X is any Algebraic Function o/"x, 1 x the Hyperbolic logarithm of x, 
 and n a positive integer. 
 By the formula 
 
 f u d v = u v y* v d u 
 we have 
 
 x-n/'(lx)-' d J c /Xdx 
 
 o \ X ^ 
 
 ^v .*v 
 
 &C. = &C. 
 
 where X', X", X"', &c. are put for/Xdx,/ dx,/ d x, &c. re- 
 
 X -X. 
 
 spectively. 
 Hence 
 /Xdx(lx) n = X / (lx) D nX // (lx) n - 1 + n.(n l)X w (lx) a - 2 &C. + C. 
 
 1 93. Required the integral of 
 where U is any function of ITS..
 
 92 A COMMENTARY ON [SECT. I. 
 
 Let u = 1 x. 
 
 Then 
 
 d x 
 
 a = _, 
 
 and substituting, the expression becomes algebraic, and therefore integra- 
 ble in many cases. 
 
 104. Required the integral of 
 
 Xdx(lx)" 
 'where n is negative. 
 
 Integrating by Parts, as it is termed, or by the formula 
 
 y*u d v = u v f\ d u 
 we get, since 
 
 (1 x) 
 
 /Xdx __ Xx 1 r___ 
 
 ./(lx) n: (n 1) (lx)"- 1+ n IV (Ix)"- 1 ' dx 
 and pursuing the method, and writing 
 
 djxjo 
 
 ~d!T~ 
 
 , _ d (X'x) 
 
 dx 
 
 &c. = &c. 
 we have 
 
 /Xdx_ Xx X'x __ r X^d 
 
 ix*" n-llx-> --- 2 J n-l...2. 
 
 or 
 
 Xx X^ m + 1 ) dx 
 
 __ __ _ __ 
 
 (n l)(lx) n -^ > (n l).(n 2)....(n m)(lx)"- m 
 
 according as n is or is not an integer, m being in the latter case the 
 greatest integer in n. 
 
 /._ __ __ , &c 
 
 X '^ (Lx)" ~n 1 t(lx) n - J + (n 2)(lx) n - 2 ^ C ' 
 
 (m + I)"- 1 ^x m dx 
 
 (n 1) (n 2). .., \J Ix 
 when m is an integer. 
 
 105. Required the integrals of 
 
 d<J d0 dO 
 
 d 6 . cos. 0, d 6 . sin. 6, d 6 . tan. 6. d 6 . sec. 0, , . -. , -. . 
 
 cos. 9 sin. & tan. 6 
 
 By Art. 26, &c. 
 
 d sin. 6 = d 6 . cos. 0, and d cos. 6 = d 6 sin. 
 
 .-./d cos. 6 = sin. + C . . . (a) 
 
 and 
 
 /d 6 sin. 6 = C cos. & . . / x , ! ^i " > -iV.-v< B . (b)
 
 BOOK I.] NEWTON'S PRINCIPLE 93 
 
 Again let tan. 6 = t ; then 
 
 1 + t 2 
 
 and 
 
 ' t d t 
 
 = C 1 . cos. 6 . ' ;- (c) 
 
 since 
 
 1 
 
 1 + t 2 = sec. 2 = 
 d 6 sec. 6 = 
 
 cos. 2 6 ' 
 Again 
 
 d 6 d cos. 6 
 
 cos. 6 ~~ \ sin. 2 
 d (sin. 6) 
 
 "1 sin. 
 
 _ l d (sin. 6) d sin. d 
 
 ~ ' 1 sin. 6 + * ' 1 + sin. 
 .../d 6 sec. 6 = l.i 
 
 = 1 . tan. (45 + ^-\ + C . . . (d) 
 
 which is the same as / . 
 
 J cos. a 
 
 Again 
 
 C~. - = fd d cosec. 
 J sin. 6 J 
 
 . =-l.tan.(45o + ^ - ^) + C ^ * J^ ^ 
 
 = l.(tan. A) + C -, ; ;'..;. ^ V . . (e) 
 Again 
 
 /yMj =/d * cot. tf =/d 6. tan. ( - t) ^ I _ () 
 
 = lcos. (*) + C(byc) 
 
 = 1 . sin. + C (f) 
 
 106. Required the integral of 
 
 sin. m 6 cos. n 6 . d 6. 
 m and n fo/wg positive or negative integers.
 
 94 A COMMENTARY ON [SECT. I. 
 
 Let sin. 6 = u ; then d 6 cos. 6 = d u and the above expression becomes 
 
 u m du (1 u 2 )^ 
 
 ..... ., m + 1 m + 1 n 1 m + n 
 
 which is mtegrable when either ^ or ^ -- 1 -- ^ = 
 
 ~ 3f X 
 
 is an integer (see 95.) If n be odd, the radical disappears ; if n be even 
 
 
 and m even also, then ^~ - = an integer ; if n be even and m odd, then 
 
 it 
 
 m + I . . _ T7 , 
 
 ^ is an integer. Whence 
 
 
 
 u m d u (1 u*)""2 
 is integrable by 95. 
 
 OTHERWISE, 
 Integrating by Parts, we have 
 
 /d 6 sin. m 6 cos. n 6= Sin ' * cos." + l 6+ ^^-r/cos."* 2 6. sin. M ~ 2 6 X d6 
 n + 1 m+1 
 
 sin *^ *~ ^ $ m 1 
 
 = -- !_^ - cos< n + 1 i -- /d x sin. m - 2 d cos. n d 
 m + n m + n* 7 
 
 and continuing the process m is diminished by 2 each time. 
 In the same way we find 
 
 cin m + 1 A fro n 1/J ri _ _1 
 
 m + n ' m + n^ 
 
 and so on. 
 
 107. Required the integrals of 
 
 d u = d 6 sin. (a 6 + b) cos. (a' 6 + b') 
 d v = d 6 sin. (a 6 + b) sin. (of 6 + b') 
 and 
 
 d w = d 6 cos. (a 6 + b) cos. (a' d -f b') 
 By the known' forms of Trigonometry we have 
 
 d u = d 6 {sin. (a + a' . d+b + b') + sin. (a a'. 0+b b')} 
 d v = d 6 {cos. (a+a'. <J + b + b 7 ) cos. (a a' . 0+b b')} 
 d w = d 6 [cos. (a + a', d+b + b') + cos. (a a' . 6+b b')} 
 Hence by 105 we have 
 
 ,f cos. (a + a / .^+ b + bQ cos, (a a 7 . + b b') \ 
 
 *l~ a + a' a a' j" 
 
 r fsin.(a + a' .0 + b + bQ sin. (a a'.d + b bQ 1 
 
 * a +a' ~^TV" " J 
 
 w _ c . , sin, (a + a / . ^ + b + bQ sin, (a^^a 7 . ^ + b bQ 
 * x ' 
 
 These integrals are very useful.
 
 BOOK I.] NEWTON'S PRINCIPLE 95 
 
 108. Required the integrals of 
 
 6 n d 6 sin. 6, and 6 n d Q cos. d. 
 Integrating by Parts we get 
 
 /'0 n Xd<J sin. 6=C n cos. <)+n 6 n ~ l sin. 0+n . (n 1) n ~ 2 cosJ &c. 
 and 
 /0 n Xd0cos.0=C + n sin. <J + n 11 - 1 cos.0 n. (n 1) n - 2 sinJ +&c. 
 
 109. Required the integrals of 
 
 X d x sin. x x 
 X d x tan. ~ l x 
 X d x sec. ~ l x 
 
 &c. 
 Integrating by Parts we have 
 
 &c. = 8tc. 
 see Art. 86. 
 
 110. Required the integral of 
 
 _ (f + g cos. 6} d d 
 ' (a + b cos. 6) n ' 
 Integrating by Parts and reducing, we have 
 
 (a g b f ) sin. d __ 1 
 
 = 
 
 __ 
 (n l)(a 2 ^ b 2 )(a + b cos. 6) n ~ l (n l)(a 2 b 2 ) 
 
 f (n l)(af bg)+(n 2)(ag bf)cos.^ 
 J (a + bcos. 0) n ~ l 
 
 which repeated, will finally produce, when n is an integer, the integral 
 required. 
 
 <U 9 (a b) tan. 5- 
 
 ..-,/ utf _ a f _i _i_r' 
 
 C< /a + bcos. 6~ V(a 2 b 2 )* V ( a 2 _b 2 )" 
 
 or 
 
 , b + a cos. ^+ sin. ^ V (b 2 a 2 ) p 
 
 ' ~ 
 
 V(b 4 a 2 )' a + b cos. 
 
 Notwithstanding the numerous forms which are integrable by the pre- 
 
 ceding methods, there are innumerable others which have hitherto resisted 
 
 all the ingenuity that has been employed to resolve them. If any such 
 
 appear in the resolution of problems, they must be expanded into con-
 
 96 A COMMENTARY ON [SECT. I 
 
 verging series, by some such method as that already delivered in Art. 101 ; 
 or with greater certainty of attaining the requisite degree of convergency, 
 by the following 
 
 METHOD OF APPROXIMATION. 
 
 111. Required to integrate between x = b, x = a, any given Indefinite 
 Difference, in a convergent series. 
 
 Let f (x) denote the exact integral of f X d x; then by Taylor's 
 Theorem 
 
 and making 
 
 h = b a 
 
 Again, make 
 
 x = a 
 then 
 
 i* il* & c 
 
 dx' dx 2 '* 
 become constants 
 
 A, A', &c. 
 and we obtain 
 
 f(b)-f(a) = A(b-a) + ^. (b-a) 2 + ^ (b a) 3 
 
 which, when b a is small compared with unity, is sufficiently conver- 
 gent for all practical purposes. 
 If b a be not small, assume 
 
 b a = p./3 
 
 p being the number of equal parts ft into which the interval b a is sup- 
 posed to be divided, in order to make /3 small compared with unity. Then 
 taking the integral between the several limits 
 a, a + ft 
 a, a + 2 /3 
 
 &c. 
 a, a + p /3
 
 BOOK L] NEWTON'S PRINCIPIA. 97 
 
 we get 
 
 f. (a + /?) -f (a) = A/3 + . + f-" . P* + &c. 
 
 <& -- . > 
 
 B' B" 
 
 A ~. O 
 
 &c. = &c. 
 
 p/, 
 
 f(a+p/3) f(a+^./3)=:P/3 + -/S 2 ^- 
 
 A, A 7 , &c. B, B', &c P, P', &c. 
 
 being the values of 
 
 V dX 
 
 x ' dT> &c ' 
 
 when for x we put 
 
 a, a + ft a + 2 ft &c. 
 Hence 
 
 f(b) f(a) = (A + B + ....P)|8 
 
 + (A' + B' + . . . . F) 172 
 
 + (A" + B" + . . . . P") 1^-3 
 
 + &c. 
 
 the integral required, the convergency of the series being of any degree 
 that may be demanded. 
 
 If /3 be taken very small, then 
 
 f (b) f (a) = (A + B + P) 18 nearly. 
 
 Ex. Required the approximate value of 
 
 /X^-'dx X (1 x n )T 
 
 m m p 
 
 between the limits of x = and x = 1, when neither ~ n r ~ + ~ 
 
 is an integer. 
 Here 
 
 X = x 1 "- 1 ^ x n )T 
 and 
 
 dX p i np 
 
 __ / m i _ ^ 1 \ v m 2/1 vn\q J -, m 2 /I ^ n ^ 
 
 j i in T~ n J j x 11 ^ A y ^ jt 1 1 ~- A j 
 
 b a= 10= 1. 
 
 Assume 1 = 10 X ft and we have for limits 
 
 J. ^ 
 
 '10' '10' * 
 VOL. I. G
 
 08 A COMMENTARY ON fSEcr. I. 
 
 Hence m being > 1, 
 
 A = 
 
 B- -^- (l- 
 " 10" 1 - 1 \ 10 
 
 &c. = &c. 
 
 p _. ( 9 \ m 
 - (TO) -TO 
 
 Hence, between the limits x = 1 and x = 
 
 /Xdx = - l - -x |(10 n l)f +(10 n 2 n ) 
 m + n 
 
 (10"_ 3 n ) + &c. + (10 n 9 n ) nearly. 
 
 We shall meet with more particular instances in the course of our 
 comments upon the text. 
 
 Hitherto the use of the Integral Calculus of Indefinite Differences has 
 not been very apparent. We have contented ourselves so far with 
 making as rapid a sketch as possible of the leading principles on which 
 the Inverse Method depends ; but we now come to its 
 
 APPLICATIONS. 
 
 112. Required to Jind the area of any curve, comprised between two 
 given values of its ordinate. 
 
 Let E c C (fig. to LEMMA II of the text) be a given or definite area 
 comprised between and C c, or and y. Then C c being fixed or De- 
 finite, let B b be considered Indefinite, or let L b = d y. Hence the 
 Indefinite Difference of the area E c C is the Indefinite area 
 
 B Ccb. 
 
 Hence if E C = x, and S denote the area E c C ; then 
 dS = BCcb=CL + Lcb 
 
 = ydx + Lcb. 
 
 But L c b is heterogeneous (see Art. 60) compared with C L or y d x. 
 .-. d S = v d x
 
 BOOK I.] NEWTON'S PRINCIPIA. 99 
 
 Hence 
 
 S=/ydx, 
 the area required. 
 
 Ex. 1. Required the area of the common parabola. 
 Here 
 
 y 2 = ax. 
 
 2 y dy 
 .-. d x = * * 
 
 a 
 
 and 
 
 and between the limits of y = r and y = r' becomes 
 
 S = (r'-r") 
 
 If m and m' be the corresponding values of x, we have 
 S = -| (r m i- 7 m') 
 
 Let r 7 = 0, then 
 
 2 
 
 = -=- of the circumscribing rectangle. 
 
 2 
 
 S =r r m (see Art. 21.) 
 
 
 
 B 
 
 Ex. 2. Take the general Parabola whose equation is 
 
 y m = a x n . 
 Here it will be found in like manner that 
 
 m-f-n 
 
 m 
 
 = . a p 
 
 m + n 
 
 between the limits of n = y = 0, and x = a, y = (3. 
 
 Hence all PARABOLAS may be squared, as it is termed ; or a square may 
 be found 'whose area shall be equal to that of any Parabola. 
 
 Ex. 3. Required the area of an HYPERBOLA comprised by its asymptote, 
 and one infinite branch. 
 
 If x, y be parallel to the asymptotes, and originate in the center 
 
 x y = ab 
 
 is the equation to the curve. 
 Hence 
 
 , a b d y 
 
 O. X ~~ a 
 
 y 8 
 
 G2
 
 100 A COMMENTARY ON [SECT. J. 
 
 and 
 
 Let at the vertex y = /S, and x = ; then the area is and 
 
 C = a b . 1 0. 
 Hence 
 
 S = ab.l. . 
 
 y 
 
 1 13. If the curve be referred to ajixed center by the radius-vector g and 
 traced-angle 6; then 
 
 For d S = the Indefinite Area contained by , and g + d=(f-fdg) ^^ - 
 
 
 
 e 2 d d P d P d 6 
 = + s | - (Art. 26) and equating homogeneous quantities we 
 
 A <& 
 
 have 
 
 . 
 
 Ex. 1. In the Spiral of Archimedes 
 S = a 6 
 
 Ex. 2. In the Trisectrix 
 
 S = 2 cos. + 1 
 
 .-. dS = /(2cos. d I) 2 do 
 which may easily be integrated. 
 
 Hence then the area of every curve could be found, if all integrations 
 were possible. By such as are possible, and the general method of ap- 
 proximation (Art. Ill) the quadrature of a curve may be effected either 
 exactly or to any required degree of accuracy. In Section VII and many 
 other parts of the Principia our author integrates Functions by means of 
 curves ; that is, he reduces them to areas, and takes it for granted that 
 such areas can be investigated. 
 
 114. Tojind the length of any curve comprised within given values of the 
 ordinate ; or To RECTIFY any curve. 
 
 Let s be the length required. Then d s = its Indefinite Chord, by 
 Art. 25 and LEMMA VII. 
 
 .-. ds = V (dx 2 + dy 2 ) 
 and 
 
 s =/*/(dx* + dy s ) ..... (a)
 
 BOOK I.] NEWTON'S PRINCIPIA. 101 
 
 Ex. 1. In the general parabola 
 
 y m = ax". 
 Hence 
 
 dx 2 = m2 lp- 2 d 2 
 
 n 2 aT 
 and 
 
 (m 2 2m 
 1 + - g y~ 
 
 n 2 a~iT 
 which is integrable by Art. 95 when either 
 
 1 1 1 
 
 n 
 ihat is, when either 
 
 In 1m 
 
 or 
 
 2*m n 2*m n 
 is an integer ; that is when either m or n is even. 
 
 The common parabola is Rectifiable, because then m = 2. In this case 
 
 ds=dyV(lH i y 2 ) (r) 
 
 Hence assuming according to Case 2 of Art. 95, 
 1 _i_ 2 = * u * 
 
 a s y - y 
 
 we get the Rational Form 
 
 u 2 d u 
 d s = 
 
 Hence by Art. 89, Case 2, 
 
 1 + Vu 
 
 s 
 
 2_ 
 
 a 
 
 2 
 
 + V u 
 _= +C. 
 
 But u = V _ . Hence by substituting and making the ne- 
 cessary reductions 
 
 G3
 
 102 
 
 A COMMENTARY ON 
 
 [SECT. I. 
 
 s = 
 
 , 
 
 + a 1 . 
 
 Let y = ; then s = and we get C = 
 and .'. between the Limits of y = and y = |8 
 
 ' + 
 
 In the Second Cubical Parabola 
 and 
 
 y 3 = a x 2 
 
 which gives at once (Art. 91) 
 
 Ex. 2. In the circle (Art. 26) 
 
 ds = 
 
 + C. 
 
 V") 
 
 V (\ y 2 ) 
 
 which admits of Integration in a series only. Expanding (1 y 2 )~ 
 by the Binomial Theorem, we have 
 
 Hence, 
 and 
 
 s = ay + 
 
 + y 4d -y + &c - 
 
 and between the limits of y = and y = - or for an arc of 30 we have 
 
 ~ 
 
 1 
 
 
 4. 
 
 s "*" 
 
 + &c. 
 
 2 r 2. 3. 2 s T 2.4.5.2 s 
 
 1 1 3 5 
 
 2 + 3. 2 4 + 5. 2 8 + 7. 2 U + 9. 2 
 .5 
 
 .0208333333 
 .0023437500 
 
 .0003487720 
 L.0000593390^ 
 
 5.7 
 
 = .5235851943 nearly.
 
 BOOK L] NEWTON'S PRINCIPIA. 103 
 
 Hence 180 of the circle whose radius is 1 or the whole circumference- 
 Tt of the circle whose diameter is 1 is 
 
 * = . 5235851943 ... X 6 nearly 
 = 3.1415111658 
 
 which is true to the fourth decimal place : or the defect is less than 
 
 10000 
 
 y taking more terms a 
 3 obtained. 
 Ex. 3. In the Ellipse 
 
 By taking more terms any required approximation to the value of IT may 
 be obtained. 
 
 a 2 a 2 v 
 
 S* i CL C -V 
 
 s =/dx . 
 
 "v a 2 x 2 
 
 where x is the abscissa referred to the center, a the semi-axis major and 
 a e the eccentricity (see Solutions to Cambridge Problems, Vol. II. p. 144.) 
 115. If the curve be referred to polar coordinates, and 6; then 
 
 s =fV (s 2 d d* +dg 2 ) . I'*'. '..'".. (b) 
 For 
 
 y = g sin. 6 
 x = m + g cos. d 
 
 and if d x 2 , d y z be thence found and substituted in the expression 
 (114. a) the result will be as above. 
 Ex. 1. In the Spiral of Archimedes 
 % = a* 
 
 .*. d s = d 
 
 *w il 
 
 see the value for s in the common parabola, Art. 1 14. 
 Ex. 2. In the logarithmic Spiral 
 
 or 
 
 6 = \. g 
 and we find 
 
 s = V~2fd = g V 2 + C. 
 
 116. Required the Volume or solid Content of any solid formed by the 
 revolution of a curve round its axis. 
 
 Let V be the volume between the values and y of the ordinate of the 
 generating curve. Then d V = a cylinder whose base is T y 2 and alti- 
 tude d x + a quantity Indefinite or heterogeneous compared with either 
 d V or the cylinder.
 
 104 A COMMENTARY ON [SECT. I. 
 
 But the cylinder = T y 2 d x. Hence equating homogeneous terms, we 
 have 
 
 d V = *y 2 dx 
 and 
 
 V = */y 2 dx (c) 
 
 Ex. 1. In the sphere (rad. = r) 
 
 y 2 __ j.2 X 2 
 
 .-. V = <r/r 2 d x */x 2 d x 
 
 and between the limits x = and r 
 
 V = -r 3 c 
 3 
 
 which gives the Hemisphere. 
 Hence for the whole sphere 
 
 Ex. 2. In the Paraboloid. 
 
 y = ax 
 .-. V = <ra x d x 
 
 and between the limits x = and a 
 
 Ex. 3. In the Ellipsoid. 
 
 .-.V = :p../(adx x 2 dx) 
 
 a' 
 and between the limits x = and a 
 
 V - 2crb * 3 - L? 
 Hence for the whole Ellipsoid 
 
 y * v 2 
 
 The formula (c) may be transformed to 
 
 V = <r y S */S d y (d)
 
 BOOK I.] NEWTON'S PRINCIPIA. 105 
 
 where S = yy d x or the area of the generating curve, which is a singular 
 expression, yS d y being also an area. 
 
 In philosophical inquiries solids of revolution are the only ones almost 
 that we meet with. Thus the Sun, Planets and Secondaries are Ellip- 
 soids of different eccentricities, or approximately such. Hence then in 
 preparation for such inquiry it would not be of great use to investigate 
 the Volumes of Bodies in general. 
 
 If x, y, z, denote the rectangular coordinates, or the perpendiculars let 
 fall from any point of a curved surface upon three planes passing through 
 a point given in position at right angles to one another, then it may easily 
 be shown by the principles upon which we have all along proceeded, 
 that 
 
 d V = d y/z d x- 
 or 
 
 = d zyy d x 
 or 
 
 = d xy~z d y_ 
 
 according as we take the base of d V in the planes to which z, y, or x is 
 respectively perpendicular 
 
 For let the Volume V be cut off by a plane passing through the point 
 in the surface and parallel to any of the coordinate planes ; then the area 
 of the plane section thus made will be 
 
 /z dx' 
 or 
 
 (e) 
 
 /y dx 
 
 or 
 
 see Art. 112. 
 
 Then another section, parallel toyz d x, orfy d x, or fz d y and at 
 the Indefinite distance d y, or d z, or d x from the former being made, 
 the Indefinite Difference of the Volume will be the portion comprised by 
 these two sections ; and the only thing then to be proved is that this por- 
 tion is = d yy*z d x or d zfy d x, or d x,/z d y. But this is easily to 
 be proved by LEMMA VII. 
 
 This, which is an easier and more comprehensible method of deducing 
 d V than the one usually given by means of Taylor's Theorem, we have 
 merely sketched ; it being incompatible with our limits to enter into de- 
 tail. To conclude we may remark that in Integrating both y z d x, and 
 f d y f z d x must be taken within the prescribed limits, first considering 
 y Definite and then x.
 
 106 A COMMENTARY ON [SECT. I. 
 
 117. To find the curved surface of a Solid of Revolution. 
 Let the curved surface taken as far as the value y of the ordinate re- 
 ferred to the axis of revolution be a, and s the length of the generating 
 curve to that point; then d a = the surface of a cylinder the radius of 
 whose base is y and circumference 2 it y, and altitude d s, by LEMMA VII. 
 and like considerations. Hence 
 
 d<r = 2-7ryds 
 and 
 
 e = 2 *fy d s .......... (a) 
 
 or 
 
 = 2 it y s 2 itfs dy ...... (b) 
 
 which latter form may be used when s is known in terms of y ; this will 
 not often be the case however. 
 Ex. In the common Paraboloid. 
 
 y 2 = a x 
 and 
 
 Let y = and j3, then a between these limits is expressed by 
 
 If the surface of any solid whatever were required, by considerations 
 similar to those by which (116. e) is established, we shall have 
 
 d<r = V (dy 2 + dz 2 )/ V (dx 2 + dz 2 ) . . . . (c) 
 and substituting for d z in V d x 2 + d z 2 its value deduced from z = f. 
 (x, y) on the supposition that y is Definite ; and in V (d y 2 + d z 2 ) its 
 value supposing x Definite. Integrate first V (d x 2 + d z 2 ) between the 
 prescribed limits supposing y Definite and then Integrate V (d y 2 + d z z ) 
 /V(dx 2 +dz 2 ) between its limits making x Definite. This last result 
 will be the surface required. 
 
 We must now close our Introduction as it relates to the Integration of 
 Functions of one Independent variable. 
 
 It remains for us to give a brief notice of the artifices by which Func- 
 tions of two Independent Variables may be Integrated. 
 
 118. Required the Integral of 
 
 Xdx + Ydy = 0, 
 where X is any function ofx, and Y a function ofy the same or different.
 
 BOOK L] NEWTON'S PRINCIPIA. 107 
 
 When each of the terms can be Integrated separately by the preceding 
 methods for functions of one variable, the above form may be Integrated, 
 and we have 
 
 /Xdx+/Ydy = C. 
 
 This is so plain as to need no illustration from examples. We shall, 
 nowever, give some to show how Integrals apparently Transcendental 
 may in particular cases, be rendered algebraic. 
 
 Ex . !. 1* + lZ_ = o. 
 
 x y 
 
 .-. 1 x + 1 y = C = 1 . C' 
 
 9 , 
 
 2 H 
 
 .'. x y = C' or = C. 
 
 d y __ 
 
 (1 x 2 ) V (1 y 2 ) ' 
 Here 
 
 sin. - l x + sin. - l y C = sin. - ' C 
 .*. C = sin. {sin. -1 x = sin. ~ ' y} 
 
 = sin. (sin. ~~ 1 x) . cos. (sin. ~ ' y ) + cos. (sin. ~~ l x) sin. (sin. ~~ * y) 
 = x. V (I y 2 ) + y V (1 x 2 ) 
 which is algebraic. 
 
 Generally if the Integral be of the form 
 
 f-'(x) + f.-'(y) = c 
 
 Then assume 
 
 C = f.-'(C) 
 and take the inverse function of f ~ l (C) and we have 
 
 C = f{f-'(x) + f- 1 (y)| 
 which when expanded will be algebraic. 
 
 119. Required the Integral of 
 
 Ydx + Xdy = 0. 
 Dividing by X Y we get 
 
 4-^-0 
 X Y 
 
 which is Integrable by art. 118. 
 
 120. Required the Integral of 
 
 Pdx + Qdy = 0; 
 
 where P and Q are each such junctions ofx. and y that the sum of the expo- 
 nents ofyi and y in every term of the equation is the same.
 
 108 A COMMENTARY ON [SECT. I.' 
 
 Let x = u y. Then if m be the constant sum of the exponents, P and 
 Q will be of the forms 
 
 U x y m U' y m 
 U and U' being functions of u. 
 
 Hence, since dx = udy + ydu, we have 
 
 U.(udy + ydu) + U'dy = 
 and 
 
 (Uu + U')dy + Uydu = 
 
 d_y " Udu 
 ' y ~ U u + U' = 
 which is Integrable by art. 118. 
 
 Ex. 1. (a x -f- b y) d y + (f x + g y) d x = 0. 
 Here 
 
 P = fx+gy, Q = ax + by 
 U= fu+ g, U' = au + b 
 . dy (fu + g)du _ Q 
 
 y fu 2 + (g + a) u + b 
 
 which being rational is Integrable by art. (88, 89) 
 Ex. 2. x d y y d x = d x V (x 2 + y 2 ) 
 
 Here 
 
 Q = x, P = y V (x 2 + y 2 ) 
 
 U' = u, U = 1 V (1 + u 8 ) 
 
 'J L+ l v V (l l + + J) )dtt = ; ; ; V " ' h 
 
 or 
 
 d y d u d u _ 
 
 y u uV(l+u 2 )" 
 
 which is Integrable by art (82, 85.) 
 These Forms are called Homogeneous. 
 121. To Integrate 
 
 (ax + by + c)dy + (mx+ny+p)dx = 0. 
 
 By assuming 
 
 by 
 and 
 
 m x 
 
 we get 
 
 + by + c = u-j 
 + n y + p = v ) 
 
 mdu adv b d v n d u 
 
 d y = -- r -- , and d x = -- r 
 
 mb na mb na 
 
 and therefore 
 
 (mu nv)du + (bv au)du = 
 which being Homogeneous is Integrable by Art. 120.
 
 BOOK I.] NEWTON'S PRINCIPIA. 109 
 
 We now come to that class of Integrals which is of the greatest use in 
 Natural Philosophy to 
 
 LINEAR EQUATIONS. 
 
 122. Required to Integrate 
 
 dy + yXdx = X'dx, 
 where X, X' are functions of X. 
 Let 
 
 y = u v. 
 Then 
 
 udv+vdu+Xuvdx= X' d x 
 Hence assuming 
 
 dv-f-vXdx = f ;" .^ i ... (a) 
 we have also 
 
 v d u = X' d x (b) 
 
 Hence 
 
 + Xdx = 
 
 .-. Iv +/Xdx = C 
 or 
 
 v = e C /Xdx 
 
 = e c X e-/ Xdx 
 = C X e-' x *. 
 Substituting for v in (b) we therefore get 
 
 1 /Xdx 
 
 du=i e X'dx 
 
 yj 
 
 which may be Integrated in many cases by Art 118. 
 
 Ex. dy + ydx = ax 3 dx. 
 Here 
 
 X = 1, X' = a x s 
 
 /Xdx = x 
 and 
 
 /X'dxe' Xdx = a/x 3 e*dx 
 
 = ae*(x s 3x 2 + 6x 6) 
 see Art. (102) 
 
 Hence 
 
 y= Ce- x + a (x 3 3x s + 6x 6)
 
 110 A COMMENTARY ON [SECT. I. 
 
 122. Required to Integrate the LINEAR Equation of the second order 
 
 dLy + X il + X'y = 
 dx 8 d x n 
 
 where X, X' are ^junctions of*. 
 
 d v 
 
 Lety = e-^ udx ; then -j-^ = ue /udx 
 a x 
 
 and .'. by substitution, 
 
 ^+u 2 +Xu+X' = 
 
 U -V 
 
 which is an equation of the first order and in certain cases may be Integ- 
 rable by some one of the preceding methods. When for instance X and 
 X' are constants and a, b roots of the equation 
 
 u 2 + Xu+ X' = 
 then it will be found that 
 
 y=Ce ax +C'e bx . 
 123. Required the Integral of 
 
 d^+X'y = X" 
 d x 
 
 we may assume the 
 
 where X" is a new function cfx. 
 
 Let y = t z ; then Differencing, and substituting, 
 
 result 
 and 
 
 , /d t\ /d t\ / Y z d zv , X" , 
 
 d ( T ) + ( -7 ) ( X + 75 . T J d x = . . . (b) 
 vdx/ \dx/ \ Z dx/ z 
 
 Hence (by 122) deriving z from (a) and substituting in (b) we have a 
 Linear Equation of the first order in terms of (T-)> whence (-5 ) may 
 be found ; and we shall thus finally obtain 
 
 Fv " y 4. r-i l v - 
 
 dx 2 r dx' x "~x 2 ' y ~ x 8 T 
 
 Here 
 
 X "5T' 
 
 ^T > A ,7z 
 
 3T 2> "X 2 I'
 
 BOOK I.] NEWTON'S PRINCIPIA. Ill 
 
 Equat. (a) becomes 
 
 d 2 z d z 1_ . . ;z 
 cfx~ 2 + dx' x "~ x 2 
 whence 
 
 wherein z = e /udx ; which becomes homogeneous when for u we put \~\ 
 Next the -variables are separated by putting (see 120) 
 
 x = v s 
 and we have 
 
 d v s 2 + s 1 
 
 V 'TjTn- i) ( 
 
 and 
 
 . v - _1 / s + 1 
 
 " s V s- r 
 
 Hence 
 
 x 2 + 1 , x 2 1 
 
 u = j 5 - r . , / u d x = 1 . - 
 x (x 2 1) * x 
 
 and 
 
 2 __ i 
 * 
 
 z = e / udx = 
 
 X 
 
 Again 
 
 e /Xdx __ e lx _ x 
 
 and 
 
 /X" e^ Xdx z d x = /a d x = a x + C 
 and 
 
 x 2 1 *(& x + C) x d x 
 
 ~T~J~ (x 2 i) 2 
 
 which being Rational may be farther integrated, and it is found that 
 finally 
 
 ax + C x 2 1 / x 
 ~~ " 
 
 Here we shall terminate our long digression. We have exposed both 
 the Direct and Inverse Calculus sufficiently to make it easy for the 
 reader to comprehend the uses we may hereafter make of them, which 
 was the main object we had in view. Without the Integral Calculus, in 
 some shape or other, it is impossible to prosecute researches in the higher 
 branches of philosophy with any chance of success ; and we accordingly 
 see Newton, partial as he seems to have been of Geometrical Synthesis, 
 frequently have recourse to its assistance. His Commentators, especially
 
 112 A COMMENTARY ON [SECT. II. 
 
 the Jesuits Le Seur and Jacquier, and Madame Chastellet (or rather 
 Clairaut), have availed themselves on all occasions of its powers. The 
 reader may anticipate, from the trouble we have given ourselves in establish- 
 ing its rules and formulae, that we also shall not be Very scrupulous in that 
 respect. Our design is, however, not perhaps exactly as he may suspect. 
 As far as the Geometrical Methods will suffice for the comments we may 
 have to offer, so far shall we use them. But if by the use of the Algo- 
 rithmic Formulae any additional truths can be elicited, or any illustrations 
 given to the text, we shall adopt them without hesitation. 
 
 SECTION II. PROP. I. 
 
 124. This Proposition is a generalization of the Law discovered by Kepler 
 from the observations of Tycho Brahe upon the motions of the planets 
 and the satellites. 
 
 " When the body has arrived at B," says Newton, " let a centripetal 
 force act at once with a strong impulse, fyc"~\ But were the force acting 
 incessantly the body will arrive in the next instant at the same point C. 
 
 For supposing the centripetal force 
 incessant, the path of the body will 
 evidently be a curve such as A B C. 
 Again, if the body move in the chord 
 A B, and A B, B C be chords de- 
 scribed in equal times, the deflection 
 from A B, produced by an impulsive 
 force acting only at B and communi- 
 cating a velocity which wouldhave been 
 generated by the incessant force in the time through A B, is C c. But 
 if the force had been incessant instead of impulsive, the body would have 
 been moving in the tangent B T at B, and in this case the deflection at the 
 end of the time through B C would have been half the space describ- 
 ed with the whole velocity generated through B C (Wood's Mech.) 
 But 
 
 CT = i Cc 
 .'. the body would still be at C.
 
 BOOK I.] NEWTON'S PRINCIPIA. 113 
 
 AN ANALYTICAL PROOF. 
 
 Let F denote the central force tending constantly to S (see Newton 
 figure), which take as the origin of the rectangular coordinates (x, y) 
 which determine the place the body is in at the end of the time t. Also 
 let be the distance of the body at that time from S, and 6 the angular 
 distance of from the axis of x. Then F being resolved parallel to the 
 axis of x, y, its components are 
 
 F.- andF. Z- 
 
 and (see Art. 46) we .-. have 
 
 d ' x _ _ F - d2y 
 
 dt 2 ' ' s ' dt 2 
 
 Hence 
 
 - -F 
 
 ' ' 
 
 y d 2 x F x_y _ x_d^ 
 
 dt 2 g dt 2 
 
 xd z y 
 
 dt 
 But 
 
 yd 2 x xd 2 y = dydx + y d 2 x dxdy xd 2 y 
 
 = d.(ydx xdy) 
 .. integrating 
 
 y d x x d v 
 
 = , = constant = c. 
 
 d t 
 
 Again, 
 
 x = f cos. 6, y = sin 0, x 2 + y 2 = g z 
 .'. d x = g d & sin. 6 + d g cos. 6 
 d y = f d 6 cos. 6 + d sin. d; 
 whence by substitution we get 
 
 ydx xdy = f d0 
 . g" d * _ 
 
 But (see Art. 1 13) 
 
 - = d . (Area of the curve) = d . A 
 
 c 
 VOL. I. H
 
 114 A COMMENTARY ON [SECT. II. 
 
 Now since the time and area commence together in the integration 
 there is no constant to be added. 
 
 .-. t = X A oc A. 
 c 
 
 Q. e. d. 
 
 125. COR. 1. PROP. II. By the comment upon LEMMA X, it appears 
 that generally 
 
 ds 
 
 V = d-t 
 
 and here, since the times of describing A B, B C, &c. are the same by 
 hypothesis, d t is given. Consequently ' 
 
 v a d s 
 
 that is the velocities at the points A, B, C, &c. are as the elemental spaces 
 described A B, B C, C D, &c. respectively. But since the area of a A 
 generally = semi-base X perpendicular, we have, in symbols, 
 
 d. A = p X d s 
 
 d. A 
 .. v oc d s oc ; 
 
 P 
 
 and since the A A B S, B C S, C D S, &c. are all equal, d A is constant, 
 and we finally get 
 
 1 c 
 
 v a or = - 
 
 P P 
 
 the constant being determinable, as will be shown presently, from the 
 nature of the curve described and the absolute attracting force of S. 
 
 1 26. COR. 2. The parallelogram C A being constructed, C V is equal and 
 parallel to A B. But A B = B c by construction and they are in the 
 same line. Therefore C V is equal and parallel to B c. Hence B V is 
 parallel to C c. But S B is also parallel to C c by construction, and 
 B V, B S have one point in common, viz. B. They therefore coincide. 
 That is B V, when produced passes through S. 
 
 127. COR. 3. The body when at B is acted on by two forces ; one in 
 the direction B c, the momentum which is measured by the product of its 
 mass and velocity, and the other the attracting single impulse in the di- 
 rection B S. These acting for an instant produce by composition the 
 momentum in the direction B C measurable by the actual velocity X mass. 
 Now these component and compound momentums being each propor- 
 tional to the product of the mass and the initial velocity of the body in 
 the directions B c, B V, and B C respectively, will be also proportional 
 to their initial velocities simply, and therefore by (125) to B V, B c, B C.
 
 BOOK I.] 
 
 NEWTON'S PRINCIPIA. 
 
 115 
 
 Hence B V measures the force which attracts the body towards S when 
 the body is at B and so on for every other position of the body. 
 
 128. COR. 1. PROP. II. In the annexed 
 figure B c = A B, C c is parallel to 
 S B, and C' c is parallel to S' B. Now 
 A S C B = S c B = S A B, and if the 
 body by an impulse of S have deflected 
 from its rectilinear course so as to be 
 in C, by the proposition the direction 
 in which the centripetal force acts is that 
 of C c or S B. But if, the body having 
 arrived at C', the A S B C' be > S A B 
 (the times of description are equal by 
 hypothesis) and .*. > S B C, the vertex 
 C' falls without the A S B C, and the 
 direction of the force along c C' or B S', 
 has clearly declined from the course 
 B S in consequentia. 
 
 The other case is readily understood 
 from this other diagram. 
 
 129. To prove that a body cannot de- 
 scribe areas proportional tothe timesround 
 two centers. 
 
 If possible let 
 
 AS'AB = AS'BC 
 and 
 
 S A B = S B C. 
 
 Then 
 
 AS 7 B C(= S' AB)= S'Bc 
 and C c is parallel to S' B. But it is 
 also parallel to S B by construction. 
 Therefore S B and S' B coincide, which 
 is contrary to hypothesis. 
 
 130. PROP. III. The demonstration of this proposition, although strictly 
 rigorous, is rather puzzling to those who read it for the first time. At least 
 so I have found it in instruction. It will perhaps be clearer when stated 
 symbolically thus : 
 
 Let the central body be called T and the revolving one L. Also lef 
 the whole force on L be F, its centripetal force be f, and the force ac- 
 
 H2
 
 116 A COMMENTARY ON [SECT. II. 
 
 celerating T be f. Then supposing a force equal to f to be applied to 
 L and T in a direction opposite to that of f , by COR. 6. of the Laws, 
 the force f will cause the body L to revolve as before, and we have 
 remaining 
 
 f = F f ' 
 or 
 
 F = f + f . 
 
 Q. e. cl. 
 
 ILLUSTRATION. 
 
 Suppose on the deck of a vessel in motion, you whirl a body round in a 
 vertical or other plane by means of a string, it is evident the centrifugal 
 force or tension of the string or the power of the hand which counteracts 
 that centrifugal force i. e. the centripetal force will not be altered by the 
 force which impels the vesseL Now the motion of the vessel gives an 
 equal one to the hand and body and in the same direction ; therefore the 
 force on the body = force on the hand + centripetal power of the hand. 
 
 131. PROP. IV. Since the motion of the body in a circle is uniform by 
 supposition, the arcs described are proportional to the times. Hence 
 
 ., , arc X radius 
 t a arc described a ^ 
 
 fit 
 
 a area of the sector. 
 
 Consequently by PROP. II. the force tends to the center of the circle. 
 
 Again the motion being equable and the body always at the same dis- 
 tance from the center of attraction, the centripetal force (F) will clearly 
 be every where the same in the same circle (see COR. 3. PROP. I.) But 
 the absolute value of the force is thus obtained. 
 
 Let the arc A B (fig. in the Glasgow edit.) be described in the time T. 
 Then by the centripetal force F, (which supposing A B indefinitely small, 
 may be considered constant,) the sagitta D B (S) will be described in 
 that time, and (Wood's Mechanics) comparing this force with gravity as 
 the unit of force put = 1, we have 
 
 S = | FT 2 
 
 
 
 g being = 32 feet. 
 But by similar triangles A B D, A B G 
 
 S = BD = ' chordAB) ' = "' rc V ) 'ult>. 
 A G 2 R
 
 BOOK I.] 
 (LEMMA VII.) 
 
 If T be given 
 
 If T = arc second 
 
 NEWTON'S PRINCIPIA. 
 T? - _ 2S - ( arc A B ) 2 
 
 117 
 
 (arcAB)- 
 
 
 
 132. COR. 1. Since the motion is uniform, the velocity is 
 
 _ arc 
 
 : T 
 
 V 2 V 2 
 
 *'* F = i~R a R ' 
 
 133. COR. 2. The Periodic Time is 
 
 circumference __ 2 # R 
 velocity v 
 
 P = 
 
 4* 2 R 
 
 gRP 2 gP 
 
 134. COR. 3, 4, 5, 6, 7. Generally let 
 P = k X R n , 
 k being a constant. 
 Then 
 
 ^ 
 P 2 ' 
 
 and 
 
 v = 
 
 F = 
 
 a 
 
 iff R 2 
 
 P = k R"- 1 " R"^ 
 
 4*'R 4 2 
 
 gk 2 R 2n " gk 2 R 2n - 1 
 
 oc 
 
 Conversely. If F a Rgn _ l ; P will a R n . 
 For (133) 
 
 'Pa / ?: oc V R zn R n . 
 
 135. COR. 8. A B, a b are similar 
 arcs, and A B, a h contemporaneous- 
 ly described and indefinitely small. M 
 
 Now ultimately 
 
 an:am::ah 2 :ab 2 
 and 
 
 a m : A M : : a b : A B 
 (LEMMA V) 
 .-. an : AM : : ah 2 : ab. AB
 
 118 
 
 A COMMENTARY ON 
 
 [SECT. II. 
 
 or 
 
 f : F 
 
 or 
 
 ah 2 . A B ! 
 
 ab 
 
 _ 
 a s 
 
 ' A B 
 
 V 2 
 A~S 
 
 ah 
 
 a s 
 
 ,T X7X 
 
 (LEMMA V) 
 
 v 
 
 AS' 
 
 And if the whole similar curves A D, a d be divided into an equal 
 number of indefinitely small equal areas A B S, B C S, &c. ; a b s, b c s, 
 &c. these will be similar, and, by composition of ratios, (P and p being 
 the whole times) 
 
 P : p : : time through A B 
 A B ab AS 
 
 time through a b 
 a s 
 
 Hence 
 
 v V 
 
 PCX A S 
 V 
 
 V 2 A S 
 <x a 
 
 AS P 2 
 
 136. COR. 9. Let A C be uniformly described, 
 and with the force considered constant, suppose 
 the body would fall to L in the same time in 
 which it would revolve to C. Then A B being 
 indefinitely small, the force down R B may be 
 considered constant, and we have (131) 
 A C 2 : AB 2 : : T 2 : T 2 
 
 AB AC 
 
 . . TP 2 T 2 
 
 AL 
 
 : : AL 
 
 A T 
 
 RB 
 
 RB (131) 
 
 A R 2 
 
 ' AD 
 
 Hence 
 
 AB 8 = AL X AD. 
 
 PROP. VI. Sagitta oc F when time is given. 
 LEMMA XI, t 2 when F is given 
 .*. when neither force nor time is given 
 
 sag. a F X t * ; 
 
 ' 
 
 Also sag. a (arc) * by
 
 BOOK I.] NEWTON'S PRINCIPIA. 119 
 
 OTHERWISE. 
 
 By LEMMA X, COR. 4, 
 
 space ipso motus initio 
 
 sag. 
 oc ^- 
 t 2 
 
 To generalize this expression, let -^- be the space described in 1" at 
 
 the surface of the Earth by Gravity. Also let the unit of force be Gravi- 
 ty. Then 
 
 t * * 2 x \ f *~ z 
 
 -, 2 sag. 2s , , 
 
 .-. F = -f- = x . (a) 
 
 gt 2 g t 2 
 
 by hypothesis. 
 
 137. COR. 1. Foc^foc- ^ R . 2 
 
 t 2 (area S P Q) 2 
 
 * S P 2 x QT 2 ' 
 
 To generalize this, let a be the area described in 1". Then the area 
 
 .,,.. S P x Q T 
 
 described m t" = a X t = - . 
 
 
 
 SP x QT 
 
 ~ 
 
 and substituting in (a) we get 
 
 8 a 2 QR ,,v 
 
 ~g~ SP x QT 2 ( ' 
 
 Agaui, if the Trajectories turn into themselves, tliere must be 
 a : I" : : A (whole Area) : T (Period. Time) 
 
 A 
 .*. a = r. 
 
 Hence by (b) we have 
 
 v - 1^ v QR / c \ 
 
 " gT 2 SP 2 X QT 2 
 
 which, in practice, is the most convenient expression. 
 
 138.COR.2. F = 
 
 Z ** O V 2 v Cl T> 2 V ' 
 
 o I X W r^ 
 139. Cor.3. F = iA! x QV2 ^ pv (e)
 
 120 A COMMENTARY ON [SECT. Jl. 
 
 Hence is got a differential expression for the force. Since 
 
 PV = 2_PJLJ 
 dp 
 
 F- 8A 'x 
 
 -T rrTZ VS 
 
 g T 2 ' 2p'pdg 
 dp 
 
 ~gT* dg 
 
 Another is the following in terms of the reciprocal of the Radius Vector 
 and the traced-angle 6. 
 Because 
 
 ' 
 
 1 dg 2 + g 2 d<? 2 
 
 p 2 
 
 , 1 
 
 T ~t 
 
 Let 
 Then 
 
 d u 
 
 d g = -- s- 
 u 2 
 
 also 
 
 - = du * 4. u s 
 p* d 2 
 
 2dp 2dud 2 u 
 
 .'. ~ = -r-rz -f- 2 u d u 
 
 p 3 d 6- 
 
 ... ^P - d2U U | U 3 
 
 and substituting in f we have 
 
 140. COR. 4. Fa _^_ x ^ a V 2 X ~ 
 
 V 2 
 at 
 
 PV 
 
 This is generalized thus. Since 
 
 V _ s P ace P Q 
 = Trn^ = ~T~ 
 
 and 
 
 F2
 
 BOOK I.] NEWTON'S PRINCIPIA. 121 
 
 A 
 
 aXt(=-~jXt) = area described 
 
 P Qx S Y 
 
 2 
 PQ _ 2A 1 
 
 t T * SY* 
 
 Hence 
 
 1 T 2 
 
 * - 
 
 SY 2 ~ 4 A 
 
 and by COR. 3. 
 
 o 
 
 F = x 
 
 From this formula we get 
 
 V 2 =-| x F x P V 
 
 ip 
 
 P V 
 = 2gFx . 
 
 But by Mechanics, if s denote the space moved through by a body 
 urged by a constant force F 
 
 V 2 = 2gF x s 
 
 P V 
 
 ............ (i) 
 
 4 
 
 that is, the space through which a body must fall when acted on by the force 
 continued constant to acquire the velocity it has at any point oj the Trajec- 
 tory , is \ of the chord of curvature at that point. 
 Also 
 
 =gFxi . . . . (k, 
 
 The next four propositions are merely examples to the preceding formulas. 
 
 141. PROP. VII. 
 
 R P 2 (= Q R x R L) : Q T 2 : : A V 2 : P V 2 
 
 QR x RL x PV 2 _ T2 
 
 A V 2 w 
 
 S P 2 
 
 and multiplying both sides by ^ p .and putting P V for R L, we have 
 
 y it 
 
 S P 2 x P V 3 __ SP 2 x QT 2 
 
 
 
 A V 2 
 
 A V 2 1 
 
 SP 2 x P V 3 S P 2 X P V 3 * 
 
 Also by (137 c.) 
 
 V - 8A * v AV 2 32*r* 1 
 
 ' gT s SP 2 xPV 3 " g T 2 SP 2 xPV 3
 
 122 A COMMENTARY ON [SECT. II 
 
 OTHERWISE. 
 
 From similar triangles we get 
 
 AV: PV:: SP: SY 
 SP x PV 
 
 .-. S Y = 
 
 AV 
 
 c pz v P V 2 
 
 .-. SY 2 x PV = - A y f - x PV 
 
 S P 2 x PV 3 
 
 AV 
 1 1 
 
 .-. F 
 
 SY'xPV SP'xPV 3 
 
 as before. 
 
 OTHERWISE. 
 
 2 22 
 
 is the equation to the circle ; whence 
 
 
 8r 
 
 
 
 gT 2 ^(r-i 
 
 OTHERWISE. 
 
 The polar equation to the circle is 
 
 2 a cos. 
 
 1 + COS. z 
 1 \ 1 COS. i 
 
 g / ~~ 2 a cos. 6 2 a 
 d u 1 / sin. 6 
 
 * - _ I . _^^. ^_m SID* 
 
 ' d 6 ~~ 2 a \cos. s tf 
 1 sin. 3 6 
 ~ 2 a cos. 2 ^ 
 
 d'u _ _l_ /3 sin. 8 . 2sin 4 <J 
 '*' dT* "" 2~a \ cos. d 
 
 _\ 
 T 8 !/ 
 
 1 sin. * 6 , 
 
 = S X -- 3-, X (3 sin. 8 
 2 a cos. 3 ^ v
 
 BOOK I.] NEWTON'S PRINCIPIA. 123 
 
 sin. 2 6 ,_ ... 1 , cos. 6 
 
 u = n ( 3 sm + o "^ + -5 
 
 2 a cos. 3 tf x 2 a cos. 6 2 a 
 
 -r- X (3 sin 2 6 sin. 4 6+ cos. 2 + cos. 4 
 
 2 a cos. 3 v 
 
 X (2sin. 2 ^ sin. 4 ^+ 1 + 12 ^in. 
 
 2 a cos. 3 6 
 1 
 
 a cos. 
 which by (139) gives 
 
 gT 2 acos. 3 ^ 
 
 +co 
 a 3 co 
 (1 + cos. 2 ^) 
 
 4A 2 (1 +cos. 2 ^) 2 
 ~gT 2> 4 a 3 cos. 5 ^ 
 
 ~ga 3 T 2 cos. 5 
 
 142. COR. 1. F sp2 x py- 3 - 
 
 But in this case 
 
 S P = P V. 
 1 _ 
 
 x > c : 
 
 gT 2 
 COR. 2. F: F:: RP 2 x PT 3 : SP 2 x PV 3 
 
 S R 
 
 SP 3 x P V 3 
 :: SP x RP 2 :- p^f v 
 
 :: SP x R P 2 : SG 3 , 
 by similar triangles. 
 
 This is true when the periodic times are the same. When they are 
 different we have 
 
 T 
 F: F:: SP x RP 2 x SG 3 , 
 
 S K A 
 
 R 
 
 where the notation explains itself. 
 143. PROP. VIII. 
 
 CP 2 : PM 2 :: PR 2 : QT* 
 and 
 
 .-. CP 2 : PM 2 :: QR x 2PM:QT 2 
 
 QT 2 _ 2PM 3 
 *' QR ~ CP 2
 
 124 A COMMENTARY ON [SECT. II. 
 
 and 
 
 QT 8 X SP 2 _2PM 3 X SP g 
 QR CP 2 
 
 CP 2 1 
 
 2PM 3 xSP* PM 3 
 Also by 137, 
 
 _ 
 
 g SP 2 x PM 3 ' 
 But 
 
 S P X velocity S P X V 
 
 ~2~~ ~2~ 
 
 V 2 CP 2 
 
 ' * ~~ ~ * T TV*-* 
 
 g PM 3 
 
 OTHERWISE. 
 
 By PROP. VII, 
 
 F 
 
 SP 2 x P V 3 
 But S P is infinite and P V = 2 P M. 
 
 1 
 
 .*. F 
 
 PM 3 ' 
 
 OTHERWISE. 
 
 The equation to the circle from any point without it is 
 
 c 2 r 2 g 8 
 
 P_ - >_ 
 2r 
 
 where c is the distance of the point from the center, and r the radius, 
 
 d r 
 
 Moreover in this case 
 
 g=c+PM=c+y 
 
 c 2 r 2 c 2 2cy y* 
 ' ' -~ 
 
 r 
 
 ... d P = c + y x f8 
 
 ' p 8 d g r c 3 y 
 
 c s y 8 '
 
 BOOK I.] NEWTON'S PRINCIPIA. 125 
 
 Hence (139) 
 
 4_a-rj 1 _ V'r 2 1 
 
 X. ^ 5 A ~~T X\ ,; 
 
 c g y g y 
 
 SCHOLiyM. 
 
 144. Generally we have 
 
 PR 2 : QT 2 : : PC 2 : P M 2 
 P P 2 O T 2 
 ,.-:T_::PC':PM' 
 
 But 
 
 P R z 
 
 - P V 
 QR " 
 
 and 
 
 PC:PM::2R(R = rad. of curvature) : P V 
 
 QT 2 _ p v PM 2 _2RxPM 
 ' QR : PC 2 : PC 
 
 2R x PM 
 
 PC 
 
 But 
 
 . QT 2 = 2AC 2 x pM3 
 
 and 
 
 1 
 
 * P~W 3 ' 
 
 From the expression (g. 139) we get 
 
 4a 2 d 2 u 
 F = x -3 T X u 2 . 
 
 or UP 
 
 But 
 
 p 2 d 6 d x 
 
 a x t = -~2~ = a x -y- 
 
 4a 2 V 2 e 4 
 
 " d tf " d x 2 ' 
 Also 
 
 ,.d = _^
 
 126 
 
 and 
 
 Hence 
 
 A COMMENTARY ON 
 
 d2u = _^ + ?4L 2 
 
 C S 
 
 = ^J. (see 69) 
 
 V 2 ? 4 d 
 
 F = 7-^ X 
 
 2 
 
 gdx 2 
 
 V 2 
 
 1 
 
 \s 
 
 r 
 
 g dx* 
 
 V 2 d'y 
 
 " g < dx2 
 This is moreover to be obtained at once from (see 48) 
 
 do * 
 t 
 
 For 
 
 [SECT. II. 
 
 (1). 
 
 T, V 2 d 2 y 
 
 K V r 
 
 M. ... x\ , Q 
 
 g dx 2 - 
 
 145. PROP. IX. Another demonstration is the following 
 
 S 
 
 T P 
 
 Let Z.PSQ = /.pSq. Then from the nature of the spiral the 
 angles at P, Q, p, q being all equal, the triangles S P Q, S p q are simi- 
 lar. Also we have the triangles R P Q, r p q similar, as likewise Q P T, 
 qpt. 
 Hence 
 
 QT 2 . qt 2 . . SP . Sl 
 Q"R '^7- K 
 
 and by LEMMA IX. 
 
 q' r : q r : : p r' 2 : p r 2 : : q r t' 2 : q t*
 
 BOOK 1.] 
 
 NEWTON'S PRINCIPIA. 
 
 127 
 
 Hence 
 
 and 
 
 q' t' _ 2 _ qt 2 
 q f r f q r * 
 
 q' f 2 Q T 2 
 
 q 7 ! 7 *' ~QR 
 
 QT 2 
 
 :: S P : S 
 a S P 
 
 ' QR 
 QT 2 x S P 2 
 
 QR 
 
 .-. F 
 
 1 
 
 SP 3 ' 
 
 OTHERWISE. 
 
 The equation to the logarithmic spiral is 
 b 
 
 m dp _b 
 
 " d ~ a 
 
 and by (f. 139) we have 
 F = 
 
 dp 4> a, s b a ; 
 
 X o T ' X X i 5~~ 
 
 g 
 
 4 a 2 , a 2 1 
 
 V 
 
 la n . m 
 
 g-b 
 
 Using the polar equation, viz. 
 
 b 
 
 X log. 
 a 
 
 - V (a 2 b ! 
 
 the force may also be found by the formula (g). 
 146. PROP. X. 
 
 P v x vG : Q v 2 : : P C 2 : CD 2 
 Qv 2 : QT 2 : : P C 2 : P F 2 
 .-. Pv x v G : QT 2 : : P C 4 : CD 2 x P F 2 
 
 CD 2 x PF 2 
 
 .-. v G : 
 
 Q T 2 
 ^ : : PC 
 
 Pv 
 
 PC 
 
 But 
 
 P V = Q R, and C D X P F = (by Conies) B C X C A 
 also 
 
 ult. v G = 2 P C. 
 
 BC 2 X C A 2 
 
 ... 2PC : 
 
 : . PC 
 
 PC 1
 
 128 A COMMENTARY ON [SECT. II. 
 
 - F_9JL_ PC pr 
 
 QT 2 X CP 2 2 BC 2 x C A 2 
 
 Also by expression (c. 137) we get 
 
 ., 8 A* PC 
 
 r FFT; X 
 
 gT 2 2BC 2 x C A 2 
 But 
 
 A = r x B C X C A 
 
 F - x P C 
 
 " gT 
 
 The additional figure represents an Hyberbola. The same reasoning 
 shows that the force, being in the center and repulsive, also in this curve, 
 a CP. 
 
 ALITER. 
 
 Take 
 
 Tu - TV 
 and 
 
 u V : vG : : DC 2 : PC 2 
 Then since 
 
 Qv 2 : Pv x vG : : D C 2 : P C 2 
 
 .-. u V : v G : : Q v 2 : P v x v G 
 .-. Q v 2 = P v x u V 
 .-. Q v 2 + u P x P v = Pvx (u V + uP) 
 
 = P v x V P. 
 But 
 
 Qv 2 = QT 2 + T 2 = QT 2 + Tu 2 
 = PQ 2 _ PT 2 + Tu 2 
 = p Q* (P T~ T u 2 ) 
 = P Q2_Pu x Pv 
 (chord PQ) 2 = Pv x VP. 
 
 Now suppose a circle touching P R in P and passing through Q to 
 cut P G in some point V. Then if Q V be joined we have 
 
 and in the AQ P v, Q V P the L. Q P V is common. They are there- 
 fore similar, and we have 
 
 P v : P Q : : P Q : P V 
 .-. P Q 2 = P v x V / P = Pvx VP 
 .-. V' P = V P 
 or the circle in question passes through V ; 
 
 .-. P V is the chord of curvature passing through C.
 
 BOOK I.] NEWTON'S PRINCIPIA. 129 
 
 Again, since 
 
 ' r> r 2 
 
 u V = v G x ^- 2 = C x v G 
 
 or 
 
 p y P u = C (P G P v) 
 and 
 
 P V, PG 
 
 being homogeneous 
 
 p 2DC 2 p p _ 2CD* 
 
 PC 2 PC 
 
 .-. (Cor. 3, PROP. VI.) 
 
 F<r PC 
 
 2 PF 2 X CD 2 ' 
 
 But since by Conies the parallelogram described about an Ellipse is 
 equal to the rectangle under its principal axes, it is constant. .. P F x 
 CD is. 
 
 and 
 
 F PC. 
 
 OTHERWISE. 
 
 By (f. 139) we have 
 
 - 
 
 4A 2 dp 
 
 - 
 
 But in the ellipse referred to its center 
 
 b 2 >* 
 
 p 2 a 2 b 2 
 
 and differentiating, and dividing by 2, there results 
 
 dp g 
 
 p 3 d g ~~ a 2 b 2 
 which gives 
 
 4A 2 t 4*r 2 
 
 F = ^ X -A-5 = nrTj X g' 
 
 In like manner may the force be found from the polar equation to the 
 ellipse, viz. 
 
 b* 
 
 - 1 _ e 2 cos.V 
 
 by means of substituting in equat. (g. 139.) 
 VOL. I. I
 
 130 A COMMENTARY ON [SECT. II. 
 
 147. COR. 1. For a geometrical proof of this converse, see the Jesuits' 
 notes, or Thorpe's Commentary. An analytical one is the following. 
 
 Let the body at the distance R from the center be projected with the 
 velocity V in a direction whose distance from the center of attraction is P. 
 Also let 
 
 F = (*> g 
 
 n being the force at the distance 1. Then (by f ) 
 4A 2 dp 
 
 F = iTr x p-a- e = " f 
 
 which gives by integration, and reduction 
 
 2 4 * 4 
 
 R and P being corresponding values of % and p. 
 But in the ellipse referred to its center we have 
 2 2 2 
 
 p 2 ' a 2 b 2 a 2 b 2 
 
 which shows that the orbit is also an ellipse with the force tending to Us 
 center, and equating homogeneous quantities, we get 
 jg-f-b 2 _j"gT 
 a s b 2 4A ! 
 
 and 
 
 1 _^gT 2 
 a 2 b 2 ~ 4A 2 
 But 
 
 A = IT a b 
 
 T= 
 
 which gives the value of the periodic time, and also shows it to be con- 
 stant. (See Cor. 2 to this Proposition.) 
 
 Having discovered that the orbit is an ellipse with the force tending to 
 tne center, from the data, we can find the actual orbit by determining its 
 semiaxes a and b. 
 
 By 140, we have 
 
 ^ 2 A 1 
 V v 
 
 T P 
 
 . a* + b _ R 2 J_ 
 
 a 2 b 2 ^gXyzpzH'ps 
 
 and 
 
 a~* IT 5 = ** g X V^P~ S
 
 BOOK L] NEWTON'S PRINCIPIA. 131 
 
 V 2 
 .-. a 2 + b 2 = R 2 + - 
 
 ^g 
 
 and 
 
 2VP 
 
 2 a b = 
 
 V r 
 
 V * 2 V 
 -- + ! 
 (* g r V 
 
 and 
 
 V* 2VP 
 - 
 
 a b= V(R 
 
 f& T \^ AC* &' 
 
 O To 
 
 which, by addition and subtraction, give a and b. 
 
 OTHERWISE 
 
 By formula (g. 139,) we have 
 
 4. A* 
 
 F = 
 
 , /d 2 u \ u, 
 
 u * ( - - + u ) = . P = 
 \d 2 / u 
 
 K P * * . 
 
 u ^rW x = 
 4 A 2 u 3 
 
 rri g 
 
 and multiplying by 2 d u, integrating and putting ^ . 2 - = M, we have 
 
 rl 11 2 T\T 
 
 To determine C, we have 
 
 d u a *_ J^ dj 2 
 
 d 6* ~ g 4 * d d 2 
 and in all curves it is easily found that 
 
 1-* = J-V (p2_ ) 
 d^ p V 
 
 . (lu2 _ g 2 P 2 _ J_ J_ 
 
 *' d tf 2 = g 2 p 2 = p 2 " ? 2 ' 
 
 Hence, when g = R, and p = P, 
 
 p^+MR 2 + C = . . ...... (3) 
 
 which gives the constant C. 
 Again from (2) we get 
 
 u d u 
 
 , 
 = 
 
 ( M Cu* u 4 ) 
 which being integrated (see Hersch's Tables, p. 160. Englished edit, 
 published by Baynes & Son, Paternoster Row) and the constants properly 
 determined will finally give g in terms of 6; whence from the equation to 
 the ellipse will be recognised the orbit and its dimensions. 
 
 12
 
 132 
 
 A COMMENTARY ON 
 
 [SECT. II. 
 
 148. COR. 2. This Cor. has already been demonstrated see (1). 
 Newton's Proof may thus be rendered a little easier. 
 By Cor. 3 and 8 of Prop. IV, in similar ellipses 
 
 T is constant. 
 
 Again for Ellipses having the same axis-major', we have 
 
 cab b 
 
 But since the forces are the same at the principal vertexes, the sagittse 
 are equal, and ultimately the arcs, which measure the velocities, are equal 
 to the ordinates, and these are as the axes-minores. Hence, a (which 
 vx SY> 
 
 b. 
 
 T oc -j- a. 1 or is constant. 
 
 Again, generally if A and B be any two ellipses whatever, and C a third 
 one similar to A, and having the same axis-major as B ; then, by what 
 has just been shown, 
 
 T in B = T in C 
 and 
 
 T in C = T in A 
 
 .-. T in B = T in A. 
 
 149. SCHOL. See the Jesuits' Notes. Also take this proof of, " If one 
 curve be related to another on the same axis bj having its ordinates in a 
 given ratio, and inclined at a given angle, the forces by which bodies are 
 made to describe these curves in the same time about the same center in 
 the axis are, in corresponding points, as the distances from the center." 
 
 S R 
 
 The construction being intelligible from the figure, we have 
 
 PN: QN: : p O : qO 
 .-. PN: pO : : QN q O 
 
 : : N T : O T ultimately.
 
 BOOK I.] 
 
 NEWTON'S PIUNCIPIA. 
 
 133 
 
 .. Tangents meet in T, 
 
 the triangles C P T, C Q T are in the ratio of P N : Q h or of parallelo- 
 grams P N O p, Q N O q ultimately, i. e. in the given ratio, and 
 C p P : C P T : : p P : P T ultimately. 
 : : NO: NT 
 : : qQ:QT 
 
 : :CQq: CQT 
 .. C p P : C q Q in a given ratio. 
 
 .*. bodies describing equal areas in equal times, are in corresponding 
 points at the same times. 
 
 .'. P p, Q q are described in the same time, and m p and k q are as the 
 forces. 
 
 Draw C R, C S parallel to P T, Q T; then 
 
 n O : 1 O 
 
 
 po 
 
 :qO 
 
 : PN: 
 
 QN: 
 
 
 .-. nO 
 
 : P 
 
 : 1O : 
 
 qO 
 
 and 
 
 
 
 
 
 
 n p 
 
 : nO 
 
 : lq : 
 
 1Q1 
 
 but 
 
 
 
 
 > 
 
 
 nO 
 
 : n R 
 
 : 1Q : 
 
 1 S j 
 
 
 (since n O 
 
 : OR 
 
 TO: 
 
 OC: 
 
 
 .-. n p 
 
 : n R 
 
 1 q : 
 
 1 S 
 
 
 .MI p 
 
 : p R 
 
 1 q : 
 
 qS 
 
 and 
 
 
 
 
 
 
 n p 
 
 i q 
 
 : p R 
 
 : q S 
 
 m p : 
 k q : 
 
 pen 
 
 qCf 
 
 
 .\ mp 
 
 : P C 
 
 k q : 
 
 f^ 
 
 or 
 
 
 
 
 
 
 Fatp 
 
 : Fatq 
 
 P C: 
 
 qC. 
 
 1 O : O S) 
 
 Q. e. d. 
 
 SECTION III. 
 
 150. PROP. XI. This proposition we shall simplify by arranging the pro- 
 portions one under another as follows 
 
 But 
 
 LxQR( = 
 
 LxPv 
 
 GvxPv 
 
 Q v 2 
 
 Qx 2 
 
 : L xPv 
 
 : 
 
 : P E 
 
 PC 
 
 
 
 AC 
 
 PC 
 
 GvxPv. 
 
 
 L 
 
 G v 
 
 :Qv 2 
 
 
 PC 2 
 
 CD 
 
 :Qx 2 
 
 
 1 
 
 1 
 
 :QT 2 
 
 
 PE 2 
 
 PF 2 
 
 
 
 CA 2 
 
 PF E 
 
 
 
 CD 2 
 
 C B 2 
 
 I 3
 
 134 A COMMENTARY ON [SECT. III. 
 
 .-.Lx QR: QT 8 : : ACxLxPC 2 xCD 8 : PCxGvxCD 2 xCB 8 
 and 
 
 QR ACx PC A C x PC AC 
 
 Q~T* ~ G v x C B 2 ~ 2 PC x C B 2 ~ 2 C B 2 
 
 QR / AC x 1 
 
 K QT 2 x SP*v- 2 CB 8 x S P 2 ) SP 2 ' 
 
 Q. e. d. 
 Hence, by expression (c) Art. 137, we have 
 
 8JL* AC 
 
 JT ^ m o 
 
 u T 2 2 C B 2 x S P 2 
 
 8r 2 a 8 b 2 a 
 
 o X 
 
 gT 2 2b 2 x g 2 
 
 2 a s 1 
 - X - 
 
 O S 
 
 where the elements a and T are determinable by observation. 
 
 OTHERWISE. 
 
 A general expression for the force (g. 139) is 
 
 T, 4 A 2 /d 2 u , \ 
 F = T -g x u 2 (, 2 - -f- uj 
 
 But the equation to the Ellipse gives 
 
 1 1 4- e cos. 6 
 u = - = ' g v- 
 
 where a is the semi-axis major and a e the eccentricity, 
 d u e sin. 6 
 
 and 
 
 d 2 u e cos. & 
 
 d 2 u 1 
 
 *d<? 2 " = a (l e 2 ) 
 and 
 
 ~ gT 2 a(l e 2 )' 
 But 
 
 A 2 = ff 2 a z b 2 = * 2 a 2 (a 2 a 4 e 8 ) 
 
 the same as before.
 
 BOOK L] NEWTON'S PRINCIPIA. 135 
 
 OTHERWISE. 
 
 Another expression is (k. 140) 
 
 T7 4A 2 dp 
 
 k v - = 
 
 Another equation to the Ellipse is also 
 
 J^_2a g 2 a J_ 
 p" 2 ~~ b 2 e ~ b" 2 "? ~~ b 1 
 
 JL 99 
 
 d p a 
 
 X 5 9 
 
 F - 4 A2 _ a __ 
 
 ' * ~~~ rri o X l^ o o 
 
 4^ 2 a 2 b 2 a 4w 2 a 3 
 
 151. PROP. XII. The same order of the proportions, which are also let- 
 tered in the same manner, as in the case of the ellipse is preserved here. 
 Moreover the equations to the Hyperbola are 
 
 
 1 + e cos. 6 
 
 and 
 
 S + % a 
 
 which will give the same values of F as before excepting that it becomes 
 negative and thereby indicates the force to be repulsive. 
 152. PROP. XIII. By Conies 
 
 4SP.Pv = Qv 2 = Qx 2 ultimately. 
 But 
 
 .-. 4SP.Q R : Qx 2 :: 1 : 1 
 and 
 
 Qx 2 : QT 2 : : SP 2 : S N 2 
 
 :: S P : S A 
 .-. 4SP.'QR : QT 2 ::SP : S A 
 
 QR 1 1_ 
 
 '* QT 2 ~ 4 S A ~ L 
 L being the latus rectum. 
 
 . F a __QA__ ex _L_ 
 
 QT 2 x S P 2 SP 2 ' 
 
 or 
 
 F = x i < b ' 137 >
 
 13G A COMMENTARY ON [SECT. III. 
 
 8a 2 1 2 P 2 V 2 1 
 = g~L X S P 2 01 gL K ST* 
 
 a being the area described by the radius-vector in a second, or P the per- 
 pendicular upon the tangent and V the corresponding velocity. 
 
 OTHERWISE. 
 
 In the parabola we have 
 
 12 22 
 
 U =- = -= (1 + COS. d) = -rf + -= COS. 6 
 
 and 
 
 J_ 4 1 
 
 o " "Y~~ ^ """ 
 
 P 8 L g 
 which give 
 
 d 2 u ^ 
 
 d 2 U ~ X 
 
 dp 21 
 
 r V 
 
 p 3 d f L g 8 
 and these giye, when substituted in 
 
 >2 V 2 /d 2 u 
 
 r * v z 2 /a - u x 
 
 F = . u 2 ( T-JV -f u ) 
 
 or \d ^ 2 / 
 
 or 
 
 P a V g dp 
 
 g V d e 
 
 the same result, viz. 
 
 2 p 2 V 2 1 
 
 _ 
 
 Newton observes that the two latter propositions may easily be deduced 
 from PROP. XL 
 
 In that we have found (Art. 150) 
 
 a 
 
 x 
 
 g T 2 B" 
 P z V 2 
 
 9 
 
 g >V 
 
 Now when the section becomes an Hyperbola the force must be repul- 
 sive the trajectory being convex towards the force, and the expression re- 
 mains the same.
 
 BOOK L] NEWTON'S PRINCIPIA. 137 
 
 Again by the property of the ellipse 
 
 K* _ v 
 
 " A a ?s a : - 
 
 4 4 
 
 which gives 
 
 a 2 I 
 
 .b 2 " L ~~4 a 
 and if c be the eccentricity 
 
 b 2 = a 2 c 2 = (a + c) X (a c) 
 
 _ 
 
 ' (a + c) X (a c) " L~~ 4~a* 
 Now when the ellipse becomes a parabola a and c are infinite, a c is 
 
 Jinite, and a + c is of the same order of infinites as a. Consequently r-j 
 
 +J 
 
 isjinite, and equating like quantities, we have 
 
 a. __2 
 
 b 2 ~ L' 
 which being substituted above gives 
 
 2P Z V 2 1 
 
 the same as before. 
 
 Again, let the Ellipse merge into a circle ; then b = a and 
 P 2 V 8 a 
 
 F = 
 
 a V 2 1 
 
 X . 
 
 g <? 2 
 
 V s 
 
 g X a 
 
 153. PROP. XIII. COR. I. For the focus, point of 'contact, and position of 
 the tangent being given, a conic section can be described having at that point 
 a given curvature. .] 
 
 For a geometrical construction see Jesuits' note, No. 268. 
 
 The elements of the Conic Section may also be thus found. 
 
 The expression for R in Art. 75 may easily be transformed to 
 
 p 
 
 for 
 
 p 2
 
 138 A COMMENTARY ON [SECT. III. 
 
 Now the general equation to conic sections being 
 
 s = r x 
 
 a 1 + e cos. 6 
 the denominator of the value of R is easily found to be 
 
 a s 
 
 which gives 
 
 R = - x *4 
 
 a p 3 
 Hence 
 
 b 2 p 3 
 - = Z- , x R 
 
 a s 3 
 
 is known. 
 
 Again, by the equation to conic sections we have 
 
 which, by aid of the above, gives 
 
 a = 2^ 
 
 And 
 
 Whence the construction is easy. 
 
 154. The Curvature is given from the Centripetal Force and Velocity being 
 given.'] 
 
 If the circle of curvature be described passing through P, Q, V, and O 
 (P V being the chord of curvature passing through the center of force, 
 and P O the diameter of curvature) ; then from the similar triangles 
 P Q R, P V Q, we get 
 
 P Q 2 
 
 " 
 
 Also from the triangles P Q T and P S Y (S Y being the perpendicu- 
 lar upon the tangent) we have 
 
 pQ _SPxQT 
 SY 
 
 and from P S Y, P V O, 
 
 2 Rx SY 
 
 P V = 
 
 SP
 
 BOOK J.l NEWTON'S PRINCIPIA. 139 
 
 whence by substitution, &c. 
 
 Q R SP 
 
 QT 2 xSP 2 ~2R x SY 2 
 
 _ 2P 2 V 2 QR V 2 xSP 
 
 g K QT 2 x SP 2 ~ llx S Y 
 which gives 
 
 SP V 2 
 
 ~g7sY x T~' 
 
 Hence, S P, S Y and g being given quantities, R is also given if V and 
 F are. 
 
 155. Two orbits which touch one another and have the same centripetal 
 Jbi'ce and velocity cannot be described.'] 
 
 This is clear from the " Principle of sufficient Reason." For it is a 
 truth axiomatic that any number of causes acting simultaneously under 
 given circumstances, viz. the absolute force, law of force, velocity, direc- 
 tion, and distance, can produce but one effect. In the present case that 
 one effect is the motion of the body in some one of the Conic Sections. 
 
 OTHERWISE. 
 
 Let the given law of force be denoted generally by f & where f means 
 any function; then (139) 
 
 F _PV dp 
 
 f ~ x\ 5 "5 
 
 g P d e 
 
 and since P and V are given 
 
 g pg 
 
 But if A be the value of F at the given distance (r) from the center to 
 the point of contact ; then 
 
 F : A : : fg : f r 
 and 
 
 F: A::fe':fr 
 
 and
 
 140 A COMMENTARY ON [SECT. III. 
 
 Hence 
 
 P* V 2 d p A . 
 
 * ' 5 1 ^~ t*~^~ ^* ^ C 
 
 g p 3 dg tr 
 
 and 
 
 P 2 V* dp' A , , 
 
 - * T e 
 
 g p' 3 dg' " fr X 
 
 and integrating, we have 
 
 P 2 V 2 fr 
 2gA 
 
 and 
 
 P 2 V 2 fr 
 
 2gA 
 
 f and yd g' f g' are evidently the same functions of g and ^, 
 which therefore assume 
 
 <p g and <f> %' ; 
 
 and adding the constant by referring to the point of contact of the two 
 orbits, and putting 
 
 we get 
 
 M x 
 
 - L j 
 "p 2 " M "" P 2 M 
 
 | m r J ff; p 
 
 jT 2 =: "M Hh P~ 2 ~M 
 
 in which equations the constants being the same, and those with which 
 g and g' are also involved, the curves which are thence descriptible are 
 identical. Q. e. d. 
 
 These explanations are sufficient to clear up the converse proposition 
 contained in this corollary. 
 
 156. It may be demonstrated generally and at once as follows : 
 By the question 
 
 1
 
 BOOK I.] NEWTON'S PRINCIPIA. 141 
 
 then 
 
 and 
 
 and substituting in (d) we have 
 
 1 -- ^_ + - 1 + J- 
 
 p 2 - r M 1 p 2 1 Mg - 
 
 But the general equation to Conic Sections is 
 
 _L 2 a I 
 
 p 2 ~ b 2 g + b 2 * 
 
 Whence the orbit is a Conic Section whose axes are determinable from 
 
 - 2 g Ar2 
 b 2 ~ M = P 2 V 2 
 
 and 
 
 _L i i 
 
 *~b 2= ~ r M " F 2 
 
 p2 p 2 y 2" ' 
 
 and the section is an Ellipse, Parabola or Hyperbola according as 
 V 2 is >, or = or < 2 g A r. 
 
 Before this subject is quitted it may not be amiss by these forms also to 
 demonstrate the converse of PROP. X, or Cor. 1, PROP. X. 
 Here 
 
 f g r= P 
 f r = r 
 
 Whence 
 
 1 
 
 p 2 ~2M n "P 2 2M* 
 But in the Conic Sections referred to the center, we Tiave 
 
 I x 
 p 2 a 2 b 2 r a 2 b 2 
 
 which shows the orbit to be an Ellipse or Hyperbola and its axes may be 
 found as before.
 
 142 
 
 A COMMENTARY ON 
 
 [SECT. III. 
 
 In the case of the Ellipse take the following geometrical solution and 
 construction 
 
 D 
 
 C, the center of force and distance C P are given. The body is projected 
 
 at P with the given velocity V. Hence P V is given, (for V 2 = -j| F . P V.) 
 
 
 
 Also the position of the tangent is given, .'. position of D C is given, and 
 
 2 C D 2 
 P V =r p _-. Hence C D is given in magnitude. Draw P F per- 
 
 pendicular to C D. Produce and take P f = CD. Join C f and bisect 
 in g. Join P g, and take g C, g f, g p, g q, all equal. Draw C p, C q. 
 These are the positions of the major and minor axis. Also \ major axis 
 = P q, minor axis = P p. 
 
 For from g describe a circle through C, f, p, q, and since C F f is a 
 right z_, it will pass through F. 
 
 .-. Pp.Pq=PF.Pf=PF.CD 
 Also 
 
 PC 2 +Pf 2 = Pg 2 +gC 2 + Pg 2 -|-gf 2 , (since base of A bisected ing) 
 or 
 
 = Pq 2 -2Pg.gq+Pp 2 + 2Pg.gp 
 = Pq a + Pp 3 
 
 /. Pp.Pq=PF.CD 1 But a and b are determined by the same 
 Pp 2 + Pq 2 = PC 2 + CD 2 / equations. .'. Pq = a, P p = b. 
 
 Also since p and F are right angles, the circle on x y will pass through p 
 and F, and ^Ppx = Cpq= CFq = xFp, because ^xFC = pFq. 
 .-. L. Pp x = z/m alternate segment. .-. P p is tangent. 
 
 Pp z = PF.Px .-. PF.Px = b. 
 
 But if in the Ellipse C x be the major axis, P F . P x = b 2 .
 
 BOOK I.] NEWTON'S PRINC1PIA. 143 
 
 .. C x is the major axis, and .-. C q is the minor axis. 
 .*. the Ellipse is constructed. 
 
 PROP. XIII, COR. 2. See Jesuits' note. The case of the body's 
 descent in a straight line to the center is here omitted by Newton, be- 
 cause it is possible in most laws of force, and is moreover reserved for a 
 full discussion in Section VII. 
 
 The value of the force is however easily obtained from 140. 
 
 157. PROP. XIV. L = 5LT! a 3J* 
 
 y tt r 
 
 a QT 2 X S P 8 by hypoth. 
 
 OTHERWISE. 
 
 By Art. 150, 
 
 F _4_A_ 8 a 8 A 2 1 
 
 ~gT 2X b 2 2 " LgT 2 * p 
 for the circle, ellipse, and hyperbola, and by 152. 
 
 2 
 
 x 
 
 2 
 
 ^g 
 
 for the parabola. 
 
 Now if /i be the value of F at distance 1, we have ^ 
 
 u, 
 
 Whence in the former case 
 
 8 A 2 2 P 2 X V 2 
 
 gLT 2 -^' -jir 
 
 and in the latter 
 
 2P 2 X V 2 
 
 ;rr = * . (b) 
 
 But 
 
 SP 2 x QT 2 
 
 : I 2 : A 2 : T 2 
 
 4 
 
 Aj S P 2 x QT 2 P^x V^ 2 
 '* T 2 = 4 ~4~~ 
 
 .-. SP 2 5 
 
 2 
 158. PROP. XIV. COR. I. By the form (a) we have 
 
 A (= * a b) = J 1 ^ X V L x 1. 
 T V L.
 
 A COMMENTARY ON [SECT. III. 
 
 159. PROP. XV. From the preceding Art. 
 
 T : 
 
 But in the ellipse 
 
 T 8 *rab 
 
 = ~' e VL * 
 
 _ 2 b 
 
 (e) 
 
 160. PROP. XVI. For explanations of the text see Jesuits' notes. 
 
 OTHERWISE. 
 
 By Art. 157 we get 
 
 for the circle, ellipse, hyperbola, and parabola. 
 
 But in the circle, L = 2 P. 
 
 1 1 
 
 ^f 
 r being its radius. 
 
 In the ellipse and hyperbola 
 L = 2_b_' 
 a 
 
 .-. V = V~i^X JL X TJT (h) 
 
 Va P 
 
 161. PROP. XVI, COR. I. By 157, 
 L = X P 2 X V 2 . 
 
 162. COR. 2. V = /<y-X 
 
 V 2 u 
 
 D being the max. or min. distance. 
 
 163. COR. 3. By Art. 160, and the preceding one, 
 
 / ^j f 
 
 v . v . . / ^~ 
 
 : : V L : V 2 D. 
 164. COR. 4. By Art. 160,
 
 BOOK IJ NEWTON'S PRINCIPIA. 145 
 
 But 
 
 2 b 2 
 L = , P = b, and r = a 
 
 ' b V a ' Va 
 
 165. COR. 6. By the equations to the parabola, ellipse, and hyper- 
 bola, viz. 
 
 T> o ^ b * 
 
 the Cor. is manifest 
 
 166. COR. 7. By Art. 160 we have 
 
 1 L 1 
 
 *T * v f * _ _ 
 
 ' 2 ' P 2 ' r 
 which by aid of the above equations to the curves proves the Cor. 
 
 OTHERWISE. 
 
 By Art. 140 generally for all curves 
 
 P V 
 
 v 2 = gFx O. 
 
 But generally 
 
 dp 
 and in the circle 
 
 P V = 2 g (rad. = s ) 
 
 ...v:v':: E_ : d _P. 
 
 S d g 
 An analogy which will give the comparison between v and v' for any 
 
 curve whose equation is given. 
 167. COR. 9. By Cor. 8, 
 
 .and 
 ex equo 
 
 VOL. I. K
 
 146 A COMMENTARY ON [SECT. III. 
 
 168. PROP. XVII. The " absolute quantity of the force" must be 
 known, viz. the value of /*, or else the actual value of V in the assumed 
 orbit will not be determinable ; i. e. 
 
 L: L': : P 2 V 2 : F 2 V' 2 
 
 will not give L'. 
 
 It must be observed that it has already been shown (Cor. 1, Prop. 
 X III) that the orbit is a conic section. 
 
 See Jesuits' notes, and also Art. 153 of this Commentary. 
 
 169. PROP. XVII, COR. 3. The two motions being compounded, the 
 position of the tangent to the new orbit will thence be given and therefore 
 die perpendicular upon it from the center. Also the new velocity. 
 Whence, as in Prop. XVII, the new orbit may be constructed. 
 
 OTHERWISE. 
 
 Let the velocity be augmented by the impulse m times. 
 
 Now, if p be the force at the distance 1, and P and V the perpendicu- 
 lar and velocity at distance (R) of projection, by 156 the general equation 
 to the new orbit is such that its semi-axes are 
 
 R R 
 
 a = ^ ., or = 
 
 2 m 2 - m 2 2 
 
 and 
 
 1 2 _ _i" A m 2 P 
 
 according as the orbit is an ellipse or hyperbola. Moreover it also 
 thence appears that when m 2 = 2, the orbit is a parabola, and that the 
 equations corresponding to these cases are 
 
 X 
 
 2 m 2 
 or 
 
 or
 
 BOOK I.] 
 
 NEWTON'S PRINCIPIA. 
 
 147 
 
 DEDUCTIONS AND ADDITIONS 
 
 TO 
 
 SECTIONS II AND III. 
 
 170. In the parabola the force acting in lines parallel to the axis, required F 
 
 4SP.QR:QT 2 ::Qv 2 :QT 2 ::YE 2 :YA 2 ::SE:SA::SP:SA 
 
 Q R_ 1 
 
 ' QT S 
 
 4 S A 
 
 , and S' P is constant, .'. F is constant. 
 
 S' 
 
 Let u be the velocity icsolved parallel to P M then since the force acts 
 perpendicular to P M, u at any point must be same as at A. .*. if P Q be 
 
 S' P O T 
 
 the velocity in the curve, Q T = u = constant quantity, and a = ~- 
 
 S'P.u 
 
 8a 2 .Q R 
 
 2u 
 
 - gS'P 2 .QT 2 - g 
 which avoids the consideration of S P being infinite ; and 
 
 (see 157; 
 
 .'. body must fall through to acquire the velocity at vertex, which agrees 
 
 TD 
 
 / /" S P 
 
 with Mechanics. (At any point V = u / Q-T-.) 
 
 171. In the cycloid required the force when acting parallel to the axis.
 
 148 
 
 A COMMENTARY ON 
 
 [SECT. III. 
 
 RP 2 : QT 2 :: ZP 1 : ZT 8 :: V F 2 : E F 8 : : VB: BE 
 
 and since the chord of curvature (C. c) = 4 P M, R P 2 = 4 P M. R Q, 
 .-. 4 P M. R Q : Q T 2 : : V B : (B E =) P M 
 
 * QT 1 ~ 4 
 
 2 ' 
 
 
 
 PM 
 
 (since S P constant) 
 
 8a 2 .QR u 8 .VB . 
 
 = g.S P*.QT 2 = 2. P M 2> u = velocit ^ P arallel to 
 
 QT* 2g, 
 
 (At any point v = u . 
 
 172. In the cycloid the force is parallel to the base 
 
 Z 
 
 \JvR 
 
 'NX QK 
 
 MU - ^i-i- 
 
 R P 2 : QT 4 :: Z P 1 : ZT 8 :: V E 2 : V M 2 : : VB: VM 
 and since C . c = 4 E M 
 
 R P 2 = 4E M.R Q, 
 .-. 4 EM. RQ: QT 2 :: VB: V M, 
 QR VB 1 
 
 " 4 E M. V M a EM. V M ' 
 
 2 ' v ^v 
 
 -) 
 
 If V M = y, F = 
 
 gy 
 
 u = velocity parallel to V B.
 
 BOOK I.] NEWTON'S PRINCIPIA. 
 
 8a 2 Q R 2u 2 .QR _Ji'jJLlL. \ 
 
 = g. S P 8 Q T 2 = gTQ T~ = 2~^7EM~V~M') 
 
 (At any point v = u . / 
 
 149 
 
 173. Find F in a parabola tending to the vertex. 
 
 TAN 
 T P : P N : : T A : A E 
 
 or 
 
 V 4 x z + y z : y : : x : 
 
 4x 2 + y 
 
 = P, (A E), 
 
 p ax ax 
 
 2 d p __ 4dx.ax 2 2axdx(4x 
 p a 2 x 4 
 
 4x 2 + 2ax, 2 
 
 ax 4 
 
 d p _ 2 x + a 
 p 3 ax 3 
 
 . d x, 
 
 .dx. 
 
 Also 
 
 g = Vx 
 
 y 2 , 
 
 a d x 
 _xdx-fydy_ x d x + 2 
 
 V x 2 -f- y 
 
 v' x 
 
 a x 
 
 dp 2 x -f- a 2 Vx 2 +ax_2v / x 2 4-ax 
 'p s dp~ ax 3 2x-J-a ax 3 
 
 . F AP 
 
 ' ^ . 
 
 K3
 
 150 A COMMENTARY ON [SECT. III. 
 
 174. Geometrically. Let P Q O be the circle of curvature, 
 
 but 
 
 but 
 
 \ 
 
 P v (C. c through the vertex of the parabola) = -r- 1 ^ 
 
 PQ 2 _ P_O . A _z 
 ' QR : AP 
 
 PQ 2 _ AJP 2 
 Q T 2 " A z " 
 QJl AP 3 
 
 *' QT 8 =: PO.Az 3 
 
 8 a 2 .Q R 8 a 8 . A P 
 
 = g.A P'.QT 2 " g.PO. A z 3 
 
 O PS Q V3 A -pS 
 
 PO.Az 3 = 2 A S.~~,- 3 . Q p 3 = 2 A S.AN 3 
 
 ox O ir 
 
 4a. AP 
 
 = g.A S.A N 3 ' 
 
 175. If the centripetal be changed into a repelling force, and the body 
 revolve in the opposite hyperbola, F a p 2
 
 BOOK I.] 
 
 NEWTON'S PRINCIPIA. 
 
 151 
 
 The body is projected in direction P R ; R Q is the deflection from the 
 Tangent due to repelling force H P, find the force. 
 
 L.Px 
 
 : L . P v 
 
 
 P x : P v : : 
 
 PE : P 
 
 C : : A C : P C 
 
 L.P v 
 
 : P v . v G 
 
 
 L : 2 P C 
 
 
 
 P.v.v G 
 
 : Q v* 
 
 
 PC 2 : CD 2 
 
 
 
 Qv 2 
 
 : Q x 2 
 
 
 1 : 1 
 
 
 
 Q x 2 
 
 : Q T s 
 
 
 PE 2 : PF 2 : 
 
 : AC 2 : 
 
 PF 2 : : CD 2 : BC 3 
 
 .-. L . P 
 
 x: QT 2 
 
 
 A C.L.PC 2 
 
 .CD 2 : 
 
 2 P C 2 .C D 2 .B C 2 
 
 
 
 
 2 B C 2 
 
 
 
 
 
 
 1 
 
 I 
 
 
 
 . . 
 
 1 
 
 A C 
 
 1 
 
 
 
 . T 
 
 
 QT 2 
 
 
 
 
 * -L 
 
 4 
 
 " Q R 
 
 
 
 
 T 
 
 * 
 
 8a 2 .Q R 
 
 
 8 a 2 1 
 
 g. H P 2 .Q T 2 ~ g.L.H P 20C HP 2 ' 
 
 L, S P 2 
 176. In any Conic Section the chord of curvature = ^"v 2 " 
 
 for 
 
 Q JP * _ QT 2 .S P 2 L.SP 2 
 : "QTR " QR.S Y 2 S Y 2 
 
 L S P 
 177. Radius of curvature = - 3 >' 
 
 for 
 
 P W = 
 
 PV.SP L.SP 3 
 
 S Y S Y 3 
 
 8 a 2 
 178. Hence in any curve F = cr^r* 
 
 * S . b 1 . 
 
 8 a 1 
 
 4a 2 .SP 
 
 
 
 g. S Y*.2B.SY" 
 SP 
 
 K4 
 
 see Art. 74.
 
 152 A COMMENTARY ON [SECT. Ill 
 
 179. Hence in Conic Sections 
 
 8a* 8 a* 8 a 2 1 
 
 = ~ 
 
 T SY'.PV - g.SY'.L.SP 1 - J7E7S~F 2 S 
 
 S Y 2 
 
 L . S P 2 
 
 180. If the chord of curvature be proved = ' v 2 - independently of 
 
 Q T s 
 
 he proof that -^ ^ = L, this general proof of the variation of force in 
 y Jrt 
 
 tonic sections might supersede Newton's ; otherwise not. 
 
 181. A body attached to a string, whose length = b, is whirled round so as 
 to describe a circle "whose center is the fixed extremity of the string parallel 
 to the horizon in T"; required the ratio of the tension to the weight. 
 
 Gravity =!,.. v of the revolving body = V g F b, if b be the length of 
 
 the string ; 
 
 V 2 
 .-. F ( = centripetal force = tension) = r- (131) 
 
 and 
 
 ~, _ circumference 2 * b V b 
 
 ~V~ = V g F b = ''Vp 1 
 
 F = g24b 
 - g T 2 ^ 
 
 .-. F : Gravity : : Tp g '. 1, or Tension : weight : : 4 * 2 b : g T*. 
 
 If Tension = 3 weight ; required T. 
 
 4-r 2 b: gT z : : 3 : 1, 
 T2 _ 4r*b 
 
 3g" 
 
 If T be given, and the tension = 3 weight, required the length of the string. 
 
 1 "~~ 4 9 ' 
 
 4 <!t 
 
 1 82. If a body suspended by a string from 
 any point describe a circle, the string describes 
 a cone ; required the time of one revolution or 
 of one oscillation. 
 
 Let A C = 1, B C = b, 
 
 The body is kept at rest by 3 forces, gra- 
 vity in the direction of A B, tension in the 
 direction C A, and the centripetal force in 
 the direction C B.
 
 BOOK I.] 
 
 As before, centripetal force = 
 
 NEWTON'S PRINCIPIA. 
 
 4*r 2 b 
 
 153 
 
 o 
 
 A 21 
 
 and centripetal force : gravity : b : V 1 * b 2 , (from A) : : 7^ ' 1 
 
 gT a 
 
 . T 2 = 
 
 4 ff 
 
 I 8 b 1 
 
 g 
 
 , ^ { 2 |j 2 ^ 
 
 .. T = 2 TT / = a constant quantity if V 1 2 b 2 
 
 be given. 
 
 .-. the time of oscillation is the same for all conical pendulums having a 
 common altitude. 
 
 183. v in the Ellipse at the perihelion : v in the circle e. d. : : n : 1, Jind 
 the major axis, excentricity, and compare its T with that in the circle, and 
 Jind the limits ofn. 
 
 Let S A = c, 
 v in the Ellipse : that in the circle e. d. : : V H P : V A C 
 
 : : V H A : V A C in this case 
 : : n : 1 by supposition, 
 
 .-. 2 AC AS = n s AC, 
 .-. A C = 
 
 Excentricity = A C A S = ^-__ n 2 
 T : Tin the circle : : A C^ : A S * 
 
 2 n 2 ' 
 c 
 
 c = 
 
 c n 
 
 2 n ! 
 
 c 2 : : 1 : (2 n 8 ) 
 
 Also n must be < V 2, 
 for if n = V 2, the orbit is a parabola 
 if n > V 2, the orbit is an hyperbola. 
 
 184. Suppose ^ of the quantity of 
 matter qfto be taken away. How 
 much would T of D be increased, and 
 what the excentricity of her new orbit ? 
 the D 's present orbit being considered 
 circular. 
 
 At any point A her direction is 
 perpendicular to S A, 
 
 .'. if the force be altered at any 
 point A, her v in the new orbit will
 
 154 A COMMENTARY ON [SECT. III. 
 
 = her v in the circle, since v = y , and S Y = S A, and a is the 
 same at A. 
 
 LetAS = c,PVatA = L, and F = jr^ p^y 
 
 in this case, 
 
 2 b 2 
 .'. 3 : 4 : : 2 c ( = L in the circle) : - ( = L in the ellipse) 
 
 _ 3 b 8 _ 3 (a e a^c 2 ) __ 3(2ac c 2 ) __ 3_c 2 
 
 t T* C i ~ ^ ~ v C- ' ---"-' - 
 
 a a a a 
 
 3c 2 
 
 a =2C 
 3c 
 
 1 
 
 c ^ /3 c\ 
 And T in the circle : T' in the ellipse : : ? T ' [-=- ) 
 
 j /3_x 
 
 : V 4 : \2/ 
 
 V 3 
 
 1 3 
 : V 2 : 2 
 
 :: V 2 : 3. 
 
 3 c 
 
 And the excentricity = a c = - c c= . 
 
 6 
 
 185. What quantity must be destroyed that T>'s T 7/zflj/ ie doubled, and 
 what the excentricity of her new orbit ? 
 Let F of :y*(new force) : : n : 1 
 
 .'. v = jj s. F . p v, and v is given, 
 
 . 2b 2 a 2 a c 2ac c 2 
 
 .. n : 1 : : : 2 c : : : c : : - - : c : : 2 a c : a 
 
 a a a 
 
 .*. n a = 2 a c, 
 
 c 
 
 = 2~^V 
 Also T in the circle : T in the ellipse : : 1 : 2 
 
 cl 
 
 :: (2 n)* : n* 
 .-. 1 : 4 : : (2 n) 3 : n .'. n = 4 (2 n) 3 , whence n.
 
 BOOK L] NEWTON'S PRINCIPIA. 
 
 And the excentricity 
 
 155 
 
 = a c r= 
 
 c (2c nc) _ c (n 1) 
 2 n~ 2 n 2 n 
 
 186. What quantity must be destroyed that 5 's orbit may become a 
 
 parabola ? 
 
 L = 4c, 
 
 .-. F : / : : 4 c : 2 c : : 2 : 1, 
 .*. $ the force must be destroyed. 
 
 187. F c =r-i' a body is projected at given D, v = v in the circle, 
 
 L. with S B = 45, find axis major, excentricity, and T. 
 Since v = v in the circle, .. the body is projected from B, 
 
 and z. S B Y= 45 ; 
 .-. z. S B C, or B S C = 45, 
 
 S B 
 
 .-. S C = S B. cos. 45 = 
 
 But 
 
 V 2' 
 
 -T. axis major 
 & J3 = JJ = ^ , 
 
 .'. axis major and excentricity are found. 
 And T may be found from Art. 159. 
 
 Y 
 
 P 
 
 P , 
 
 188. Prove that the angular v round H : that round S : : S P : H.P. 
 This is called Seth Ward's Hypothesis. 
 
 In the ellipse. Let P m, p n, be perpendicular to S p, H P, 
 .*. p in = Increment of S P = Decrement of H P = P n 
 .*. triangles P m p, P n p, are equal, 
 
 .*. P m = p n, and angular v ,. 
 
 189. Similarly in the hyperbola. 
 
 Angular v of S P : angular vofSY::PV:2SP 
 
 2CD' 
 AC 
 C D 2 
 
 :2SP 
 
 : AC 
 
 SP 
 HP : A C.
 
 156 A COMMENTARY ON [SECT. III. 
 
 190. Compare the times of falling to the center of the logarithmic spiral 
 from different points. 
 
 The times are as the areas. 
 
 P 
 
 c s 
 
 d . area =- g , (6 = L. C S P), for d . area = 
 
 2 
 
 Also T^Ufi = ^ = tan. L. Y P T = tan. , (a being constant) = a 
 d e 
 
 g 2 d ___ a . g. d g 
 *'"~2~ ~~2~ 
 
 a. P* 
 .-. area = T- oc g 2 5 (for when g = 0, area = 0, .*. Cor. = 0) 
 
 .*. if P, p, be points given, 
 T from P to center : t from p to center : : S P * : S p 2 . 
 
 191. Compare \ in a logarithmic spiral with that in a circle, e. d. 
 
 .-. if F be given, V a 
 
 .: v in spiral : v in the circle : : V P V in spiral : V 2 S P : : 1 : 1. 
 192. Compare T in a logarithmic spiral with that in a circle, e. d. 
 
 m . whole area a P 2 a p z 
 
 1 in spiral = : r-rr = s ov = -. 
 
 area in 1 4 . v . S Y 2 v . g . sin. a 
 
 ~2~ 
 
 rr,, . . , whole area ST g 2 2WP 2 2crp 
 
 T x in circle = r^- = grw = = = 
 
 area in I" v. S Y v . g v 
 
 2 
 
 ...T:T':: a g> . - :^- g : :^- : 2 : : a : 4 . sin. a. 
 2 v . g . sin. a v 2 sin. a 
 
 : : tan. : 4 T . sin. a : : 1 : 4 w cos. a.
 
 BOOK I.] 
 
 NEWTON'S PRINCIPIA. 
 
 157 
 
 1-92. In the Ellipse compare the time from the mean distance to the Aphe- 
 lion, with the time from the mean distance to the Perihelion. Also given the 
 Excentricity, tojind the difference of the times, and conversely. 
 
 Circle 
 A D V is described on A V. 
 
 2i 
 
 T of passing through Aphelion : t through Perihelion 
 :: SB V: SB A 
 :: SDV: SD A 
 
 Let Q = quadrant C D V, 
 . T- t- O 4- a " ae 
 
 t..Q + -- 
 
 DC - SC :CDV- DC ' SC 
 
 a> ae 
 
 2 
 
 .-. (T+ t =) P: T t: : 2Q:a. ae 
 P a. a e 
 
 . T t 
 
 J. A L 
 
 2Q 
 
 whence T t, or, if T t be given, a e may be found. 
 
 193. If the perihelion distance of a comet in a parabola = 64, 's mean 
 distance = 100, compare its velocity at the extremity of L with 's velocity 
 at mean distance. 
 
 Since moves in an ellipse, v at the mean distance = that in the circle 
 e . d . and v in the parabola at the extremity of L 
 
 : v in the circle rad. 2 S A : : V 2 : 1 
 v in the circle rad. 2 S A 
 
 : v in the circle rad. AC:: VAC : V S A 
 . v in the parabola at L 
 
 : v in the ellipse at B : : V~2TA~C : V~S~A72 
 
 :: 10 V 2 : 8 V 2 
 : : 5 : 4 
 
 194. What is the difference between L of a parabola and ellipse, having 
 the same < st distance = 1, and axis major of the ellipse = 300? Compare 
 the \ at the extremity oflu and < st distances. 
 
 In the parabola L = 4 A S = 4.
 
 158 
 
 A COMMENTARY ON 
 
 [SECT. III. 
 
 In the ellipse L' = 
 
 2B C 
 A C 
 
 = AC2 
 
 (2 AC. AS AS 2 ) = 
 
 150 
 
 600 1 
 150 
 
 .-. L L' = 4 
 
 J_ 
 150' 
 
 V 300 : V 299. 
 
 d L\ 
 
 V* 150J 
 
 \ in the parabola at A : v in the circle rad. S A : : V 2 : 1 
 v in the circle rad. S A : v in the ellipse e. d. : : VAC: VHP 
 
 : : VTC : V 2AC SA 
 .. v in the parabola at A : v in the ellipse e. d. ; 
 Similarly compare v s . at the extremity of Lat. R. 
 1 95. Suppose a body to oscillate in a 
 whole cycloidal arc, compare the tension 
 of the string at the lowest point with 
 the weight of the body. 
 
 The tension of the string arises 
 from two causes, the weight of the 
 body, and the centrifugal force. At 
 V we may consider the body revolving 
 in the circular arc rad. D V, .. the 
 centrifugal = centripetal force. Now 
 the velocity at V = that down C V by the force of grav. 
 
 = that with which the body revolves in the circle rad. 
 
 2C V. 
 
 .*. grav. : centrifugal force ::!:!, 
 .-. tension : grav. : : 2 : 1 
 
 196. Suppose the body to oscillate c ^ 
 
 through the quadrant A B, compare the 
 tension at B with the weight. 
 
 AtBthestring will be in the direction of 
 gravity; .*. the whole weight will stretch 
 the string; .'. the tension will = centrifugal \ 
 force + weight. Now the centrifugal 
 force = centripetal force with which the 
 body would revolve in the circle e. d. 
 
 
 
 And v in the circle = V 2 g . F .
 
 BOOK I.] 
 
 NEWTON'S PRINCIPIA. 
 
 159 
 
 F v 
 
 F ~^R- 
 
 CB 
 
 in this case, 
 
 also v' at B from grav. = V 2 g . C B, grav. = 1. 
 
 v'2 
 
 .'. grav. = 1 = 
 
 2g C B 
 
 v , v , 
 
 V : grav ' : : 2^CB : ^~C~B : 
 
 2: 1, 
 
 since v 
 
 ' 
 
 v. 
 
 /. tension : grav. : : 3 : 1. 
 
 197. A body vibrates in a circular arc 
 from the center C ; through what arc must 
 it vibrate so that at the lowest point the 
 tension of the string = 2 X weight? 
 
 v from grav. = v d . N V, (if P 
 be the point required) v' of revo- 
 
 A CV 
 
 lution in the circle = v d . -= . 
 
 .-. centrifugal force : grav. : : v : v f : ; J - : V N V 
 .*. centrifugal force + grav. (= tension) rgrav. : : J -^ |- V 
 
 V 
 
 : VNV 
 
 : : 2 : 1 by supposition. 
 
 /CV = v~WT, 
 
 .-. N V = 
 
 C V 
 
 198. There is a hollow vessel inform 
 of an inverted paraboloid down 'which 
 a body descends, the pressure at lowest 
 point = n . Weight, Jindfrom what point 
 it must descend. 
 
 At any point P, the body is in the 
 same situation as if suspended from G, 
 P G being normal, and revolving in the 
 circle whose rad. G P. Now P G = 
 V 4 A S . S P, .-. at A, P G =
 
 160 
 
 A COMMENTARY ON 
 
 [SECT. Ill, 
 
 v / 4AS 8 = 2AS. Also v 2 at A with which the body revolves = 
 
 .. centrifugal force = ^ - r~ o 
 2 g A S 
 
 v 
 and grav. = - j-, if h '= height fallen from. 
 
 But the whole pressure arises from grav. -f- centrifugal force, and = n . grav. 
 .. centrifugal force -f grav. : grav. : : n : 1 
 
 or 
 
 1 1 1 
 
 AS H h : TT : 
 
 1 1 
 
 . _. __ n _ I 1 
 
 AS' h "" l ' J) 
 
 .-. h = n 1 . A S. 
 
 199. Compare the time (T') in 'which a body de- 
 scribes 90 of anomaly in a parabola with T in the 
 circle rad. = S A. 
 
 Time through A L : 1 : : area A S L : a in 1" 
 
 . T = * A S> SL - 4 A S * 
 a 3 a 
 
 T in the circle rad. S A : 1 : : whole circle : a' in 1" 
 
 *A S 2 
 
 A S 9 
 
 .-. T' : T : : 
 
 3 a ' a? 
 
 and 
 
 a -.a':: VL: V 2 AS:: V 4> A S : V 2 AS:: V 2: I 
 
 4 
 
 .-. T' : T : : 
 
 : IT : : 2 V 2 : 3 it. 
 
 3 V 2 
 
 Compare the time of describing 90 in the parabola A L with that in the 
 parabola A 1, (fig. same.) 
 
 t : T in the circle rad. S A : : 4 : 3 V 2 . v 
 
 T in the circle S A : T' in the circle rad. <rA::SA^:dA^ 
 (since T 2 R s ) 
 
 T : t' through 1 A : : 3 V 2 . v : 4 
 
 .-. t through S A : t' through a A : : S A % : : a A &. 
 See Sect. VI. Prop. XXX.
 
 BOOK I.] 
 
 NEWTON'S PRINCIPIA. 
 
 161 
 
 200. Draw the diameter P p such that the time through P V p : time 
 through p A P : : n : 1, force . 
 
 Describe the circle on A V. 
 
 t = 
 
 Let t = time through P V p, and T the periodic time 
 
 n _ PVpS Q V q S circle + A Q q S 
 n + ] ellipse " circle circle 
 
 - circle , S R. 2 C Q 
 ~ 2 + 2 
 
 it a 
 
 circle 
 
 + a e . sin. u . a 
 
 ., (u = excentric anomaly) 
 
 = -5- + e . sin. u 
 
 tdt 
 
 .'. n it = n -f 1 . ( + 
 
 e sn. 
 
 .e sn. u 
 
 sin. u =r 
 
 n 1 
 
 n -f 1 ' 2e 
 
 which determines u, &c. 
 
 201. The Moon revolves round the Earth in 30 days, the mean distance 
 from the Earth = 240,000 miles. Jupiter's Moon revolves in day, the 
 mean distance from Jupiter = 240,000 miles. Compare the absolute forces 
 of Jupiter and the Earth. 
 
 VOL. I. L
 
 162 A COMMENTARY ON [SECT. III. 
 
 A* 
 T - , A being the major axis of the ellipse, 
 
 .*. If A be given, /<*; 
 
 Mass of Jupiter T * of the Earth's Moon __ 30 * _ 14,400 
 "*" Mass of the Earth "" T" of Jupiter's Moon _]_ 1 * 
 
 4 2 
 
 202. A Comet at perihelion is 400 times as near to the Sun as the Earth 
 at its mean distance. Compare their velocities at those points. 
 
 Velocity 2 of the Comet __ F. 4 A S J^ _4 F 1 
 
 Velocity 2 of the Earth " F' . 2 B S =: F' * 2 . 400 ~ = F' ' 200 
 
 400~ s 
 
 1 2 '200 
 
 = 800 
 
 V V 2 . 20 30 
 
 .. = , = -7- nearly. 
 
 v 1 1 
 
 203. Compare the Masses of the Sun and Earth, having the mean distance 
 of the Earth from the Sun = 400, the distance of the Moon from the Earth, 
 and Earth's P d . = 13. the Moon's P*. 
 
 /A QC 
 
 * T~I j ' 
 
 Mass of the Sun 400 8 _1^ 64,000,000 
 *'* Mass of the Earth ~~ "I 7 " ' IF 2 ~ " ~T69~ 
 
 1 
 
 1 a 
 
 204. If the force - g , 'where x is the distance from the center 
 
 A X. 
 
 1 a 
 
 of force, it mil be centripetal whilst - 2 > 3 , or x > a ; there will be 
 
 A. A 
 
 1 a 
 
 a point of contrary flexure in the orbit when z =. ^ , or x = a, and 
 
 X. X 
 
 afterwards when x < a, /^^ ybrce tuz'/Z 6e repulsive, and the curve change 
 its direction.
 
 BOOK I.] 
 
 NEWTON'S PRINCIPIA. 
 
 163 
 
 205. The body revolving in an ellipse, at 
 B the force becomes n times as great. Find 
 the new orbit, and under what values ofn it 
 "will be a parabola, ellipse, or hyperbola. 
 
 S being one focus since the force 
 
 ct -^ -- i, the other focus must lie 
 distance * 
 
 in B H produced both ways, since 
 S B, H B, make equal angles with 
 
 the tangent. V 2 = - F.PV = -- F. 2 A C in the original ellipse, or 
 
 & 
 
 = n F . P V in the new orbit 
 
 .-.2ACzzn.PVzrn. 
 
 2 SB. h B 
 g-g h - B 
 
 .-. (S B + h B) A C = 2 n. S B. h B, 
 .-. AC 2 +hB.AC = 2nAC.hB, 
 
 . hB = AC 
 
 If 2 n 1 = 0, or n = , the orbit is a parabola ; if n > , the orbit 
 is an ellipse ; if n < , the orbit is an hyperbola. 
 
 Let S C in the original ellipse be given = B C, 
 
 .-. S B H = right angle, and S B or A C = B h . cot. B S h 
 whence the direction of a a', the new major axis ; also 
 
 , Sh VBh 2 SB* 
 
 a a' = S B + B h, and S c = -^- = - - . 
 
 A 
 
 If the orbit in the parabola a a' be parallel to B h, and L . R = 2 S B, 
 since S B h = right angle. 
 
 206. Suppose a Comet in its or- 
 bit to impel the Earth from a cir- 
 cular orbit in a direction making 
 an acute angle with the Earth's 
 distance from the Sun, the velo- 
 city-after impact being to the velo- 
 city before : : V~3 : V 2. Find 
 the alteration in the length of the 
 year. 
 
 Since V 3 : V 2 < ratio than ^2:1, .*. the new orbit will be an 
 ellipse. 
 
 L2
 
 164 A COMMENTARY ON [SECT. III. 
 
 Vj . _3 . J 3 ^ V 2 S P. H P H P 
 v 2 : : 2 := 2 S P " A C. 2 S P " AHC 
 
 _ 2 A C S P 
 
 AC 
 .-. 3AC = 4AC 2SP 
 
 .-. 2 S P = A C 
 
 Tin ellipse 2^ S P* 8 
 
 . * _ ^- _ _._, 
 
 * T' in circle " p I ' ' 3 
 
 207. ^4 body revolves in an ellipse, at any given point the farce becomes 
 diminished by ^ th part. Find the new orbit. 
 
 B' 
 
 .*. in this case P V , 
 F 
 
 P V in ellipse 1 n n 1 
 ' pvinneworbit " I n 
 
 But 
 
 11 
 
 v z in conic section p v n 1 ' P V 
 v 2 in circle e. d. = 2~S~P = ~2~S~P~ 
 
 n HP 
 
 n 1* A C 
 
 if- i . H P = AC, the new orbit is a Circle 
 
 = 2 A C, Parabola 
 
 < 2 A C, Ellipse 
 
 > 2 A C, Hyperbola. 
 
 If - -y = 2, or n = 2, then when the orbit is a circle or an ellipse, P 
 
 must be between a, B ; when the orbit is a parabola, P must be at B ; 
 when the .orbit is an hyperbola, P must be between B, A.
 
 BOOK I.] NEWTON'S PRINCIPIA. 165 
 
 208. If the curvature and inclination of the tangent to the radius be the 
 same at two points in the curve, the forces at those points are inversely as the 
 radii 2 . 
 
 P - 8a 2 _ 8a 2 _ 8 a 2 1 
 
 "g.SY 2 . PVg.SY.S P.R~g.sin.0SP 2 .R C SI r2> 
 This applies to the extremities of major axis in an ellipse (or circle) in 
 the center of force in the axis. 
 
 209. Required the angular velocity of '. 
 By 46, 6 being the traced-angle, 
 
 d< 
 
 = d t* 
 
 But by Prop. I. or Art. 124, 
 
 dt: T::dA: A 
 
 2 
 2 A 
 
 or 
 
 P X V 
 
 (a) 
 
 210. Required the Centrifugal Force (p) in any orbit. 
 
 When the revolving body is at any distance g from the center of force, 
 the Centrifugal Force, which arises from its inertia or tendency to persevere 
 in the direction of the tangent (most authors erroneously attribute this force 
 to the angular motion, see Vince's Flux. p. 283) is clearly the same as it 
 would be were the body with the same Centripetal Force revolving in a 
 circle whose radius is . Moreover, since in a circle the body is always 
 at the same distance from the center, the Centrifugal Force must always 
 be equal to the Centripetal Force. 
 
 But in the circle 
 
 QT 2 = Q R x 2SP 
 
 and .. by 137 we have 
 
 F- 8A2 X _L_ -tA." v 1 
 
 "gT" 2 " 2SP 3 ~gT* * f 3 
 
 or 
 
 P 2 V 2 1 
 
 X "5" 
 
 g ? 
 
 P and V belonging to the orbit. 
 
 L 3
 
 166 A COMMENTARY ON [SECT. III. 
 
 Hence then 
 
 P 2 V 2 1 
 
 ? = ~ x a 
 
 Hence also and by 209, 
 And 139, 
 
 t: f t., . . . ........ (c) 
 
 P S 
 
 211. Required the angular velocity of the perpendicular upon the 
 tangent. 
 
 If two consecutive points in the curve be taken ; tangents, perpendiculars 
 and the circle of curvature be described as in Art. 74, it will readily ap- 
 pear that the incremental angle (d -vp) described by p =r that described 
 by the radius of curvature. It will also be seen that 
 
 But from similar triangles 
 
 P V : 2 R : : p : g. 
 .-. d 6 : d ^ : : P V : 2 s 
 P V being the chord of curvature. 
 
 . Hence 
 
 d d * 
 
 or 
 
 or 
 
 Ex. 1. In the circle P V = 2 f ; whence 
 
 PxV 
 
 - = = *" 
 
 Ex. 2. In the other Conic Sections, we have 
 
 j,l rl f "P~"\7 \ C V7; 
 
 -x 2? 
 
 (d] 
 
 PV 
 2P X V 
 
 (e) 
 
 " S X P V 
 
 P x v dp 
 
 tf) 
 
 TO ~~ 
 
 ~
 
 BOOK L] NEWTON'S PRINCIPIA. 167 
 
 which gives by taking the logarithms 
 
 2 lp = lb 2 -f Ig l(2a + g) 
 and (17 a.) 
 
 2 dp .!_ dj d g 2 a d g 
 
 P S ^ a + g (2 a + g) 
 
 whence 
 
 aP X V 
 
 tf ^r* . . * 
 
 212. Required the Paracentric Velocity in an orbit. 
 It readily appears from the fig. that 
 
 ds:dg::g: V g 2 p s . 
 
 .*. If u denote the velocity towards the center, we have 
 d gx d s V g 2 p 2 
 
 A _ \s f 
 
 ( Q J\ _ 
 
 a V~ dt/ " dt 
 
 = Z_2<_y x v e* v* (125) 
 
 or 
 
 2 A 
 T X 
 
 Also since 
 
 1 g ! d<) 2 -f.clg ! 
 
 p 2 
 
 u = PV x -^f-, (k) 
 
 g 2 d d 
 
 213. To find where in an orbit the Paracentric Velocity is a maximum. 
 
 From the equation to the curve substitute in the expression (212. g) 
 for p 2 , then put d u = 0, and the resulting value of g will give the posi- 
 tion required. 
 
 Thus in the ellipse 
 
 and 
 
 u 2 = P' V 2 X (?4 - = max. 
 
 D 
 
 2 a 1 1 
 
 -p~^ 
 L4
 
 168 A COMMENTARY ON [SECT. III. 
 
 2a_dj 2_dj_ 
 
 bv f 3 
 
 and 
 
 b 2 __ Latus- Rectum 
 S =: T : 2 
 
 or the point required is the extremity of the Latus- Rectum. 
 
 OTHERWISE. 
 
 Generally, It neither increases nor decreases when F = <p. Hence 
 when u = max. (see 210) 
 
 d p _ d g 
 
 "F : 7 
 
 which is also got from putting 
 
 d (u 2 ) = 
 in the expression 212. h. 
 
 214. Tojlnd where the angular velocity increases fastest. 
 
 By Art. 209 and 125, 
 
 < a) 
 
 But from similar triangles 
 p: V ( f 2 
 
 = max. 
 
 ,,v 
 
 . ..... (b) 
 
 P p g 
 
 either of which equations, by aid of that to the curve, will give the point 
 required. 
 
 Ex. In the ellipse 
 
 and -j = gives
 
 BOOK I.] NEWTON'S PRINCIPIA. 169 
 
 which gives 
 
 f = ^a+-^ V (49 a 1 48 b 2 ) 
 
 for the maxima or minima positions. 
 If the equation 
 
 = X 
 
 a 1 + e cos. 6 
 and the first form be used, we have 
 
 dp a e 
 
 - = rr X s sin. 6 
 a 6 b 2 
 
 and 
 
 sin. 6 
 T- = max. = m. 
 
 f' 
 
 Whence and from d m = 0, we get finally 
 
 C o S 6 - + /( * 
 
 8 e ^f *64 e 2 
 
 215. Tojind 'where the Linear Velocity increases fastest. 
 Here 
 
 dv 
 
 HI = max> 
 But (125) 
 
 PX V 
 v = 
 
 P 
 and 
 
 Pdf 
 
 
 PxV~ P X V ^~^~ 
 dv P 2 V 2 Vfp s ^p ? ) dp 
 
 dt "~ e P 3 d 
 
 . * x (g 2 -P 2 ) x d P 
 
 * * ~ ~ 3 j 
 
 or S = max. = m. 
 
 Fx V( g '~p') J 
 
 and 
 
 d m = 
 will give the point required.
 
 170 A COMMENTARY ON [SECT. IV. 
 
 Thus in the ellipse 
 
 2ag 5 
 
 "TiT " "" .Tfi" 
 which gives 
 
 d m 4 IQ a b g g 4 6 b g g 5 
 
 whence the maxircfa and minima positions. 
 
 In the case of the parabola, a is indefinitely great and the equation 
 becomes 
 
 4 a 2 g a b 2 = 
 
 .*. * ss. a- X T* * Latus-Rectum. 
 b a ID 
 
 Many other problems respecting velocities, &c. might be here added. 
 But instead of dwelling longer upon such matters, which are rather 
 curious than useful, and at best only calculated to exercise the student, 
 I shall refer him to my Solutions of the Cambridge Problems, where -he 
 will find a great number of them as well as of problems of great and 
 essential importance. 
 
 SECTION IV. 
 
 216. PROP. XVIII. If the two points P, p, be given, then circles whose 
 centers are P, p, and radii AB+SP, AB+Sp, might be described 
 intersecting in H. 
 
 If the positions of two tangents T R, t r be given, then perpendiculars 
 S T, S t must be let fall and doubled, and from V and v with radii each 
 = A B, circles must be described intersecting in H. 
 
 Having thus in either of the three cases determined the other focus H, 
 the ellipse may be described mechanically, by taking a thread = A B in 
 length, fixing its ends in S and H, and running the pen all round so as to 
 stretch the string.
 
 BOOK T.I NEWTON'S PRINCIPIA. 171 
 
 This proposition may thus be demonstrated analytically. 
 
 1st. Let the focus S, the tangent T R, and the point P be given in 
 position ; and the axis-major be given in length, viz. 2 a. Then the per- 
 pendicular S T ( = p), and the radius-vector S P ( = g) are known. 
 
 But the equation to Conic Sections is 
 
 whence b is found. 
 
 Also the distance (2 c) between the foci is got by making p = f , thence 
 finding g and therefore c = a If . 
 
 This gives the other focus ; and the two foci being known, and the axis- 
 major, the curve is easily constructed. 
 
 217. 2d. Let two tangents T R, t r, and the focus S be given in position. 
 
 Then making S the origin of coordinates, the equations to the trajectory 
 are 
 
 + e cos. - 
 a being the inclination of the axis-major to that of the abscissae. 
 
 Now calling the angles which the tangents make with the axis of the ab- 
 scissae T and T', by 31 we have 
 
 tan.T = iy. 
 d x 
 
 But 
 
 x = cos. 6, y = sin. 6 
 whence 
 
 , __ d g sin. 6 + g d 6 cos. 6 
 d cos. 6 g d sin. 6 
 
 -$4- tan. 6 + I 
 
 g d d 
 
 Also from equations (a) we easily get 
 dp a e 
 
 t 
 
 tan. 
 
 * \ / 1 \ 
 
 f d = p- ssm - (1 
 
 b-. . (s) 
 
 ae S 
 sin. (5 a) = X V (2ag f 2 b 8 ) . . (3) 
 
 d C P 
 
 and 
 
 
 (4)
 
 172 A COMMENTARY ON [SECT. IV. 
 
 and putting 
 
 R = V (2a z b 2 ) . . . . (5) 
 we have 
 
 R tan, 6 tan. a 
 
 s- = tan. (d a) = - - . (6) 
 
 b * a g 1 + tan. a . tan. 
 
 which gives tan. 6 in terms of a, b, f, and tan. a. 
 
 Hence by successive substitutions by means of these several expres- 
 sions tan. T may be found in terms of a, b, p, tan. a, all of which are given 
 except b and tan. a. Let, therefore, 
 
 tan. T = f (a, p, b, tan. a). 
 In like manner we also get 
 
 tan. T' = f (a, p', b, tan. a) 
 p' belonging to the tangent whose inclination to the axis is T. 
 
 From these two equations b and tan. a may be found, which give 
 c= V a 2 b 2 and a, or the distance between the foci and the position 
 of the axis-major j which being known the Trajectory is easily con- 
 structed. 
 
 218. 3d. Let the focus and two points in the curve be given in posi- 
 tion, &c. 
 
 Then the corresponding radii f, /, and traced angles 6, tf, in the 
 equations 
 
 a(l e 2 ) 
 
 1 + e cos. (6 a) 
 
 a(l-e 2 ) 
 
 1 + e cos. (tf a) 
 are given ; and by the formula 
 
 cos. (6 a) = cos. 6 . cos. a + sin. 6 sin. a 
 
 2 a e and a or the distance between the foci and the position of the axis- 
 major may hence be found. 
 
 This is much less concise than Newton's geometrical method. But it 
 may still be useful to students to know both of them. 
 
 219. PROP. XIX. To make this clearer we will state the three cases 
 separately. 
 
 Case 1. Let a point P and tangent T R be given. 
 
 Then the figure in the text being taken, we double the perpendicular 
 S T, describe the circle F G, and draw F I touching the circle in F and 
 passing through V. But this last step is thus effected. Join V P, sup- 
 pose it to cut the circle in M (not shown in the fig.), and take 
 V F 2 = VM x (V P + PM). 
 
 The rest is easy.
 
 BOOK I.] 
 
 NEWTON'S PRINCIPIA. 
 
 173 
 
 Case. 2. Let two tangents be given. Then V and v being determined 
 the locus of them is the directrix. Whence the rest is plain. 
 
 K 
 
 Case 3. Let two points (P, p) be given. Describe from P and p the 
 circles F G, f g intersecting in the focus S. Then draw F f a common 
 tangent to them, &c. 
 
 But this is done by describing from P with a radius = S P S p, a 
 circle F' G', by drawing from p the tangent p F' as in the other case (or 
 by describing a semicircle upon P p, so as to intersect F' G' in F') by 
 producing P F' to F, and drawing F f parallel to F' p. 
 
 See my Solutions of the Cambridge Problems, vol. I. Geometry, where 
 tangencies are fully treated.
 
 174 A COMMENTARY ON [SECT. IV. 
 
 These three cases may easily be deduced analytically from the general 
 solution above ; or in the same way may more simply be done at once, 
 from the equations 
 
 a L L 1 
 
 4 *' f " 2 " 1 + cos. (6 a) * 
 
 220. PROP. XX. Case 1. Given in species'] means the same as " simi- 
 lar" in the 5th LEMMA. 
 
 Since the Trajectory is given in species t &c.] From p. 36 it seems that 
 the ratio of the axes 2 a, 2 b is given in similar ellipses, and thence the 
 same is easily shown of hyperbolas. Hence, since 
 
 2 c being the distance between the foci, if = m, a given quantity, we 
 have 
 
 __ = 5 T = V (1 + m 2 ) = e, 
 
 a a 
 
 which is also given. 
 
 With the centers B, C, &c.] 
 
 The common tangent L K is drawn as in 219. 
 
 Cases 2. 3. See Jesuits' Notes. 
 
 OTHERWISE. 
 
 221. Case 1. Let the two points B, C and the focus S be given. 
 
 Then 
 
 + a(l e 2 ) ^ 
 
 ' 1 + e cos. (6 a)( ,jv 
 
 , _ + a(l e 2 ) ( 
 * : = 1 + e cos. (tf a.}) 
 
 a being the inclination of the axis of abscissas to the axis major. 
 But since the trajectory is given in species 
 
 e = is known, 
 a 
 
 and in equations (1), g, 6 ; g', ^, are given. 
 Hence, therefore, by the form 
 
 cos . (Q a ) = cos. 6 . cos. a + sin. 6 sin. a, 
 a and a, or the semi- axis-major and its position are found; 
 
 also c = a e is known ; 
 which gives the construction.
 
 BOOK IJ NEWTON'S PRINCIPIA. 175 
 
 Case 2. By proceeding as in 220, in which expressions (e) will be 
 known, both a, a e, and a may be found. 
 Case 3. In this case 
 
 b* s a 2 x(l e 2 )g 
 
 |-fc X. _ _ * _ M _ __ V^ '_ 9 
 
 ~ 2 a ~ 
 
 will give a. Hence c = a e is known and 
 
 + a(l--e 2 ) 
 * 1 + e cos. (d a) 
 gives a. 
 
 Case 4. Since the trajectory to be described must be similar to a given 
 one whose a' and c' are given, 
 
 is known (217). 
 
 Also and 6 belonging to the given point are known. 
 Hence we have 
 
 
 1 + e cos. (6 a) 
 
 And by means of the condition of touching the given line, another 
 equation involving a, a may be found (see 217) which with the former 
 will give a and a. 
 
 222. SCHOLIUM TO PROP. XXI. 
 
 Given three points in the Trajectory and the focus to construct it. 
 
 ANOTHER SOLUTION. 
 
 Let the coordinates to the three points be f, 6 ; ^', (f ; f ", 6", and a the 
 angle between the major axis and that of the abscissas. Then 
 
 + a. (1 e 2 ) 
 
 f- e cos. (d a) 
 
 4. fl i \ ~ 2^ 
 
 Tail e ; 
 
 p ~~" 
 
 j f e cusi. ^o a,j 
 j /i _ 2\ 
 
 Y (A) 
 
 1 + e COS. (Q' a) 
 
 ~ 1 + e cos. (if 1 a) - 
 and eliminating + a ( 1 e 2 ) we get 
 
 S =e.cos. (^ a) ecos. (d a)") , 
 ' =e. cos. (0" a) e cos. (6 } )
 
 176 A COMMENTARY ON [SECT. V. 
 
 from which eliminating e, there results 
 
 g' . COS. (^ a) g COS. (6 a) g" COS. (6" a) g COS. (& a) 
 Hence by the formula 
 
 cos. (P Q) = cos. P . cos. Q + sin. P . sin. Q 
 
 (g g')g" cos. 0" (g g") g' cos. 
 tan. a .1 ,, . ... ... ,. . 
 
 which gives a. 
 
 Hence by means of equations (B) e will be known ; and then by substi- 
 tution in eq. (A), a is known. 
 
 SECTION V. 
 
 The preliminary LEMMAS of this section are rendered sufficiently intel- 
 ligible by the Commentary of the Jesuits P.P. Le Seur, &c. 
 
 Moreover we shall be brief in our comments upon it (as we have been 
 upon the former section) for the reason that at Cambridge, the focus of 
 mathematical learning, the students scarcely even touch upon these sub- 
 jects, but pass at once from the third to the sixth section. 
 
 223. PROP. XXII. 
 
 This proposition may be analytically resolved as follows : 
 
 The general equation to a conic section is that of two dimensions (see 
 Wood's Alg. Part IV.) viz. 
 
 in which if A, B, C, D, E were given the curve could be constructed. 
 Now since five points are given by the question, let their coordinates be 
 a, |8; a, /3; a, /3; a, /3; a, /3. 
 
 11 22 33 44 
 
 These being substituted for x, y, in the above equation will give us five 
 simple equations, involving the five unknown quantities A, B, C, D, E, 
 which may therefore be easily determined ; and then the* trajectory is 
 easily constructed by the ordinary rules (see Wood's Alg. Lacroix's Diff. 
 Cal. &c.) 
 
 224. PROP. XXIII. The analytical determination of the trajectory 
 from these conditions is also easy. 
 Let 
 
 a, /3 ; a, /3 ; a, ,3 ; a, 
 
 11 22 33
 
 BOOK I.] NEWTON'S PRINCIPIA. 177 
 
 be the coordinates of the given point. Also let the tangent given in posi- 
 tion be determinable from the equation 
 
 y 7 = m x' + n (a) 
 
 in which m, n are given. 
 
 Then first substituting the above given values of the coordinates in 
 
 y 2 + Axy + Bx z + Cy+Dx+ E = . . . (b) 
 we get four simple equations involving the five unknown quantities 
 A, B, C, D, E ; and secondly since the inclination of the curve to the axis 
 of abscissas is the same at the point of contact as that of the tangent, 
 
 d y _dy' 
 
 <Tx ~~ dx' 
 
 y = y' 
 
 Ay+ 2Bx + D 
 
 *. ~~ in 
 
 2y+ Ax+ C 
 
 and substituting in this and the general equation for y its value 
 
 y' = m x + n 
 we have 
 
 A(mx + n)+2Bx + D __ 
 
 2(mx + n) + Ax+C 
 and 
 
 from the former of which 
 
 n A + mC + D 
 
 2(m 2 -fmA + B) 
 and from the latter 
 
 X = -2(m'+!nA+B) X (n A + mC +D + 2mn 
 + v^(nA + mC + D + 2mn) z (n 2 + nC + E)(m 2 + mA + B}} 
 and equating these and reducing the result we get 
 
 4,m 2 n 2 = (nA + mC + D+ 2 m n) 2 (n*+n C + E) (m z +m A+B) 
 and this again reduces to 
 
 + 2mCD nBC mAE BE+ 3mn 2 A 
 + 3nm 2 C + 4mnD -n 8 B m 2 E n 2 m 2 = 
 
 which is a fifth equation involving A, B, C, D, E. 
 
 From these five equations let the five unknown quantities be determined, 
 
 and then construct eq. (b) by the customary methods. 
 
 M
 
 178 A COMMENTARY ON [SECT. V. 
 
 225. PROP. XXIV. 
 
 OTHERWISE. 
 
 Let 
 
 a,/5; a', /3'; ", 0" 
 be the coordinates of the three given points, and 
 
 y' = m x' 4- n 
 
 y"= m'x'' + n' 
 
 the equations to the two tangents. Then substituting in the general 
 equation for Conic Sections these pairs of values of x, y, we get three 
 simple equations involving the unknown coefficients A, B, C, D, E ; and 
 from the conditions of contact, viz. 
 
 d x d x' 
 
 y = y' 
 
 x =x' 
 
 We also have two other equations (see 224) involving the same five un- 
 knowns, whence by the usual methods they may be found, and then the 
 trajectory constructed. 
 
 226. PROP. XXV. 
 
 Proceeding as in the last two articles, we shall get two simple equations 
 and three quadratics involving A, B, C, D, E, from whence to find them 
 and construct the trajectory. 
 
 227. PROP. XXVI. 
 
 In this case we shall have one simple equation and four quadratics to 
 find A, B, C, D, E, with, and wherewith to describe the orbit. 
 
 228. PROP. XXVII. 
 
 In the last case of the five tangents we shall have five quadratics, 
 wherewith to determine the coefficients of the general equation, and to 
 construct.
 
 BOOK I.] NEWTON'S PRINCIPIA. 179 
 
 SECTION VI. 
 229. PROP. XXX. 
 
 OTHERWISE. 
 
 After a body has moved t" from the vertex of the parabola, let it be re- 
 quired to find its position. 
 
 If A be the area described in that time by the radius vector, and P, V 
 the perpendicular or the tangent and velocity at any point, by 124 and 
 
 125 we have 
 and by 157, 
 L being the latus-rectum. 
 
 c P V 
 
 A _ - X t _ x t 
 
 2 V 2 
 But 
 
 ASP = AOP SOP=f AOxPO SOxPO 
 
 = fxy i.(x r)y 
 where r = A S, &c. (see 21) and 
 y 2 = 4rx 
 
 2 '' ~~ 24r T 2 
 
 V 
 
 O J 
 
 .'.y 3 + 12r 2 y = 12 r t v'g/zr 
 by the resolution of which y may be found and therefore the position of P. 
 
 OTHERWISE. 
 
 230. By 46 and 125, 
 
 ds pds 
 
 dt = = 
 
 v c 
 
 Also 
 
 i d g 
 
 M2
 
 180 A COMMENTARY ON [SECT. VI. 
 
 which is an expression of general use in determining the time in terms of 
 the radius vector, &c. 
 In the parabola 
 
 whence 
 
 c V (f r) 
 and integrating by parts 
 
 2V r 2Vr 
 
 But 
 
 c = P V = V.2gf*T (229) 
 
 which gives 
 
 whence we have and the point required. 
 
 By the last Article the value of M in Newton's Assumption is easily 
 obtained, and is 
 
 M = A = ' x /*. 
 
 4r 4 -N 2r 
 
 231. COR. 1. This readily appears upon drawing S Q the semi-latus- 
 rectum and by drawing through its point of bisection a perpendicular to 
 GH. 
 
 232. COR. 2. This proportion can easily be obtained as in the note of 
 the Jesuits, by taking the ratio of the increments of G H and of the curve 
 at the vertex ; or the absolute value of the velocity of H is directly got 
 thus. 
 
 d.GH SdM 3 /g,i 
 
 v / o . 
 
 dt dt " 4N 2r 
 
 Also the velocity in the curve is given by (see 140)
 
 BOOK I.] NEWTON'S PRINCIPIA. 181 
 
 and at the vertex g = r, 
 
 r 
 .-. v : v' : : 3 : 8. 
 
 233. COR. 3. Either A P, or S P being bisected, &c. will determine 
 the point H and therefore 
 
 t /lixGH. 
 
 3V gp 
 
 234. LEMMA XXVIII. That an oval cannot be squared is differently 
 demonstrated by several authors. See Vince's Fluxions, p. 356; also 
 Waring. 
 
 235. PROP. XXXI. This is rendered somewhat easier by the follow- 
 ing arrangement of the proportions : 
 
 If G is taken so that 
 
 O G : O A : : O A : O S 
 or 
 
 'or AZ 
 OG = ^T 
 
 and 
 
 GK: 2*OG::t:T 
 
 or 
 
 rv 2*xOA 2 t 
 
 Cr K = - Q-g - X rp ....... (a) 
 
 Then, &c. &c. For 
 
 ASP = ASQx- 
 
 a 
 
 = i. X (OQA OQS) 
 
 H 
 
 AQ OQ x SR) 
 
 But 
 
 S R : sin. A Q : : S O : O A 
 :: OA : OG:: AQ: FG 
 QR _ A Q sin. A Q 
 
 FG 
 and 
 
 M3
 
 182 A COMMENTARY ON SECT. VI. 
 
 = |x(FG-sin.AQ) 
 
 ..ASP = 2X (FG sin.AQ) 
 
 ....... (b) 
 
 (see the Jesuits' note q.) which is identical with (a), since 
 
 J_ _ A S P 
 
 T ~ Ellipse 
 _ ASP 
 Trab 
 
 OTHERWISE. 
 
 236. By 230 we have 
 
 But in the ellipse 
 
 (2a ? b 2 
 and putting 
 
 g a = u 
 it becomes 
 
 At b . (a + u) d u 
 - c V(a 2 e 2 u 2 ) 
 2 a e being the excentricity. 
 Hence 
 
 _ b a f du Jb r udu 
 
 ~~^J Va*e z u 24 " c-/ Va 
 
 c a e c 
 
 Let t = 0, when u = a e ; then 
 
 P _ b a it 
 
 and we get 
 
 ba 
 
 2 e 2 u 2 ) 
 u 2 ) + C.
 
 BOOK I.] NEWTON'S PRINCIPIA. 183 
 
 which is the known form of the equation to the Trochoid, t being the ab- 
 scissa, &c. 
 
 Hence by approximation or by construction u and therefore may be 
 found, which will give the place of the body in the trajectory. 
 
 It need hardly be observed that (157) 
 
 OTHERWISE. 
 
 237. 
 
 but in the ellipse 
 
 1 
 
 a 1 + e cos. 6 
 
 b 4 dd 
 
 .-. d t = - - X 
 
 a 2 c ( 1 + e cos - 2 
 and (see Hirsch's Tables, or art. 110) 
 
 a 2 (l e 2 ) C 1 .e + cos.d esin.0 1 
 
 t ' _ L v J __ cos ' _ _ __ _ y. 
 
 c ''^(l e 2 ) '1+ecos.^ 1 + ecos. O 
 
 which also indicates the Trochoid. 
 To simplify this expression let 
 
 . e + cos. 6 
 cos. ~ - - = u 
 1 e cos. 6 
 
 then 
 
 e + cos. 
 
 1 + e cos. 6 
 and 
 
 = COS.. U 
 
 e cos. u 
 cos. 6 = 
 
 e cos. u 1 
 Hence 
 
 V (\ e 2 ) 
 
 sin. o = 
 
 I e cos. u 
 and 
 
 e sin. & e sin. u 
 
 l + ecos.0 V (\ e 2 ) 
 
 a* V (1 e 2 ) 
 
 .'. t = s ' X f u e sin. ul 
 
 c 
 
 But (157) 
 
 c = PV = b.= V(l e 2 )
 
 184 A COMMENTARY ON [SECT. VI. 
 
 3 
 
 a 2 
 .'. t = - X (u e sin. u) 
 
 Let 
 
 V gp n 
 Then 
 
 n t =. u e sin. u ......... (1) 
 
 Again, 6 may be better expressed in terms of u, thus 
 
 21 ~ cos ^ l-cos.ul + e u 
 
 - 
 2 1 + cos.0- 1 e 1 +cos.u" 1 e 2 
 
 Moreover is expressible in terms of u, for 
 a ( i _ e z\ 
 
 f-=-fi -^ = a(l ecos.u) ..... (3) 
 1 + e cos. 6 
 
 In these three equations, n t is called the Mean Anomaly ; u the 
 Excentric Anomaly, (because it = the angle at the center of the ellipse 
 subtended by the ordinate of the circle described upon the axis-major 
 corresponding to that of the ellipse) ; and 6 the True Anomaly. 
 
 238. SCHOLIUM. 
 
 Newton says that " the approximation is founded on the Theorem that 
 TJie area APSaAQ SF, SF being the perpendicular let fall from 
 S upon O Q."] 
 
 First we have 
 
 APR=AQRx 
 
 a 
 
 SPR=SQRx 
 
 a 
 
 .-.ASP = ASQx 
 
 a 
 
 But 
 
 A SQ = AO Q SOQ 
 
 = | A Q x AO SF x OQ 
 
 = i AO x (AQ SF). 
 
 .-. A S P = x (A Q S F) 
 u' being the L. A O Q. 
 
 = X (a u' a e sin. u') ..... (1)
 
 BOOK I.] NEWTON'S PRINCIPLE 185 
 
 (Hence is suggested this easy determination of eq. 1. 237. 
 
 For 3 b . 
 
 _- ASp l(au-aes.n.u) 
 
 v ^~ JL 
 
 x\ *n i ^ , - 
 
 Ellipse \/ ^ g 
 
 f 
 
 X (u e sin. u). ) 
 
 Again, supposing u' an approximate value of u, let 
 
 / .1 E 
 u = u +- 
 
 Then, by the Theorem, we have 
 
 2A b Sp = A q S O X sin. A q 
 
 = AQ + Qq+ SOx sin. (A Q + Q q) 
 to radius 1. 
 
 But A Q being an approximate value of A q, Q q is small compared 
 with A O, and we have 
 
 sin. ( A Q + Q q) = sin. A Q cos. Q q + cos. A Q sin. Q q 
 = sin. A Q + Q q cos. A Q nearly. 
 
 J_ 
 
 L , ? AQ+ SOsin.AQ) x -, nearly 
 
 b /I L ^. 
 
 which points out the use of these assumptions 
 
 N' = ^A^P = 11 x area of the Ellipse 
 
 B ' = s o 
 
 and 
 
 D' = S O . sin. A Q = B' sin. A Q 
 
 L'- -1 
 " SO 
 
 Then 
 
 in which it is easily seen B', N 7 , D', L' 
 are identical with B, N, D, L. 
 
 Hence
 
 186 
 
 A COMMENTARY ON 
 
 [SECT. VII. 
 
 Having augmented or diminished the assumed arc A Q by E, then re- 
 peat the process, and thus find successively 
 G, I, &c. 
 
 For a developement of the other mode of approximation in this 
 Scholium, see the Jesuits' note 386. Also see Woodhouse's Plane Astro- 
 nomy for other methods. 
 
 SECTION VII. 
 
 239. PROP. XXXII. F -T. 2 Determine the spaces which a 
 
 distance 
 
 body descending from A in a straight line towards the center of 
 force describes in a given time. p 
 
 If the body did not fall in a straight line to the center, it would 
 describe some conic section round the center of force, as focus 
 
 f ellipse ~\ 
 
 (which would be < parabola > if the velocity at any point were to S 
 (_ hyperbola ) 
 
 the velocity in the circle, the same distance and force, in 
 
 V 2 : 1.) 
 
 (I) Let the Conic Section be an Ellipse A R P B. 
 
 Describe a circle on Major Axis A B, draw 
 C P D through the place of the body perpendi- 
 cular to A B. 
 
 The time of describing A P a area A S P a 
 area A S D, whatever may be the excentricity 
 of the ellipse. 
 
 Let the Axis Minor of the ellipse be diminish- 
 ed sine limite and the ellipse becomes a straight 
 line ultimately, A B being constant, and since 
 A S . S B = (Minor Axis) 2 = 0, and A S finite 
 .-.SB = 0, or B ultimately comes to S, and 
 
 time d . A C a area A D B. .. if A D B be taken proportional to time, 
 C is found by the ordinate D C. 
 
 (T . A C a area ADBaADO + ODBaarcAD + CD 
 
 /. take 6 + sin. & proportional to time, and D and C are determined.)
 
 BOOK I.] 
 
 NEWTON'S PRINCIPIA. 
 
 187 
 
 (Hence 
 
 the time down A O 
 T.OB 
 
 ^-4- 1 
 
 * 
 cr - + 
 
 1 -5-1 TT-1 
 
 il + i 
 
 7 18 9 . . 
 
 = ~ = ~2 near W 
 
 
 A 
 
 N. B. The time in this case is the time 
 from the beginning of the fall, or the time 
 from A. 
 
 (II) Let the conic section be the hyperbola 
 B F P. Describe a rectangular hyperbola on 
 Major Axis A B. 
 
 T a area S B F P or area SEED. 
 
 Let the Minor Axis be diminished sine 
 limite, and the hyperbola becomes a straight 
 line, and T a area B D E. 
 
 N. The time in this case is the time from 
 the end of motion or time to S. 
 
 Let the conic section be the parabola B F P. 
 Describe any fixed parabola BED. 
 
 T a area S B F P a area SEED. 
 
 Let L . R. of B F P be diminished sine 
 limite the parabola becomes a straight line, 
 and T a area B D E. 
 
 N. The time in this case is the time from 
 the end of motion, or tune to S. 
 
 Objection to Newton's method. If a 
 straight line be considered as an evanescent 
 conic section, when the body comes to peri- 
 helion i. e. to the center it ought to return to aphelion i. e. to the original 
 point, whereas it will go through the center to the distance below the 
 center = the original point. 
 
 240. We shall find by Prop. XXXIX, that the distance from a center from 
 which the body must fall, acted on by a ble force, to acquire the velocity such 
 as to make it describe an ellipse = A B (finite distance), for the hyperbola 
 = A B, for the parabola = a . 
 
 241. Case 1. vdv = g^dx, f= force distance 1, 
 
 B 
 
 if a be the original point 
 
 a x 
 
 V a 
 
 dx . 
 
 V ax x*
 
 188 
 
 A COMMENTARY ON 
 2** ; 
 
 [SECT. VII. 
 
 \ 
 
 Vax-x* 
 
 A/ax x 
 
 .-. t = 
 
 + C, when t = 0, x = a, 
 
 V a x x z + /circumference 
 r 
 
 vers. -'x") 
 1 
 
 rad - - ( 
 ~ 2) 
 
 V CT It V 
 
 n r* 
 
 if the circle be described on B A = a, 
 
 = ^/_ a _. 4 ..( 
 
 a 4 /CD.OB AD. OP 
 
 r> "t" o 
 
 .BAD. 
 
 Case 2. v * = 2 g /ce. . - , if a be an original point, 
 a x 
 
 x d x 
 
 2 ' 
 
 w . V a x + x 
 for t in this case is the time to the center, not the time from the original 
 point, 
 
 , d x d x 
 .*. d t = , or d t = 
 
 v ' v 
 
 Now if with the Major Axis A B = a, we describe the rectangular 
 hyperbola, 
 
 C 
 
 D 
 
 E 
 
 we have
 
 BOOK 1J NEWTON'S PRINCIPIA. 189 
 
 d.BED = d.BEDC d.ABDC=ydx ~'^ y = ^^"^1? 
 
 <i to 
 
 Vax + x 2 /a. \ , axdx 
 
 / 2 
 
 .. t from B = / .BED, for they begin or end together at B. 
 
 ^ 2 g p 
 
 IX 
 
 Case 3. v z = 2 g p - , if a be oc, 
 
 X 
 
 dx v'xdx ,. 
 
 .vats = .r , t being time to B. 
 
 v * 
 
 C, when t = 0, x = 0, .-. C - 0. 
 
 . 
 
 v 2 g /j,o 
 
 Describe a parabola on the line of fall, vertex B, L . R. = any fixed 
 distance a, 
 
 1 2V2 
 
 x . x = 
 
 V 
 
 a x . x = 
 
 2 V2 
 
 V ag /(i 
 2 V 2 
 
 .. curvili- 
 
 Hence in general, in Newton Prop. XXXII, t = 
 
 V a g 
 
 near area, a being L . R. of the figure described. 
 
 T , 2 (Ax. Min.) 1 . r . 
 
 in the evanescent conic sections, L . R. = V lyr-^ > * it Ax, 
 
 Min. be indefinitely small, L. R. will be indefinitely small with respect to the 
 Ax. Min. The chord of curvature at the finite distance from A to B is ulti- 
 
 mately finite, for P V = 
 
 ' but at A or B, P V = L, = in- 
 
 finitesimal of the second order. Hence S B is also ultimately of the second 
 order, for at B, S B = L. - . 
 
 l| 
 
 2~TS 
 PROP. XXXIII. Force a 
 
 \ in the circle distance S C V S A 
 
 (distance) * 
 
 = - JL5 in the ellipse and hyperbola. 
 
 . / O A * * f
 
 190 A COMMENTARY ON [SECT. VII. 
 
 /V VHP VAC 
 
 ^ = .--..-. . = ;=== when the conic section becomes a straight line j 
 
 ~2~ V 2~~ 
 
 NEWTONS METHOD. 
 
 A 
 
 V* 
 
 L 
 
 SY 8 L SP 
 
 2 AO 
 
 C.CB 
 
 ~ 2SP ~ 2 ' SY* 
 
 SP 
 AO 2 2 A O 
 
 
 CP 2 
 
 /Min. Ax.\ * 2/Min. Ax.\ * 
 
 L 
 
 ( 2 ) \ 2 ) 
 
 AO 
 L _ A O.C P 8 
 '*2 :: AC.C B 
 V s _ AJXCP^.SP 
 ' V* == AC.CB.SY 8 ' 
 
 but 
 
 CjO __ B O 
 BO"" TO* 
 
 CO C B comp. in the ellipse 
 *" B~O ~" FT ' div. in the hyperbola, 
 
 A C _ CT div. in the ellipse _ C P 
 B O ~" B T ' ' comp. in the hyperbola B Q ' 
 AC 2 _ CP 2 
 *' AO Z ~ BQ 2 ' 
 BQ 2 .AC AQ.CP 1 
 AO AC 
 
 V 2 BQ 2 .AC.SP 
 '" v 1 ~ AO.B C.S Y 2 ' 
 but ultimately 
 
 BQ = SY, SP = BC, 
 
 , V 2 in a straight line A C 
 
 .*. ultimately s-. -r % \ = -T~?^ 
 
 v z in the circle A O 
 
 AC 
 
 .-.-= / 
 
 V V 
 
 COR. 1. It appeared in the proof that 
 
 AO' 
 
 AC C T
 
 BOOK L] NEWTON'S PRINCIPIA. 191 
 
 AC CT 
 
 .-.ultimately^ = 3^. 
 
 (This will be used to prove next Prop.) 
 COR. 2. Let C come to O, then A C = A O and V = v, 
 .. the velocity in the circle = the velocity acquired by falling externally 
 
 through distance = rad. towards the center of the force a ?: , . 
 
 distance 2 
 
 242. Find actual Velocity at C. 
 
 V 2 atC _ AC 
 
 v 2 in the circle distance B C B A* 
 
 2 
 _ 2 AC , _ 2 AC gft, Rr 
 
 B A * B A * BC 2 ' 
 
 if ft, = the force at distance 1, 
 
 . v 2 9 a IL __ AJrL. 
 
 g ^B A.B C' 
 
 .-. V = V 2gt*. Va ~ x ,ifBA = a, B C = x. 
 V a x 
 
 TT V space described 
 
 If a is given, V a r . 
 
 V space to be described 
 
 In descents from different points, 
 
 __ V space described 
 
 ; . 
 
 V space to be described x initial height 
 In descents from different points to different centers, 
 
 V space described X absolute force 
 V a , 
 
 V space to be described X initial height 
 
 243. Otherwise, v d v = ~ d x , 
 
 O | V 
 
 .-.v 2 = 2 g p . , when a is positive, as in the ellipse 
 
 o ^L v 
 
 = 2 g fjt, . - , when a is negative as in the hyperbola 
 
 *.! X. 
 
 = 2 g & . , when a is a , as in parabola 
 
 (when x = 0, v is infinite) 
 V 2 in the circle radius x (in the ellipse and hyperbola) 
 
 2 a x . , 1V a 
 
 in the ellipse, =
 
 192 
 
 A COMMENTARY ON 
 
 [SECT. VII. 
 
 ~ = a + x in the hyperbola, = 
 
 (I) 
 
 V * in the circle radius = (in the parabola) = 
 
 ff /i X 2 
 
 = . = 
 
 = in the parabola. 
 
 
 244. In the hyperbola not evanescent 
 Velocity at the infinite distance S A 
 velocity at A " S Y 
 
 finite R., but when the hyperbola van- 
 ishes, S Y ultimately = Min. Ax. for 
 
 S Y S C 
 
 -jjr- -p- p > and ultimately S C = 
 
 AS 
 
 A C, and b C = A C, .-. ultimately S Y = A b = C B, .. ultimately 
 S Y infinitesimal of the first order 
 S~A ~ - of the 2d order 
 
 velocity at A 
 velocity at oc distance 
 
 245. PROP. XXXIV. 
 
 the parabola. 
 
 Velocity at C - J_ f 
 
 velocity in the circle, distance S C T ' 
 
 2 
 
 S P 
 
 For the velocity in the parabola at P = velocity in the circle what- 
 ever be L . R . of the parabola. 
 
 246. PROP. XXXV. Force ex J r -. 
 
 (distance) 2 
 
 The same things being assumed, the area swept out by the indefinite 
 
 T "R 
 
 radius S D in fig. D E S = area of a circular sector (rad. = ^ 
 
 IB 
 
 of fig.) uniformly described about the center S in the same time. 
 Whilst the falling body describes C c indefinitely small, let K k be the 
 arc described by the body uniformly revolving in the circle. 
 
 Case 1 . If D E S be an ellipse or rectangular hyperbola, '- = - , 
 
 Cc 
 
 Dd 
 
 CJD 
 
 S Y 
 
 CT 
 DT 
 DT 
 
 TS
 
 BOOK I.] NEWTON'S PRINCIPIA. 193 
 
 Cc.CD CT AC 
 ' Dd.SY = TS = AD ultimatelv - 
 (Cor. Prop. XXXIII.) 
 But 
 
 velocity at C V A C 
 
 v in the circle rad. S C \f A O 
 
 v in the circle rad. S C _ , S K /A O 
 
 vin the circle rad. SK ~~ V S~C ~" -V ~S~C 
 / velocity at C N Cc _ /A~C _ A C 
 ' \v in the circle rad. S K/ ~ K k ~ V' S C == CD 
 .-. Cc. CD = Kk. AC 
 
 Kk. A C _ AC 
 '' D d . S Y " A~O ' 
 .-. AO. Kk = Dd. S Y, 
 .'. the area S K k = the area S D d, 
 /. the nascent areas traced out by S D and S K are equal 
 .'. the sums of these areas are equal. 
 
 Case 2. If D E S be a parabola S K = ^t. 
 
 fit 
 
 As above 
 
 Cc.CD _ C T ^ 
 D d . S Y ~ T S = 1 
 
 also , . . , . , S C 
 
 velocity m the circle - 
 C c __ _ velocity at C __ _ _ 2_ 
 
 Kk ~~ velocity in the circle L. R ~~ velocity in the circle L . R 
 
 ~~ ~ 
 
 V~S~6 " CD 
 
 2 2 
 
 .-. Cc.CD = 2.Kk.SK 
 .-. K k . S K = D d . S Y. 
 
 247. PROP. XXXVI. Force oc ___!__- 
 
 (distance) 2 
 
 To determine the times of descent of a body falling from the given (and 
 .-.Jinite} altitude A S 
 
 On A S describe a circle and an equal circle round the center S. 
 
 From any point of descent C erect the ordinate C D, join S D. Make 
 the sector O S K = the area A D S (O K = A D + D C) the body 
 will fall from A to C in the time of describing O K about the center S 
 
 VOL. I. N
 
 194 
 
 A COMMENTARY ON 
 
 [SECT. VII. 
 
 uniformly, the force -JT- ^. Also S K being given, the period 
 in the circle may be found, (P = / - - . it. SK 2 ), and the time through 
 
 o 
 
 O K 
 
 O K = P . -. . .-. the time through O K is known. .-. the time 
 
 circumference 
 
 through A C is known. 
 
 248. Find the time in which a Planet would Jail from any point in its 
 orbit to the Sun. 
 
 Time of fall = time of describing 
 
 O K H, S O = 
 
 J^.lV,^ *.. V..V, ^XV^V, V *^ *.* _ r ~"W~ *" ".V, V^^V, *CIV.. ^ V *_, ^ 
 
 period in the ellipse ~ period in the circle rad. AC " " A C ^ 
 
 .*. the time of fall = \ . P . 
 be considered a circle 
 
 and the time of fall 
 
 4 V 2 
 
 , P= period of the planet. If the orbit 
 
 I j 
 
 ~ V'S 
 
 V 2 4 
 
 = P. -^- = P. -s- nearly. 
 
 = nearly.
 
 BOOK I.] 
 
 NEWTON'S PRINCIPIA. 
 
 195 
 
 249. The time down A C a (arc 
 = A D + C D), a C L, if the cy- 
 cloid be described on A S. Hence, 
 having given the place of a body at a 
 given time, we can determine the 
 place at another given time. 
 
 .. 
 time d. A C 
 
 Draw the ordinate m 1 ; 1 c will deter- 
 mine c the place of the body. 
 
 250. PROP. XXXVII. To determine the times of ascent and descent of a 
 
 body projected upwards or downwards from a given point, F a .- ^. 
 
 distance 
 Let the body move off from the point G with a given velocity. Let 
 
 r~. -, -j = -7-, (V and v known, .'. m known). 
 
 v * m the circle e. d. 1 ' v 
 
 To determine the point A, take 
 
 GA 
 
 S A 
 
 GA 
 
 m 
 
 
 ' G A + G S 
 GA 
 
 m 
 
 -\ must be des- 
 >cribed on the 
 
 ' G S ~ 2 m 2 
 
 .*. if m 8 = 2, G A is + andx , .'. the parabola 
 
 if m z <2, G A is + and fin. .. the circle __ 
 
 if m*> 2, G A is and fin. .'. the rectangular hyperbola ) axis S A. 
 
 With the center S and rad. = - of the conic section, describe the 
 
 circle k K H, and erecting the ordinates G I, C D, c d, from any places 
 of the body, the body will describe G C, G c, in times of describing the 
 areas S K k, S K k', which are respectively = S I D, S I d. 
 
 251. PROP. XXXVIII. Force distance. 
 
 Let a body fall from A to any point C, 
 by a force tending to S, and g . as the 
 distance. Time a arc AD, and V acquir- 
 ed a C D. Conceive a body to fall in an 
 evanescent ellipse about S as the center. 
 /. the time down A P or A C 
 
 a ASP a ASD a AD. 
 
 a A D for the same descent, i. e. when 
 A is given. 
 
 D
 
 196 A COMMENTARY ON 
 
 The velocity at any point P 
 
 [SECT. VII. 
 
 a V F. P V 
 
 a / S P. 
 
 ultimately. 
 
 B 
 
 a CD. 
 
 COR. 1. T. from A to S = period in an evanescent ellipse. 
 
 = period in the circle A D E. 
 = T. through A E. 
 
 COR. 2. T. from different altitudes to 
 S a time of describing different quadrants 
 about S as the center a 1. 
 
 N. In the common cycloid A C S it is 
 proved in Mechanics that ifSca=SCA 
 and the circle be described on 2 . Sea, 
 and if a c = AC, the space fallen through, 
 then the time through A C a arc a d, 
 and V acquired a c d, which is analogous 
 to Newton's Prop. 
 
 Newton's Prop, might be proved in the 
 same way that the properties of the cycloid 
 are proved. 
 
 OTHERWISE. 
 
 252. vdv = g^x.dx, 
 
 .*. v 2 = 2 g p (a 2 x 2 ), if a = the height fallen from 
 .. v = V 2gp. V a 8 x * = V2gp. CD. 
 d x 1 
 
 d x 
 
 d t = = 
 
 .'. t = + 
 
 arc 
 
 */2gp V a 
 /cos. = x 
 
 ("J x )+c,c = o, 
 
 Vrad. r= a/ 
 
 - .AD. 
 
 a V2g/jt, 
 
 .*. velocity a sine of the arc whose versed sine = space, and the arc 
 a time, (rad. = original distance.) 
 253. The velocity is velocity from ajinite altitude. 
 If the velocity had been that from infinity, it would have been infinite
 
 BOOK I.] NEWTON'S PRINCIPIA. 197 
 
 d x x 
 
 and constant. .*. d t = -- , and t = - ^ ._ + C, when t = 0, 
 
 a. V g /^ 
 
 =r V g p. a, a = a. 
 
 x = a, .'. c is finite, .'. t = C = . . 
 
 V g p 
 
 Similarly if the velocity had been > velocity from infinity, it would 
 have been infinite. 
 
 254. PROP. XXXIX. Force a (distance) n , or any function of distance. 
 
 Assuming any oc n . of the centripetal force, and also that quadratures of 
 all curves can be determined (i. e. that all fluents can be taken) ; Re- 
 quired the velocity of a body, when ascending or descending perpendicu- 
 larly, at different points, and the time in which a body will arrive at any 
 point. 
 
 (The proof of the Prop, is inverse. Newton assumes the area A B F D 
 to ot V * and A D to space described, whence he shows that the force 
 D F the ordinate. Conversely, he concludes, ifFDF, ABFD 
 V 2 .) 
 
 v 2 /vd voc/F. ds. 
 
 Let D E be a small given increment of space, and I a corresponding 
 increment of velocity. By hypothesis 
 
 ABFD V* V 2 
 
 _ 
 AB G E " v' 2 V 2 +2V.I + I 2 
 
 ABFD V 2 V 
 
 DF-Q-E 2V.I + P 2T7I 
 
 But 
 
 ABFDocV 2 .-. DFGE2V.I 
 
 .-. D E . D F ultimately, a 2 . V . I 
 
 _-, 2V. I I.V 
 a ~~~ 
 
 ultimatel y- 
 
 But in motions where the forces are constant if I be the velocity gene- 
 rated in T, F -~, 5 (F -, -) and if S be the space described with uni- 
 
 form velocity V in T, ^- = -_,- , (d t = ) . Also when the force is 
 
 O 1 V / 
 
 I. V 
 
 a ble , the same holds for nascent spaces. .. F ' , and D E re- 
 
 presents S. .'. D F represents F. 
 
 N 3
 
 J98 
 
 Let D L a 
 D E 
 
 A COMMENTARY ON 
 
 [SECT. VII. 
 
 
 __ 
 V A B F D 
 
 - , ... D L M E ultimately = D L . D E 
 v 
 
 a -^- a time through D E ultimately. 
 
 .. Increment of the area A T V M E increment of the time down A D. 
 .-. A T V M E T. 
 
 (Since A B F D vanishes at A, .-. A T is an asymptote to the time 
 curve. And since E M becomes indefinitely small when A B F D is in- 
 finite, .'. A E is also an asymptote.) 
 
 255. COR. 1. Let a body fall from P, and be acted on by a constant 
 force given. If the velocity at D = the velocity of a body falling by the 
 action of a ble fote, then A, the point of fall, will be found by making 
 ABFD = PQRD. 
 
 For 
 
 ABFD V , _ 
 Tnr ^ 1 = by Prop. 
 
 DFGE 
 
 DJP 
 DR 
 
 I 
 
 D R SE 
 
 if i be the increment of the velocity generated through D E by a constant 
 force. 
 
 DRSE V 2 (V + i) 2 2i .. 
 
 V 7 " - = T ultimatelv - 
 
 PQ-RD 
 ABFD 
 
 _ 
 
 ' PQRD 1 * 
 
 256. COR. 2. If a body be projected up or down in a straight line 
 from the center of force with a given velocity, and the law of force given ; 
 Find the velocity at any other point E'. Take E' g' for the force at E'.
 
 BOOK I.] NEWTON'S PRINCIPIA. 199 
 
 velocity at E' = velocity at D. V/PQRDDF g /E \ + if pro- 
 
 V P Q R D 
 
 jected down, if projected up. 
 
 For V P Q R P D F g 7 E' 
 ' V PQRD 
 
 257. COR. 3. Find the time through D 
 
 Take E' m inversely proportional to V PQRD + DFg'E' (or 
 to the velocity at E'). 
 
 T.PD \/"PD V~TV 
 
 p 
 
 PD 
 
 T.PD _ 2 PD _ 2 PD. PL 
 ''T.DE = DE DLME 
 
 also 
 
 T.D Eby fief orce ^ DLME 
 
 T . D E' by do. = DTTnTE' 
 
 but T . D E by a constant force = T . D E by a ble force since the velo- 
 cities at D are equal (d t = ) 
 
 T. PD _ 2PD.DL 
 '* T.DE 7 = DLmE 7 
 
 d v 
 258. It is taken for granted in Prop. XXXIX, that F a ^- (46), 
 
 d s d v 
 
 and that v = -5 , whence it follows that if c . F = T , d v = c . F. d t, 
 
 Cl L Cl L 
 
 and vdv = cF.ds. 
 
 .-. v 2 = 2c/Fds 
 
 Newton represents/ F d s by the area A B F D, whose ordinate D I 
 always = F. 
 
 d s d s 
 
 d t = = 
 
 ,.t=/ 
 
 v V2c./Fds 
 d 
 
 s 
 
 N4
 
 200 A COMMENTARY ON [SECT. VII. 
 
 "dl 
 
 Newton represents f - - by the area A B T U M E. whose or- 
 J 
 
 dinate D L always = --^ 
 
 _J ).. 
 
 . A D T? T~\ J 
 J\. JJ JT J_/' 
 
 In COR. 1. If F' be a constant force V 2 = 2 g F' . P D, by Mechanics 
 
 but 
 
 V 2 = 2c./Fds 
 And F'. P D or P Q R D is proved =/F d s or A B F D, 
 
 .-. c = g 
 
 and 
 
 v* = 2g./Fds. 
 
 T C 9 velocity at E' __ V/F d s when s = A E' 
 " velocity at D " " V/F d s when s = A D 
 V A B 7 E' 
 
 V A B F D ' 
 
 In COR. 3. t= time through D '=/.=/ ds =DLmE / , 
 
 2PD 
 
 T = time through P D = xr -^ , = 
 
 VatD V2g.PQ 
 
 = 2 PD. DL 
 T^_ 2 PD. D L 
 
 '** T = D L m E' 
 
 259. The force a x n . 
 
 .*. v d v = g ju, x n d x, /- = the force distance 1. 
 
 = n + 1 (; 
 
 if a be the original height. 
 
 Let n be positive. 
 
 V from a finite distance to the center is finite \ 
 V from x to a finite distance is infinite. / 
 
 Let n be negative but less than 1. 
 
 V from a finite distance to the center is finite 1 
 V from x to a finite distance is infinite. J 
 
 Let n = 1 the above Integral fails, because x disappears, which 
 cannot be.
 
 BOOK!.] NEWTON'S PRINCIPI A. 201 
 
 dx 
 
 v d v = e / - 
 x 
 
 .-.v = 2 g/ ,.l a 
 
 X 
 
 .*. V from a finite distance to the center is infinite 1 
 V from GO to a finite distance is infinite. / 
 
 1 x 
 
 But the log. of an infinite quantity is cc ly less than the quantity itself when 
 
 X, 
 
 x is infinite = - A . Diff. and it becomes = = . 
 x x 
 
 Let n be negative and greater than 1. 
 
 V from a finite distance to the center is infinite 1 
 V from GO to a finite distance is finite. / 
 
 260. If the force be constant, the velocity-curve is a straight line parallel 
 to the line of fall, as Q R in Prop. XXXIX. 
 
 DEDUCTIONS. 
 
 261. To find under what laws of -force the velocity from x to a finite 
 distance will be infinite or finite, and from a finite distance to. the center 
 will be finite or infinite. 
 
 If (1) F a x 2 , V a V~&*~ ~x* 
 
 (2) x V a 2 x 2 
 
 1 V a x 
 
 I 
 
 x 
 
 1 
 
 (4) 
 
 A 
 
 J 
 
 x* "9 ax 
 
 ] /o2 
 
 x 3 *V a* x 
 
 1 /a"- 1 ' 
 
 In the former cases, or in all cases where F x some direct power of 
 distance, the velocity acquired in falling from oo to a finite distance or to 
 the center will be infinite, and from a finite distance to the center will be 
 finite.
 
 202 
 
 A COMMENTARY ON 
 
 [SECT. VII, 
 
 In the 4th case, the velocity from oo to a finite, and from a finite dis- 
 tance to the center will be infinite. 
 
 In the following cases, when the force a as some inverse power of 
 distance, the velocity from co to a finite distance will be finite, for 
 
 .a"- 1 x"- 1 _ / L_ 
 \l a -^ 11 - 1 = 'N/x n - ri 
 
 when a is infinite. And the velocity from a finite distance to the center 
 will be infinite, for _ 
 
 ~ r 
 
 / 
 
 V 
 
 when x = 0. 
 
 262. On the Velocity and Time-Curves. 
 
 B 
 
 (4) 
 
 H 
 
 (1) Let F a D, the area which represents V 2 becomes a A. 
 For D F a D C. 
 
 (2) Let Fa V D, .*. D F 2 a D C and V-curve is a parabola. 
 
 (3) Let F a D 2 , /. D F a D C 2 , and V-curve is a parabola the 
 axis parallel to A B. 
 
 (4) Let F a -=^ , /. D F a ~-~ , .'. V-curve is an hyperbola referred 
 
 to the asymptotes A C, C H. 
 
 (5) If F a D, and be repulsive, V s a DC.DF a DC 2 , 
 
 .-.V a D C, .'. the ordinate of the time curve a -y- a -~-~ , 
 .-. T-curve is an hyperbola between asymptotes. 
 
 (6) If a body fall from co distance, and F a ^-5 , V a -jj , 
 
 .-. the ordinate of the time-curve D, .'. T-curve is a straight line. 
 
 (7) If a body fall from co , and F cc =y^ , V a -= , 
 
 .-. the ordinate of T-curve x> V D C, .'. T-curve is a parabola. 
 
 (8) If a body fall from x, and F a 
 
 V a 
 
 jj- 2 
 
 2 , 
 
 .-. the ordinate of T-curve a D C 2 , /. T-curve is a parabola as in case 3.
 
 BOOK I.] 
 
 NEWTON'S PRINCIPIA. 
 
 203 
 
 EXTERNAL AND INTERNAL FALLS. 
 
 263. Find the external fall in the ellipse, the force in the focus. 
 
 Let x P be the space required to acquire the velocity in the curve at P. 
 
 V*down Px __ Px 
 
 V 2 in- the circle distance S P ~~ S x 
 
 ~2 
 
 V z in the circle distance S P A a 
 
 V z in the ellipse at P = 2. H P 
 
 V 2 down P x Aa. Px 
 
 '"' V s in the ellipse at P ~ Sx. H P 
 
 . _? H? 
 
 "' Sx " A a 
 
 Pjc _ HP 
 
 '* S P " S P 
 
 .-. P x = H P 
 
 .-. S x = SP + Px = Aa, and the locus of x is the circle on 2 A a, 
 the center S. 
 
 264. Find the internal fall in the ellipse, the force in the focus. 
 
 Px 
 
 V * in the circle S x 
 
 SP 
 2 
 
 V 8 in the circle S x _ S P f 1 
 
 V 2 in the circle SP ~ S~x ' e distance'
 
 204 
 
 A COMMENTARY ON 
 
 [SECT. VII. 
 
 V 2 in the circle S P _ Aa 
 V 2 in the ellipse at P ~ 2 H P 
 
 V 8 down P x Px. Aa . 
 
 '* V 2 in the ellipse at P " Sx . H P 
 
 Pjc _ HP 
 
 '* S x = A a 
 
 P_x HP 
 
 ''SP~Aa + HP 
 
 Describe a circle from H with the radius A a. Produce P H to the 
 circumference in F. Join F S. Draw H x parallel to F S. 
 265. Generally. 
 
 For external falls. 
 
 V 2 down P x __ 2g.areaAB FD Newton's fig. 
 
 V 2 in the circle distance S P ~ g F . S P F = force at distance S P 
 
 V 2 in the circle S P 2 S P 
 
 V 2 in the curve at P 
 V 2 down P x 
 
 P V 
 4. A BFD 
 
 ,_,. , 
 
 Find the area in general 
 
 ) 
 > 
 ) 
 
 ' V 2 in the curve ~ F . P V 
 .-. 4. A BFD - F. P V 
 
 f ordinate = F 
 
 -{ , 
 
 (^ abscissa = space 
 
 In the general expression make the distance from the center = S P, 
 and a the original height, S x will be found. 
 266. For internal falls. 
 
 V g down P x 2 g . A B F D Newton's fig. 
 
 2gF. STT~F = force at P 
 
 2 S P 
 
 V 2 in the circle S P 
 V 2 in the circle S P 
 
 V 2 in the curve at P 
 V 2 down P x 
 
 PV 
 4 A B FD 
 
 ' V 2 in the curve at P " F . P V 
 /. if the velocities are equal, 4ABFD = F.PV.
 
 BOOK L] NEWTON'S PRINCIPIA. 
 
 267. Ex. For internal and external falls. 
 
 205 
 
 In the ellipse the force tending to the center. 
 
 In this case, D F a D S. Take A B for the force at A. Join B S. 
 
 /. D F is the force at D, and the area A B F D = ^-5 (A B + D F) 
 
 = AS SD.AB+ DF. Let i* equal the absolute force at the dis- 
 tance 1. Let S A = a, S D = x, .-. A B = a p 
 
 D F = x/i 
 
 .-. A B F D = 
 
 a x. a -f x 
 
 and 
 
 cr 
 
 or 
 
 4ABFD = F.PV, 
 
 CD 2 
 
 a 2 x * = C P . --- in the ellipse, 
 
 a 2 x 2 = C D . 
 
 For the external fall, make x = C P, then a = Cx, and C x 2 C P 2 = C D 2 , 
 or Cx 2 =CP 2 -r-CD 2 
 
 = AC 2 + BC 2 
 = AB 2 
 .-. C x = A B. 
 For the internal fall, make a = C P, then x = C x', and 
 
 CP 2 Cx /2 = CD 2 , 
 or 
 
 Cx /2 = C P 2 CD 2 , 
 
 .-. C x x = V C P 2 CD 2 . 
 
 268. Similarly, in all cases where the velocity in the curve is quadrable, 
 without the Integral Calculus we may find internal and external falls. 
 But generally the process must be by that method.
 
 206 
 
 A COMMENTARY ON 
 
 [SECT. VII. 
 
 Thus in the above Ex. 
 
 vclv = g^x.dx 
 .-. v 2 = gp (a 2 x 2 ) 
 
 269. And in general, 
 
 V " = 
 
 Also 
 
 , as above, &c. 
 
 (a n + l x n + 1 ), if the force oc x ". 
 ' "2 ^ dp 
 
 * n + 1 v 'dp 
 
 And to find the external fall, make x = g, and from the equation find a, 
 the distance required. 
 
 And to find the internal fall make a = r, and from the equation find x, 
 the distance required. 
 
 270. Find the external fall in the hyperbola, the force ex ^-^from the focus. 
 
 O 
 
 V 1 down O P : V s in the circle rad. S P : : O P : _ 
 
 8 
 
 V s in the circle S P : V 2 in the hyperbola at P : : A C : H P
 
 BOOK I.] NEWTON'S PJRINCIPIA. 207 
 
 .-. V 2 down OP: V 2 in the hyperbola : : A C . O P : S H P 
 
 .-. 2AC.OP = SO.HP 
 .-. 2AC.SO 2AC.SP=:SO.HP 
 
 __ 2 A r 
 
 - 
 
 _ 
 
 HP SAC 
 
 To find what this denotes, find the actual velocity in the hyperbola. 
 Let the force = /3, at a distance = r, .*. the force at the distance 
 
 Also 
 
 V 2 in the circle S P _ j8. r 2 x ._ |3 x 2 
 2 g x 2 ' ~2 " ~2~x" 
 
 V 2 in the hyperbola _ (2 a -f x) /3 r " 
 2 g a . 2 x 
 
 - _ 4. _ 
 
 x 2 a 
 
 V * S r 2 V 2 
 
 But ^T when the body has been projected from o> = -- 1- ^ of 
 <^g x ^ g 
 
 y a fl r 2 y 2 
 
 projection from oo , .*. ^ of projection from oo = ^ = down 2 a, 
 ^ g / a & g 
 
 /S r 2 
 F being constant and = 7 5, or = V 2 from oo to O', when S O'=:2 A C. 
 
 .*. V in the hyperbola is such as would be acquired by the body ascend- 
 ing from the distance x to GD by the action of force considered as repul- 
 sive, and then being projected from OD back to O', S O being = 2 A C. 
 
 In the opposite hyperbola the velocity is found in the same way, the 
 
 - 2HC. S P 
 
 lorce repulsive, p externally = rj-^ . 
 
 ^i A v^ Jrl L 
 
 271. Internal fall. 
 
 V 2 down P O : V 2 in the circle rad. SO: : P O : ^-? 
 
 35 
 
 V 2 in the circle S O : V 2 in the circle S P : : S P : S O 
 V 2 in the circle S P : V 2 in the hyperbola at P : : A C : H P 
 
 .-. V* down P O : V 2 in the hyperbola : : A C . P O : SO ' HP 
 
 m 
 
 .: 2 A C. PO = S O. H P 
 or 
 
 2AC(SP SO) = SO.HP 
 
 a n 2 A c. SP 
 
 ' bU = 2AC + HP'
 
 208 
 
 A COMMENTARY ON 
 
 [SECT. VII. 
 
 and 
 
 PO = S P SO = 
 
 S P. H P 
 
 2 AC + HP' 
 
 Hence make HE = 2 A C, join S E, and draw H O parallel to S E. 
 
 Hence the external and internal falls are found, by making V acquired 
 down a certain space p with a ble force equal that down . P V by a 
 constant force, P V being known from the curve. 
 
 272. Find how far the body must fall externally to the cir- 
 cumference to acquire V in the circle, F distance towards the 
 center of the circle. 
 
 Let OC = p, OB = x, OA=a, C being the point re- 
 quired from which a body falls. 
 
 Let the force at A = 1, .'. the force at B = 
 
 a 
 
 vdv= g.F.dx, (for the velocity increases as x decreases) 
 
 C T 
 
 Br 
 
 A 
 
 Q- 
 p . 
 
 O 
 
 .-.v 2 = s-.x 2 + C 
 and when v = 0, x = p, 
 
 .-.C= .P 2 
 
 .-. v* = -| (p 2 
 a vr 
 
 - - 
 and when x = a, 
 
 v 2 at A = 
 But 
 
 v 2 at A = 2 
 
 a 
 ' ~2~ 
 
 the force at A being constant, and 
 a PV 
 2 : 4 
 
 ... p 2 _ a 2 = a 2 , .-. p 2 = 2 a 2 , .-. p = V 2 . a. 
 273. Find hovofar the body must fall internally from the circumference to 
 acquire V in the circle, F a distance towards the center of the circle. 
 
 Let P be the point to which the body must fall, O A = a, O P = p, 
 
 O Q = x, F at A = ], .-. the force at Q = . 
 
 a
 
 BOOK 1.1 NEWTON'S PRINCIPIA. 209 
 
 x , 
 
 /. v d v = g . . d x 
 e a 
 
 = -- .x 2 + C, 
 a 
 
 and when v = 0, x = a, 
 
 .-. C = --.a 2 
 a 
 
 .-.v 2 = (a 2 x 2 ) 
 and when x == p, 
 
 v z = (a 2 p ! )from a ble force 
 a 
 
 and 
 
 v 2 = g . a, from the constant force 1 at A. 
 
 .'.a 2 P 2 = a 2 , .'. p = 0, .. the body falls from the circumference 
 to the center. 
 
 274. Similarly, when F a -p . 
 
 distance 
 
 O C, or p externally = a V e, (e = base of hyp. log.) 
 and 
 
 OP, or p internally ^ . ' 
 
 V e 
 
 275. When F a -,. 
 
 distance z ' 
 p externally = 2 a 
 
 2 a 
 
 p internally = - - . 
 3 
 
 276, When F a ^ 
 
 distance s ' 
 p externally = x . 
 
 p internally = ~/~a 
 
 v & 
 
 277. When F a -p- - == . 
 
 distance " + ' 
 
 ii 
 
 p externally = a ^1 - - 
 
 n 
 / 9 
 
 p internally = a 
 
 2 + n 
 
 If the force be repulsive, the velocity increases as the distance increases, 
 .. v d v = gF.dx 
 
 Vor, I. O
 
 210 A COMMENTARY ON [SECT. VII. 
 
 278. Find how far a body must fall externally to any point P in the 
 parabola, to acquire v in the curve. F a :p-, , towards the focus. 
 
 P V = 4 S P = c, S Q = p, S B = x, S P = a, force at P = 1, 
 
 ^O 
 
 a- ^ 
 
 .-. vd v = g .dx 
 
 when v = 0, x = p 
 
 f^ _ o 
 
 IT' 
 
 v2 = 2 g a* (i- i) = 2 g a 2 (-1 - 1) at P, 
 but 
 
 v z = 2g.-^- = 2ga, 
 
 L _L i. 
 
 a p " " a ' 
 
 * 
 
 279. Similarly, internally, p = -^ . 
 
 280. In the ellipse, F a 5 towards a focus 
 
 p externally = P H + P S. (.'. describe a circle with the center S, rad. = 2 A C) 
 PH. PS 
 
 (Hence V at P = V in the circle e. d.) 
 
 281. In the hyperbola, F a -^ towards focus 
 
 p externally = 2 A C (Hence V at P = V in the circle e. d.) 
 
 P H P S 
 
 p internally = o A r a. P H " (^ ence V at P = V in the circle e.d., p. 190) 
 
 282. In the ellipse F cc D from the center 
 pexternally= V A C z + B C 2 , (= A B)} (Hence construction) 
 
 or (= V CD* + CP 2 ) 
 
 (Hence also V at P = V in the circle radius C P, when C D = C P) 
 p internally = V C P 2 CD 8 .
 
 BOOK I.] 
 
 NEWTON'S PRINCIPIA. 
 
 211 
 
 (Hence if C P = C V, p = 0, and V at P = V in the circle e. d., as 
 was deduced before) 
 
 (If C P < C D, p impossible, .-. the body cannot fall from any distance 
 to C and thus acquire the V in the curve) 
 
 283. In the ellipse, F a D from the center. 
 
 External fall. 
 
 The velocity-curve is a straight line, (since D F a C D, also 
 since F = 0, when C P = 0, this straight line comes to C, as 
 C d h, V a VCObaCO, O being the point fallen from, to acquire 
 Vat P. 
 
 .-. V from O to C : V from P to C : : O C : P C 
 
 Also since vdv = gF.dx, and if the force at the distance 1 = 1, 
 the force at x = x. .. v d v = g x d x, and integrating and correct- 
 ing, v 2 = g (p * x 2 ), where p = the distance fallen from. 
 
 .*. v a V p z x 2 , and if a circle be described, with center C, rad. C O 
 a P N (the right sine of the arc whose versed P O is the space fallen 
 through). 
 
 .-. V from O to P : V from O to C : : P N : (C M =) O C 
 and 
 
 V from P to C : V in the circle rad. C P : : 1 
 (for if P v = i P C, v d = C d P) and 
 
 V hi the circle C P : V in the ellipse : 
 Compounding the 4 ratios, 
 
 V down O P : V in the ellipse : : P N : C D 
 .-. Take P N = C D, and 
 
 V down O P = V in the ellipse, 
 
 1 
 
 C P : C D. 
 
 .-. C O = CN= V C P 2 + CD'. 
 02
 
 212 
 
 Internal fall. 
 
 A COMMENTARY ON 
 
 [SECT. II. 
 
 V in the ellipse : V in the circle rad. C P : : C D : C P 
 
 V in the circle : V down C P : : 1 : 1 
 
 V down C P : V down P O : : (C M =) C P : O N 
 
 .-. V in the ellipse : V down P O : : C D : O N 
 
 .-. Take O N = C D, and V in the curve = V down P O, and C O 
 
 V C P 2 C D z . 
 
 284. Find the point in tlie ellipse, the force in the center, where V = tlte 
 
 oelocily in the circle, e. d. 
 
 D 
 
 In this case C P = C D, whence4he construction. 
 Join A B, describe - 
 
 on it, bisect the circumference in D', join 
 
 B D', A D'. From C with A D 7 cut the ellipse in P. 
 
 .-. 2 CP 2 = C P 2 + CD' 
 
 .-. C P 2 C D J . (C P will pass through E.) 
 
 A simpler construction is to bisect A B in E, B M in F, then C P is 
 the diameter to the ordinate A B, and from the triangles C E B, C F B, 
 C F is parallel to A B, .'. C D' is a conjugate to C P and = C P. 
 
 p externally (to which body must 
 
 285. In the hyperbola, 
 force repulsive, oc D, from the center 
 
 rise from P,)= V C D z + C P 2 
 p internally (to which body must 
 
 rise from the center) = V C P-C D* 
 (Hence if the hyperbola be rectangular p internally = 0, or the body must 
 rise through C P.)
 
 BOOK I.] NEWTON'S PRINCIPIA. 213 
 
 286. In any curve, F oc - n+ - i ,Jind p externally. 
 
 where a = S P, c = P V. 
 287. If the curve be a logarithmic spiral, c = 2 a, 
 
 / a \ I 
 ' P = a D 
 
 = a 
 
 also F a ^ , ( .. p = a 
 .-. n = 2 J 
 
 288. In any curve, F oc ^ . . ,find p internamy. 
 
 \j ' 
 
 / a \I/ 4a+ 1 N 
 
 p = a / \ 2 . (p n = -: ) 
 
 n c J v r 4 a + n c/ 
 \ a + - / 
 4 / 
 
 289. If the curve be a logarithmic spiral, c = 2 a, n = 2, 
 
 / a \ ! 
 
 - . 
 
 V 2 
 290. If the curve be a circle, F in the circumference, c = a, and n = 4, 
 
 (o 1 
 
 - J 4 = x 
 
 and p internally = a 
 
 
 291. In the ellipse, F a ^- z from focus. External fall. 
 
 A S C 
 \ 2 down O P : V 2 in the circle radius S P : : O P : -g- , Sect. VII. 
 
 V e in the circle S P : V 2 in the ellipse at P : : A C : II P, 
 
 O3
 
 214 
 
 A COMMENTARY ON [SECT. VII. 
 
 SO. HP 
 
 .-. V 2 down O P : V* in the ellipse : : A C . O P : 
 
 .-. S O = 
 
 ... s o = - 
 
 .-. 2 A C.OP = SO. HP 
 
 2AC.OP 2 AC. SO 2 A C.SP 
 
 H P 
 
 Internal fall. 
 
 2 A C H P 
 
 = 2 A C. 
 
 E 
 
 V 2 down P O : V 8 in the circle radius S O : : P O : - -? , 
 
 SJ 
 
 V 2 in the circle S O : V 2 in the circle S P : : S P : S O 
 V * in the circle S P : V 2 in the ellipse at P : : A C : H P 
 
 .-. V 2 down P O : V 2 in the ellipse : : P O . A C : 
 
 .-. 2PO.AC = SO.HP 
 .-. 2SP. A C 2SO.AC = SO.HP 
 
 2 AC.SP 
 
 SO. HP 
 
 .-. S O = 
 
 F a 
 
 2 AC + HP 
 
 Hence, make H E = 2 A C, join S E, and draw H O parallel to E S. 
 292. External fall in the parabola, T O 
 
 =r-$ from focus. 
 
 E 
 
 V 2 d . O P : V z in the circle radius S P 
 SO 
 
 : : O P : 
 
 , Sect. VII. 
 
 V * in the circle S P : V 2 in the parabola 
 atP:: 1 : 2,
 
 BOOK I.] NEWTON'S PRINCIP1A. 215 
 
 .-. V 2 down O P : V 2 in the parabola : : O P : S O 
 .-. O P = S O, .-. S O = a 
 
 Internal fall. 
 
 V 2 down O P : V 2 in the circle S O : : O P : ? - 
 
 A 
 
 V 2 in the circle S O : V 2 in the circle S P : : S P : S O 
 V 2 in the circle S P : V 2 in the parabola at P: : 1:2 
 
 .-, V 2 down OP: V 2 in the parabola : : O P : S O, 
 .-. O P = S O, 
 
 V = V down - = V down S P = V . down E P = V of a body describ- 
 
 ing the parabola by a constant vertical force = force at P. 
 293. Find the external Jail so that the velocity* ac- 
 quired = n' . velocity in the curve, F a x n . 
 
 v d v =: g /, . x n . d x, (f' = force distance 1), 
 
 ... v 8 = ~~r . (a n + 1 x a + l ) a = original height, 
 
 TT9 . ,, P d P & ., 2 p d P 
 
 V 2 in the curve = a IL . l ~. t = $-.. c. u c = ^ s 
 dp 2 dp 
 
 .'. n.i*. c = -. a "*_ x , orn.c = -^. a n + x 
 
 Make x = S P = g, and from the equation we get a, which = S x. 
 For the internal fall, make a = S P = g, and from the equation we get 
 x, which = S x'. 
 
 294. Find the external fall in a LEMNISCATA. 
 
 (x 2 + y 2 ) 2 = a 2 (x 2 y 2 ) 
 
 is a rectangular equation whence we must get a polar one 
 Let z. N S P = 6, 
 
 .*. y = g. sin. 0, x = g. cos. 6, g 2 = (x 2 + y 2 ) 
 .-. 4 = a z . (g * (cos. s 6 sin. 2 fl)) = a 2 g 2 .cos. 2 0, 
 .. g 2 = a 2 . cos. 2 
 
 / s \ 
 
 .'. 2 6 = A (cos. = 5-r), 
 
 \ cl / 
 
 .-. 2 d 6 = ^ 2 -^ = 
 
 /it-t.' 
 
 V a" 
 
 O4
 
 216 A COMMENTARY ON [SECT. VII. 
 
 - li! - a ' 4 g 4 
 
 ''d^ = g 2 
 but in general 
 
 , _ 
 ' 
 
 _ 
 
 dj_* 
 h 8 
 
 = in this case *-j ; 
 
 1 a 4 
 
 V 2 ~T 6) 
 
 2dp 6^a^ 
 
 " P 3 dg= T"" 
 
 .. force to S a -TJ 
 v d v = ftfif . d x, 
 
 Also 
 
 _ 2pdg ._ a 2 _2.g a* _ 8 g 
 
 dp 3g 2 ' " 
 
 f ' ' 3 6 ' g 6 
 
 Make x in the formula above = g, 
 
 i ^ _L 
 
 'V "~a 6 =: g 6 ' 
 
 .. f. = 0, .*. a is infinite. 
 
 a 6
 
 BOOK I.] 
 
 NEWTON'S PRINCIPIA. 
 
 217 
 
 295. Find the force and external fall in an EPICYCLOID 
 
 CY 2 =CP 2 YP 2 =CP 2 CA 2 . 
 
 YB 
 CB 
 
 Let 
 
 CY = p, 
 
 = g, CB = c, CA=b, 
 
 _ n 
 
 .-. c*p 2 = 
 
 b 2 c 
 
 b 2 p 
 
 b* 
 
 p* - c 2 (g 2 b 2 )' 
 _ 2dp _ c 2 b 2 / 2dg.gx 
 p 3 ~ " c 2 U_bW 
 
 P< 
 .-. force 
 
 (f'-b 2 ) 2 p* 
 
 (as in the Involute of the circle which is an Epicycloid, when the radius 
 of the rota becomes infinite.) 
 
 Having got n offeree, we can easily get the external (or internal) fall. 
 296. Find in what cases we can integrate for the Velocity and Time. 
 Case 1. Let force a x n , 
 
 .-. v d v = g p . x n d x, 
 
 n + 1 
 d x 
 
 v 
 
 l ) 
 
 t- /*~ dx - / n + 1 
 . . l~ / zr . 
 
 * v *^i 20 /A 
 
 dx 
 
 V(a n + l x n +') 
 
 Now in general we can integrate x m dx.(a -f bx" 1 ) , when 
 
 m + 1 . , , m 
 is whole or 
 
 n n q 
 
 . . in this case, we can integrate, when - 
 Let 
 
 whole. 
 
 1 
 
 = p any whole number 
 
 .*. n = P- , (p being positive), (a)
 
 218 A COMMENTARY ON [SECT. VII. f 
 
 Let 
 
 1 
 
 o = P> 
 
 n + 1 2 
 
 1 . . 1 _ g P + 1 
 
 " n + 1 ~~ P h 2 ~ 2 
 
 n+1 = 2-Frr' 
 
 i -t- z p 
 
 .*. these formulae admit only and 1 for integer positive values of n, and 
 no positive fractional values. .'. we can integrate when F a x, or Fa 1. 
 
 297. Case 2. Let force 
 
 x ' 
 
 d x 
 
 .*. v d v = s p 
 
 O ' v n 
 
 x 
 
 . y2 _ 2g/* /a^_-x-. 
 - n 1' V a^'x"- 1 "-/ 
 
 ._ i\ v 1 n _ lo*i^l ^!v ~v - 
 
 . > /* ux y. u A * a / 
 
 L / "" ~ 4 ' ff 
 
 J v N g P J Va, n ~~ l x n ~' 
 
 n 1 
 
 , j 
 
 
 2 2 ' " o 
 
 in which case we can integrate, when - j = , or * , whole, 
 
 ii * * i n ~ ~ - -- j 
 
 i. e. if - -\ or v , be whole. 
 
 2 nr n 1, n 1 
 
 Let z = p, any whole positive No., 
 
 Let-1 
 
 1 , 
 
 1 _ 2 p 1 
 *' n - T = 2 
 
 .. n 1 = - 
 
 .. these formulae admit any values of n, in which the numerator ex- 
 ceeds the denominator by 1, or in which the numerator and denominator 
 
 are any two successive odd numbers, the numerator being the greater. 
 
 1111 
 integrate, when F ^ , , - , , &c. \ 
 
 X X g- X j X f 
 
 .*. we can
 
 BOOK I.] NEWTON'S PRINCIPIA. 219 
 
 298. Case 3. The formulas (a') (/3'), in which p is positive, cannot be- 
 come negative. But the formulae (a) and (/3) may. From which we can 
 
 integrate, when F oc __ & c . 
 
 x xf xf ' xf- 
 
 or when F oc - - & c . 
 
 x'x'xf'x 
 
 299. Wfien the force a x n , fold a n . of limes from different altitudes 
 
 force. Find the same, force a s . 
 
 s 
 
 Fa x ", .. v d v = g /A x n d x, 
 
 to the center of force. Find the same, force a s . 
 
 t 
 
 _ x n+1 
 
 i. dx dx i,-L-r n+ 1 ,. 
 
 .*. d t = -- a which is of -- s dimensions, 
 
 v n + 1 _ 
 
 .*. t will be of -- dimensions. 
 
 and when x = 0, t will a 
 
 a n ---I ' 
 
 T7 1 1 ?-+! 
 
 ^-^..ta^^oca 
 t a f ~ dx - or ~ dx 
 
 y va^ 1 x^ 1 a 
 
 ^ , 1 
 
 +&c.j 
 
 . _ 
 
 2 n+ 
 when t = 0, x = a, 
 
 
 5' J 
 
 .-. when x = 0, t a r ^ pT cc y
 
 220 A COMMENTARY ON [SECT. VII. 
 
 1 5L+J 
 
 when n is negative t oe r a a 2 . 
 & a ~~ n ~ 1 
 
 2 
 
 COR. If n be positive and greater than 1, the greater the altitude, the 
 less the time to the center. 
 
 300. A body is projected tip P A with the velocity V from the given 
 po'nt A, force in S y^^Jind the height to which the body will rise. 
 
 vdv = g /* x n d x, 
 for the velocity decreases as x increases, A 
 
 when v = V, x = a, 
 
 . . 
 
 a"* 1 V 2 v 2 
 
 .... 
 n + 1 
 
 Let v = 0, 
 a"* 1 ) = V 2 
 
 '*' X ~~ \ 2 gft 
 
 L 
 
 COR. Let n = 2, and V = the velocity down , force at A con- 
 
 ff 
 
 slant, = velocity in the circle distance S A. 
 
 2 gfjt, / 
 2 ff /t 2 
 
 *^ 
 
 _. a - 
 a a 2 ' a a
 
 BOOK I.] NEWTON'S PRINCIPIA. 221 
 
 SECTION VIII. 
 
 301. PROP. XLI. Resolving the centripetal force I N. or D E (F) 
 into the tangential one IT (F') and the perpendicular one T N, we 
 have (46) 
 
 I N : I T : : F : F : : : 
 
 d t d t 
 
 .-. d v : d v' : : d t x I N : d t' x I T. 
 But since (46) 
 
 dt = i- s ,dt' = ii' 
 
 v v' 
 
 and by hypothesis 
 
 v = * 
 
 .-. d t : d t' : : d s : d s' : : I N : I K 
 .-. dv: dv' : : IN 2 : IK x IT 
 
 : : 1 : 1 
 or 
 
 d v = d v', 
 
 &c. &c. 
 
 OTHERWISE. 
 
 302. By 46, we have generally 
 vdv = gFds 
 
 s being the direction of the force F. Hence if s' be the straight line and 
 s the trajectory, &c. we have 
 
 vdv =.- gFds 
 v' d v' = g F' d s' 
 ... v 2 V a = 2g/Fd s 
 
 v'*_ V' = 2g/F'd s' 
 
 V and V being the given values of v and v' at given distances by which 
 the integrals are corrected. 
 
 Now since the central body is the same at the same distance the central 
 force must be the same in both curve and line. Therefore, resolving F
 
 222 A COMMENTARY ON [SECT. VIII. 
 
 when at the distance s into the tangential and perpendicular forces, we 
 have 
 
 TV 17 IT T. 'IN 
 
 I?' F v F x 
 
 C IN ' C IK 
 
 n d S 
 
 = F x 
 
 ds' 
 
 .-. F' d s' = F d s 
 and 
 
 v" V' 2 = 2g/Fds = v 2 V 2 
 
 which shows that if the velocities be the same at any two equal distances> 
 they are equal at all equal distances i. e. if 
 
 V = V 
 then 
 
 v = v'. 
 303. COR. 2. By Prop. XXXIX, 
 
 v 2 A B GE. 
 But in the curve 
 
 y a F a A 11 - 1 
 .*. y d x a A n - ! d A 
 Therefore (112) 
 
 ABGE=/ydx--~ + C 
 
 a 
 
 P n A 
 
 n 
 Hence 
 
 v 2 a P n A". 
 
 OTHERWISE. 
 
 304. Generally (46) 
 
 vdv = gFds 
 and if 
 
 F = ft s 11 - 1 
 then 
 
 n 
 But when v = 0, let s = P ; then 
 
 and 
 
 C = P n .
 
 BOOK I.] NEWTON'S PRINCIPIA. 223 
 
 in which s is any quantity whatever and may therefore be the radius vector 
 of the Trajectory A ; that is 
 
 V 2 = - (pn _ A n\ or = r /J)n __ ) 
 
 n v n v> 
 
 in more convenient notation. 
 
 N. B. From this formula may be found the spaces through which a 
 body must fall externally to acquire the velocity in the curve (286, &c.) 
 
 305. PROP. XLI. Given the centripetal farce to construct the Trajec- 
 tory r , and to jind the time of describing any portion of it. 
 
 By Prop. XXXIX, 
 
 v = V~2~jr. V A B F D = i? (46) = ~ 
 But 
 
 dt = ICK 
 
 Area 2 Ara 
 
 = -^p yr- (P being the perpendicular upon the 
 
 i xx V 
 
 tangent when the velocity is V. See 125, &c.) 
 
 Moreover, if V be the velocity at V, by Prop. XXXIX, 
 
 V = V~~2"jf . V A B L V. 
 Whence 
 
 PVABLV IK 
 
 V ABFD = " x 
 
 A ' KN 
 
 /. putting 
 
 Z = 
 
 PVABLV/ _ Q _ PX V) 
 
 V. A 4/inTA/ v ' 
 
 A V A ' V 2 g A 
 
 we liave 
 
 ABFD : Z 8 : : IK 2 : KN 
 
 .-. A BFD Z 2 c Z 2 : : I K 2 K N 2 : KN 2 
 and 
 
 V A B F D Z* : Z = -- : : I N : K N 
 
 Q x I N 
 
 v X7 
 ' A X K N = 
 
 Also since similar triangles are to one another in the duplicate ratio or 
 their homologous sides 
 
 YXxXC = AxKNx -'
 
 24 A COMMENTARY ON [SECT. V11I. 
 
 Q x CX* x I N 
 
 and putting 
 
 A 8 V (ABFD Z ! ) 
 
 = ]>b = 
 
 
 y = 
 
 2 V (A BFD Z 8 ) 
 
 Q X CX* 
 
 2 A 2 V (ABFD Z 
 
 Area VCI=/ydx = VDba| . 
 
 AreaVCX=/y / dx = VDcaJ 
 Now (124) 
 
 2VCI 2VDba 
 
 : P x V " P X V" 
 or 
 
 2 VDba 
 
 " V 2g. P X VABLV 
 the time of describing V I. 
 Also, if L. V C I = 6, we have 
 
 vrv XVXCV_<XCV^ 
 ca = VCX= - ~g~~ ~ = -- % - 
 
 SVDca 
 
 (5) 
 
 (6) 
 
 which gives the Trajectory. 
 
 306. To express equations (5) and (6) in terms ofg and 6, ( = A). 
 First 
 
 and 
 
 t l 
 
 > 
 
 v 2 P 2 X V
 
 BOOK I.] NEWTON'S PRINCIPIAs 225 
 
 Hence 
 
 - PxVg 
 
 ~ 
 
 and 
 
 P 3 xV 
 
 ~ 
 
 V ( S 2 v* P 2 V E ) 
 
 VDba-^A 
 
 ~~J 
 
 *v* p*v) 
 
 and 
 
 P 3 V 
 
 _ 
 2 f Vtf'v 2 P 2 V*} 
 
 .. t - d 
 
 - 
 
 *v 2 P 2 V 2 ) 
 But by Prop. XL. 
 
 the integral being taken from v = 0, or from g =D, D being the same as 
 P in 304. 
 
 gFdg P 2 V 2 ) 
 PxVdg 
 
 . f 
 ' ~^ 
 
 - 
 
 307. Tojlnd t awff m ^rrns o/*g anc? p. 
 Since (125) 
 
 . f 
 
 -y 
 
 and 
 
 /- A 
 
 (io) 
 
 But previous to using these forms we must find the equation to the tra- 
 jectory, thus ( 139) 
 
 P 2 V 2 do 
 
 x -41- = F = f w 
 
 g P & 
 
 f denoting the law of force.
 
 226 
 
 or 
 
 A COMMENTARY ON 
 
 p2 ys 
 
 [SECT. VIII. 
 
 P * = 
 
 f f 
 
 i 
 308. To these different methods the following are examples : 
 
 1st. Let F <x = p s . Then (see 304) 
 
 .-. v 2 = gMD' r) 
 
 and if P and V belong to an apse or when P = g ; 
 
 S S. 
 
 L-l'- 
 
 = u. Then we easily get 
 
 and making t = at an apse or when g = P, we find 
 
 P ?-! 
 
 - 1 1 
 
 - ~~~2~ 
 2 * 
 
 C =^ sin. - x . 
 
 (1) 
 
 Also 
 
 P 
 
 and assuming 
 P2 _D 2 
 " 2 
 we get 
 
 sin.-
 
 BOOK I.] NEWTON'S PRINCIPIA. 227 
 
 and making = 0, when f = P we find 
 
 C =sin.- 1 l= 1-. 
 
 Also 
 
 V= V gi*. V (D 2 P 2 ) 
 
 = ' -i ^ 
 
 2 
 
 = cos. 20=2 cos. 2 6 1 
 which gives 
 
 P 2 (D 2 -P*) 
 
 P*-_(2 P 2 D 2 )cos. a tf ^ ' 
 
 Now the equation to the elh'pse, g and ^ being referred to its center, is 
 
 ..= ^__ 
 
 1 e 8 cos. * 6 
 
 Therefore the trajectory is an ellipse the center of force being in its 
 center, and we have its semiaxes from 
 b 2 = D 2 P 2 
 
 c* a 2 b 2 2P l D 2 
 
 ,2 _ 
 
 t 
 
 VIZ. 
 
 b=V(D 2 P 2 )") 
 
 and V (3) 
 
 a = P J 
 
 which latter value was already assumed. 
 Tojind the Periodic time. 
 From (3) it appears that when 
 
 and substituting in (1) we have 
 
 P2
 
 228 A COMMENTARY ON [SECT. VIII 
 
 But 
 
 i / t \ ^ ff 
 sm.- l ( 1) = . 
 
 and 
 
 T = -~- .......... .... (4) 
 
 V gp 
 
 which has already been found otherwise (see 147). 
 
 To apply (9) and (10) of 307 to this example we must first integrate 
 (11) where f f = fig; that is since 
 
 But 
 
 V 2 = g^(D 2 P 2 ) 
 
 PZ (T)2 _ T>2\ 
 n 2 ' \ - ' 
 
 * 
 
 which also indicates an ellipse referred to its center, the equation being 
 generally 
 
 a 2 b 2 
 
 2 _ 
 - 
 
 Hence 
 
 P 2 (D 2 P*) 
 
 p d 
 
 the same as before. 
 
 With regard to 0, the axes of the ellipse being known from (5) we have 
 the polar equation, viz. 
 
 b 2 
 
 " 
 
 1 e 2 cos. 2 6 ' 
 
 309. Ex. 2. Let F = -^- . Then (304) 
 
 1 
 
 V =
 
 BOOK I.] NEWTON'S PRINCIPIA. 
 
 and 
 
 P and V belonging to an apse. 
 
 " ~" J V 2 vH, V (De e 2 D P 
 
 which, adding and subtracting -^ , transforms to 
 
 t ' 
 
 4 
 D 
 
 and making 5 = u 
 
 u + -a- 
 ' 
 
 g/* 
 
 (see 86). 
 Let t = 0, when g = P. Then 
 
 D D 
 
 _ *-'. : i i _ _ v 
 
 ~~ 
 
 22 
 
 <= Vfex } ^-f-i 
 
 "" 
 
 Also 
 
 D / du 
 
 But assuming 
 
 the above becomes rationalized, and we readily find 
 
 P3
 
 230 A COMMENTARY ON [SECT. VIII. 
 
 -f )+ 
 
 and making 6 = 0, when = P, or when u = P -- - , we get 
 
 Hence, since moreover 
 
 or 
 
 D V- D ( e ---? 
 
 " 2 ) ' 2 V 2 
 
 zz sin. 
 
 + - -- 
 
 2 2 ' V 2 / 
 
 = sin. ( 6 Jf. -5-! = cos. 
 
 = a P.(D-P) x _ _1_ . . . . ( 2 ) 
 
 1 + (l jj) cos. 
 
 But the equation to the ellipse referred to its focus is 
 
 b 2 1 
 
 /* -. . 
 
 a 1 + e cos - 
 b'_ 2P(D P) 
 ' a : D 
 
 and 
 
 r^ 2 Q 2 jrj 2 ^ 
 
 -~ ' a ~" ~~~ s ~~ \ "~~ ri
 
 BOOK L] NEWTON'S PRINCIPIA. 231 
 
 b4P 4P*4P 
 
 > 
 
 b^ 2 
 D 
 
 -T X D 
 
 (3) 
 
 and f ' ' ' 
 
 b = V P x (D P). 
 
 To find the Periodic Time ; let 6 = *. Then ? = 2a P=D P, 
 and equation (1) gives 
 T 
 
 see 159. 
 
 OTHERWISE. 
 
 2 
 
 First find the Trajectory by formula (11. 307) ; then substitute for ~ 2 
 in 9 and 10, &c. 
 
 310. Required the Time and Trajectory when F= s 
 
 By 304, 
 
 v 2 = g^x (D- 2 ?~ 2 ) 
 
 "D 8 ? 2 
 
 .. if V and P belong to an apse we have 
 g tt D 2 P 2 
 
 D~ 2 ^ 
 D 
 
 V P* s 
 
 P4,
 
 232 A COMMENTARY ON [SECT. VI 11. 
 
 = -= X (C + V P*-- f) 
 
 V gA 6 
 
 and taking t = at an apse or when ? = P, C = 0, 
 
 t = -J2= X VT^Tp ...... (1) 
 
 ^ g^ 
 also 
 
 But 
 
 f d * - J- x ll 
 
 y f v (P s s *) - + p x v- 
 
 and 
 
 = ^-^ x V(D'-P'). 
 
 . i V (P 8 g g )+P , 
 " ~~~ 
 
 ' V (D 8 P 2 ) 
 and making 6 = at the apse or where e P, 
 
 = -l. = 
 
 __ 
 - V D 8 P s 
 
 . Po 
 
 ' -e V(D 2 P 2 ) 
 which gives 
 
 2 p e V r - D2 - 
 
 - ' 2Pfl 
 
 311. Required the Trajectwy and circumstances of motion <when 
 
 or for any inverse latso of the distance. 
 
 The readiest method is this ; By (1 1) 307, if r, and P be the values of 
 p and p for the given velocity V (P is no longer an apsidal distance) 
 
 P2V2 2 /; r n 1 __ on 1 
 
 v - v 2 -4- ^ x r g ri) 
 
 I" - 3 _l 
 
 the equation to the Trajectory. 
 Also since 
 
 vdv = gFdf
 
 BOOK I.] 
 
 Hence 
 
 and if we put 
 
 NEWTON'S PRINCIPIA. 
 
 2 f Uf 
 
 " 
 
 to 
 
 Va = 7 - 2 f/\ . (from oo to 
 (n IJr 11 - 1 v 
 
 in which ra may be > = or < 1 we easily get 
 
 "- 1 
 
 P 2 
 
 / m 
 
 A. / - 1 
 
 ^ m - 1 
 
 P 
 
 x 
 
 m 
 
 m= 1 
 
 m 
 
 P = 
 
 m 1 
 
 If r ~ n 
 
 N Vr=m~~* 
 
 . . m< 1 
 
 7b Jind 6 on this hypothesis. 
 We have (307) 
 
 which gives by substitution 
 P X- 
 
 n--3 
 2 
 
 m 
 
 r f 
 
 233 
 
 (2) 
 
 m= 
 
 dd = +r 
 
 I 
 
 m 
 
 - V 1 m 
 
 X PX 
 
 n--^ 
 
 a a 
 
 the positive or negative sign being used according as the body ascends or 
 descends. 
 
 Ex. If n = 2, we get 
 
 VTY1 A 
 
 - P X 
 m I r X 

 
 234 A COMMENTARY ON [SECT. VIII. 
 
 P i 
 P= - S m = 1 
 
 the equations to the ellipse, parabola and hyperbola respectively. 
 Also we have correspondingly 
 
 r 7-J5L_Pdg 
 ,1,-+ Wm-1 
 
 ^m 1 s m 1' 
 
 j . 
 = +r P. 
 
 m / m 
 
 which are easily integrated. 
 
 Ex. 2. Let n = 3. Then we get 
 
 / ni T> f i 
 
 P = J - r x P x - * - 3 . . m> 1 
 
 f ............. m = 1 
 
 p = -, X P X - - . . m < 1 
 
 - 
 
 . Prx - - - p2 - g- . m> 
 
 - 
 
 312. In the first of these values of i, m P ! may be > = or < r 2 . 
 (1). Let m P 2 > r 2 . Then (see 86) 
 
 and at an apse or when r = P 
 
 6 = + J -5U, X P X sec.- 1 X . . . (a) 
 *i m 1 "
 
 BOOK I.] NEWTON'S PRINCIPIA. 235 
 
 for 
 
 / m 1 1 1 
 
 / __ : _ _-_ __ |i| _ , 
 
 N m P' r 2 ~" P " r ' 
 (2) Let m P 2 = r ! . Then we have 
 
 V (m 1) , M 
 
 p = - i - L - ........ (b) 
 
 r rS 
 
 a. 4- v / . 
 
 - V m 1 */ 2 
 
 V (m 1) 
 
 2 /I 1 
 X ( 
 
 ~ V m 1 V r 
 
 ^- r . '. (c) 
 
 - V m 1 g 
 
 which indicates the Reciprocal or Hyperbolic Spiral. 
 (3) LetmP 2 be<r 2 . Then 
 
 < d > 
 
 + r p / m r _ ii 
 
 V ml / / , r.-mP 
 
 / e^0? 2 + m _i 
 
 =+rP 
 
 ** r 2 mP 3 , 
 
 J._ _ -- -+C 
 
 m v i r m-. -- , 
 
 * 2 
 
 2 mP 2 *g Vm .(r 2 P) V (r 2 m P 2 ) 
 at an apse r = P ; and then 
 
 Thus the first of the values of has been split into three, and integrat- 
 ing the other two we also get 
 
 Pr 
 
 P 2 ) *r 
 m 
 
 
 /" 
 
 / 
 / 
 
 _ 
 
 ^TTH
 
 236 
 
 A COMMENTARY ON 
 
 /r* mP 2 
 
 = +rP. 
 
 =+rP, 
 
 m 
 
 m 
 
 [SECT! VIII. 
 2 mP 
 
 1 m 
 
 2 mP" 
 
 c 
 
 m r -v^i 3 mP~I m.gV- V^r* mP) 
 
 X 1. - 
 
 _-mP' g V(m.r 2 *) -/ (r 8 mP 2 ) 
 and if t is measured from an apse or r = P it reduces to 
 
 m 
 
 313. Hence recapitulating we have these pairs of equations, viz. 
 
 p 
 
 (i,p = 
 
 v ' " 
 
 or 
 
 m 
 
 x sec - 1 
 ' 
 
 1 
 
 7b construct the Trajectory, 
 put = 0, then 
 
 ? = P= SA; 
 let g = co, then 
 
 m 
 
 and 
 
 and taking A S B, A S B' for these values of ^, 
 and S B, S B x for those of p and drawing B Z, 
 B' 7J at right angles we have two asymptotes ; S C being found by put- 
 ting 6 = ir. Thus and by the rules in (35, 36, 37, 38.) the curve may 
 be traced in all its ramifications. 
 
 2. p = 
 
 and 
 
 V (m 1)
 
 BOOK I.] 
 
 NEWTON'S PRINCIPLE. 
 
 23? 
 
 This equation becomes more simple when 
 we make d originate from g = cc ; for then 
 it is 
 
 ,- r' 1 
 
 ' </(m-^l) X s 
 
 and following the above hinted method the 
 curve, viz. the Reciprocal Spiral, may easily be 
 described as in the annexed diagram. 
 
 \.* 
 
 and 
 
 m 
 
 mP 
 
 1 
 
 I V(m 
 
 mP) ^(r 2 mP 
 
 S Vm(r 2 P 2 ) V (r 8 mP 2 ) 
 and when 6 is measured from an apse or when P' = r 
 
 * = p -Vr^;- lV(ri V 8) ~ r 
 
 Whence may easily be traced this figure.* 
 
 From which may be described the Logarithmic Spiral.^ 
 
 T- / m 
 5. = P./- - x 
 
 
 m 
 
 m r ^(r 2 mP flT^.f) V (r 2 m P) 
 
 Xl> 
 
 2 mP > 
 
 .r 2 - ? 2 ) V (r 2 m P 2 )
 
 238 
 or 
 
 A COMMENTARY ON 
 
 [SECT. VIIT 
 
 / _m j r V (g 2 r 8 ) 
 
 m g 
 
 when P = r. 
 
 Whence this spiral. 
 
 These several spirals are called Cotes' Spirals, 
 because he was the first to construct them as 
 Trajectories. 
 
 314. If n = 4. Then the Trajectory, &c. 
 are had by the following equations, viz. 
 
 ' 
 
 m 
 
 315. If n = 5. Then 
 p = P V'm 
 
 d 6 r P 
 
 V (m 1 .f 2 +r 4 ) 
 
 m 
 
 v 
 
 m 
 
 which as well as the former expression is not integrable by the usual 
 methods. 
 When 
 
 m -r. r 
 
 ""m 1 
 is a perfect square, or when 
 
 m 1 
 
 m 
 
 2 P4 
 
 then we have 
 
 m __ i - 4 (m 1) * 
 = +r F 
 
 "2(m 
 
 Therefore (87) 
 
 / m P 
 
 m 
 
 2 (m 
 - 
 
 L-+C 
 
 2(m I)
 
 BOOK I.] 
 
 NEWTON'S PRINCIPIA. 
 
 5 \ / 
 
 '(2.m~^T.g 2 m P 2 ) 
 2 (m 1) + P V m 
 
 V(mP 2 2.m^l.g 2 ) 
 and these being constructed will be as subjoined. 
 
 Q 
 
 * /ovxi 
 
 a = r V 2 Xl 
 
 , ! 
 
 a d or = r V 21. 
 
 316. COR. 1. OTHERWISE. 
 
 To find the apses of an orbit where F = -^ . 
 Let 
 
 Then 
 
 P = f- 
 
 m 
 
 m 
 
 n 1 
 n 3 
 
 m 
 
 - -=0m>l 
 1 
 
 , m = 1 
 
 and 
 
 I 
 
 m 
 
 . . . m < 1 
 
 239 
 
 . . . A 
 
 which being resolved all the possible values of will be discovered in each 
 case, and thence by substituting in $, we get the position as well as the 
 number of apses. 
 
 Ex. 1. Let n = 2. Then 
 
 2 r mP 2 
 
 ft * ,1- . _^r O - -,- ^__- - ^~* I 1 
 
 h m i if m 1 " 
 L 
 
 L 
 
 4 
 
 mP 2 
 
 r " 4 
 
 1 
 
 1 m 
 

 
 240 A COMMENTARY ON [SECT. VIII. 
 
 which give 
 
 r , r 2 + 4 m P 2 . (m 1) 
 
 ~ 2 (m 1) V ~ 4 (m I) 2 
 L 
 : T 
 
 and 
 
 ^ r_ /r 2 4m P 2 .(l m) 
 
 S ~~ 2 (1 m) ^ <V 4 . (1 m) 2 
 
 Whence in the ellipse and hyperbola there are two apses (force in the 
 focus) ; in the former lying on different sides of the focus ; in the latter 
 both lying on the same side of the focus, as is seen by substituting the 
 values off in those of 6. Also there is but one in the parabola. 
 
 Ex. 2. Let n = 3. Then eq. (A) become 
 
 which indicate two apses in the same straight line, and on different sides 
 
 of the center, whose position will be given by hence finding 6 ; 
 
 2 
 
 T 
 
 (2) g = t = GO 
 
 po 
 
 because r is > P, 
 
 whence there is no apse. 
 
 r 2 _ m pa 
 
 which gives two apses, r 2 being > m P 2 because m is < 1 and P < r ; 
 and their position is found from 0. 
 
 317. COR. 2. This is done also by the equation 
 
 P 
 
 p = g. sin. f, or sin. <p = 
 
 <f> being the L. required. 
 
 Ex. When n = 3, and m = 1, we have (4. 313) 
 
 P 
 
 P = S 
 r s 
 
 P 
 
 .-. sin. <f> = ^ 
 
 .. <p is constant, a known property of the logarithmic spiral. 
 
 318. To find when the body reaches the center of force. 
 
 Put in the equations to the Trajectory involving p, g ; or g, 6 
 
 s = 0. 
 Ex. 1. When n = 3, in all the five cases it is found that 
 
 p =
 
 BOOK I.] NEWTON'S PRINCIPIA. 241 
 
 and 
 
 Ex. 2. When n = 5 in the particular case of 315, the former value of 
 6 becomes impossible, being the logarithm of a negative quantity, and the 
 latter is = co . 
 
 319. Tojind when the Trajectory has an asymptotic circle. 
 
 If at an apse for 6 = co the velocity be the same as that in a circle at 
 the same distance (166), or if when 
 
 6 = oo 
 and 
 
 P = g 
 
 we also have 
 
 p ._ d p 
 
 then it is clear there is an asymptotic circle. 
 Examples are in hypothesis of 315. 
 
 320. To find the number of revolutions from an apse to = oo . 
 
 Let ^ be the value of 6 when g = p or at an apse, and 0" when 
 I = 00 . Then 
 
 v = = the number of revolutions required. 
 
 2 it 
 
 Ex. By 313, we have 
 
 / m x_ 
 ~ Vm 1 ' 2~ 
 
 . v - m 
 
 
 
 1* 
 
 321. COR. 3. First let V R S be an hyperbola whose equation, x being 
 measured from C, is 
 
 Then 
 
 V C R = y -^ -/y d x 
 But 
 
 a 2 ) 
 
 x 2 dx 
 
 = x x a 
 
 ** ~ * J * o o\ 
 
 a a y V(x 2 a 2 ) 
 
 VOL. I.
 
 242 A COMMENTARY ON [SECT. VIII. 
 
 * 
 
 a 2 ) 
 
 =^x V (x 2 -a')_Vdx V (x'-a) - - f , a> . d * 
 a a j &J A/X* 
 
 .. 2/y d x = xV(x*-a)-abl. 
 and 
 
 Again 
 
 =CP=CT=x subtangent 
 
 = x 
 
 dy 
 x 2 a s a 
 
 x x 
 
 and substituting for x in (1) we have 
 
 vcK = ir 
 
 a ^ 
 
 - 
 
 N being a constant quantity. 
 322. Hence conversely 
 
 and differentiating ( 17) we get 
 d u 8 4 
 
 and again differentiating (d 8 being constant) 
 d g u 4 
 
 _ 
 dtf 2 = a 2 b 2 N 8 
 
 Hence (139) 
 
 >< 
 
 (1) 
 
 322. By the text it would appear that the body must proceed from V 
 in a direction perpendicular to C V i. e. that V is an apse. 
 From ( 1 ) 322, we easily get 
 
 2
 
 BOOK I.] NEWTON'S PRINCIPIA. 243 
 
 and since generally 
 
 U E _ L. ( 2 n 2 ^ 
 d0 2 ~ p 2 ' K 
 
 4 
 
 ' v n 2 v (a. 2 e 2 ^ o 2 n * 
 
 a 4 b 2 N 2 ^ * ( a ) 8 
 
 .... (1) 
 
 which is another equation to the trajectory involving the perpendicular 
 upon the tangent. 
 Now at an apse 
 
 P = 8 
 
 and substituting in equation (1) we get easily 
 
 8 = a 
 which shows V to be an apse. 
 
 OTHERWISE. 
 
 Put d f = 0, for f is then = max. or min. 
 
 324. With a proper velocity.'} 
 
 The velocity with which the body must be projected from V is found 
 from 
 
 vdv = gFdf. 
 
 325. Descend to the center]. When 
 
 g = 0, p = (1. 323) and ^ = oo (2. 321). 
 
 326. Secondly, let V R S be an ellipse, whose equation referred to the 
 center C is 
 
 Then 
 
 and as above, integrating by parts, 
 
 v A/ / a 2 v 2\ ft 2 fl v 
 
 -T // 9\ xvid. x i a / u A. 
 
 /dxv^(a 2 -x 2 ) =- -^- - +2-/V (a 2 -x 2 ) 
 
 Q2
 
 A COMMENTARY ON [SECT. VIII. 
 
 x V (a 2 x') . a 
 
 a b 
 
 
 
 f=x+NT=x+ .~y dx 
 
 - x L a ' x ' __ j 
 x " "x" 
 
 and 
 
 abN . 
 
 rr sin. ~' - 
 
 . a it 26 
 
 .'. sin. - * = 
 
 2 abN 
 
 a . fir 2 6 \ 26 
 
 cos. 
 
 - - "S --- U~Xf . - i ^TT 
 
 p \2 abN/ abN 
 
 and 
 
 2 6 
 e = a sec. , x . .......... (2) 
 
 a b .N 
 
 Conversely by the expression for F in 139, we have 
 
 F p 
 
 327. To Jind when the body is at an apse> either proceed as in 323, 
 or put 
 
 dg = 0. 
 
 TJ m\ A d x . sin. x 
 
 By (27) d. sec. x = - s - 
 cos. 2 x 
 
 sin, d 
 
 " cos. 2 6 
 or 
 
 6= 
 that is the point V is an apse. 
 
 328. The proper velocity of projection is easily found as indicated 
 in 324. 
 
 329. Will ascend perpetually and go off to infinity.'] 
 From (2) 327, we learn that when 
 
 2 6 _ 
 aTTN " 2 
 
 f is CD; 
 also that g can never = 0.
 
 BOOK L] NEWTON'S PRINCIPIA. 245 
 
 330. When the force is changed from centripetal to centrifugal, the 
 sign of its expression (139) must be changed. 
 
 331. PROP. XLII. The preceding comments together with the Jesuits' 
 notes will render this proposition easily intelligible. 
 
 The expression (139) 
 
 _ P*V* dp 
 
 A ~~ /\ Q i 
 
 g P 3 df 
 
 or rather (307) 
 
 pz ya 
 
 in which P and V are given, will lead to a more direct and convenient 
 resolution of the problem. 
 
 It must, however, be remarked, that the difference between the first 
 part of Prop. XLI. and this, is that the force itself is given in the former 
 and only the law of force in the latter. That is, if for instance F = ^ g n ~ *, 
 in the former /, is given, in the latter not. But since V is given in the 
 latter, we have /A from 304. 
 
 SECTION IX. 
 
 332. PROP. XLIII. To malce a body move in an orbit revolving about 
 the center of force, in the same way as in the same orbit quiescent} 
 that is, To adjust the angular velocity of the orbit, and centripetal force 
 so that the body may be at any time at the same point in the revolving 
 orbit as in the orbit at rest, and tend to the same center. 
 
 That it may tend to the same center (see Prop. II), the area of the new 
 orbit in a fixed plane (V C p) must a time a area in the given orbit 
 ( V C P) ; and since these areas begin together their increments must also 
 be proportional, that is (see fig. next Prop.) 
 CPxKRocCpxkr 
 But 
 
 KR = CK x Z-KCP 
 k r= Ck x z-kCp 
 and CP= Cp, andCK = Ck 
 
 .-. ZLKCPakCp 
 
 and the angles V C P, V C p begin together 
 .-. /iVCP a /iVCp. 
 Q3
 
 24-6 A COMMENTARY ON [SECT. IX. 
 
 Hence in order that the centripetal force in the new orbit may tend to 
 C, it is necessary that 
 
 L. V C p QC L. V C P. 
 
 Again, taking always 
 
 CP = Cp 
 and 
 
 VCp: VCP:: G: F 
 
 G : F being an invariable ratio, the equation to the locus of p or the orbit 
 in fixed space can be determined; and thence (by 137, 139, or by Cor. 
 1, 2, 3 of Prop. VI) may be found the centripetal force in that locus. 
 
 333. Tojind the orbit infixed space or the locus of p. 
 
 Let the equation to the given orbit V C P be 
 
 where g = C P, and 6 = V C P, and f means any function ; then that of 
 the locus is 
 
 which will give the orbit required. 
 
 OTHERWISE. 
 
 Let p' be the perpendicular upon the tangent in the given orbit, and p 
 that in the locus ; then it is easily got by drawing the incremental figures 
 and simifar triangles (see fig. Prop. XLIV) that 
 K R : k r : : F : G 
 kr :pr :: p : V ( z p 2 ) 
 pr :PR:: 1 : 1 
 PR:KR:: V ( s * p' 2 ) : p' 
 whence 
 
 1: 1 :: F.p V( s * p' 2 ) : Gp' V (? 2 p s ) 
 and 
 
 " FV + (G 2 F 2 ) p' 2 
 
 334. Ex. 1. Let the given Trajectory be the ellipse with the force in 
 its focus ; then 
 
 P /2 = ^-> ^d S = ?'_* I "" e '] 
 2 a * 1 + ecos. (f 
 
 and therefore 
 
 2 _ b 2 G 2 (2a g)g 2 
 
 IT ^ t_ Q / /"^ o "t^ 2 \ l TJ* 2 / O 2 \
 
 BOOK L] NEWTON'S PRINCIPIA. 247 
 
 and 
 
 a. (1 e 2 ) 
 
 S ~~ / F \ ' 
 
 1 + e cos. ^ -p- n 
 
 Hence since the force is (139) 
 
 P 2 V 2 ,/dj_u , \ 
 
 and here we have 
 
 F 
 a(l e *) u = 1 + e cos. -~- 
 
 2F a F' 
 
 h aG 2 (l e 2 ) U ~ G' 2 
 
 and again differentiating, &c. we have 
 
 F 2 , G 2 F 2 
 
 = G s a(l e 2 ) ' ~G^ 
 
 But if 2 R = latus-rectum we have 
 
 R=L = L! = a 
 
 .'. the force in the new orbit is 
 
 P* V 2 (F^ 2 R G* R F 2 
 
 > 2 " 3 
 
 gRG 2 
 335. Ex. 2. Generally let the equations to the given trajectory be 
 
 and 
 
 Then since 
 
 G it 
 
 'Mr' 
 
 d'u , F 2 d 2 u u 
 
 F 2 /d'u , x F< 
 
 ^> I ^.^ ^L 11 1 [_ n -_ - If 
 
 ~ G 2 \d0 2 ^ / ~ G 2 
 
 and if the centripetal forces in the given trajectory and locus be named 
 X, X', by 139 we have 
 
 '&XL - iL! gX G 2 F 2 J_ 
 Q4
 
 248 A COMMENTARY ON [SECT. IX. 
 
 1 x 
 
 3 / ' * v ' 
 
 OS 
 
 Also from (2. 333) we have 
 
 J^ F_* J_ G 2 Fa J^ 
 
 T* == G 2 X " ~ G* '" r 
 
 L! > 
 
 ~ 
 
 .-. by 139 
 
 gX' ._F*gX G* 
 
 p 2 y a p/ a y / a < 
 
 the same as before. 
 
 This second general example includes the first, as well as Prop. XLIV, 
 &c. of the text. 
 
 336. Another determination of the force tending to C and which shall 
 make the body describe the locus of p. 
 
 First, as before, we must show that in order to make the force X tend 
 to C, the ratio L. V C P : L. V C p must be constant or . = F : G. 
 
 Next, since they begin together the corresponding angular velocities 
 w, u f of C P, C p are in th*t same ratio ; i e. 
 
 : / : : F : G. 
 
 Now in order to exactly counteract the centrifugal force which arises 
 from the angular motion of the orbit, we must add the same quantity to 
 the centripetal force. Hence if f>, <f>' denote the centrifugal forces in the 
 given orbit and the locus, we have 
 
 X' = X + <? ? 
 
 X being the force in the given orbit. 
 But (210) 
 
 P 2 V * 1 
 
 f _ v 
 A 5 
 
 g f 
 
 and 
 
 a w 2 
 when * is given. 
 
 , _ a/ * G 3 _ P 2 V G 2 1 
 
 .'. <p <f> X -Y p X v-j - X 7 X -,- 
 
 
 x 
 
 g 
 
 G 2 F 2 
 
 pays (^2 _ pa j 
 
 X H x rg x -- . . . . . (1)
 
 BOOK I.] NEWTON'S PRINCIPJA. 219 
 
 or 
 
 P'V 2 / d 2 u G 2 
 
 x 
 
 / 2 u , 
 
 ( u -dT- + u 
 
 or 
 
 x J 
 
 ' 
 
 g 
 
 837. PROP. XLIV. Take u p, u k similar and equal to V P and V K ; 
 also 
 
 m r : k r : : L. V C p : V C P. 
 Then since always C P = p c, we have 
 
 p r = P R. 
 
 Resolve the motions P K, p k into P R, R K and p r, r k. Then 
 RK(=rk):rm::^VCP:^VCp 
 
 and therefore when the centripetal forces PR, p r are equal, the body 
 would be at m. But if 
 
 P Cn:pCk::VCp:VCP 
 and 
 
 Cn = Ck 
 
 the body will really be in n. 
 
 Kence the difference of the forces is 
 
 mkxms (mr kr).(mr+kr) 
 
 m n = - = s - - s - : - - . 
 m t m t 
 
 But since the triangles p C k, p C n are given, 
 
 K r a m r a 
 
 -^ 
 Cp 
 
 1 1 
 
 .. m n oe T^J . X - . 
 C p* m t 
 
 Again since 
 
 P Ck:pCn::PCK:pCn::VCP:VCp 
 
 : : k r : m r by construction 
 : : p C k : p C m ultimately 
 .'. p C n = p C m 
 and m n ultimately passes through the center. Consequently 
 
 m t = 2 C p ultimately 
 and 
 
 1 
 
 m n <x -^ 3 
 Cp
 
 250 A COMMENTARY ON [SECT IX. 
 
 OTHERWISE. 
 
 338. By 336, 
 
 X' X = ?' <f> 
 
 P*V* G 2 F 2 J_ 
 
 339. Jb /race /fo variations of sign ofmn. 
 
 If the orbit move in consequentia, that is in the same direction as C P, 
 the new centrifugal force would throw the body farther from the center, 
 that is 
 
 Cmis>CnorCk . 
 or m n is positive. 
 
 Again, when the orbit is projected in antecedentia with a velocity < 
 than twice that of C P, the velocity of C p is less than that of C P. 
 Therefore 
 
 C m is < C n 
 or m n is negative. 
 
 Again, when the orbit is projected in antecedentia with a velocity = 
 twice that of C P, the angular velocity of the orbit just counteracts the 
 velocity of C P, and 
 
 m n = 0. 
 
 And finally, when the orbit is projected in antecedentia with a velocity 
 > 2 vel. of.C P, the velocity of C p is > vel. of C P or C m is > C n, or 
 m n is positive. 
 
 OTHERWISE. 
 
 By 338, 
 
 m n oc qf tp 
 
 ' * w 2 
 But 
 
 ' = a W 
 
 W being the angular velocity of the orbit. 
 .-. m n + 2 W W+ W 2 
 
 oc+ 2 u + W 
 >f- or being used according as W is in consequentia or antecedentia.
 
 BOOK L] NEWTON'S PRINCIPIA. 251 
 
 Hence m n is positive or negative according as W is positive, and nega- 
 tive and > 2 w ; or negative and < 2 u. That is, &c. &c. 
 
 Also when W is negative and = 2 w, m = 0. Therefore, &c. 
 
 340. COR. 1. Let D be the difference of the forces in the orbit and in 
 the locus, and f the force in the circle K R, we have 
 
 D: f : : m n : z r 
 
 m k X m s . r k 2 
 m t ' 2kc 
 
 (m r + r k) (m r r k) f r k 2 
 2 k c ' 2~kc 
 
 : : m r 2 r k 2 : r k * 
 :: G 2 F 2 : F 2 . 
 
 341. COR. 2. In the ellipse with the force in the focus, we have 
 
 2 R G 2 __ R F 2 
 A 
 
 a __ f. 
 
 2 
 
 For (C V being put = T) 
 
 v 2 v' 2 
 
 Force at V in Ellipse : Do. in circle : : -: - , y. TT : . _,/ 
 
 chord P V P V 
 
 1 J_ 
 : 2 R : 2 T 
 :: T: R 
 
 Also F in Circle : m n at V : : F 2 : G 2 F 2 
 
 m n at V : m n at p : : 7 ' i-, 
 T 3 A 3 
 
 F2 RG 2 ~R F 
 
 .*. F at V in ellipse : m n at p : 
 Hence 
 
 we have 
 
 F 2 
 
 F in ellipse at V = ^ 
 
 and 
 
 RG 2 RF* 
 
 and 
 
 X' = X + m n 
 
 a FJ RG 2 RF* 
 
 see 834.
 
 252 A COMMENTARY ON [SECT. IX. 
 
 OTHERWISE. 
 
 342. By 836, 
 
 pz v 8 G z _ F 8 1 
 
 v/ _ V _i_ v _ _ v 
 
 ~~ 2 X " 
 
 But 
 
 and 
 
 P* V 8 L 
 
 -JL =>|V:k *t> (157) 
 
 . X' " . fF 2 . G 8 -F 8 1 
 - F' x i F " ~? J 
 
 343. COR. 3. In the ellipse with the force in the center. 
 
 2 A RG 8 RF 2 
 
 QC 
 
 V 
 
 For here X a A and the force generally a ^j-^ (140) 
 
 f Force in ellipse at V : Force in circle at V : : T : R 
 . J Fin circle : m n at V ::F 8 :G 2 F 2 
 
 (.m n at V : m n at p : : T^T, : -r- 3 
 
 L A 
 
 F 2 RG 8 RF* 
 
 F. ,,. , -i- JC rl -, IV \J JLI A 
 in ellipse at V : m n at p : : 7^-3 . T : -r- 
 
 F 2 A 
 .*. assuming F in ellipse at P = f^j- > we have 
 
 F in ellipse at V = ^ X T 
 
 and 
 
 RG 8 R F 8 
 .. m n = -T-J 
 
 .-. X' a X + m n o , &c. 
 
 OTHERWISE. 
 
 . P 8 V 8 4 (Area of Ellipse) 
 
 344. X = p P, and -- = s - 7^5 . ,., 
 
 g g (Period) 8 
 
 4 T* a 8 b 2 
 
 = /r > i\t 
 g( Period) 2
 
 BOOK I.] NEWTON'S PRINCIPIA. 353 
 
 Therefore by 336 
 
 G 2 p 2 i 
 
 X' = ^ + ^ab 2 x --pj-- X ^ 
 
 "P + - 3 | 
 
 
 RG g RF* 
 
 ) 
 / 
 
 F 2 
 345. COR. 4. Generally let X be the force at P, V ~ at V, R 
 
 radius of curvature in V, C V = T, &c. 
 
 V R G 2 V R F 2 
 
 X' OC X -f A3 
 
 A 
 
 For 
 
 /- F in orbit at V : F' in circle at V : : T : R 
 < F' :mnatV ::F 8 : G 2 F 2 
 
 (.m n at V : m n : : A 3 : T 3 
 
 VF 2 G 2 F* 
 
 .-. F in orbit at V : m n : : -1^- : V R. 
 
 .*. since by the assumption 
 
 TT T7 1 2 
 
 F in orbit at V == 
 
 VR(G 2 
 
 .-. m n = 
 and 
 
 OTHERWISE. 
 
 This may better be done after 336, where it must be observed V is not 
 the same as the indeterminate quantity V in this corollary. 
 346. COR. 5. The equation to the new orbit is (333) 
 
 2 _ _ 
 
 = 2 2/2 
 
 + (G 2 F 2 )p 
 p' belonging to the given orbit. 
 
 Ex. 1. Let the given orbit be a common parabola. 
 Then 
 
 2 G 2 rg 8 
 
 - F 2 g + (G 2 F s )f 
 and the new force is obtained from 336.
 
 254 A COMMENTARY ON [SECT. IX. 
 
 Ex. 2. Let the given orbit be any one of Cotes' Spirals, whose general 
 equation is 
 
 Then the equation of 333 becomes 
 
 which being of the same form as the former shows the locus to be similar 
 in each case to the given spiral. 
 
 This is also evident from the law of force being in each case the same 
 (see 336) viz. 
 
 P*V 8 G*-F 2 
 
 .. 
 
 3 ~ 2 S 
 
 G 
 
 Ex. 3. If the given orbit be a circle, the new one is also. 
 Ex. 4. Let the given trajectory be a straight line. 
 Here p' is constant. Therefore 
 
 G* p /2 X g* 
 
 the equation to the elliptic spiral, &c. &c. 
 
 Ex. 5. Let the given orbit be a circle with the force in its circumference. 
 Here 
 
 and we have from 333 
 
 2 Gy 
 
 = 4r 2 F 2 + (G 2 F 2 )g 2 ' 
 
 Ex. 6. Let the given orbit be an ellipse with force in the focus. 
 Here 
 
 P" = o^ 
 
 2 a g 
 
 and this gives 
 
 , G 2 b 2 g 8 
 
 P " F 1 g(2a ? )+ b s (G s - F 1 )'
 
 BOOK L] NEWTON'S PRINCIPIA. 255 
 
 347. To find the points of contrary jlesure, in the locus put 
 
 dp = 0; 
 and this gives in the case of the ellipse 
 
 b* F 2 G 2 
 
 OTHERWISE. 
 
 In passing from convex to concave towards the center, the force in the 
 locus must have changed signs. That is, at the point of contrary flexure, 
 the force equals nothing or in this same case 
 
 F* A + RG 2 RF 2 = 
 
 V A = Jr 8 X(F 2 -G*) 
 b* F 2 G 2 
 
 a ' F 2 
 
 And generally by (336) we have in the case of a contrary flexure 
 
 X' = x p2 YZ x G2 ~ F ' x _L - o 
 
 g F 8 ' S 3 = 
 
 which will give all the points of that nature in the locus. 
 
 348. To fold the points where the locus and given Trajectory intersect 
 one another. 
 
 It is clear that at such points 
 
 g = f, and d' = 2 W * 6 
 W being any integer whatever. But 
 
 * _. G 4 _ m * 
 r - f ' 
 
 2 W* 
 
 This is independent of either the Trajectory or Locus. 
 349. To Jind the number of such intersections during an entire revolution 
 ofCP. 
 
 Since & cannot be > 2 9 
 W is < m + 1 and also < m 1 
 .-. 2 W is < 2 m. 
 
 2 G 
 
 Or the number required is the greatest integer in 2 m or -p- 
 
 This is also independent of either Trajectory or Locus.
 
 25G 
 
 A COMMENTARY ON 
 
 [SECT. IX. 
 
 350. To fold the number and position of the double points of the Locus, 
 i. e. of those points where it cuts or touches itself. 
 
 Having obtained the equation to the Locus find its singular points 
 whether double, triple, &c. by the usual methods ; or more simply, 
 consider the double points which are owing to apses and pairs of equal 
 values of C P, one on one side of C V and the other on the other, thus : 
 
 The given Trajectory V W being V 
 
 symmetrical on either side of V W, let 
 W be the point in the locus correspond- 
 ing to W. Join C W and produce 
 it indefinitely both ways. Then it is 
 clear that W is an apse,* also that the 
 angle subtended by V v' x' W is 
 
 /-i 
 
 the greatest whole number in - 
 
 G 
 
 w being 
 
 a'\ 
 
 supposes the motion to be in consequentia). Hence it appears that where- 
 ever the Locus cuts the line C W' there is a double point or an apse, and 
 
 also that there are w + 1 such points. 
 
 f~\ 
 Ex. 1. Let -=;- = 2 ; i. e. let the orbit move in conse- 
 
 K 
 
 quentia with a velocity = the velocity of C P. Then L. 
 V C y 7 = 0, y' coincides with V, and the double points 
 are y' V, x' and W. 1 
 
 The course of the Locus is indicated by the order of 
 the figures 1, 2, 3, 4. 
 
 Ex. 2. Let ^ = 3- 
 
 Then the Locus resembles this figure, I, 2, 3, 
 4, 5, 6. showing the course of the curve in which 
 V, x 7 , A, W are double points and also apses. 
 
 Ex. 3. Let -p = 4. 
 
 Then this figure sufficiently traces the Locus. 
 Its five double points, viz. V, x', A, B, W are 
 also apses. 
 
 Higher integer values of -^ will give the Locus
 
 BOOK I.I 
 
 NEWTON'S PRINCIPIA. 
 
 257 
 
 *~\ 
 still more complicated. If -p, be not integer, the 
 
 figure will be as in the first of this article, the 
 
 double points lying out of the line C V. More- 
 f-\ 
 
 over if -^ be less than 1, or if the orbit move in 
 c 
 
 antecedentia this method must be somewhat 
 varied, but not greatly. These and other curio- 
 sities hence deducible, we leave to the student. 
 
 351. To investigate the motion of (p) when the 
 ellipse, the force being in the focus, moves in ante- 
 cedentia with a velocity = velocity of C P in 
 consequentia. 
 
 Since hi this case 
 
 G = 
 .-. (333) also 
 
 p = 
 
 or the Locus is the straight line C V. 
 Also (342) 
 
 p r F 2 R_F'v 
 
 - - 
 
 = A* X 
 
 R 
 
 Hence 
 
 vdvoc X'dgoc 
 
 
 ex 
 
 R 
 
 . , axis maior , , , , 
 
 (where jj" = ^ an ^ Stops n 
 
 2g 1+e 2 A 2 = 0, 
 or when 
 
 g = 1 + e. 
 
 Hence then the body moves in a straight line C V, the force increasing 
 3 
 to of the latus-rectum from the center, when it = max. Then it 
 
 T) 
 
 decreases until the distance = or R. Here the centrifugal force pre- 
 
 if 
 
 vails, but the velocity being then = max. the body goes forward till the 
 
 VOL. I. R
 
 258 A COMMENTARY ON [SECT. IX. 
 
 distance = the least distance when v = 0, and afterwards it is repelled 
 and so on in inh'nitum. 
 
 352. To Jind when the velocity in the Locus = max. or min. 
 Since in either case 
 
 d.v 2 = 2vdv = 
 and 
 
 v d v = X' d ? 
 .-. X' = 
 .-. (336) 
 
 pa y 2 Q2 F 2 1 
 
 x + ~~jT~ "F 2 " x f 3 = 
 Ex. In the ellipse with the force in the focus, we have (342) 
 
 r + RGt I7 R] 
 
 El j R G 2 RF 2 
 
 '; e* " f 3 
 
 .-. f = R x 
 
 : a 
 
 F 2 
 *_G 2 
 
 b 2 L 
 
 If G = 0, v = max. when e = ^- or when P is at the extre- 
 
 a 2 
 
 mity of the latus-rectum. 
 
 If F = 2 G, v = max. when $ = R . - -^f-r~ 
 
 lat. rectum. 
 
 353. To find when the force X' in the Locus max. or min. 
 Put d X' = 0, which gives (see 336) 
 
 _ 3 P 2 V 2 G 2 F 2 1 
 ~g~ F 2 ' ?* 
 
 Ex. In the ellipse 
 
 and (157) 
 
 2F 2 dg 3RGdg 3RF 2 dg 
 
 g3 ,4 
 
 which gives 
 
 3 R F 2 G 8
 
 BOOK I.] NEWTON'S PRINCIPIA. 
 
 Hence when 
 
 G = 
 
 Xi 3 R 
 
 = max. when f = =-- . 
 
 IE 
 
 When = R, and G = 0. Then 
 
 Y *L 2 R F ' 
 
 = R 2 "^T" 
 
 When F = 2 G, or the ellipse moves in consequentia with the velo- 
 city of C p ; then 
 
 X = max. when 
 
 3JI 4 G 2 G 2 j) 
 
 2 ' ~ 4G 2 : 8 
 
 354. COR. 6. Since the given trajectory is a straight line and the center 
 of force C not in it, this force cannot act at all upon the body, or (336) 
 
 X = 0. 
 Hence in this case 
 
 p* V 2 G 2 F 2 1 
 
 x - v v . 
 
 -*- ^ /> 4 
 
 fif r 6 
 
 where P =r C V and V the given uniform velocity along V P. 
 In this case the Locus is found as in 346. 
 
 355. If the given Trajectory is a circle, it is clear that the Locus of p 
 is likewise a circle, the radius-vector being in both cases invariable. 
 
 356. PKOP. XLV. The orbits (round the same center of force) acquire 
 the same form, if the centripetal forces by which they are described at equal 
 altitudes be rendered proportional.] 
 
 Let f and f be two forces, then if at all equal altitudes 
 
 f a f 
 
 the orbits are of the same form. 
 For (46) 
 
 f ? 
 
 f a T-J a T-I a 
 
 dt 2 * dt S P 2 x QT* 
 
 1 1 
 
 a 
 
 QT 2 SP 2 x d 
 1 
 
 * d 6 2 ' 
 
 J_ _L 
 
 '* d 6 2 a d 6' " 
 and 
 
 d 6 a d (/. 
 
 R 2
 
 260 A COMMENTARY ON [SECT. IX. 
 
 But they begin together and therefore 
 
 6 a tf 
 and 
 
 S = (- 
 Hence it is clear the orbits have the same form, and hence is also sug- 
 
 gested the necessity for making the angles 6, tf proportional. 
 
 /-< 
 Hence then X 7 , and X being given, we can find -~- such as shall ren- 
 
 der the Trajectory traced by p, very nearly a circle. This is done ap- 
 proximately by considering the given fixed orbit nearly a circle, and 
 equating as in 336. 
 
 357. Ex. 1. To Jind the angle between the apsides when X' is constant. 
 
 In this case (342) 
 
 Y , . g 3 F 2 e + RG 2 RF 2 
 X' a 1 a -4 a - - 
 
 g 3 3 
 
 Now making g = T x, where x is indefinitely diminishable, and 
 equating, we have 
 
 (T x) 3 = F 2 T F 2 x + RG 2 RF 2 
 
 = T 3 3T 2 x + 3Tx 2 x 3 
 and equating homologous terms (6) 
 
 T 3 =F 2 T+RG 2 RF 2 =F 2 x (T 
 and 
 
 F 2 = 3 T 2 
 
 G_ 2 T 3 T R 
 ' F 2 ~ R F 2 R 
 
 T 3 T R 
 
 ~ 3 RT 2 R 
 
 T T R _ 3 R 2T 
 3 R R 3 R 
 
 = nearly 
 
 since R is = T nearly. 
 
 Hence when F = 180 = 
 
 = G = - 
 
 the angle between the apsides of the Locus in which the force is constant. 
 358. Ex. 2. Let X 7 a g n ~ a . Then as before 
 
 (T x) n = F 2 (T x) + RG 2 RF 2 
 and expanding and equating homologous terms 
 
 T n_ F 2 T + RG RF*
 
 BOOK I.] NEWTON'S PRINCIPIA. 261 
 
 and 
 
 But since T nearly = R 
 
 T&-1 _. Q2 
 
 '* F 2 = ~n~ 
 
 and when F = <x 
 
 y = G = w 
 
 Thus when n 3 = 1, we have 
 
 y = - = = 90. 
 7 V 4 2 
 
 When n 3 = 1, n = 2, and 7 = ^ = 127. 16'. 45". 
 
 When n 3 = y,n = 4 , and 7 = 2 cr = 360. 
 
 4 4 
 
 359. Let- X' a b e m c g n . Then 
 
 b.(T x) m + c(T x) n = F 2 .(T x)+ R.(G 2 F 2 ) 
 and expanding and equating homologous terms we get 
 
 bT m + cT n = F 2 (T R) + R G 2 
 and 
 
 bm T m ~ 1 + cn T"- 1 = F 2 . 
 But R being nearly = T, we have 
 
 b T- 1 + cT"- 1 = G 2 
 m G_ 2 _ bT 1 "- 1 + c T n - } b T ro + cT n 
 
 which is more simply expressed by putting T = 1. Then we have 
 
 G_ 8 _ b + c 
 
 Y z ~ mb + nc 
 and when F = it 
 
 b + c 
 
 360. COR. 1. Given the L. between the apsides to Jind the force, 
 Let n : m : : 360 : 2 7 
 
 : : 180 = v : 7 
 
 m 
 
 .'. 7 = r 
 n 
 
 ButifX'oc gp- 3 
 
 V 
 
 7 = 
 R3
 
 262 A COMMENTARY ON [SECT. IX. 
 
 n2 
 
 .'. X 7 a ^Ts 
 
 Ex. 1. If n : m : : 1 : 1, 
 
 as in the ellipse about the focus. 
 
 2. If n : m : : 363 : 360 
 
 X'oc 
 
 3. Ifn : m : : 1 : 2 
 
 And so on. 
 
 Again if X' 5 
 
 and the body having reached one apse can never reach another. 
 
 1 
 
 IfX'oc 
 
 -q 
 
 .*. the body never reaches another apse, and since the centrifugal force 
 a ; , if the body depart from an apse and centrifugal force be > centri- 
 petal force, then centrifugal is always > centripetal force and the body 
 will continue to ascend in infmitum. 
 
 Again if at an apse the centrifugal be < the centripetal force, the centri- 
 fugal is aWays < centripetal force and the body will descend to the center. 
 
 The same is true if X' a and in all these cases, if 
 
 centrifugal = centripetal 
 the body describes a circle. 
 
 361. COR. 2. First let us compare the force -j- z c A, belonging to 
 
 A. 
 
 the moon's orbit, with 
 
 F_* R G 2 R F 2 
 A 2 + A 3 
 
 Since the moon's apse proceeds, (n m) is positive.
 
 BOOK I.] NEWTON'S PRINCIP1A. 263 
 
 1 
 A 
 
 .*. c A does not correspond to n m and .. -- 2 does not correspond 
 
 F 2 
 
 Now 
 
 A c A 4 b A m c A 
 
 .-. X' a A~T^ J a A 
 
 1 4c _ F 
 ' i_2 = G 2 
 RG 2 RF 2 _ 1 4c 3cR 
 
 A 3 A 3 A 3 
 
 F* 1 4c , 1 
 
 ..-, = -,- and not -p 
 
 and 
 
 3c R 
 
 Hence also 
 
 * ~ C 
 
 7 = 
 
 . &c. &c. &c. 
 
 . 
 1 4 c. 
 
 362. To determine the angle between the apsides generally. 
 Let 
 
 f ( A) meaning any function whatever of A. Then for Trajectories which 
 are nearly circular, put 
 
 f(A) F 2 A + R.(G 2 F ) 
 A 3 A s 
 
 .-. f. A = F 2 A + R(G 2 F 2 ) 
 or 
 
 f.(T x) =; F 2 (T x) + R(G 2 F 2 ) 
 But expanding f (T x) by Maclaurin's Theorem (32) 
 
 u = f (T x) =U U x x + IT" & c - 
 
 iS 
 
 TL U' &c. being the values of u. -T . -, 5 &c. 
 
 d x d x 2 
 
 when x = 0, and therefore independent of x. Hence comparing 
 homologous terms (6) we have 
 
 U = F 2 T -f R (G 2 F 2 ) 
 
 U' = F 2 
 
 E4
 
 264, 
 
 A COMMENTARY ON 
 
 [SECT. IX. 
 
 Also since R = T nearly 
 U = TG 2 
 G 2 U 
 
 F 2 ~" T.U' 
 Hence when F = T, the angle between the apsides is 
 
 / U " 
 *VT7TJ' 
 
 or 
 
 ,U 
 
 * N' U' 
 making T = 1. 
 
 Ex. 1. Let f (A) = b A m + c A n = u 
 Then 
 
 -3 = mbA m - 1 + nc A 11 - 1 , 
 d x 
 
 Hence since A = T when x = 
 
 U'= mbT" 1 - 1 + nc T 11 - 1 
 GJ _ bT m + cT n 
 "'* F 2 *~ mbT ra +ncT n 
 
 or 
 
 and 
 
 G_ 2 b+ c 
 
 F 2 ~ m b -f n c 
 
 / b+ c 
 
 y = * / 1 
 
 V m b + n c 
 
 as in 359. 
 
 Ex. 2. Let f . (A) = b A m + c A n + e A r + 
 
 and 
 
 .-. U = bT m +cT"+eT r + &c. 
 
 
 
 T X U 7 = m b T ra + n c T n + r e T r + &c. 
 . G^ _ b T m + c T n + e T r + &c. 
 **' F 2 ~mbT m +ncT n +reT r +&c. 
 
 or 
 
 c+e+f+&c. 
 
 when T = 1. 
 Also 
 
 &c. 
 
 7 = 
 
 b + c + e 
 
 in b + n c -J- r e + 
 
 (D 
 
 (2) 
 
 &c.
 
 BOOK I,] NEWTON'S PRINCIPIA. 265 
 
 Ex.3. Let -^ = a A = u. 
 
 Here (17) 
 
 du 
 
 d. x ^ 
 
 Hence 
 
 U = T 2 a T x (3 + Tla) 
 T X U' = T 3 a T (3 + Tla) 
 G 2 1 
 
 F 2 = T X (3 + T 1 a) 
 and when T = 1 
 
 G 2 1 
 
 F 2 " 3 +la 
 
 y - t I 1 
 
 V3 + la 
 
 Hence if a = e the hyperbolic base, since 1 e = 1, we have 
 
 Ex.4. Letf(A) = e A = u. 
 Then 
 
 d u 
 
 - fk ^ 
 
 dx- 
 
 .-. U = e T 
 and 
 
 T.U' = Te T 
 
 G_ 2 J_ 
 ' p 2 =: T 
 .'. y = v. 
 
 Ex. 5. Let ^J- = sin. A. 
 
 u = f (A) = A 3 sin. A 
 .% U = T 3 sin. T 
 and 
 
 3-? = 3 A 2 sin. A + A 3 cos. A 
 d x 
 
 .-. TU / z=3T 3 sin.T + T 4 cos.T 
 
 . 1 _ sin. T 
 
 ' F 2 "~ 3 sin. T + T cos. T 
 
 sin. T 
 
 Tcos.T*
 
 266 A COMMENTARY ON [SECT. IX. 
 
 = . Then 
 4 
 
 7 = 
 
 363. To prove that 
 
 bA m +cA n 1 mb + nc a 
 
 i j 
 b+c 
 
 = b.(l x) m + c.(l x) n 
 = b + c (m b + n c) x + &c. 
 
 1 / mb+nc Q \ 
 
 = vr~i 1 ! i x + &c. ) 
 
 b + c\ b + c / 
 
 1 mb+ n c 
 
 = u-7 X(l X) 
 
 b + c v 
 
 m b +_n_c 
 . A 
 
 "b + c 
 
 364. To Jind the apsides when the excentricity is infinitely great. 
 
 Make 
 
 2 q : V (n + 1) : : velocity in the curve : velocity in the circle of the 
 same distance a. 
 
 Then (306) it easily appears that when F n 
 
 n + 3 
 ' 
 
 and 
 
 gives the equation to the apsides, viz. 
 
 (a + i ? n + 1 )^ 2 q 2 a n + J (a 2 g 2 ) = 
 whose roots are 
 
 a (and a when n is odd) and a positive and negative quantity (and when 
 n is odd another negative quantity). 
 Now when q = 
 
 ( a n + l_gn + l) 2 = 
 
 two of whose roots are 0, 0, and the roots above-mentioned consequently 
 arise from q, which will be very small when q is. 
 Again since 
 
 when q and g are both very small 
 
 ?'
 
 BOOK I.] NEWTON'S PRINCIPIA. 267 
 
 and 
 
 = + a q. 
 
 /. the lower apsidal distance is a q. 
 A nearer approximation is 
 
 Hence 
 
 n + 3 
 
 - qa 2 
 
 f V(f 2 a*q 2 + e) X Q 
 
 where /3 contains q 4 &c. &c., and this must be integrated from g = b to 
 
 I = a (b = a q). 
 
 But since in the variation of from b to c, Q may be considered con- 
 stant, we get 
 
 a c 
 
 6 = sec. - '. -*- + C = sec. - l . . 
 aq a q 
 
 and 
 
 ir 3v 5* ,. 
 
 7 = --, -g- , , &c. ultimately 
 
 the apsidal distances required. 
 
 Next let 
 
 Then again, make 
 
 v : v in a circle of the same distance : : q V 2 : V (n 1) 
 and we get (306) 
 
 o V a n-l s 3-n 
 
 and for the apsidal distances 
 
 n-l 
 
 _ 
 
 which gives (n > 1 and < 3) 
 
 2 
 
 f = a q 3 n' 
 Hence 
 
 =/; 
 
 ,3-.n_ q 2 a 3-.n + ) x Q 
 
 l r a qd^ 
 
 VQ V . v7? 3 ~ n o 2 a 3 - n 
 
 _ q a
 
 268 A COMMENTARY ON SECT. IX. 
 
 and 
 
 3 n 
 
 2 c 2 v 3 
 
 7 = 7i sec. - 
 
 ' 
 
 3^n 3_n-3_ n - 3 _ n > 
 
 qa 2 
 
 Hence, the orbit being indefinitely excentric, when 
 
 It 
 
 F g . ... we have . . . . 7 = 
 
 1 
 
 for 
 
 F 
 
 any number 
 
 7 = 
 
 FaJL . . . 7= * 
 
 g 2 
 
 Fa _J ^^x^ 
 
 g number between 1 and 2 y 2 * 
 
 F a pt?3 7>*- 
 
 But by the principles of this 9th Section when the excentricity is inde- 
 finitely small, and F cc n 
 
 7 = 
 
 (see 358), and when 
 
 V (n + 3) 
 
 7 " -v/ (3 n) 
 Wherefore when n is > 1 
 
 7 increases as the excentricity from 
 
 to -^ . 
 
 -v/ (3 + n) 2 
 When F oc g 
 
 y =r Is the same for all excentricities. 
 
 IB 
 
 When F a g - "? 
 y decreases as the excentricity increases from 
 
 ir * 
 
 V(3 n) to 2" 
 
 which is also true for F . 
 
 g
 
 BOOK i.j NEWTON'S PRINCIPIA. 269 
 
 When Fot__^_ 2 
 y decreases as the excentricity increases from 
 
 K It 
 
 to 
 
 V(3 n) w S iT 
 When F -\ 
 
 When F a 
 
 7 increases with the excentricity from 
 
 to 
 
 V (3 n) 3 n* 
 If the above concise view of the method of finding the apsides in this 
 particular case, the opposite of the one in the text, should prove obscure ; 
 the student is referred to the original paper from which it is drawn, viz. 
 a very able one in the Cambridge Philosophical Transactions, Vol. I, 
 Part I, p. 179, by Mr. Whewell. 
 
 365. We shall terminate our remarks upon this Section by a brief dis- 
 cussion of the general apsidal equations, or rather a recapitulation of re- 
 sults the details being developed in Leybourne's Mathematical Repository, 
 by Mr. Dawson of Sedburgh. 
 
 It will have been seen that the equation of the apsides is of the form 
 
 x n Ax m B = (1) 
 
 the equation of Limits to which is (see Wood's Algeb.) 
 
 nx n-i mAx m - 1 = (2) 
 
 and gives 
 
 /m A \ n 
 
 x = (IT A ) 
 
 i 
 
 m 
 
 If n and m are even and A positive, <; has two values, and the number 
 of real roots cannot exceed 4 in that case. 
 
 Multiply (1) by n and (2) by x and then we have 
 
 (m n)Ax m nB = 
 which gives 
 
 and this will give two other limits if A, B be positive and m even ; and if 
 (1) have two real roots they must each = x.
 
 270 A COMMENTARY ON [SECT. X. 
 
 If m, n be even and B, A positive, there will be two pairs of equal roots. 
 Make them so and we get 
 
 (m-n) A ./.). B .-. = 
 
 n n - m \m/ 
 
 which will give the number of real roots. 
 
 (1). If n be even and B positive there are two real roots. 
 
 (2). If n be even, m odd, and B negative and (M), the coefficient to 
 A n , negative, there are two ; otherwise none. . 
 
 (3). If n, m, be even, A, B, negative, there are no real roots. 
 
 (4). If m, n be even, B negative, and A positive, and (M) positive there 
 are four real roots ; otherwise none. 
 
 (5). If m, n be odd, and (M) positive there will be three or one real. 
 
 (6). If m be even, n odd, and A, B have the same sign, there will be 
 but one. 
 
 (7). If m be even, n odd, and A, B have different signs, and M's sign 
 differs from B's, there will be three or only one. 
 
 (8). If 
 
 x n + An ra B = 
 then 
 
 n 
 
 is positive, and it must be > B, and the whole must be positive. 
 If 
 
 x n Ax m + B = 
 the result is negative. 
 
 SECTION X. 
 
 366. PROP. XL VI. The shortest line that can be drawn to a plane 
 from a given point is the perpendicular let fall upon it. For since 
 Q C S = right , any line Q S which subtends it must be > than either 
 of the others in the same triangle, or S C is <C than any other S C. 
 
 A familiar application of this proposition is this: 
 
 367. Let S Q be a sling with a body Q at the end ofit^ and by the hand 
 S lei it be whirled so as to describe a right cone whose altitude is S C, and 
 base the circle whose radius is Q C; required the time of a revolution. 
 
 Let S C = h, S Q = 1, Q C = r = VI* h 2 .
 
 BOOK I.] NEWTON'S PRINCIPIA. 271 
 
 Then if F denote the resolved part of the tension S Q in the direction 
 Q C, or that part which would cause the body to describe the circle P Q, 
 and gravity be denoted by 1, we have 
 F : l : : r : h 
 
 But by 134, or Prop. IV, 
 
 4< T 2 V r r 
 
 Fx P* = * x =-*- x P 
 
 the time required. 
 
 If the time of revolution (P) be observed, then h may be hence obtained. 
 
 If a body were to revolve round a circle in a paraboloidal surface, whose 
 axis is vertical, then the reaction of the surface in the direction of the 
 normal will correspond to the tension of the string, and the subnormal, 
 which is constant, will represent h. Consequently the times of all such 
 revolutions is constant for every such circle. 
 
 368. PROP. XLVII. When the excentricity of the ellipse is indefi- 
 nitely diminished it becomes a straight line in the limit, &c. &c. &c. 
 
 369. SCHOLIUM. In these cases it is sufficient to consider the motion 
 in the generating curves.] 
 
 Since the surface is supposed perfectly smooth, whilst the body moves 
 through the generating curve, the surface, always in contact with the 
 body, may revolve about the axis of the curve with any velocity whatever, 
 without deranging in the least the motion of the body ; and thus by ad- 
 justing the angular velocity of the surface, the body may be made to trace 
 any proposed path on the surface. 
 
 If the surface were not perfectly smooth the friction would give the 
 body a tangential velocity, and thence a centrifugal force, which would 
 cause a departure from both the curve and surface, unless opposed by 
 their material ; and even then in consequence of the resolved pressure a 
 rise or fall in the surface. 
 
 Hence it is clear that the time of describing any portion of a path in a 
 surface of revolution, is equal to the time of describing the corresponding 
 portion of the generating curve. 
 
 Thus when the force is in the center of a sphere, and whilst this force 
 causes the body to describe a fixed great-circle, the sphere itself revolves 
 with a uniform angular velocity, the path described by the body on the 
 
 surface of the sphere will be the Spiral of Pappus. 
 
 . 

 
 272 
 
 A COMMENTARY ON 
 
 [SECT, X. 
 
 370. PROP. XL VIII and XLIX. In the Epicycloid and Hypocycloid, 
 
 s: 2vers. -:: 
 
 SHI 
 
 : R 
 
 where s is any arc of the curve, s' the corresponding one of the wheel, and R 
 the radius of the globe and r that of the wheel, the + sign being used for 
 the former and in the Hypocycloid. (See Jesuits' notes.) 
 
 OTHERWISE. 
 
 If p be the perpendicular let fall from C upon the tangent V P, we 
 have from similar triangles in the Epicycloid and Hypocycloid 
 
 PY: CB:: VY: VC 
 
 or 
 
 g 2 p 2 :R 2 :: (R + 2r) 2 p 2 : (R + 2r) 2 
 which gives 
 
 n a _ /p j- o r \e v, g 2 R 8 /l\ 
 
 - V 1V JL r V * /n u. OT\ t T? 2 v 1 / 
 
 (2) 
 
 Now from the incremental figure of a curve we have generally 
 d s 
 
 But 
 
 R 
 
 R 
 
 ,.ds = 
 
 . 
 = X 
 
 and integrathig from 
 
 s = 0, when = R 
 we get 
 
 V (R + 2r) 2 
 
 which is easily transformed to the proportion enunciated. 
 
 The subsequent propositions of this section shall now be headed by a 
 succinct view of the analytical method of treating the same subject 
 
 371. Generally, A body being constrained to move along a given curve by 
 known forces, required its velocity. 
 
 Let the body P move along the curve 
 P A, referred to the coordinates x, y 
 originating in A ; and let the forces be 
 resolved into others which shall act 
 parallel to x, y and call the respective 
 aggregates X, Y. Besides these we 
 have to consider the reaction (R) of the
 
 BOOK I.] NEWTON'S PRINCIPIA. 273 
 
 curve along the normal P K, which being resolved into the same .direc- 
 tions gives (d s, being the element of the curve) 
 
 dx i T> dy 
 R -j , and R -v* 
 d s d s 
 
 Hence the whole forces along x and y are (see 46) 
 d s x _ Y i "R d y~ 
 
 TTT ** T "> TT 
 
 dt 2 L "ds 
 
 Again, eliminating R, we get 
 
 and 
 
 But 
 
 ..v'=2/(Xdx + Ydy) ...... (1) 
 
 Hence it appears that The velocity is independent of the reaction of the 
 curve. 
 
 372. If the force be constant and in parallel lines, such as gravity, and 
 x be vertical ; then 
 
 X = -g 
 and 
 
 Y = 
 and we have 
 
 v 2 = 2/ gdx 
 = 2g(c x) 
 = 2g(h-x) 
 
 h being the value of x, when v = ; and the height from which it begins to 
 fall. 
 
 373. To determine the motion in a common cycloid, when the force is gravity. 
 The equation to the curve A P is 
 
 f2r x 
 
 x 
 
 r being the radius of the generating circle. 
 
 /2r 
 
 .*. ds =r dx . / - 
 -V x 
 
 VOL. I. S
 
 A COMMENTARY ON [SECT. X. 
 
 and 
 
 r.V(h X) <V g V(hx X*) 
 
 t being = 0, when x = h. 
 
 Hence the whole time of descent to the lowest point is 
 
 T r 
 
 ~o~ = * *J 
 
 ^ g 
 
 which also gives the time of an oscillation. 
 
 374. Required the time of an oscillation in a small circular arc. 
 Here 
 
 y = V(2rx x 2 ) 
 r being the radius of the circle, and 
 r d x 
 
 d - 
 
 = 
 
 (2 r x x 2 ) 
 2g V(h x) 
 
 V 2g V [h x) (2 r x 
 
 r dx 
 ^f 
 
 f(hx x 2 )(2r x)} 
 to integrate which, put 
 
 / x 
 6 = sin. ~ x / v- , 
 
 'V h 
 
 d x 
 
 /. d 6 = 
 
 2 V Oix x 8 ) 
 
 and since 
 
 Vx 
 h 
 
 x = h sin. * d, 2 r x = 2r h sin. 2 6 
 
 = 2r (1 a 2 sin. 2 0), 3 2 being put=g- % 
 .-. d t = / X y /'i_a'2 s i n 2^) ' 
 
 ON * 
 
 Now since the circular arc is small, h is small ; and therefore 3 is so. 
 And by expanding the denominator we get
 
 BOOK I.] NEWTON'S PRINCIPIA. 275 
 
 and integrating by parts or by the formula 
 
 .,,.. 1 '. m I/,, 
 
 / cl d . sin. m = cos. 6 sin. m ~ J 6 4- fa 6 sin. m ~ z 6 
 
 m m 
 
 and taking it from 
 
 6 to 8 = 
 we get 
 
 f. d d sin. m 6 z= m 1 f d 6 sin. m ~ 2 
 m 
 
 the accented^ denoting the Definite Integration from = 0, to 6= * . 
 In like manner 
 
 // Q v SID* """" 8 /.dp S1H* 9 
 
 m 2 J ' 
 and so on to 
 
 Hence 
 
 (ra 1) (m 3) 1 
 
 m 
 
 and 
 
 V (1 5 2 sin. 
 
 /~7~71 .o .. p . from 
 v ( 1 5 2 sin. 2 ^ 
 
 is the same as 
 
 ^ (l_3 2 sin. 2 0) from " 2 
 0= 
 whence then 
 
 and taking the first term only as an approximate value 
 
 which equals the time down a cycloidal arc whose radius is -7-. 
 If we take two terms we have 
 
 1 = TV 
 
 T" 
 
 S2
 
 276 A COMMENTARY ON [SECT. X. 
 
 375. To determine the velocity and time in a Hypocycloid, the force 
 tending to the center of the globe and . 
 
 By (370) 
 the equation to the Hypocycloid is 
 
 -R' D 2 
 
 by hypothesis. 
 
 Now calling the force tending to the center F, we have 
 
 X= F x -,Y = Fx y - 
 
 .-.v 2 = C 2/Fd f .......... (1) 
 
 But by the supposition 
 
 F =i* s 
 ...v s = A*(h 2 -^) .......... (2) 
 
 Hence 
 
 ds _ V R 2 D' _ gdg 
 
 :: : 
 
 To integrate it, put 
 2 D 2 = u 2 
 
 h f* = h 2 D 2 u 2 
 and 
 
 dt=- v r- p ' - du 
 
 ; V fjt, V (h 2 D 2 u 2 ) 
 
 V(R 2 D 2 ) /g 2 D 8 
 
 i - * i p/^c ^* / g 
 
 RV/t Vh 2 D 2 * 
 
 Hence making g D, we have 
 Oscill. 
 
 ~ 2V R 2 ^ 
 
 2 
 
 376. Since h does not enter the above expression the descents are 
 Isochronous. 
 
 We also have it in another form, viz. 
 
 T //_r_ r 2 x 
 
 2 - w \n^~Tni)'
 
 BOOK I.] 
 
 NEWTON'S PRINCIPIA. 
 
 277 
 
 If R p = g or force of gravity and R be large compared with b, 
 T r 
 
 the same^as in the common cycloid. 
 
 377. Required to Jind the value of the reaction R, when a body is con- 
 strained to move along a given curve. 
 
 As before (46) 
 
 d ' x _ Y , R d y 
 
 ~j"7 AT** 3 
 
 dt 2 dx 
 
 d2 y_ v p dx 
 
 j ^ - = 1C rt -3 . 
 d t 2 ds 
 
 Hence 
 
 dyd 2 x dxd z y VJ T7 . , 
 
 -2 - TJ - = Xdy Ydx+Rds 
 
 p Xdy Ydx , dyd 2 x dxdy 
 
 ~~d7~ dt 2 ds 
 
 But if r be the radius of curvature, we have (74) 
 
 ds 3 
 
 r = 
 
 Hence 
 
 dyd 8 x dxd s y' 
 
 n Ydx Xdy , dsj 
 ~T1~ h rdt 
 
 Another expression is 
 
 or 
 
 _ Ydx Xdy v 8 ' 
 
 Ydx Xdy 
 
 ds 
 
 (1) 
 
 (2) 
 
 <f> being the ceRtrifugal force. 
 
 If the body be acted on by gravity only 
 
 R = 
 
 or 
 
 or 
 
 gdy . 
 = j" + f 
 
 ds 
 If the body be moved by a constant force in the origin of x, y, we have 
 
 = F g d 6. 
 
 S3
 
 278 
 
 A COMMENTARY ON 
 
 [SECT. X. 
 
 for 
 
 xdy y d x = f 2 d 
 
 378. 
 cycloid. 
 
 .-. R = 
 
 h 
 
 ds 
 
 or 
 
 or 
 
 ds 
 
 Fgdd v* 
 d s r 
 
 (4) 
 
 the tension of the string in the oscillation of a common 
 
 Here 
 
 but 
 
 dy , d s 
 
 d y = d x J 
 
 d s = d x 
 
 2 a x 
 
 2a 
 
 d y 2 a x 
 
 cTs ~ V 2l~ 
 
 and 
 
 x) 
 
 2a-x + g (h - x) 
 
 2 a 
 
 
 
 V2 a V(2 a x) 
 
 2 a + h 2 x 
 
 = g ' 2 2 ax) ' 
 
 When x = h 
 
 When x = 
 
 When moreover h = 2 a, the pressure at A the lowest point is =: 2 g. 
 379. To find the tension when the body oscillates in a circular arc by 
 gravity. 
 
 R 2 a h V (2 a h) 
 
 ~ g ' V (4. a 2 2 ah) " g V (2 a) 
 
 P 2 a + h _ /, . h
 
 BOOK I.] NEWTON'S PR1NCIPIA. 279 
 
 Here 
 
 (c x) d x 
 H v zz J 
 
 7 " V(2cx x*) 
 
 c d x 
 
 d s 
 
 V (2 ex x 2 ) 
 
 d_y __ c x 
 
 d x c 
 
 r = c 
 ^J5 = 2 g (h x) 
 
 R - c ~ x + 2 g < h " *) 
 c c 
 
 c + 2 h 3 x 
 
 = g. . 
 
 When x = 
 
 c + 2h 
 
 JL v ~* 
 
 c 
 
 3 g or h = c. 
 If it fall through the whole semicircle from the highest point 
 
 h = 2 c, 
 and 
 
 R = 5g 
 
 or the tension at the lowest point is five times the weight. 
 When this tension = 0, 
 
 c + 2 h 3 x = 0, or x = C + Q 2 - . 
 
 u 
 
 A body moving along a curve whose plane is vertical will quit it when 
 
 R = 
 that is when 
 
 c + 2h 
 
 v 
 
 and then proceed to describe a parabola. 
 
 / 380. To Jlnd the motion of a body upon a surface of revolution^ when 
 
 acted on by forces in a plane passing through the axis. 
 
 Referring the surface to three rectangular axes x, y, z, one of which (z) 
 is the axis of revolution, another is also situated in the plane of forces, and 
 the third perpendicular to the other two. 
 
 Let the forces which act in the plane be resolved into two, one parallel 
 to the axis of revolution Z, and the other F, into the direction of the 
 radius-vector, projected upon the plane perpendicular to this axis. Then, 
 
 S4,
 
 280 
 
 A COMMENTARY ON 
 
 [SECT. X. 
 
 calling this projected radius f, and resolving the reaction R (which also 
 takes place in the same plane as the forces) into the same directions, these 
 components are 
 
 supposing ds:V'(dz 
 off' is 
 
 ds 
 
 d~I 
 
 f 2 ) and the whole force in the direction 
 
 cU 
 
 ds 
 
 and resolving this again parallel to x and y, we have 
 
 \ 
 
 F + R 
 
 t 2 
 d 2 y 
 
 - 
 
 and 
 
 d 2 z 
 
 dz 
 
 y R 
 
 = -- Z +R dT 
 
 Hence we get 
 
 x d* y y d g x __ n __ A xd y y d x 
 
 dt ! 
 
 dt 
 
 and 
 
 dxd*x+dyd 2 y+dzd*z__ -p, xdx+ y dy 
 
 dt ! 
 
 . 
 
 d s 
 
 d s 
 
 Which, since 
 
 x d x + y d y 
 
 e 
 
 Again 
 
 dz 2 _ d_z 
 dt 2 " d 
 
 (D 
 
 (2) 
 
 and from the nature of the section of the surface made by a plane passing 
 through the axis and body, -r is known in terms off. Let therefore 
 dz
 
 BOOK I.] NEWTON'S PRINCIPI A. 281 
 
 and we have 
 
 d_z_ 2 2 dg 2 
 d t 2 " P dt 2 ' 
 Also let the angle corresponding to g be 6, then 
 
 xdy ydx = 2 d 
 and 
 
 dx 2 + dy* = dg 2 + e 2 d0 2 , 
 and substituting the equations (2) and (3) become 
 
 d. 1^1 = 
 d t 
 
 d p 2 e z d 6 Z d 
 
 Integrating the first we have 
 
 g 2 d 6 = h d t 
 
 h being the arbitrary constant. 
 or 
 
 (4) 
 
 The second can be integrated when 
 
 2 F d 2 Z d z 
 
 is integrable. Now if for F, Z, z we substitute their values in terms of & 
 the expression will become a function of and its integral will be also a 
 function of g. Let therefore 
 
 fCFdg + Zdz) = Q 
 and we get 
 
 which gives, putting for d t its value 
 
 ..UP 
 
 P 2 )hdg 
 
 -h 2 } 
 Hence also 
 
 dt = - 
 
 If the force be always parallel to the axis, we have 
 
 F = 
 and if also Z be a constant force, or if 
 
 we then have 
 
 /u\ 
 Z ~ g Z . ..... .. (Oj
 
 282 A COMMENTARY ON [SECT. X. 
 
 Z being to be expressed in terms off. 
 
 381. Tojind under what circumstances a body will describe a circle on a 
 surface of revolution. 
 
 For this purpose it must always move in a plane perpendicular to the 
 axis of revolution ; *, z will be constant; also (Prop. IV) 
 g cos. 6 = x 
 
 d 2 x g cos. 6 d d 
 
 " dT 2 " : dt 2 
 
 Also 
 
 gd 6 
 
 v - 
 
 d t 
 
 d 2 x v 2 cos. 6 
 
 *' dT 1 : ~ * 
 
 Hence as in the last art. 
 
 If the force be gravity acting vertically along z, we have 
 
 Z = g 
 v 2 d z 
 
 f **C 
 
 Hence may be found the time of revolution of a Conical Pendulum. 
 (See also 367.) 
 
 382. To determine the motion of a body moving so as not to describe a 
 circle, when acted on by gravity. 
 Here 
 
 Q = gz 
 and 
 
 C 2 Q = 2 g. (k z) 
 k being an arbitrary quantity. 
 Also 
 
 g 2 = 2 r z z* 
 z being measured from the surface. 
 
 .. g d = (r z) d z 
 and 
 
 p 2 r*
 
 BOOK I.] NEWTON'S PRINCIPIA. 283 
 
 Hence (380) 
 
 (k z}. (2 rz z s ) 
 In order that 
 
 the denominator of the above must be put =: ; i. e. 
 
 2 g (k z) (2 r z z 2 ) h 2 = 
 or 
 
 h 2 
 z '_(k + 2 r) z 2 -f 2k rz ^ = 
 
 2 
 which has two possible roots ; because as the body moves, it will reach 
 
 one highest and one lowest point, and therefore two places when 
 
 i- z -o 
 
 dt ' 
 Hence the equation has also a third root. Suppose these roots to be 
 
 a , ft 7 
 
 where a is the greatest value of z, and /3 the least, which occur during the 
 body's motion. 
 Hence 
 
 , _ __ r d z _ _ 
 
 : V (2g) V (-_ z ).(z_/3)( 7 -z)- 
 To integrate which let 
 
 Then 
 
 d z 
 
 _ dz 
 
 Also 
 
 z 
 
 sin. l 9 = 
 
 -; 24 
 
 a /3 
 
 /. z = /S + (a |3) sin. * 6 
 and 
 
 y z = 7 {(3 + (a 0) sin. * i 
 = ( 7 _ |3) U _3 2 sin. 2 ^, 
 if 
 
 'a S 
 
 3 = 
 
 f3
 
 284 A COMMENTARY ON [SECT. X. 
 
 2 rd 6 
 
 .*. a t = ===== 
 
 V2g. (y -). V[l d 2 sin.*6] 
 
 which is to be integrated from z = (8, to z = a ; that is from 
 6 = to d = ~ 
 
 this expanded hi the same way as in 374 gives 
 
 2r 
 
 t = 
 
 V2g( 7 i 
 
 which is the time of a whole oscillation from the least to the greatest 
 distance. 
 Also 
 
 h d t h d t 
 
 d 6 = = 
 
 2rz z 2 
 and 6 is hence known in terms of z. 
 
 383. A body acted on by gravity moves on a surface of revolution whose 
 axis is vertical : when its path is nearly circular, it is required to find the 
 angle between the apsides of the path projected in the plane qfx, y. 
 
 In this case 
 
 /Zdz = gz = Q 
 and if at an apse 
 
 e = a, z = k 
 we have 
 (C 2gk)a t h 2 = 
 
 Hence (380) 
 
 V(l + P 8 )hdg 
 
 x h 2 (a 2 
 
 -~ z)f " 
 
 + 
 
 \ 
 h d 
 
 Let L = _L+ 
 
 e a 
 
 V (1 + p 2 ) hd
 
 BOOK I.] NEWTON'S PRINCIPIA. 285 
 
 2g(k z) h(i(V) 
 z 9 w 
 
 dO 2 h 2 (l + p a ) 
 
 It is requisite to express the right-hand side of this equation in terms 
 of 
 
 Now since at an apse we have 
 
 = 0, z = k, and g = a 
 we have generally 
 
 dz d 2 zw 2 
 
 the values of the differential coefficients being taken for 
 
 = (see 32) 
 And 
 
 d z = p d = p * d 
 d 2 z = 2 p g d d g * d <w d p 
 or, making 
 
 d p = q d f 
 
 And if p/ and q, be the values which p and q assume when u = 0, 
 $ = a, we have for that case, 
 
 ~ s = (2 P/ + q,a) a 3 
 
 Z = k p, a 2 * + (2 p + q, a) a 3 . "- &c. 
 
 Also 
 
 2 u 
 
 1 /! \ 2 i 
 
 g 2 " U ' ^ ' a 2 
 Hence 
 
 U. 
 
 a 
 
 2 g ( P/ a 2 - (2 P/ + q/ a) a 3 . + &c.)- h 2 ( + W 2 ). 
 
 becomes 
 
 But when a body moves in a circle of radius = a, we have 
 
 h 2 = gg s p = ga' P/ 
 
 in this case. And when the body moves nearly in a circle, h z will have 
 nearly this value. If we put 
 
 h 2 = (1 + 3)ga 3 P/ 
 we shall finally have to put 
 
 3 =
 
 A COMMENTARY ON fSEcx.X. 
 
 in order to get the ultimate angle when the orbit becomes indefinitely near 
 a circle. Hence we may put 
 
 . h z = g a 3 p, 
 and 
 
 2g(k-z)-h(l 3 - -L) 
 
 r 
 
 becomes 
 
 - *3ga 3 p, + ga'qjo, 2 + &c. 
 
 in which the higher powers of u may be neglected in comparison of u 2 ; 
 . du * _ _ ga 3 (3 P/ + q.a)^ 2 _ (3 P/ + q/ a) * 
 d*" h*(l+p*) P,(1 + P') 
 
 (S P/ + q/ a)a, 2 
 
 P/(l +P/ 2 ) 
 
 again omitting powers above u 2 : for p = p; -j- A u + &c. 
 Differentiate and divide by 2 d , aijd we have 
 
 suppose; of which the integral is taken so that 
 
 6 = 0, when a = 
 is 
 
 u = C sin. 6 V N. 
 
 And u passes from to its greatest value, and consequently g passes 
 from the value a. to another maximum or minimum, while the arc 6 V N 
 passes from to ST. Hence, for the angle A between the apsides we have 
 
 v "" 
 
 ' r ~ VN 
 
 where 
 
 _ 3 p, + q, a 
 
 384. Let the surface be a sphere and let the path described be nearly a 
 circle ; to Jind the horizontal angle between the apsides. 
 
 Supposing the origin to be at the lowest point of the surface, we have 
 z = r - V (r 2 s 2 ) 
 d z _ 
 
 _ dp _ r 2 
 
 q= d f 
 
 * P/ =
 
 BOOK I.I NEWTON'S PRINCIPIA. 287 
 
 N _ 4 r 2 3 a 
 . r 
 
 Hence the angle between the apsides is 
 
 or r 
 
 A = 
 
 V(4r 2 3 a 2 )' 
 
 The motion of a point on a spherical surface is manifestly the same as 
 the motion of a simple pendulum or heavy body, suspended by an inex- 
 tensible string from a fixed point ; the body being considered as a point 
 and the string without weight. If the pendulum begin to move in a ver- 
 tical plane, it will go on oscillating in the same plane in the manner al- 
 ready considered. But if the pendulum have any lateral motion it will 
 go on revolving about the lowest point, and generally alternately ap- 
 proaching to it, and receding from it. By a proper adjustment of the velocity 
 and direction it may describe a circle (134) ; and if the velocity when it 
 is moving parallel to the horizon be nearly equal to the velocity in a cir- 
 cle, it will describe a curve little differing from a circle. In this case we 
 can find the angle between the greatest and least distances, by the for- 
 mula just deduced. 
 Since 
 
 A = 
 
 V (4 r * 3 a 2 ) 
 
 if a = 0, A -^y the apsides are 90 from each other, which also ap- 
 pears from observing that when the amplitude of the pendulum's revolu- 
 tion is very small, the force is nearly as the distance ; and the body de- 
 scribes ellipses nearly ; of which the lowest point is the center. 
 
 If a = r, 
 
 A = * = 180 
 
 this is when the pendulum string is horizontal ; and requires an infinite 
 velocity. 
 
 If a = - ; so that the string is inclined 30 to the vertical ; 
 
 38 
 
 A = . '- = 99 50'. 
 V 13
 
 288 A COMMENTARY ON [SECT. X. 
 
 r* 
 If a * = ; so that the string is inclined 45 to the vertical : 
 
 
 
 A = * = 113. 56'. 
 
 3 r 2 
 
 If a z = - - ; so that the string is inclined 60 to the vertical ; 
 4 
 
 A = -^ = 136 nearly. 
 
 385. Let the surface be an inverted cone, with its axis vertical : to find 
 the horizontal angle between the apsides when the orbit is nearly a circle. 
 
 Let r be the radius of the circle and 7 the angle which the slant side 
 makes with the horizon. Then 
 
 z tan. 7 
 p = tan. 7 
 
 .-.N= 
 
 tan. 7. sec. 2 7 
 and 
 
 A = 
 
 cos. 7-^3* 
 If 7 = 60 
 
 A = ^ = 120. 
 
 386. Let the surface be an inverted paraboloid whose parameter is c. 
 g 2 = c z 
 
 dz 2g 
 
 ' p : : "37 ~ 
 
 2 
 c 
 
 6 a 2 a 
 
 XT c c 4 c * 
 
 .. N = 
 
 2 a / 4 a 2 \ "~ c 2 + 4 a 2 ' 
 
 c 
 
 (' + 
 
 M 
 
 If a = ~ , or the body revolve at the extremity of the focal ordinate, 
 
 
 
 N = 2 
 and 
 
 A = V^ 2 '
 
 BOOK I ] NEWTON'S PRINCIPIA. 281) 
 
 387. When a body moves on a conical surface, acted on by a force tend- 
 ing to the vertex : its motion in the surface will be the same, as if the sur- 
 face were unwrapped, and made plane, the force remaining at the vertex. 
 
 Measuring the radius-vector (g) from the vertex, let the force be F, 
 and the angle which the slant side makes with the base = y : then 
 z = g tan. y 
 p = tan. y 
 1 + p 2 = sec. z y 
 also 
 
 Q=/(Fd f + Zdz) = 
 Hence (380) 
 
 , . _ _ sec, y h d g 
 = 
 
 or putting 
 
 h' cos. y for h 
 
 d tf sec. y for d 6 
 and 
 
 tf cos. 7 for g 
 we have 
 
 _ 
 - j V{(C 2/F 
 
 Now d tf is the differential of the angle described along the conical sur- 
 face, and it appears that the relation between & and g 7 will be the same as 
 in a plane, where a body is acted upon by a central force F. For in that 
 case we have 
 
 and integrating 
 
 h /2 <3' 2 h/2 
 
 - C 
 
 ' 
 
 which agrees with the equation just found. 
 
 388. When a body moves on a surface of revolution, to Jind the reac- 
 tion R. 
 
 Take the three original equations (380) and multiply them by x d z, 
 y d z, g d ; and the two first become 
 
 x d ' x d z _ _ Fx'dz _ R dz 2 x 2 
 d t 2 g ds ' 
 
 y d 2 y dz _ _ F / y 2 d z __ R dz 2 yj 
 
 d t 2 g ds ' g 
 
 VOL. 1. T
 
 290 A COMMENTARY ON [SECT. X. 
 
 add these, observing that 
 
 x' + y 2 = g 2 
 
 and we have 
 
 (xd'x+yd 2 y)dz _ _ __ d zj 
 
 d t 2 ' ds 
 
 Also the third is 
 
 g d g d 8 z , K d g 2 
 
 -dt~ -Zgdg + Rg-^. 
 
 Subtract this, observing that dz* + dg 2 = ds 2 , and we have 
 
 (xd 2 x -f- yd 2 y) dz g d g d 2 z 
 dt 2 
 
 g (Z d g F' d z) R g d s. 
 But 
 
 x 2 + y 2 = g 2 
 
 xdx+ydy = g d g 
 xd 2 x + yd 2 y + dx 2 +dy 2 = gd 2 g + dg 2 . 
 
 Hence 
 
 (dg 2 dx 2 dy 2 ) dz g d z d 2 g g d gd 2 z 
 dt 2 dt 2 
 
 g (Z d f F d z) R g d s 
 and 
 
 dg 2 = ds 2 dz 2 . 
 Hence 
 
 R Z d g F d z dgd 2 z dzd 2 g 
 
 ds dt 2 ds 
 
 (dx 2 + dy 2 + dz 2 ds 2 ) dz 
 
 gdt 2 ds 
 Now if r be the radius of curvature, we have (74) 
 
 ds 3 
 
 : dgd 2 z dzd 2 g 
 and 
 
 d x 2 + dy 2 + dz 2 = d tf 2 
 a being the arc described. 
 Hence 
 
 p Z d g F d z ds 2 
 
 ds h FdT* 
 
 d g 2 d s 2 (^z 
 gdt 2 'd~s 
 Here it is manifest that 
 
 d s*
 
 BOOK I.] NEWTON'S PRINCIPI A. 291 
 
 is the square of the velocity resolved into the generating curve, and that 
 
 d a- d s- 
 
 dt 2 
 
 is the square of the velocity resolved perpendicular to f. The two last 
 terms which involve these quantities, form that part of the resistance 
 which is due to the centrifugal force ; the first term is that which arises 
 from the resolved part of the forces. 
 
 From this expression we know the value of R ; for we have, as before 
 
 Also 
 
 dj,2 d s 2 _ g*dd a _ hj 
 ~~dt 8 ~~dT 2 ~ : 7~ 2 
 
 Hence 
 
 889. To find the tension of a pendulum moving in a spherical surface. 
 
 *= V (2rz z 2 ) 
 
 d o _ r z 
 
 d~z ~~ V (2rz z 2 ) 
 
 d s _ r 
 dg r z 
 
 d s r 
 
 d z ~~ V(2rz z 2 ) "~ 
 Hence 
 
 _ g (r+2k 3z) 
 
 r 
 
 and hence it is the same as that of the pendulum oscillating in a vertical 
 plane with the same velocity at the same distances. 
 
 390. Tojindtlie Velocity, Reaction, and Motion of a lody upon any 
 surface whatever. 
 
 Let R be the reaction of the surface, which is in the direction of a nor- 
 mal to it at each point. Also let *, ', i" be the angles which this normal 
 
 T2
 
 292 
 
 A COMMENTARY ON 
 
 [SECT. X. 
 
 makes with the axes of x, y, z respectively ; we shall then have, consider- 
 ing the resolved parts of R among the forces which act on the point 
 
 -3 , = X + R COS. 
 
 at- 
 
 Now the nature of the surface is expressed by an equation between 
 x, y, z : and if we suppose that we have deduced from this equation 
 dz = pdx + qdy 
 
 d z , d z 
 where p = and q = 
 
 p and q being taken on the supposition of y and x being constants respec- 
 tively ; we have for the equations to the normal of the points whose co- 
 ordinates are 
 
 x, y, z 
 
 x' x + p(z' z) = 0\ 
 y y + q(z' z) = OJ 
 
 x', y', T! being coordinates to any point in the normal (see Lacroix, 
 No. 143.) 
 
 Hence it appears that if P K be the normal, 
 P G, P H its projections on planes parallel to 
 x z, y z respectively. 
 The equation of P G is 
 
 x' x + p . (2! z) = 0, 
 and hence 
 
 GN + pPN= 
 and 
 
 GN = p.PN. 
 Similarly the equation of P H is 
 
 whence 
 
 y 
 
 And hence, 
 
 HN+ q.PN = 
 
 HN = q.PN. 
 
 irr>u 
 COS. =r COS. K P h = - 
 
 GN 
 
 NG-+
 
 BOOK I.] NEWTON'S PRINCIPIA. 
 
 P 
 
 cos. e' = cos. K P g = p-.j| 
 HN 
 
 V(PN 2 
 
 q 
 
 Whence, since 
 cos. 3 f + cos - a *' + cos - * s" = 1 
 
 cos. 2 ?" = V ( 1 cos. 2 ? cos. 2 ') 
 _ _ 1 _ 
 = V(l+p2 + q2)- 
 
 Substituting these values ; multiplying by d x, d y, d z respectively, in 
 the three equations ; and observing that 
 
 dz pdx qdy = 
 we have 
 
 and integrating 
 
 and if this can be integrated, we have the velocity. 
 
 If we take the three original equations, and multiply them respectively 
 by p, q, and 1, and then add, we obtain 
 
 d g x d 2 y d^z_ 
 
 ~ P dt 2 ~~ q *dt 2 + dt 2 ~ 
 P X qY+ Z + R V(l + p 2 +q 2 ). 
 But 
 
 d z = p d x -f- q d y. 
 Hence 
 
 d 2 z d 2 x d 2 ydpdx + dqdy 
 
 d7*- p dT 2 + q dT 2 ~ t ~~dT 2 ~~ 
 
 Substituting this on the first side of the above equation, and taking 
 the value of R, we find 
 
 _ pX-f qY Z dpdx + dqdy 
 - V(l+p 2 4 q 2 ) "*" dt 2 V(l+p 2 +q 2 ) 
 
 If in the three original equations we eliminate R, we find two second 
 differential equations, involving the known forces 
 
 X,Y, Z 
 
 T3
 
 A COMMENTARY ON [SECT. X. 
 
 and p, q, which are also known when the surface is known, combining 
 with these the equation to the surface, by which z is known in terms of 
 x, y, we have equations from which we can find the relation between the 
 time and the three coordinates. 
 
 391. To find the path which a body "will describe upon a given surface, 
 \a-hen acted itpon by no force. 
 
 In this case we must make 
 
 X, Y, Z each = 0. 
 
 Then, if we multiply the three equations of the last art. respectively by 
 
 -(qdz + dy), pdz + dx, qdx pdy 
 and add them, we find, 
 
 (qdz + dy)d 2 x + (pdz+dx) d 2 y + (qdx pdy)d 2 z 
 
 /- (q d z + d y) cos. t ~\ 
 = Rdt 2 -|+ (pdz+dx) cos. t' | 
 \.+ (qdx pdy) cos. t" J 
 or putting for cos. , cos. ', cos. i" their values 
 
 Hence, for the curve described in this case, we have 
 
 (pdz+dx)d 2 y = (pdy qd x) d 2 z+(q d z + dy) d 2 x. 
 This equation expresses a relation between x, y, z, without any regard 
 to the time. Hence, we may suppose x the independent variable, and 
 d 2 x := ; whence we have 
 
 (p d z + d x) d 2 y = (p d y q d x) d 2 z. 
 This equation, combined with 
 
 dz = pdx + qdy, 
 
 gives the curve described, where the body is left to itself, and moves along 
 the surface. 
 
 The curve thus described is the shortest line which can be drawn from 
 one of its points to another, upon the surface. 
 
 The velocity is constant as appears from the equation 
 
 v 2 = 2/(Xdx + Ydy + Zdz). 
 
 By methods somewhat similar we might determine the motion of a point 
 upon a given curve of double curvature, or such as lies not in one plane 
 when acted upon by given forces. 
 
 392. Tojlnd the curve of 'equal pressure ', or that on 'which a body descend- 
 ing bij the force of gravity, presses equally at all points.
 
 BOOK I.] 
 
 NEWTON'S PRINCIPIA. 
 
 Let A M be the vertical abscissa = x, M P the hori- 
 zontal ordinate = y ; the arc of the curve s, the time t, 
 and the radius of curvature at P = r, r being positive 
 when the curve is concave to the axis ; then R being the 
 reaction at P, we have by what has preceded. 
 
 R _g d y , ds * m 
 
 TT + Tdfi 
 
 But if H M be the height due to the velocity at P, 
 A H = h, we have 
 ds 2 
 
 dt 
 
 = 2 g (h - x). 
 
 Also, if we suppose d s constant, we have (74) 
 d s d x 
 
 
 and if the constant value of R be k, equation ( 1 ) becomes 
 k ^gdy 2g(h-x)d 2 y 
 
 d s d s d x 
 
 A/ /h ^ 2 
 
 ~ ' x ' 
 
 295 
 H 
 
 M 
 
 Ji dx A/ /h ^ d 2 y d y d x 
 
 " "g *2 V (h x)~ ' x; '"aT"~dl'2V"(h x) 
 
 The right-hand side is obviously the differential of 
 
 (h 
 
 ds 1 
 
 hence, integrating 
 k 
 
 dy _ _k_ 
 
 d s " g 
 
 V (h x) 
 If C = 0, the curve becomes a straight line inclined to the horizoi^, 
 
 which obviously answers the condition. The sine of inclination is . 
 
 o 
 In other cases the curve is found by equation (2), putting 
 
 V(dx 2 +dy 2 ) fords 
 and integrating. 
 
 If we differentiate equation (2), d s being constant, we have 
 
 y_ 
 
 Cdx 
 
 ds 
 
 2 (h x) 2 
 dsdx_ 2(h x) 
 
 And if C be positive, r is positive, and the curve is concave to the axis.
 
 296 
 
 A COMMENTARY ON 
 
 [SECT. X. 
 
 We have the curve parallel to the axis, as at C, when ^ = 0, that is, 
 
 ds 
 
 , 
 when = 
 
 g 
 
 ; when 
 
 -x) 
 x = h 
 
 When x increases beyond this, the curve approaches the axis, and -p^ 
 
 is negative ; it can never become < 1 ; hence B the limit of x is 
 found by making 
 
 i._ c _ 
 
 or 
 
 C 2 g 2 
 
 If k be < g, as the curve descends towards Z, it approximates perpe- 
 tually to the inclination, the sine of which is . 
 
 o 
 
 If k be > g there will be a point at which the curve becomes horizontal. 
 
 C is known from (2), (3), if we knew the pressure or the radius of cur- 
 vature at a given point. 
 
 If C be negative, the curve is convex to the axis. In this case the part 
 of the pressure arising from centrifugal force diminishes the part arising 
 from gravity, and k must be less than g. 
 
 393. To find the curve which cuts a given assemblage of curves, so as to 
 make them Synchronous, or descriptible ly the force of gravity in the same 
 time. 
 
 Let A P, A P', A P", &c. be curves of the 
 same kind, referred to a common base A D, 
 and differing only in their parameters, (or the 
 constants in their equations, such as the radius 
 of a circle, the axes of an ellipse, &c.) 
 
 Let the vertical A M = x, M P (horizontal) 
 = y ; y and x being connected by an equation E 
 involving a. The time down A P is 
 
 /- dx 
 
 the integral being taken between 
 
 x = and x = A M ; 
 and this must be the same for all curves, whatever (a) may be.
 
 BOOK L] NEWTON'S PRINCIPIA. 297 
 
 Hence, we may put 
 
 /ds _ , 
 ~ 
 
 k being a constant quantity, and in differentiating, we must suppose (a) 
 variable as well as x and s. 
 Let 
 
 d s = pd x 
 
 p being a function of x, and a which will be of dimensions, because d x, 
 and d s are quantities of the same dimensions. Hence 
 
 P dx _u 
 
 and differentiating 
 
 Pf X x +qda = .......... (2) 
 
 V(2gx) n 
 
 Now, since p is of dimensions in x, and a, it is easily seen that 
 
 f P dx 
 J 
 
 is a function whose dimensions in x and a are , because the dimensions 
 of an expression are increased by 1 in integrating. Hence by a known 
 property of homogeneous functions, \v e have 
 
 P* .+qa=|k; 
 V (2gx) n 
 
 p V x 
 
 2a aV(2g) 
 substituting this in equation (2) it becomes 
 
 pdx kda pda V x __ ... 
 
 V(2gx) H ' 2 a ~a V (2 g) " 
 
 in which, if we put for (a) its value in x and y, we have an equation to the 
 curve P P' F'. 
 
 If the given time (k) be that of falling down a vertical height (h), we 
 have 
 
 and hence, equation (3) becomes 
 
 p (a d x x d a) -f d a V (h x) = . . . . (4} 
 
 Ex. Let the curves A P, A P', A P" be all cycloids of which the bases 
 coincide Moith A D. 
 
 Let C D be the axis of any one of these cycloids and = 2 a, a being 
 the radius of the generating circle. If C N = x', we shall have as before 
 
 ds = dx' /'?-? 
 
 "V X r
 
 298 A COMMENTARY ON [SECT. X. 
 
 and since 
 
 x' = 2 a x 
 
 2a 
 
 ds =r dx ^ I- 
 
 2a x* 
 Hence 
 
 2a 
 
 and equation (4) becomes 
 
 V_(2a)(adx-xda) _ 
 
 V 2 a x t ' ' 
 
 Let = u 
 a 
 
 so that 
 
 adx x d a = a 2 du 
 
 x = au; 
 and substituting 
 
 a z du V 2 . 
 
 du V 2 da V h 
 
 V(2u-uV a f 
 
 . V 2 x vers. - l u 2 J - = C (6) 
 
 *l a 
 
 When a is infinite, the portion A P of the cycloid becomes a vertical 
 line, and 
 
 x = h, .-. u = 0, .-. C = 0. 
 
 Hence 
 
 Xo |-> 
 / 40 11 / i*-\ 
 
 = vers. / (7) 
 
 a % a 
 
 From this equation (a) should be eliminated by the equation to the 
 cycloid, which is 
 
 y = a vers. ~ l V (2 a x x 2 ) . . . . (8) 
 
 and we should have the equation to the curve required. 
 Substituting in (8) from (7), we have 
 
 y= V (2 ah) V(2ax x 2 ) 
 , _da\^h xda + adx xdx 
 
 ' ~V~(2a)~ V (2 ax x 2 ) 
 and eliminating d a by (5) 
 
 dj^ 2 a x . 2 a x 
 
 dx~ V(2ax x 2 ) ~ ">i x
 
 BOOK I.] 
 
 NEWTON'S PRINCIPIA. 
 
 299 
 
 But differentiating (8) supposing (a) constant, we have in the cycloid 
 x 
 
 And hence (31) the curve P P' P" cuts the cycloids all at right angles, 
 the subnormal of the former coinciding with the subtangent of the latter, 
 each being 
 
 2a x 
 
 y 
 
 ' 
 
 AOO 
 
 The curve P P' P" will meet A D in the point B, such that the given 
 time is that of describing the whole cycloid A B. It will meet the vertical 
 line in E, so that the body falls through A E in the given time. 
 
 394. If instead of supposing all the cycloids 
 to meet in the point A, we suppose them all to 
 pass through any point C, their bases still being 
 in the same line A D ; a curve P P' drawn so 
 that the times down P C, P 7 C, &c. are all 
 equal, will cut all the cycloids at right angles. 
 This may easily be demonstrated. 
 
 395. Tojind Tautochronous curves or those down which to a given Jtxed 
 point a body descending all distances shall move in the same time. 
 
 (1) let the force be constant and act in parallel lines. 
 
 Let A the lowest point be the fixed point, D that 
 from which the body falls, A B vertical, B D, M P 
 horizontal. A M = x, A P = s, A B = h, and the 
 constant force = g. 
 
 Then the velocity at P is 
 
 v= 
 
 x) 
 
 and 
 
 ds 
 
 v '" V 2g V (h x) 
 
 and the whole time of descent will be found by integrating this from 
 x = h, to x = 0. 
 
 Now, since the time is to be the same, from whatever point D the body 
 falls, that is whatever be h, the integral just mentioned, taken between the 
 limits, must be independent of h. That is, if we take the integral so as 
 to vanish when 
 
 x = 
 
 and then put h for x, h will disappear altogether from the result. This 
 must manifestly arise from its being possible to put the result in a form
 
 300 ' A COMMENTARY ON [SECT. X. 
 
 involving only -p- , as r-^ , &c. ; that is from its being of dimensions in 
 
 x and h. 
 Let 
 
 ds = p dx 
 
 where p depends only on the curve, and does not involve h. Then, we 
 have 
 
 ,._A P d *- 
 
 _ Af P dx j_ ] Pxdx 1.3 px'dx 
 
 + ~" + 
 
 nncl from what has been said, it is evident, that each of the quantities 
 
 /p d x /-pxdx /px n dx 
 1 > J 5~~ > 7 2n>l" 
 
 -- 
 
 h 
 
 must be of the form 
 
 C X 2 
 
 -2 n + 1 > 
 
 h 2 
 that is 
 
 8n + I 
 
 f p x n d x must = c x 2 ; 
 hence 
 
 2 n + I iJLr. 1 
 p x n d x = jj ex a d x ; 
 
 23 
 
 _ 2n + 1 _c_ 
 
 P o :* A 
 x 2 
 
 or if 
 
 2n + 1 
 
 2 C ~ a 
 
 and 
 
 a 
 
 d s = dx ./ 
 x 
 
 which is a property of the cycloid. 
 
 Without expanding, the thing may thus be proved. If p be a function 
 
 of m dimensions in x, , r- - : is of in A dimensions : and as the 
 V (h x) 
 
 dimensions of an expression are increased by 1 in integrating 
 
 f P dx 
 
 J v (h x)
 
 BOOK I.] 
 
 NEWTON'S PKINCIPIA. 
 
 301 
 
 is of m + 1 dimensions in x, and when h is put for x, of m -f dimen- 
 sions in h. But it ought to be independent of h or of dimensions 
 Hence 
 
 m+ \ = 
 
 .-. p = a * x ~ * 
 as before. 
 
 396. (2) Let the force tend to a center and vary as any function of the 
 distance. Required the Tautochronous Curve. 
 
 Let S be the center of force, A the point to 
 which the body must descend ; D the point from 
 which it descends. Let also 
 
 SA = e, SD = f, S P = ?, AP = s 
 P being any point whatever. 
 Now we have 
 
 v 2 = C : 
 or if 
 
 the velocity being when g f. 
 
 Hence the time of describing D A is 
 
 t = /_ d?_ 
 
 taken from g = f, to g = e. And since the time must be the same what- 
 ever is D, the integral so taken must be independent of f. 
 Let 
 
 pf f e = h 
 
 d s = p d z 
 p depending on the nature of the curve, and not involving f. Then 
 
 ,, 
 (h 
 
 = f , ,, - r , from z = h to z = 
 J V (h z) 
 
 5 from z = to z = h. 
 
 ri - 
 J V (h z) 
 
 And this must be independent of f, and therefore of <p f, and of h 
 Hence, after taking the integral the result must be when z = 0, and 
 independent of h, when h is put for z. Therefore it must be of dimen- 
 sions in z and h. But if p be of n dimensions in z, or if 
 p = cz n 
 
 V (h z) 
 
 will be of n \ dimensions,
 
 302 A COMMENTARY ON [SECT. X. 
 
 and 
 
 /-, - r of n 4- i dimensions. 
 V (h z) 
 
 Hence, n + = 0, n = , and 
 
 C 
 
 P = V 
 
 Therefore 
 
 1C / C 
 
 ds = dz / = P e d g . / - 7 ; 
 > z V p g p e' 
 
 whence the curve is known. 
 
 If & be the angle A S O, we have 
 ds 2 = dg 2 + g 2 dd 2 
 and 
 
 whence may be found a polar equation to the curve. 
 
 397. Ex. 1. Let the force vary as the distance, and be attractive. 
 Then 
 
 F = & , 9 g = A* s 2 5 
 z = pg pe = /i(f 2 e 2 ); 
 dz = 
 
 4 c 
 
 
 when P = e, -j is infinite or the curve is perpendicular to S A at A. 
 
 d 8 
 If S Y, perpendicular upon the tangent P Y, be called p, we have 
 
 g 2 ~ ds 2 
 
 -i_it! 
 
 ds 2 
 
 = i_ii=-^ 
 
 4c/*g 2 
 
 t _ 
 
 4c,<* 
 
 If e = 0, or the body descend to the center, this gives the logarithmic 
 spiral. 
 
 In other cases let 
 
 -
 
 BOOK I.] 
 
 and 
 
 .. 4 c p = 
 
 NEWTON'S PRINCIPIA. 
 
 a 2 e 2 
 
 303 
 
 the equation to the Hypocycloid (370) 
 
 If 4 c (Jt, = 1, the curve becomes a straight line, to which S A is per- 
 pendicular at A. 
 
 If 4 c /* be > 1 the curve will be concave to the center and go off to 
 infinity. 
 
 398. Ex. 2. Let the force vary inversely as the square of the distance. 
 Then 
 
 Uf 
 
 Tr* ^ 
 
 and as before we shall find 
 
 2 fjt, c e 
 
 399. A body being acted upon by a force in parallel lines, in its descent 
 from one point to another, to find the Brachystochron, or the curve of quick- 
 est descent between them. 
 
 Let A, B be the given points, and A O P Q B 
 the required cui've. Since the time down 
 A O P Q B is less than down any other curve, if 
 we take another as A O p Q B, which coincides 
 with the former, except for the arc O P Q, we 
 shall have 
 
 Time down A O : T. O P Q + T. Q B, less than 
 Time down A O+T. O p Q + T. Q B 
 
 and if the times down Q B be the same on the two suppositions, we shall 
 have 
 
 T. O P Q less than the time down any other arc O p Q. 
 
 The times down Q B will be the same in the two cases if the velocity 
 at Q be the same. But we know that the velocity acquired at Q is the 
 same, whether the body descend down 
 
 A O P Q, or A O p Q. 
 
 Hence it appears that if the time down A O P Q B be a minimum, the 
 time down any portion O P Q is also a minimum.
 
 304 A COMMENTARY ON [SECT. X. 
 
 Let a vertical line of abscissas be taken in the direction of the force; 
 and perpendicular ordinates, O L, P M, Q N be drawn, it being sup- 
 posed that 
 
 L M = M N. 
 
 Then, if L M, M N be taken indefinitely small, we may consider them 
 as representing the differential of x : On this supposition, O P, P Q, will 
 represent the differentials of the curve, and the velocity may be supposed 
 constant in O P, and in P Q. Let 
 
 AL = x, LO = y, OA = s, 
 
 and let d x, d y, d s be the differentials of the abscissa, ordinate, and 
 curve at Q, and v the velocity there ; and d x 7 , d y', d s', v' be the cor- 
 responding quantities at P. Hence the time of describing O P Q will 
 be (46) 
 
 d_s d s' 
 
 V + ~V 
 
 which is a minimum ; and consequently its differential = 0. This dif- 
 ferential is that which arises from supposing P to assume any position as 
 p out of the curve O P Q; and as the differentials indicated by d arise 
 from supposing P to vary its position along the curve O P Q, we shall 
 use 3 to indicate the differentiation, on hypothesis of passing from one 
 curve to another, or the variations of the quantities to which it is 
 prefixed. 
 
 We shall also suppose p to be in the line M P, so that d x is not sup- 
 posed to vary. These considerations being introduced, we may pro- 
 ceed thus, 
 
 And v, v' are the same whether we take O P Q, or O p Q ; for the 
 velocity at p = velocity at P. Hence 
 
 a v = 0, 3 v' = 
 and 
 
 3ds b d s' 
 -p - -. 0. 
 v v' 
 
 Now 
 
 d s 2 = d x 2 + d y 8 
 .*. ds3ds = dy3dy, 
 (for 3 d x = 0). 
 Similarly 
 
 d s' o d s' = d y' a d y .
 
 BOOK I.] NEWTON'S PRINCIPIA. 305 
 
 Substituting the value of 3 d s, a d s' which these equations give, 
 we have 
 
 dy3dy dy'3dy' ._ 
 
 v d s v' d s' 
 
 And since the points O, Q, remain fixed during the variation of P's 
 position, we have 
 
 d y + d y' = const. 
 
 3 d y' = 3 d y. 
 Substituting, and omitting 3 d y, 
 
 At 
 
 =0 . 
 
 v d s v' d s' 
 
 Or, since the two terms belong to the successive points O, P, their 
 difference will be the differential indicated by d ; hence, 
 
 dy 
 
 ' vd s ~~ 
 
 .-. f- = const. . . . (2) 
 
 v d s 
 
 Which is the property of the curve ; and v being known in terms of x, 
 we may determine its nature. 
 Let the force be gravity ; then 
 
 v= V(2gx); 
 
 d y 
 
 -i 7-7"^ s const. 
 
 ds v' (2gx) 
 
 dy 1 
 
 ds V x ~~ V a 
 a being a constant. 
 
 . iy - / 
 
 'ds ~ V a 
 
 which is a property of the cycloid, of which the axis is parallel to x, 
 and of which the base passes through the point from which the body 
 falls. 
 
 If the body fall from a given point to another given point, setting off 
 with the velocity acquired down a given height; the curve of quickest 
 descent is a cycloid, of which the base coincides with the horizontal line, 
 from which the body acquires its velocity. 
 
 400. If a body be acted on by gravity, the curve of its quickest descent 
 from a given point to a given curve, cuts the lattei' at right angles. 
 
 Let A be the given point, and B M the given curve ; A B the curve of 
 quickest descent cuts B M at right angles. 
 
 VOL. I. U
 
 306 
 
 A COMMENTARY ON 
 
 [SECT. X. 
 
 It is manifest the curve A B must be a cycloid, for 
 otherwise a cycloid might be drawn from A to B, in 
 which the descent would be shorter. If possible, let 
 A Q be the cycloid of quickest descent, the angle 
 A Q B being acute. Draw another cycloid A P, and 
 let P P' be the curve which cuts A P, A Q so as to 
 make the arcs A P, A P' synchronous. Then (394) P P' 
 is perpendicular to A Q, and therefore manifestly P' is 
 between A and Q, and the time down A P is less than the time down 
 A Q ; therefore, this latter is not the curve of quickest descent. Hence, 
 if A Q be not perpendicular to B M, it is not the curve of quickest 
 descent. 
 
 The cycloid which is perpendicular to B M may be the cycloid of 
 longest descent from A to B M. 
 
 401. If a body be acted on by gravity, and if A B be the 
 curve of quickest descent from the curve A L to the point B ; 
 A T, the tangent of A L at A, is parallel to B V, a perpen- 
 dicular to the curve A B at B. 
 
 If B V be not parallel to A T, draw B X parallel to 
 A T, and falling between B V and A. In the curve A L 
 take a point a near to A. Let a B be the cycloid of quick- 
 est descent from the point a to the point B; and Bb being 
 taken equal and parallel to a A, let A b be a cycloid equal 
 and similar to a B. Since A B V is a right angle, the 
 curve B P, which cuts off A P synchronous to A B, has B V for a tan- 
 gent. Also, ultimately A a coincides with A T, and therefore B b with 
 B X. Hence B is between A and P. Hence, the time down A b is less 
 than the time down A P, and therefore, than that down A B. And 
 hence the time down a B (which is the same as that down A b) is less 
 than that down A B. Hence, if B V be not parallel to A T, A B is not 
 the line of quickest descent from A L to B. 
 
 402. Supposing a body to be acted on by any forces whatever, to determine 
 the Brachystochron. 
 
 Making the same notations and suppositions as before, A L, L O. (see 
 a preceding figure) being any rectangular coordinates ; since, as before, 
 the time down O P Q is a minimum, we have
 
 BOOK I.] NEWTON'S PRINCIPIA. 307 
 
 ads ads 7 dsSv ds'av' _ 
 
 v v' v 2 V 2 
 
 Now as before we also have 
 
 d yady 
 ads = ^5 - *- 
 ds 
 
 supposing a d x = 0, and 
 
 ad^-dy'-ad/- d/.ady 
 
 ~dV~ ds' 
 
 dv = 
 for v is the velocity at O and does not vary by altering the curve. 
 
 v' = v + d v 
 
 dv' = av + adv = adv. 
 Hence 
 
 dyady _ d y' a d y d s / a d v _ 
 
 v d s v' d s' -v' 2 
 
 Also 
 
 __ _ 
 
 v'~v+dv"~"v v 2 ' 
 
 for d v \ &c. must be omitted. Substituting this in the second term of 
 the above equation, we have 
 
 dy. ady _ d y 7 a d y d y 7 d v a d y _ ds' d d v _ 
 
 vds vds' v 2 ds / ~V~~ 
 
 or 
 
 /d/ d y\ 1_ dy r .dv _ ch/ a d v _ 
 ~ \d~?~d~W' v + "oTs 7 7v T ~v 72 " ' ~ 
 Now as before 
 
 .__ . 
 
 d s' d s ' d s * 
 And in the other terms we may, since O, P, are indefinitely near, put 
 
 d s, d y, v for d s', d y', v' : 
 if we do this, and multiply by v, we have 
 dy dy.dv ds adv 
 d! ds.v v'ady- 
 which will give the nature of the curve. 
 
 If the forces which act on the body at O, be equivalent to X in the 
 
 direction of x, and Y in the direction of y, we have (371) 
 vdv=Xdx+Ydy 
 
 Xdx+ Ydy 
 .-. d v = - -- * 
 
 v 
 
 Yady 
 
 .*. a d v = - * 
 
 v 
 
 U2
 
 308 
 
 A COMMENTARY ON 
 
 [SECT. X. 
 
 because 3 v = 0, # d x = ; also X and Y are functions of A L, and L O, 
 and therefore not affected by d. 
 
 Substituting these values in the equation to the curve, we have 
 , dy dy Xdx+Ydy . ds Y . 
 
 Q j j Q -f- . " 
 
 dsds v a vv 
 
 or 
 
 dy dx Xdy Ydx_ A 
 
 Q . "j ~~ ~j U 
 
 d s d s v 2 
 
 which will give the nature of the curve. 
 
 If r be the radius of curvature, and d s constant, we have (from 74) 
 
 d s d x 
 
 r 
 
 r being positive when the curve is convex to A M ; 
 
 and hence 
 
 , d y d x 
 d. -3-*- = 
 
 d s r 
 
 v_ 2 _ Xdy Ydx 
 r d s 
 
 V 
 
 The quantity is the centrifugal force (210), and therefore that part 
 
 ~, ,. , f v j 
 
 of the pressure which arises from it. And 
 
 Ydx. 
 
 . , 
 
 is the pressure 
 
 which arises from resolving the forces perpendicular to the axis. Hence, 
 it appears then in the Brachystochron for any given forces, the parts of 
 the pressure which arise from the given forces and from the centrifugal 
 force must be equal. 
 
 403. If we suppose the force to tend to a center S, 
 which may be assumed to be in the line A M, and F 
 to be the whole force ; also if 
 
 SA=a, S P = g, SY = p; 
 then we have 
 
 Xdy Ydx 
 
 ds 
 
 = force in P S resolved parallel to 
 
 and 
 
 v 2 = C 2/gFd> 
 -2g/Fdg = Fp 
 
 r S 
 
 also 
 
 dp
 
 BOOK L] NEWTON'S PRINCIPIA. 309 
 
 2dp_ 2Fdg 
 
 p = C-2g/Fd f 
 and integrating 
 
 whence the relation of p and g is known. 
 If the body begin to descend from A 
 C 2g/Fdg = 
 when g = a. 
 
 404. Ex. 1. Let the force vary directly as the distance. 
 Here 
 
 which agrees with the equation to the Hypocycloid (370). 
 
 405. Ex. 2. Let the force vary inversely as the square of the distance ; 
 then 
 
 2 /a C x a P 2 a 
 
 T\ * ^"" * f* * _ 
 
 a f 
 
 by supposition. 
 
 e 3 + c 2 p c 2 a 
 
 c V (a g).dg 
 'g ^ (S 3 + c 2 ^ c*a) 
 
 When g = a, d d = ; when 
 
 3 + c? f c Ea = 
 
 d ^ is infinite, and the curve is perpendicular to the radius as at B. This 
 equation has only one real root. 
 
 If we have c = , S B = ~ 
 <& & 
 
 B being an apse. 
 
 U3
 
 810 A COMMENTARY ON [SECT. X. 
 
 J.I C 5 ; < O Jt$ ^^ 
 
 5 ; < "; ; 1 
 
 n s + n n 2 + 1 
 
 406. When a body moves on a given surface^ to determine the Brachy- 
 stochron. 
 
 Let x, y, z be rectangular coordinates, x being vertical ; and as before 
 let d s, d s' be two successive elements of the curve ; and let 
 
 d x, d y, d z, 
 dx', dy', dz' 
 
 be the corresponding elements of x, y, z ; then since the minimum pro- 
 perty will be true of the indefinitely small portion of the curve, we have 
 as before, supposing v, v' the velocities, 
 
 ds ds' 
 
 H -- - =. mm. 
 
 The variations indicated by 5 are those which arise, supposing d x, d x' 
 to be equal and constant, and d y, d z, d y', d z' to vary 
 Now 
 
 d s 2 = dx 2 -f dy 2 + dz 2 
 
 .. d s a d s = d y a d y -\-dzddz. 
 Similarly 
 
 d s' a d s' = d y' 3 d y' + d z' d d z. 
 
 Also, the extremities of the arc 
 
 d s + d s' 
 being fixed, we have 
 
 d y + d y' = const. 
 .-. ady + Sdy' = 
 
 d z + d z' = const 
 .-. o d z + S d z' = 0. 
 Hence 
 
 ..- (2) 
 
 -adz 
 
 d s' d s' 
 
 And the surface is defined by an equation between x, y, z, which we 
 
 may call 
 
 L = 0.
 
 BOOK I.] NEWTON'S PRINCIPLE 311 
 
 Let this differentiated give 
 
 dz=pdx + qdy ....... (3) 
 
 Hence, since d x, p, q are not affected by 8 
 
 3dz = q5dy ......... (4) 
 
 For the sake of simplicity, we will suppose the body to be acted on 
 only by a force in the direction of x, so that v, v' will depend on x alone, 
 and will not be affected by the variation of d y, d z. Hence, we have by (1) 
 
 3ds ad s'_ 
 +-^ 
 
 which, by substituting from (2) becomes 
 
 y' d y 1 * ,f dz/ d z 1 * i 
 
 i / -- j \- a d y + -! -j / -- ;y P d z = o. 
 
 v d s' vdsj \ v' d s' vdsj 
 
 Therefore we shall have, as before 
 
 .-- . ,- 
 
 v d s v d s 
 
 and by equation (4), this becomes 
 
 d.4f- + qd.4^ = <) ...... (5) 
 
 v d s v d s 
 
 whence the equation to the curve is known. 
 
 If we suppose the body not to be acted on by any force, v will be con- 
 stant, and the path described will manifestly be the shortest line which 
 can be drawn on the given surface, and will be determined by 
 
 d.^+q.d.^=0 ... . (6) 
 
 d s d s 
 
 If we suppose d s to be constant, we have 
 
 d 2 y + qd 2 z = 
 
 which agrees with the equation there deduced for the path, when the 
 body is acted on by no forces. 
 
 Hence, it appears that when a body moves along a surface undisturbed, 
 it will describe the shortest line which can be drawn on that surface, be- 
 tween any points of its path. 
 
 407. Let P and Q be two bodies, of 'which the Jirst hangs 
 from affixed point and the second from the Jirst by means of 
 inextensible strings A P, P Q; it is required to determine the 
 small oscillations. 
 Let 
 
 A M = x, M P = y, 
 
 A N = x', N Q = y' 
 
 A P = a, P Q = a' 
 
 , mass of P = p, of Q = (jf 
 
 tension of A P =p,of PQ = p'. </J -- ~N 
 
 U4
 
 312 A COMMENTARY ON [SECT. X. 
 
 Then resolving the forces p, p', we have 
 
 y pg _y 
 
 y = pg y y pg _y v 
 
 t s /*' a' /* * a r 
 
 / _ _p^g /-y f 
 
 t 2 " ^ ' a' 
 
 d 
 By combining these with the equations in x, x' and with the two 
 
 = a 8 , 
 
 we should, by eliminating p, p' find the motion. But when the oscilla- 
 tions are small, we may approximate in a more simple manner. 
 
 Let /3, /3' be the initial values of y, y'. Then manifestly, p, p' will de- 
 pend on the initial position of the bodies, and on their position at the time 
 t : and hence we may suppose 
 
 p = M + P|S + Q/3' + R y + S y' + &c. 
 and similarly for p'. 
 
 Now, in the equations of motion above, p, p' are multiplied by y, y' y 
 which, since the oscillations are very small are also very small quantities, 
 (viz. of the order /3). Hence their products with /3 will be of the order 
 B\ and may be neglected, and we may suppose p reduced to its first 
 term M. 
 
 M is the tension of A P, when j3, $' &c. are all = 0. Hence it is the 
 tension when P, Q, hang at rest from A, and consequently 
 M = p + fi!. 
 
 Similarly, the first term of p', which may be put for it is m'. Substi- 
 tuting these values and dividing by g, equations (1) become 
 
 d 2 y ' f (if p + P'\ P' 
 
 > ___ i _L ______ I v -t- V 
 
 gdt 2 W P a ) y T pa' 3 
 
 "dT 2 = ~a 7 ~~"a 7 
 Multiply the second of these equations by X and add it to the first, and 
 we have 
 
 gdt 2 , ~ \p~af + ~p~a a 7 ") y " " W " ~~ AMI'/ y 
 
 and manifestly this can be solved if the second member can be put in 
 the form 
 
 k . (y + X y') 
 that is, if 
 
 , p f p -\- p' x 
 ~ p af pa, af
 
 BOOK I.] NEWTON'S PRINCIPIA. 313 
 
 kx-- * --*- 
 
 K A _ . . 
 
 a' u, a 
 
 or 
 
 a /& a 
 
 Eliminating X we have 
 
 (a'k-l)a'k ' = (a'k-1) (* + *' + * 
 & \ p a . V./* 
 
 Hence 
 
 a (i a. 
 
 From this equation we obtain two values of k. Let these be de- 
 noted by 
 
 'k, 2 k 
 and let the corresponding values of X, be 
 
 Then, we have these equations. 
 
 and it is easily seen that the integrals of these equations are 
 y + ^y' = 'C cos. t V ('kg) + L D sin. t V ( J k g) 
 y + 2 X y' = 2 C cos. t V ( 2 k g) + 2 D sin. t V ( 2 k g) 
 'C, 1 D, 2 C, 2 D being arbitrary constants. But we may suppose 
 1 C = J E cos. J e 
 1 D = J E sin. ! e 
 
 2 C = 2 E cos. 2 e 
 D 2 = 2 E sin. 2 e 
 By introducing these values we find 
 
 y + 'A y/ = E cos. {t V ('k g) + e} l (5) 
 
 y + *X y' = 2 E cos. [t V ( 2 k g) + 2 e 
 From these we easily find 
 
 cos - 
 
 1 21? I * ' ' ("/ 
 
 y'=ixrr^x COSt {t v ( lk g)+ le ^ +^zr^ cos - & v ( 2k g)+ 2e U 
 
 The arbitrary quantities 1 E, J e, 8z:c. depend on the initial position and
 
 314 A COMMENTARY ON [SECT. X. 
 
 velocity of the points. If the velocities of P, Q = 0, when t = 0, we 
 shall have 
 
 'E, 2 e, each = 
 as appears by taking the Differentials of y, y 7 . 
 
 If either of the two % 2 E be = 0, we shall have (supposing the latter 
 case and omitting *e) 
 
 2 x 'E 
 y = 2 - j-cos. t V ('kg) 
 
 Hence it appears that the oscillations in this case are symmetrical : that 
 is, the bodies P, Q come to the vertical line at the same time, have similar 
 and equal .motions on the two sides of it, and reach their greatest dis- 
 tances from it at the same time. It is .easy to see that in this case, the 
 motion has the same law of time and velocity as in a cycloidal pendulum ; 
 and the time of an oscillation, in this case, extends from when t = to 
 when t V ( : k g) = it. Also if /3, /3' be the greatest horizontal deviation 
 of P, Q, we shall have 
 
 y = j8. cos. t V (*kg) 
 y 7 = /S'.cos. t V ('kg). 
 
 In order to find the original relation of /3, /3', (the oscillations will be 
 symmetrical if the forces which urge P, Q to the vertical be as P M, Q N, 
 as is easily seen. Hence the conditions for symmetrical oscillation might 
 be determined by finding the position of P, Q that this might originally 
 be the relation of the forces) that the oscillations may be of this kind, the 
 original velocities being 0, we must have by equation (5) since 2 E = 0. 
 
 p + 2 x p = o. 
 
 Similarly, if we had 
 
 p + J x p = o 
 
 we should have 'E = 0, and the oscillations would be symmetrical, and 
 would employ a time 
 
 V( 2 kg)' 
 
 When neither of these relations obtains, the oscillations may be consi- 
 dered as compounded of two in the following manner : Suppose that we 
 put 
 
 y = Hcos. t V ('kg) + Kcos. t V ( 2 kg) ... (7) 
 omitting l e, 2 e, and altering the constants hi equation (6) ; and suppose 
 that we take 
 
 M p = H . cos. t V ( l k g) ;
 
 BOOK L] NEWTON'S PRINCIPIA. 315 
 
 Then p will oscillate about M according to the law of a cycloidal pen- 
 dulum (neglecting the vertical motion). Also 
 p P will - K . cos. t V ( 2 k g). 
 
 Hence, P oscillates about p according to a similar law, while p oscil- 
 lates about M. And in the same way, we may have a point q so moved, 
 that Q shall oscillate about q in a time 
 
 V( 2 kg) 
 while q oscillates about N in a time 
 
 And hence, the motion of the pendulum A P Q is compounded of the 
 motion A p q oscillating symmetrically about a vertical line, and of A P Q 
 oscillating symmetrically about A p q, as if that were a fixed vertical line. 
 
 When a pendulum oscillates in this manner it will never return exactly 
 to its original position if V 'k, V 2 k are incommensurable. 
 
 If V 'k, V 2 k are commensurable so that we have 
 
 m V 'k = n V * k 
 
 m and n being whole numbers, the pendulum will at certain intervals, re- 
 turn to its original position. For let 
 
 t V ( 'k g) = 2 n <r 
 then 
 
 t V ( 2 k g) = 2 m it 
 and by (7) 
 
 y = H cos. 2 n it + K . cos. 2 m it 
 
 = H + K, 
 which is the same as when 
 
 t = 0. 
 And similarly, after an interval such that 
 
 t V ( l k g) = 4 n v, 6 n r, &c. 
 
 the pendulum will return to its original position, having described in the 
 intermediate times, similar cycles of oscillations. 
 408. Ex. Let /*' = p, 
 a' = a 
 
 to determine the oscillations. 
 Here equation (4) becomes 
 
 a*k* 4 ak = 2 
 and 
 
 a k = 2 + V 2.
 
 316 A COMMENTARY ON 
 
 Also, by equation (3) 
 
 [SECT. X 
 
 ak = 3 X 
 .-. x = 1 + V 2, 2 X = 1 V 2. 
 
 Hence, in order that the oscillations may be symmetrical, we must 
 either have 
 
 /3 + (I + V 2) & = 0, whence ft' = ( V 2 1) ft 
 or 
 
 j3 ( V 2 1) ft? = 0, whence /3' = ( V 2 + 1) 0. 
 The two arrangements indicated by these equations are thus repre- 
 sented. 
 
 Q' N Q 
 The first corresponds to 
 
 P = (V 2 + 1)0 
 or 
 
 QN=(V2+1)PM. 
 
 In this case, the pendulum will oscillate into the position A P' Q', simi- 
 larly situated on the other side of the line ; and the time of this complete 
 oscillation will be 
 
 V (2 
 
 it i a 
 
 V 2V 
 
 In the other case, corresponding to 
 
 /3' = (V 2- I) ft 
 
 Q is on the other side of the vertical line, and 
 QN = (V2 1)PM. 
 
 The pendulum oscillates into the position A P' Q', the point O remain 
 ing always in the vertical line ; and the time of an oscillation is 
 
 it la 
 
 V(2 + V2)Vi" 
 
 The lengths of simple pendulums which would oscillate respectively in 
 these times would be 
 
 a a 
 
 2 V 2 
 
 and 
 
 2 + V 2
 
 BOOK I.] 
 
 NEWTON'S PRINCIPIA. 
 
 317 
 
 or 
 
 1 .707 a and .293 a. 
 
 If neither of these arrangements exist originally, let |8, /3' be the origi- 
 nal values of y, y' when t is 0. Then making t = in equation (5), we 
 have 
 
 'E = |3+ (V 2 + 1)18' 
 and 
 
 And these being known, we have the motion by equation (6). 
 
 409. Any number of material points P 1} P 2 , P 3 . . . Q, 
 hang by means of a string without 'weight, from a point 
 A ; it is required to determine their small oscillations in 
 a vertical plane. 
 
 Let A N be a vertical abscissa, and PI MI, P 2 M 2 , 
 &c. horizontal ordinates ; so that 
 
 A M! = Xj, A M 2 = x 2 , &c. 
 
 P! Mi = yi, P 2 M 2 = yg, &c. 
 
 A P! = a,, P! P 2 = a 2 , &c. 
 
 tension of A P t = p 1} of PiP 2 = p 2 , &c. 
 
 mass of PX = /j, of P 2 = (L& &c. 
 
 Hence, we have three equations, by resolving the forces parallel to the 
 horizon. 
 
 PI g ^ _y_i_ 4. P^g m yg yi 
 
 g ys 3 
 
 dt 1 
 d 2 \ 
 
 dt 2 
 
 . r* Q j 6 j i I 
 
 y- 
 t 2 
 
 Psg y.s : 
 
 + 
 
 34 
 
 i i 
 
 j> . . . . (1) 
 
 d y n _ p n g f y n - 
 
 d t ft a 
 
 And as in the last, it will appear that p 1} p 2 , &c. may, for these small 
 oscillations, be considered constant, and the same as in the state of rest. 
 Hence if 
 
 then 
 
 pi = M, p 2 = M A*i5 PS = M /*, ^ &c. 
 Also, dividing by g, and arranging, the above equations may be put in 
 this form :
 
 318 
 
 A COMMENTARY ON 
 
 X. 
 
 3i 
 
 P2 y2 
 
 P3 
 
 P3 
 
 33 
 
 d 2 y 2 _ pg yi / p 2 
 
 g d t 2 to 2 ^ ^to a 2 
 
 d2 y3 ._ Psya /p 3 PM L P*y* 
 
 . ,1 . 2 .. "~ I "1" J J3 T "~ 
 
 g Cl t /*3 3 3 x/Og 83 /AS a4/ ,^3 3.1 
 
 gdt' 
 
 _ Pny n _: 
 
 p n y n 
 
 At a 
 
 in n 
 
 (1) 
 
 The first and last of these equations become symmetrical with the rest 
 if we observe that 
 
 y = 
 and 
 
 Pn+l = 0. 
 
 Now if we multiply these equations respectively by 
 
 1, X, X', X", &c. 
 and add them, we have 
 
 d'yi + X d 2 y g + X' d 2 y 3 4- &c. _ 
 ~gd~t*~ 
 
 P2 M 
 
 ( PJL_ 
 
 I to 3l 
 
 to 
 
 2 a 2 /ct2 a 
 
 P3 , P 
 
 to a s to 
 
 | 
 
 *n-i a n ^ n a n 
 
 and this will be integrable, if the right-hand side of the equation be redu- 
 cible to this form 
 
 k (y t + X y 2 + X' y s + &c.). 
 That is, if 
 
 V _ Pi .J. ?2 
 
 lY ~ 
 
 a. 2 
 
 /J> 3 a 
 
 + J?*-) + ^P* 
 
 ^ a 4 / to a 4 
 
 /"n - 1 a n 
 
 4 n 3n 
 
 (S)
 
 BOOK L] NEWTON'S PRINCIPIA. 319 
 
 If we now eliminate 
 
 X, X', X", &C. 
 
 from these n equations, it is easily seen that we shall have an equation of 
 n dimensions in k. 
 Let 
 
 l k, 2 k, 3 k n k 
 
 be the n values of k ; then for each of these there is a value of 
 \ f \" \ f " 
 
 /. , A , A 
 
 easily deducible from equations (3), which we may represent by 
 
 'X, 'X', 'X", &c. 
 2 X', 2 X", 2 X'", &C. 
 
 Hence we have these equations by taking corresponding values X and k, 
 
 &c .) 
 
 o 
 
 and so on, making n equations. 
 
 Integrating each of these equations we get, as in the last problem 
 
 yi + * y 2 + '*' y a + &c. = 'E cos. ft V ('k g) + ej | , 5) 
 
 yi + 2 * y 2 + 2 *' ya + &c. = 2 E cos . ft v ( 2 k g ) + 2 e} / 
 
 *E, 2 E, &c. 'e, 2 e, &c. being arbitrary constants. 
 
 From these n simple equations, we can, without difficulty, obtain the n 
 
 quantities yj, y 2 , &c. And it is manifest that the results will be of this 
 
 form 
 
 y^'Hj cos. ft V ( J kg) + 1 e}+ 2 H 1 cos.ftV( 2 kg) + 2 e} + &c.-j 
 ygrz^cos-ft V ( l kg) + 'e] + 2 H 2 cos.ft V( 2 kg) + 2 e} + &c. V . . . (6) 
 &c. = &c. ) 
 
 where 1 H 1 , 1 H 2 , &c. must be deduced from /Sj, /3 2 , &c. the original values 
 
 ofyu y 2 , &c. 
 
 If the points have no initial velocities (i. e. when t = 0) we shall have 
 1 E = 0, 2 E = 0, &c. 
 
 We may have symmetrical oscillations in the following manner. If, 
 of the quantities J E, 2 E, 3 E, &c. all vanish except one, for instance n E ; we 
 have 
 
 yi + I Xy 2 + 1 X'y 3 + &c. = 
 
 yi + ^y 2 + 2 ^y3 + &c. = o 
 3 ^ &c. = o 
 
 .... (7) 
 omitting n E.
 
 320 A COMMENTARY ON .[SECT. X. 
 
 From the n 1 of these equations, it appears that y 2 , y 3 , &c. are in a 
 given ratio to y t ; and hence 
 
 is a given multiple of y l and = m y t suppose. Hence, we have 
 
 m y l = n E cos. V ( n k g) ; 
 or, omitting the index n, which is now unnecessary, 
 
 m yj = E cos. t V (k g). 
 Also if y 2 = eg yi, 
 
 m y a = E e 2 cos. t V (k g) 
 and similarly for y 3 &c. 
 
 Hence, it appears that in this case the oscillations are symmetrical. All 
 the points come into the vertical line at the same time, and move similar- 
 ly, and contemporaneously on the two sides of it. The 'relation among 
 the original ordinates ft, /3 2 , /3 3 , &c. which must subsist in order that the 
 oscillations may be of this kind, is given by the n 1 equations (7), 
 
 &c. = 
 &c. = 
 &c. = 
 &c. = &c. 
 
 These give the proportion of & # 2 , &c ; the arbitrary constant n E, in 
 the remaining equation, gives the actual quantity of the original displace- 
 ment 
 
 Also, we may take any one of the quantities J E, S E, 3 E, &c. for that 
 which does not vanish ; and hence obtain, in a different way, such a sys- 
 tem of n 1 equations as has just been described. Hence, there are n 
 different relations among ft /3 2 , &c. or n different modes of arrangement, 
 in which the points may be placed, so as to oscillate symmetrically. 
 
 ( We might here also find these positions, which give symmetrical oscil- 
 lations, by requiring the force in each of the ordinates P t MI, P 2 M 2 to 
 be as the distance; in which case the points P 1? P 2 , &c. would all come 
 to the vertical at the same time. 
 
 If the quantities V : k, V 2 k have one common measure, there will be 
 a time after which the pendulum will come into its original position. And 
 it will describe similar successive cycles of vibrations. If these quantities 
 be not commensurable, no portion of its motion will be similar to any 
 preceding portion.) 
 
 The time of oscillation in each of these arrangements is easily known ; 
 the equation 
 
 m yj = n E cos. t V ( n k g)
 
 BOOK I.] NEWTON'S PRINCIPIA. 321 
 
 shows that an oscillation employs a time 
 
 And hence, if all the roots *k, 2 k, 3 k, &c. be different, the tune is dif- 
 ferent for each different arrangement. 
 
 If the initial arrangement of the points be different from all those thus 
 obtained, the oscillations of the pendulum may always be considered as 
 compounded of n symmetrical oscillations. That is, if an imaginary pen- 
 dulum oscillate symmetrically about the vertical line in % time 
 
 and a second imaginary pendulum oscillate about the place of the first, 
 considered as a fixed line, in the time 
 
 and a third about the second, in the same manner, in the tune 
 
 and so on; the n th pendulum may always be made to coincide per- 
 petually with the real pendulum, by properly adjusting the amplitudes of 
 the imaginary oscillations. This appears by considering the equations 
 (6), viz. 
 
 yx = 'Hi cos. t V ( l k g) + 2 Hi cos. t V ( 2 k g) + &c. 
 &c. = &c. 
 
 This principle of the coexistence of vibrations is applicable in all cases 
 where the vibrations are indefinitely small. In all such cases each set of 
 symmetrical vibrations takes place, and affects the system as if that were 
 the only motion which it experienced. 
 
 A familiar instance of this principle is seen in the manner in which the 
 circular vibrations, produced by dropping stones into still water, spread 
 from their respective centers, and cross without disfiguring each other. 
 
 If the oscillations be not all made in one vertical plane, we may take a 
 horizontal ordinate z perpendicular to y. The oscillations in the direc- 
 tion of y will be the same as before, and there will be similar results ob- 
 tained with respect to the oscillations in the direction of z. 
 
 We have supposed that the motion in the direction of x, the vertical 
 axis, may be neglected, which is true when the oscillations are very 
 small. 
 
 410. Ex. Let there be three bodies all equal (each = ^), and also their 
 distances & 19 a 2 , a 3 all equal (each = a). 
 
 VOL. I. X
 
 322 A COMMENTARY ON [SECT. X. 
 
 Here 
 
 and equations (3) become 
 
 ak = 5 2 X 
 akx = 2 + 3 X X' 
 a k X' = X + X'. 
 Eliminating k, we have 
 
 5X 2X 2 = 2 + 3X X', 
 5 X' 2 X X' = X + X', 
 or 
 
 X' = 2X 2 2X 2, 
 4 X' 2 X X' = X 
 
 ... X' = X 
 
 or 
 
 2 X 4 
 
 .. (2X 2 2X 2)(2X 4) = X 
 
 -X + 2 =0, 
 
 4 
 
 which may be solved by Trigonometrical Tables. We shall find three 
 values of X. 
 
 Hence, we have a value of X' corresponding to each value of X ; and 
 then by equations (7) 
 
 . 
 
 13 + 2 X& + 2 X' J 3 3 =OJ 
 whence we find /3 2 , /3 3 in terms of &. 
 We shall thus find 
 
 & = 2. 295 & 
 or 
 
 13. = 1.348/3, 
 or 
 
 13,= .64,313, 
 according as we take the different values of X. 
 
 And the times of oscillation in each case will be found by taking tlie 
 value of 
 
 a k = 5 2 X; 
 
 that value of X being taken which is not used in equation (7'). For the 
 time of oscillation will be given by making 
 
 t V (k g) = *. 
 If the values of /3 1} J3 2 , /3 3 have not this initial relation, the oscillations
 
 BOOK I.I 
 
 NEWTON'S PRINCIPIA. 
 
 323 
 
 will be compounded in a manner similar to that described in the example 
 for two bodies only. 
 
 411. A flexible chain, of uniform thickness, hangs from a fixed point : 
 to find its initial form, that its small oscillations may be symmetrical. 
 
 Let A M, the vertical abscissa = x ; M P the hori- 
 zontal ordinate = y; A P = s, and the whole length 
 A C = a; 
 
 .-. A P = a s. 
 
 And as before, the tension at P, when the oscillations 
 are small, will be the weight of P C, and may be represent- 
 ed by a s. This tension will act in the direction of a 
 tangent at P, and hence the part of it in the direction 
 P M will be 
 
 tension X i* 
 d s 
 
 or 
 
 (a s) JJ. 
 
 d s 
 
 Now, if we take any portion P Q = h, we shall find the horizontal 
 force at Q in the same manner. For the point Q, supposing d s constant 
 
 dy , dy 
 
 - becomes 3-* -f- -j - 2 . 
 
 ds d s d s * 1 
 
 d 2 y h , d 3 y h 2 
 - + 
 
 
 
 ^ = &c. 
 s 3 1.2 
 
 (see 32). 
 
 Also, the tension will be a s + h. Hence the horizontal force in 
 the direction N Q, is 
 
 Subtracting from this the force in P M, we have the force on P Q 
 horizontally. 
 
 and the mass of P Q being represented by h, the accelerating force 
 
 (= -B\ is found. But since the different points of P Q move 
 V. mass / 
 
 with different velocities, this expression is only applicable when h is inde- 
 finitely small. Hence, supposing Q to approach to and coincide with P, 
 we have, when h vanishes 
 
 accelerating force on P = (a s) ^. 
 X 2 
 
 ds*
 
 324 A COMMENTARY ON [SECT. X. 
 
 But since the oscillations are indefinitely small, x coincides with s and 
 we have 
 
 d 2 v d v 
 accelerating force on P = (a x) -j ^ -r-^-. 
 
 Now, in order that the oscillations may be symmetrical, this force must 
 be in the direction P M, and proportional to P M, in which case all the 
 points of A C, will come to the vertical A B at once. Hence, we must 
 have 
 
 (a x)^_ ^ = kdy . (1) 
 
 1 dx 2 d x 
 
 k being some constant quantity to be determined. 
 
 This equation cannot be integrated in finite terms. To obtain a 
 series let 
 
 y = A+ B.(a x)+ C(a x) 2 + &c. 
 
 ..^-| = B 2C(a x) 3D(a x) 2 
 
 .-. ? = 1. 2. C + 2. 3 D (a x) + &c. 
 Hence 
 
 gives 
 
 = 1. 2. C (a x) + 2. 3 D (a x) 2 + &c. 
 
 + B + 2 C (a x) + 3 D (a x) 2 + &c. 
 + kA + kB(a x) + kC(a x) 2 + &c. 
 Equating coefficients ; we have 
 
 B = k A, 
 2 2 C= kB 
 3 2 D = k C 
 &c. = &c. 
 .-. B = k A 
 
 r k 2 A 
 c = -5T- 
 
 D = 
 
 2 
 k 3 A 
 
 2 2 .3 2 
 &c. = &c. 
 and 
 
 v = 
 
 ..(2)
 
 BOOK I.] NEWTON'S PBINCIPIA. 325 
 
 Here 
 
 A is B C, the value of y when x = a. When x = 0, y = ; 
 
 lr S a 2 V 3 fl 3 
 
 .M-ka + ^| -L_l_ + &c. = ..... (3) 
 
 From this equation (k) may be found. The equation has an infinite 
 number of dimensions, and hence k will have an infinite number of values, 
 which we may call 
 
 i]f SI, a]f 1 
 
 ft, PL) ... A. . . J, 
 
 and these give an infinite number of initial forms, for which the chain 
 may perform symmetrical oscillations. 
 
 The time of oscillation for each of these forms will be found thus. At 
 the distance y, the force is k g y : hence by what has preceded, the time 
 to the vertical is 
 
 2 V (k g) 
 and the time of oscillation is 
 
 ^ (k g) ' 
 (The greatest value of k a is about 1.44 (Euler Com. Acad. Petrop. 
 
 torn. viii. p. 43). And the tune of oscillation for this value is the same as 
 
 g 
 
 that of a simple pendulum, whose length is a nearly.) 
 
 B 
 
 The points where the curve cuts the axis will be found by putting y = 0. 
 Hence taking the value n k of k, we have 
 
 nlfZ/j, Y \ 2 n k 3 (a. v^ * 
 
 = l_^k(a-x)+- ^ -+ 23* +&C ' 
 which will manifestly be verified, if 
 
 n k (a x) = 'k a 
 or 
 
 n k (a x) = 2 k a 
 or 
 
 *k(a x) = 3 ka 
 
 &c. = &c. 
 
 because l k a, 2 k a, &c. are roots of equation (3). 
 That is if 
 
 x = a (l 5J-) or = a (l ^ or = &c. 
 
 Suppose l k, 2 k, 3 k, &c. to be the roots in the order of their magnitude 
 x k being the least. 
 
 Then if for n k, we take % all these values of x will be negative, and 
 the curve will never cut the vertical axis below A. 
 
 X3
 
 
 326 A COMMENTARY ON [SECT. X. 
 
 If for n k, we take *k, all the values of x will be negative except the 
 first ; therefore, the curve will cut A B In one point. If we take 3 k, all 
 the values will be negative except the two first, and the curve cuts A B 
 in two points ; and so on. 
 
 Hence, the forms for which the oscillations will be 
 symmetrical, are of the kind thus represented. 
 
 And there are an infinite number of them, each 
 cutting the axis in a different number of points. 
 
 If we represent equation (2) in this manner 
 
 y = A p (k, x) 
 it is evident that 
 
 y = 'A 9 (% x) 
 y = *A f (% x) 
 * &c. = &c. 
 will each satisfy equation (1). Hence as before, if we put 
 
 y = 'A f ( J k, x) + 2 A <p ( 2 k, x) + &c. 
 
 and if 1 A, *A, &c. can be so assumed that this shall represent a given 
 initial form of the chain, its oscillations shall be compounded of as many 
 coexisting symmetrical ones, as there are terms *A, 2 A, &c. 
 
 We shall now terminate this long digression upon constrained mo- 
 tion. The reader who wishes for more complete information may con- 
 sult Whewell's Dynamics, one of the most useful and elegant treatises 
 ever written, the various speculations of Euler in the work above quoted, 
 or rather the comprehensive methods of Lagrange in his Mecanique 
 Analytique. 
 
 We now proceed to simplify the text of this Xth Section. 
 
 412. PROP. L. First, S R Q is formed by an entire revolution of the 
 generating circle or wheel, whose diameter ie O R, upon the globe 
 SOQ. 
 
 413. Secondly, by taking 
 
 C A : C O : : C O : C R 
 we have 
 
 CA:CO::CA CO: CO CR 
 
 : : A O : O R 
 
 and therefore if C S be joined and produced to meet the exterior globe 
 in D, we have also 
 
 A D : S O (: : C A : C O) : : A O : O R. 
 But 
 
 S O = the semi-circumference of the wheel O R = '^
 
 BOOK I.] NEWTON'S PR1NCIPIA. 327 
 
 K A O 
 .'.AD = ' =% the circumference of the wheel whose diameter is 
 
 A O. That is S is the vertex of the Hypocycloid A S 9 and A S is per- 
 pendicular in S to C S. But O S is also perpendicular to C S. There- 
 fore A S touches O S in S, &c. 
 
 414. The similar Jigures A S, S R.] 
 
 By 39 it readily appears that Hypocycloitls are similar when 
 
 R : r : : R' : r' 
 R and r being the radii of the globe and wheel ; that is when 
 
 C A: AO ::CO : O R 
 or when 
 
 C A : C O : : C O : C R 
 .'. A S, S R are similar 
 
 415. V B, V W are equal to O A, O R.] . 
 
 If B be not in the circumference AD let C V meet it in B'. Then 
 V P being a tangent at P, and since the element of the curve A P is the 
 same as would be generated by the revolution of B' P around B' as a 
 center, and .*. B' P is perpendicular both to the curve and its tangent 
 P V, therefore P B, P B' and .-. B, B' coincide. That is 
 
 V B := O A. 
 
 Also if the wheel O R describes O V whilst A O describes A B, the 
 angular velocity B P in each must be the same, although at first, viz. at 
 O and A, they are at right angles to each other. Hence when they shall 
 have arrived at V and B their distances from C B must be complements 
 of each other. But 
 
 z.TVW = BVP=-^ PBV 
 
 j& 
 
 .'. T V is a chord in the wheel O R, and 
 
 .-. V W = O R. 
 See also the Jesuits' note. 
 
 OTHERWISE. 
 
 416. Construct the curve S P, to which the radius of curvature to every 
 point of S R Q is a tangent ; or which is the same, find S A the Locus of 
 the Centers of Curvature to S R Q. 
 
 Hence is suggested the following generalization of the Problem, viz. 
 
 417. To make a body oscillate in any given curve. 
 
 Let S R Q (Newton's fig.) the given curve be symmetrical on both sides 
 
 X4
 
 328 A. COMMENTARY ON [SECT. X. 
 
 of R. Then if x, y be the rectangular coordinates referred to the vertex 
 R, and a, /3 those of the centers of curvature (P) we have 
 
 r * = P T 2 = (y ft) 2 + (x ) 2 . 
 Hence, the contact being of the second order (74) 
 
 X -_+ (y_0)J*I-0 ...... . . (1) 
 
 and 
 
 d v x d 2 v 
 
 These two equations by means of that of the given curve, will give us 
 8 in terms of , or the equation to the Locus of the centers of curvature. 
 
 Let S A be the Locus corresponding to S R, and A Q the other half. 
 Then suspending a body from A attached to a string whose length is R A, 
 when this string shall be stretched into any position APT, it is evident 
 that P being the point where the string quits the locus is a tangent to it, 
 and that T is a point in S R Q. 
 
 Ex. 1. Let S R Q be the common parabola. 
 
 Here 
 
 y 2 rr 2 a X 
 
 d y a 
 ' d x y 
 
 d 2 y a d y a 2 
 
 2 " 2 '" 3 "" 
 
 dx 2 y 2 'dx~ y 3 2xy 
 
 .. substituting we get 
 
 x + (y /3) . -- = 
 
 p) 
 
 and 
 
 i + . - ( v B] a = 
 t- 2x U P; 2xy 
 
 ... x a + ~(l + A) -4^ =0 = 3x + a 
 
 or 
 
 a = 3 x 
 and 
 
 a = 3 x + a-\ 
 
 _ i2L J 
 
 a ) 
 
 But 
 
 y 
 
 4 x 2 . y 2 _ 8 x 3 
 
 = Sax 
 
 8* = 
 p a z a
 
 BOOK I.] NEWTON'S PRINCIPIA. 329 
 
 __ 
 ' a 27 ~ 27 a * ' ' ' > 
 
 Now when @ = 0, a = a ; which shows that A R the length of the 
 string must equal a. Also making A the origin of abscissas, that is, aug- 
 menting a by a, we have 
 
 the equation to the semicubical parabola A S, A Q, which may be traced 
 by the ordinary rules (35, &c.); and thereby the body be made to oscillate 
 in the common parabola S Q R. 
 
 Ex. 2. Let S R Q be an ellipse. 
 
 Then, referring to its center, instead of the vertex, 
 
 or 
 
 a 2 y 2 + b 2 x 2 = a 2 b 2 
 
 T + b 2 x = 
 d x 
 
 and 
 
 J d x dx 2 
 
 These give 
 
 dj _ b 2 x 
 dx a 2 y 
 
 and 
 
 dx 2 " a 2 y 3 ' 
 Hence 
 
 _ (a 2 b z )x 3 
 
 a 4 
 and 
 
 (a*-b 2 )y 3 
 
 b 4 
 Hence substituting the values of y and x in 
 
 a 2 y 2 + b s x 2 = a 2 b 2 
 we get 
 
 b \f / a a \f , 
 
 + T.* 
 
 the equation to the Locus of the centers of curvature.
 
 330 
 
 A COMMENTARY ON 
 
 [SECT. X. 
 
 In the annexed figure let 
 
 SC = b, CR = a 
 C M = x, T M = y. 
 Then 
 
 P N = |S, C N = . 
 And to construct A S' by points, first put 
 
 (3 = 
 whence by equation (a) 
 
 = + 
 
 a 2 b 
 
 a 
 the value of A C. Let 
 
 a - 
 then 
 
 a 2 b 
 
 b 
 the value of S' C or C Q'. 
 
 Hence to make a body oscillate in the semi-ellipse S R Q we must 
 take a pendulum of the length A R, (part = A P S' flexible, and part 
 = S S' rigid ; because S S' is horizontal, and no string however stretched 
 can be horizontal see Whewell's Mechanics,) and suspend it at A. 
 Then A P being in contact with the Locus AS', P T will also touch 
 A S in P, &c. &c. 
 
 Ex. 3. Let S R Q be the common cycloid , 
 
 The equation to the cycloid is 
 
 iy J 2 T y /f?_i_ 
 
 r ~ W \ V 
 
 <L? / 
 
 dx ~ V 
 
 d'y _ r_ 
 
 * "H a ""*' ~ 
 
 dx ! 
 
 whence it is found that 
 
 Hence 
 
 -y 2 ) 
 
 and 
 
 dS 
 
 _ y _ / 
 
 dx ~ dx ~ V 
 
 2r y 
 
 . 1?- / y / ft 
 
 "da N 2r y ~ V 2 r + |8 
 
 which is also the equation of a cycloid, of which the generating circle is
 
 BOOK I.] NEWTON'S PRINCIPIA. 331 
 
 precisely the same as the former, the only difference consisting in a change 
 of sign of the ordinate, and of the origin of the abscissae. 
 
 The rest of this section is rendered sufficiently intelligible by the 
 Notes of P. P. Le Seur and Jacquier ; and by the ample supplementary 
 matter we have inserted. 
 
 SECTION XL 
 
 417. PROP. LVII. Two bodies attracting one another, describe round 
 each other and round the center of gravity similar figures. 
 
 Q 
 
 Since the mutual actions will not affect the center of gravity, the bodies 
 will always lie in a straight line passing through C, and their distances 
 from C will always be in the same proportion. 
 
 .-. S G : T C : : P C : Q C 
 and 
 
 ., the figures described round each other are similar. 
 
 Also if T t be taken = S P, the figure which P seems to describe 
 round S will be t Q, and 
 
 Tt: TQ:: S P: TQ 
 :: CP: CQ 
 and 
 
 .. the figures t Q, P Q, are similar ; and the figure which S seems to 
 describe round P is similar, and equal to the figure which P seems to 
 describe round S. 
 
 418. PROP. LVIII. If S remained at rest, a figure might be de- 
 scribed by P round S, similar and equal to the figures which P and S 
 seem to describe round each other, and by an equal force.
 
 332 
 
 A COMMENTARY ON 
 
 [SECT. XL 
 
 R 
 
 P 8 ^ 
 
 Curves are supposed similar and Q R, q r indefinitely small. Let P and 
 p be projected in directions P R, p r (making equal angles C P R, s p r) 
 with such velocities that 
 
 V sp V pq 
 j _ 
 Then ( sii 
 
 snce 
 
 ds\ 
 
 dt = ) 
 v ) 
 
 t p q V PQ V~p~q ~ V~qr 
 
 c 
 But in the beginning of the motion f = 
 
 ** 
 
 2 
 
 F _ Q R _qr }_ 
 ' f : : ~qT ' Q R = 1 ' 
 
 The same thing takes place if the center of gravity and the whole system 
 move uniformly forward in a straight line in fixed space. 
 
 419. COR. 1. IfF oc D, the bodies will describe round the common 
 center of gravity, and round each other, concentric ellipses, for such would 
 be described by P round S at rest with the same force. 
 
 Conversely, if the figures be ellipses concentric, F D. 
 
 420. COR. 2. If F oc - the figures will be conic sections, the foci in 
 
 the centers of force, and the converse. 
 
 421. COR. 3. Equal areas are described round the center of gravity, 
 and round each other, in equal times. 
 
 V 
 
 422. COR. 3. Otherwise. Since the curves are similar, the areas, bounded 
 by similar parts of the curves, are similar or proportional. 
 
 .-. spq : C P Q : : sp 2 : C P 2 : : (S + P) 8 : s 2 in a given ratio;
 
 BOOK 1.1 NEWTON'S PRINCIPIA. 333 
 
 and T. through s p q : T. through CPQ:: VS+P: V S, in a given ratio 
 and .-. : : T. through spv: T.through CP V 
 
 .-. T. through C P Q : T. through C P V : : T. through spq : T.through spv 
 
 : : s p q : s p v (by Sect. II.) 
 ::CPQ:CPV 
 
 .. the areas described round C are proportional to the times, and the 
 areas described round each other in the same times, which are similar to 
 the areas round C, are also proportional to the times. 
 
 423. PROP. LIX. The period in the figure described in last Prop. 
 : the period round C : : V S + P : V~S ; for the times through similar 
 arcs p q, P Q, are in that proportion. 
 
 424. PROP. LX. The major axis of an ellipse which P seems to de- 
 scribe round S in motion (Force oc =^\ : major axis of an ellipse which 
 
 \ J J 
 
 would be described by P in the same time round S at rest : : S + P : first 
 of two mean proportionals between S -f- P and S. 
 
 Let A = major axis of an ellipse described (or seemed to be described) 
 round S in motion, and which is similar and equal to the ellipse de- 
 scribed in Prop. LVIII. 
 
 Let x = major axis of an ellipse which would be described round S at 
 rest in the same time. 
 
 period in ellipse round S in motion V S /T , T TV\ 
 
 .-. - ;-. TP i-r, = (Prop. LIX) 
 
 period in same ellipse round feat rest V S + P 
 
 and by Sect. Ill, 
 
 period in ellipse round S at rest _ A * 
 period in required ellipse round S at rest ~~ x I 
 
 period in ellipse round S in motion A ^ V S 
 
 ' period in required ellipse round S at rest ~~ f . ^ , p 
 
 but these periods are to be equal, 
 
 .-.A 3 S = x 3 .S~TP 
 
 .*. A : x : : V S -f P: V S : : S + P: first of two mean proportionals 
 
 (for if a, a r, a r *, a r 3 , be proportionals, V a : V a r 3 : : a : a r.) 
 
 425. At what mean distance from the earth would the moon revolve 
 round the earth at rest, in the same time as she now revolves round the 
 earth in motion ? This is easily resolved. 
 
 426. PROP. LXI. The bodies will move as if acted upon by bodies at 
 the center of gravity with the same force, and the law of force with re-
 
 334 A COMMENTARY ON [SECT. XI. 
 
 spect to the distances from the center of gravity will be the same as with 
 respect to the distances from each other. 
 
 For the force is always in the line of the center of gravity, and .*. the 
 bodies will be acted upon as if it came from the center of gravity. 
 
 And the distance from the center of gravity is in a given ratio to the 
 distance from each other, .. the forces which are the same functions of 
 these distances will be proportional. . 
 
 427. PROP. LXII. Problem of two bodies with no initial Velocities. 
 
 F ex . Two bodies are let fall towards each other. Determine the 
 
 motions. 
 
 The center of gravity will remain at rest, and the bodies will move as 
 if acted on by bodies placed at the center of gravity, (and exerting the 
 same force at any given distance that the real bodies exert), 
 
 .-. the motions may be determined by the 7th Sect. 
 
 428. PROP. LXIII. Problem of two bodies with given initial Velo- 
 cities. 
 
 F cc . Two bodies are projected in given directions, with given 
 
 velocities. Determine the motions. 
 
 The motion of the center of gravity is known from the velocities and 
 directions of projection. Subtract the velocity of the center of gravity 
 from each of the given velocities, and the remainders will be the velocities 
 with which the bodies will move in respect of each other, and of the cen- 
 ter of gravity, as if the center of gravity were at rest. Hence since they 
 are acted upon as if by bodies at the center of gravity, (whose magnitudes 
 are determined by the equality of the forces), the motions may be deter- 
 mined by Prop. XVII, Sect. Ill, (velocities being supposed to be acquired 
 down the finite distance), if the directions of projection do not tend to the 
 center, or by Prop. XXXVII, Sect. VII, if they tend to or directly from 
 the center. Thus the motions of the bodies with respect to the center of 
 gravity will be determined, and these motions compounded with the uni- 
 form motion of the center of gravity will determine the motions of the 
 bodies in absolute space. 
 
 429. PROP. LXIV. F <x D, determine the, motions of any number of 
 bodies attracting each other.
 
 BOOK I.] NEWTON'S PRINCIPIA. 335 
 
 T and L will describe concentric 
 ellipses round D. 
 
 Now add a third body S. 
 
 Attraction of S on T riiay be re- 
 presented by the distance T S, and 
 on L by L S, (attraction at distance 
 being 1) resolve T S, L S, into 
 T D, D S ; L D, D S, whereof the 
 parts T D, L D, being in given 
 ratios to the whole, T L, L T, will 
 only increase the forces with which 
 L and T act on each other, and 
 
 the bodies L and T will continue to describe ellipses (as far as respects 
 these new forces) but with accelerated velocities, (for in similar parts of 
 similar figures V 2 F.R Prop. IV. Cor. 1 and 8.) The remaining 
 forces D S, and D S, being equal and parallel, will not alter the relative 
 motions of the bodies L and T, .*. they will continue to describe ellipses 
 round D, which will move towards the line I K, but will be impeded in 
 its approach by making the bodies S and D (D being T + L) describe 
 concentric ellipses round the center of gravity C, being projected with 
 proper velocities, in opposite and parallel directions. Now add a fourth 
 body V, and all the previous motions will continue the same, only accel- 
 erated, and C and V will describe ellipses round B, being projected with 
 proper velocities. 
 
 And so on, for any number of bodies. 
 
 Also the periods in all the ellipses will be the same, for the accelerating 
 force on T = L.TL + S .TD = (T+L). TD + S. TD = (T+L +.S). 
 T D, i. e. when a third body S is added, T is acted on as if by the sum 
 of the three bodies at the distance T D, and the accelerating force on D 
 towards C = S. S D = S.C S+ S.D C = (T + L).DC + S. D C 
 = (T + L + S). D C. 
 
 .*. accelerating force on T towards D : do. on D towards C : : T D : D C 
 
 .*. the absolute accelerating forces on T and D are equal, or T and D 
 move as if they revolved round a common center, the absolute force the 
 same, and varying as the distance from the center, i. e. they describe el- 
 lipses, in the same periods. 
 
 Similarly when a fourth body V is added, T, L, D, S, C, and V, move 
 as if the four bodies were placed at D, C, B, i. e. as if the absolute forces 
 were the same, and with forces proportional to their respective distances 
 from the centers of gravity, and .'. in equal periods.
 
 336 
 
 A COMMENTARY ON 
 
 [SECT. XI. 
 
 And so on, for any number of bodies. 
 
 430. PROP. LXVI. S and P revolve round T, S in the exterior orbit, 
 P in the interior, 
 
 F oc , find when P will describe round T an orbit nearest to the 
 
 ellipse, and areas most nearly proportional to the times. 
 
 (1st.) Let S, P, revolve round the greatest body T in the same plane. 
 Take K S for the force of S on P at the mean distance S K, 
 and 
 
 L S = S K . 
 
 = force at P, 
 
 resolve L S into L M, M S, 
 
 L M is parallel to P T, and .'. tends to the center T, .. P will con- 
 tinue to describe areas round T proportional to the times, as when acted 
 
 on only by P T, but since L M does not oc p~^i > tne sum of L M and 
 
 P T will not s 2 , .. the form of the elliptic orbit P A B will be 
 disturbed by this force, L M, M S neither tends from P to the center 
 
 T, nor oc 
 
 1 
 
 , .'. from the force M S both the proportionality of areas 
 
 to times, and the elliptic form of the orbit, will be disturbed, and the 
 elliptic form on two accounts, because M S does not tend to C, and be- 
 1 
 
 cause it does not 
 
 PT 
 
 .*. the areas will be most proportional to the times, when the force 
 M S is least, and the elliptic form will be most complete, when the forces 
 M S, L M, but particularly L M, are least. 
 
 Now let the force of S on T = N S, then this first part of the force 
 M S being common to P and T will not affect their mutual motions, .*. the
 
 BOOK I.] 
 
 NEWTON'S PRINCIPIA. 
 
 3S7 
 
 disturbing forces will be least when L M, M N, are least, or L M remain- 
 ing, when M N is least, i. e. when the forces of S on P and T are nearly 
 equal, or S N nearly = S K. 
 
 (2dly) Let S and P revolve round T in different planes. 
 
 Then L M will act as before. 
 
 But M N acting parallel to T S, when S is not in the line of the Nodes, 
 (and M N does not pass through T), will cause a disturbance not only 
 in the longitude as before, but also in the latitude, by deflecting P from 
 the plane of its orbit. And this disturbance will be least, when M N is 
 least, or S N nearly = S K. 
 
 431. COR. 1. If more bodies revolve round the greatest body T, the 
 motion of the inmost body P will be least disturbed when T is attracted 
 by the others equally, according to the distances, as they are attracted by 
 each other. 
 
 432. COR. 2. In the system of T, if the attractions of any two on the 
 
 third be as ^ > P w ^l describe areas round T with greater velocity near 
 
 conjunction and opposition, than near the quadratures. 
 
 433. To prove this, the following investigation is necessary. 
 
 Take 1 S to represent the attraction of S on P, 
 nS T, 
 
 Then the disturbing forces are 1 m (parallel to P T-) and m n. 
 Now 
 
 l ( for ce jj- 2 ), 
 S 
 
 SI = 
 
 R* 2Rrcos. A + r 2 ' 
 
 (R = ST,r = 
 
 VOL. I. 
 
 
 
 k 
 
 ' ** 
 
 
 V R 2 
 
 2 R r cos. A 
 
 + r' 
 
 SP~ (R 2 2 
 S.R 
 
 R 
 
 r cos. A 
 
 + r 2 ) 
 
 pjf, 2rcos. 
 
 A 
 
 + 
 
 . 
 
 r 2 \ / 
 Y 
 
 2 
 
 r cos. 
 
 ^ + 
 
 r 8 
 
 1 ^ l R 
 
 I 
 
 
 R 
 
 R*
 
 338 A COMMENTARY ON [SECT XT. 
 
 S / 2r 
 '~ C 
 
 S . 3/2r r*\ 3. 5 2rcos. A 
 
 2T4 R 
 S/. 3r /3 3.5 * 
 
 - cos ' A ~ ~ cos ' 
 
 r 2 x * x 
 IT) &C ') 
 
 _S_ /. 3 r. cos. A 
 ~R*{ 1 ' ~R 
 
 where R is indefinitely great with respect to r. 
 Also 
 
 Qm Q S / 3 r cos. A\ S S.Srcos. 
 
 Sn =E - 2 (l+ --^ - )- Rz = -jp 
 
 ultimately 
 
 and lm = Sl.^- =^(R 2 2Rr cos. A + r *) 
 
 = TT-( RS 2Rrcos. A + r 2 )- 1 
 
 - SIT + s - 2rz &c 
 
 - 3 4 > 
 
 = -~- ultimately. 
 
 434. Call 1 m the addititious force 
 and m n the ablatitious force 
 and m n = 1 m 3 cos. A. 
 Resolve m n into m q, q n. 
 
 The part of the ablatitious force which acts in the direction m q 
 = m n . cos. A 
 3 . S . r. cos. 2 A 
 
 R 
 
 = central ablatitious force. 
 
 The tangential part = m n . sin. A = R 3 . sin. A . cos. A 
 
 3 S r 
 
 o "^T s ^ n ' 2 A = tangential ablatitious force 
 
 < IV. 
 
 . , , ff . . ,. . T,^ , S.r 3.S.r.cos. 2 A 
 *. the whole force in the direction PT = lm mq = ^- 3 . ^5-3 
 
 = ^| (13 cos. 2 A) and the 
 
 o o 
 
 whole force in the direction of the Tangent = q n = . ^ . sin. 2 A. 
 435. Hence COR. 2. is manifest, for of the four forces acting on P, the
 
 BOOK I.] 
 
 NEWTON'S PRINCIPIA. 
 
 339 
 
 three first, namely, attraction of T, addititious force, and central ablatiti- 
 ous force, do not disturb the equable description of areas, but the fourth 
 or tangential ablatitious force does, and this is + from A to B, from B 
 to C, + from C to D, from D to A. /. the velocity is accelerated from A 
 to B, and retarded from B to C, /. it is greatest at B. Similarly it is a 
 maximum at D. And it is a minimum at A and C. This is Cor. 3. 
 436. To otherwise calculate the central and tangential ablititious forces. 
 
 On account of the great distance of S, S M, P L may be considered 
 parallel, and 
 
 .-. P T = L M, and S P = S K = S T. 
 .. the ablatitious force = 3 P T. sin. 6 = 3 P K. 
 Take P m = 3 P K, and resolve it into P n, n m. 
 P n = P m . sin. 6 = 3 P T. sin. 2 6 = central ablatitious force 
 
 n m = P m . co& & = 3 P T. sin. 6 cos. 6 = -^ . P T. sin. 2 6 = tangential 
 
 3p 
 
 ablatitious force. 
 
 The same conclusions may be got in terms of 1 m from the fig. in Art 
 433, which would be better. 
 
 437. Find the disturbing force on P in the direction P T. 
 
 This = (addititious + central ablatitious) force = 1 m + 3 1 m . sin. 2 d 
 \ cos. 2 
 
 i i f 
 
 = lm-31m( 
 
 I 3 cos. 2 
 
 438. To Jlnd the mean disturbing force of S during a whole revolution 
 in the direction P T. 
 
 Let P T at the mean distance = m, then 1 m r 
 
 Y2 
 
 13 cos. 2
 
 340 A COMMENTARY ON [SECT. XI. 
 
 1m m 
 
 r= = -5- since cos. 2 is destroyed during a whole revo- 
 
 
 
 lution. 
 
 439. The disturbing forces on P are 
 
 S r 
 
 (1) addititious = -^- r = A. 
 
 (2) ablatitious 8 . A . sin. 6 
 
 3 . A 
 which is (1) tangential ablatitious force . cos. 2 6 
 
 
 
 and (2) central ablatitious force = 3 A . - 
 
 2 
 
 3 A 3 A 
 
 .*. whole disturbing force in the direction P T = A 1 . cos. 2 & 
 
 si iii 
 
 A , 3 A 
 
 = ^ H o~ cos - ^ 6. 
 
 But in a whole revolution cos. 2 6 will destroy itself, .'. the whole dis- 
 turbing force in the direction P T in a complete revolution is ablatitious 
 and = addititious force. 
 
 S r 
 The whole force in the direction P T = ^-j- (I 3 sin. 2 6) (Art. 433) 
 
 S r / 3 3 
 
 multiply this by d.0, and the integral = -^y {$ 6 + . sin. 2 i\ 
 
 = sum of the disturbing forces ; and this when 6= v becomes ^-j- . ~ . 
 
 1 i. & 
 
 This must be divided by T, and it gives the mean disturbing force act- 
 ing on P in the direction of radius vector = -^*, . 
 
 440. The 2d COR. will appear from Art. 433 and 434. 
 
 3 
 
 For the tangential ablatitious force = . sin 2 6 . x addititious force, 
 
 IB 
 
 .'. this force will accelerate the description of the areas from the quadra* 
 tures to the syzygies and retard it from the syzygies to the quadratures, 
 since in the former case sin. 2 & is +, and in the latter 
 
 441. COR. 3 is contained in COR. 2. (Hence the Variation in as- 
 tronomy.)
 
 BOOK I.] 
 
 NEWTON'S PRINCIPIA. 
 
 341 
 
 442. P V is equivalent to P T, T V, and accelerates the motion ; 
 p V is equivalent to p T, T V, and retards the motion. 
 
 443. COR. 4. Cast, par., the curve is of greater curvature in the quadra- 
 tures than in the syzygies. 
 
 For since the velocity is greatest in the syzygies, (and the central abla- 
 titious force being the greatest, the remaining force of P to T is the least) 
 the body will be less deflected from a right line, and the orbit will be less 
 curved. The contrary takes place in the quadratures. 
 
 444. The whole force from S in the direction P T= ^4 (1 3 sin. 2 6} 
 
 T 
 
 (see 433) and the force from T in the direction P T = . 
 
 T S r 
 /. the whole force in the direction P T = + -^- (1 3 sin. 2 6) 
 
 T Jit 
 
 T S r 
 and at A this becomes 5- + ~-^ 
 
 * * l< 3 
 
 at B 
 
 at C 
 
 atD 
 
 R 
 
 2. S.r 
 R 3 
 
 T.,^1 
 
 j.2 ' R 3 
 
 R 
 
 2 S.r 
 R 3 
 
 (for though sin. 270 is , yet its syzygy is +). 
 
 Thus it appears that on two accounts the orbit is more curved in the 
 quadratures than in the syzygies, and assumes the form of an ellipse at 
 the major axis A C. 
 
 Y3
 
 342 A COMMENTARY ON [SECT. XL 
 
 .. the body is at a greater distance from the center in the quadratures 
 than in the syzygies, which is Cor. 5. 
 
 44-5. COR. 5. Hence the body P, cast, par., will recede farther from 
 T in the quadratures than in the syzygies ; for since the orbit is less 
 curved in the syzygies than in the quadratures, it is evident that the body 
 must be farther from the center in the quadratures than in the syzygies. 
 
 446. COR. 6. The addititious central force is greater than the ablati- 
 tious from Q 7 to P, and from P' to Q, but less from P to P', and from 
 Q to Q', .. on the whole, the central attraction is diminished. But it 
 may be said, that the areas are accelerated towards B and D, and .*. the 
 time through P P' may not exceed the time through P' Q, or the time 
 through Q Q' exceed that through Q' P. But in all the corollories, since 
 the errors are very small, when we are seeking the quantity of an error, 
 and have ascertained it without taking into account some other error, 
 there will be ^an error in our error, but this error in the error will be an 
 error of the second order, and may .. be neglected. 
 
 The attraction of P to T being diminished in the course of a revolution, 
 the absolute force towards T is diminished, (being diminished by the 
 
 S r r 2 
 
 mean disturbing force ^5-^ , 439,) .-. the period which , is 
 
 increased, supposing r constant. 
 
 But as T approaches S (which it will do from its higher apse to the 
 
 lower) R is diminished, the disturbing force (which involves ^-) will be 
 
 increased, and the gravity of P to T still more diminished, and .. r will 
 be increased ; .'. on both accounts (the diminution of f and increase of r) 
 the period will be increased. 
 
 (Thus the period of the moon round the earth is shorter in summer 
 than in winter. Hence the Annual equation in astronomy.) 
 
 When T recedes from S, R is increased, and the disturbing force di- 
 minished and r diminished. .'. the period will be diminished (not in com- 
 parison with the period round T if there were no body S, but in compari- 
 son with what the period was before, from the actual disturbance.) 
 
 T 1 Q 
 
 447. COR. 6. The whole force of P to T in the quadratures = -f--^ 
 
 . T 2 S r 
 
 the syzygies = ft ^j 
 
 . . on the whole the attraction of P to T is diminished in a revolution. 
 For the ablatitious force in the syzygies equals twice the addititious force 
 in the quadratures.
 
 BOOK I.] NEWTON'S PRINCIPIA. 343 
 
 At a certain point the ablatitious force = the addititious ; when 
 
 1 = 3 sin. 2 t 
 or 
 
 sin. 6=^ 
 
 and 
 
 A = 55, &c. 
 
 P 
 
 (the whole force being then = ) 
 
 Up to this point from the quadratures the addititious force is greater 
 than the ablatitious force, and from this point to one equally distant from 
 the syzygies on the other side, the ablatitious is greater than the addititious ; 
 .. in a whole revolution P's gravity to T is diminished. 
 
 Again since T alternately approaches to and recedes from S, the radius 
 
 TL 
 
 P T is increased when T approaches S, and the period oc ---- -^ 
 
 V absolute force 
 
 and since f is diminished, and .*. r increased, .s the periodic time is in- 
 creased on both accounts, (for f is diminished by the increase of the dis- 
 
 turbing forces which involve -r^.J If the distance of S be diminished, the 
 
 absolute force of S on P will be increased, .\thedisturbingforces which a ^^1 
 
 from S are increased, and P's gravity to T diminished, and .. the periodic 
 
 s. 
 time is increased in a greater ratio than r 2 (because of the diminution of 
 
 r f . 
 fin the expression rr) and when the distance of S is increased, the dis- 
 
 turbing force will be diminished, (but still the attraction of P to T will be 
 diminished by the disturbance of S) and r will be decreased, .*. the 
 
 5 
 
 period will be diminished in a less ratio than r ? . 
 
 448. COR. 7. To find the effect of the disturbing force on the motion 
 of the apsides of P's orbit during one whole revolution. 
 
 Whole force in the direction P T = ^ 2 + ^ (1 3 cos. 2 A) 
 
 T S Tr4-Tcr 4 
 
 = 1 3 + T.c.r, (if T.c = - 3 (l_3cos.'A)=^ t-Llf -, 
 
 .*. the L. between the apsides =180 . "*" , c by the IXth Sect, which 
 
 1 + 4 c J 
 
 is less than 180 when c is positive, i. e. from Q' to P and from P' to P, 
 
 Y-l
 
 344 
 
 A COMMENTARY ON 
 
 [SECT. XI. 
 
 (fig. (446,)) and greater than 180 when c is negative, i. e. from P to F 
 and from Q to Q', 
 
 .'. upon the whole the apsides are progressive, (regressive in the quadra- 
 tures and progressive in the syzygies) ; 
 
 T 3Sr 
 force = j p-y- = force in conjunction 
 
 R 
 
 3.Sr 
 
 -7g W, = force in opposition 
 
 Now 
 
 R 3 T 3Sr 3 _R 3 T 
 
 - and 
 
 r 2 R 
 
 r' 2 R J 
 
 differ most from a and -.-= 
 r 2 r /2 
 
 when r is least with respect to r f , 
 
 which is the case when the Apsides are in the syzygies. 
 But 
 
 R 3 T+ Sr 3 R 3 T+ Sr 78 
 r 2 R 3 r / *R*~ 
 
 differ least from 2 and when r is most nearly equal to r 7 , 
 
 449. COR. 7. Ex. Find the angle from the quadratures, when the apses 
 are stationary. 
 
 Draw P m parallel to T S, and = 3 P K, m n perpendicular to T P, 
 resolve P m into P n, n m, whereof n m neither increases nor diminishes 
 the accelerating force of P to T, but P n lessens that force, .-. when P n 
 = P T, the accelerating force of P is neither increased nor diminished, 
 and the apses are quiescent, 
 
 by the triangles PT:PK::PM = 3PK:Pn = PT 
 .*. in the required position 3 P K 2 = P T 1 
 or
 
 BOOK I.] NEWTON'S PRINCIPIA. 345 
 
 or 
 
 6 = 35 26'. 
 The addititious force P T P n is a maximum in quadratures. 
 
 F or P T : P K : : 3 P K : P n = 
 
 3 P K 8 
 /. P T Pn = PT -- , which is a maximum when P K = 0, 
 
 or the body is in syzygy. 
 
 450. COR. 8. Since the progression or regression of the Apsides de- 
 pends on the decrement of the force in a greater or less ratio than D 2 , from 
 the lower apse to the upper, and on a similar increment from the upper 
 to the lower, (by the IXth Sect.), and is .'. greatest when the proportion 
 of the force in the upper apse to that in the lower, recedes the most from the 
 inverse square of D, it is manifest that the Apsides progress the fastest from 
 the ablatitious force, when they are in the syzygies, (because the whole forces 
 in conjunction and opposition, i. e. at the upper and lower apses being 
 
 T 2 S r 
 
 -g -- 5-3- , when the apsides are in the syzygies and when r is greatest 
 
 T 
 
 at the upper apse, being least, and the negative part of the expression 
 
 2 S r 
 P 3 being greatest, the whole expression is .*. least, and when r is least, 
 
 T 
 
 at the lower apse, ^ being greatest, and the negative part least, .*. the 
 
 whole expression is greatest, and .. the disproportion between the forces at 
 the upper and lower apse is greatest), and that they regress the slowest 
 
 T S r 
 
 in that case from the addititious force, (for -. + ,-3-^ , which is the whole 
 
 r Jtv 
 
 force in the quadratures, both before and after conjunction, r being the 
 semi minor axis in each case, differs least from the inverse square) ; there- 
 fore, on the whole the progression in the course of a revolution is greatest 
 when the apsides are in the syzygies, 
 
 Similarly the regression is greatest when the apsides are in the quadra- 
 tures, but still it is not equal to the progression in the course of the re- 
 volution. 
 
 451. COR. 8. Let the apsides be in the syzygies, and let the force 
 at the upper apse : that at the lower, : : D E : A B, DA'
 
 346 A COMMENTARY ON [SECT. XL 
 
 being the curve whose ordinate is inversely d' D a E 
 
 as the distance * from C, .'. these forces being 
 diminished, the force D E at the upper apse 
 
 2 r S 
 by the greatest quantity -^ 3 , and the force 
 
 A B at the lower apse by the least quantity 
 2r , s a ' Aa 
 
 R 3 -; the curve a d which is the new force 
 
 curve has its ordinates decreasing in a 
 greater ratio than -r-g . 
 
 Let the apsides be in the quadratures, then the force E D will be increased 
 
 S r 
 by the greatest quantity vj-y, and the force A B by the least quantity 
 
 S r' 
 
 5-7- , .*. the curve of d' which is the new force curve will have its 
 
 rt 
 
 ordinates decreasing in a less ratio than =r- 2 
 
 45 1. COR. 9. Suppose the line of apsides to be in quadratures, then while the 
 body moves from a higher to a lower apse, it is acted on by a force which 
 
 does not increase so fast as -^- (for the force = r-rr-; , .'. the 
 
 D 2 r i R 3 
 
 numerator decreases as the denominator increases), .*. the orbit will be 
 exterior to the elliptic orbit and the excentricity will be decreased. Also as 
 
 S r 
 the descent is caused by the force -rr-j- (I 3 cos. 2 A), the less this 
 
 T 
 
 force is with respect to - 2 , the less will the excentricity be diminished. 
 
 Now while the line of the apsides moves from the line of quadratures, the 
 
 S r 
 force p 3 (1 3 cos. 2 A) is diminished, and when it is inclined at L. 35 
 
 16' the disturbing force = 0, and .*. at those four points the excentricity 
 is unaltered. After this, it may be shown in the same manner that the 
 excentricity will be continually increased until the line of apsides coin- 
 cides with the line of syzygies. Here it is a maximum, since the disturb- 
 ing force is negative. Afterwards it will decrease as before it increased 
 until the line of apsides again coincides with the quadrature, and then the 
 excentricity = maximum. 
 (Hence Evection in Astron.)
 
 BOOK I.] NEWTON'S PRINCIPIA. 347 
 
 452. LEMMA. To calculate that part of the ablatitious force which is 
 employed in drawing P from the plane of its orbit. 
 
 Let A = angular distance from syzygy. 
 
 Q = angular distance of nodes from syzygy. 
 I = inclination of orbit to orbit of S and T. 
 
 3 S r 
 
 Then the force required = R 3 . cos. A . sin. Q . sin. I. (not quite 
 
 accurately.) 
 
 When P is in quadratures, this force vanishes, since oos. A = 0. 
 
 When nodes are in syzygy, since sin. Q = 0, 
 
 quadratures, this force (cset. par.) = maxi- 
 mum, since sin. Q = sin. 90 = rad. 
 
 453. COR. 12. The effects produced by the disturbing forces are all 
 greater when P is in conjunction than when in opposition. 
 
 For they involve ,,-g , .*. when R is least, they are greatest. 
 
 454. COR. 13. Let S be supposed so great that the system P and T re- 
 volve round S fixed. Then the disturbing forces will be of the same kind 
 as before, when we supposed S to revolve round T-at rest. 
 
 The only difference will be in the magnitude of these forces, which will 
 be increased in the same ratio as S is increased. 
 
 455. COR. 14. If we suppose the different systems in which S and S T 
 oc, but P T and P and T remain the same, and the period (p) of P round 
 
 T remains the same, all the errors - oc ' 3 , if A = density of S, 
 
 and d its diameter, 
 
 oc 5 3 , if A given, and d = apparent diam. 
 also 
 
 1 S 
 p- 2 ^- 3 if P = period of T round S, 
 
 .. the errors oc p^ . 
 
 These are the linear errors, and angular errors oc in the same ratio, 
 since P T is given. 
 
 456. COR. 15. If S and T be varied in the same ratio, 
 
 S T 
 Accelerating force of S : that of T : : ^ : 2 the same ratio as before. 
 
 .. the disturbances remain the same as before. 
 
 (The same will hold if R and r be also varied proportionally.) 
 .-. the linear errors described in P's orbit oc P T, (since they involve r), 
 
 if P T oc, the rest remaining constant.
 
 31-8 A COMMENTARY ON [SECT. XJ. 
 
 c ~ m linear errors P T 
 
 also the angular errors of P as seen from T oc - , >T -- oc __ oc 1, 
 
 and are .*. the same in the two systems. 
 
 The similar linear errors oc f . T 2 , .-. P T oc f . T 2 , and f 
 
 P T P T 
 
 Tp-j- , but f ex accelerating force of T on P oc ~ , (p = period of P 
 
 round T,) 
 
 ..Tp and .. oc P 
 
 3 
 
 COR. 14. In the systems 
 
 S, T, P, Radii R, r Periods P, p 
 S', T, P - R', r -- PVp. 
 
 Linear errors dato t. in 1st. : do. in second : : p- 2 : p^ 
 
 .*. angular errors in the period of P - : - : : p- : ^-^ . 
 
 COR. 15. In the systems 
 
 S, T, P, - R, r - P, p 
 S', T, P - R', r' - F, p', 
 
 S' T' R' r' 
 so that -rr = rfr and -, , = - 
 o 1 lv r 
 
 . E. -EL 
 
 * P' '- p' ' 
 
 Linear errors in a revolution of P in 1 st. : do. in second : : r : r' 
 angular errors : ::!:!. 
 
 COR. 16. In the systems 
 
 S, T, P, - R, r - P, p 
 
 S,T',P, - R,!' - P,p'. 
 
 Linear errors in a revolution of P in 1st. : do in second : : r p 2 : r* p' 
 angular errors in a revolution of P : : : p * : p' *. 
 
 To compare the systems 
 
 (1) S, T, P - R, r - P, p 
 
 (2) S', T', P' - R', r 7 - P', p'. 
 Assume the system 
 
 (3) S', T, P - R', r - P', p 
 
 /. by (14) angular errors in P S revolution in (1) : in (3) : : ^- t : p^, 
 
 by (16) angular errors in (3) : in (2) : : p 2 : p' 2 
 
 p 2 p /2 
 therefore errors in (1) : in (2) : : - 2 : 7-5.
 
 BOOK I.] NEWTON'S PRINCIPIA. 349 
 
 Or assume the system (3) 2, T, P e, r II, p 
 2 T Q r 
 - = 
 
 .-. the errors in ( 1) : errors in (3) : : - 2 : 
 (3) : (2) 
 
 I 1 
 
 S 
 
 2 S 
 
 T> 3 
 
 p 2 * -Q 2 ' ' 
 
 : 1 : 1 
 
 SO/ "R 3 
 O it 
 
 R 3 ' 
 
 g 3 " 2 
 
 S T' 
 
 *7 3 " 
 
 R 3 
 
 lO i -LV 
 C .. 3 C/ 
 
 ^ 
 
 * C!/ * nr* 
 o 1 
 
 .p 2 . 
 
 ' R' 3 * 
 P' 2 
 
 457. COR. 16. In the different systems the mean angular errors of 
 P a ~ whether we consider the motion of apses or of nodes (or errors 
 
 in latitude and longitude.) 
 
 For first, suppose every thing in the two different systems to be the same 
 except P T, .*. p will vary. Divide the whole times p, p', into the same 
 number of indefinitely small portions proportional to the wholes. Then if 
 the position of P be given, the disturbing forces all oc each other cc P T ; 
 and the space a f . T 2 , .. the linear errors generated in any two corre- 
 sponding portions of time oc P T . p 2 . 
 
 .. the angular errors generated in these portions, as seen from T, a p 2 . 
 
 .*. Cornp . the periodic angular errors as seen from T x p 2 . 
 
 Now by Cor. 14, if in two different systems P T and .. p be the same, 
 every thing else varying, the angular errors generated in a given time, as in 
 
 P*ptt 
 
 .. neutris datis, in different systems the angular errors generated in the 
 
 time p a ^ . 
 Now 
 
 D " . i" . . E! 2. 
 
 P A p2 p2> 
 
 .*. the angular errors generated in 1" (or the mean angular errors) or J^. 
 Hence the mean motion of the nodes as seen from T ex mean motion 
 of the apses, for each a ^ . 
 
 458. COR. 17. 
 
 Mean addititious force : mean force of P on T : : p 2 : P 2 . 
 For 
 
 mean addititious force : force of S on T : : P T : S T,
 
 350 
 
 A COMMENTARY ON 
 
 [SECT. XL 
 
 force of S on T : mean force of T on P: : 
 
 S 
 
 ST PT 
 
 P. 
 
 ,. rad.\ 
 
 force oc -) 
 
 P 2 ; 
 
 .. mean addititious force : mean force of T on P: : p 2 : P 2 
 
 .. ablatitious force : mean force of T on P: : 3 cos. 6* p 2 : 
 Similarly, the tangential and central ablatitious and all the forces may 
 be found in terms of the mean force of T on P. 
 
 459. PROP. LXVII. Things being as in Prop. LXVI, S describes 
 the areas more nearly proportional to the times, and the orbit more ellipti- 
 cal round the center of gravity of P and T than round T. 
 
 T 
 
 For the forces on S are 
 
 PS 
 
 TS 
 
 .'. the direction of the compound force lies between S P, S T ; and T 
 attracts S more than P. 
 
 .-. it lies nearer T than P, and .'. nearer C the center of gravity of T 
 and P. 
 
 .*. the areas round C are more proportional to the times, than when 
 round T. 
 
 Also as S P increases or decreases, S C increases or decreases, but S T 
 remains the same ; .*. the compound force is more nearly proportional to 
 the inverse square of S C than of S T; .*. also the orbit round C is more 
 nearly elliptic (having C in the focus) than the orbit round T.
 
 A 
 
 SECOND COMMENTARY 
 
 ON 
 
 SECTION XI. 
 
 460. To find the axis major of an ellipse, whose periodic time round 
 S at rest would equal the periodic time of P round S in motion. 
 
 Let A equal the axis major of an ellipse described round P at rest 
 equal the axis major of P Q v. 
 
 Let x equal the axis major required, 
 
 P. T. of P round S in motion : p S at rest : : V S : V S + P 
 
 5 5 
 
 P. T. of p in the elliptic axis A : P. T. in the elliptic axis x : : A 2 : x * 
 /. P. T. of P round S in motion : P.T.in the el.ax. x : : V A^S : Vx 3 (S+P). 
 By hyp. the 1st term equals the 2d, 
 
 .-. A 3 S = x 3 . S + P 
 
 .-. A:x::(S+P)3: S*. 
 
 461. PROP. LXIII. Having given the velocity, places, and directions 
 of two bodies attracted to their common center of gravity, the forces vary- 
 ing inversely as the distance 2 , to determine the actual motions of bodies in 
 fixed space. 
 
 Since the initial motions of the bodies are given, the motions of the center 
 of gravity are given. And the bodies describe the same moveable curve 
 round the center of gravity as if the center were at rest, while the center 
 moves uniformly in a right line. 
 
 * Take therefore the motion of the center proportional to the time, 
 i. e. proportional to the area described in moveable orbits. 
 
 * Since a body describes some curve in fixed space, it describes areas in proportion to the times 
 in this curve, and since the center moves uniformly forward, the spaco described by it is in pro- 
 portion to the time, therefore, &c.
 
 352 
 
 A COMMENTARY ON 
 
 [SECT. XI. 
 
 462. Ex. 1. Let the body P describe a circle round C, while the center C 
 moves uniformly forward. Take C G : C P : : v of C : v of P, and with the 
 
 center C and rad. C G describe a circle G C N, and suppose it to move 
 round along G H, then P will describe the trochoid P L T, and when P 
 has described the semicircle P A B, P will be at the summit of the trochoid 
 
 .. every point of the semicircumference G F N will have touched G H, 
 
 .-. G H equals the semicircumference G F N, 
 
 .-. v of P : v of C : : P A B semicircumference: C 11 = G F N semicircle 
 * : : C P : C G Q. e. d. 
 
 463. Ex. 2. Let the moveable curve 
 be a parabola, and let the center of gravity 
 move in the direction of its primitive 
 axis. When the body is at the vertex 
 A', let S' be the position of the center 
 of gravity, and while S' has described 
 uniformly S' S, let A have described the 
 arc of the parabola A P. 
 
 Let A? N = x, N P = y, be the ab-A' 8' 
 scissa and ordinate of the curve A P in fixed space. 
 
 Let 4 p equal the parameter of the parabola A P. 
 
 4p 
 
 4p 
 
 SN = AN A S = A N p = / p' 
 
 4 p 
 
 .4 p 
 
 AreaASP=ANP SNP=|ANx N P N S X NP 
 
 ? X-l 1 y 8 4-p'y y 3 -f I2p 8 y 
 
 ~ s '4p~~* 4~p~ 24 p 
 
 By Prop. S' S GO A S P ; therefore they are in some given ratio. 
 
 T . y 3 +J2p 2 y 4- p x y* 
 
 Let A S P : S' S : : a : b : : - :! r J i : J i- 
 
 24 p 4 p 
 
 If C P = C G the curve in fixed space becomes the common cycloid. 
 If C P > C G the ollongated trochoid.
 
 BOOK I.] 
 
 NEWTON'S PRINGIPIA. 
 
 353 
 
 .. y 3 + 12p s y = 4pax a y ! 
 .-. y'+ ay 2 + 12 p 2 y 4 pa x = 0. 
 Equation to the curve in fixed space. 
 464- Ex. 3. * Let B B' be the orbit of the earth round the sun, M A 
 
 that of the moon round the earth, then the moon will, during a revolution, 
 trace out a contracted or protracted epicycloid according as A L has a 
 greater or less circumference than A M, and the orbit of the moon round 
 the sun will consist of twelve epicycloids, and it will be always concave to 
 the sun. For 
 
 F of the earth to the sun : F of the mdbn to the earth : : 
 
 P* 
 
 400 
 
 1 
 
 "(365) 2 ' (27) 2 
 
 in a greater ratio than 2 : 1. But the force of the earth to the sun is 
 nearly equal to the force of the moon to the sun, .'. the force of the moon 
 to the earth, .-. the deflection to the sun will always be within the tan- 
 gential or the curve is always concave towards the sun. 
 
 465. PROP. LXVI. If three bodies attract each other with forces 
 
 varying inversely as the square of the distance, but the two least revolve 
 
 To determine the nature of the curve described by the moon with respect to the sun. 
 VOL. I. z
 
 354 A COMMENTARY ON [SECT. XL 
 
 about the greatest, the innermost of the two will more nearly describe the 
 areas proportional to the time, and a figure more nearly similar to an el- 
 lipse, if the greatest body be attracted by the others, than if it were at rest, 
 or than if it were attracted much more or much less than the other bodies. 
 (L M : P T : : S L : S P, 
 PT 
 
 .-. L M a 
 
 SP 3 ' 
 PT x SL SK 3 x PT 
 
 SP s* 
 
 .-. SK 1 : SP' :: SL : S P). 
 
 Let P and S revolve in the same plane about the greatest body T, and 
 P describe the orbit P A B, and S, E S E. Take S K the mean distance 
 of P from S, and let S K represent the attraction of P to S at that dis- 
 tance. Take S L : S K : : S R 2 : S P 2 , and S L will represent the 
 attraction of S on P at the distance S P. Resolve it into two S M, and 
 L M parallel to P T, and P will be acted upon by three forces P T, L M, 
 S M. The first force P T tends to T', and varies inversely as the dis- 
 tance % .*. P ought by this force to describe an ellipse, whose focus is T. 
 The second, L M, being parallel to P T may be made to coincide with it 
 in this direction, and .*. the body P will still, being acted upon by a centri- 
 petal force to T, describe areas proportional to the time. But since L M 
 does not vary inversely as P T, it will make P describe a curve different 
 from an ellipse, and .'. the longer L M is compared with P T, the more 
 will the curves differ from an ellipse. The third force S M, being neither 
 in the direction P T, nor varying in the inverse square of the distance, will 
 make the body no longer describe areas in proportion to the times, and the 
 curve differ more from the form of an ellipse. The body P will .*. describe 
 areas most nearly proportional to the times, when this third force is a 
 minimum, and P A B will approach nearest to the form of an ellipse, when 
 both second and third forces are minima. Now let S N represent the 
 attraction of S on T towards S, and if S N and S M were equal, P and 
 T being equally attracted in parallel directions would have relatively the 
 same situation, and if S N be greater or less then S M, their difference 
 M "N is the disturbing force, and the body P will approach most nearly 
 the equable description of areas, and P A B to the form of an ellipse, 
 when M N is either nothing or a minimum. 
 
 Case 2. If the bodies P and S revolve about T in different planes, L M 
 being parallel to P S will have the same effect as before, and will not
 
 BOOK I.] 
 
 NEWTON'S PKINCIPIA. 
 
 355 
 
 tend to move P from its plane. But N M acting in a different plane, 
 will tend to draw P out of its plane, besides disturbing the equable des- 
 cription of areas, &c. and as before this disturbing force is a minimum, 
 when M N is a minimum, or when S N = nearly S K. 
 
 466. To estimate the magnitude of the disturbing forces on P, when P 
 moves in a circular orbit, and in the same plane with S and T. 
 
 Let the angle from the quadratures P C T = 6 t 
 
 S T = d, P T = r, F at the distance (a) = M, 
 FonP = ^l > 
 
 .'. From P in the direction SP: P T : : S P : PT 
 
 PT 
 
 /. F in the direction P T = ^* 2 
 
 o P 2 
 
 But S P 2 = d 2 + r 2 2 d r sin. 6, 
 .-. F hi the direction P T = 
 
 SP' 
 
 M a*r 
 
 Ma 2 r 
 
 2 d r sin. 6} f 
 , r * 2 d r sin. 6 
 
 = A nearly, since d being indefinitely great compared with r 
 in the expansion, all the terms may be neglected except two. First 4- 
 vanishes when compared with ~ , ... the addititious force in the direction 
 
 T --= A. By proportion as before, force in the direction S T 
 Ma 2 ST Ma 2 d 
 
 d 3 (1 + fr'2dr sin. 4, 
 
 SP 1 SP 
 
 Ma 2 
 
 -} 
 
 3 r 8 2 d r sin. 6 
 
 Ma 2 
 d 1 
 
 3 Mar 3 M a r sin. 6 
 2d 4 ' + d 1
 
 356 
 
 A COMMENTARY ON 
 
 [SECT. XL 
 
 r *u r * OT- Ma* 3 Ma 8 r 
 .-. force in the direction S T = -^ J r, sin. nearly, since 
 
 -i-r vanishes when compared with -. . and the force of S on T = A- , 
 d 4 d 3 d * 
 
 .. . . ,-, Ma ! 3 Ma r . Ma 8 
 .. ablatitious F = p 1 p sin. 6 j-j 
 
 = 3 A . sin. 0. 
 
 in 
 
 If P T equal the addititious force, then the ablatitious force equals 3 P K, 
 for P K : P T : : sin. 6 : (1 = r), 
 
 .-. 3 P K = 3 P T. sin. d = 3 A . sin. 6. 
 To resolve the ablatitious force. Take 
 
 P m : P n : : P T : T K : : 1 : cos. 6, 
 
 .*. P n = P m X cos. 6 = 3 A X sin. 6 cos. 6 = 
 
 3 A 
 
 . sin. 2 6 
 
 m n = P m X PK = 3A. sin. *6 = 3 A . 1 ~ cos - 2 ^ 
 
 ss 
 
 .. the disturbing forces of S on P are 
 
 1. The addititious force = ^ = A. 
 
 d 3 
 
 2. The ablatitious force which is resolved into the tangential part 
 = ^- . sin. 2 6, and that in the direction T P = 3 A . ~ - , 
 
 .*. whole disturbing force in the direction P T = A 3 A . - 
 
 o A o A A. . o A. j . i i 
 
 = A 5 | ^ . cos. 2 d = | . cos. 2 6, and in the whole 
 
 revolution the positive cosine destroys the negative, therefore the whole 
 disturbing force in a complete revolution is ablatitious, and equal to one 
 half of the mean addititious force. 
 
 467. To compare N M and L M. 
 
 L M : P T : : (S L = 
 
 SK 
 
 : S P, 
 
 .-. L M = g p , x P T
 
 BOOK I.] 
 
 NEWTON'S PRINCIPIA. 
 
 SK 3 SP 3 
 
 357 
 
 X ST 
 
 _SK 3 -(SK-KP) 3 
 3 
 
 S P 
 
 SK 3 SK 3 + 3SR 2 x KP _ _ . 
 = g-pi X S T nearly 
 
 3SK* x PK 3SK 3 
 
 = o-pl X S r nearly = 
 
 o JL o x^ 
 
 = -<ror X P T X sin. 6, 
 
 PK 
 
 .-. M N : L M : : 1 : 3 sin. 6. 
 
 468. Next let S and P revolve about T in different planes, and let 
 N P N' be P's orbit, N N' the line of the nodes. Take T K in T S = 
 3 A . sin. 6. Pass a plane through T K and turn it round till it is per- 
 pendicular to P's orbit. Let T e be the intersection of it with P's orbit. 
 Produce T E and draw K F perpendicular to it, .-. K F is perpendicular 
 to the plane of P's orbit, and therefore perpendicular to every line meet- 
 ing it in that orbit, T in the plane of S's orbit ; draw K H perpendicular 
 to N' N produced ; join H F, then F H K equals the inclination of the 
 planes of the two orbits. For K H T, K F T, K F H being all right angles, 
 
 KT S = KH 2 + HT 1 
 K F* + H* = K F 2 + FH 2 + H T 2 , 
 * .-. F T * = F H ! + H T , 
 
 .*. F H is perpendicular to H T. 
 Since PT = A, TK = Ax sin. 6 
 
 Let the angle KHT=T, HTK 
 from S y z. 
 
 = angular distance of the line of the 
 
 Z3
 
 358 
 
 A COMMENTARY ON 
 
 [SECT. XI. 
 
 PT 
 TK 
 KH 
 PT 
 
 TK 
 KH 
 KF 
 
 3 sin. i 
 sin. <p 
 sin. T, 
 
 K F : : 1 : 3 sin. 6 . sin. p . sin. T, 
 .% ablatitious force perpendicular to P's orbit = K F 
 = 3 P T X sin. 6. sin. <p x s-in. T = 3 A X sin. 6. sin. 9 X sin. T. 
 2d. Hence it appears that there are four forces acting on P. 
 
 C 
 
 1. Attraction of P to T oc , . 
 
 r 2 
 
 2. Addititious F in the direction P T = 
 
 M a 
 
 3. Ablatitious F in the direction P T = 
 
 3 Ma 2 r 
 
 sin. z 6. 
 
 4. Tangential part of the ablatitious force = f . - -p . sin. ! 6. 
 
 Of these the three first acting in the direction of the radius-vector do 
 not disturb the equable description of areas, the fourth acting in the di- 
 rection of a tangent at P does interrupt it. 
 
 Since the tangential part of F is formed by the revolution of P M = 3 A X 
 sin. 6 at C, 6 = 0, therefore P m = 0, and consequently the tangential 
 F = ; from C to A, P n is in consequentia, and therefore accelerates 
 the body P at A, it again equals 0, and from A to D is in antecedentia, 
 and therefore retards P ; from D to B it accelerates; from B to C it re- 
 tards. 
 
 Therefore the velocity of P is greatest at A and B, because these are 
 the points at which the accelerations cease and retardations begin, and 
 the velocity is least at D and C. To find the velocity gained by the ac- 
 tion of the tangential force.* 
 
 dZ = Fdx = A. sin. 2 6 d 6 
 
 F in the direction P T is a maximum at the quadrature, because the ablatitious F in the 
 quadrature is 0, and at every other point it is something.
 
 BOOK I.] NEWTON'S PRINCIPIA. 359 
 
 sin. 20X2^ = (cos. 2 6)', 
 
 v 2 
 .. Z = - = Cor. f A. cos. 2 6. 
 
 But when = 0, the tangential F = 0, and no velocity is produced, 
 .-. cos. 2 4 = R = 1, 
 
 yZ g ^ 
 
 .'. g = T- (1 cos. 26) = I A. 2 sin. 2 6, 
 
 .-. v 2 = 3 g A. sin. 2 0, 
 .. v = V 3 g A. sin. 0, 
 .*. v' a (sin. d)', 
 
 .. whole f on the moon at the mean distance : f of S on T : : - z : p^ 
 and the force of S on T : add. f at the mean distance (m) : : -p : -p , 
 .*. whole f at the mean distance : m : : P * : p * and p- t x whole f &c. = m. 
 
 / 
 
 Now f on the moon at any distance (r) = j L- 3 and at the mean 
 
 r & Q 
 
 distance (1) = f pTs = f ~ , 
 
 2 f* m r^ * 
 
 /. m = -p^ ^-p^ , 
 2p 2 f 
 
 .'. m = 
 
 2P 2 + p 
 
 p f p * f 
 and therefore nearly = * o~p~* 
 
 f p 2 2 p 4 ) 
 .. m r (which equals the addititious force) = f. r. \ p- t p-j- 1 . 
 
 469. To compare the ablatitious and addititious forces upon the moon, 
 with the force of gravity upon the earth's surface. (Newton, Vol. III. 
 Prop. XXV.) 
 
 add. f : f of Son T : : P T : S T 
 
 S T P V P T 
 
 f of S on T : f of the earth on the moon : : -p-j- : ^ = r , 
 
 .. add. f : f of the earth on the moon : : p 2 : P z 
 f of the earth on the moon : force of gravity :: 1 : 60 2 , 
 
 .*. add. f : force of gravity 
 
 Also ablat. f : addititious force 
 
 .. ablat. f : addititious force 
 
 : p ! : P 2 . 60 2 . . . (l) 
 
 : 3 P K : P T, 
 
 : 3 P K.p e : 60 2 . PT. P*. (2) 
 
 470. COR. 2. In a system of three bodies S, P, T, force a ^, the 
 
 Z4
 
 360 A COMMENTARY ON [SECT. XI. 
 
 body P will describe greater areas in a given time at the syzygies than at 
 the quadrature. 
 
 The tangent ablatitious f = f . P T . sin. 2 6 ; therefore this force will 
 accelerate the description of areas from quadratures to syzygies and retard 
 it from syzygies to quadratures, since in the former case sin. 2 6 is positive, 
 and in the latter negative. 
 
 COR. 3. is contained in Cor. 2. 
 
 The first quadrant d. sin. being positive the velocity increases, 
 in the second d. sin. negative the velocity decreases, &c. for the 1st Cor. 
 2d Cor. &c. 
 
 Also v is a maximum when sin. 6 is a maximum, i. e. at A and B. 
 
 471. COR. 4. The curvature of P's orbit is greater in quadratures than 
 in the syzygy. 
 
 Ma 2 , Ma 2 r 3Ma 2 r ' _ ,. 
 
 The whole F on P = -r" H -n o , 3 (1 cos. 2 0) X 
 
 r 2 d 3 2 d 3 
 
 /3 M a 3 r . sin. 2 0\ 
 \ 2d 3 ~)' 
 
 In quadratures sin. 20=0, 
 
 F M-t * 2 Ma2r 
 r 2 d 3 
 
 And in syz. 20= 180, 
 
 .*. sin. 20 = 0, cos. 2 &= I 
 
 3Ma 2 r 3Ma s r 
 
 .'. . , . . ( 1 cos. 20)= 3-5 > 
 
 2 a. 3 a 
 
 Ma 1 2Ma 2 r 
 
 .*. the whole F on P in the syz. = ^ 
 
 r : 
 
 d 
 
 .". F is greater in the quadratures than in the syzygies ; and the velocity 
 is greater in the syzygies than in the quadratures. 
 
 1 F 
 
 But the curvature a p-^ a v 2 , .. is greatest in the quadratures and 
 
 least in the syzygies. 
 
 472. COR. 5- Since the curvature of P's orbit is greatest in the quadra- 
 ture and least in the syzygy, the circular orbit must assume the form of an 
 ellipse whose major axis is C D and minor A B- 
 
 /. P recedes farther from T in the quadrature than in the syzygy. 
 
 473. COR- 6. 
 
 Mn 2 Ma 2 r 3Ma 2 r 
 The whole F on P in the line P T= ^f- + fr 1 ^" ' sltt '*' 
 
 . Ma 2 , M a*r 
 = in quad. 3- + 3
 
 BOOK I.] 
 
 NEWTON'S PRINCIPIA. 
 
 361 
 
 Ma 8 2 M a 2 r 
 and in syz. = - -ri 
 
 r 2 jJ3 
 
 let the ablatitious force on P equal the addititious, and 
 
 Ma s r 3 M a 2 r 
 
 sin. 2 6 
 
 .-. sin. d = =- sin. 35 . 1 6. 
 V 3 
 
 Therefore up to this point from quadrature the ablatitious force is less 
 than the addititious, and from this to one equally distant from the other 
 point of quadrature, the ablatitious is greater than the addititious, therefore 
 in a whole revolution the gravity of P to T is diminutive from what it 
 
 R* 
 
 would be if the orbit were circular or if S did not act, and P a . , . ^ 
 
 V abl. F 
 
 and since the action of S is alternately increased or diminished, therefore 
 P ex from what it would be were P T constant, both on account of the 
 variation, and of the absolute force. 
 
 474. COR. 7. * Let P revolve round T in an elliptic orbit, the force on 
 
 ' Ma s Ma 2 r b 
 
 ^ -- \- ^ + c r. 
 
 . 
 
 P in the quad. = 8 
 
 R 
 
 B 
 
 " G + 180 / and since the number is greater than the de- 
 
 V b + 4 c 
 
 nomination G is less than 180. .. the apsides are regressive if the same 
 effect is produced as long as the addititious force is greater than the abla- 
 titious, i. e. through 35. 16'. 
 
 The force on P in the syz. = Mj 
 
 2 M 
 
 a'r 
 
 Since P oc 
 
 R 
 
 ^^ and in winter the sun is nearer the earth than in summer, 
 
 \/ ablatitious force 
 
 R is increased in winter, and A is diminished, therefore the lunar months are shorter in winter 
 than in summer.
 
 362 A COMMENTARY ON [Siscr. XI. 
 
 .*. in the syz. the apsides are progressive, and since ./* will be 
 
 ah improper fraction as long as the ablatitious force is greater than the 
 addititious, and when the disturbing forces are equal, m c = n c, therefore 
 G = 180, i. e. the line of apsides is at rest (or it lies in V C produced 
 9th.) .. since they are regressive through 141. 4' and progressive 
 2 18. 56' they are on the whole progressive. 
 
 To find the effect produced by the tangential ablatitious force, on the 
 velocity of P in its orbit. Assume u = velocity of a body at the mean 
 
 distance 1, then = velocity at any other distance r nearly, the orbit 
 
 being nearly circular. 
 
 Let v be the true velocity of P at any distance (r), vdv = gFdx 
 
 ( = 16 ^ . For the tangent ablatitious f = f . P T . 2 6 9 and x' = r t>') 
 
 = 3 P T . m r . sin. 20.0', 
 
 .-. v 2 = 3PTmr cos. 2 & + C, 
 and 
 
 u 2 
 r 2 ' 
 
 u 2 
 
 . v 2 . frc 
 
 . . V _ ^ 8 - - OCC. 
 
 Hence it appears that the velocity is greatest in syzygy and least in 
 quadrature, since in the former case, cos. 2 6 is greatest and negative, and 
 in the latter, greatest and positive. 
 
 To find the increment of the moon's velocity by the tangential force 
 while she moves from quadrature to syzygy. 
 
 v 2 = 3 PT.m.r. cos. 2 6 + C, 
 
 but (v) the increment = 0, when 6 = 0, 
 .-. C = 3 P T . m . r, 
 
 .-. v 8 = 3PT.m.r(l cos. 2 6) = 6 P T. m. r. sin. 8 6 t 
 and when 6 = 90, or the body is in syzygy v z = 6 P T m . r. 
 
 475. COR. 6. Since the gravity of P to T is twice as much diminished 
 in syzygy as it is increased in quadrature, by the action of the disturbing 
 force S, the gravity of P to T during a whole revolution is diminished. 
 Now the disturbing forces depend on the proportion between P T and 
 T S, and therefore they become less or greater as T S becomes greater
 
 BOOK I.] NEWTON'S PRINCIPIA. 363 
 
 or less. If therefore T approach S, the gravity of P to T will be still 
 more diminished, and therefore P T will be the increment. 
 
 R ts 
 
 Now P . T cc ; since, therefore, when S T is di- 
 
 V absolute force 
 
 minished, R is increased and the absolute force diminished (for the ab- 
 solute force to T is diminished by the increase of the disturbing force) the 
 P . T is increased. In the same way when S T is increased the P . T is 
 diminished, therefore P. T is increased or diminished according as S T 
 is diminished or increased. Hence per. t of the moon is shorter in winter 
 than in summer. 
 
 OTHERWISE. 
 
 476. COR. 7. To find the effect of the disturbing force on the motion 
 
 of the apsides of P's orbit during a whole revolution. 
 
 f 
 Let f = gravity of P to T at the mean distance (1), then = gravity 
 
 of P at any other distance r. 
 
 f f 
 
 Now in quadrature the whole force of P to T = + add. f = 2 + r 
 
 f r 4- r 4 /f+1 
 
 ^ -- 
 
 
 and with this force the distance of the apsides = 180 / 
 
 which is less than 180, therefore the apsides are regressive when the 
 
 f 
 body is in quadrature. Now in syz. the whole force of P to T = 
 
 f r _ 2 r 4 
 2 r = - 3 - , therefore the distance between the apsides = 180 
 
 _ 2 
 
 which is greater than 180, therefore the apsides are progressive 
 
 when the body is in syzygy. 
 
 But as the force (2 r) which causes the progression in syzygy is double 
 the force (r) which causes the regression in quadrature, the progressive 
 motion in syzygy is greater than the regressive motion in the quadrature. 
 Hence, upon the whole, the motion of the apsides will be progressive 
 during a whole revolution. 
 
 At any other point, the motion of the apsides will be progressive or 
 
 P T 3 P T 
 
 retrograde, according as the whole central force -- ~ | -- ~ . cos. 2 d 
 
 .- ' 
 
 is negative or positive.
 
 364 
 
 A COMMENTARY ON 
 
 [SECT. XL 
 
 477. COR. 8. To calculate the disturbing force when P's orbit is ex- 
 centric. 
 
 P T 3 P T 
 
 The whole central disturbing force = --- \- - cos. 2 6 = 
 
 
 H s . cos. 2 & (m is the mean add. f). Now r = - 
 
 1 
 
 2 2 
 
 = by div. 1 e 2 + e . cos. u 
 
 e 2 
 volving e 3 , &c. = 1 ( ; + e . cos. u -f ^ . cos. 2 u ; therefore the 
 
 e cos. u 
 cos. 2 u, &c. neglecting terms in- 
 
 o 2 
 
 whole central disturbing force = 
 
 m m e 
 
 T "r A 
 
 m 
 
 cos. u 
 
 m e * cos. 2 u . 3 3 m e * 3 
 
 j H -g- m cos. 2 6 . cos. 2 6 + m e . cos. u . cos. 2 & 
 
 + | m e . cos. 2 u . cos. 2 6. 
 
 478. COR. 8. It has been shown that the apsides are progressive in 
 syzygy in consequence of the ablatitious force, and that they are regres- 
 sive in quadrature from the effect of the ablatitious force, and also, that 
 they are upon the whole progressive. It follows, therefore, that the 
 greater the excess of the ablatitious over the addititious force, the more will 
 the apsides be progressive in the course of a revolution. Now in any 
 position m M of the line of the apsides, the excess of the ablatitious in 
 conjunction 2 A T in opposition = T B, therefore the whole excess 
 = 2 A B. Again, the excess of the addititious above the ablatitious force 
 in quadrature = C D. Therefore the apsides in a whole revolution will 
 be retrograde if 2 A B be less than C D, and progressive if 2 A B be 
 greater than C D. Also their progression will be greater, the greater the 
 excess of 2 A B above C D ; but the excess is the greatest when M m is 
 in syzygy, for then A B is greatest and C D the least. Also, when M m 
 is in syzygy the apsides being progressive are moving in the same direc- 
 tion with S, and therefore will remain for some length of time in syzygy. 
 Again, when the apsides are in quadrature A B = P p, and C D = M m,
 
 BOOK L] NEWTON'S PRINCIPIA. 365 
 
 but if the orbit be nearly circular, 2 A B is greater than C D ; therefore 
 the apsides are still in a whole revolution progressive, though not so 
 
 much as in the former case. 
 
 F 
 
 In orbits nearly circular it follows from G = = when F a A?- 3 , 
 
 V r 
 
 that if the force vary in a greater ratio than the inverse square, the 
 apsides are progressive. If therefore in the inverse square they are sta- 
 tionary, if in a less ratio they are regressive. Now from quadrature to 
 35 a force which oc the distance is added to one varying inversely as 
 the square, therefore the compound varies in a less ratio than the inverse 
 square, therefore the apsides are regressive up to this point. At this point 
 
 F cc -r: ~ , therefore they are stationary. From this to 35 from 
 
 distance 
 
 another D a quantity varying as the distance is subtracted from one 
 varying inversely as the square, therefore the resulting quantity varies 
 in a greater ratio than the inverse square, therefore the apsides are 
 progressive through 218. 
 
 OTHERWISE. 
 
 479. COR. 8. It has been shown that the apsides are progressive in 
 syzygyin consequence of the ablatitious force, and that they are regressive 
 in the quadratures on account of the addititious force, and they are on the 
 whole progressive, because the ablatitious force is on the whole greater 
 than the addititious. .-. the greater the excess of the ablatitious force 
 above the addititious the more will be the apsides progressive. 
 
 In any position of the line A B in conjunction the excess of the ablati- 
 tious force above the addititious is 2 FT, in opposition 2 p t. .'. the whole 
 excess in the syzygies = 2 P p. In the quadratures at C the ablatitious 
 force vanishes. .*. the excess of the addititious = additious = C T. 
 .'. the whole addititious in the quadratures = C D. 
 
 Now the apsides will, in the whole revolution, be progressive or regres- 
 sive, according as 2 P p is greater or less than C D, and then the progres- 
 sion will be greatest in that position of the line of the apses when 2 P p 
 CD is the greatest, i. e. when A B is in the syzygy, for then 2 P p = 
 2 A B, the greatest line in the ellipse, and C D = R r = ordinate = 
 least through the focus. .*. 2 P p CD is a maximum. Also when 
 A B is in the syzygy, the line of apsides being progressive, will move the 
 same way as S. .*. it will remain in the syzygy longer, and on this account 
 the apsides will be more progressive. But when the apsides are in the 
 quadratures S P = R r and C D = A B, and the orbit being nearly 
 circular, R r nearly equals A B. .'. 2 P p C D is positive, and the
 
 366 A COMMENTARY ON [.SECT. XI. 
 
 apsides are progressive on the whole, though not so much as in the last 
 case ; and the apsides being regressive in tfce quadratures move in the op- 
 posite direction to S, .'. are sooner out of the quadratures, .*. the regres- 
 sion in the quadrature is less than the progression in the syzygy. 
 
 480. COR. 9. LEMMA. If from a quantity which cc -^ any quantity 
 be subtracted which a A the remainder will vary in a higher ratio than 
 
 the inverse square of A, but if to a quantity varying, as ^-5 another be 
 
 A 
 
 added which a A, the sum will vary in a lower ratio than ^ . 
 
 I I c A.* 
 
 If be diminished C A = 7-5 . If A increases 1 c A s 
 
 A 2 A 2 . 
 
 decreases, and -r-^ increases. .*. the quantity decreases, 1 c A increases 
 
 1 
 
 and -T-r increases. .'. increases from both these accounts. .'. the whole 
 
 A 1 
 
 quantity varies in a higher ratio than -^ . 
 
 1 4- c A 2 
 
 If C A be added -r-g , as A is increased the numerator increases, 
 
 and -r-s decreases. .'. the quantity does not decrease so fast as - lr - . and 
 A 2 A* 
 
 if A be diminished 1 + c A * is diminished, and -^ increased. .. the 
 quantity is not increased as fast as -j- t . .'. &c. Q. e. d. 
 
 OTHERWISE. 
 
 481. COR. 9. To find the effect of the disturbing force on the excen- 
 tricity of P's orbit. If P were acted on by a force oc -rj , the excentricity 
 of its orbit would not be altered. But since P is acted on by a force vary- 
 ing partly as r 8 and partly as the distance, the excentricity will continual- 
 ly vary. 
 
 Suppose the line of the apsides to coincide with the quadrature, then 
 while the body moves from the higher to the lower apse, it is acted upon 
 
 by a force which does not increase so fast as -p , for the force at the quad- 
 
 f 
 rature = + m r, and .*. the body will describe an orbit exterior to the 
 
 elliptic which would be described by the force a - . Hence the body
 
 BOOK L] NEWTON'S PRINCIPIA. 367 
 
 will be farther from the focus at the lower apse than it would have been 
 had it moved in an elliptic orbit, or the excentricity is diminished. Also 
 as the decrease in excentricity is caused by the force (m r), the less this 
 
 f 
 force is with respect to z , the less will be the diminution of excentricity. 
 
 Now while the line of apsides moves from the line of quadratures, the force 
 (m r) is diminished, and when it is inclined at an angle of 35 16' the 
 disturbing force is nothing, and .. at those four points the excentricity 
 remains unaltered. After this it may be shown in the same manner that 
 the excentricity will be continually increased, until the line of apsides 
 coincides with the syzygies. Hence it is a maximum, since the disturbing 
 force in these is negative. Afterwards it will decrease as before it in- 
 creased, until the line of apsides again coincides with the line of quadra- 
 ture, and the excentricity is a minimum. 
 
 COR. 14. Let P T = r, S T = d, f = force of T on P at the distance 
 1, g = force of S on T at the distance, then the ablatitious force 
 
 3 r sin. d ... . 
 
 = 2 P ; if.', the position of P be given, and d varies, the ablati- 
 tious force oc - . But when the position of P is given, the ablatitious 
 
 addititious : : in a given ratio, .'. addititious force oc -^ , or the dis- 
 turbing force oc ^-5 . Hence if the absolute force of S should oc the dis- 
 turbing force oc A 3 ' . Let P = the periodical time of T about S, 
 
 A = density, d = diameter of the sun, then the 
 
 A * 3 1 
 
 absolute force oc A 3 , then the disturbing force a - _. 3 cc p-^ oc A (ap- 
 parent diameter) 3 of the sun. Or since P T is constant, the linear as well 
 as the angular errors a in the same ratio. 
 
 483. COR. 15. If the bodies S and T either remain unchanged, or their 
 absolute forces are changed in any given ratio, and the magnitude of the 
 orbits described by S and P be so changed that they remain similar to 
 what they were before, and their inclination be unaltered, since the accel- 
 
 c rr)rp i.-r co absolute force of T 
 crating force of P to T : accelerating force of S : : p~T^ : 
 
 absolute force of S , ., , , c .-, , 
 
 q-rf^ > and the numerators and denominators or me last 
 
 terms are changed in the same given ratio, the accelerating forces remain 
 in the same ratio as before, and the linear or angular errors oc as before,
 
 368 
 
 A COMMENTARY ON 
 
 [SECT. XL 
 
 i e- as the diameter of the orbits, and the times of those errors oc P T's 
 of the bodies. 
 
 COR. 16. Hence if the forms and inclinations of the orbits remain, and 
 the magnitude of the forces and the distances of the bodies be changed ; to 
 find the variation of the errors and the times of the errors. In Cor. 14. 
 
 it was shown, how that when P T remained constant, the errors a -p- 2 . 
 
 Now let P T also a , then since the addititious force in a given position 
 of P a P T, and in a given position of P the addititious : ablatitious in 
 a given ratio. 
 
 COK. If a body in an ellipse be acted upon bv a force which varies 
 in a ratio greater than the inverse 
 square of the distance, it will in de- 
 scending from the higher apse B to the 
 lower apse A, be drawn nearer to the 
 
 center. .*. as S is fixed, the excen- 
 tricity is increased, and from A to B 
 the excentricity will be increased 
 also, because the force decreases the faster the distance 8 increases. 
 
 484. (Con. 10.) Let the plane of P's orbit be inclined to the plane of T's 
 orbit remaining fixed. Then the addititious force being parallel to P T, 
 is in the same plane with it, and .'. does not alter the inclination of the 
 plane. But the ablatitious force acting from P to S may be resolved into 
 two, one parallel, arid one perpendicular to the plane of P's orbit. The 
 force perpendicular to P's orbit = 3 A X sin. 6 X sin. Q X sin. T 
 when 6 = perpendicular distance of P from the quadratures, Q = angular 
 distance of the line of the nodes from the syzygy, T = first inclination of 
 the planes. 
 
 Hence when the line of the nodes is-in the syzygy, 6 = 0, .-. sin. = 
 .-. no force acts perpendicular to the plane, 
 and the inclination is not changed. When 
 
 the line of the nodes is in the quadratures, 
 6 = 90, .*. sin. is a maximum, .. force per- 
 pendicular produces the greatest change 
 in the inclination, and sin. d being posi- 
 tive from C to D, the force to change the 
 inclination continually acts from C to D 
 pulling the plane down from D to C. Sin. & 
 is negative, .'. force which before was posi- 
 
 N 
 
 H
 
 BOOK I.] NEWTON'S PRINCIPIA. 369 
 
 tive pulling down to the plane of S's orbit (or to the plane of the paper) 
 now is negative, and .. pulls up to the plane of the paper. But P's orbit is 
 now below the plane of the paper, .. force still acts to change the inclina- 
 tion. "Now since the force from C to D 'continually draws P towards the 
 plane of S's orbit, P will arrive at that plane before it gets to D. 
 
 If the nodes be in the octants past the quadrature, that is between C 
 and A. Then from N to D, sin. 6 being positive, the inclination is di- 
 minished, and from D to N' increased, .. inclination is diminished through 
 270, and increased through 90, .'. in this, as in the former case, it is 
 more diminished than increased. When the nodes are in the octants be- 
 fore the quadratures, i. e. in G H, inclination is decreased from H to C, 
 diminished from C to N, (and at N the body having got to the highest 
 point) increased from N to D, diminished from D' to N', and increased 
 from 2 N' to H, .*. inclination is increased through 270, and diminished 
 through 90, .*. it is increased upon the whole. Now the inclination of 
 P's orbit is a maximum when the force perpendicular to it is a minimum, 
 i. e. when (by expression) the line of the nodes is in the syzygies. When 
 is the quadratures, and the body is in the syzygies, the least it is increased 
 when the apsides move from the syzygies to the quadratures ; it is dimin- 
 ished and again increased as they return to the syzygies. 
 
 485. (CoR. 11.) While P moves from the quadrature in C, the nodes 
 being in the quadrature it is drawn towards S, and .*. comes to the plane 
 of S's orbit at a point nearer S than N or D, i. e. cuts the plane before it 
 arrives at the node. .'. in this case the line of the nodes is regressive. In 
 the syzygies the nodes rest, and in the points between the syzygies and 
 quadratures, they are sometimes progressive and sometimes regressive, 
 but on the whole regressive; .*. they are either retrograde or stationary. 
 
 486. (Con. 12.) All the errors mentioned in the preceding corollaries are 
 greater in the syzygies than in any other points, because the disturbing 
 force is greater at the conjunction and opposition. 
 
 487. (CoR. 13.) And since in deducing the preceding corollaries, no re- 
 gard was had to the magnitude of S, the principles are true if S be so 
 great that P and T revolve about it, and since S is increased, the disturbing 
 force is increased ; .. irregularities will be greater than they were before. 
 
 488. (CoR. 14.) L M = ^^ = N N M = 3 ^f' r sin. 6, .-. in 
 a given position of P, if P T remain unaltered, the forces N M and L M 
 
 Voi~ I. A a
 
 370 A COMMENTARY ON [SECT. XI. 
 
 cc p X absolute force oc rp ~r- 2 of T for (sect. 3 . P * ac 
 
 whether the absolute force vary or be constant. Let D = diameter of S, 
 a = density of S, and attractive force of S a magnitude or quantity of 
 matter cc D 3 d, 
 
 .-. forces L M and N M oc 
 
 d 
 
 But r- = apparent diameter of S, 
 
 .-. forces (apparent diameter) 3 d another expression. 
 
 489. (Con. 15.) Let another body as P' revolve round T' in an orbit 
 similar to the orbit of P round T, while T' is carried round S' in an orbit 
 similar to that of T round S, and let the orbit of P' be equally inclined to 
 that of T" with the orbit P to that of T. Let A, a, be the absolute forces 
 of S, T, A', a', of S', T, 
 
 A a 
 
 accelerating force of P by S : that of P by T : : o~p"z : v> T r, , 
 
 and the orbits being similar 
 
 A' a 
 
 accelerating force of P' by S' : that of P' by T' : : p, 2 : -pr^i , 
 
 .. if A' : a' : : A : a, and the orbits being similar, 
 
 S P : PT* : : S' P' : FT', 
 accelerating force of P' by S' : that of P' by T' 
 : : force on P by S : force on P' by T', 
 and the errors due to the disturbing forces in the case of P are as 
 
 A A' 
 
 0- X r, in the case of P' and S' are as , ,p /3 X R, 
 
 .. linear errors in the first case : that in the second : : r : R. 
 
 sin. errors 
 Angular errors <x ^ , 
 
 angular errors in the first case : that in the second ::!:!. 
 
 linear errors 
 
 Now Cor. 2. Lem. X. T s a 
 
 TC 
 
 anjrt 
 
 QC 
 
 .-. T * oc angular errors, 
 .-. angular errors : 360 : : T 2 : P 2 , 
 
 .-. T * QC P 2 X angular errors, 
 .-. T oc P for = angular errors.
 
 BOOK I.] NEWTON'S PRINCIPIA. 371 
 
 490. (Con. 16.) Suppose the forces of S, P T, ST to vary in any man- 
 ner, it is required to compare the angular eiTors that P describes in simi- 
 lar, and similarly situated orbits. Suppose the force of S and T to be 
 constant, .\ addititious force cc P T, .'. if two bodies describe in similar 
 orbits = evanescent arcs. Linear errors oc p 2 X P T. 
 
 .-. angular errors oc p 2 (p = per. time of P round T, P = that of T 
 round S). But by Cor. 14. if P T be given, the absolute force of A and 
 SToc. 
 
 Angular errors cc a- 
 
 .. if P T, S T and the absolute force alternately vary, 
 
 angular errors cc ~- , 
 -P = per. time of P round T) M a 2 r 
 
 /P = per. time of P round T > ., 
 
 c rr> ^ n r force x 
 ^p = per. time of T round S J 
 
 linear errors 
 angular errors x 
 
 .-. lin. errors cc force T* 
 
 /. angular errors oc -= 
 
 -= 
 * 
 
 -= 
 
 d* X r 
 
 M a 2 
 Now the errors d t X p = whole angular errors cc . 
 
 .. error d t oo -L thence the mean motion of the apsides cc mean motion 
 oi the nodes, for each x -^ , for each error is formed by forces varying as 
 
 proof of the preceding corollaries, both the disturbing forces, and .. the 
 errors produced by them in a given time will cc P T. Let P describe an 
 indefinite small angle about T (in a given position of P), then the linear 
 errors generated in that time cc force T P time 2 , but the time of describ- 
 ing = angles about T x whole periodic time (p), .'. linear errors cc 
 P T p 2 , and as the same is true for every small portion, similar; the 
 linear errors during a whole revolution x P T p 2 . Angular errors 
 
 . 
 
 cc - j - * .. oc p * .'. when S T, P T, and the absolute force vary, the 
 
 r\ 2 ciUSOlUtC t) ^ T) ' 
 
 angular errors oc p-j a -- ^ , 3 a ^ rp-, (when the absolute force is 
 
 A a2
 
 372 A COMMENTARY ON [SECT. XI. 
 
 given.) Now the error in any given timex p varies the whole errors during 
 a revolution a ^- z . .-. the errors in any given time a jf- g . Hence the 
 mean motion of the apsides of P's orbit varies the mean motion of the 
 nodes, and each will a ~^- 3 the excentricities and inclination being small 
 
 and remaining the same. 
 
 491. (Con. 17.) To compare the disturbing forces with the force of 
 PtoT. 
 
 absolute F a 
 
 F of S on T : F of P on T 
 
 P * a 
 
 S T s ' T P 
 absolute F m A. S T . aT P 
 
 axis major 3 S S 3 ' T P 5 
 
 . S JT . TP . d_ 
 p 2 ' * : p z 
 
 mean add. F : F of S on T : 
 
 .-. mean add. F : F P on T : : p 2 : P . 
 
 492. To compare the densities of different planets. 
 
 Let P and P' be the periodic times of A and B, r and r' their distances 
 from the body round which they revolve. 
 
 F of A to S : F of B to S : : ^- z : ~ 
 
 quantity of matter in A do. in B D 3 of A X density ^ D 3 of Bx density 
 distance 2 'distance 2 ' distance" distance* 
 
 3 2 
 
 where S and S 7 represent the apparent diameters of the two planets. 
 
 493. In what part of the moon's orbit is her gravity towards the earth 
 unaffected by the action of the sun. 
 
 r ,_Ma 2 , Ma'r 3 Ma'r tl~cos. 8 4 L 3 Ma'r . 
 "T" """d*"" ~d~ 2 ' 2 d 3 
 
 M a 8 
 and when it is acted upon only by the force of gravity = - for die 
 
 other forces then have no effect.
 
 BOOK I.] 
 
 Ma 8 
 
 NEWTON'S PRINCIPIA. 
 
 373 
 
 1 - 
 1 
 
 Let x = shi. 
 
 Q 
 
 Ma 
 
 2 r 1 
 
 - cos. 
 
 2 
 
 6 , 
 
 3 M 
 
 a 
 
 
 
 
 
 
 + 
 
 2 " 
 
 a 7 
 
 
 d 3 
 
 
 2 
 
 
 
 | - 
 
 - cos. 2 6 
 
 I 
 
 > 
 
 
 5 
 
 
 3 
 
 
 3 
 
 
 
 J < 
 3 
 
 
 
 
 2 
 
 + 
 
 - cos. 2 
 
 6 + 
 
 2 
 
 sm. 
 
 26 = 
 
 
 
 3 
 
 
 3 1 
 
 sin. 2 
 
 C 
 
 
 3 . 
 
 
 2 
 
 
 2 ' 
 
 2 
 
 
 
 2 Sm< 
 
 ~ ( 
 
 sin. 6 = 
 
 = 
 
 + 1 I sin. * 6 + I X 2 sin. 6 x cos. 4 = 0) 
 
 and 
 
 1 ^ + 3x V 1 x = 
 
 0. 
 
 An equation from which x may be found. 
 
 494. LEMMA. If a body moving towards a plane given in position, be 
 acted upon by a force perpendicular to its motion lending towards that plane, 
 the inclination of the orbit to the plane will be increased. Again, if the body 
 be moving from the plane, and the force acts from the plane, the inclina- 
 tion is also increased. But if the body be moving towards the plane, and 
 the force tends from the plane, or if the body be moving from the plane, 
 and the force tends towards the plane, the inclination of the orbit to the 
 plane is diminished. 
 
 495. To calculate that part of the ablatitious tangential force which is 
 employed in drawing P from the plane of its orbit. 
 
 Let the dotted line upon the ecliptic N A P N' be that part of P's orbit 
 which lies above it. Let C D be the intersection of a plane drawn per- 
 pendicular to the ecliptic ; P K perpendicular to this plane, and there- 
 
 Aa3
 
 374 
 
 A COMMENTARY ON 
 
 [SECT. XI. 
 
 fore parallel to the ecliptic. Take T F = 3 P K ; join P F and it will 
 represent the disturbing force of the sun. Draw P i a tangent to, and 
 F i perpendicular to the plane of the orbit. Complete the rectangle i m, 
 and P F may be resolved into P m, P i, of which P m is the effective force 
 to alter the inclination. Draw the plane F G i perpendicular to N N' ; 
 then F G is perpendicular to N N'. Also F i G is a right angle. As- 
 sume P T tabular rad. Then 
 
 P T T F : : R : 3g-\ .-. P T : P m : : R 3 : 3 g. s. i 
 T F F G : : R : s 5- PT.3g.s.i 
 
 FG Pm:: R:i )' R 3 
 
 g = sin. 6 =r sin. L dist. from quad. 
 
 s = sin. <p = sin. z_ dist. of nodes from syz. 
 
 i = sin. F T i = sin. F G i = sin. inclination of orbit to ecliptic. 
 
 Hence the force to draw P from its orbit = ' ^ when P is in 
 
 R 3 
 
 the quadratures. Since g vanishes this force vanishes. When the nodes 
 are in the syzygies s vanishes, and when in the quadratures this force is a 
 maximum. Since s = rad. cotan- parte- 
 
 496. To calculate the quantity of the forces. 
 
 Let S T = d, P T = r, the mean distance from T = 1. The force 
 
 S A 
 
 of T on P at the mean distance = f ; the force of S on P at the mean 
 distance = g. 
 
 Then the force S T = ^ , and the force S T : f. P T : : d : r, 
 .-. force P T = -, hence the add. f = |^; ablat. f = -^ sin. 6 9 the 
 mean add. force at distance 1 = A> l ^ e central ablat. = -jj- sin. 2 0, the 
 
 tangential ablat. f = ?A-? . sin. 2 6.
 
 BOOK I.] NEWTON'S PRINCIPIA. 375 
 
 _ _ rr F 3 ff r 
 
 The whole disturbing force of S on P = - -4ft \- ^ 3 . cos. 2 d ; the 
 
 t Cl vi Cl 
 
 or r rn 
 
 mean disturbing f = ^ 3 - (since cos. 2 6 vanishes) = by supposi- 
 
 ^ ci n 
 
 tion. 
 
 f g r 3 K r 
 Hence we have the whole gravitation of P to T = z ^-3-5 + A3 - X 
 
 r ci (\ d 
 / 
 
 cos. 2 0. and the mean = -p - (since cos. 2 vanishes). 
 
 r s 2 d 1 
 
 PROBLEM. 
 
 497. Required the whole effect, and also the mean effect of the sun to 
 
 diminish the lunar gravity; and show that if P and p be the periodic 
 times of the earth and moon, f the earth's attraction at the mean 
 distance of the moon, r the radius-vector of the moon's orbit ; the additi- 
 
 fp 2 P 4 I * 
 tious force will be nearly represented by the formula -j _-- w p*J 
 
 P n=3 P T. sin. 6, and P T 3 P T . sin. 8 6 = ^ + -|- P T x 
 
 cos. 26 = whole diminution of gravity of the moon, and the mean di- 
 
 P T g r 
 
 mmution = -J *? 3 by supposition. 
 
 Again, 
 
 P 1 a d 3 
 ab. f d , rj 
 
 *'~d r a F~ 2 ' V)d * seq> 
 
 498. To find the central and ablatitious tangential forces. 
 
 C 
 
 tn 
 
 7\rv\ 
 
 Take Pm = 3PK = 3PT. sin. 6 = ablatitious force. 
 Then P n = P m . sin. d = 3 P T . sin. * 6 = central force 
 m n = P m . cos. 6 = 3 P T . sin. 6 . cos. 6 
 
 = | . P T sin. 2 6 = tangential ablatitious force. 
 To find what is the disturbing force of S on P. 
 
 A a-l
 
 S7t> A COMMENTARY ON [SECT. XL 
 
 The disturbing force = P T 3 P T . sin. 6 = ("" 1+ | COS>2 ^ x 
 
 P T = ^ + -| P T. cos. 2 *. 
 
 To find the mean disturbing force of S during a whole revolution. 
 
 P T 3 
 
 Let P T at the mean distance = m, then -- - -- 1- . P T cos. 2 6 
 
 *-* 
 
 TJ rp 
 
 - since cos. 2 6 is destroyed during a whole revolution. 
 
 2 
 499. To find the disturbing force in syzygy. 
 
 SAT AT = 2AT = disturbing force in syzygy; 
 the force in quadrature is wholly effective and equal P T, 
 .-. force in quadrature : fin syzygy : : P T : 2 P T : : 1 : 2. 
 
 To find that point in P's orbit when the force of P to T is neither 
 increased nor diminished by the force of S to T. 
 
 In this point Pn= PTorSPT sin. * 6 = P T, 
 
 .. sin. 6 = - 
 V 3 
 
 and 
 
 6 = 35 16'. 
 
 To find when the central ablatitious force is a maximum. 
 
 P n = 3 P T . sin. z 6 = maximum, 
 .'. d . (sin. * 6) or 2 sin. 6 . cos. & d 6 = 0, 
 
 .*. sin. 6 . cos. 6 = 0, 
 or 
 
 sin. 6 . V 1 sin. * 6 = 0, 
 and 
 
 sin. 0=1, 
 
 or the body is in opposition. 
 Then (Prop. LVIII, LIX,) 
 
 T 2 : t 2 : : S P : C P : : S + P : S 
 and 
 
 T 8 : t 2 :: A 3 : x s 
 
 . A 3 : x' :: S+ P : S 
 
 and 
 
 A : x :: (S+ P)* : S*. 
 
 500. PROB. Hence to correct for the axis major of the moon's orbit- 
 Let S be the earth, P the moon, and let per. t of a body moving in a 
 secondary at the earth's surface be found, and also the periodic time of
 
 BOOK I.] NEWTON'S PRINCIPIA. 377 
 
 the moon. Then we may find the axis major of the moon's orbit round 
 the earth supposed at rest rz x, by supposition. Then the corrected axis 
 
 or axis major round the earth in motion : x : : (S + P) * : S * 
 
 (S+ 
 .*. axis major round the earth in motion = x . ^ - - 
 
 Hence to compare the quantity of matter in the earth and moon, 
 
 y : x : : V S + P : V S 
 
 ...y3_ x 3 . X 3 .. p . g< 
 
 501. To define the addititious and ablatitious forces. Let S T repre- 
 
 sent the attractive force of T to S. Take 
 
 S L : S T : : ^ : ^: : S T* : S P' 
 o Jr o JL 
 
 and S L will represent the attractive force of P to S. Resolve this into 
 S M, and L M ; then L M, that part of the force in the direction P T 
 is called the addititious force, and SM S T = NMis the ablatitious 
 force. 
 
 502. To compare these forces. 
 
 Since S L : S T : : S T 2 : S P 2 , .-. S L = |^ = attractive force of 
 
 C rr> 3 Q< np 4 
 
 P to S in the direction S P, and S P : S T : : ^-p^ : -^-p- 3 = attractive 
 
 force of P to S in the direction T S = S T 4 (S T P K)~ =ST 
 + 3 P K = S M nearly, 
 
 .-. 3PK = TM = PL = ablatitious force = 3 P T . sin. 6. 
 
 o np 3 Q T 1 3 
 
 P T = attractive force of P to S in the direction L M = P T nearly. 
 Hence the addititious force : ablatitious force : : P T : 3 P T . sin. & : 1 
 
 : 3 sin. 0. Q. e. d.
 
 BOOK III. 
 
 1. PROP. I. All secondaries are found to describe areas round the 
 primary proportional to the time, and these periodic times to be to each 
 other in the sesquiplicate ratio of their radii. Therefore the center of 
 
 force is in the primary, and the force a =~r- t . 
 
 2. PROP. II. In the same way, it may be proved, that the sun is the 
 
 center of force to the primaries, and that the forces oc -p ^ . Also the 
 
 dist. 2 
 
 Aphelion points are nearly at rest, which would not be the case if the 
 force varied in a greater or less ratio than the inverse square of the dis- 
 tance, by principles of the 9th Section, Book 1st. 
 
 3. PROP. III. The foregoing applies to the moon. The motion of the 
 moon's apogee is very slow about 3 3' in a revolution, whence the force 
 
 will ip-rz sis It was proved in the 9th Section, that if the ablatitious 
 
 force of the sun were to the centripetal force of the earth : : 1 : 357.45, 
 that the motion of the moon's apogee would be the real motion. 
 .*. the ablatitious force of the sun : centripetal force : : 2 : 357.45 
 
 : : 1 : 178 fg. 
 
 This being very small may be neglected, the remainder oc ^~z 
 
 4. COR. The mean force of the earth on the moon : force of attraction 
 
 29 . 17 29 
 
 The centripetal force at the distance of the moon : centripetal force at 
 the earth : : 1 : D 8 . 
 
 5. PROP. IV. By the best observations, the distance of the moon from 
 the earth equals about 60 semidiameters of the earth in syzygies. If the 
 moon or any heavy body at the same distance were deprived of motion in 
 the space of one minute, it would fall through a space = 16 ,V feet. For the
 
 380 
 
 A COMMENTARY ON 
 
 [BOOK III. 
 
 deflexion from the tangent in the same time = 1 6 ^ feet. Therefore the 
 space fallen through at the surface of the earth in 1" s 16 ^ feet. 
 For 60" : t : : D : 1, 
 
 __ ?2L' _ i// 
 : 60 : ' 
 
 thence the moon is retained in its orbit by the force of the earth's gravity 
 like heavy bodies on the earth's surface. 
 
 6. PROP. XIX. By the figure of the earth, the force of gravity at 
 
 the pole : force of gravity at the equator : : 289 : 288. Suppose A B Q q 
 a spheroid revolving, the lesser diameter P Q, and A C Q q c a a canal 
 filled with water. Then the weight of the arm Q q c C : ditto of 
 A a c C : : 288 : 289. The centrifugal force at the equator, therefore 1 
 suppose g^ of the weight. 
 
 Again, supposing the ratio of the diameters to be 100 : 101. By com- 
 putation, the attraction to the earth at Q : attraction to a sphere whose 
 radius = Q C : : 126 : 125. And the attraction to a sphere whose ra- 
 dius A C : attraction of a spheroid at A formed by the revolution of an 
 ellipse about its major axis : : 126 : 125. 
 
 The attraction to the earth at A is a mean proportional between the at- 
 tractions to the sphere whose radius = A C, and the oblong spheroid, 
 since the attraction varies as the quantity of matter, and the quantity of 
 matter in the oblate spheroid is a mean to the quantities of matter in the 
 oblong spheroid and the circumscribing sphere. 
 
 Hence the attraction to the sphere whose radius = A C : attraction to 
 the earth at A : : 126 : 125 . 
 
 .'. attraction to the earth at the pole : attraction to the earth at the equa- 
 tor : : 501 : 500. 
 
 Now the weights in the canals a whole weights a magnitudes X gra-
 
 BOOK III.] NEWTON'S PRINClPIA. 381 
 
 vity, therefore the weight of the equatorial arm : weight of the polar 
 : : 500 X 101 : 501 X 100 
 : : 505 : 501. 
 
 4 
 
 Therefore the centrifugal force at the equator supports VXP to make an 
 
 oOo 
 
 equilibrium. 
 
 But the centrifugal force of the earth supports 
 
 1 
 
 = ^ e excess f tne equatorial over the polar 
 
 radius. 
 
 Hence the equatorial radius : polar : : 1 + : 1 
 
 : : 230 : 229. 
 Again, since when the times of rotation and density are different the 
 
 V* 
 
 difference of the diameter <x -5 - , and that the time of the earth's rota- 
 
 dens. 
 
 tion = 23h. 56'. 
 
 The time of Jupiter's rotation = 9h. 56'. 
 
 The ratio of the squares of the velocity are as 29 : 5, and the density 
 of the earth : density of Jupiter : : 400 : 94.5. 
 
 29 400 1 
 
 d the difference of Jupiter's diameter is as X r-= X 
 
 on 400 1 
 .-. d : Jupiter's least diameter : : ~ X ^-r-= X ^^ : : 29 X 80 : 94.5 X 229 
 
 O t/4?O 
 
 : : 2320 : 21640 
 : : 232 : 2164 
 : : 1 : 9J 
 
 The polar diameter : equatorial diameter : : 9| : 10^ 
 
 ON THE TIDES. 
 
 7. THE PHENOMENA OF THE TIDES. 
 
 1. The interval between two succeeding high waters is 12 hours 25 
 minutes. The diminution varies nearly as the squares of the times from 
 high water. 
 
 2. Twenty-four hours 50 minutes may be called the lunar day. The 
 interval between two complete tides, the tide day. The first may be call-
 
 38<2 
 
 A COMMENTARY ON 
 
 [BOOK III. 
 
 ed the snperior, the other inferior, and at the time of new moon, the 
 morning and evening. 
 
 3. The high water is when the moon is in S. W. to us. The highest tide 
 at Brest is a day and a half after full or change. The third full sea after 
 the high water at the full moon is the highest ; the third after quadrature 
 is the lowest or neap tide. 
 
 4. Also the highest spring tide is when the moon is in perigee, the next 
 spring tide is the lowest, since the moon is nearly in the apogee. 
 
 5. In winter the spring tides are greater than in summer, and from the 
 same reasoning the neap tides are lower. 
 
 6. In north latitude, when the moon's declination is north, that tide in 
 which the moon is above the horizon is greater than the other of the same 
 day in which the moon is below the horizon. The contrary will take 
 place if either the observer be in south latitude or the moon's declination 
 south. 
 
 7. PROP. I. Suppose P to be any 
 particle attracted towards a center E, 
 and let the gravity of E to S be repre- 
 sented by E S. Draw B A perpendi- 
 cular to E S, which will therefore re- 
 present the diameter of the plane of il- 
 lumination. Draw Q P N perpendicu- 
 lar to B A, P M perpendicular to E C. 
 Then take E I = 3 P N, and join P I, 
 P I will represent the disturbing force 
 of P. PI may be resolved into the 
 two P E, P Q, of which P E is counter- 
 balanced by an equal and opposite force, 
 P Q acts in the direction N P. 
 
 Hence if the whole body be supposed 
 to be fluid, the fluid in the canal N P 
 will lose its equilibrium, and therefore 
 cannot remain at rest. Now, the equi- 
 librium may be restored by adding a 
 small portion P p to the canal, or by 
 supposing the water to subside round 
 the circle B A, and to be collected to- 
 wards O and C, so that the earth may put on the form of a prolate sphe- 
 roid, whose axis is in the line O C, and poles in O and C, which may be- 
 
 Q
 
 BOOK III.] NEWTON'S PRINCIPIA. 383 
 
 the case since the forces which are superadded a N P, or the distance 
 from B A, so that this mass may acquire such a protuberancy at O and C, 
 that the force at O shall be to the force at B : : E A : E C ; and by the 
 above formula 
 
 __ 5C = EC E A 
 r '" 4 g E A 
 
 8. PROP. II. Let W equal the terrestrial gravitation of C ; G equal its 
 gravitation to the sun; F the disturbing force of a particle acting at O and 
 C ; S and E the quantities of matter in the sun and earth. 
 
 3 S C 
 
 .-. F : W : : 
 
 CS J X CG'CE" 3 
 
 Since the gravitation to the sun ,. t 
 
 CS 2 :ES 2 :: ES: CG 
 
 .-. CG x C S 2 = ES 3 . 
 
 . F- W-- 3 S E 
 ' ES 3 * C E 3 
 
 and 
 
 E : S : : 1 : 338343 
 
 E C:ES: : 1 : 23668 
 
 q C "P 
 
 F W 
 . . r . vv. 
 
 .-. 4 W : 5 F : : C E : E C E A. 
 
 4 d 3d 
 
 Attraction to the pole : attraction to the equator : : 1 --- : 1 -- - 
 
 5 ft 
 
 Quantity of matter at the pole : do. at equator : : 1 : 1 d. 
 
 Weight of the polar arm : weight of the equatorial arm : : 1 -- : 1 -- 
 
 .. Excess of the polar = attractive force : weight of the equator or 
 mean weight W : : p- : 1 
 
 - 
 
 " 4W 
 
 9. PROP. III. Let A E a Q be the spheroid, B E b Q the inscribed
 
 384 
 
 A COMMENTARY ON 
 
 [BOOK. III. 
 
 sphere, A G a g the circumscribed sphere, and D F d f the sphere equal 
 (in capacity) to the spheroid. 
 
 K 
 
 Then since spheres and spheroids are equal to of their circumscribing 
 cylinder, and that the spheroid = sphere D F d f. 
 CF Z X CD = CE 2 x CA 
 CE ! : CF 2 :: CD: C A, 
 
 and make 
 
 but 
 
 Also 
 
 CE:CF::CF:Cx 
 .-. C E 2 : C F 2 : : C E : Cx 
 .-. CD:CA::CE:Cx 
 .-. CD:CE::CA:Cx 
 
 C D = C E nearly 
 .-. C A = C x. 
 
 E x = 2 E F nearly 
 .-. A D = 2 E F.* 
 
 LetCE = a,CF==a + x, 
 
 .-. E x = 2 x nearly. 
 
 = a -j- 2 x nearly
 
 BOOK III.] NEWTON'S PRINCIPIA. 385 
 
 PROP. IV. By the triangles p I L, C I N, 
 
 A B: IL::r f : (cos.) 2 L. T C A 
 .-. I L = A B x (cos.) * L, I C A = S X (cos.) * x 
 (if S = A B and x = angular distance from the sun's place.) 
 
 Again, 
 
 G E : K I : : r * : (sin.) L. T C A 
 
 .-. K I = S x (sin.) * K. 
 COR. 1. The elevation of a spheroid above the level of the undisturbed 
 
 S 
 
 ocean = 1 i 1 m = S X (cos.) 2 x - = S X (cos.) 2 x 
 
 o 
 
 The depression of the same = S X (sin.) 8 x S = S X (sin.) 2 x |. 
 COR. 2. The spheroid cuts the sphere equal in capacity to itself in a 
 
 S 
 point where S X (cos.) 8 x = = 0, or (cos.) 2 x = \. 
 
 O 
 
 .-. cos. x = .57734, &c. 
 =r cos. 54. 44'. 
 
 10. PROP. V. The unequal gravitation of the earth to the moon is 
 (4000) 3 times greater than towards the sun. 
 
 Let M equal the elevation above the inscribed sphere at the pole of 
 the spheroid, 7 equal the angular distance from the pole. 
 
 .*. the elevation above the equally capacious sphere = Mx (cos.) 2 y ^ 
 
 the depression -- = M X (sin.) * 7 |. 
 Hence the effect of the joint action of the sun and moon is equal to the 
 sum or difference of their separate actions. 
 
 .-. the elevation at any place = S X (cos.) 3 x + M X (cos.) 2 7 & S + M" 
 the depression - = S X (sin.) e x + M X (sin.) 2 7 f S+M. 
 
 1. Suppose the sun and moon in the same place in the heavens. 
 Then the elevation at the pole = S + M S + M = f S + M, and 
 the depression at the equator = S + M S -f M = & S + M, 
 
 .'. the elevation above the inscribed sphere =. S + M. 
 
 2. Suppose the moon to be in the quadratures. 
 
 The elevation at S = S S + M = f S i M. 
 
 the depression at M = S f S + M = $ S | M, 
 the elevation at S above the inscribed sphere = S M, 
 the elevation at M (by the same reasoning) = M S. 
 But (by observation) it is found that it is high water under the moon 
 when it is in the quadratures, also that the depression at S is below the 
 natural level of the ocean ; hence M is more than twice S, and although 
 VOL. I. B b
 
 386 
 
 A COMMENTARY ON 
 
 [BooK III. 
 
 the high water is never directly under the sun or moon, when the moon is 
 in the quadratures high water is always 6 hours after the high water at 
 full or change. 
 
 Suppose the moon to be in neither of the former positions. 
 
 Then the place of high water is where the elevation = maximum, 
 or when IS X cos. * x + M X cos. 2 y = maximum, 
 and since 
 
 cos. * x =r + cos. 2 x, 
 and 
 
 cos. y = + \ cos. 2 y, 
 
 elevation = maximum, when S X cos. 2 x + M X cos. 2 y = max- 
 imum. 
 
 Therefore, let A B S D be a great circle of the earth passing through 
 S and M, (those places on its surface which have the sun and moon in the 
 zenith). Join C M, cutting the circle described on C S in (m). Make 
 S d : d a : : force of the moon : force of the sun (which force is supposed
 
 BOOK III.] NEWTON'S PRINCIPIA. 387 
 
 known). Join ma, m d, and let H be any point on the surface of the 
 ocean. Join C H cutting the circle C m S in (h) ; draw the diameter 
 h d h', and draw m t, a x perpendicular to h h', and a y parallel to it. 
 Then 
 
 M = Sd, S= ad 
 and 
 
 and 
 
 ^adx = /iSdH = 2x. 
 .. d t = M X cos. 2 y, d x = .S X cos. 2 x, 
 .*. elevation = maximum when t x = a y == maximum, 
 or when a y = a m, i. e. when h h' is parallel to a m, hence 
 
 CONSTRUCTION. 
 
 Make 
 
 S d : d a : : M : S, 
 
 and join m a, draw h h' parallel to a m, and from C draw C h H cutting 
 the surface of the ocean in H, which is the point of high water. 
 
 Again, through h' draw L C h', meeting the circle in L, L'; these are 
 the points of low water. For let 
 
 LCS = u, LCM = z. 
 
 cos. L. a dx = cos. A S d h' = cos. 2 . S C h' = cos. 2 u = d x 
 and 
 
 cos. 2 z = cos. 2 L C M = d t. 
 .. S X cos. 2 u + M X cos. 2 z = max. 
 
 COR. If d f be drawn perpendicular to a m, a m represents the whole 
 difference between high and low water, a f equals the point effected by the 
 sun, m f that by the moon. 
 For 
 
 sin. 2 u = cos. 2 x, 
 sin. * y = cos. 2 x. 
 
 .. elevation + depression = S X : cos. 2 x + M X : cos. * y | 
 
 + S X cos. 2 x 'i 
 
 + M X : cos. 2 y f = S X : 2 cos. * x 1 -f. M X : 2 cos. 2 y 1 
 
 = S X cos. 2 x + M X cos. 2 y 
 and 
 
 d t = M X cos. 2 y 
 d x = S X cos. 2 x. 
 Bb2
 
 883 
 
 A COMMENTARY ON 
 
 [Boon III. 
 
 12. Conclusions deduced from the above (supposing that both the sun 
 and moon are in the equator.) 
 
 M' 
 
 1. At new and full moon, high water will be at noon and midnight. 
 For in this case C M, a m, C S, d h, C H, all coincide. 
 
 2. When the moon is in the quadrature at B, the place of high water is 
 also at B under the moon, when the moon is on the meridian, for C M is 
 perpendicular to C S, (m) coincides with C, (a m) with (a C), d h with 
 dC. 
 
 3. While the moon passes from the syzygy to the quadrature the place 
 of high water follows the moon's place, and is to the westward of it, over- 
 takes the moon at the quadratures, and is again overtaken at the next 
 syzygy. Hence in the first and third quadrants high water is after noon 
 or midnight, but before the moon's southing, and in second and fourth vice 
 versa. 
 
 4. t. M C H = max. when S C H = 45. S d h' = 90. and m' a 
 perpendicular to S C, and A a m' d = max., and a m' d m' d h'= 2 y'.
 
 BOOK III.] NEWTON'S PRINCIPIA. 389 
 
 Hence in the octants, the motion of the high water = moon's easterly 
 motion ; in syzygy it is slower ; in quadratures faster. Therefore the tide 
 day in the octants = 24h. 50' = the lunar day ; in syzygy it is less = 24h. 
 35'; in quadratures = 25h. 25'. 
 
 For take any point (u) near (m), draw u a, u d, and d i parallel to a u 
 and with the center (a) and radius a u, describe an arc (u v) which may 
 be considered as a straight line' perpendicular to am; u m and i h are 
 respectively equal to the motions of M and H, and by triangles u m v, 
 
 dmf. 
 
 um:ih::ma:mf. 
 
 Therefore the synodic motion of the moon's place : synodic motion ot 
 high water : : m a : m f. 
 
 COR. 1. At new or full moon, m a coincides with S a, and m f with S d j 
 at the quadratures, m a coincides with C a, and m f with C d ; therefore 
 the retardation of the tides at new or full moon : retardation at quadra- 
 tures ::Sa:Ca::M + S:M S. 
 
 COB. 2. In the octants, m a is perpendicular to S a, therefore m a, m f 
 coincide. Therefore the synodic motion of high water equals the synodic 
 motion of the moon. 
 
 COR. 3. The variation of the tide during a lunation is represented by 
 (m a) ; at S, m a = S a, at C = C a. 
 
 Therefore the spring tide : neap tide : : M + S : M S. 
 
 COR. 4. The sun contributes to the elevation, till the high water is in 
 the octants, after which (a f) is v e, therefore the sun diminishes the 
 elevation. 
 
 COR. 5. Let m u be a given arc of the moon's synodic motion, m v is 
 the difference between the tides m a, u a corresponding to it. 
 Therefore by the triangles m u v, m d f. 
 
 mu:mv::md:df. 
 
 .*. m v oc d f ; 
 and since 
 
 m d : d f : : r : sin. d m f : : r : sin. m d h : : r : sin. 2 M C H 
 m v a sip. 2 arc M H. 
 
 13. PROP. VI. In the triangle m d a, m d, d a and L, m d a are known 
 when the proportion M : S is known and the moon's elongation. 
 
 Let the angle m d a = a, 
 and make 
 
 M + S : M S : : tan. a : tan. b 
 
 Bb3
 
 390 A COMMENTARY ON [BOOK III. 
 
 then 
 
 a b a -f- b 
 
 y : ~2~' S ~2~ 
 For 
 
 M + S:M S::md + da:md da 
 
 mad+amd mad amd 
 : : tan. : tan. 
 
 2 x -f 2 y 2 x 2 y 
 
 :: tan. -^ : tan. 
 
 : : tan. x -f y : tan. x y 
 
 : : tan. a : tan. b, 
 .'. x + y : x y : : a : b, 
 .-. 2 x = a + b, 2 y = a b, 
 a + b 
 
 ' x = -i- 
 
 and 
 
 a b 
 
 y = -8-- 
 
 14. PROP. VII. To find the proportion between the accelerating forces 
 of the moon and sun. 1st. By comparing the tide day at new and full 
 moon with the tide day at quadratures. 
 
 35 : 85 : : M : S, 
 
 Also, at the time of the greatest separation of high water from the moon 
 in the triangle m' d a, m d : d a : : r : sin. 2 y : : M : S, 
 
 *- jj = sin. 2 y, 
 
 at the octants y is found =12 30', 
 
 c 
 
 , " cin 9^ 
 
 .'. *fr si n. > , 
 M 
 
 .-. M : S : : 5 : 2} nearly. 
 / Hence taking this as the mean proportion at the mean distances of the 
 
 moon and sun (if the earth =1) the moon = . 
 
 COR. 1. If the disturbing forces were equal there would be no high or 
 low water at quadratures, but there would be an elevation above the in- 
 scribed spheroid all round the circle, passing through the sun and moon 
 = 1 M + S.
 
 BOOK III.] NEWTON'S PRINCIPIA. 391 
 
 COR. The gravitation of the sun produces an elevation of 24 inches, 
 the gravitation of the moon produces an elevation of 58 inches. 
 .. the spring tide = 82 inches, and the neap tide = 33| inches. 
 
 15. COR. 3. Though M : S : : 5 : 2, this ratio varies nearly from (6 : 2) 
 to 4 : 2, for supposing the sun and moon's distance each =: 1000. 
 
 In January, the distance of the sun = 983, perigee distance of the 
 moon = 945. 
 
 In July, the distance of the sun = 1017, apogee distance of the moon 
 = 1055. 
 
 Disturbing force oc .pr- 3 ; hence 
 
 
 ,S 
 
 M 
 
 apogee 
 
 1.901 
 
 4.258 
 
 mean 
 
 2 
 
 5 
 
 perigee 
 
 2.105 
 
 5.925.* 
 
 
 
 5 c 
 
 A 3 d 3 
 
 The general expression is M = 
 
 
 
 To find the general expression above. 
 
 Disturbing force of different bodies (See Newton, Sect, llth, p. 66, 
 
 Cor. 14.) a jL, 
 
 .-. disturbing force S : disturbing force at mean distance : : D 3 : A 3 
 disturbing force M : disturbing force at mean distance : : d 3 : 5 3 , 
 
 , M 5 _d^ a 3 
 " S ; 2 '"' D 3 ' A~ 3 ' 
 
 M J3 A_ 
 "S" ~ 2 X D 
 
 Cl c\ X TTV 3 X * 3 > 
 
 TV/T 5 A 3 d 3 
 
 ' M = a" x S x D " x 3~ 3 
 (or supposing that the absolute force of the sun and moon are the same). 
 
 16. PROP. VIII. Let N Q S E be the earth, N S its axis, E Q its equa- 
 tor, O its center ; let the moon be in the direction O M having the de- 
 clination B Q. 
 
 * The solar force may be neglected, but the variation of the moon's distance, and proportion- 
 ally the variation of its action, produces am effect on the times, and a much greater on the heighta 
 of the tides. 
 
 Bb4
 
 392 
 
 A COMMENTARY ON 
 
 [BooK III. 
 
 Let D be any point on the surface of the earth, D C L its parallel of 
 latitude, N D S its meridian ; and let B' F b' f be the elliptical spheroid 
 of the ocean, having its poles in O M, and its equator F O f. 
 
 As the point D is carried along its parallel of latitude, it will pass 
 through all the states of the tide, having high water at C and L, and low 
 water when it comes to (d) the intersection of its parallel of latitude with 
 the equator of the watery spheroid. 
 
 Draw the meridian N d G cutting the terrestrial equator in G. Then 
 the arc Q G (converted into lunar hours) will give the duration of the 
 ebb of the superior tide, G E in the same way the flood of the inferior. 
 N. B., the whole tide G Q C, consisting of the ebb Q G, and the flood 
 G Q is more than four times G O greater than the inferior tide. 
 
 COR. If the spheroid touch the sphere in F and f, C C' is the height 
 at C, L L' the height at L, hence if L' q be a concentric circle C' q will 
 be the difference of superior and inferior tides. 
 
 CONCLUSIONS DRAWN FROM PROP. VIII. 
 
 1. If the moon has no declination, the duration of the inferior and su- 
 perior tides is equal for one day over all the earth. 
 
 2. If the moon has declination, the duration of the superior will be 
 longer or shorter than the duration of the inferior according as the 
 moon's declination and the latitude of the place are of the same or differ- 
 ent denominations. 
 
 3. When the moon's declination equals the colatitude or exceeds it,
 
 BOOK III.] NEWTON'S PRINCIPIA. 393 
 
 there will only be a superior or inferior tide in the same day, (the paral- 
 lel of latitude passing through f or between N and f.) 
 
 4. The sin. of arc G O = tan. of latitude X tan. declination. 
 For 
 
 rad. : cot. d O G : : tan. d G : sin. G O, 
 .-. sin. G O = cot. d O G X tan. G d 
 
 = tan. declination X tan. latitude. 
 
 17. PROP. IX. With the center C and radius C Q (representing the 
 
 P 
 
 M 
 
 whole elevation of the lunar tide) describe a circle which may represent 
 the terrestrial meridian of any place, whose poles are P, p, and equator 
 E Q. Bisect P C in O, and round O describe a circle P B C D ; let M 
 be the place on the earth's surface which has the moon in its zenith, Z 
 the place of the observer. Draw M C m, cutting the small circle in A, 
 and Z C N cutting the small circle in B ; draw the diameter BOD and 
 A I parallel to E Q, draw A F, G H, IK perpendicular to B D, and 
 join ID, A B, AD, and through I draw C M' cutting the meridian in 
 M'. Then after a diurnal revolution the moon will come into the 
 situation M', and the angle M' C N ( = the nadir distance) = supplement 
 the angle ICB = ^IDB. 
 
 Also the .ADB = BCA = zenith distance of the moon.
 
 394 A COMMENTARY ON [BooK III. 
 
 Hence D F, D K oc cos. * of the zenith and nadir distances to rad. D B. 
 a elevation of the superior and inferior tides. 
 
 CONCLUSIONS FROM PROP. IX. 
 
 1. The greatest tides are when the moon is in the zenith or nadir of the 
 observer. For in this case (when M approaches to Z) A and I move to- 
 wards D, B, and F coincides with B ; but in this case, the medium tide 
 which is represented by D H (an arithmetic mean to D K, D F) is di- 
 minished. 
 
 If Z approach to M, D and I separate ; and hence, the superior and 
 inferior and the medium tides all increase. 
 
 2. If the moon be in the equator, the inferior and superior tides are 
 equal, and equal M X (cos) * latitude. For since A and I coincide with 
 C, and F and K with (i) D i = D B X (cos.) 8 B D C = M X (cos.) 
 latitude. 
 
 3. If the observer be in the equator, the superior and inferior tides are 
 equal every where, and = M X (cos.) 2 of the declination of the moon. 
 For B coincides with C, and F and K with G ; P G = P C X cos. * of 
 the moon's declination = M x (cos.) 2 of the moon's declination. 
 
 4. The superior tides are greater or less than the inferior, according as 
 the moon and place of the observer are on the same or different sides of 
 the equator. 
 
 5. If the colatitude of the place equal the moon's declination or is less 
 than it, there will be no superior or inferior tide, according as the latitude 
 and the declination have the same or different denominations. For when 
 P Z=M Q, D coincides with I, and if it be less than M Q, D falls between 
 I and C, so that Z will not pass through the equator of the watery spheroid. 
 
 6. At the pole there are no diurnal tides, but a rise and subsidence 
 of the water twice in the month, owing to the moon's declining to both 
 sides of the equator. 
 
 18. PROP. X. To find the value of the mean tide. 
 A G = sin. 2 declination (to rad. = O C.) 
 and 
 
 O G = cos. 2 declination (to the same radius). 
 
 M 
 .-.OH = cos. 2 declination X cos. 2 lat. X ~g-, 
 
 .-.DH= OD + OH 
 
 1 + cos. 2 lat. X cos. 2 declination 
 = M X
 
 BOOK III.] NEWTON'S PIUNCIPIA. 395 
 
 Now as the moon's declination never exceeds 30, the cos. 2 declination 
 is always -f- v 2 , and never greater than \ ; if the latitude be less than 45, 
 the cos. 2 lat. is -J- v e, after which it becomes v e. 
 
 Hence 
 
 1. The mean tide is equally affected by north and south declination of 
 the moon. 
 
 2. If the latitude = 45, the mean tide M. 
 
 3. If the lat. be less than 45, the mean tide decreases as the declina- 
 tion increases. 
 
 4. If the latitude be greater than 45, the mean tide decreases as the 
 declination diminishes. 
 
 . -rf , , . , *! + cos. 2 declination 
 
 5. If the latitude = 0, the mean tide = M X
 
 BOOK I. 
 
 SECTION XII. 
 
 503. PROP. LXX. To find the attraction on a particle placed within 
 
 a spherical surface, force a 8 , -jr , . 
 
 distance 2 
 
 Let P be a particle, and through P draw H P K, 
 I P L making a very small angle, and let them 
 revolve and generate conical surfaces I P H, H 
 L P K. Now since the angles at P are equal 
 and the angles at H and L are also equal (for 
 both are on the same segment of the circle), 
 therefore the triangles H I P, P L K, are similar. 
 
 .-. HI:KL::HP:PL 
 
 Now since the surface of a cone GC (slant side) 2 , 
 
 .. surface intercepted by revolution of I P H : that of L P K : 
 
 
 
 and attractions of each particle in I P H : that of L P K : 
 
 ' HP'KL* 
 
 but the whole attraction of P oc the number of particles X attraction of 
 each, 
 
 HI* K L* 
 
 1 .-. the whole attraction on P from H I : from K L : : ^j T 't : v~-r't 
 
 HI Jv JL 
 
 :: J : 1; 
 
 and the same may be proved of any other part of the spherical surface ; 
 .'. P is at rest. 
 
 504. PROP. LXXL To find the attraction on a particle placed without 
 a spherical surface, force oc g - -p ,.
 
 398 
 
 A COMMENTARY ON 
 
 [SECT. XII. 
 
 Let A B, a b, be two equal spherical surfaces, and let P, p be two 
 particles at any distances P S, p s from their centers; draw P H K, 
 
 K 
 
 P I L very near each other, and S F D, S E perpendicular upon them, and 
 from (p) draw p h k, p i 1, so that h k, i 1 may equal H K, I L respective- 
 ly, and s f d, s e, i r perpendiculars upon them may equal S F D, S E, 
 I R respectively ; then ultimately PE = PF = pe = pf, and D F 
 = d f. Draw I Q, i q perpendicular upon P S, p s. 
 Now 
 
 and 
 
 PI:PF 
 
 p f : p i : 
 
 Again 
 
 and 
 
 PI:PS::IQ: SF 
 
 PI pf:pi.PF::IR:ir::IH:ih 
 
 _ .-. P I . p s : p i . P S : : I Q : i q 
 ps:pi::sf:iq 
 .-. P P. p f . p s : (p i) 2 . P F . P S : : I Q. I H : i q. i h 
 
 : : circumfer. of circle rad. I Q X I H : circumfer. of circle rad i q X i h 
 : : annulus described by revolution of 1 Q : that by revolution of i q. 
 Now 
 
 attraction on 1st annulus : attraction on 2d 
 
 And 
 
 1st annulus a 2d annulus 
 distance? * distance 2 
 PP.pf.ps (pi)'.PF. PS 
 
 PI 1 (pi) 2 
 
 :: pf. ps :PF.PS. 
 
 attraction on the annulus : attraction in the direction P S : : P I : P Q 
 
 :: PS: PF 
 
 P F 
 
 .. attraction in direction PS = p f. p s. p-~ 
 
 .-. whole att n . of P to S : whole att n . of (p) to s : : p f . p s . ,5-5 : P F . P S 2- 
 
 Jr o ps 
 
 1 * 
 
 P S s ' p s 1
 
 BooKl.] NEWTON'S PRINCIPIA. 399 
 
 and the same may be proved of all the annul! of which the surfaces are 
 
 composed, and therefore the attraction of P a - O r cc -r-. - ; from 
 
 P S 2 distance* 
 
 the center. 
 
 COR. The attraction of the particles within the surface on P equals the 
 attraction of the particles without the surface- 
 For K L : I H :': P L : P I : : L N : I Q. 
 
 .. annulus described by I H : annulus described by K L 
 
 ::IQ.IH:KL.LN::PP:PL 2 
 
 .. attraction on the annulus I H : attraction on the annulus K L 
 
 PI 2 PL 
 
 PI* PL 
 
 and so on for every other annulus, and one set of annuli equals the part 
 within the surface, and the other set equals the part without. 
 
 506. PROP. LXXII. To find the attraction on a particle placed with- 
 
 out a solid sphere, force oc g - -^ - ; . 
 
 distance 2 
 
 Let the sphere be supposed to be made up of spherical surfaces, and 
 
 the attraction of these surfaces upon P will <x -rr -- ;, and therefore 
 
 distance * 
 
 the whole attractions 
 
 number of surfaces content of sphere diameter 3 
 
 PIS 1 " PS 2 PS 2 
 
 and if P S bear a given ratio to the diameter, then 
 
 diameter 3 
 
 the whole attraction on P cc -~ ; a diameter- 
 
 diameter z 
 
 507. PROP. LXXIII. To find the attraction on the particle placed 
 within- 
 
 Let P be the particle ; with rad. S P describe 
 the interior sphere P Q ; then by Prop. LXX. 
 (considering the sphere to be made of spherical 
 surfaces,) the attraction of all the particles con- 
 tained between the circumferences of the two 
 circles on P will be nothing, inasmuch as they 
 are equal on each side of P, and the attraction 
 
 PS 3 
 of the other part by the last Prop, oc - cc P S.
 
 400 
 
 A COMMENTARY ON 
 
 [SECT. XIJ. 
 
 508. PROP. LXXIV. If the attractions of the particles of a sphere 
 oc r > and two similar spheres attract each other, then the spheres 
 
 will attract with a force a 8 as -^ -, of their centers. 
 
 distance 2 
 
 1 
 
 For the attraction of each, particle oc -^ ^ from the center of the 
 
 attracting sphere (A), and therefore with respect to the attracted particle 
 the attracting sphere is the same as if all its particles were concentrated 
 in its center. Hence the attraction of each particle in (A) upon the 
 
 whole of (B) will a -JT 1 of each particle in B from the center of P, 
 
 and if all the particles in B were concentrated in the center, the attraction 
 would be the same ; and hence the attractions of A and B upon each other 
 will be the same as if each of them were concentrated in its center, and 
 1 
 
 therefore oc 
 
 distance 2 ' 
 
 509. PROP. LXXVI. Let the spheres attract each other, and let 
 them not be homogeneous, but let them be homogeneous at correspond- 
 ing distances from the center, then they attract each other with forces 
 1 
 
 a*. 
 
 distance * * 
 
 G 
 
 II 
 
 Suppose any number of spheres C D and E F, I K and L M, &c. to 
 be concentric with the spheres A B, G H, respectively; and let C D and 
 I K, E F and L M be homogeneous respectively ; then each of these 
 
 spheres will attract each other with forces at*, -r . Now suppase 
 
 distance * 
 
 the original spheres to be made up by the addition and subtraction of 
 similar and homogeneous spheres, each of these spheres attracting each
 
 BOOK I.] 
 
 other with a force a g . -rr 
 
 distance 8 
 each other in the same ratio. 
 
 401 
 
 ; then the sum or differences will attract 
 
 510. PROP. LXXVII. Let the force cc distance, to find the attraction 
 of a sphere on a particle placed without or within it. 
 
 Let P be the particle, S the center, draw two planes E F, e f, equally 
 distant from S ; let H be a particle in the plane E F, then the attraction 
 of H on P a HP, and therefore the attraction in the direction S P ex 
 P G, and the attraction of the sum of the particles in E F on P towards 
 S oc circle E F . P G, and the attraction of the sum of the particles in 
 (e f) on P towards S oc circle e f . P g, therefore the whole attraction of 
 E F, e f, oc circle EF(PG+Pg) circle E F . 2 P S, therefore the 
 whole attraction of the sphere a sphere X P S. 
 
 When P is within the sphere, the attraction of the circle E F on P to- 
 wards S <x circle E F . P G, and the attraction of the circle (e f ) towards 
 S oc circle e f . P g, and the difference of these attractions on the whole 
 attraction to S a circle E F (P g P G) ac circle E F. 2 P S. There- 
 fore the whole attraction of the sphere on P a sphere X P S. 
 
 511. LEMMA XXIX. If any arc be described with the center S, rad. 
 
 S B, and with the center P, two circles be described very near each other 
 
 VOL. I. C c
 
 402 
 
 A COMMENTARY ON 
 
 [SECT. XII. 
 
 cutting, first, the circle in E, e, and P S in F, f ; and E D, e d, be drawn 
 perpendicular to P S, then ultimately, 
 
 Dd: Ff: : PE: PS. 
 
 DE: ES 
 
 : : D E : S G : : P E : P S. 
 
 512. PROP. LXXIX. Let a solid be generated by the revolutions of ai 
 evanescent lamina E F f e round the axis P S, then the force with which 
 the solid attracts Pa D E s . F f X force of each particle. 
 
 For 
 Dd:Ee:: 
 
 DT:ET 
 
 and 
 
 
 Ee:Ff :: 
 
 E e : e r 
 
 D r! F f : : 
 
 
 Draw E D, e d perpendiculars upon PS; let e d intersect E F in r ; 
 draw r n perpendicular upon E D. Then E r : n r : : P E : ED, .*. 
 E r . E D = n r . P S = D d . P E, .-.the annular surface generated by 
 the revolution of Era Er.EDa Dd.PE, and (P E remaining the 
 same) a D d. But the attraction of this annular surface on P a D d . 
 P E, and the attraction in the direction P E : the attraction in the direc- 
 tion P S : : P E : P D, 
 
 PD 
 
 .'. the attraction in the direction P S a 
 
 PE 
 
 .Dd. PE a PD.Dd 
 
 and the whole attraction on P of the surface described by E F a sum of 
 the PD.Dd. 
 
 Let P E = r, D F = x, 
 .-. P D = r x, 
 
 . P D . D d = r d x x d x, 
 
 .'.sumofPD.DdSrr/rdx xdx _ 2rx x * 
 
 DE< 
 
 a DE*, 
 
 2. 2 
 
 and therefore the attraction of lamina a D E 2 . F f X force of each particle.
 
 BOOK I.] 
 
 NEWTON'S PRINCIPIA. 
 
 403 
 
 D E* P S 
 513. PROP. LXXX. Take D N proportional to rr X force 
 
 of each particle at the distance P E, or if ^ represent that force, let D N 
 
 T~\ "C 1 2 P Q 
 
 ' , then the area traced out by D N will be proportional to 
 
 the whole attraction of the sphere. 
 
 For the attraction of lamina EFfeocDE 2 . F fx force of each parti- 
 
 D E 2 PS 
 cle <x (LEMMA XXIII) ~-^ . D d x force of each particle, or 
 
 DE 2 . PS 
 
 D d, /. D N . D d a attraction of lamina E F f e, and the 
 
 P E. V 
 
 sum of these areas or area A N B will represent the whole attraction of 
 the sphere on P. 
 
 514. PROP. LXXXI. To find the area A N B. 
 
 P _\B 
 
 Draw the tangent P H and H I perpendicular on P S, and bisect P I 
 in L ; then 
 
 Cc2
 
 404, A COMMENTARY ON [SECT. XII. 
 
 PE 8 = P S 2 + SE 2 + 2PS.SD 
 But 
 
 SE 1 = SH 2 = PS. SI, 
 .-. PE 2 = PS 2 + PS.SI + 2PS.SD 
 = PSPS + SI + 2SD$ 
 = P S {(P I + I S) + S I + 2 S Df 
 
 DE'=SE 2 SD 3 =SE 2 (LD LS) 2 
 = SE 2 LD 2 LS 2 + 2LD.LS 
 = 2LD.LS LD 2 (LS + SE)(LS SE) 
 = 2LD.LS LD 2 LB.LA, 
 
 ~ XT DE 2 .PS 2LD.LS.PS 
 .'. D N cc n ^ ^.r oc 
 
 PE.V V2SD.PS.V 
 LD 2 . PS LB.LA. PS 
 
 V2LD.PS.V V2LD.PS.V 
 
 and hence if V be given, D N may be represented in terms of L D and 
 known quantities. 
 
 515. Ex. 1. Let the force cc -^ ; to find the area A N B. 
 
 distance 
 
 Since F oe -^ a i , .-. V a P E, 
 
 distance V 
 
 2LS.LD.PS LD 2 .PS AL.LB.PS 
 " Di 2LD.PS 2LD.PS 2LD.PS 
 
 T LD AL.LB 
 ~2~ 8L.D ' 
 
 T G ^ , LD.Dd AL.LB. Dd 
 .-. D N . D d, or d . area a L S . D d T ^ , 
 
 & / Li L) 
 
 .'. area AND between the values of L A and L B 
 
 -TSfTB LA^ LB 2 -LA" AL.LB LB 
 =sL,b. ( (l,iJ j- 2 
 
 Now 
 
 LB LA 8 = (LB + LA).(LB LA) 
 
 = (LS + AS + LS AS)AB = 2LS.AB, 
 
 *xrr AT* 2LS.AB AL.LB,LB 
 
 .-. area AND = LS.AB ^ 1 =j .- 
 
 4 2 L A 
 
 LS.AB AL.LB ,LB 
 ~2~ ~2~ 1 ITA'
 
 BOOK I.] 
 
 NEWTON'S PRINCIP1A. 
 
 405 
 
 516. To construct this area. 
 
 To the points L, A, B erect L 1, A a, B b, 
 perpendiculars, and let A a = L B, and B b 
 = L A, through the points (a), (b), de- 
 scribe an hyperbola to which L 1, L B are 
 asymptotes. Then by property of the hy- 
 perbola, AL.Aa = LD.DF, 
 
 A L. Aa A L.LB 
 
 .-. D F = 
 .-.DF.Dd = 
 
 LD LD 
 AL.LB.Dd 
 L~D ' 
 
 .-. area AaFD=/DF.Dd = AL.L B/L D, 
 
 T B 
 
 /. hyperbolic area A a f b B = A L . L B 
 
 The area AaBb = Bb.AB 
 Bb.AB 
 
 A B a 
 
 iB 
 
 an + Bb 
 2 - ' 
 
 Aa + Bb 
 = - 2 - 
 
 LB + L A 
 
 . A B = L S . A B, 
 
 .. area a f b a = area A a B b area A a f b B 
 
 T -i-k f L 13 
 
 517. Ex. 2. Let the force a 
 
 = LS. AB 
 1 
 
 distance 3 ' 
 
 "J LA' 
 
 to find the area A N B. 
 
 but 
 
 Let V = 
 .-. D N = 
 
 V.PE = 
 .-. D N = 
 
 P 
 
 2 AS*' 
 2LS.LD.PS 
 
 LD 2 . PS AL.LB.PS 
 
 PE.V PE.V 
 
 PE 4PS.LD' 
 
 PE.V 
 
 PS 
 
 2AS 2 ' 
 SI.LS 
 
 2 AS 2 
 SI 
 2 
 
 LD 
 .-./DN.x' = Si.LS/LD 
 
 AL.LB.SI, LJ)t 
 
 2LD 2 b> sr 
 
 SI.LD . AL.LB.SI 
 
 _ C T T C 
 O * MJt Q 
 
 = SI.LS 
 
 2 2LD 
 
 .'. area between the values of L A and L B 
 LB SI.(LB LA) /LB.SI AL.SI 
 
 LB 
 LA 
 
 SLAB. 
 
 Cc3
 
 406 
 
 A COMMENTARY ON 
 
 [SECT. xir. 
 
 To construct this area. 
 1 a 
 
 S Dd 
 
 Take S I = S s, and describe a hyperbola passing through a, s, b, to which 
 L 1, L B are asymptotes ; then as in the former case, the area A a n b B 
 
 .-. the area ANBrrSI.LS /*-? SLAB. 
 
 'LA 
 
 518. PROP. LXXXII. Let I be a particle within the sphere, and P 
 the same particle without the sphere, and take 
 
 S P : S A : : S A : S I, 
 
 then will the attracting power of the sphere on I : attracting power of the 
 sphere on P 
 
 : : V S I. V force on I : V S P. V force on P. 
 D N force on the point P : D' N' force on the point I 
 
 D E* P S DE g I S 
 
 PE.V IE.V 
 
 ;: PS.IE.V: IS. PE.V. 
 
 Let 
 
 V : V' :: P E n ; IE",
 
 NEWTON'S PRINCIPIA. 
 
 DN : D'N' :: PS.IE.IE n :IS.PE.EE, 
 
 BOOK L] 
 then 
 
 but 
 
 P S : S E : : S E : S I, 
 and the angle at S is common, 
 
 .. triangles P S E, I S E are similar, 
 .-. P E : I E : : P S : S E : : S E : S I, 
 .-. DN : D'N' :: PS.SE.IE" : PS.SI.PE", 
 :: SE.IE n : SI.PE" 
 
 407 
 
 VST'.IE": 
 
 ?: VSF : SI* VSI.PSf. 
 
 519. PROP. LXXXIII. To find the attraction of a segment of aspheie 
 upon a corpuscle placed within its centre. 
 
 Draw the circle F E G with the 
 center P, let R B S be the segment of 
 the sphere, and let the attraction of the 
 spherical lamina E F G upon P be 
 proportional to F N, then the area de- 
 
 scribed by F N <x whole attraction of 
 
 the segment to P. 
 
 Now the surface of the segment 
 E F G a P F D F, and the content 
 of the lamina whose thickness is O cc 
 PF D F O. 
 
 Let F oc -p- and the attraction on P of the particle in that 
 
 distance n 
 
 spherical lamina, oc (Prop. LXXIII.) -n-=nT~ 
 
 cc 
 
 cc 
 
 (2PF F D F D 2 ) O 
 PF n 
 
 2 FD O FD 2 O 
 p-p-n^rr ' p jm > 
 
 .-. if F N be taken proportional to Fn-1 p-^ , the area traced 
 out by F N will be the whole attraction on P. 
 
 520. PROP. LXXXIV. To find the attraction when the body is placed 
 in the axis of the segment, but not in the center of the sphere. 
 
 Pel
 
 408 
 
 A COMMENTARY ON 
 
 [SECT. XIII. 
 
 Describe a circle with the radius P E, and the segment cut off by the 
 revolution of this circle E F K round P B, will have P in its center, and 
 
 the attraction on P of this part may be found by the preceding Proposi- 
 tion, and of the other part by PROP. LXXXI. and the sum of these at- 
 tractions will be the whole attraction on P. 
 
 SECTION XIII. 
 
 521. PROF. LXXXV. If the attraction of a body on a particle placed 
 iu contact with it, be much greater than if the particle were removed at 
 any the least distance from contact, the force of the attraction of the par- 
 ticles a in a higher ratio than that of -p , . 
 
 distance z 
 
 For if the force a -r. g , and the particle be placed at any distance 
 
 from the sphere, then the attraction oc -^ , from the center of the 
 
 distance 2 
 
 sphere, and .. is not sensibly increased by being placed in contact with 
 the sphere, and it is still less increased when the force a in a less ratio 
 
 than that of -^ r, and it is indifferent whether the sphere be homo- 
 distance * 
 
 geneous or not ; if it be homogeneous at equal distances, or whether the 
 body be placed within or without the sphere, the attraction still varying in 
 the same ratio, or whether any parts of this orbit remote from the point of 
 contact be taken away, and be supplied by other parts, whether attractive 
 or not, .-. so far as attraction is concerned, the attracting power of this 
 sphere, and of any other body will not sensibly differ ; .-.if the pheno-
 
 BOOK I.] NEWTON'S PRINCIPIA. 409 
 
 mena stated in the Proposition be observed, the force must vary in a higher 
 
 ratio than that of ~r. - . 
 distance 2 
 
 522. PROP. LXXXVI. If the attraction of the particles cc in a higher 
 
 ratio than T . - - , or oc -p - - , then the attraction of a body placed 
 distance 3 distan ce 3 
 
 in contact with any body, is much greater than if they were separated 
 even by an evanescent distance. 
 
 For if the force of each particle of the sphere oc in a higher ratio than 
 
 that of T - = , the attraction of the sphere on the particle is indefinitely 
 distance 3 
 
 increased by their being placed in contact, and the same is the case for 
 any meniscus of a sphere ; and by the addition and subtraction of attrac- 
 tive particles to a sphere, the body may assume any given figure, and 
 .*. the increase or decrease of the attraction of this body will not be sensi- 
 bly different from the attraction of a sphere, if the body be placed in con- 
 tact with it. 
 
 523. PROP. LXXXVII. Let two similar bodies, composed of particles 
 equally attractive, be placed at proportional distances from two particles 
 which are also proportional to the bodies themselves, then the accelerat- 
 ing attractions of corpuscles to the attracting bodies will be proportional 
 to the whole bodies of which they are a part, and in which they are simi- 
 larly situated. 
 
 For if the bodies be supposed to consist of particles which are propor- 
 tional to the bodies themselves, then the attraction of each particle in one 
 body : the attraction of each particle in the other body, : : the attraction 
 of all the particles in the first body : the attraction of all the particles in 
 the second body, which is the Proposition. 
 
 COB. Let the attracting forces oc -^ - - , then the attraction of a 
 
 particle in a body whose side is A : B 
 
 A 3 : B 3 
 
 * distance n from A ' distance n from B 
 
 A 3 B^ 
 : : A n : B n 
 
 __ 
 
 A n ~ 3 B n ~ 3 ' 
 if the distances cc as A and B.
 
 410 
 
 A COMMENTARY ON 
 
 [SECT. XIII. 
 
 524. PROP. LXXXVIII. If the particles of any body attract with a 
 force a distance, then the whole body will be acted upon by a particle 
 without it, in the same manner as if all the particles of which the body is 
 composed, were concentrated in its center of gravity. 
 
 Let R S T V be the body, Z the par- 
 tide without it, let A and B be any 
 two particles of the body, G their cen- 
 ter of gravity, then A A G = B B G, 
 and then the forces of Z of these parti- 
 cles GC A A Z, B B Z, and these 
 forces may be resolved into A A G + 
 A G Z, B B G + B G Z, and A A G 
 being = B B G and acting in opposite 
 directions, they will destroy each other, 
 and .'. force of Z upon A and B will be 
 
 proportional to A Z G + B Z G, or to (A + B) Z G, .-. particles A 
 and B will be equally acted upon by Z, whether they be at A and B, or 
 collected in their center of gravity. And if there be three bodies A, B, 
 C, the same may be proved of the center of gravity of A and B (G) and 
 C, and .*. of A, B, and C, and so on for all the particles of which the 
 body is composed, or for the body itself. 
 
 525. PROP. LXXXIX. The same applies to any number of bodies 
 acting upon a particle, the force of each body being the same as if it 
 were collected in its center of gravity, and the force of the whole system 
 of bodies being the same as if the several centers of gravity were collected 
 in the common center of the whole. 
 
 526. PROP. XC. Let a body be placed in a perpendicular to the plane 
 of a given circle drawn from its center ; to find the attraction of the circu- 
 lar area upon the body. 
 
 With the center A, radius = A D, let 
 a circle be supposed to be described, to 
 whose plane A P is perpendicular. From 
 any point E in this circle draw P E, in 
 P A or it produced take P F = P E, and 
 draw F K perpendicular to P F, and let 
 F K oc attracting force at E on P. Let 
 1 K L be the curve described by the point 
 K, and let I K L meet A D in L, take 
 P H = P D, and draw H I perpendicular
 
 BOOK I.] NEWTON'S PRINCIPIA. 411 
 
 to P H meeting this curve in I, then the attraction on P of the circle 
 oc A P the area A H I L. 
 
 For take E e an evanescent part of A D, and join P e, draw e C per- 
 pendicular upon P E, .-. E e : E C : : P E : A E, .-. E e . A E = E C x 
 P E cc annulus described by A E, and the attraction of that annulus in 
 
 P A 
 
 the direction P A oc E C . P E . ^-^ X force of each particle at E cc E C X 
 
 f sLi 
 
 P A X force of each particle at E, but E C = F f, .-. F K . F f cc E C x 
 the force of each particle at E, .'. attraction of the annulus in the direction 
 PA a P A . F f. F K, and .-. P A x sum of the areas F K . Ff or P A 
 the area A H I L is proportional to the attraction of the whole part de- 
 scribed by the revolution of A E. 
 
 527. COR. 1. Let the force of each particle cc -r- i> at P F =r x, 
 
 let b = force at the distance a, 
 
 ba 2 
 
 .. F K the force at the distance x = 
 
 * 
 
 x 2 
 
 b a 2 dx 
 
 .-. attraction = PA.FK.Ff=PAy 
 
 cc P A ^cc A ^ F , 
 
 and between the values of P A and P H, the attraction 
 
 apA A-pir a I "^H- 
 
 528. COR. 2. Let the force cc -p , then T K = V > 
 
 distance a x n 
 
 .'. attraction = P A /* d x cc r X r + Cor., 
 
 -/x 11 n 1 x"' 
 
 and between the values of P A and P H, 
 
 attraction = 
 
 1 PA 
 
 a PAn-^PH"- 1 ' 
 
 529. COR. 3. Let the diameter of a circle become infinite, or P H 
 
 oc OD, then the attraction cc ~ j . 
 
 Jr A " " 
 
 530. PROP. XCI. To find the attraction on a particle placed in the 
 axis produced of a regular solid.
 
 412 
 
 A COMMENTARY ON 
 * R 
 
 [SECT. XIII. 
 
 Let P be a body situated in the axis A B of the curve D E C G, by 
 the revolution of which the solid is generated. Let any circle It F S 
 perpendicular to the axis, cut the solid, and in the semidiameter F S of 
 the solid, take F K proportional to the attraction of the circle on P, then 
 F K . F f a attraction of the solid whose base = circle R F S, and depth 
 = F f, let I K L be the curve traced out by F K, .-. A L K F a at- 
 traction of the solid. 
 
 COR. 1. Let the solid be a cylinder, the force varying as r . . 
 
 J J distance 2 
 
 Then the attraction of the circle R F S, or F K which is proportional 
 
 P F 
 
 to that attraction a 1 ^-J . 
 
 i rl 
 
 Let P F = x, F R = b, 
 
 .-. area a x Vx 2 + b * .
 
 BOOK I.] 
 
 NEWTON'S PRINCIPIA. 
 
 413 
 
 Now if P A = x, attraction = 0, 
 .-. Cor. = P D P A, 
 .. whole attraction = P B 
 = AB 
 
 Let A B = oo = P E = P D, 
 .'. atraction = A B. 
 
 PE+PD PA 
 P E + P D. 
 
 531. COR. 3. Let the body P be placed 
 within a spheroid, let a spheroidical shell 
 be included between the two similar 
 spheroids DOG, K N I, and let the 
 spheroid be described round S which 
 will pass through P, and which is simi- 
 lar to the original spheroid, draw D P E, 
 F P G, very near each other. Now P D 
 = BE, PF = CG, PH = BI, PK 
 = CL. 
 
 .-. F K = L G, and D H = I E, 
 
 and the parts of the spheroidical shell which are intercepted between these 
 lines, are of equal thickness, as also the conical frustums intercepted by 
 the revolution of these lines, and 
 
 .*. attraction on P by the part D K : . . . . G I 
 number of particles in D K ... G* 
 
 ~~ 
 
 G 
 
 E 
 
 P G 
 
 
 and the same may be proved of every other part of a spheroidical shell, and 
 .'. body is not at all attracted by it; and the same may be proved of all the 
 other spheroidical shells which are included between the spheroids, A O G, 
 and C P M, and .'. P is not affected by the parts external to C P M, and 
 ,-. (Prop. LXXII.), 
 
 attraction on P : attraction on A : : PS: AS. 
 
 532. PROP. XCII1. To find the attraction of a body placed without an 
 infinite solid, the force of each particle varying as -TT- - , where n is 
 greater than 3. 
 
 Let C be the body, and let G L, H M, K O, &c. be the attractions 
 at the several infinite planes of which a solid is composed on the
 
 414 
 
 A COMMENTARY ON 
 
 [SECT XIII. 
 
 body C; then the area G L O K equals the whole attraction of a solid 
 on C. 
 
 L 
 
 M 
 
 
 
 
 
 
 -N 
 
 
 
 
 
 G 
 
 H 
 
 I 
 
 K 
 
 
 I 
 
 in 
 
 n 
 
 o 
 
 
 Now if the force a T . . 
 
 distance" 
 
 Then 
 
 H M a CHn _, (Cor. 3. Prop. XC) 
 .-./HM.dx ./^ a - * + Cor. 
 
 C G"- 3 C H n ~ 3 
 
 and if H C = oo 
 
 then the area G L O K a -^ 
 
 1 
 
 GO- 3 * 
 Case 2. Let a body be placed within the solid. 
 
 
 N 
 C 
 
 O 
 I K 
 
 
 
 
 o 
 
 
 G 
 
 Let C be the place of the body, and take C K = C G ; the part of 
 the solid between G and K will have no effect on the body C, and there- 
 fore it is attracted to remain as if it were placed without it at the distance 
 CK. 
 
 1 1 
 
 .'. attraction cc 
 
 GC 
 
 C K n ~ 3 CG n ~ 3 *
 
 BOOK L] 
 
 NEWTON'S PRINCIPIA. 
 
 415 
 
 SECTION XIV. 
 
 534. PROP. XCIV. Let a body move through a similar medium, ter- 
 minated by parallel plane surfaces, and let the body, in its passage through 
 this medium, be attracted by a force varying according to any law of its 
 distance from the plane of incidence. Then will the sine of inclination be 
 to the sine of refraction in a given ratio. 
 
 Let A a, B b be the planes which terminate the medium, and G H be 
 the direction of the body's incidence, and I R that of its emergence. 
 
 Case 1. Let the force to the plane A a be constant, then the body will 
 describe a parabola, the force acting parallel to I R, which will be a diameter 
 of the parabola described. H M will be a tangent to the parabola, and if 
 K I be produced I L will also be a tangent to the parabola at I. Let K I 
 produced meet G M in L v.ith the center L, and distance L I describe 
 a circle cutting I R in N, and draw L O perpendicular to I R. Now by a 
 property of the parabola M I =. I v, 
 
 .-. M L = H L, .-. M O = O R, and .-. M N = I R. 
 
 The angle L M I=the angle of incidence, and the angle M I L = sup- 
 plement of M I K = supplemental angle of emergence. 
 Now 
 
 L. MI = MH 2 = 4 ML 2
 
 416 A COMMENTARY ON [SECT. XIV. 
 
 but 
 
 MN.MI=MI.IR=MQ.MP=ML+LQ.ML LQ 
 
 = ML 8 LQ* 
 . M j = ML 2 -LQ* 
 
 .-. L:IR::4ML 8 : ML 2 LQ 
 but L and I R are given 
 
 .-.4 ML 2 a ML 8 LQ* 
 .-.ML 2 ocLQ 2 a LI 2 
 
 .*. M L a L I or sin. refraction : sin. inclination in a given ratio. 
 Case 2. Let the force vary according to . 
 any law of distance from A a. g~ 
 
 Divide the medium by parallel planes A a, <f~ 
 B b, C c, D d, &c. and let the planes be at *) 
 evanescent distances from each other, and 
 let the force in passing from A a to B b, 
 from B b to C c, from C c to D d, &c. be 
 uniform. 
 
 .*. sin. I at H : sin. R at H : : a : b 
 sin. R or I at I : sin. R at K : : c : d 
 sin. R or I at K : sin. R at R : : e : f, and so on. 
 
 .*. sin. I at H : sin. R at R : : a . c . e : b . d . f and in a constant pro- 
 portion. 
 
 535. PROP. XCV. The velocity of a particle before incidence : velocity 
 after emergence : : sin. emergence : sin. incidence. 
 
 G 
 
 \- b 
 
 D 
 
 '\T 
 
 K 
 
 Take A H = I d, and draw A G, d K perpendicular upon A a, D d, 
 meeting the directions of incidence and emergence in G, K. Let the 
 motion of the body be resolved into the two G A, A H, Id, d k, the ve-
 
 BOOK I.] NEWTON'S PRINCIPIA. 417 
 
 locity perpendicular to A a cannot alter the motion in the direction A a ; 
 therefore the body will describe G H, I K in the same time as the spaces 
 A H, I d are described, that is, it will describe G H, I K in equal times 
 before the incidence and after the emergence. 
 
 Velocity before incidence : velocity after emergence : : G H : I K 
 
 A H . Id 
 
 * sin. incidence ' sin. emergence 
 : : sin. emergence : sin. incidence. 
 
 536. PROP. XCVI. Let the velocity before incidence be greater than 
 the velocity after emergence, then, by inclining the direction of the inci- 
 dent particle perpetually, the ray will be refracted back again in a similar 
 curve, and the angle of reflection will equal the angle of incidence. 
 
 fi f \ 
 
 AX" 
 
 ii/a 
 
 B \P 
 
 p / i. 
 
 C ^^Q *1 
 
 / c 
 
 D \^ K^' 
 
 d 
 
 E 
 
 e 
 
 Let the medium be separated by parallel planes A a, B b, C c, D d, 
 E e, &c. and since the velocity before incidence is greater than the 
 velocity after emergence. .. sin. of emergence is greater than sin. of in- 
 cidence. .-. H P, P Q, Q Rj &c. will continually make a less angle with 
 H a, P b, Q c, R d, &c. till at last it coincides with it as at R ; and after 
 this it will be reflected back again and describe the curve R q p h g simi- 
 lar to R Q P H G, and the angle of emergence at h will equal the angle 
 of incidence at H. 
 
 537. PROP. XCVII. Let sin. incidence : sin. refraction in a given ra- 
 dio, and let the rays diverge from a given point ; to find the surface of 
 medium so that they may be refracted to another given point. 
 
 Let A be the focus of incident, B of refracted rays, and let C D E 
 be the suiface which it is required to determine. Take D E a small arc, 
 
 VOL. I. D d
 
 418 A COMMENTARY ON [SECT. XIV. 
 
 and draw E F, E G perpendiculars upon A D and D B ; then D F, D G 
 are the sines of incidence and refraction ; or increment of A D : decrement 
 of B D : : sin. incidence : sin. refraction. Take .*. a point C in the axis 
 through which the curve ought to pass, and let C M : C N : : sin. inci- 
 dence : sin. refraction, and points where the circles described with radii 
 A M, B N intersect each other will trace out the curve. 
 
 538. COR. 1. If A and B be either of them at an infinite distance or at 
 any assigned situation, all the curves, which are the loci of D in different 
 situations of A and B with respect to C, will be traced out by t'lis 
 process. 
 
 lv 
 
 AC B 
 
 539. COR. 2. Describe circles with radii A C and C B, meeting A D, 
 B D in P and Q ; then P D : D Q : : sin. incidence : sin. refraction, since 
 P D, D Q are the increments of B C and A C.
 
 BOOK II. 
 
 SECTION I. 
 
 1. PROP. I. Suppose the resistance <x velocity, and supposing the whole 
 time to be divided into equal portions, the motion lost will velocity, and 
 oc space described. Therefore by composition, the whole decrement of the 
 velocity cc space described. 
 
 COR. Hence the whole velocity at the beginning of motion : that part 
 which is lost : : the whole space which the velocity can describe : space 
 already described. 
 
 2. PROP. II. Suppose the resistance ex velocity. 
 
 Case 1. Suppose the whole time to be divided into equal portions, and 
 at the beginning of each portion, the force of resistance to make a single 
 impulse which will a velocity, and the decrement of the velocity 
 oc resistance in a given time, oc velocity. Therefore the velocities 
 at the beginning of the respective portions of time will be in a con- 
 tinued progression. Now suppose the portions of time to be diminished 
 sine limite, and then the number increased ad infinitum, then the force of 
 resistance will act constantly, and the velocity at the beginning of equal 
 successive portions of time will be in geometric progression. 
 
 Case 2. The spaces described will be as the decrements of the velocity 
 oc velocity. 
 
 3. COR. 1. Hence if the time be represented by any line and be divid- 
 ed into equal portions, and ordinates be drawn perpendicular to this 
 line in geometric progression, the ordinates will represent the velocities, 
 and the area of the curve which is the logarithmic curve, will be as the 
 spaces described. 
 
 Dd2
 
 420 
 
 A COMMENTARY ON 
 
 [SECT. I. 
 
 Suppose L S T to be the logarithmic curve to the asymptote A Z. 
 A L, the velocity of the body at the beginning of the motion. 
 
 P Q 
 
 h H 
 
 K Z 
 
 The space described in the time A H with the first velocity continued 
 uniform : space described in the resisting medium, in the same time : : 
 A H P L : area A L S H : : rect. A L X PL: rect. A L X P S * 
 
 : : P L : P S (if A L = subtan. of the curve). 
 
 Also since H S, K T representing the velocities in the times A H, A K ; 
 P S, Q T are the velocities lost, and therefore a spaces described. 
 
 4. COR. 1. Suppose the resistance as well as the velocity at the begin- 
 
 ning of the motion to be represented by the line C A, and after any time by 
 the line C D. The area A B G D will be as the time, and A D as the 
 space described. 
 
 For if A B G D increase in arithmetical progression the areas being 
 the hyperbolic logarithms of the abscissas, the abscissa will decrease in 
 geometrical progression, and therefore A D will increase in the same 
 proportion. 
 
 5. PROP. III. Let the force of gravity be represented by the rectangle 
 
 * Let the subtangent = M. Then the whole area of the curve = M X A L. 
 
 .-. the area ALSH = MXAL MXHS=MXPS=ALXPS.
 
 BOOK II.] 
 
 NEWTON'S PRINCIPIA. 
 
 421 
 
 BACH, and the force of resistance at the beginning of the motion by 
 the rectangle B A D E on the other side of A B. 
 
 D d A I i 
 
 Describe the hyperbola G B K between the asymptotes A C and C H 
 cutting the perpendiculars D E, d e, in G and g. 
 
 Then if the body ascend in the time represented by the area D G g d, 
 the body will describe a space proportional to the area E G g e, and the 
 whole space through which* it can ascend will be proportional to the area 
 EGB. 
 
 If the body descend in the time A B K I, the area described is B F K. 
 
 For suppose the whole area of the parallelogram B A C H to be di- 
 
 A a K L M N 
 
 vided into portions, which shall be as the increments of the velocity in 
 equal times, therefore A k, A 1, A m, A n, &c. will oc velocity, and there- 
 fore a resistances at the beginning of the respective times. 
 
 Let A C : A K : : force of gravity : resistance at the beginning of the 
 second portion of time, then the parallelograms B A C H, k K C H, &c. 
 will represent the absolute forces on the body, and will decrease in geome- 
 trical progression. Hence if the lines K k, L 1, &c. be produced to meet 
 
 Dd3
 
 A COMMENTARY ON [SECT. I. 
 
 the curve in q, r, &c. these hyperbolic areas being all equal will repre- 
 sent the times, and also the force of gravity which is constant. But the 
 area B A K q : area Bqk::Kq:4kq:: AC: ^ A K : : force of 
 gravity : resistance in the middle of the first portion of time. 
 
 In the same way, the areas q K L r, r L M s, &c. are to the areas 
 q k 1 r, r 1 m s, &c. as the force of gravity to the force of resistance in the mid- 
 die of the second, third, &c. portions of time. And since the first term is 
 constant and proportional to the third, the second is proportional to the 
 fourth, similarly as to the velocities, and therefore to the spaces described. 
 
 .. by composition B k q, B r 1, B s m, &c. will be as the whole spaces 
 described, Q. e. d. 
 
 The same may be proved of the ascent of the body in the same way. 
 
 6. COR. 1. The greatest velocity which the body can acquire : the velo- 
 city acquired in any given time : : force of gravity : force of resistance 
 at the end of the given time. 
 
 7. COR. 2. The times are logarithms of the velocities. 
 
 8. COR. 4. The space described by the body is the difference of the space 
 representing the time, and the area representing the velocity, which at the 
 beginning of the motion are mutually equal to each other. 
 
 Suppose the resistance to a velocity. 
 
 r v 
 
 '. c 2 : v 2 : : r : j- = retarding force corresponding with the velocity (v) 
 .-. vdv= g X T - X dx, 
 
 \s 
 
 c* dv 
 .-. dx = -- x- 
 
 g v 
 
 .-. x = b X 1 v + C, 
 
 .'. x = b X 1 
 v 
 
 __dx^_ bdv 
 
 v v 2 ' 
 
 .-. t = b X --- f- Cor. 
 v 
 
 1111 
 
 __ u X ~ cc' ~~ 
 
 V C V C 
 
 .. the times being in geomelncal progression, the velocities C, d, E, &c. 
 will be in the same inverse geometrical progression. 
 
 Also the spaces will be in arithmetical progression.
 
 BOOK II.] 
 
 NEWTON'S PRINCIPIA. 
 
 423 
 
 9. PROP. IV. Let D P be the direction of the projectile, and let it 
 represent the initial velocity ; draw C P perpendicular to C D, and 
 
 N 
 
 E 
 
 R A 
 
 F C 
 
 let D A : A C : : resistance : gravity. Also D P : C P : : resistance : 
 gravity, .-. DAxDP:CPxCA::R:G. Between D C, C P de- 
 scribe a hyperbola cutting D G and A B perpendicular to D C in G and B, 
 from R draw R V perpendicular cutting D P in V and the hyperbola in T, 
 complete the parallelogram G K C D and make N : Q B : : C D : C P. 
 Take 
 
 G T t ., G T E I 
 
 r = ^ or R r = 
 
 N 
 
 N 
 
 for since 
 
 R V = 
 
 N : QB::CD:CP::DR:RV, 
 D R x QB 
 
 and 
 
 GTEI DR X QB GTt 
 
 ~N~ ~N- 
 
 in the time represented by D R T G the body will be at (r), and the great- 
 est altitude = a, and the velocity ex r L. 
 
 For the motion may be resolved into two, ascending and lateral. The 
 lateral motion is represented by D R, and the motion in ascent by R r, 
 which 
 
 a DR X QB GTt, 
 or 
 
 DRxAB DG.RT 
 
 Ddi
 
 424 
 
 A COMMENTARY ON 
 
 [SECT. II. 
 
 or 
 
 D R X A B D R x AQ 
 
 ~~N~ 
 D R : R r : : N : A B A Q, or Q B 
 
 : : C D : C P, 
 
 : : lateral motion . ascending motion at the beginning, 
 (r) will be the place of the body required. 
 
 SECTION II. 
 
 10. PROP. V. Suppose the resistance to vary as the velocity 2 . 
 Then as before, the decrement of velocity a resistance oc velocity ? . 
 
 D 
 
 AKI,M T D 
 
 Let the whole time A D be divided into a great number of equal por- 
 tions, and draw the ordinates A B, K k, L 1, M m, &c. to the hyperbola 
 described between the two rectangular asymptotes, C H, CD; then by the 
 property of the hyperbola, 
 
 A B : K k : : C K : C A, 
 .-. AB Kk:Kk::AK:CA 
 
 ::ABxAK:ABxCA. 
 .-. AB KkaABxKk. 
 In the same way 
 
 Kk LI a Kk 2 , &c. 
 
 or 
 
 A B \ K k , L 1 2 , &c. 
 
 are proportional to their differences. 
 
 .\ velocities will decrease in the same proportion. Also the spaces de- 
 scribed are represented by the areas described by the ordinates ; hence in
 
 BOOK II.] 
 
 NEWTON'S PRINCiPIA. 
 
 425 
 
 the time A M the space described may be represented by the whole area 
 AMmB. 
 
 Now suppose the lines C A, C K, &c. and similarly A K, K L, &c. in 
 geometrical progression, then the ordinates will decrease in the inverse 
 geometrical progression, and the spaces will be all equal to each other. 
 
 Q. e. d. 
 
 ] 1. COR. 1. The space described in the resisting medium : the space de- 
 scribed with the first velocity continued uniform for the time AD:: the 
 hyperbolic area A D G B : rectangle A B X AD. 
 
 12. COR. 3. The first resistance equals the centripetal force which would 
 generate the first velocity in the time A C, for if the tangent B T be drawn 
 to the hyperbola at B, since the hyperbola is rectangular A T = A C, and 
 with the first resistance continued uniform for the time A C the whole 
 velocity A B would be destroyed, which is the time in which the same ve- 
 locity would be generated by a force equal the first resistance. For the 
 first decrement is A B K k, and in equal times there would be equal de- 
 crements of velocity. 
 
 13. COR. 4. The first resistance : force of gravity : : velocity generated 
 by the force equal the first resistance in the time A C : velocity generated 
 by the force of gravity in the same time. 
 
 14. COR. 5. Vice versa, if this ratio is given, every thing else may be 
 found. 
 
 15. PROP. VIII. Let C A represent the force of gravity, A K the resis- 
 tance, .*. C K represents the absolute force at any time (if the body de- 
 scend) ; A P, a mean proportional to A C and A K, represents the velo- 
 city ; K L, P Q are contemporaneous increments of the resistance and 
 the velocity. 
 
 Then since 
 
 AP 2 aAK, KLa2APxPQxAPxKC,
 
 426 
 
 A COMMENTARY ON 
 
 [SECT. II. 
 
 the increment of velocity oc force when the time is given, 
 
 .-. K L x K N a A P x K C x K N, 
 
 .*. ultimately K L O N (equal the increment of the hyperbolic area) 
 ce A P a velocity, a space described, and the whole hyperbolic area = 
 the sum of all the K L O Ns which are proportional to the velocity, and 
 .*. space described. .. If the whole hyperbolic area be divided into equal 
 portions the absolute force C A, C I, C K, &c. are in geometrical pro- 
 gression. Q. e. d. 
 
 16. COR. 1. Hence if the space described be represented by a hyper- 
 bolic area, the force of gravity, velocity, and resistance, may be repre- 
 sented by lines which are in continued proportion. 
 
 17. COR. 2. The greatest velocity = A C. 
 
 18. COR. 3. If the resistance is known for a given velocity, the greatest 
 velocity : given velocity : : V force of gravity : V given resistance. 
 
 19. PROP. IX. Let A C represent the greatest velocity, and A D be per- 
 
 pendicular and equal to it. With the center D and radius A D describe 
 the quadrant A t E and the hyperbola A V Z. Draw the radii D P, D p. 
 Then 
 
 Case 1. If the body ascend ; draw D v q near to D p, .-. since the sector 
 and the triangle are small, 
 
 D v t : Dpq: : D t* : Dp 2 
 
 .-.Dvt ocSA?
 
 BOOK II.] NEWTON'S PRINCIPIA. 427 
 
 A D X p q P_JL 
 
 AD 2 + AD x A K a C K 
 
 oc increment of the time. 
 
 .-. by composition, the whole sector oc whole time till the whole 
 V= 0." 
 
 Case 2. If the body descend; as before 
 
 D V T: D PQ: : D T 2 : D P 2 
 
 :: DX 2 : D A s : : T X 2 : A P z 
 : : DX 2 TX 2 : DA 2 AP S 
 : : AD 2 : AD 2 AD X AK 
 : : A D : C K. 
 By the property of the hyperbola, 
 
 TX 2 = D X 2 D A 2 
 .-. D A 2 = DX 2 TX 2 
 
 DPQ p Q 
 
 AD x CK C 
 
 oc increment of the time. 
 
 .*. by composition, the whole time of descent till the body acquire its 
 greatest V = the whole hyperbolic sector DAT. 
 
 20. COR. 1. If A B = i A C. 
 
 The space which the descending body describes in any time : space 
 which it would describe in a non-resisting medium to acquire the greatest 
 velocity : : area ABNKrAATD, which represents the time. For 
 since AC:AP::AP:AK 
 
 KL:| PQ:: A P: AC 
 
 and 
 
 KN: AC ::AB:CK 
 .-. KLON:DTV::AP:AC 
 
 : : vel. of the body at any time : the greatest vel. 
 Hence the increments of the areas oc velocity oc spaces described. 
 .*. by composition the whole A B N K : sector A T D : : space described 
 to acquire any velocity : space described in a non-resisting medium 'for 
 the same time. 
 
 21. COR. 2. In the same way, if the body ascend, the space described 
 till the velocity = A p : space through which a body would move : : 
 A B n k : A D t. 
 
 22. COR. 3. Also, the velocity of a body falling for the time A T D : 
 velocity which a body would acquire in a non-resisting medium in the 
 same time : : A A D P : sector T D A ; for since the force is constant,
 
 428 A COMMENTARY ON [SECT. II. 
 
 the velocity in a non-resisting medium oc time, and the force in a resist- 
 ing medium ocAPccAADP. 
 
 23. COR. 4. In the same waj, the velocity in the ascent : velocity with which 
 a body should move, to lose its whole motion in the same time : : A A p D 
 : sector A t D : : A p : arc A t. 
 
 For let A Y be any other velocity acquired in a non-resisting medium 
 in the same time with A P. 
 
 /. A P : A C : : A P D : this area 
 and 
 
 AP:AC::APD:ACD. 
 
 Therefore the area which represents the time of acquiring the greatest 
 velocity in a non-resisting medium = A C D. 
 
 In the same way, let Ay be velocity lost in a non-resisting medium in 
 the same time as A p in a resisting medium. 
 
 .*. Ap:Ay::AApD: area which represents the time of losing the 
 velocity A p. 
 
 .*. time of losing the velocity A y =r A A p D. 
 
 24. COR. 5. Hence the time in which a falling body would acquire the 
 velocity A P : time in which, in a non-resisting medium, it would acquire 
 the greatest velocity : : sector A D T : A C A D. 
 
 Also the time in which it would lose the velocity A p : time in which, 
 in a non-resisting medium, it would lose the same velocity : : arc A t : 
 tangent A p. 
 
 25. COR. 6. Hence the time being given, the space described in ascent 
 or descent may be known, for the greatest velocity which the body can 
 acquire is constant, therefore the time in which a body falling in a non- 
 resisting medium, would acquire that velocity is also known. Then the 
 sector ADTorADtrAADC:: given time : time just found; there- 
 fore tho velocity A P is known or A p. 
 
 Then the area ABNKorABnkrADTorADt:: space sought 
 for : space which the body would describe uniformly with its greatest 
 velocity. 
 
 26. COR. 7. Hence vice versa, if the space be given, the tune will be 
 known.
 
 BOOK II.] NEWTON'S PRINCIPIA. 429 
 
 27. PROP. X. Let P F Q be the curve meeting the plane P Q. Let 
 
 T 
 
 B C D E Q 
 
 G, H, I, K be the points in the curve, draw the ordinates ; let B C = C D 
 = D E, &c. 
 
 Draw H N, G L tangents at H and G, meeting the ordinates produced 
 in L and N, complete the parallelogram C H M D. Then the times 
 a V L H and V N I, and the velocities cc G H and H I, and the times 
 
 x ; let T and t =r times, and the velocities cc 
 
 C 1 
 
 T-T T 
 
 and -- , therefore 
 
 the decrement of the velocity arising from the retardation of resistance and 
 
 C 1 TT H T 
 
 the acceleration of gravity <x , also the accelerating force of 
 
 gravity would cause a body to describe 2 I N in the same time, therefore 
 
 the increment of the velocity from G = 
 
 2NI 
 
 , again the arc is increased 
 
 M I x N I 
 
 by the space = H I H N = R I = Q-^ , therefore the de- 
 
 crement from the resistance alone = 
 
 HI 
 GH HI 
 T t 
 
 2 M I x N I 
 t x HI 
 
 GHxt UT 2MIxNI OXTT 
 resistance : gravity : : ^ H I -f TT-T ~ : 2 N I. 
 
 T 
 
 HI 
 
 Again, let 
 
 and 
 
 A B, C D, C E, &c. be o + o, 2o, 3o, &c. 
 C H = P 
 
 M I := Q o + R o 2 + S o 3 + &c. 
 
 .-. D I = P Q o + &c. 
 EK = P_2Qo 4Ro s &c.
 
 430 A COMMENTARY ON [SECT. II. 
 
 (BG CH) S + BC 8 (= GH S ) = o 2 + Q 8 o' + 3QRo 3 -f &c. 
 
 .-. G H 8 = 1 + Q 2 x o e + 3 Q R o 3 , 
 
 .-. GH= V 1 + Q'x 0+-2L1 
 
 1 + Q 2 
 
 and 
 
 Subtract from C H the sum G B and D I, and R o 2 and R o 2 + 
 3 S o 3 will be the remainder, equal to the sagittae of the arcs, and which 
 are proportional to L H and N I, and therefore, in the subtracted num- 
 ber of the times, 
 
 t R + 3 S o R + fSo ,3So 
 
 *' T a A/ -R- " * 2R " X l + TIT 
 
 . G_ H JU - o v-nro- 2 + QRo * x i + ^ So -* 
 
 T " V 14- Q 8 2 R 
 
 HI = o. V 1 + 
 
 3So QRo ! 
 -t" "TR" " r 
 
 Q 
 
 -o vMcy 4- | , 
 
 V * + 2 R f 2 R X 
 
 S l + Q* 
 
 MIxNI Ro g x Qo + Ro 2 + &c. 
 H I 
 
 UT ,2MIxNI 
 /. resistance : gravity : : ^p H 1 H rpr : 2 N 1 
 
 : :3 S V 1 + Q 2 : 4 R 2 . 
 Tlie velocity is equal to that in the parabola whose diameter = H C, 
 
 H N* 1 + O * 
 
 and the lat. rect = or . The resistance oc density x V 2 , 
 
 c , , resistance 3 S V I + Q* . . R 
 
 therefore the density - y^ a - 4 PZ - directly 
 
 directly 
 
 . 
 R V I + Q 4 
 
 28. Ex. 1. Let it be a circular arc, CH = e, AQ = n, AC 
 CD = o, 
 .-.DP =r n* a+o 2 = n s a 2 2ao o s =e 2 2ao
 
 BOOK II.] 
 
 NEWTON'S PRINCIPIA. 
 
 431 
 
 - e 
 
 ao n z o 
 - - -^pp 
 
 a n- o 
 
 
 - , 
 
 and therefore 
 
 P = e, Q=-,R = 7r - 5 , S = 
 
 a n 
 
 .*. density oc 
 
 R v' 1 + Q 
 
 oc 
 
 an 2 2 e 
 
 X , 
 
 n ' 
 
 a a sin. 
 
 a oc a oc tangent. 
 
 n e e cos. 
 
 3 a n 2 n n 4 
 The resistance : gravity : : X -r : : 3 a : 2 n. 
 
 
 29. Ex. 2. Of the hyperbola. 
 
 P I X b = P D ', 
 .-. put P C = a, C D = o, Q P = c, 
 
 .. a+oXc a o = ac a* 2ao -f co o* 
 
 ac a* 2a-f c 
 
 
 and since there is no fourth term, 
 
 S = 0, 
 
 .*. draw y =r 0. 
 30. PROP. XIII. Suppose the resistance to V + V 2 . 
 
 Q P 
 
 D F 
 
 Case 1. Suppose the body to ascend ; with the center D and rad. D B,
 
 432 
 
 A COMMENTARY ON 
 
 [SECT. II. 
 
 describe the quadrant B T F ; draw B P an indefi nite line perpendicular 
 to B D, and parallel to D F. Let A P represent the velocity ; join D P, 
 D A, and draw D Q near D P. 
 
 ,\ resistance AP* + 2BAxAP, suppose gravity D A% 
 
 .*. decrement of V gravity + resistance AD 2 +AP 2 +2BAxAP. 
 
 a D P 2 
 
 D P Q ( P Q) : D T V : : D P 8 : D T 2 , 
 .-. D T V a D T 2 a i, 
 therefore the whole sector E T D, is proportional to the time. 
 
 Case 2. Suppose the force of gravity proportional to a less quantity 
 than DA 2 , draw B D perpendicular to B P, and let the force of gravity 
 
 P Q 
 
 D 
 
 FG 
 
 a A B 2 B D 2 . Draw D F parallel to P B and = D B and with the 
 center D axis-major = \ axis-minor = D B, describe a hyperbola 
 from the vertex F, cutting A D produced in E, and D P, D Q in T, V. 
 
 Now since the body is supposed to ascend. 
 
 The decrement of the velocity a A P 2 + 2 A B X AP-f-AB 2 
 BD 2 a BP 2 BD 2 (B P 2 = AP 2 + AB 2 -r-2AB x BP). 
 
 Also, DTV:DPQ::DT 2 :DP 2 (by similar triangles) 
 
 : : T G s : B D 2 (T G perpendicular to G) 
 ::D F 2 : PB 2 D B 2 . 
 
 Now D P Q oc decrement of velocity a P B 2 D B 2 , 
 .*. DTVacDF*al a increment of the time, since the time flows uni- 
 formly.
 
 BOOK II.] NEWTON'S PRINCIPIA. 
 
 Case 3. If the body descend ; let gravity B D * A B *. 
 
 433 
 
 With center D and vertex B, describe the rectangular hyperbola B T V, 
 cutting the lines D A, D P, D Q produced in E, T, V. 
 
 The increment of V BD 1 AB 2 2 AB x A P A P 
 a BD 2 (AB+AP) 2 aBD J BP 2 
 
 DTV:DPQ(PQ)::DT 2 :DP S 
 
 ::GT 2 :BP::GD 2 BD 2 : BP* 
 r:GD 2 :BD 2 ::BD 2 :BD BP 2 , 
 
 .-. DT Va BD 2 1, 
 .'. the whole sector E D T oc time. 
 
 31. COR. With the center C and distance D A describe an arc similar 
 toBT. 
 
 Then the velocity A P : the velocity which in the time E D t a body 
 would lose or acquire in a non-resisting medium : : A D A P : sector 
 ADt. 
 
 For V in a non- resisting medium a time. 
 
 32. In the case of the ascent, 
 
 Let the force of gravity <x I. Resistance oc 2 a v -|- v 2 
 .'. d v a 1 + 2 a v + v 2 
 
 d v 
 
 .*. -; 2 oc time. 
 
 1 -j- 2 a. v -j- v* 
 
 .'. by Demoivre's first formula, 
 f. or time = 
 when 
 
 
 
 o i == ~i x 
 
 2a y-f v* g 2 
 
 arc - 
 
 tangent = v -f- a 
 
 Vot. I. 
 
 Ee
 
 A COMMENTARY ON [SECT. I If. 
 
 The whole time .. when v = = -. x cir. arc rad. = a 
 
 S 
 
 and tangent = a -f C. 
 
 .. cor', time = X cir. arc rad. = g and tangent v -f a cir. arc rad. 
 
 o 
 
 = g and tangent a. 
 
 .'. the time of ascent = sector EDT g ! = 1 a*. 
 33. In the case of descent, 
 
 dv oc 1 2av v s 
 let 
 
 v + a = x 
 .-. d v = d x 
 .-. v*-}-2av-t-a 2 = x 2 
 
 .-. l+ a 2 _x 2 = l_2av v z 
 
 r ~- :L ?+C,(g 2 = :l + a^) 
 
 Time = 0, v =r 0, 
 /. x = a 
 
 1 /-ff + x /C -f- ^ 
 
 .'. Cor 1 , tune = ~ x f^ 1 --- p^JL . 
 
 2g J g_x J g a 
 
 34- PROP. XIV. Take A C proportional to gravity, and A K to the 
 resistance on contrary sides if the body ascend, and vice versa. 
 Between the asymptotes describe a hyperbola, &c. &c. 
 Draw A b perpendicular to C A, and 
 
 Ab:DB::DB 2 :4BA x A C. 
 
 The area A b N K increases or decreases in arithmetic progression it 
 the forces be taken in geometric progression. 
 Now 
 
 A K cc resistance aSJBAP + AP 2 . 
 Let 
 
 T , 2BAP + AP* 
 AK = - ~ - 
 
 Lt 
 
 2BA X PQ+2APX PQ 
 - - -
 
 BOOK II.] 
 
 NEWTON'S PRINCIPIA. 
 
 435 
 
 D 
 
 Ab:LO::CK:CA 
 DB:Ab::4BAx CA:DB 
 
 TQ- 
 ~ 
 
 KT r^xT- 
 
 . . K JL U JN = 
 
 X PQxBD 3 
 
 . -n . - T^T? - ?r 
 
 4B A X CK X Z 
 
 Case 1. Suppose the body to ascend, 
 
 AB 2 + BD 
 
 gravity a A B + B D 2 = 
 
 E e 2
 
 1H6 A COMMENTARY ON [SECT. IV. 
 
 A K - AP ' + 2BAP 
 Z 
 
 .-.DP = CKx Z. 
 ..DT*:DP*::DB:CKx Z 
 
 and in the other two cases the same result will obtain. 
 Make 
 
 DTV = DBx m. 
 
 .-.DB X m:DB X PQ::DB s :CKxZ 
 .-. BD 3 xPQ = 2BDxmxCKxZ. 
 
 A0 
 A B 
 
 AKVir r* T \r B P A B x B D x m A r> 
 .. A b N K D rVrr - -^-g - - a A P . oc velocity. 
 
 .'. it will represent the space. 
 
 SECTION IV. 
 
 35. PROP. XV. LEMMA. The 
 
 /. O P Q = a rectangle =r L O Q R 
 and 
 
 . S P Q = L. of the spiral = 4. S Q R 
 .-. i- O P S = . O Q S. 
 
 .*. the circle which passes through the points P, S, O, also passes 
 
 circle 
 through Q. Also when Q coincides with P, this - touches the spiral. 
 
 8 
 .-. c. P S O L. in a - - whose diameter = P O. 
 
 MB
 
 BOOK II.] NEWTON'S PR1NCIPIA. 437 
 
 Also 
 
 T Q : P Q : : P Q : 2 P S.. 
 r. PQ 2 = 2PS x TO 
 
 which also follows from the general property of every curve. 
 PQ 2 rr P V X QR. 
 
 36. Hence the resistance ex density X square of the velocity. 
 
 37. Density a -^ > centripetal force a density 2 -rr- ; . 
 
 distance distance 2 
 
 Then produce S Q to V so that S V = S P, and let P Q be an arc 
 described in a small lime, P R described in twice that time, .. the decre 
 ments of the arcs from what would be described in a non-resisting me- 
 dium a T 2 . 
 
 .*. decrement of the arc P Q = ^ decrement of the arc P R 
 
 .-. decrement of the arc P Q = \ R r (if Q S r = area P S Q). 
 
 For let P q, q v be arcs described (in the same time as P Q, Q R) in a 
 non-resisting medium, 
 
 P S q PSQ^rQSqzzqSv Q S r 
 
 = r S v Q Sq 
 .'. 2QSq = rSv 
 
 .. if S T ultimately = S t be the perpendicular on the tangents 
 ST X Qq = ^ S t X rv 
 
 .-. 2 Q q = r v 
 and 
 
 R v = 4 Q q. 
 .-. 2 Q q = R r. 
 Hence 
 
 Resistance : centripetal force : : | R r : T Q, 
 Also 
 
 T Q X S P 2 a time 2 , (Newt. Sect. II.) 
 .-. P Q 2 X S P a time ' 
 .'. time a P Q x 
 
 .-.V a 
 also 
 
 V at Q oc
 
 438 A COMMENTARY ON [SECT. IV. 
 
 P Q : Q R : : V~S~Q : V~S~P 
 
 :: SQ: V SQ X SP 
 P Q : Q r : : S Q : S P 
 
 since the areas are equal, and the angles at P and Q are equal. 
 .'. P Q : Rr::SQ:SP V S Q x S P 
 
 : : S Q : V Q 
 For 
 
 SQ = S P VQ 
 ..SQxSP = SP' VQx SP 
 
 .-. V Q ultimately = S P V S P x S Q 
 
 r> . , decrement of V R r 
 
 Resistance : = a 
 
 time 2 P Q 2 x S P 
 
 i V O 
 
 GC 
 
 PQxSQxSP 
 VQ: PQ: : OS: PO 
 
 and 
 
 O S 
 
 S Q= SP oc 
 
 O P x S P 2 
 
 OS 
 
 .. density X square of the velocity oc resistance oc 
 
 \y P 
 
 1 X o i 
 
 OS 
 
 ' dens ">' * OPXSP 
 
 O S 
 and in the logarithmic spiral -^^ is constant. 
 
 .-. density oc - . Q. e. d. 
 
 38. Con. 1. V in spiral = V in the circle in a non -resisting medium at 
 .he same distance. 
 
 39. COR. 3. Resistance : centripetal force : : R r : T Q 
 
 j 
 
 SQ SP 
 
 ::*VQ:PQ 
 : : I O S : O P. 
 
 .*. the ratio of resistance to the centripetal force is known if the spiral be 
 given, and vice versa. 
 
 40. COR- 4. If the resistance exceed * the centripetal force, the body 
 cannot move in this spiral. For if the resistance equal the centripetal
 
 BOOK II.] NEWTON'S PRINCIPIA. 43!) 
 
 force, O S = O P, .*. the body will descend to the center in a straight 
 line P S. 
 
 V of descent in a straight line : V in a non-resisting medium of de- 
 scent in an evanescent parabola : : 1 : V 2 ; for V in the spiral = V in the 
 circle at the same distance, V in the parabola = V in the circle at 
 f distance. 
 
 Hence since time a -^ , 
 
 time of descent in the 1st case : that in 2d : : V 2 : 1. 
 
 41. COR. 5. V in the spiral P Q R = V in the line P S at the same 
 distance. Also 
 
 P Q R : P S in a given ratio : : P S : P T : : O P: O S 
 .-. time of descending P Q R : that of P S : : O P : O S.* 
 Length of the spiral = T P = sector of the L. T P S. 
 
 a:b::b:c::c:d::d:e 
 a + b + c + &c. : b + c + d + &c. : : a : b 
 
 .-. a + b + c + &c. : a : : a : a b. 
 
 42. COR. 6. If with the center S and any two given radii, two 
 circles be described, the number of revolutions which the body makes 
 between the two circumferences in the different spirals <x tangent of the 
 
 P S 
 angle of the spiral a ^ ~ . 
 
 The time of describing the revolution : time down the difference of the 
 radii : : length of the revolution : that difference. 
 
 2d a 4th, 
 .'. time a length of the revolution a secant of the angle of the spiral 
 
 OJ> 
 a O S' 
 
 pq:pt::Sp:Sy 
 
 .'. d w = 
 
 p d x 
 ^ r 2 p 
 
 X <1 X 
 
 d w : : : x : p. 
 
 r 2 p 2 
 
 2 Y/ r a p 2 
 
 r. c 4
 
 440 A COMMENTARY ON [SECT. IV. 
 
 43. Con. 7. Suppose a body to revolve as in the proposition, and to cut 
 
 K 
 
 the radius in the points A, B, C, D, the intersections by the nature of the 
 spiral are in continued proportion. 
 
 ,. r , . perimeters described 
 1 lines o! revolution oc - 
 
 and velocity a 
 
 1_ 
 
 V distance 
 
 a A S^, B S*, CS'i 
 
 53 
 
 .-. the whole time : time of one revolution ::AS 2 -j-BS^ + &c. : A S 
 
 ::AS*:AS* BS*. 
 44. PROP. XVI. Suppose the centripetal force x ^ p n + i> 
 
 time a P Q x S P * 
 and velocity oc 
 
 S P 2 
 
 PQ : Q R :: S Q* : SP 
 
 Qr : PQ :: SP : SQ 
 
 Qr : QR : : SQ^- 1 : S 
 
 .-. Qr : Rr : : SQ?- 1 : S 
 
 SP2- 
 
 For 
 
 : : S Q : l~^~fn . V Q. 
 
 S P = SQ+ VQ,
 
 BOOK II.] NEWTON'S PRINCIPIA. 441 
 
 - 1 + 1. VQ x SQ*~ 2 + &c. 
 
 ... SQ*- 1 SP*- 1 = 1 " x VQ x SQ?- 2 . 
 
 IB 
 
 Then as before it may be proved, if the spiral be given, that the density 
 g-p. Q. e. d. 
 45. Cou. 1. 
 
 Resistance : centripetal force : : 1 n . O S : O P, 
 for the resistance : centripetal force : : | II r : T Q 
 
 - x VQx PQ PQ ! 
 
 as Q ~"2lTp 
 
 x VQ:PQ 
 
 :: 1_| X OS: OP. 
 
 46. COR. 2. If n -f 1 = 3, 1 ~ = 0, 
 
 <w 
 
 .'. resistance = 0. 
 COR. 3. If n + 1 be greater than 3, the resistance is propelling. 
 
 SECTION VI. 
 
 47. PROP. XXIV. The distances of any bodies' centers of oscillation from 
 the axis of motion being the same, the quantities of matter x weight 
 X squares of the times of oscillation in vacua. 
 
 For the velocity generated oo ~ . Force on bodies at 
 
 quantities or matter 
 
 equal distances from the lowest points GO weights, times of describing 
 corresponding parts of the motion x whole time of oscillation, 
 
 ., force X time of oscil. 
 
 .-. quantities ot matter x , --. 
 
 velocities 
 
 x weights X squares of the times, 
 since the velocities generated x -: for equal spaces. 
 
 48. COR. 1. Hence the times being the same, the quantities of matter 
 x weights. 
 
 Con. 2. If the weights be the same, the quantities of matter x time 2 . 
 
 1 
 
 COR. 3. If the quantities of matter be the same, the weights x 
 
 tune
 
 442 A COMMENTARY ON [SECT. VJ 
 
 49. Cou. 4. Generally the accelerating force oc eig .\ ts of matter 
 
 quantities 
 
 and L oo T T z , 
 
 WxT* 
 ,.Loo_ Q , 
 
 L 
 
 .-. if W and Q be given L oo T 2 . 
 If T and Q be given L oo W. 
 
 c weight X time* of oscillation 
 50. COR. 5. generally the quantity of matter cc j r- - . 
 
 O 
 
 51 PROP. XXV. Let A B be the arc which a body would describe in a 
 
 non-resisting medium in any time. Then the accelerating force at any 
 point D GO C D ; let C D represent it, and since the resistance oo time, 
 it may be represented by the arc C o. 
 
 .*. the accelerating force in a resisting medium of any body d, ~ o d. 
 
 Take 
 
 od:CD::eB:CB. 
 
 Therefore at the beginning of motion, the accelerating force will be in 
 this ratio, .*. the initial velocities and spaces described will be in the same 
 ratio, .'. the spaces to be described will also be in the same ratio, and 
 vanish together, .*. the bodies will arrive at the same time at the points 
 C and o. 
 
 In the same way when the bodies ascend, it may be proved that they 
 will arrive at their highest points at the same time. .*. If A B : a B in 
 the ratio C B : o B, the oscillations in a non-resisting and resisting me- 
 dium will be isochronous. Q. e. d.
 
 BOOK II.] 
 
 NEWTON'S PRINCIPIA. 
 
 443 
 
 COR. The greatest velocity in a resisting medium is at the point o. 
 The expression for the ^ time of an oscillation in vacuo, or time of de- 
 scent down to the lowest point oc quadrant whose radius = 1. Now 
 
 suppose the body to move in a resisting medium when the resistance 
 : force of gravity : : r : 1. 
 
 Then vdv = gFdx + grdz = gd 2 x + grdz. Now by 
 
 o 7 
 
 a property of the cycloid, if -x be the axis, d x : d z : : x : T : : z : a, 
 
 d ii 
 
 z dz 
 
 .*. v d v = xzdz + grdz 
 
 a & 
 
 = -f-X z* + grz, 
 
 Now 
 
 2grz+C. 
 
 z = d, v =r o, 
 
 .-. v 2 = ^ xd 2 z 2 2 g r X d z 
 a 
 
 = f- x d 2 4ard + Sadrz ~z\ 
 
 X 2 
 
 =^ x 
 
 dz 
 
 Vd * 2 a r d + 2 a r z z *. 
 
 Assume 
 
 z a r = y, 
 ..,z J 2 a r z -f a * r 2 = y *, 
 
 .-. 2 a r z z 2 = a 2 v * y 2 , 
 jl * 2 a r d + 2 a r z z ! = (d a r) 5 y 2 = (b s y *.)
 
 444 A COMMENTARY ON [SECT. VI. 
 
 and 
 
 d z = d y 
 
 ,1 1 r v ~ d y 
 
 - J g ' Vb'-y< 
 
 /. t = f X circular arc, radius = 1, 
 
 J % 
 and 
 
 cos. = , + C and C = o. 
 
 d a r 
 
 /. the whole time of descent to the lowest point = f - X circular arc 
 
 , a r . .... ,. 
 
 whose cos. = -, . .'. time in vacuo : time in resisting medium 
 
 d ar 
 
 : : quadrant : arc whose cos. = -. ' 
 
 a r 
 
 * , 
 
 r 
 
 COR. 1. Time of descent to the point of greatest acceleration is constant, 
 for in that case z = a r, 
 
 .-. t = C X quadrant, for d v = 0, 
 
 o 
 
 .-. v d v = 0, 
 .-. gzdz + garz = 0, 
 
 .-. z = a r, 
 . z : r : : a : 1. 
 
 COR. 2. To find the excess of arc in descent above that in ascent. 
 vdv= -fgTdx-fgrdz, 
 
 g z d z 
 . .-. v d v = *? g r d z 
 
 s 
 
 v 2 m z* 
 f . T m-r grz + C, 
 
 .-. v'= & (d ! z 1 ) (z d) X 2 a r 
 
 & 
 
 = g. x (d f 2 a r d) (2 a r z z ') 
 
 which when the body arrives to the highest point = 0, 
 
 d 2 Sard 2arz z 2 = 0, 
 ... t l _ o a r d = z 2 + 2 a r z, 
 .-. z + a r = d a r, 
 .-. z = d 2 a r, 
 .-. d z = 2 a r.
 
 BOOK II.] 
 
 NEWTON'S PRINCIPIA. 
 
 445 
 
 52. PROP. XXVI. Since V oc arc, and resistance a V, resistance a arc. 
 .. Accelerating force in the resisting medium oc arcs. 
 
 Also the increments or decrements of V a accelerating force. 
 
 .*. the V will always a arc. 
 
 But in the beginning of the motion, the forces which oo arcs will generate 
 velocities which are proportional to the arcs to be described. .-. the velo- 
 cities will always CD arcs to be described. 
 
 .*. the times of oscillation will be constant. 
 
 53. PROP- XXVIII. Let C B be the arc described in the descent, C a 
 in the ascent. 
 
 .-. A a = the difference (if A C = C B) 
 Force of gravity at D : resistance : : C D : C O. 
 C A = CB 
 Oa = O B 
 .-. CA OaorAa eO = CB OB = CO 
 
 .'. C O = A a 
 
 .'. Force of gravity at D : resistance : : C D = A a 
 .'*. At the beginning of the motion, 
 
 Force of gravity : resistance : : 2 C B : A a 
 
 : : 2 length of pendulum : A a. 
 
 54. PROB. To find the resistance on a thread of a sensible thickness. 
 
 Resistance cc V s X D 2 of suspended globe. 
 
 .*. resistance on the whole thread : resistance on the globe C : :
 
 446 A COMMENTARY ON [SECT. VI. 
 
 ::2a 3 b'. (a b) ' : a 3 r*c ! r *c 2 . (a 2 b) 3 , c = a + r. 
 
 :: a 3 b'. (a b)' : 3 a 1 r 2 c 8 b b a b 2 r c s + 4 b 3 r ! c % 
 :: a'b .(a b) 8 : Sa'r'c* babr*c 2 + 4b s r s c z , 
 .*. resistance on the thread : whole resistance 
 ::a 3 b. (a b) : rc.(3a bab + 4b). 
 
 COR. If the thickness (b) be small when compared with the length (a) 
 3a 2 bab-f 4 b 2 = 3 a * bab -f 3 b * (nearly) = 3. (a b) 2 . 
 /. Resistance on the whole thread : resistance on the globe 
 
 : : a 3 b: 3r 2 c* 
 and 
 
 Resistance on the thread : whole resistance to the pendulum 
 
 : : a'b: a s b + 3r ! c 2 . 
 
 Suppose, instead of a globe, a cylinder be suspended whose ax. rr 2 r. 
 Now by differentials 
 
 the resistance on the circumference : resistance on the base : : 2 : 3. 
 
 By composition the resistance to the cylinder : resistance on the square 
 = 2 r : : 2 : 3. 
 
 Resistance a x * x x , 
 .. resistance ax 3 , 
 
 .'. resistance to the whole thread oc x 3 . 
 Resistance on A E a (a 2 b) 3 if 2 b = E D. 
 .. Resistance on the thread : resistance of the globe 
 
 : : 16. a'b 2 . (a b) 2 : 3 p . a 3 (a 2 b; 3 xr 2 . (a + i) 2 . 
 
 55. PROP. XXIX. B a is the whole arc of oscillation. In the line O Q 
 take four points S, P, Q, R, so that if O K, S T, P I, Q E be erected
 
 BOOK II.] 
 
 NEWTON'S PRINC1PIA. 
 
 447 
 
 perpendiculars to O Q meeting a rectangular hyperbola between the 
 asymptotes O Q, OK in T, I, G, E, and through I, K F be drawn 
 
 o s p P RQ M 
 
 parallel to O Q, meeting Q E produced in F. The area P I E Q may 
 be : area P I S T : : C B : C a. Also IEF:ILT::OR:OS. 
 
 Draw M N perpendicular to O Q meeting the hyperbola in N, so that 
 P L M N may be proportional to C Z, and P 1 G R to C D. 
 
 Then the resistance : gravity : : ? X TEF 1GH:PINM. 
 
 Now since the force cc distance, the arcs and forces are as the hyper- 
 bolic areas. .*. D d is proportional to R r G g. 
 
 Now by taking the differentials the increment of (/YQ T E F I G 
 Rr. IE 
 
 = G II g Ii 
 
 H G I E 
 O P x P I 
 
 OQ 
 OR 
 
 :RrxGR::HG -^~ : 
 
 ., OR x 
 
 PxPI (ORxHG = ORxHR 
 = PIHR=PIRG+IGH)::PIRG+IGH- 
 
 5-|xIEF:OPxPI. 
 
 Now if Y = 5-~|x I E F I G H, the increment Y a P I G R Y. 
 
 Let V = the whole from gravity. .. V R = actual accelerating 
 force. .'. Increment of the velocity a V R X increment of the time. 
 As the resistance cc V 2 the increment of resistance cc V X increment of 
 
 , . , . increment of the space T / 
 
 the velocity, and the velocity a -. - ^-, -. - . .*. Increment ol 
 
 increment ot the time 
 
 resistance cc V R if the space be given, cc P I G R Z, if Z be the 
 area which represents the resistance R e. 
 
 Since the increment Y a PIGR Y, and the increment of Z
 
 448 
 
 A COMMENTARY ON 
 
 [SECT. VIII. 
 
 ooPIGR Z. IfY and Z be equal at the beginning of the motion and 
 begin at the same time by the addition of equal increments, they will still 
 remain equal, and vanish at the same time. 
 
 Now both Z and Y begin and end when resistance = 0, i. e. when 
 OR 
 
 .1 EF IGH = 
 
 or 
 
 OQ 
 
 ILT 
 OS 
 O R x I E F 
 
 x OR I G H = 0. 
 -IG H = Z 
 
 OQ 
 
 .-. Resistance : gravity : : ~| . I E F I G H : P M N I. 
 
 SECTION VIII. 
 66. PROP. XLIV. The friction not being considered, suppose the mean 
 
 K 
 
 M 
 
 E 
 
 3 
 
 F 
 
 '****' 
 
 
 
 
 , , 
 
 0' 
 
 
 
 A 
 
 
 
 B C 
 
 
 
 D 
 
 
 
 G 
 
 
 H 
 
 
 N 
 
 altitude of the water in the two arms of the vessel to be A B, C D. Then 
 when the water in the arm K L has ascended to E F, the water in the arm 
 M N will descend to G H, and the moving force of the water equals the 
 excess of the water in one arm above the water in the other, equals twice 
 A E F B. Let V P be a pendulum, R S a cycloid = length of the 
 canal, and P Q = A E. The accelerating force of the water : whole 
 weight : : A E or P Q : P R.
 
 BOOK II.] NEWTON'S PRINCIPIA. 449 
 
 Also, the accelerating force of P through the arc P Q : whole weight 
 of P : : P Q : P R; therefore the accelerating force of the water and P 
 cc the weights. Therefore if P equal the weight of the water in the canal, 
 the vibration of the water in the canal will be similar and cotemporaneous 
 with the oscillations of P in the cycloid. 
 
 COR. 1. Hence the vibrations of the water are isochronous. 
 
 COR. 2. If the length of the canal equal twice the length of the 
 pendulum which oscillates in seconds; the vibrations will also be performed 
 in seconds. 
 
 COR. 3. The time of a vibration will <x V L. 
 Let the length = L, A E = a, 
 
 then the accelerating force : whole weight : : 2 a : L, 
 
 .*. accelerating force = -=- ; 
 
 2 A 
 
 ,\ when the surface is at 0, the accelerating force = ~ 
 
 Put E = x, 
 
 A = a x, 
 
 , . c 2 a 2 x 
 .*. accelerating torce = = -, 
 
 , g . 2 a d x 2 x d x 
 
 v d v =z 
 
 . . U V 
 
 ga 
 
 + cor", and cor". = 0, 
 
 v t = 0, x = 0, 
 .-. ifp = 3. 14159, &c. 
 
 X cir. arc rad. =a, and vers. = x 
 
 t = 
 
 .*. time of one entire vibration = p x J = time of one entire vi- 
 
 \r * Gf 
 
 L 
 
 2 
 
 t3 
 
 bration of a pendulum whose length = . 
 
 Vox. I. Ff
 
 450 A COMMENTARY ON [SECT. VIII. 
 
 57. COR. 1. Since the distance (a) above the quiescent surface does 
 not enter into the expression. The time will be the same, whatever be 
 the value of A E. 
 
 58. COR. 2. The greatest velocity is at A = ,J~ x a > a T~ a /"? 
 
 ^f Li Li ^1 - 
 
 L fc* 
 
 t 
 
 59. PROP. XLVII. Let E, F, G be three physical points in the line 
 B C, which are equally distant ; E e, F f, 
 
 G g the spaces through which they move S -j-D 
 
 during the time of one vibration. Let e, <p, y 
 be their place at any time. Make P S = 
 E e, and bisect it in O, and with center O 
 and radius O P = O S, describe a circle. 
 Let the circumference of this circle repre- H \Z 
 sent the time of one vibration, so that in 
 the time P H or P H S h, if H L or h 1 
 be drawn perpendicular to P S and E be 
 
 taken = P L or P 1, E may be found in g. 
 
 E ; suppose this the nature of the medium. 
 Take in the circumference P H S h, the arcs 
 HI, IK, hi, i k which may bear the 
 
 same ratio to the circumference of the circle as E F or F G to 
 B C. Draw I M, K N or i m, k n perpendicular to P S. Hence 
 PI, or P H S i will represent the motion of F . and P K or ^ - 
 PHSkthatof G . E *, F?, Gy = PL, P M, P Nor PI, E!! 
 P m, P n respectively. 
 
 - - "R 
 
 Hence eyorEG+Gy E = GE LN = expan- 
 sion at s 7 ; or = E G + 1 n. 
 
 .. in going, expansion : mean expansion : : G E L N : E G 
 In returning, : : : E G + In : E G 
 
 Now join I O, and draw K r perpendicular to H L, H K r, 
 I O M are similar triangles, since the AKHr = KOk= 
 I O i = A I O P and A at r and M = 90, 
 .-. L N : K H : : I M : I O or O P, and by supposition K H : 
 EG:: circumference PSLP:BC::OP:V = radius of 
 the circle whose circumference = B C. 
 
 .*. by composition L N : G E : : I M : V. 
 
 .'. expansion : mean expansion : : V I M : V, 
 
 J-A
 
 BOOK II.] NEWTON'S PRINCIPIA. 451 
 
 .-. elasticity : mean elasticity : : _ -- : - . In the same way, for the 
 
 _ j -- 
 
 points E and G, the ratio will be y _J H : ~ a y^ N : -^ 
 
 : : excess of elasticity of E : mean elasticity 
 
 _ HL KN _ 1 
 : V 2 HLxV KN xV + HLxKN : T 
 
 : : H L K N : V. 
 Now 
 
 V a 1. 
 
 .*. the excess of E's elasticity oc H L K N, and since H L K N 
 = H r : H K : : O M : O P, 
 
 .-. H L K N a O M, 
 
 /. excess of E's elasticity oc O M. 
 
 Since E and G exert themselves in opposite directions by the arc's ten- 
 dency to dilate, this excess is the acceleratinsr force of e 7, .. accelerating- 
 force oo O M.* 
 
 ON THE HARMONIC CURVE. 
 
 Since the ordinates in the harmonic curve drawn perpendicular to the 
 axis are in a constant ratio, the subtenses of the angle of contact will be 
 
 in the same given ratio. Now the subtenses oc 5- ~ - , and when 
 
 rad. 01 curv. 
 
 the curve performs very small vibrations, the arcs are nearly equal. 
 
 Now the curv. oc 3- , .. subtense cc curvature. 
 rad. 
 
 Hence the accelerating force on any point of the string cc curvature at 
 that point. 
 
 * Now bisect F f in fi, 
 
 .-. O M = a $ 
 For 
 
 OM=OP PM=nF F0=fl<p 
 
 i. e. the accelerating force oc distance from fl the middle point. Q. e. d. 
 
 Ff?
 
 452 A COMMENTARY ON [SECT. VIII. 
 
 To find the equation to the harmonic curve. 
 
 C S 
 
 Let A C be the axis of the harmonic curve C B A, D the middle point, 
 draw B D perpendicular cutting the curve in B; draw P M perpendi- 
 cular to B D cutting the curve in P, and cutting the quadrant described 
 with the center D and radius D B in N. Draw P S perpendicular to A C. 
 Put 
 
 BD = a, PM = y, BM = x, 
 .-. D M = a x = P S. 
 
 r = rad. of curv. at B, B P = z, 
 
 .'. rad. of curv. = 
 Now 
 
 d z d x ,.- , , x 
 
 (if d K be constant). 
 
 d*y 
 
 B D : P S : : curvature at B : curvature at P 
 : : rad. of cur. at P : rad. at B 
 
 or 
 
 a : a 
 
 d z d x 
 
 X * 
 fi . 1 
 
 d* y 
 
 Now 
 
 .'. r a d * y + adzdx xdxdz = 0, 
 
 X ' ci 7 
 
 .'. rady+adzx = + C. 
 
 x = 0, d y = d x, 
 z = + C = C, 
 
 x * d z 
 
 .. rady + axdz 
 
 = r a d z. 
 
 Put 
 
 /.rady = ra b* d z, 
 -. r 2 a 2 dy'= (ra b 8 ) 1 X d 
 
 2rab ! dy*+ b 4 dy*,
 
 ROOK II.] NEWTON'S PRINCIPIA. 453 
 
 .-. (ra b 2 ) 2 xdx 2 = 2 r a b'dy* b 4 dy 2 , 
 
 .-. r 8 a 2 dx l = 2 r ab'dy* 
 if (b) be small compared to (a), 
 
 r adx* 
 '^ = -2-b^' 
 
 V r a xdx /_r_ adx __ 
 
 = V 2 a x x 1 ~ V a ' V 2ax x 2 ' 
 
 .. y = / x circular arc whose rad. = a a and vers. = x 
 -I- C, and cor n . = 0, 
 
 because when y = 0, x = 0, 
 
 .. arc = 0. 
 
 .-. C D = J~ X quadrant B N E, 
 'and therefore 
 
 CD 
 
 lr_ 
 V a == 
 
 y : 
 
 B N E' 
 
 B N X I Z 
 B N E 
 
 60. PROP. XLIX. Put A = attraction of a homogeneous atmosphere 
 when the weight and density equal the weight and density of the medium 
 through which the physical line E G is supposed to vibrate. Then every 
 thing remaining as in Prop. XLVII. the vibration of the line E G will 
 be performed in the same times as the vibrations in a cycloid, whose 
 length P S, since in each case they would move according to the same 
 law, and through the same space. Also, if A be the length of a pendulum, 
 since T a V L 
 
 The time of a vibration : time of oscillation of a pendulum A 
 : : V~P~O : V^k. 
 
 Also (PROP. XLVIL), the accelerating force of EG in medium : ac- 
 celerating force in cycloid 
 
 ::AxHK:VxEG; 
 since H K : G E : : P O : V. 
 
 :: PO X A : V 2 . 
 F f 3
 
 454 A COMMENTARY ON [SECT VIII. 
 
 Now 
 
 ^ X fJ T~ wnen k * s gi ven ' 
 
 .. the time of vibration : time of oscillation of the pendulum A 
 : : V : A 
 : 1 B C : circumference of a circle rad. = A. 
 
 Now B C = space described in the time of one vibration, therefore 
 the circumference of the circle of radius A = space described in the time 
 of the oscillation of a pendulum whose length =r A. 
 
 Since the time of vibration : time of describing a space r= circum- 
 ference of the circle whose rad. = A : : B C : that circumference. 
 
 COR. 1. The velocity equals that acquired down half the altitude of 
 A. For in the same time, with this velocity uniform, the body would de- 
 scribe A ; and since the time down half A : time of an oscillation : : r : 
 circumference. In the time of an oscillation the body would describe the 
 circumference. 
 
 COR. 2. Since the comparative force or weight GC density X attraction 
 
 of a homogeneous atmosphere, A GO ^ r- , and the velocity co V A. 
 
 V elastic force 
 cc . % 
 
 V density 
 
 SCHOLIUM. 
 
 61. PROP. XLIX. Sound is produced by the pulses of air, which 
 theory is confirmed, 1st, from the vibrations of solid bodies opposed to it. 
 2d. from the coincidence of theory with experiment, with respect to the 
 velocity of sound. 
 
 The specific gravity of air : that of mercury : : 1 : 11890. 
 
 Now since the alt. <x , .-. 1 : 11890 : : 30 inches : 29725 feet = 
 
 sp. gr. 
 
 altitude of the homogeneous atmosphere. Hence a pendulum whose 
 length = 29725, will perform an oscillation in 190", in which time by 
 Prop. XLIX, sound will move over 186768 feet, therefore in V sound 
 will describe 979 feet. This computation does not take into considera- 
 tion the solidity of the particles of air, through which sound is pro- 
 pagated instantly. Now suppose the particles of air to have the same 
 density as the pafticles of water, then the diameter of each particle : dis-
 
 BOOK I.] NEWTON'S PRINCIPIA. 455 
 
 tance between their centers : : 1 : 9, or 1 : 10 nearly. (For if there are 
 two cubes of air and water equal to each other, D the diameter of the par- 
 ticles, S the interval between them, S + D = the side of the cube, and if 
 N = N. N S + N D = N. in the side of the cube, N. in the cube 
 x> N 3 . Also, if M be the N. in the cube of water, M D the side of the 
 cube and the N. in the cube oc M 3 . 
 
 Put 1 : A : : N 3 : M 3 , 
 
 .-. M = A * N, 
 By Proposition 
 
 .-. S = D x A 
 
 i*-l, 
 
 .'. S : D : : 1 
 
 l* I :1, 
 
 .'. S + D : D : : i 
 
 V.*: 1 : : 9 
 
 or 10: 1 if A = 1000). 
 
 Now the space described by sound : space which the air occupies : : 9 : 11, 
 
 979 
 /. space to be added = -^- = 108 or the velocity of sound is 1088 
 
 feet per 1". 
 
 Again, also the elasticity of air is increased by vapours. Hence since 
 
 the velocity a i : if the density remain the same the velocitv 
 
 J V density 
 
 cc V elasticity. Hence if the air be supposed to consist of 11 feet, 10 of 
 air, and I of vapour, the elasticity will be increased in the ratio of 11 : 10, 
 therefore the velocity will be increased in the ratio of 11| : 10| or 21 : 20, 
 therefore the velocity of sound will altogether be 1142 feet per 1", which 
 is the same as found by experiment. 
 
 In summer the air being more elastic than in winter, sound will be 
 propagated with a greater velocity than in winter. The above calculation 
 relates to the mean elasticity of the air which is in spring and autumn. 
 Hence may be found the intervals of pulses of the air. 
 
 By experiment, a tube whose length is five Paris feet, was observed to 
 give the same sound as a chord which vibrated 100 times in 1", and in 
 the same time sound moves through 1070 feet, therefore the interval of 
 the pulses of air = 10.7 or about twice the length of the pipe. 
 
 Ff 4
 
 456 A COMMENTARY ON [SECT: VJ1T. 
 
 62. On the vibrations of a harmonic string. 
 
 The force with which a string tends to the center of the curve : force 
 which stretches the string : : length : radius of curvature. Let P p be a 
 
 B 
 
 GO 
 
 small portion of the string, O the center of the curve ; join O P, O p, and 
 draw P t, p t, tangents at P and p meeting in t, complete the parallelo- 
 gram P t p r. Join t r, then P t, p t represent the stretching force of 
 the string, which may be resolved into P x, t x and p x, t x of which 
 P x, p x destroy each other, and 2 t x = force with which the string 
 tends to the center O. Now the . t P r = | /L P O p, .*. . t P x = L. 
 P O p, .*. t r : P t : : P p : O P, i. e. the force with which any particle 
 moves towards the center of the curve : force which stretches it : : length 
 : radius. 
 
 63. To find the times of vibration of a harmonic string. 
 
 Let w = weight of the string. L = length. 
 D d : L : : weight D d : w 
 
 i.* TTA i D d X w 
 
 .. weight of D d = T
 
 BOOK II.]. NEWTON'S PRINCIPIA. 457 
 
 Also 
 
 j^ i 
 -D d : - = rad. of curve : : the moving force of D d : P 
 
 p a 
 
 .-. the moving force of D d = p X Dd X ap' 
 
 Li W 
 
 .% accelerating force = P X D d X a p* X T ^ _ 
 
 L s Dd X w 
 
 P X ap* 
 
 Lw. 
 if D O = x, D C = a, O C = a x, 
 
 p p 2 3 x 
 - 
 
 .'. the accelerating force at O = 
 
 Lw 
 
 g. Pp _ 
 ... v d s = fe r X a d x x d x 
 
 t i-. W 
 
 .-. v =J g P* X V2ax x'. 
 V L w 
 
 .-. C and 1 = 0, 
 
 d x / L w d x 
 
 dx / 
 
 .-. d t = - - = J 
 
 v V 
 
 g P p* V 2a x x 
 
 L w . ii 
 
 -p 3 X cir. arc rad. = 1 
 
 and 
 
 x 
 
 vers. sine == , 
 a 
 
 when x = a, 
 t = 0. 
 
 / L w / L w v P 
 
 * *. 5 rX quadrant = / ^ 5- X 
 
 -VgPp* VgPp* 2 
 
 / L w 
 = ^ x X /-^p- 2 
 
 .*. time of a vibration = / =r- 1" 
 
 _l. 
 
 .-. number of vibrations in 1" = ./-#' . 
 
 V L w 
 
 COR. Time of vibration = time of the oscillation of a pendulum whose 
 
 , L w 
 
 len S th =-p^r-
 
 458 
 
 For this time = ./ 
 
 A COMMENTARY, &c. 
 
 [SECT. IX. 
 
 ~ . 
 g P 
 
 64. PROP. LI. Let A F be a cylinder moving in a fluid round a 
 fixed axis in S, and suppose the fluid divided into a great number of solid 
 
 Q 
 
 orbs of the same thickness. Then the disturbing force a translation of 
 parts X surfaces. Now the disturbing forces are constant. .*. Transla- 
 tion of parts, from the defect of lubricity a -p-^ . Now the differ- 
 
 r . u . translation 1 ~ . .- , 
 
 ence ol the angular motions a r . - a -3 5 . On A Q draw 
 
 distance d stance 2 
 
 1 
 
 A a, B b, C c, &c. : : -- r - 
 
 then the sum of the differences will 
 
 distance 2 ' 
 a hyperbolic area. 
 
 .*. periodic time a i : oc |- r r . a distance. 
 
 angular motion hyperbolic area 
 
 In the same way, if they were globes or spheres, the periodic time 
 would vary as the distance *. 
 
 END OF THE FIRST VOLUME.
 
 COMMENTARY 
 
 NEWTON'S PRINCIPIA. 
 
 A SUPPLEMENTARY VOLUME. 
 
 DESIGNED FOR THE USE OF STUDENTS AT THE UNIVERSITIES. 
 
 BY 
 
 J. M. F. WRIGHT, A. B. 
 
 LATE SCHOLAR OF TRINITY COLLEGE, CAMBRIDGE, AUTHOR OF SOLUTIONS 
 OF THE CAMBRIDGE PROBLEMS, &C. &C. 
 
 IN TWO VOLUMES. 
 VOL. II. 
 
 LONDON: 
 
 PRINTED FOR T. T. & J. TEGG, 73, CHEAPSIDE; 
 AND RICHARD GRIFFIN & CO., GLASGOW. 
 
 MDCCCXXXIII.
 
 GLASGOW: 
 
 GEORGE BROOKM\X, PIUNTKIt, TILLAI ILLU.
 
 INTRODUCTION 
 
 TO 
 
 VOLUME II. 
 
 AND TO THE 
 
 ANALYTICAL GEOMETRY 
 
 1. To determine the position of a point in fixed space. 
 
 Assume any point A in fixed space as known and immoveable, and let 
 
 Z' z 
 
 three fixed planes of indefinite extent, be taken at right angles to one 
 another and passing through A. Then shall their intersections A X', 
 A Y', A Z' pass through A and be at right angles to one another.
 
 u INTRODUCTION. 
 
 This being premised, let P be any point in fixed space; from P draw 
 P z parallel to A Z, and from z where it meets the plane X A Y, draw 
 z x, z y parallel to A Y, AX respectively. Make 
 A x = x, A y = y, P z = z. 
 
 Then it is evident that if x, y, z are given, the point P can be found 
 practically by taking A x = x, A y = y, drawing x z, y z parallel to 
 AY, AX; lastly, from their intersection, making z P parallel to A Z 
 and equal to z. Hence x, y, z determine the position of the point P. 
 
 The lines x, y, z are called the rectangular coordinates of the point P ; 
 the point A the origin of coordinates ; the lines AX, AY, A Z the axes 
 of coordinates, A X being further designated the axis of x, AY the axis 
 of y, and A Z the axis of z ; and the planes X A Y, Z A X, Z A Y co- 
 ordinate planes. 
 
 These coordinate planes are respectively denoted by 
 plane (x, y), plane (x, z), plane (y, z) ; 
 
 and in like manner, any point whose coordinates are x, y, z is denoted 
 briefly by 
 
 point (x, y, z). 
 
 If the coordinates x, y, z when measured along AX, AY, A Z be 
 always considered positive ; when measured in the opposite directions, 
 viz. along A X' A Y', A Z', they must be taken negatively. Thus ac- 
 cordingly as P is in the spaces 
 
 ZAXY, ZAYX', ZAX'Y', ZAY'X; 
 Z'AXY, Z'AYX', Z'AX'Y', Z'AY'X, 
 the point P will be denoted by 
 
 point (x, y, z), point ( x, y, z), point ( x, y, z), point (x, y, z); 
 point (x, y, - z), point (- x, y, - z), point (- x, - y, - z), point (x, - y, - z) 
 respectively. 
 
 2. Given the position of two points (a, /3, 7), (a', /3', "/} in Jixcd space, 
 to find the distance between them. 
 
 The distance P P' is evidently the diagonal of a rectangular parallele- 
 piped whose three edges are parallel to A X, A Y, A Z and equal to 
 
 as a', /3s/3', 7 s/. 
 Hence 
 
 P P' = V (a a')* + 08 0r+ (7 /)" .... (1) 
 the distance required. 
 
 Hence if P' coincides with A or ', /3', / equal zero, 
 
 P A = V a* + P* + 7 * (2)
 
 ANALYTICAL GEOMETRY. iii 
 
 3. Calling the distance of any point P (x, y, z) from the origin A of 
 coordinates the radiits-vector, and denoting it by g, suppose it inclined to 
 the axes A X, A Y, A Z or to the planes (y, /), (x, z), (x, y), by the 
 angles X, Y, Z. 
 
 Then it is easily seen that 
 
 x = g cos. X, y = g cos. Y, z = cos. Z ...... (3) 
 
 Hence (see 2) 
 
 : 22 < COS> Y = * 2 2 ' 
 
 so that when the coordinates of a point are given, the angles which the ra- 
 dius-vector makes 'with each of the axes may hence be found. 
 Again, adding together the squares of equations (3), we have 
 
 (x ' + y 2 + z 8 ) = 2 (cos. 2 X + cos. 2 Y + cos. ! Z). 
 But 
 
 ? 2 = x 2 + y + z 2 (see2), 
 
 .-. cos. 2 X + cos. 2 Y + cos. Z = 1 ..... (5) 
 which shows that when two of these angles are given the other may be 
 found. 
 
 4. Given two points in space, viz. (, /S, 7), (a* /3', /) awe? owe o/* ^/;e 
 coordinates of any other point (x, y, z) m Me straight line that passes 
 through them, to determine this other point , that is, required the equations 
 to a straight line given in space. 
 
 The distances of the point (a, (3, y) from the points (a', /3', /), and 
 (x, y, z) are respectively, (see 2) 
 
 and 
 
 PQ= V ( x) * + y) + (7 z) '. 
 But from similar triangles, we get 
 
 (7-z) 2 : (PQ) 2 :: (7 - /) 2 : (P P) 2 
 whence 
 
 which gives 
 
 "J( a r+(l5 j80 8 Hy z)= ( 7 /).{( x) 2 + (/3 y) 2 } 
 
 But since a, a' are independent of /3, /3' and vice versa, the two first 
 terms of the equation, 
 ( a _a ) 2 . ( 7 -z) 2 - -(y-77 (-x) 2 ( 7 - 7 J (/3-y) ! + (/3-/3') 2 (/-z) =
 
 iv INTRODUCTION. 
 
 are essentially different from the last. Consequently by (6 vol. 1.) 
 
 (/3 /3') (y z) 2 = (y y'} z (13 y) 2 
 
 which give 
 
 
 y / 
 
 y = ^-4 (-*') 
 
 (6) 
 
 These results may be otherwise obtained; thus, pgp',is the projection 
 of the given line on the plane (x, y) &c. as in fig. 
 
 Z 
 
 Hence 
 
 Also 
 
 p q p' 
 
 z y:/ y::pq:pp' 
 
 : : m n : m p' 
 
 : : y : /3' 
 
 z "y": / y::pq:pp'::pr:pm 
 
 : : a x : a a'. 
 
 Hence the general forms of the equations to a straight line given in 
 space, not considering signs, are 
 
 z = a x -f 
 z = a' y + b' 
 
 To find where the straight line meets the planes, (x, y), (x, z), (y, z), 
 we make 
 
 z = 0, y =t 0, x = 0, 
 which give 
 

 
 ANALYTICAL GEOMETRY. 
 
 z = b' 
 
 b' 
 
 X = 
 
 a 
 z = b 
 
 b b' 
 y = a' 
 which determine the points required. 
 
 To find when the straight line is parallel to the planes, (x, y), (x, z), 
 (y, z), we must make z, y, x, respectively constant, and the equations be- 
 come of the form 
 
 Z = C \ /gx 
 
 a' y = a x + b b' / 
 
 To find when the straight line is perpendicular to the planes, (x, y), 
 (x, z) (y, z), or parallel to the axes of z, y, x, we must assume x, y; 
 x, z; y, z; respectively constant, and z, y, x, will be any whatever. 
 
 To find the equations to a straight line passing through the origin of 
 coordinates ; we have, since x = 0, and y = 0, when z = 0, 
 z = 
 z = a'y. 
 
 5. To Jind the conditions that two straight lines in Jixed space may inter- 
 Sect one another ; and also their point of intersection. 
 
 Let their equations be 
 
 z = ax -f A ) 
 z = by + BJ 
 z = a' x + A' \ 
 z= b'y+ B'f 
 
 from which eliminating x, y, z, we get the equation of condition 
 a' A a A' b' B b B' 
 a' a b' b 
 
 Also when this condition is fulfilled, the point is found from 
 
 A A'- B B' a' A A' /ln > 
 
 x = , - , y = p r-, z = 7 . . . (10) 
 
 a' a ' b b a' a 
 
 6. To Jind the angle I, at which these lines intersect. 
 
 Take an isosceles triangle, whose equal sides measured along these 
 lines equal 1, and let the side opposite the angle required be called i ; 
 then it is evident that 
 
 cos. I = 1 | i 2 
 
 a 3
 
 ' > 
 '/ 
 
 vi INTRODUCTION. 
 
 But if at the extremities of the line i, the points in the intersecting lines 
 be (x', y', z'), (x", y", z"), then (see 2) 
 
 .-. 2 cos. I = 2 J(x' x") 2 + (y 7 y") * + (z' z") *} 
 But by the equations to the straight lines, we have (5) 
 
 z' = a x' -f A ") 
 
 z'=by' + B/ 
 
 z" = a' x" + A' 
 z"=b'y" + B' 
 
 and by the construction, and Art. 2, if (x, y, z) be the point of intersec- 
 tion, 
 
 (X-XT + (y_y") 8 + (z-z") 2 = 
 Also at the point of intersection, 
 
 z = by + B = b'y + B'J 
 
 From these several equations we easily get 
 z z' = a (x a') 
 
 , a ,. 
 
 z z"= a'(x x") 
 
 y y"=g/(x x x/ ) 
 
 whence by substitution, 
 
 . / v v /\ s i_ Q z /-v v / ^ * a. - fv v 7 ^ * 1 
 
 ^A A. y -j- a ^A A ; -f . ^A. A. j 
 
 ( X _ x '/)t + a ' s (x - x") ! + pi (x-x") 2 = 1 
 which give 
 
 X X' = =- 
 
 X X" = 
 
 Hence
 
 ANALYTICAL GEOMETRY. vii 
 
 Also, since 
 
 and 
 
 z z' = a (x x') 
 z z" = a' (x x") 
 we have 
 
 '" a ' ' 2 <' 
 
 . _ _ 
 
 , a * b/2 'i n , &7t ~ bb/ ' 
 
 a 8 la- 
 
 aa' 
 
 Hence by adding these squares together we get 
 
 ( 
 
 2 cos. 1=2-1 + 1 
 
 which gives 
 
 cos. 1= _ _ ..... (J1) 
 
 ( 
 
 ^ 
 
 t 
 
 Tliis result may be obtained with less trouble by drawing straight lines 
 from the origin of coordinates, parallel to the intersecting lines ; and then 
 finding the cosine of the angle formed by these new lines. For the new 
 angle is equal to the one sought, and the equations simplify into 
 z' = ax x = b y', z" = a' x" = V y 
 z=ax = by, z=a'x =b'y 
 X' 2 + y'2 + z' 2 = 1 
 X" 2 + y'* + Z " 2 = 1 
 
 From the above general expression for the angle formed by two inter- 
 secting lines, many particular consequences may be deduced. 
 
 For instance, required the conditions requisite that two straight lines 
 given in space may intersect at right angles. 
 
 That they intersect at all, this equation must be fulfilled, (see 5) 
 a' A a A' _ b 7 B b B'; 
 of _ a b' b
 
 viii INTRODUCTION. 
 
 and that being the case, in order for them to intersect at right angles, 
 we have 
 
 I = , cos. I =. 
 
 and therefore 
 
 a a' 
 
 7. In the preceding No. the angle between two intersecting lines is 
 expressed in a function of the rectangular coordinates, which determine 
 the positions of those lines. But since the lines themselves would be 
 given in parallel position, if their inclinations to the planes, (x, y), (x, z), 
 (y, z), were given, it may be required, from other data, to find the same 
 angle. 
 
 Hence denoting generally the complements of the inclinations of a 
 straight line to the planes, (x, y), (x, z), (y, z), by Z, Y, X, the problem 
 may be stated and resolved, as follows : 
 
 Required the angle made by the two straight lines, whose angles of inclina- 
 tion are Z, Y, X ; Z', Y/, X'. 
 
 Let two lines be drawn, from the origin of the coordinates, parallel 
 to given lines. These make the same angles with the coordinate planes, 
 and with one another, as the given lines. Moreover, making an isosceles 
 triangle, whose vertex is the origin, and equal sides equal unity, we have^ 
 as in (6), 
 
 cos. I = 1 Ji = l_L(x x') 8 + (y y') 2 + (z zT? ' 
 the points in the straight lines equally distant from the origin being 
 (x, y, z), (x', y', z'). 
 But in this case, 
 
 x 2 + y 2 + z 2 = 1 
 x' 2 + y' 2 + z' 2 = 1 
 and 
 
 x = cos. X, y = cos. Y, z = cos. Z 
 x' = cos. X', y' = cos. Y', z' = cos. 71 
 . cos. I = x x' + y y' -f z z' 
 
 = cos. X. cos. X' + cos. Y. cos. Y' + cos. Z. cos. Z'. . (13) 
 Hence when the lines pass through the origin of coordinates, the same 
 expression for their mutual inclination holds good ; but at the same time 
 X, Y, Z ; X', Y', Z', not only mean the complements of the inclinations 
 to the planes as above described, but also the inclinations of the lines to 
 the axes of coordinates of x, y, z, respectively.
 
 ANALYTICAL GEOMETRY. ix 
 
 8. Given the length (L) of a straight line and the complements of its in- 
 clinations to the planes (x, y), (x, z), (y z), viz. Z, Y, X, tojind the lengths 
 of its projections upon those planes. 
 
 By the figure in (4) it is easily seen that 
 
 L projected on the plane (x, y) = L. sin. Z-\ 
 
 (x, z) = L. sin. Y V . . . (14) 
 
 (y, z) = L. sin. Xj 
 
 9. Instead of determining the parallelism or direction of a straight line 
 in space by the angles Z, Y, X, it is more concise to do it by means of 
 Z (for instance) and the angle 6 which its projection on the plane (x, y) 
 makes with the axis of x. 
 
 For, drawing a line parallel to the given line from the origin of the co- 
 ordinates, the projection of this line is parallel to that of the given line, 
 and letting fall from any point (x, y, z) of the new line, perpendiculars 
 upon the plane (x, y) and upon the axes of x and of y, it easily appears, 
 that 
 
 x = L cos. X = (L sin. Z) cos. 6 (see No. 8) 
 y = L. cos. Y = (L. sin. Z) sin. 6 
 which give 
 
 cos. X = sin. Z. cos. 6\ . . 
 
 cos. Y = sin. Z . sin. 6 J 
 Hence the expression (13) assumes this form, 
 
 cos. I = sin. Z . sin. Z' (cos. 6 cos. ^ + sin. 6 sin. 6') + cos. Z cos. Z' 
 
 = sin. Z. sin. Z'cos. (6 6') + cos. Z.cos. 71 . . . . (16) 
 which may easily be adapted to logarithmic computation. 
 The expression (5) is. merely verified by the substitution. 
 
 10. Given the angle of intersection (I) between two lines in space and 
 their inclinations to the plane (x, y), to Jind the angle at which their pro- 
 jections upon that plane intersect one another. 
 
 If, as above, Z, Z' be the complements of the inclinations of the lines 
 upon the plane, and 6, (f the inclinations of the projections to the axis of 
 x, we have from (16) 
 
 cos. I cos. Z . cos* Z' 
 
 cos. (tf if) = : fj -. = (17) 
 
 sin. Z . sin. Z' 
 
 This result indicates that I, Z, Z' are sides of a spherical triangle 
 (radius = 1), d ^ being the angle subtended by I. The form may at 
 once indeed be obtained by taking the origin of coordinates as the center 
 of the sphere, and radii to pass through the angles of the spherical tri- 
 angle, measured along the axis of z, and along lines parallel to the 
 given lines.
 
 x INTRODUCTION. 
 
 Having considered at some length the mode of determining the posi- 
 tion and properties of points and straight lines in fixed space, we proceed 
 to treat, in like manner, of planes. 
 
 It is evident that the position of a plane is fixed or determinate in posi- 
 tion when three of its points are known.. Hence is suggested the follow- 
 ing problem. 
 
 11. Having given the three points (a, /5, y), (a', 3', /), (a", /3", /') in 
 space, tojind the equation to the plane passing through them ; that is, to 
 Jind the relation between the coordinates of any other point in the plane. 
 
 Suppose the plane to make with the planes (z, y), (z, x) the intersec- 
 
 V 
 
 tions or traces B D, B C respectively, and let P be any point whatever 
 in the plane ; then through P draw P Q in that plane parallel to B D, 
 &c. as above. Then 
 
 z QN = PQ' = QQ' cot. D B Z 
 
 = y cot. D B Z. 
 But 
 
 QN = AB NA. cot. C B A 
 
 = A B + x cot. C B Z, 
 .. z = A B + x cot. C B Z + y cot D B Z. 
 
 Consequently if we put A B = g, and denote by (X, Z), (Y, Z) the 
 inclinations to A Z of the traces in the planes of (x, z), (y, z) respectively, 
 we have 
 
 z = g + x cot. (X, Z) + y cot. (Y, Z) . . . . (18) 
 Hence the form of the equation to the plane is generally 
 
 z=Ax-fBy+C (19)
 
 ANALYTICAL GEOMETRY. xi 
 
 Now to find these constants there are given the coordinates of three 
 points of the plane; that is 
 
 7 = A +B/3 + C 
 / = A a' + B 0' + C 
 
 7" = A a" + B /3" + C 
 from which we get 
 
 A - y/ y 
 
 - 
 
 B = |L0 t ^"-%> + "-!"" = C0t ' (Y, Z) ' (21) 
 c = $" (7 ' / ) + g (/ " /Y> + ff (/' 7 ") ^ <(22) 
 
 Hence when the trace coincides with the axis of x, we have 
 
 C = 0, 
 and 
 
 A = cot. ~ = 
 
 E 
 
 " (7 <*' / ) + (/ a// /' a/ ) + ' (/' 7 a") = <O ,, 
 
 7 jS' / /3 + / /3" 7" /S 7 + /' 7 /3" = ) ' 
 
 T3 _ \ / * \/ / / I \' i"^ / * \ / / / / rt >| \ 
 
 ~ $"' a(B' a,' $ + a' $" a" {? + a." $ a /S" ^ 24 > 
 
 and the equation to the plane becomes 
 
 z = B y (25) 
 
 When the plane is parallel to the plane (x, y), 
 
 A = 0, B = 0, ' 
 and. 
 
 z = C < 26 ) 
 
 from which, by means of A = 0, B = 0, any two of the quantities 7, 7', y" 
 
 being eliminated, the value of C will be somewhat simplified. 
 
 Hence also will easily be deduced a number of other particular results 
 connected with the theory of the plane, the point, and the straight line, of 
 which the following are some. 
 
 To find the projections on the planes (x, y), (x, z), (y, z) of the intersec- 
 tion of the planes, 
 
 Eliminating z, we have 
 
 (A A')x + (B B')y + C C' - .... (27) 
 which is the equation to the projection on (x, y).
 
 xii INTRODUCTION. 
 
 Eliminating x, we get 
 
 (A 7 A)z + (AB / A'B)y + AC' A' C = .... (28) 
 which is the equation to the projection on the plane (y, z). 
 And in like manner, we obtain 
 
 (B' B)z + (A'B AB')x + BC 7 B'C = .... (29) 
 for the projection on the plane (x, z). 
 
 To find the conditions requisite that a plane and straight line shall be 
 parallel or coincide. 
 
 Let the equations to the straight line and plane be 
 x = a z -f A~ 
 y = bz + BJ 
 z = A' x + B' y + C'. 
 Then by substitution in the latter, we get 
 
 z(A / a + B'b 1) + A'A + B'B + C' = 0. 
 
 Now if the straight line and plane have only one point common, we 
 should thus at once have the coordinates to that point. 
 
 Also if the straight line coincide with the plane in the above equation, 
 z is indeterminate, and (Art. 6. vol. I,) 
 
 A' a + B' b 1 = 0, A' A + B' B + C' = . . . (27) 
 
 But finally if the straight line is merely to be parallel to the plane, the 
 
 above conditions ought to be fulfilled even when the plane and line are 
 
 moved parallelly up to the origin or when A, B, C' are zero. The only 
 
 condition in this case is 
 
 A' a + B' b = 1 (28) 
 
 To Jtnd the conditions that a straight line be perpendicular to a plane 
 
 Since the straight line is to be perpendicular to the given plane, the 
 plane which projects it upon (x, y) is at right angles both to the plane 
 (x, y) and to the given plane. The intersection, therefore, of the plane 
 (x, y) and the given plane is perpendicular to the projecting plane. Hence 
 the trace of the given plane upon (x, y) is perpendicular to the projec- 
 tion on (x, y) of the given straight line. But the equations of the traces 
 of the plane on (x, z), (y, z), are 
 
 z= Ax+ C, z = By+ C-v 
 or ' t9Q\ 
 
 }_ c i cr ^^ 
 
 and if those of the perpendicular be 
 
 x = a z + A,l 
 y = b z -f B, J
 
 ANALYTICAL GEOMETRY. xiii 
 
 it is easily seen from (11) or at once, that the condition of these traces 
 being at right angles to the projections, are 
 A + a = 0, A + b = 0. 
 
 To draw a straight line passing through a given point (, /3, 7) at right 
 angles to a given plane. 
 
 The equations to the straight line, are clearly 
 
 x a + A (z 7) = 0, y /3 + B (z - 7) = 0. . . . (30) 
 To find the distance of a given point (a, /3, y] from a given plane. 
 The distance is (30) evidently, when (x, y, z) is the common point in 
 the plane and perpendicular 
 
 (z ?)* + (y 0)' + (x ) 2 = (z 7) V1 + A 2 + B*. 
 But the equation to the plane then also subsists, viz. 
 
 z = Ax + By+C 
 from which, and the equations to the perpendicular, we have 
 
 z y= C 7 + A a + B/3, 
 therefore the distance required is 
 
 C 7 + A + Bg 
 A* + B' 
 
 To Jind the angle I formed by two planes 
 z = Ax + By+C, 
 z = A' x + B' y + C'. 
 
 If from the origin perpendiculars be let fall upon the planes, the angle 
 which they make is equal to that of the planes themselves. Hence, if 
 generally, the equations to a line passing through the origin be 
 x = a z ) 
 y = bz J 
 
 the conditions that it shall be perpendicular to the first plane are 
 A + a = 0, 
 B + b = 0, 
 and for the other plane 
 
 A' + a = 0, 
 B' + b = 0. 
 Hence the equations to these perpendiculars are 
 
 x + Az = 0\ 
 
 y + Bz = Oj 
 x + A r z = \ 
 y + B' z = 0, J
 
 * v INTRODUCTION. 
 
 which may also be deduced from the forms (30). 
 Hence from (11) we get 
 
 = V (1+ A 2 + B*) V(l + A" + #"5 ' ' ' (32 > 
 
 Hence to find the inclination (E) of a plane with the plane (x, y). 
 We make the second plane coincident with (x, y), which gives 
 
 A' = 0, B' = 0, 
 and therefore 
 
 cos -*= V(I+A' + B-) ...... < 83 > 
 
 In like manner may the inclinations (), (j) of a plane 
 
 z = Ax + By + C 
 to the planes (x, z), (y, z) be expressed by 
 
 ""^Vd + A' + B-tf ...... (M> 
 
 cos ' " = V(1 + A + B')) 
 Hence 
 
 cos. 2 s + cos. 2 ^ + cos. 2 q = 1 ...... (35) 
 
 Hence also, if /, ', 93' be the inclinations of another plane to (x, y) s 
 (x, z), (y, z). 
 
 COS. I = COS. S COS. J 7 + COS. COS. ' + COS. n COS. rf . . . (36) 
 
 Tojtnd the inclination vofa straight line x = a z + A', y = b z + B', 
 
 The angle required is that which it makes with its projection upon the 
 plane. If we let fall from any part of the straight line a perpendicular 
 upon the plane, the angle of these two lines will be the complement of u. 
 From the origin, draw any straight line whatever, viz. x = a' z, y = b 7 z. 
 Then in order that it may be perpendicular to the plane, we must have 
 
 a' = A, b 7 = B. 
 
 The angle which this makes with the given line can be found from (11); 
 consequently by that expression 
 
 1 A a B b /Qiy . 
 
 : V (1 + a* + b 2 ) V (1 + A*+ B*) 
 
 Hence we easily find that the angles made by this line and the coor- 
 dinate planes (x, y), (x, z), (y, z), viz. Z, Y, X are found from 
 
 7- 1 
 
 = '
 
 cos. Y = 
 
 ANALYTICAL GEOMETRY. xv 
 
 b 
 
 cos. X = , . . , a , , . - . . . (38) 
 
 v ( 1 -J- a -p o ) 
 
 which agrees with what is done in (3). 
 
 TRANSFORMATION OF COORDINATES. 
 
 12. To transfer the origin of coordinates to the point (a, j3, y) V&ktoit 
 changing their direction. 
 
 Let it be premised that instead of supposing the coordinate planes at 
 right angles to one another, we shall here suppose them to make any 
 angles whatever with each other. In this case the axes cease to be rec- 
 tangular, but the coordinates x, y, z are still drawn parallel to the axes. 
 
 This being understood, assume 
 
 x = x' + , y = / + ft z = z' + y (39) 
 
 and substitute in the expression involving x, y, z. The result will contain 
 x', y', z' the coordinates referred to the origin (a, >, y). 
 
 When the substitution is made, the signs of a, jS, y as explained in (1), 
 must be attended to. 
 
 13. To change the direction of the axes from rectangular, without 
 affecting the origin. 
 
 Conceive three new axes A x', A y', A z', the first axes being supposed 
 rectangular, and these having any given arbitrary direction whatever. 
 Take any point ; draw the coordinates x', y', z' of this point, and project 
 them upon the axis A X. The abscissa x will equal the sum, taken with 
 their proper signs, of these three projections, (as is easily seen by drawing 
 the figure) ; but if (x x'), (y, y'), (z, z') denote the angles between the 
 axes A x, A x' ; A y, A y' ; A z, A z' respectively ; these projections 
 are 
 
 x' cos. (x' x), y' cos. (y' x), z' cos. (z' x). 
 
 In like manner we proceed with the other axes, and therefore get 
 x == x' cos. (x' x) + y' cos. (y' x) + z' cos. (2! x) -\ 
 y = y' cos. (y y) + z' cos. (z' y) + x' cos. (x' y) > . . . (40) 
 z = z' cos. (z' z) + y' cos. (y' z) + x' cos. (x' z)J
 
 XVI 
 
 INTRODUCTION. 
 
 Since (x'x), (x'y), (x'z) are the angles which the staight line A x', 
 makes with the rectangular axes of x, y, z, we have (5) 
 cos. * (x' x) -f. cos. * (x' y) -f- cos. 
 cos. 
 
 . * (x' x) -f. cos. * (x' y) -f- cos. * x' z = 1 ~\ 
 
 . 2 (y'xj l+ cos. 8 (y'y) +cos. 2 (y / z)= 1 V ... (41; 
 
 . * (z' x) + cos. 2 (z' y) 4. cos. * (z' x) = 1 ) 
 
 s.(y / z)^ 
 s.(z'z) >. (42) 
 .z'z) ) 
 
 We also have from (13) p. 
 
 cos.(xy)=cos.(x'x)cos.(y / x)+cos.(x'y)cos.(y / y)+cos.(x / z)cos.(y / z) 
 cos.(x'z') = cos.(x'x)cos.(z'x) + cos.(x'y)cos.(z'y) + cos.(x'z)cos. 
 cos.(y'z') = cos.(y'x)cos.(z'x) + cos.)y'y)cos.(z'y)-f-cos.(y'z)cos.(z'z) 
 
 The equations (40) and (41), contain the nine angles which the axes of 
 x', y', z' make with the axes of x, y, z. 
 
 Since the equations (41) determine three of these angles only, six of 
 them remain arbitrary. Also when the new system is likewise rectangu- 
 lar, or cos. (x'y') = cos. (x'z') cos. (y' z'} = 1, three others of the 
 arbitraries are determined by equations (42). Hence in that case there 
 remain but three arbitrary angles. 
 
 14. Required to transform the rectangular axe of coordinates to other 
 rectangular axes, having the same origin t but too of which shall be situated 
 in a given plane. 
 
 Let the given plane be Y' A C, of which the trace in the plane (z, x) is 
 
 A C. At the distance A C describe the arcs C Y', C x, x x' in the planes 
 C A Y', (z, x), and X' A X. Then if one of the new axes of the coordi- 
 nates be A X', its position and that of the other two, A Y', A Z', will be 
 determined by C x 7 = p, C x = 4, and the spherical angle x C x' = 6 = 
 inclination of the given plane to the plane (z, x). 
 
 Hence to transform the axes, it only remains to express the angles 
 (x'x), (y'x), &c. which enter the equations (40) in terms of $ -^ and p.
 
 ANALYTICAL GEOMETRY. xvii 
 
 By spherics 
 
 cos. (x'x) =: cos. 4 cos. -f. sin. 4 sin. cos. 6. 
 In like manner forming other spherical triangles, we get 
 cos. (/ x) = cos. (90 + 0) cos. 4/ -f- sin. 4 sin. (90 + 0) cos. 6 
 cos. (x' y) = cos. (90 + 4) cos. p + sin. (90 + 4) sin. p cos. 6 
 cos. (y'y) = cos. (90+4)c 
 So that we obtain these four equations 
 
 cos. (x' x) = cos. 4 cos. p + sin. 4 sin. p cos 
 cos. (y' x) = sin. 4 sin. 
 
 n. -y sin. p cos. ^ 
 sin. 4 cos. f cos. t 
 cos. 4* sin. p^-bs. &C 
 s. 4- cos. cos. -J 
 
 cos. (x' y) = sin. 4 cos. P + cos ' " : ~ - j ~- **" 
 cos. (y' y) = sin. 4 sin- P + cos. 4 
 Again by spherics, (since A Z is perpendicular to A C, and the inclin- 
 ation of the planes Z'A C, (x, y) is 90 6) we have 
 cos (z' x) = sin. 4 sin. $ \ 
 cos. (z 'y) = cos. 4 sin. 6 J " * 
 
 And by considering that the angle between the planes Z A C, Z A X', = 
 90 + 0, by spherics/we also get 
 
 cos. (x'z) = sin. p sin. 6 *\ 
 
 cos. (y'z) = cos. p sin. 6 V (45) 
 
 COS. (z'z) rr COS. 6 J 
 
 which give the nine coefficients of equations (40). 
 
 Equations (41), (42) will also hereby be satisfied when the systems are 
 rectangular. 
 
 15. To find the section of a surface made by a plane. 
 
 The last transformation of axes is of great use in determining the na- 
 ture of the section of a surface, made by a plane, or of the section made 
 by any two surfaces, plane or not, provided the section lies in one plane ; 
 for having transformed the axes to others, A Z, A X', A Y, the two lat- 
 ter lying in the plane of the section, by the equations (40), and the de- 
 terminations of the last article, by putting z' = in the equation to the 
 surface, we have that of the section at once. It is better, however, to 
 make z ;= in the equations (40), and to seek directly the values of 
 cos. (x'x), cos., (y'x), &c. The equations (40) thus become 
 
 x = x y cos. 4" + y' sm - 4 cos - 6 ") 
 
 y = x' sin. 4 y cos 4" cos. 6 v ..... (46) 
 
 z = y' sin. 6 ) 
 
 16. To determine the nature and position of all surfaces of the second 
 order ,- or to discuss the general equation of the second order, viz. 
 
 ax * + by* + cz z + 2dxy + 2exz + 2fyz + gx + hy +iz = k . . (a) 
 First simplify itby such a transformation of coordinates as shall destroy
 
 xviii INTRODUCTION. 
 
 the terms in x y, x z, y z ; the axes from rectangular vrill become oblique, 
 by substituting the values (40), and the nine angles which enter these, 
 being subjected to the conditions (41), there will remain six of them 
 arbitrary, which we may dispose of in an infinity of ways. Equate to 
 zero the coefficients of the terms in x' y', x' z', y' z\ 
 
 But if it be required that the new axes shall be also rectangular, as this 
 condition will be expressed by putting each of the equations (42) equal 
 zero, the six arbitrary angles will be reduced to three, which the three 
 coefficients to be destroyed will make known, and the problem will thus 
 be determined. 
 
 This investigation will be rendered easier by the following process : 
 Let x = a z, y =r /3 z be [the equations of the axis of x' ; then for 
 brevity making 
 
 : V (I + a* + /3*) 
 we find that (3) 
 
 cos. (x 7 x = a 1, cos. (x 7 y) = /3 1, Cos. x' z = 1. 
 
 Reasoning thus also as to the equations x == a'z, y = /3' z of the axis 
 of y 7 , and the same for the axis of z', we get 
 
 cos. (y'x) = a'l', cos. (y'y) = /3'1', cos. (y'z) = 1' 
 cos. (z' x) = a" 1", cos. (z' y) = j8" 1", cos. (z' z) = 1". 
 Hence by substitution the equations (40) become 
 x = 1 a x' + 1' a' y' + 1" a' 
 y = l/3x' + I'Py 
 z = 1 x' + 1' y' 
 The nine angles of the problem are replaced by the six unknowns a, 
 a', a", /3, /3', (3", provided the equations (41) are thereby also satisfied. 
 
 Substitute therefore these values of x, y, z, in the general equation of 
 the 2d degree, and equate to zero the coefficients of x' y', x' z', y' z', and 
 we get 
 
 ( aa + d/3 + e) a! + (d a -f b /3 + f) & + ea + f/3+c = 
 (a a + d + e) a" + (d + b |3 + f) $" + e a + f /3 + c 
 (a " + d/3" + e) a! + (da' 7 + b/3"+ f) & + e a" + f/3" + 
 One of these equations may be found without the others, and by making 
 the substitution only in part. Moreover from the symmetry of the pro- 
 cess the other two equations may be found from this one. Eliminate a', 
 3' from the first of them, and the equations x = a! z, y = |S' z, of the 
 axis of y'; the resulting equation 
 
 (a a + d |3 + e) x + (d a + b B + f) y + (e a + f 8 + c) z = . . (b) 
 is that of a plane (19). 
 
 1" a" zf i 
 1" /3" z' V 
 1" 2!. ) 
 
 = oj 
 
 = 0/
 
 ANALYTICAL GEOMETRY. xix 
 
 But the first equation is the condition which destroys the term x'y't 
 since if we only consider it, , /?, ', /3', may be any whatever that will 
 satisfy it ; it suffices therefore that the axis of y' be traced in the plane 
 above alluded to, in order that the transformed equations may not contain 
 any term in x' y. 
 
 In the same manner eliminating a", /3", from the 2d equation by means 
 of the equations of the axis of z', viz. x = a" z, y = /3" z, we shall have 
 a plane such, that if we take for the axis of z' every straight line which it 
 will there trace out, the transformed equation will not contain the term in 
 x' z\ But, from the form of the two first equations, it is evident that this 
 second plane is the same as the first : therefore, if we there trace the axes 
 of y' and z' at pleasure, this plane will be that of y' and z', and the 
 transformed equation will have no terms involving x' y or x z'. The 
 direction of these axes in the plane being any whatever, we have an in- 
 finity of systems which will serve this purpose ; the equation (b) will be 
 that of a plane parallel to the plane which bisects all the parallels to x, 
 and which is therefore called the Diametrical Plane. 
 
 Again, if we wish to make the term in y' z disappear, the third equa- 
 tion will give ', @, and there ai*e an infinity of oblique axes which will 
 answer the three required conditions. But make x', y 1 , a', rectangular. 
 The axis^of x' must be perpendicular to the plane (y z') whose equa- 
 tion we have just found ; and that x = a z, y = /3 z, may be the equa- 
 tions (see equations b) we must have 
 
 aa + d/3 + e = (e + f/3 + c) a . . . . (c) 
 d + b + f = (ea + f/3 + c)/3 . . . , (d) 
 
 Substituting in (c) the value of a found from (d) we get 
 { (a b)fe + (f 2 e 2 ) d }/3 3 
 
 + { (a b) (c b)e+ (2d 2 f 2 e')e + (2c a b)fd} /3 
 
 + { (c a) (c b) d+ (2e 2 f 2 d 2 ) d + (2b a c) f e } 
 
 + (a c) fd + (f 2 d 2 ) e = 0. 
 
 This equation of the 3d degree gives for /3 at least one real root ; hence 
 the equation (d) gives one for a; so that the axis of x' is determined so as 
 to be perpendicular to the plane (y', z',) and to be free from terms in 
 x' z', and y' z'. It remains to make in this plane (y*, z',) the axes at right 
 angles and such that the term x' y' may also disappear. But it is evident 
 that we shall find at the same time a plane (x, z',) such that the axis of y' 
 is perpendicular to it, and also that the terms in x' y, / y are not involved. 
 But it happens that the conditions for making the axis of y' perpendicular 
 to this plane are both (c) and (d) so that the same equation of the 3d de- 
 
 2
 
 xx INTRODUCTION. 
 
 gree must give also fi. The same holds good for the axis of z. Conse- 
 quently the three roots of the equation /3 are all real, and are the values 
 of /3, /?, {%'. Therefore a, a', a", are given by the equation (d). Hence, 
 There is only one system of rectangular axes which eliminates x' y', x' z', 
 x'y'; and thej-e exists me in all cases. These axes are called the Princi- 
 iial axes of the Surface. 
 
 Let us analyze the case which the cubic in /3 presents. 
 
 1. If we make 
 
 (a b)fe + (f 2 _e 2 )d = 
 
 the equation is deprived' of its first term. This shows that then one of 
 the roots of /3 is infinite, as well as that a derived from equation (d) be- 
 comes e a -f- f j8 = 0. The corresponding angles are right angles. One 
 of the aKes, that of z' for instance, falls upon the plane (x, y), and we 
 obtain its equation by eliminating a and ]8 from the equations x = a z, 
 y = /3 z, which equation is 
 
 ex + fy = 
 
 The directions of y', z' are given by the equation in /3 reduced to a 
 quadrature. 
 
 Sndly. If besides this first coefficient the second is also = 0, by substi- 
 tuting b, found from the above equation, in the factor of /3 2 , it reduces to 
 the last term of the equation in /3, viz. 
 
 (a c) fd + (f 2 d ! ) e = 0. 
 
 These two equations express the condition required. But the coeffi- 
 cient of 8 is deduced from that of /3 2 by changing b into c and d into e, 
 and the same holds for the first and last term of the equation in 0. 
 Therefore the cubic equation is >*lso thus satisfied. There exists therefore 
 an infinity of rectangular systems, which destroy the terms in x' y', x' z', 
 y' z'. Eliminating a and b from equations (c) and (d) by aid of the above 
 two equations of condition, we find that they are the product of fa d 
 and e|3 d by the common factor eda + fd/3 + fe. These factors 
 are therefore nothing ; and eliminating a and /3, we find 
 
 fx = dz, ey = dz, e d x + f d y + f e z = 0. 
 
 The two first are the equations of one of the axes. The third that oi 
 a plane which is perpendicular to it, and in which are traced the two 
 other axes under arbitrary directions. This plane will cut the surface in 
 a curve rherein all the rectangular axes are principal, which curve is 
 therefore a circle, the only one of curves of the second order which has 
 that property. The surface is one then of revolution round the axis 
 whose equations we have just given.
 
 ANALYTICAL GEOMETRY. xxi 
 
 The equation once freed from the three rectangles, becomes of the 
 form 
 
 kz* + my*-{-nx 2 + qx-t-q'y + q''z = h . . . . (e) 
 
 The terms of the first dimension are evidently destroyed by removing 
 the origin (39). It is clear this can be effected, except in the case 
 where one of the squares x 2 , y 2 , z * is deficient. We shall examine these 
 cases separately. First, let us discuss the equation 
 
 kz* + my 2 + nx 8 = h ....... (f) 
 
 Every straight line passing through the origin, cuts the surface in two 
 points at equal distances on both sides, since the equation remains the same 
 after having changed the signs of x, y, z. The origin being in the middle 
 of all the chords drawn through this point is a center. The surface therefore 
 has the property of possessing a center whenever the transformed equation 
 has the squares of all the variables. 
 
 We shall always take n positive : it remains to examine the cases where 
 k and m are both positive, both negative, or of different signs. 
 
 If in the equation (f) k, m, and n, are all positive, h is also positive ; 
 and if h is nothing, we have x = 0, y 0, z =: 0, and the surface has 
 but one point. 
 
 But when h is positive by making x, y, or z, separately equal zero, we 
 find the equations to an ellipse, curves which result from the section of 
 the surface in question by the three coordinate planes. Every plane 
 parallel to them gives also an ellipse, and it will be easy to show the 
 same of all plane sections. Hence the surface is termed an Ellip- 
 soid. 
 
 The lengthy A, B, C, of the three principal axes are obtained by find- 
 ing the sections of the surface through the axes of x, y, and z. Th,:e 
 give 
 
 kC 2 = h, mB z = h, nA c = h. 
 
 from which eliminating k, m and n, and substituting in equation (f) we gel 
 
 2 
 
 --'- 1 ") 
 
 A 2 ' f. (47) 
 
 A'B'z 2 + A*C 2 y 2 + B* C 2 x 2 = A 2 B* C 2 J 
 which is the equation to an Ellipsoid referred to its center and principal 
 axes. 
 
 We may conceive this surface to be generated by an ellipse, traced in 
 the plane (x, y), moving parallel to itself, whilst its two axes vary, the 
 curve sliding along another ellipse, traced in the plane (x, z) as a direct- 
 
 4 3
 
 INTRODUCTION. 
 
 rix. If two of the quantities A, B, C, are equal, we liave an ellipsoid of 
 revolution. If all three are equal,, we have a sphere. 
 
 Now suppose k negative, and in and h positive or 
 k z 2 my 8 ax 2 = h. 
 
 Making x or y equal zero, we perceive that the sections by the planes 
 (y z) and (x z), are hyperbolas, whose axis of z is the second axis. All 
 planes passing through the axis of z, give this same curve. Hence the 
 surface is called an Jiyperboloid. Sections parallel to the plane (x y) are 
 always real ellipses, A, B, C V 1 designating the lengths upon the 
 axes from the origin, the equation is the same as the above equation ex- 
 cepting the first term becoming negative. 
 
 Lastly, when k and h are negative 
 
 kz 2 + my 2 + nx 2 = h; 
 
 all the planes which pass through the axis of z cut the surface in hyper- 
 bolas, whose axis of z is the first axis. The plane (x y) does not meet 
 the surface and its parallels passing through the opposite limits, give 
 ellipses. This is a hyperboloid also, but -having two vertexes about the 
 axis of z. The equation in A, B, C is still the same as above, excepting 
 that the term in z* is the only positive one. 
 
 When h = 0, we have, in these two cases, 
 
 k 2J = my 2 + nx 2 (48) 
 
 the equation to a cone, which cone is the same to these hyperboloids that 
 an asymptote is to hyperbolas. 
 
 It remains to consider the case of k and m being negative. But by a sim- 
 ple inversion in the axes, this is referred to the two preceding ones. The 
 hyperboloid in this case has one or two vertexes about the axis of x ac- 
 cording as h is negative or positive. 
 
 When the equation (e) is deprived of one of the squares, of x * for in- 
 stance, by transferring the origin, we may disengage that equation from 
 the constant term and from those in y and z ; thus it becomes 
 
 kz 2 + my 2 = hx (49) 
 
 The sections due to the planes (x z), (x y) are parabolas in the same 
 or opposite directions according to the signs of k, m, h ; the planes par- 
 allel to these give also parabolas. The planes parallel to that of (y z) 
 give ellipses or parabolas according to the sign of m. Ine surface is an 
 elliptic paraboloid in the one case, and a hyperbolic paraboloid in the 
 other case. When k = m, it is a paraboloid of revohtticn. 
 
 "When h = 0, the equation takes the form 
 a J z 2 + b 2 y 2 =
 
 ANALYTICAL GEOMETRY. xxiii 
 
 according to the signs of k and m. In the one case we have 
 z = 0, y = 
 
 whatever may be the value of x, and the surface reduces to the axis of x, 
 In the other case. 
 
 (a z + b y) (a z by) = 
 
 shows that we make another factor equal zero ; thus we have the system 
 of two planes which increase along the axis of x. 
 
 When the equation (e) is deprived of two squares, for instance of x 2 , 
 y *, by transferring the origin parallelly to z, we reduce the equation to 
 
 kz 2 + py + qx = (50) 
 
 The sections made by the planes drawn according to the axis of z, are 
 parabolas. The plane (x y) and its parallels give straight lines par- 
 allel to them. The surface is, therefore, a cylinder whose base is a para- 
 bola, or a parabolic cylinder. 
 
 If the three squares in (e) are wanting, it will be that of a plane. 
 
 It is easy to recognise the case where the proposed equation is decom- 
 posable into rational factors ; the case where it is formed of positive 
 squares, which resolve into two equations representing the sector of two 
 planes. 
 
 PARTIAL DIFFERENCES. 
 
 17. If u = f (x, y, z, &c.) denote any function of the variable x, y, z, 
 &c. d u be taken on the supposition that y, z, &c. are constant, then is the 
 result termed the partial difference of u relative to x, and is thus written 
 
 d u 
 
 u\ 
 
 T ) 
 d x/ 
 
 dx. 
 *d x/ 
 
 Similarly 
 
 d u\ /d u 
 
 denote the partial differences of u relatively to y, z, &c. respectively. 
 
 Now since the total difference of u arises from the increase or decrease 
 of its variables, it is evident that 
 
 Z+& , ...(51)
 
 INTRODUCTION. 
 
 But, by the general principle laid down in (6) Vol. I, this result may 
 be demonstrated as follows ; Let 
 
 u + du = A + Adx+Bdy +Cdz +&c. 
 
 A'dx 2 + B'dy 2 + C'dz 2 + &c.| 
 + M d x . d y + N d x . d z + &c. J 
 + A" d x d + &c. 
 Then equating quantities of the same nature, \ve have 
 
 du = Adx-fBdy+Cdz + &c. 
 
 Hence when d y, d z, &c. = 0, or when y, z, &c. are considered con- 
 slant 
 
 d u = A d x 
 or according to the above notation 
 
 A = 
 
 In the same manner it is shown, that 
 
 r - u 
 
 
 &c. 
 Hence 
 
 du = - dx + E dy + d. + &c. ,s before. 
 
 Ex. 1. u = xyz. &c. 
 du\ /cl\ 
 
 ar) = y z> (-ay) = XZi 
 
 .: du = yzdx + xzdy + xydz 
 
 du dx dy.dz 
 or - = -- + i + - . 
 
 u x y z 
 
 Ex. 2. u = x y z, &c. Here as above 
 
 ^ = l2E + i_y + li+ &c . 
 
 u x y z 
 
 And in like manner the total difference of any function of any number 
 of variables may be found, viz. by first taking the partial differences, as in 
 the rules laid down in the Comments upon the first section of the first 
 book of the Principia. 
 
 But this is not the only use of partial differences. They are frequently 
 used to abbreviate expressions.* Thus, in p. 13, and 14, Vol. II. we
 
 ANALYTICAL GEOMETRY. xxv 
 
 learn that the actions of M, p, &", &c. upon p resolved parallel to x, 
 amount to 
 
 > j'(x' x) jt"(x" x) 
 
 d f - [(x'- 
 
 MX 
 
 _ 
 
 [(x'"_ x)* + (y'" y)+ (z-' 
 retaining the notation there adopted. 
 
 But if we make 
 
 JL Krr 
 
 ' 
 
 and generally 
 
 '_ y)t + ( Z '_ Z) 2 = g 
 0, 1 
 
 and then assume 
 
 i ' " 
 X = + + &c (A) 
 
 S S 
 
 0,1 0,2 
 
 + ^L + ZOL. + &c (B) 
 
 S 
 
 2 1,3 
 
 +-^- + ftJL - + &c. . . . (C) 
 
 f e 
 
 2, 2,4 
 
 &C. 
 
 we get 
 
 U, IL (x' X) It fJt," (X" X) 
 
 = S + ^-3 + 
 
 0,1 , 0,2 
 
 ^ 
 
 + 
 
 , - , 
 
 /dxx __./dAx v , /* ft (/ y) , f- 1*" (f y) 
 
 VdyJ - \-dT) - ~p~ -p- 
 
 0, 1 0,2 
 
 _ it,p'(zfz) 11 1*" (z z) 
 
 - ~^~ ^" ~ 
 
 0, 1 0, 2 
 
 We also get 
 
 dx\ ^^( x> x ) f&B\ 
 
 dxJ "?" VdxV 
 
 0,1 
 
 ^A(,"(x" X) A>>" K *') 
 
 ~^~ ~~ 
 
 0,2 1, 2 
 
 x'" x) ^>"'(x'" x) ff(t'(x'" x") /d D 
 3 ~"~~ 3 h 
 
 0, 3 1, 3 2, 3
 
 INTRODUCTION. 
 
 Hence since (13) has one term less than (A); (C) one term less than 
 (B) ; and so on ; it is evident that 
 
 ) +*<=- <)-(!) - Or*) - 
 
 and therefore that 
 
 2.( d ) = (!L) + (1^) + (1M + &c. = 
 
 See p. 15, Vol. II. 
 
 Hence then X is so assumed that the sum of its partial differences re* 
 lative to x, x', x" &c. shall equal zero, and at the same time abbreviate 
 the expression for the forces upon fi along x from the above complex 
 formula into 
 
 d (g + x) m J[_/d_X\ MX 
 dt* /a \dx/ j 3 ' 
 
 and the same may be said relatively to the forces resolved parallel to 
 y, z, &c. &c. 
 
 Another consequence of this assumption is 
 
 X (Ty) + x ' (dy) + &C ' = y (^) + y/ (ff?) + & " 
 
 d 
 
 For 
 
 d X x ttti'(x' x)y ^X'(x" x)y ft ^"(TS."' x) y & 
 
 dirJ - ' ~p~ "7^" f 3 
 
 0, 1 0, 2 0, 3 
 
 puTtf x')y' ^VV xpy' ^' (x x) y 
 
 -- ~ 
 
 1,2 1,3 
 
 v f d x ^ _^'V"(x >> '-x") y" . /aV"(x""-x") y" , g _ ^"(x"- 
 
 1 ~ "~ 3 3 
 
 2,3 2,4 0,2 1,2 
 &C. 
 
 Hence it is evident that 
 
 _ ^'(x x)(y y') /.^(x" x) (y y") & 
 
 ~~ - 3 "" 3 T^ 
 
 dx / 
 
 0, 1 0, 2 
 
 ^y (x" x) (y y") ^"(x'" x) (y y") 
 
 -~ - 
 
 12 12 
 
 /i'y(x" x") (y y") ^V'Cx"" x") (y" y'") & 
 
 T ~Ts T ' 3 T txl " 
 
 2, 3 2. 4 
 
 &C.
 
 ANALYTICAL GEOMETRY. xxvii 
 
 In like manner it is found that 
 
 . 
 3- 
 
 0,1 *, 2 
 
 >"(y" y') (xx") /jV"(y'" y ) (s x'") 
 
 ? 3 
 
 f t 3 
 
 which is also perceptible from the substitution in the above equation of 
 y for x, x for y; y' for X', x' for y' ; and so on. 
 
 But 
 
 , (/-y) (x x') = (x x) (y y') 
 
 (y" y) (x x") = (x" x) (y y") 
 
 &c. 
 consequently 
 
 See p. 16. For similar uses of partial differences, see also pp. 22, and 
 105. 
 
 CHANGE OF THE INDEPENDENT VARIABLE. 
 
 When an expression is given containing differential coefficients, such 
 
 06 
 
 iy ilz&c 
 
 dx j dx 2& 
 
 in which the first differential only of x and its powers are to be found, it 
 shows that the differential had been taken on the supposition that dx is 
 constant, or that d 2 x = 0, d s x = 0, and so on. But it may be re- 
 quired to transform this expression to another in which d * x, d 3 x shall 
 appear, and in which d y shall be constant, or from which d 2 y, &c. shall 
 be excluded. This is performed as follows : 
 For instance if we have the expression 
 
 I dx 8 d_y 
 d* y dx 
 dx* . 
 
 the differential coefficients 
 
 dy (Py 
 dx' dx"
 
 xxviii INTRODUCTION. 
 
 may be eliminated by means of the equation of the curve to which we 
 mean to apply that expression. For instance, from the equation to a 
 parabola y = a x *, we derive the values of 
 
 dy i d* y 
 -j-^- and -j j { 
 d x dx z 
 
 which being substituted in the above formula, these differential coefficients 
 will disappear. If we consider 
 
 dy d^y 
 <Tx J dx' 
 
 unknown, we must in general have two equations to eliminate them from 
 one formula, and these equations will be given by twice differentiating the 
 equation to the curve. 
 
 When by algebriacal operations, d x ceases to be placed underneath 
 d y, as in this form 
 
 __ 
 
 dx* + dy* y d'y 
 
 the substitution is effected by regarding d x, d y, d * y as unknown ; but 
 then in order to eliminate them, there must be in general the same 
 number of equations as of unknowns, and consequently it would seem the 
 elimination cannot be accomplished, because by means of the equation to 
 the curve, only two of the equations between d x, d y, d * y can be ob- 
 tained. It must be remarked, however, that when by means of these two 
 equations we shall have eliminated d y and d z y, there will remain a com- 
 mon factor d x *, which will also vanish. For example, if the curve is 
 always a parabola represented by the equation y = ax , by differentiat- 
 ing twice we obtain 
 
 dy = 2axdxOd 2 y = 2a dx* 
 
 and these being substituted in the formula immediately above, we shall 
 obtain, after suppressing the common factor d x 2 , 
 y(l + 4a*x') 
 
 4a 2 x 2 Say 
 
 The reason why d x * becomes a common factor is perceptible at once, 
 for when from a formula which primitively contained 
 
 d'y dy 
 dx" dx' 
 
 d z v 
 we have taken away the denominator of -. * all the terms independent 
 
 of V-^ and T^ must acquire the factor d x * ; then the terms which 
 dx 2 d x 
 
 d 2 v d y 
 
 were affected by -7 a do not contain d x, whilst those affected by j-
 
 ANALYTICAL GEOMETRY. xxix 
 
 contain d x. When we afterwards differentiate the equation of the curve, 
 and obtain results of the form dy = M d x, d 2 y = Ndx 2 , these values 
 being substituted in the terms in d 2 y, and in dy dx, will change them, 
 as likewise the other terms, into products of d x 2 . 
 
 What has been said of a formula containing differentials of the two first 
 orders applying equally to those in which these differentials rise to supe- 
 rior orders, it thence follows that by differentiating the equation of the 
 curve as often as is necessary, we can always make disappear from the 
 expression proposed, the differentials therein contained. 
 
 The same will also hold good if, beside these differentials which we have 
 just been considering, the formula contain terms in d * x, in d * x, &c. ; 
 for suppose that there enter the formula these differentials d x, d y, d 2 x, 
 d 2 y and that by twice differentiating the equation represented by y = f x, 
 we obtain these equations 
 
 F (x, y, d y, d x) = 
 F(x,y, dx, dy, d'x, d'y) = 0, 
 
 vre can only find two of the three differentials d y, d 2 x, d 2 y, and we see 
 it will be impossible to eliminate all the differentials of the formula ; there 
 is therefore a condition tacitly expressed by the differential d 2 x ; it is 
 that the variable x is itself considered a function of a third variable which 
 does not enter the formula, and which we call the independent variable. 
 This will become manifest if we observe, that the equation y = f x may 
 be derived from the system of two equations 
 
 x = F t, y = (f t 
 from which we may eliminate t. Thus the equation 
 
 (x c) s 
 y = a ' b^ 
 is derived from the system of two equations 
 
 x = b t + c, y = a t 2 , 
 
 and we see that x and y must vary by virtue of the variation which t may 
 undergo. But this hypothesis that x and y vary as t alters, supposes that 
 there are relations between x and t, and between y and t. One of these 
 relations is arbitrary, for the equation which we represent generally by 
 y = f x, for example 
 
 ^ / \ 
 
 y : = %- ( x c ) > 
 
 if we substitute between x and t, the arbitrary relation,
 
 INTRODUCTION. 
 
 this value being put in the equation 
 
 y = * (x c) f 
 
 will change it to 
 
 (t 3 c 3 ) 8 
 
 y " a b*c 4 * 
 
 an equation which, being combined with this, 
 
 t 3 
 
 X = ; , 
 
 C 8 
 
 ought to reproduce by elimination, 
 
 the only condition which we ought to regard in the selection of the varia- 
 ble t. 
 
 We may therefore determine the independent variable t at pleasure. 
 For example, we may take the chord, the arc, the abscissa or ordinate 
 for this independent variable ; if t represent the arc of the curve, we 
 have 
 
 t = V (dx + dy 2 ); 
 
 if t denote the chord and the origin be at the vertex of the curve, we 
 have 
 
 lastly, if t be the abscissa or ordinate of the curve, we shall have 
 
 t = x, or t = y. 
 
 The choice of one of the three hypotheses or of any other, becoming in- 
 dispensible in order that the formula which contains the differentials, may 
 be delivered from them, if we do not always adopt it, it is even then tacitly 
 supposed that the independent variable has been determined. For ex- 
 ample, in the usual case where a formula contains only the differentials 
 d x, d y, d* y, d 3 y, &c. the hypothesis is that the independent variable 
 t has been taken for the abscissa, for then it results that 
 
 t - x ^ -- 1 
 
 * df 
 
 = o, 
 
 and we see that the formula does not contain the second, third, &c. dif- 
 ferentials.
 
 ANALYTICAL GEOMETRY. xxxi 
 
 To establish this formula, in all its generality, we must, as above, sup- 
 pose x and y to be functions of a third variable t, and then we have 
 
 d y _ d y d x 
 dt := d~^'dT ; 
 from which we get 
 
 dy 
 
 4* =41 ........... (53) 
 
 dx d_x 
 
 d~t 
 
 taking the second differential of y and operating upon the second membei 
 as if a fraction, we shall get 
 
 d x d 2 y d y d 8 x 
 d*y _ d~t ' dl cfT* d t 
 d x d x 2 
 
 dT* 
 
 In this expression, d t has two uses ; the one is to indicate that it is 
 the independent variable, and the other to enter as a sign of algebra. 
 In the second relation only will it be considered, if we keep in view that 
 t is the independent variable. Then supposing d t 2 the common factor, 
 the above expression simplifies into 
 
 d 8 y _ dxd 2 y dyd'x 
 "dT : dx 2 ~ 
 
 and dividing by d x, it will become 
 
 d* y _ d x d* y d y d 2 x 
 d^ ~ dx 3 
 
 Operating in the same way upon the equation (53), we see that in 
 taking t as the independent variable, the second member of the equation 
 ought to become identical with the first ; consequently we have only one 
 change to make in the formula which contains the differential coefficients 
 
 dyd 2 y . d z y, 
 
 , 2 , viz. to replace - s by 
 
 < 54) 
 
 To apply these considerations to the radius of curvature which is given 
 by the equation See p. 61. vol. I.) 
 
 H 
 
 IV = -r 
 
 d z y
 
 xxxii INTRODUCTION. 
 
 if we wish to have the value of R, in the case where t shall be the inde- 
 pendent variable, we must change the equation to 
 
 R = 
 
 d x d* y d y d s x* 
 
 dx 3 
 and observing that the numerator amounts to 
 
 (dx* + dy V 
 
 dx 3 
 we shall have 
 
 R- 
 
 - 
 
 dxd*y dy*d*x 
 This value of R supposes therefore that x and y are functions of a third 
 independent variable. But if x be considered this variable, that is to say, 
 if t = x, we shall have d 2 x =0, and the expression again reverts to the 
 common one 
 
 . (i + i^) 1 
 
 I fl "5 -I- (I V I * V fl V "/ 
 
 Rl U -V I \J V J \. A. f 
 1 T - rs; ' . 
 
 d x d 2 y d' y 
 
 d x* 
 
 But if, instead of x for the independent variable, we wish to have the 
 ordinate y, this condition is expressed by y = t ; and differentiating this 
 equation twice, we have 
 
 The first of these equations merely indicates that y is the independent 
 variable, which effects no change in the formula ; but the second shows 
 us that d* y ought to be zero, and then the equation (55) becomes 
 
 R= _* l ...... (56) 
 
 d y d 2 x 
 
 We next remark, that when x is the independent variable, and 
 consequently d * x = 0, this equation indicates that d x is constant. 
 Whence it follows, that generally the independent variable has always 
 a constant differential. 
 
 Lastly, if we take the arc for the independent variable, we shall have 
 
 dt = V(dx ! + dy); 
 Hence, we easily deduce 
 
 d*' dy'__ .. 
 dT' + dt' 
 
 v
 
 ANALYTICAL GEOMETRY. xxxiii 
 
 differentiating this equation, we shall regard d t as constant, since t is the 
 independent variable ; we get 
 
 2dx d'x 2dy d g y 
 ~dt^~ dt " 
 
 which gives 
 
 dxd 2 x = dy d 2 y 
 
 Consequently, if we substitute the value of d s x, or that of d ~ y, in the 
 equation (55), we shall have in the first case 
 
 R( Cl X *T~ Q V 1 t \f ( d X "4 ^* V ) i / ^ *s\ 
 
 = TA j t( i d X = i -5-5-= J ' d X . . (a7) 
 
 (dx + dy 2 ) d*y d*y 
 
 and in the second case, 
 
 (d x 2 -f- d y *) 2 V (d x a -f- d y 2 ) , 
 
 K = 7-5 , J ' . .. d y = j-5 *-* d y . (58) 
 
 (d x + d y l ) d ^ x J d i x 
 
 In what precedes, we have only considered the two differential coeffi- 
 cients 
 
 d y d 2 y 
 
 cTx' HIT 2 ' 
 
 but if the formula contain coefficients of a higher order, we must, by 
 means analogous to those here used, determine the values of 
 
 ^of^ &c 
 
 df> wl i .3 CXI* 
 
 x d d x^ 
 
 which will belong to the case where x and y are functions of a third in- 
 dependent variable. 
 
 PROPERTIES OF HOMOGENEOUS FUNCTIONS. 
 
 IfMdx -f Ndy-f Pdt-f- &* = d z, be a homogeneous function of 
 any number of variables^ x, y, t, &c. in which the dimension of each term is 
 n, then is 
 
 MX + Ny -f Pt + &c. = nz. 
 
 For let M d x + N d y be the differential of a homogeneous function 
 z between two variables x and y ; if we represent by n the sum of the 
 exponents of the variables, in one of the terms which compose this func- 
 tion, we shall have therefore the equation 
 Mdx + Ndy = dz. 
 
 Making ^ = q, we shall find (vol. I.) 
 
 F (q) X X n = z ; 
 c
 
 INTRODUCTION. 
 
 and replacing, in the above equation, y by its value q x, and colling M' 
 N', what M and N then become, that equation transforms to 
 
 M' d x + N' d. q x = d z ; 
 and substituting the value of z, we shall have 
 
 M' d x + 1ST d (q z) = d (x n F. q.) 
 But d (q z) = q d x + x d q. Therefore 
 
 (M' + N'q) dx + N'xdq = d (x n F. q). 
 
 But, (M* + N' q) d x being the differential of x n F q relatively to x, we 
 have (Art 6. vol. 1.) 
 
 M' + N'q = nx"- 1 X F.q. 
 If in this equation y be put for q x, it will become 
 
 M + N- = nx'-'F.q, 
 
 or 
 
 Mx-fNy = nz. 
 
 This theorem is applicable to homogeneous functions of any number of 
 variables ; for if we have, for example, the equation 
 Mdx+ Ndy+ Pdt = dz, 
 in which the dimension is n in every term, it will suffice to make 
 
 y t 
 
 = q , = r 
 x x 
 
 to prove, by reasoning analogous to the above, that we get z r= x" F (q, r), 
 and, consequently, that 
 
 MX + Ny + Ft = nz (59) 
 
 and so on for more variables. 
 
 THEORY OF ARBITRARY CONSTANTS. 
 
 An equation V = between x, y, and constants, may be considered as 
 the complete integral of a certain differential equation, of which the order 
 depends on the number of constants contained in V = 0. These constants 
 are named arbitrary constants, because if one of them is represented by a, 
 and V or one of its differentials is put under the form f (x, y) = a, we see 
 that a will be nothing else than the arbitrary constant given by the integra- 
 tion of d f (x, y). Hence, if the differential equation in question is of the 
 order n, each integration introducing an arbitrary constant, we have 
 V = 0, which is given by the last of three integrations, and contains, at
 
 ANALYTICAL GEOMETRY. xxxv 
 
 least, n arbitrary constants more than the given differential equation. Lei 
 therefore 
 
 F (x,y) = 0, F (x,y, jj-2) = , F (x,y,j}|, ^) = & ' 00 
 
 be the primitive equation of a differential equation of the second order 
 and its immediate differentials. 
 
 Hence we may eliminate from the two first of these three equations, 
 the constants a and b, and obtain 
 
 If, without knowing F (x, y) = 0, we find these equations, it will be 
 
 sufficient to -eliminate from them ** > to obtain F (x, y) = 0, which will 
 
 be the complete integral, since it will contain the arbitrary constants a, b. 
 If, on the contrary, we eliminate these two constants between the 
 above three equations, we shall arrive at an equation which, containing 
 the same differential coefficients, may be denoted by 
 
 But each of the equations (b) will give the same. In fact, by eliminating 
 the constant contained in one of these equations and its immediate differ- 
 ential, we shall obtain separately two equations of the second order, 
 which do not differ from equation (c) otherwise than the values of x and 
 of y are not the same in both. Hence it follows, that a differential equa- 
 tion of the second order may result from two equations of the first order 
 which are necessarily different, since the arbitrary constant of the one is 
 different from that of the other. The equations (b) are what we call the 
 first integrals of the equation (c), which is independent, and the equation 
 F (x, y) = is the second integral of it. 
 
 Take, for example, the equation y = a x + b, which, because of its 
 two constants, may be regarded as the primitive equation of an equation 
 of the second order. Hence, by differentiation, and then by elimination 
 of a, we get 
 
 dy dy , 
 
 - r ^- = a,y = x- r ^- + b. 
 dx dx 
 
 These two first integrals of the equation of the second order which we 
 are seeking, being differentiated each in particular, conduct equally, by 
 
 d 2 y 
 the elimination of a, b, to the independent equation -. -, = 0. In the 
 
 Cl A 
 
 c3
 
 xxxvi INTRODUCTION. 
 
 case where the number of constants exceeds that of the required arbitrary 
 constants, the surplus constants, being connected with the same equations, 
 do not acquire any new relation. Required, for instance, the equation of 
 the second order, whose primitive is 
 
 y = Jax 8 + bx + c = 0; 
 differentiating we get 
 
 ^ = ax + b. 
 dx 
 
 The elimination of a, and then that of b, from these equations, give 
 separately these two first integrals 
 
 ^2 = ax + b, y = x^ ax 2 + c . . . (d) 
 
 dx ' 3 dx 
 
 Combining them each with their immediate differentials, we arrive, 
 
 d 2 v 
 by two different ways, at -, - 2 = a. If, on the contrary, we had elimi- 
 
 Cl X 
 
 nated the third constant a between the primitive equation and its imme- 
 diate differential, that would not have produced a different result; for 
 we should have arrived at the same result as that which would lead to 
 the elimination of a from the equations (d), and we should then have 
 
 jo _j 
 
 fallen upon the equation x -r \ = -*- b, an equation which reduces 
 
 d*y 
 to - ^ = a by combining it with the first of the equations (d). 
 
 Let us apply these considerations to a differential equation of the third 
 order : differentiating three times successively the equation F (x, y) = 0, 
 we shall have 
 
 These equations admitting the same values for each of the arbitrary 
 constants contained by F (x, y) = 0, we may generally eliminate these 
 constants between this latter equation and the three preceding ones, and 
 obtain a result which we shall denote by 
 
 dy d 2 y d 3 y\ 
 
 y'di>d/3r) = ...... <> 
 
 This will be the differential equation of the third order of F (x, y) = 0, 
 and whose three arbitrary constants are eliminated ; reciprocally, 
 F (x, y) =r 0, will be the third integral of the equation (e). 
 
 If we eliminate successively each of the arbitrary constants from the
 
 ANALYTICAL GEOMETRY. xxxvii 
 
 equation F (x, y) = 0, and that which we have derived by differentiation, 
 we shall obtain three equations of the first order, which will be the second 
 integrals of the equation (e). 
 
 Finally, if we eliminate two of the three arbitrary constants by means 
 of the equation F (x, y) = 0, and the equations which we deduce by two 
 successive differentiations, that is to say, if we eliminate these constants 
 from the equations 
 
 F (x >y ) = 0, F (*, y , fa = 0, F (*, y , & &) = . . (0 
 
 we shall get, successively, in the equation which arises from the elimina- 
 tion, one of the three arbitrary constants ; consequently, we shall have as 
 many equations as arbitrary constants. Let a, b, c, be these arbitrary 
 constants. Then the equations in question, considered only with regard 
 to the arbitrary constants which they contain, may be represented by 
 
 <p c =. 0, <p b c= 0, <f> a = (g) 
 
 Since the equations (f ) all aid in the elimination which gives us one of 
 these last equations, it thence follows that the equations (g) will each be 
 of the second order ; we call them the first integrals of the equation (e). 
 
 Generally, a differential equation of an order n will have a number n 
 of first integrals, which will contain therefore the differential coefficients 
 
 d v d n ~ 1 v 
 from -r^ to -, 5^ inclusively; that is to say, a number n-l of differential 
 
 O X Cl X 
 
 coefficients ; and we see that then, when these equations are all known, 
 to obtain the primitive equation it will suffice to eliminate from these equa- 
 tions the several differential coefficients. 
 
 PARTICULAR SOLUTIONS OF DIFFERENTIAL EQUATIONS. 
 
 It is easily seen that a particular integral may always be deduced from 
 the complete integral, by giving a suitable value to the arbitrary con- 
 stant. 
 
 For example, if we have given the equation 
 
 xdx + ydy = d y V x * + y 2 a*, 
 whose complete integral is 
 
 y + c = V (x 2 + y 2 a j ), 
 whence (for convenience, by rationalizing,) we get 
 
 (a-x).-j^+2xy^ + x = .... (1.) 
 c2
 
 xxxviii INTRODUCTION. 
 
 and the complete integral becomes 
 
 2 cy + c 1 x 2 + a 1 = .... (i) 
 
 Hence, in taking for c an arbitrary constant value c = 2 a, we shall 
 obtain this particular integral 
 
 2 c y + 5 a 2 x 2 = 0, 
 
 which will have the property of satisfying the proposed equation (h) as 
 well also as the complete integral. In fact, we shall derive from this 
 particular integral 
 
 x' 5 a* d y x 
 ~~2~c" ' cfx =: 7 ; 
 these values reduce the proposed to 
 
 an equation which becomes identical, by substituting in the second mem- 
 ber, the value of c *, which gives the relation c = 2 a. Let 
 
 Mdx + Ndy = 0, 
 
 be a differential equation of the first order of a function of two variables 
 x and y ; we may conceive this equation as derived by the elimination of 
 a constant c from a certain equation of the same order, which we shall 
 represent by 
 
 mdx-f-ndy = 0, 
 and the complete integral 
 
 F (x, y, c) = 0, 
 
 which we shall designate by u. But, since every thing is reduced to 
 taking the constant c such, that the equation 
 
 Mdx+Ndy = 0, 
 
 may be the result of elimination, we perceive that is at the same time 
 permitted to vary the constant c, provided the equation 
 
 M d x + N d y = 0, 
 holds good ; in this case, the complete integral 
 
 F (x, y, c) = 
 
 will take a greater generality, and will represent an infinity of curves of 
 the same kind, differing from one another by a parameter, that is, by a 
 constant. 
 
 Suppose therefore that the complete integral being differentiated, by 
 considering c as the variable, we have obtained
 
 ANALYTICAL GEOMETRY. xxxix 
 
 an equation which, for brevity, we shall write 
 
 dy = pdx + qdc (k) 
 
 Hence it is clear, that if p remaining finite, q d c is nothing, the result 
 of the elimination of c as a variable from 
 
 F (x, y, c) = 0, 
 
 and the equation (k), will be the same as that arising from c considered 
 constant, from 
 
 F (x, y, c) = 0, 
 and the equation 
 
 d y = p d x 
 (this result is on the hypothesis 
 
 Mdx+Ndy = 0), 
 for the equation (k), since 
 
 q d c = 0, 
 does not differ from 
 
 dy = p dx; 
 but in order to have 
 
 q d c = 0, 
 
 one of the factors of this equation ss constant, that is to say, that we 
 have 
 
 d c = 0, or q = 0. 
 
 In the first case, d c = 0, gives c = constant, since that takes place 
 for particular integrals ; the second case, only therefore conducts to a par- 
 ticular solution. But, q being the coefficient of d c of the equation (k), 
 we see that q = 0, gives 
 
 *y = . 
 
 dx 
 
 This equation will contain c or be independent of it. If it contain c, 
 there will be two cases ; either the equation q = 0, will contain only c 
 and constants, or this equation will contain c with variables. In the first 
 case, the equation q = 0, will still give c = constant, and in the second case, 
 it will give c = f (x, y) ; this value being substituted in the equation 
 F (x, y, c) = 0, will change it into another function of x, y, which will 
 satisfy the proposed, without being comprised in its complete integral, 
 and consequently will be a singular solution ; but we shall have a parti- 
 cular integral if the equation c =r f (x, y), by means of the complete in- 
 tegral, is reduced to a constant. 
 
 c 4
 
 xj INTRODUCTION. 
 
 When the factor q = from the equation q d c = not containing 
 the arbitrary constant c, we shall perceive whether the equation q = 
 gives rise to a particular solution, by combining this equation with the 
 complete integral. For example, if from q = 0, we get x = M, and put 
 this value in the complete integral F (x, y, c) = 0, we shall obtain 
 c = constant = B or c = f y ; 
 
 In the first case, q = 0, gives a particular integral ; for by changing c 
 into B in the complete integral, we only give a particular value to the 
 constant, which is the same as when we pass from the complete integral 
 to a particular integral. In the second case, on the contrary, the value 
 f y introduced instead of c in the complete integral, will establish between 
 x and y a relation different from that which was found by merely re- 
 placing c by an arbitrary constant. In this case, therefore, we shall have 
 a particular solution. What has been said of y, applies equally to x. 
 
 It happens sometimes that the value of c presents itself under the form 
 
 : this indicates a factor common to the equations u and U which is ex- 
 
 traneous to them, and which must be made to disappear. 
 
 Let us apply this theory to the research of particular solutions, when 
 the complete integral is given. 
 Let the equation be 
 
 y d x x'd y = aV'(dx 2 + dy l ) 
 of which the complete integral is thus found. 
 Dividing the equation by d x, and making 
 dy 
 
 ai = p 
 
 we obtain 
 
 y px = a V(l + p). 
 
 Then differentiating relatively to x and to p, we get 
 
 apdp 
 
 - P dx--xd P = ' 2 ; 
 
 observing that 
 
 dy = pdx, 
 
 this equation reduces to 
 
 apdp 
 
 * d P+ V(l + p') = 
 
 and this is satisfied by making d p = 0. This hypothesis gives p = con- 
 stant = c, a value which being put in the above equation gives
 
 ANALYTICAL GEOMETRY. xli 
 
 y ex = aV(l + c*) ........ (1) 
 
 This equation containing an arbitrary constant c, which is not to be 
 found in the proposed equation, is the complete integral of it. 
 
 This being accomplished, the part q d c of the equation d y = p d x -f- 
 q d c will be obtained by differentiating the last equation relatively to c 
 regarded as the only variable. Operating thus we shall have 
 
 a c d c 
 
 xdc+ = 0; 
 
 consequently the coefficients of d c, equated to zero, will give us 
 
 ac 
 
 
 c') 
 To find the value of c, we have 
 
 (1 + c ! ) L x 2 = a ! c 2 , 
 which gives 
 
 x ' a 
 
 and 
 
 by means of this last equation, eliminating the radical of the equation (m) 
 we shall thus obtain 
 
 This value and that of V(l + c 2 ) being substituted in the equation (H 
 will give us 
 
 x 8 a 2 
 
 V(a z x 2 ) = : V(a 2 x') 
 whence is derived 
 
 y = V(a'-x 2 ), 
 
 an equation which, being squared, will give us 
 
 y 2 = a' x 2 ; 
 
 and we see that this equation is a particular solution, for by differentiating 
 it we obtain 
 
 xd x 
 
 dy = - ; 
 
 this value and that of V(x 2 + y*), being substituted in the equation 
 originally proposed, reduce it to 
 
 a 2 = a e . 
 In the application which we have just given, we have determined the
 
 xhi INTRODUCTION. 
 
 value of c by equating to zero the differential coefficient (-p-) This 
 
 process may sometimes prove insufficient. In fact, the equation 
 
 dy = p dx + q dc 
 being put under this form 
 
 Adx + Bdy + Cdc = 
 where A, B, C, are functions of x and y, we shall thence obtain 
 
 dy = -gdx gdc ........ (o) 
 
 dx = -dy -^-dc ........ (p) 
 
 and we perceive that if all that has been said of y considered a function of 
 x, is applied to x considered a function of y, the value of the coefficient of 
 d c will not be the same, and that it will suffice merely that any factor of B 
 destroys in C another factor than that which may destroy a factor of A, 
 in order that the value of the coefficient of d c, on both hypotheses, may 
 appear entirely different. Thus although very often the equations 
 
 1=0,1=0 
 
 give for c the same value, that will not always happen ; the reason of 
 which is, that when we shall have determined c by means of the equation 
 
 dx 
 
 it will not be useless to see whether the hypothesis of -3 gives the same 
 
 result. 
 
 Clairaut was the first to remark a general class of equations susceptible 
 of a particular solution ; these equations are contained in the form 
 
 dy _, dy 
 
 ' = &***& 
 
 an equation which we shall represent by 
 
 y = px + Fp ......... (r) 
 
 By differentiating it, we shall find 
 
 dy = pdx + xdp + (-jy) dp; 
 this equation, since d y = pdx, becomes
 
 ANALYTICAL GEOMETRY. xliii 
 
 and since d p is a common factor, it may be thus written : 
 
 We satisfy this equation by making d p = 0, which gives p = const. 
 = c; consequently, by substituting this value in the equation (r) we 
 shall find 
 
 y = ex + Fc. 
 
 This equation is the complete integral of the equation proposed, since 
 an arbitrary constant c has been introduced by integration. If we differ- 
 entiate relatively to c we shall get 
 
 dFc 
 
 Consequently, by equating to zero the coefficients of d c, we have 
 
 dFc 
 ~d^~ 
 
 which being substituted in the complete integral, will give the particular 
 solution. 
 
 THE INTEGRATION OF EQUATIONS OF PARTIAL DIFFERENCES. 
 
 An equation which subsists between the differential coefficients, com- 
 bined with variables and constants, is, in general, a partial differential 
 equation, or an equation of partial differences. These equations are thus 
 named, because the notation of the differential coefficients which they 
 contain indicates that the differentiation can only be effected partially ; 
 that is to say, by regarding certain variables as constant. This supposes, 
 therefore, that the function proposed contains only one variable. 
 
 The first equation which we shall integrate is this ; viz. 
 
 i If contrary to the hypothesis, z instead of being a function of two vari- 
 ables x, y, contains only x, we shall have an ordinary differential equation, 
 which, being integrated, will give 
 
 z = ax + c 
 
 but, in the present case, z being a function of x and of y, the T/S con- 
 tained in z have been made to disappear by differentiation, since differen-
 
 xhv INTRODUCTION. 
 
 tinting relatively to x, we have considered y as constant. We ought, 
 therefore, when integrating, to preserve the same hypothesis, and suppose 
 that the arbitrary constant is in general a function of y ; consequently, we 
 shall have for the integral of the proposed equation 
 
 z = ax + py. 
 Required to integrate the equation 
 
 in which X ' ls any function of x. Multiplying by d x, and integrating, 
 we get 
 
 z =/Xdx + py. 
 
 For example, if the function X were x 2 + a 2 , the integral would be 
 
 x 3 
 z = + a 2 x + p y . 
 
 In like manner, it is found that the integral of 
 
 is 
 
 z = x Y + p y . 
 
 Similarly, we shall integrate every equation in which (-T ) is equal to 
 
 a function of two variables x, y. If, for example, 
 
 (d z\ x 
 
 d x/ V ay + x 2 ' 
 
 considering y as constant, we integrate by the ordinary rules, making the 
 arbitrary constant a function of y. This gives 
 
 z = (ay + x 2 ) + py. 
 Finally, if we wish to integrate the equation 
 
 f^\ - ! 
 
 Vdx/ ' V(y 2 x 1 ) 
 
 regarding y as constant, we get 
 
 z = sin.-'y + py. 
 Generally to integrate the equation 
 
 we shall take the integral relatively to x, and adding to it an arbitrary 
 function of y, as the constant, to complete it, we shall find 
 
 z = /T(x, y) dx + <p y.
 
 ANALYTICAL GEOMETRY. xlv 
 
 Now let us consider the equations of partial differences which contain 
 two differential coefficients of the first order ; and let the equation be 
 
 in which M and N represent given functions of x, y. Hence 
 
 /d z\ M /d 
 
 \d^) '' ~~W 
 
 substituting this value in the formula 
 
 which has no other meaning than to express the condition that z is a 
 function of x and of y, we obtain 
 
 rd zx f , M , 
 
 or 
 
 /dz\ Ndx Mdy 
 
 d z = ( -T ) TTT * . 
 
 \dx/ N 
 
 Let X be the factor proper to make Ndx Mdya complete differ- 
 ential d s ; we shall have 
 
 X (N d x M d y) = d s. 
 
 By means of this equation, we shall eliminate Ndx Mdy from the 
 preceding equation, and we shall obtain 
 
 1 /d 
 
 Finally, if we remark that the value of f -r ) is indeterminate, we may 
 
 take it such that ^ . (-j ) d s may be integrable, which would make it 
 
 a function of s ; for we know that the differential of every given function 
 of s must be of the form F s . d s. It therefore follows, that we may 
 assume 
 
 an equation which will change the preceding one into 
 
 dz = Fs.ds 
 which gives 
 
 Z = S.
 
 xlvi INTRODUCTION. 
 
 Integrating by this method the equation 
 /d z\ /d z\ 
 
 x (a^) -y Grf) = 
 
 we have in this case 
 
 M = -y, 
 and 
 
 N = x; 
 consequently 
 
 d s = X (x d x + y d y). 
 
 It is evident that the factor necessary to make this integrable is z. 
 Substituting this for X and integrating, we get 
 
 s = x 2 + y 1 . 
 Hence the integral of the proposed equation is 
 
 z = p(x = + y 2 ). 
 Now let us consider the equation 
 
 in which P, Q, R are functions of the variables x, y, z ; dividing it by P 
 and making 
 
 Q - M ^ - N 
 
 p ivi ? TT * 
 
 we shall put it under this form : 
 
 (*-> -&)+-'" 
 
 and again making 
 
 /d z\ 
 
 (n) = P' 
 
 and 
 
 d 
 
 it becomes 
 
 p + M q + N = ............ (a) 
 
 Tliis equation establishes a relation between the coefficients p and q ot 
 the general formula 
 
 = pdx + qdy; 
 
 without which relation p and q would be perfectly arbitrary, for as it has 
 been already observed, this formula has no other meaning than to indicate 
 that z is a function of two variables x, y, and that function may be auy
 
 ANALYTICAL GEOMETRY. xlvii 
 
 whatever ; so that we ought to regard p and q as indeterminate in this last 
 equation. Eliminating p from it, we shall obtain 
 dz + Ndx = q(dy Mdx) 
 
 and q will remain always indeterminate. Hence the two members of this 
 equation are heterogeneous (See Art. 6. vol. 1), and consequently 
 
 dz + Ndx = 0, dy Mdx = ..... (b) 
 
 If P, Q, R do not contain the variable z, it will be the same of M and 
 
 N; so that the second of these equations will be an equation of two varia- 
 
 bles x and y, and may become a complete differential by means of a factor 
 
 ?.. This gives 
 
 X (d y M d x) = 0. 
 
 The integral of this equation will be a function of x and of y, to wlncn 
 we must add an arbitrary constant s ; so that we shall have 
 
 F(x, y) = s; 
 whence we derive 
 
 y = f (x, s). 
 
 Such will be the value of y given us by the second of the above equa- 
 tions; and to show that they subsist simultaneously we must substitute 
 this value in the first of them. But although the variable y is not shown, 
 it is contained in N. This substitution of the value of y just found, 
 amounts to considering y in the first equation as a function of x and of 
 the arbitrary constant s. Integrating therefore this first equation on that 
 hypothesis we find 
 
 z = f N d x + <p s. 
 To give an example of this integration, take the equation 
 
 and comparing it with the general equation (a), we have 
 
 These values being substituted in the equations (b) will change them to 
 d z V (x 1 -f- y 2 ) d x = 0, d y 2- d x = ..... (c) 
 
 Let X be the factor necessary to make the last of these integrable, and 
 we have 
 
 x(dy-I-dx) =0, 
 
 or rather 
 
 x d y y d x __ 
 
 A . -- - - - U ,
 
 xlviii INTRODUCTION. 
 
 which is integrable when X = ; for then the integral is -J = constant. 
 
 x * 
 
 Put therefore 
 
 x 
 
 and consequently 
 
 y = s x. 
 
 By means of this value of y, we change the first of the equations 
 (c) into 
 
 V x 2 s 2 x 2 , 
 d z a . d x = 0, 
 
 3C 
 
 or rather into 
 
 dz = adxV r (l + s t ). 
 Integrating on the supposition that s is constant, we shall obtain 
 
 z = a/dx V (1 + s s ) + ?s 
 and consequently 
 
 z = a x V (1 + s 2 ) + <p s. 
 
 Substituting for s its value we get 
 z = a x \ 
 
 x 
 
 In the more general case where the coefficients P, Q, R of the equation 
 contain the three variables x, y, z it may happen that the equations 
 (.b) contain only the variables which are visible, and which consequently 
 we may put under the forms 
 
 d z = f (x, z) d x = 0, d y = F (x, y) d x. 
 
 These equations may be treated distinctly, by writing them as above, 
 
 z =/f(x, z)dx + z, y =/F(x,y)dx + *y 
 for then we see we may make z constant in the first equation and y in 
 the second ; contradictory hypotheses, since one of three coordinates 
 x, y, z cannot be supposed constant in the first equation without its being 
 not constant in the second. 
 
 Let us now see in what way the equations (b) may be integrated in the 
 case where they only contain the variables which are seen in them. 
 
 Let fj, and X be the factors which make the equations (b) integrable. 
 If their integrals thus obtained be denoted by U and by V, we have 
 A(dz + Ndx) = dU, Ao(dy Mdx) =r d V.
 
 ANALYTICAL GEOMETRY. xlix 
 
 By means of these values the above equation will become 
 
 dU = qdV ........ (d). 
 
 Since the first member of this equation is a complete differential the 
 
 second is also a complete differential, which requires q to be a function 
 
 / 
 
 of V. Represent this function by <f> V. Then the equation (d) will 
 become 
 
 dU = pV.dV 
 which gives, by integrating, 
 
 U = *'V. 
 Take, for example, the equation 
 
 (d z\ . /d z\ 
 
 n) + x (dy) = y z; 
 
 which being written thus, viz. 
 
 d z\ x /d z\ z 
 
 ~~~ = 
 
 we compare it with the equation 
 
 and obtain 
 
 M = *, N = - 
 y' x 
 
 By means of these values the equations (b) becomes 
 
 dz -.dx = 0, dy ~dx = 0; 
 x y 
 
 which reduce to 
 
 xdz zdx = 0,ydy xdx = 0. 
 
 The factors necessary to make these integrable are evidently s and 2. 
 
 JL 
 
 gr 
 
 Substituting which and integrating, we find and y * X s for the in- 
 
 jC 
 
 tegrals. Putting, therefore, these values for U and V in the equation 
 U = * V, we shall obtain, for the integral of the proposed equation, 
 
 It must be remarked, that, if we had eliminated q instead of p, the equa- 
 tions (b) would have been replaced by these 
 
 Mdz + Ndy = 0,dy Mdx = 0. . . . (e) 
 and since all that has been said of equations (b) applies equally to these, 
 
 d
 
 / INTRODUCTION. 
 
 it follows that, in the case where the first of equations (b) was not in- 
 tegrable, we may replace those equations by the system of equations (e), 
 which amounts to employing the first of the equations (e) instead of the 
 first of the equations (b). 
 For instance, if we had 
 
 fd 
 
 az 
 
 this equation being divided by a z and compared with 
 
 will give us 
 
 o 
 
 -Mf X XT X Y 
 
 M = , N = J - 
 
 and the equations (b) will become 
 
 a z 
 
 , 
 a z a 
 
 which reduce to 
 
 azdz-f-xydx = 0,ady + xdx,=:0 . . . (f) 
 The first of these equations, which, containing three variables, is not 
 immediately integrable, we replace by the first of the equations (e), and 
 we shall have, instead of the equations (f), these 
 
 y =0,ady+xdx=0; 
 
 . c * 
 
 which reduce to 
 
 2ydy 2zdz = 0,2ady + 2xdx = 0; 
 equations, whose integrals are 
 
 y 2 z 8 , 2 a y + x *" 
 
 These values being substituted for U and V, will give us 
 y 2 z 2 = <f> (2 a y + x 2 ). 
 
 It may be remarked, that the first of equations (e) is nothing else than 
 the result of the elimination of d x from the equations (b) . 
 
 Generally we may eliminate every variable contained in the coefficients 
 M, N, and in a word, combine these equations after any manner what- 
 ever ; if after having performed these operations, and we obtain two in- 
 tegrals, represented by U = a, V r= b, a and b being arbitrary constants, 
 we can always conclude that the integral is U = <t> V. In fact, since 
 a and b are two arbitrary constants, having taken b at pleasure, we may 
 compose a in terms of b in any way whatsoever; which is tantamount to 
 saying that we may take for a an arbitrary function of b. This condition 
 will be expressed by the equations a = f (b). Consequently, we shall
 
 ANALYTICAL GEOMETRY. li 
 
 have the equations U = <f> b, V = b, in which x, y, z represent the same 
 coordinates. If we eliminate (b) from these equations, we shall obtain 
 U = 9 V. 
 
 This equation also shows us that in making V = b, we ought to have 
 U = f b = constant ; that is to say, that U and V are at the same time 
 constant; without which a and b would depend upon one another, where- 
 as the function <f> is arbitrary. But this is precisely the condition expressed 
 by the equations U = a, V == b. 
 
 To give an application of this theorem, let 
 d z\ /d z\ 
 
 Dividing by z x and comparing it with the general equation we 
 have 
 
 M = , N = ?; 
 
 X ZX 
 
 and the equations (b) give us 
 
 d z -_ d x = 0, d y + - -d x = 
 zx x 
 
 or 
 
 zxdz y * d x = 0, xdy-fydz= 0. 
 
 The first of these equations containing three variables we shall not at- 
 tempt its integration in that state; but if we substitute in it for y d x its 
 value derived from the second equation, it will acquire a common factor 
 x, which being suppressed, the equation becomes 
 
 zdz + ydy = 0, 
 
 and we perceive that by multiplying by 2 it becomes integrable. r i he 
 other equation is already integrable, and by integrating we find 
 
 z ! + y * = a, xy=b, 
 whence we conclude that 
 
 z 2 + V s = <f> x y. 
 
 We shall conclude what we have to say upon equations of partial differ- 
 ences of the first order, by the solution of this problem. 
 
 Given an equation 'which contains an arbitrary function of one or more 
 variables, tojind the equation of partial differences which produced it. 
 Suppose we have 
 
 z= F(x' + y s ). 
 Make 
 
 x z + y 2 =u .......... (0 
 
 and the equation becomes 
 
 z = Fu. 
 it
 
 lii INTRODUCTION. 
 
 The differential of F u must be of the form f u . d u. Conse- 
 quently 
 
 d z = d u. p u 
 
 If we take the differential of z I'elatively to x only, that is to say, in 
 regarding y as constant, we ought to take also d u on the same 
 hypothesis. Consequently, dividing the preceding equation by d x, 
 we get 
 
 /d z\ /d u\ * 
 
 > u. 
 
 d z\ /d u\ 
 
 ) = (cf)' u - 
 
 Again, considering x as constant and y as variable, we shall similarly 
 find 
 
 d z\ /d u 
 
 y 
 
 But the values of these coefficients are found from the equation (f), 
 which gives 
 
 (d u\ 
 TX) = 
 
 Hence our equations become 
 
 /d z\ / d z \ 
 
 ( j - ) = 2 x f u , ( - ) = 2 y 9 u ; 
 
 \dx/ \ dy / J ' 
 
 and eliminating p u from these, we get the equation required ; viz. 
 
 As another example, take this equation 
 
 z 8 + 2 ax = F ( y). 
 Making 
 
 x y = u, 
 It becomes 
 
 z*-j-2ax = Fu 
 aiid differ ntiating, we get 
 
 d (z * 4- 2 a x) = d u <p u . 
 Then taking the differential relatively to x, we have 
 
 x 
 and similarly, with regard to y, we get 
 
 d z\ /d u
 
 ANALYTICAL GEOMETRY. "liii 
 
 But since 
 
 x y = u 
 
 A u 
 
 which, being substituted in the above equation, gives us 
 
 and eliminating p u from these, we have the equation required ; viz. 
 
 d z\ /d z\ a 
 + 
 
 We now come to 
 
 EQUATIONS OF PARTIAL DIFFERENCES OF THF, SECOND ORDER. 
 
 An equation of Partial Differences of the second order in which z is a 
 function of two variables x, y ought always to contain one or more of the 
 differential coefficients 
 
 /d 2 z\ /d 2 z\ / d ~ z 
 " 
 
 / z\ / ~ z \ 
 * Vd7"V' Vd x.d y/ 
 
 independently of the differential coefficients which enter equations of the 
 first order. 
 
 We shall merely integrate the simplest equations of this kind, and shall 
 begin with this, viz. 
 
 Multiplying by d x and integrating relatively to x we add to the inte- 
 gral an arbitrary function of y ; and we shall thus get 
 
 /dz\ 
 
 (dx) = ^ 
 
 Again multiplying by d x and integrating, the integral will be com- 
 pleted when we add another arbitrary function of y, viz. -^ y. We thus 
 obtain 
 
 z = xpy + ^y. 
 Now let us integrate the equation. 
 
 . P 
 
 dx 
 
 43
 
 liv INTRODUCTION. 
 
 in which P is any function of x, y. Operating as before we first obtain 
 
 (li) = 
 and the second integration gives us 
 
 z = /f/Pdx + 
 In the same manner we integrate 
 
 _ 
 
 dyV 
 
 and find 
 
 z =/{px + /Pdy} dy 
 The equation 
 
 must be integrated first relatively to one of the variables, and then rela- 
 tively to the other, which will give 
 
 z =/{?y + /Pdx} dy + px. 
 In general, similarly may be treated the several equations 
 
 /d n z\ .p / d n z \ n / d B z \ 
 
 VdTV ^ Vdxdy"- 1 / W> Vdx 2 dy^>) = K) &c ' 
 
 in which P, Q, R, &c. are functions of x, y, which gives place to a series 
 of integrations, introducing for each of them an arbitrary function. 
 
 One of the next easiest equations to integrate is this : 
 
 in which P and Q will always denote two functions of x and y. 
 Make 
 
 and the proposed will transform to 
 
 To integrate this, we consider x constant, and then it contains only 
 two variables y and u, and it will be of the same form as the equation 
 
 dy + Py dx = Qdx 
 whose integral (see Vol. 1. p. 109) is 
 
 y = e-/ pd J/Qe/ pd *dx + C}. 
 Hence our equation gives 
 
 u se-'^M/Qe'Vdy + fxj.
 
 ANALYTICAL GEOMETRY. 
 
 But 
 
 Hence by integration we get 
 
 z = /{ e-/*(/Qe'"*dy) + pxjdy + 4*. 
 By the same method we may integrate 
 
 VdlTdy) + P CdlD = Q ' dT^ + P (df) = Q ' 
 
 in which P, Q represent functions of x, and because of the divisor d x d y, 
 we perceive that the value of z will not contain arbitrary functions of the 
 same variable. 
 
 THE DETERMINATION OF THE ARBITRARY FUNCTIONS WHICH ENTER 
 
 THE INTEGRALS OF EQUATIONS OF PARTIAL DIFFERENCES O.Y 
 
 THE FIRST ORDER. 
 
 The arbitrary functions which complete the integrals of equations of 
 partial differences, ought to be given by the conditions arising from the 
 nature of the problems from which originated these equations ; problems 
 generally belonging to the physical branches of the Mathematics. 
 
 But in order to keep in view the subject we are discussing, we shall 
 limit ourselves to considerations purely analytical, and we shall first seek 
 what are the conditions contained in the equation 
 
 Since z is a function of x, y, this equation may be ^,,msidered as that of 
 a surface. This surface, from the nature of its equation, has the following 
 
 property, that (-T ) must always be constant. Hence it follows that 
 
 every section of this surface made by a plane parallel to that of x, y is a 
 straight line. In fact, whatever may be the nature of this section, if we 
 divide it into an infinity of parts, these, to a small extent, may be con- 
 sidered straight lines, and will represent the elements of the section, one 
 of these elements making with a parallel to the axis of abscissae, an angle 
 
 whose tangent is (-7-;) Since this angle is constant, it follows that all 
 the angles formed in like manner by the elements of the curve, with pnr- 
 
 OL i
 
 Ivi INTRODUCTION. 
 
 allels to the axis of abscissae will be equal. Which proves that the sec- 
 tion in question is a straight line. 
 
 We might arrive at the same result by considering the integral of the 
 equation 
 
 . = a 
 
 d x/ 
 
 which we know to be 
 
 z = ax + f y, 
 
 since for all the points <of the surface which in the cutting plane, the or- 
 dinate is equal to a constant c. Replacing therefore <p y by f c, and 
 making p c rr C, the above equation becomes 
 
 z = ax + C; 
 
 this equation being that of a straight line, shows that the section is a 
 straight line. 
 
 The same holding good relatively to other cutting planes which may be 
 drawn parallel to that of x, z, we conclude that all these planes will cut the 
 surface in straight lines, which will be parallel, since they will each form 
 with a parallel to the axis of x, an angle whose tangent is a. 
 
 If, however, we make x = 0, the equation z = ax + <p y reduces to 
 z = py, and will be that of a curve traced upon the plane of y, z; this 
 curve containing all the points of the surface whose coordinates are x = 0, 
 will meet the plane in a point whose coordinate is x =0; and since we 
 have also y = c, the third coordinate by means of the equation 
 
 z = ax -f C 
 will be 
 
 z = C. 
 
 What has been said of this one plane, applies equally to all others 
 which are parallel to it, and it thence results that through all the points 
 of the curve whose equation is z = <p y, and which is traced in the plane 
 of y, z, will pass straight lines parallel to the axis of x. This is ex- 
 pressed by the equations 
 
 'd 
 
 and 
 
 z = a x + <p y ; 
 
 and since this condition is always fulfilled, whatever may be the figure of 
 the curve whose equation is z := <p y, we see that this curve is arbi- 
 trary. 
 
 From what precedes, it follows that the curve whose equation is z = py,
 
 ANALYTICAL GEOMETRY. 
 
 Ivii 
 
 may be composed of arcs of different curves, which unite at their extre- 
 mities, as in this diagram 
 
 or which have a break off in their course, as in this figure. 
 
 ,N 
 
 In the first case the curve is discontinuous, and in the second it is dis- 
 contiguous. We may remark that in this last case, two different ordinates 
 P M, P N corresponding to the same abscissa A P; finally, it is possible, 
 that without being discontiguous, the curve may be composed of an in- 
 finite series of arcs indefinitely small, which belong each of them to 
 different curves ; in this case, the curve is irregular, as will be, for 
 instance, the flourishes of the pen made at random ; but in whatever way 
 it is formed, the curve whose equation is z = <p y, it will suffice, to con- 
 struct the surface, to make a straight line move parallelly with this condi- 
 tion, that its general point shall trace out the curve whose equation is 
 
 z = ?y, 
 
 and vhich is traced at random upon the plane of y, z. 
 If instead of the equation 
 
 On) = - 
 
 we had 
 
 in which X was a function of x, then in drawing a plane parallel to the 
 plane (x, z), the surface will be cut by it no longer in a straight line, as 
 in the preceding case. In fact, for every point taken in this section, the 
 tangent of the angle formed by the element produced of the section, with 
 a parallel to the axis of x, will be equal to a function X of the abscissa x 
 of this point ; and since the abscissa x is different for every point :t foJ-
 
 Iviii INTRODUCTION. 
 
 lows that this angle will be different at each point of the section, which 
 section, therefore, is no longer, as before, a straight line. The surface 
 will be constructed, as before, by moving the section parallelly, so that its 
 point may ride continually in the curve whose equation is z = p y. 
 
 Suppose now that in the preceding equation, instead of X we have a 
 function, P of x, and of y. The equation 
 
 fd z\ p 
 r > 
 
 containing three variables will belong still to a curve surface. If we cut 
 this surface by a plane parallel to that of x, z, we shall have a section in 
 
 which y will be constant ; and since in all its points (T~) will be equal 
 
 to a function of the variable x, this section must be a curve, as in the pre- 
 ceding case. The equation 
 
 C ^ =P 
 
 being integrated, we shall have for that of the surface 
 
 z =r/Pdx + py; 
 
 if in this equation we give successively to y the increasing values y', y", 
 y'", &c. and make P', P", P"', &c. what the function P becomes in these 
 cases, we shall have the equations 
 
 z =/P / dx + py', z =/P"dx + p y" 
 Z = /P"'dx + py"', z = /P'"'dx + py"" &C, 
 and we see that these equations will belong to curves of the same nature, 
 but different in form, since the values of the constant y will not be the 
 same. These curves are nothing else than the sections of the surface 
 made by planes parallel to the plane (x, z) ; and in meeting the plane 
 (y, .z) they will form a curve whose equation will be obtained by equating 
 to zero, the value of x in that of the surface. Call the value of/Pdx, 
 in this case, Y, and we shall have 
 
 and we perceive that by reason of p y, the curve determined by this equa- 
 tion must be arbitrary. Thus, having traced at pleasure a curve, Q R S, 
 upon the plane (y, z), if we represent by R L the section whose equation 
 
 13 z =y P'd x -f py', we shall move this section, always keeping the ex-
 
 ANALYTICAL GEOMETRY. lix 
 
 tremity R applied to the curve Q R S ; but so that this section as it 
 moves, may assume the successive forms determined by the above group 
 of equations, and we shall thus construct the surface to which will belong 
 the equation 
 
 as) = 
 
 Finally let us consider the general equation 
 
 whose integral is U = <p V. Since U = a, V = b, each of these equa- 
 tions subsisting between three coordinates, we may regard them as be- 
 longing to two surfaces ; and since the coordinates are common, they 
 ought to belong to the curve of intersection of the two surfaces. This 
 being shown, a and b being arbitrary constants, if in U = a, we give to 
 X and y the values x', y' we shall obtain for z, a function of x', of y 7 and 
 of a, which will determine a point of the surface whose equation is U = a. 
 This point, which is any whatever, will vary in position if we give succes- 
 sively different values to the arbitrary constant a, which amounts to say- 
 ing that by making a vary, we shall pass the surface whose equation is 
 U = a, through a new system of points. This applies equally to V = b, 
 and we conclude that the curve of intersection of the two surfaces will 
 change continually in position, and consequently will describe a curved 
 surface in which a, b may be considered as two coordinates ; and since 
 the relation a = <p b which connects these two coordinates, is arbitrary 
 we perceive that the determination of the function <f>~ amounts to making 
 a surface pass through a curve traced arbitrarily. 
 
 To show how this sort of problems may conduct to analytical condi- 
 tions, let us examine what is the surface whose equation is 
 d z\ /d 
 
 We have seen that this equation being integrated gives 
 Reciprocally we hence derive 
 
 If we cut the surface by a plane parallel to the plane (x, y) the equation 
 of the section will be 
 
 ' 2 , * 
 
 x 2 + y 8 = <D c; 
 and representing by a * the constant <l> c, we shall have 
 
 x* + y* = a 2 . 
 This equation belongs to the circle. Consequently the surface will
 
 Ix INTRODUCTION. 
 
 have this property, viz. that every section made by a plane parallel to the 
 plane (x, y) will be a circle. 
 
 This property is also indicated by the equation 
 d 
 
 for this equation gives 
 
 dy 
 ~ y dx* 
 
 This equation shows us that the subnormal ought to be always equal to 
 the abscissa which is the property of the circle. 
 
 The equation z = <p (x 2 -f- y 1 ) showing merely that all the sections 
 parallel to the plane (x, y) are circles, it follows thence that the law ac- 
 cording to which the radii of these sections ought to increase, is not 
 comprised in this equation, and that consequently, every surface of revo- 
 lution will satisfy the problem ; for we know that in this sort of surfaces, 
 the sections parallel to the plane (x, y) are always circles, and it is need- 
 less to say that the generatrix which, during a revolution, describes the 
 surface, may be a curve discontinued, discontiguous, regular or irregular. 
 
 Let us therefore investigate the surface for which this generatrix will 
 be a parabola A N, and suppose that, in this hypothesis, the surface is 
 cut by a plane A B, vrhich shall pass through the axis of z ; the trace of 
 
 B 
 
 this plane upon the plane (x, y) will be a straight line A L, which, being 
 drawn through the origin, will have the equation y =r a x ; if we repre- 
 sent by t the hypothenuse of the right angled triangle A P Q, constructed 
 upon the plane (x, y) we shall have 
 
 t = x s + y'i 
 
 but t being the abscissa of the parabola A M, of which Q M = z is the 
 ordinate, we have, by the nature of the curve, 
 
 t * = b z. 
 Putting for t * its value x * + y *, we get 
 
 z = \ (y'-f x'),orz = *-(a'x' + x') = !-x'(l + n ') ;
 
 ANALYTICAL GEOMETRY. Ixi 
 
 and making 
 
 A (a + 1) = m, 
 
 we shall obtain 
 
 z = mx*; 
 
 so that the condition prescribed in the hypothesis, where the generatrix 
 is a parabola, is that we ought to have 
 
 z = m x *, when y = a x. 
 
 Let us now investigate, by means of these conditions, the arbitrary 
 function which enters the equation z = <p (x s -f y*). For that pur- 
 pose, we shall represent by U the quantity x* + y a , which is effected by 
 the symbol p, and the equation then becomes 
 
 z = pU; 
 and we shall have the three equations 
 
 x* + y 2 = U, y = ax, z = mx 2 . 
 
 By means of the two first we eliminate y and obtain the value of x * 
 which being put into the third, will give 
 
 Lt m ; ; - 
 
 1 
 
 an equation which reduces to 
 
 the value of z being substituted in the equation z = <f> U, will change 
 it to 
 
 b 
 and putting the value of U in this equation, we shall find that 
 
 and we see that the function is determined. Substituting this value of 
 <f> (x * + y *) in the equation z <p (x * + y 2 ), we get 
 
 - b t x T y ; , 
 
 for the integral sought, an equation which has the property required, 
 since the hypothesis of y = ax gives 
 
 z = m x 2 . 
 
 This process is general ; for, supposing the conditions which determine 
 the arbitrary constant to be that the integral gives F (x, y, z) = 0, when 
 we have f (x, y, z) =0, we shall obtain a third equation by equating to
 
 1X11 
 
 INTRODUCTION. 
 
 U the quantity which follows p, and then by eliminating, successively, 
 two of the variables x, y, z, we shall obtain each of these variables in a 
 function of U ; putting these values in the integral, we shall get an equa- 
 tion whose first member is <f> U, and whose second member is a compound 
 expression in terms of U ; restoring the value of U in terms of the vari- 
 bles, the arbitrary function will be determined. 
 
 THE ARBITRARY FUNCTIONS WHICH ENTER THE INTEGRALS OF THE 
 EQUATIONS OF PARTIAL DIFFERENCES OF THE SECOND ORDER. 
 
 Equations of partial differences of the second order conduct to integrals 
 which contain two arbitrary functions ; the determination of these func- 
 tions amounts to making the surface pass through two curves which may 
 be discontinuous or discontiguous. For example, take the equation 
 
 d * 
 
 ( z\ 
 JV-0 = 
 
 whose integral has been found to be 
 
 Let A x, A y, A z, be the axis of coordinates ; if we draw a plane 
 
 K L parallel to the plane (x, z), the section of the surface by this plane 
 will be a straight line ; since, for all the points of this section, y being 
 equal to A p, if we represent A p by a constant c, the quantities f y, ^ y 
 will become p c, <4> c, and, consequently, may be replaced by two con- 
 stants, a, b, so that the equation 
 
 z = xpy -f 4y
 
 ANALYTICAL GEOMETRY. hciii 
 
 will become 
 
 z = a x -f- b, 
 and this is the equation to the section made by the plane K L. 
 
 To find the point where this section meets the plane (y, z) make 
 x =r 0, and the equation above gives z = -vj/ y, which indicates a curve 
 a m b, traced upon the plane (y, z). It will be easy to show that the 
 section meets the curve a m b in a point m ; and since this section is a 
 straight line, it is only requisite, to find the position of it, to find a second 
 point. For that purpose, observe that when x = 0, the first equation 
 reduces to 
 
 whilst, when x = 1, the same equation reduces to 
 
 z = 9 y + + y> 
 
 Making, as above, y = Ap = c, these two values of z will become 
 
 z = b, z = a + b, 
 
 and determining two points m and r, taken upon the same section, in r 
 we know to be in a straight line. To construct these points we thus pro- 
 ceed : we shall arbitrarily trace upon the plane (y, z) the curve a m b, 
 and through the point p, where the cutting plane K L meets the axis of 
 y, raise the perpendicular pm = b, which will be an ordinate to the 
 curve ; we shall then take at the intersection H L of the cutting plane, 
 and the plane (x, y), the part p p' equal to unity, and through the point 
 p', we shall draw a plane parallel to the plane (y, z), and in this plane 
 construct the curve a' m' b', after the modulus of the curve a m b, and so 
 as to be similarly disposed ; then the ordinate m' p' will be equal to m p ; 
 and if we produce m' p' by m' r, which will represent a, we shall deter- 
 mine the point r of the section. 
 
 If, by a second process, we then produce all the ordinates of the curve 
 a' m' b', we shall construct a new curve a' r 7 b', which will be such, that 
 drawing through this curve and through a m b, a plane parallel to the 
 plane (x, z), the two points where the curves meet, will belong to the 
 same section of the surface. 
 
 From what precedes, it follows that the surface may be constructed, by 
 moving the straight line m r so as continually to touch the two curves, 
 a m b, a 7 m' b'. 
 
 This example suffices to show that the determination of the arbitrary 
 functions which complete the integrals of equations of partial differences 
 of the second order, is the same as making the surface pass through two 
 curves, which, as well as the functions themselves, may be discontinuous, 
 discontiguous, regular or irregular.
 
 kir INTRODUCTION 
 
 CALCULUS OF VARIATIONS. 
 
 If we have given a function Z F, (x, y, y 7 , y"), wherein y', y" mean 
 
 /cl yx /d * yx 
 \d~x> ^dx*> 
 
 y itself being a function of x, it may be required to make L have certain 
 properties, (such as that of being a maximum, for instance) whether by 
 assigning to x, y numerical values, or by establishing relations between 
 these variables, and connecting them by equations. When the equation 
 y = p x is given, we may then deduce y, y', y" . . . in terms of x and sub- 
 stituting, we have the form 
 
 Z = f x. 
 
 By the known rules of the differential calculus, we may assign the values 
 of x, when we make of x a maximum or minimum. Thus we determine what 
 are the points of a given curve, for which the proposed function Z, is 
 greater or less than for every other point of the same curve. 
 
 But if the equation y = <f> x is not given, then taking successively for 
 <f> x different forms, the function Z = f x will, at the same time, assume 
 different functions of x. It may be proposed to assign to f x such a 
 form as shall make Z greater or less than every other form of p 'X.yfor the 
 same numerical value of x "whatever it may be in other respects. This latter 
 species of problem belongs to the calculus of variations. This theory 
 relates not to maxima and minima only; but we shall confine our- 
 selves to these considerations, because it will suffice to make known all 
 the rules of the calculus. We must always bear in mind, that the varia- 
 bles x, y are not independent, but that the equation y = px is unknown, 
 and that we only suppose it given to facilitate the resolution of the prob 
 lem. We must consider x as any quantity whatever which remains the same 
 for all the differential forms of <p x ; the forms of p, f ', p" . . . . are therefore 
 variable, whilst x is constant. 
 
 In Z = F (x, y, y', y". . .) put y + k for y, y' + k', for y'. . . , k being 
 an arbitrary function of x, and k', k," . . . the quantities 
 
 dl L (1 ' k 
 dx' dx l " 
 
 But, Z will become 
 
 Z, = F (x, y + k, y' + k', y" + k," . - .)
 
 ANALYTICAL GEOMETRY. Ixv 
 
 Taylor's theorem holds good whether the quantities x, y, k be depen- 
 dent or independent. Hence we have 
 
 so that we may consider x, y, y', y" . . . as so many independent variables. 
 The nature of the question requires that the equation y = px should 
 be determined, so that for the same value of x, we may have always 
 Z y > Z, or T L t < Z : reasoning as in the ordinary maxima and minima, 
 we perceive that the terms of the first order must equal zero, or that we 
 have 
 
 Since k is arbitrary for every value of x, and it is not necessary that its 
 value or its form should remain the same, when x varies or is constant, 
 k', k" . . is as well arbitrary as k. For we may suppose for any value 
 x = X that k = a + b (x X) + c (x ) * + &c., X, a, b, c . . . 
 being taken at pleasure ; and since this equation, and its differentials, 
 ought to hold good, whatever is x, they ought also to subsist when 
 x = X, which gives k a, k' = b, k" = c, &c. Hence the equation 
 Z, = Z + . . . cannot be satisfied when a, b, c . . . are considered inde- 
 pendent, unless (see 6, vol. I.) 
 
 n being the highest order of y in Z. These different equations subsist 
 simultaneously, whatever may be the value of x ; and if so, there ought 
 to be a maximum or minimum ; and the relation which then subsists be- 
 tween x, y will be the equation sought, viz. y = 9 x, which will have the 
 property of making Z greater or less than every other relation between 
 x and y can make it. We can distinguish the maximum from the mini- 
 mum from the signs of the terms of the second order, as in vol. I. 
 p. (31.) 
 
 But if all these equations give different relations between x, y, the 
 problem will be impossible in the state of generality which we have 
 ascribed to it ; and if it happen that some only of these equations subsist 
 mutually, then the function Z will have maxima and minima, relative to 
 some of the quantities y, y', y" . .. without their being common to them 
 all. The equations which thus subsist, will give the relative maxima and 
 minima. And if we wish to make X a maximum or minimum only relatively
 
 ixvi INTRODUCTION. 
 
 1 lo one of the quantities y, y', y" . . . , since then we have only one equa- 
 tion to satisfy, the problem will be always possible. 
 
 From the preceding considerations it follows, that first, the quantities 
 X, y depend upon one another, and that, nevertheless, we ought to make 
 them vary, as if they were independent, for this is but an artifice to get 
 the more readily at the result. 
 
 Secondly, that these variations are not indefinitely small ; and if we em- 
 ploy the differential calculus to obtain them, it is only an expeditious 
 means of getting the second term of the developement, the only one 
 which is here necessary. 
 
 Let us apply these general notions to some examples. 
 
 Ex. 1. Take, upon the axis of x of a curve, two abscissas m, n; and 
 draw indefinite parallels to the axis of y. Let y = p x be the equation 
 of this curve: if through any point whatever, we draw a tangent, it will 
 cut the parallels in points whose ordinates are 
 
 1 = y + y' (m x), h = y -f y' (n x) . 
 
 If the form of <p is given, every thing else is known ; but if it is not 
 given, it may be asked, what is the curve which has the property of 
 having for each point of tangency, the product of these two ordinates less 
 than for every other curve. 
 
 Here we have 1 X h ; or 
 
 Z = { y -x (m x) y'} + [ y + (n - x) y'} . 
 
 From the enunciation of the problem, the curves 'which pass through tlie 
 same point (x, y) have tangents taking different directions, and that which 
 is required, ought to have a tangent, such that the condition Z = maximum 
 is fulfilled. We may consider x and y constant; whence 
 
 /d Z \ _ 2 y' 2 x in n 1 1 
 
 \ d yv " y ~~ (x m) (x n) ~x m x n " 
 
 Then integrating we get 
 
 y* = C(x m) (x n). 
 
 The curve is an ellipse or a hyperbola, according as C is positive or 
 negative ; the vertexes are given by x = m, x n ; in the first case, the 
 product h X 1 or Z is a maximum) because y" is negative; in. the second, 
 Z is a minimum or rather a negative maximum ; this product is moreover 
 constant, and 1 h = \ C (m n) 8 , the square of the semi-axis. 
 
 Ex. 2. What is the curve for which, in each of its points, the square of 
 the subnormal added to the abscissa is a minimum ? 
 
 We have in this case
 
 ANALYTICAL GEOMETRY. Ixvii 
 
 whence we get two equations subsisting mutually by making 
 
 y y' + x = 
 and thence 
 
 . x ! + y 2 = r *. 
 
 Therefore all the circles described from the origin as a center wi I alone 
 satisfy the equation. 
 
 The theory just expounded has not been greatly extended ; but it serves 
 as a preliminary developement of great use for the*comprehension of a 
 far more interesting problem which remains to be considered. This re- 
 quires all the preceding reasonings to be applied to a function of the form 
 f Z: the sign y indicates the function Z to be a differential and that after 
 having integrated it between prescribed limits it is required to endow it 
 with the preceding properties. The difficulty here to be overcome is that 
 of resolving the problem without integrating. 
 
 When a body is in motion, we may compare together either the differ- 
 ent points of the body in one of its positions or the plane occupied suc- 
 cessively by a given point. In the first case, the body is considered fixed, 
 and the symbol d will relate to the change of the coordinates of its surface ; 
 in the second, we must express by a convenient symbol, variations alto- 
 gether independent of the first, which shall be denoted by d. When we 
 consider a curve immoveabie, or even variable, but taken in one of its po- 
 sitions, d x, d y . . . announce a comparison between its coordinates ; but 
 to consider the different planes which the same point of a curve occupies, 
 the curve varying in form according to any law whatever, we shall write 8 
 x, 3 y ... which denote the increments considered under this point of view, 
 and are functions of x, y , . . In like manner, d x becoming d (x -f 6 x) 
 will increase by d d x ; d 2 x will increase by d 2 d x, &c. 
 
 Observe that the variations indicated by the symbol d are finite, and 
 wholly independent of those which d represents ; the operations to which 
 these symbols relate being equally independent, the order in which they 
 are used must be equally a matter of indifference as to the result. So 
 that we have 
 
 3. d x = d. 3 x 
 d 2 . 8 x = 5 . d z \ 
 
 &c. 
 
 /S U = * U. 
 and so on. 
 
 It remains to establish relations between x, y, 7. . . .such thatyZ may 
 be a maximum or a minimum letween given limits. That the calculus may 
 be rendered the more symmetrical, we shall not suppose any differential
 
 Ixviii INTRODUCTION 
 
 constant ; moreover we shall only introduce three variables because it will 
 be easy to generalise the result To abridge the labour of the process, 
 make 
 
 d x = x y , d * x = x //} &c. 
 so that 
 
 z = F (x, X/ , X// , . . . y^ y,, y,,, . . . z, Z/ , z// . . .) 
 
 Now x, y and z receiving the arbitrary and finite increments d x, d y, 
 3 z, d x or x, becomes 
 
 d (x + 3 x) = d x + a d x or x, + 3 x,. 
 
 In the same manner, x /7 increases by 3 x,, and so on ; so that develop- 
 ing Z, by Taylor's theorem, and integrating f Z becomes 
 
 '' 
 
 The condition of a maximum or minimum requires the integral of the 
 terms of the first order to be zero between given limits whatever may be 
 3 x, 3 y, 3 z as we have already seen. Take the differential of the known 
 function Z considering x, x /? x 7/ . . . y, y /} y /7 . . . as so many independent 
 variables; we shall have 
 
 dZ=mdx + ndx / + pd x/ ,+... Mdy + Ndy y . . .+ /udz + v d z, . . . 
 m, n ... M, N ... /,... being the coefficients of the partial differences 
 of Z relatively to x, x, . . . y, y, . . . z, z,, . . . considered as so many varia- 
 bles ; these are therefore known functions for each proposed value of Z. 
 Performing this differentiation exactly in the same manner by the symbol 
 3, we have 
 
 3Z = m3x + n3d x+ p 3d'x + q 3 d 3 x + . . . 
 + M3y + N3dy + P3d*y + q 3 d 3 y + . . . 
 
 But this known quantity, whose number of terms is limited, is precisely 
 that which is under the sign f, in the terms of the first order of the de- 
 velopement : so that the required condition of max. or win. is that 
 
 /az = o, 
 
 between given limits, whatever may be the variations 3 x, 3 y, 3 z. Ob- 
 serve, that here, as before, the differential calculus is only employed as a 
 means of obtaining easily the assemblage of terms to be equated to zero ; 
 so that the variations are still any whatever and finite.
 
 ANALYTICAL GEOMETRY. 
 
 We have said that d . 3 x may be put for d . 3 x ; thus the first line is 
 equivalent to 
 
 m5x + n.d3x-f-p.d 8 3x+q.d 3 3x + &c. 
 m, n . . . contains differentials, so that the defect of homogeneity is here 
 only apparent. To integrate this, we shall see that it is necessary to 
 disengage from the symbol f as often as possible, the terms which con- 
 tain d d. To effect this, we integrate by parts which gives 
 
 /nd3x = n. 3x /dn.3x 
 /p.d 2 3x = pd3x d p 3x+/d ? p3x 
 /qd 3 3x==qd 2 3x dq.d3x-f d 2 q.dx /d 3 q. 3x 
 
 &c. 
 
 Collecting these results, we have this series, the law of which is easily 
 recognised ; viz. 
 
 /(m d n + d * p d 3 q + d * r . . .) 3 x 
 + (n dp + d 2 q d 3 r + d 4 s . . .) 3 x 
 + (p dq + d 2 r d 3 s + d 4 t ...)d3x 
 + (q-dr+..,)d 2 3x 
 + &c. 
 
 
 
 The integral of (A) ory\ 3 z = , becomes therefore 
 
 -dn + d 2 p-...)3x+(M-dN+d 2 P-...)3y+(/x-d^...)3z} = 
 
 (n-dp + d 2 q...)3x-f(N-dP+d 2 Q-...)3y + (K-d*...)dz 
 (p-dq + d 2 r...)d3x+(P-dQ + ...)d3y + (*-d % ...)d 3 z 
 +(q-dr...) d 2 3x ...+ K = 
 
 K being the arbitrary constant. The equation has been split into two, 
 because the terms which remain under the sign f cannot be integrated, at 
 least whilst 3 x, 3 y, 3 z are arbitrary. In the same manner, if the nature 
 of the question does not establish some relation between 3 x, 3 y, 3 z, the 
 independence of these variations requires also that equation (B) shall again 
 make three others ; viz. 
 
 C 
 
 .-{ 
 (, 
 
 0= m d n + d 2 p d 3 q 
 0=M dN+d 2 P d 3 Q+d 4 R ...V. .(D) 
 
 4 r . . . } 
 4 R ...V. . 
 
 4 ^ . . . ) 
 
 Consequently, to find the relations between x, y, z, which make y Z a 
 maximum, we must take the differential of the given function Z by con- 
 sidering x, y, z, d x, d y, d z, d 2 x, ... as so many independent vari- 
 ables, and use the letter 3 to signify their increase ; this is what is termed 
 taking the variation of Z. Comparing the result with the equation (A), 
 we shall observe the values of m, M, ^, n, N . . . in terms of x, y, z, and 
 
 3
 
 Ixx INTRODUCTION. 
 
 their differences expressed by d. We must then substitute these in the 
 equations (C), (D) ; the first refers to the limits between which the 
 maximum should subsist; the equations (D) constitute the relations re- 
 quired; they are the differentials of x, y, z, and, excepting a case of 
 absurdity, may form distinct conditions, since they will determine nume- 
 rical values for the variables. If the question proposed relate to Geo- 
 metry, these, equations are those of a curve or of a surface, to which 
 belongs the required property. 
 
 As the integration is effected and should be taken between given limits, 
 the terms which remain and compose the equation (C) belong to these 
 limits : it is become of the form K + L = 0, L being a function of 
 x y> z > 3 x, 3 y, 3 z . . . Mark with one and two accents the numerical 
 values of these variables at the first and second limit. Then, since the 
 integral is to be taken between these limits, we must mark the different 
 terms of L which compose the equation C, first with one, and then with 
 two accents ; take the first result from the second and equate the differ- 
 ence to zero ; so that the equation 
 
 L,, - L, = 
 
 contains no variables, because x, d x . . . will have taken the values 
 x,, 3 x, . . . x /y , 3 x x/ . . . assigned by the limits of the integration. We 
 must remember that these accents merely belong to the limits of the 
 integral. 
 
 There are to be considered four separate cases. 
 
 1. If the limits are given andjixed, that is to say, if the extreme values 
 of x, y, z are constant, since 3 x /} d 3 x, . . . d x /x , d 3 x //5 &c. are zero, all 
 the terms of L, and L,, are zero, and the equation (C) is satisfied. Thus 
 we determine the constants which integration introduces into the equations 
 (D), by the conditions conferred by the limits. 
 
 2. Jf the limits are arbitrary and independent, then each of the coeffi- 
 cients 3 x, , 3 x y/ . . . in the equation (C) is zero in particular. 
 
 3. If there exist equations of condition, (which signifies geometrically 
 that the curve required is terminated at points which are not fixed, but 
 which are situated upon two given curves or surfaces,) for the limits, that 
 is to say, if the nature of the question connects together by equations, 
 some of the quantities x,, y,, z,, x,,, y //5 z,, we use the differentials of these 
 equations to obtain more variations 3 x /5 3 y,, 3 z x , d x //5 &c. in functions 
 of the others ; substituting in L y/ L, = 0, these variations will be re- 
 duced to the least number possible : the last being absolutely independent, 
 the equation will split again into many others by equating separately their 
 coefficients to zero.
 
 ANALYTICAL GEOMETRY. Ixxi 
 
 Instead of this process, we may adopt the following one, which is more 
 elegant. Let 
 
 u ss 0, v = 0, &c. 
 
 be the given equations of condition ; we shall multiply their variations 
 3 u, 3 v ... by the indeterminates X, X'. . . This will give X3u4.X'3v + ... 
 a known function of 3 x /} 3 x,,, d y, . . . Adding this sum to L,, L,, we 
 shall get 
 
 L /y L, + X 3 u + X' d v + . . . = . . . . (E). 
 Consider all the variations 3 x,, 8 x //} ... as independent, and equate 
 their coefficients separately to zero. Then we shall eliminate the inde- 
 terminates X, X'. . . from these equations. By this process, we shall arrive 
 at the same result as by the former one ; for we have only made legiti- 
 mate operations, and we shall obtain the same number of final equations. 
 
 It must be observed, that we are not to conclude from u = 0, v r= 0, 
 that at the limits we have d u = 0, dv = 0; these conditions are inde- 
 pendent, and may easily not coexist. In the contrary case, we must 
 consider d u = 0, d v = 0, as new conditions, and besides X 3 u, we 
 must also take X' 3 d u . . . 
 
 4. Nothing need be said as to the case where one of the limits is fixed 
 o 
 
 and the other subject to certain conditions, or even altogether arbitrary, 
 because it is included in the three preceding ones. 
 
 It may happen also that the nature of the question subjects the varia- 
 tions 3 x, 3 y, 3 z, to certain conditions, given by the equations 
 
 t = 0, 6 = 0, 
 
 and independently of limits ; thus, for example, when the required curve 
 is to be traced upon a given curve surface. Then the equation (B) will 
 not split into three equations, and the equations (D) will not subsist. We 
 must first reduce, as follows, the variations to the smallest number possi- 
 ble in the formula (B), by means of the equations of condition, and equate 
 to zero the coefficients of the variations that remain ; or, which is tanta- 
 mount, add to (B) the terms X3e + X'30 + ...; then split this equation 
 into others by considering 3 x, 3 y, 3 z as independent ; and finally elimi- 
 nate X, X' ... 
 
 It must be observed, that, in particular cases, it is often preferable to 
 make, upon the given function Z, all the operations which have produced 
 the equations (B), (C) instead of comparing each particular case with the 
 general formulae above given. 
 
 Such are the general principles of the calculus of variations: let us 
 illustrate it with examples. 
 
 * 4
 
 Ixxii 
 
 INTRODUCTION. 
 
 Ex. 1. Wliat is the curve C M K of which the length M K, comprised 
 between the given radii-vectors A M, A K is the least possible. 
 
 A D P 
 
 We have, (vol. I, p. 000)> if r be the radius-vector, 
 
 s =/(i; s <H S + d 2 ) = Z 
 
 it is required to find the relation r = <f> 6, which ^renders Z a minimum 
 the variation is 
 
 dl 8 .3 r*d<>.ada--dr.od r 
 
 V (r ! d a + d r ) 
 
 Comparing with equation (A), where we suppose x = r, y = 6, we 
 
 have 
 
 m = 
 
 rd 
 
 -a ,n=^,M = 0,N = r -^- 
 d s d s d s 
 
 the equations (D) are 
 
 rd 6 
 
 ds 
 
 . /d r\ r 2 d 6 
 
 ~ = d (dl) -dT- = c ' 
 
 Eliminating d d, and then d s, from these equations, and ds*= r'dtf 2 ; 
 + d r *, we perceive that they subsist mutually or agree ; so that it is 
 sufficient to integrate one of them. But the perpendicular A I let fall 
 from the origin A upon any tangent whatever. T M is 
 A J = A M + sin. A M T = r sin, /3, 
 which is equivalent, as we easily find, to 
 
 r tan. & 
 
 which gives 
 
 V (1 + tan. z ]8) 
 
 r z d 6 
 
 r'd tf 
 ds 
 
 = c; 
 
 V (r * d 6 * + d r ') 
 
 and since this perpendicular is here constant, the required line is a 
 straight line. The limits M and K being indeterminate, the equations 
 (C) are unnecessary. 
 
 Ex. 2. Tojind the shortest line between two given points, or two given 
 curves.
 
 ANALYTICAL GEOMETRY. Ixxiii 
 
 The length s of the line is 
 
 /Z = /V(dx* + dy' + dz'). 
 It is required to make this quantity a minimum ; we have 
 
 as d s r d s 
 
 ind comparing with the formula (A), we find 
 
 the other coefficients P, p, it . . . are zero. The equations (D) become, 
 therefore, in this case, 
 
 whence, by integrating 
 
 dx = ads,dy = b d s, d z = cds. 
 Squaring and adding, we get 
 
 a s + b 8 + c 2 = 1, 
 
 a condition that the constants a, b, c must fulfil in order that these equa- 
 tions may simultaneously subsist.- By division, we find 
 
 d y __ b d z __ 
 dx~~a'dx~~a* 
 whence 
 
 b x = a y + a', c x = a z + b'; 
 
 the projections of the line required are therefore straight lines the line is 
 therefore itself a straight line. 
 
 To find the position of it, we must know the five constants a, b, c, 
 a', b'. If it be required to find the shortest distance between two given 
 fixed points (x,, y,, z,), (X A , y //5 z y/ ), it is evident that 5, x, d x //? d y, . . . are 
 zero, and that the equation (C) then holds good. Subjecting our two 
 equations to the condition of being satisfied when we substitute therein 
 x /} x //5 y, . . . for x, y, z, we shall obtain four equations, which, with 
 a 2 + b* + c 8 =l, determine the five necessary constants. 
 
 Suppose that the second limit is a fixed point (x /7 , y //5 z ;/ ), in the plane 
 (x, y), and the first a curve passing through the point (x /} y 7 Z 7 ), and also 
 situated in this plane ; the equation 
 
 b x = a y + a' 
 then suffices. Let y, = f x, be the equation of the curve ; hence 
 
 3y x = A dx,; 
 the equation (C) becomes ,
 
 Jxxiv INTRODUCTION. 
 
 and since the second limit is fixed it is sufficient to combine together the 
 equations 
 
 3y, = A3x, 
 
 d x, a x, + d y, 3 y, = 0. 
 Eliminating d y, we get 
 
 d x, + A d y, = 0. 
 We might also have multiplied the equation of condition 
 
 a y, A 8 x, = 
 
 by the indeterminate X, and have added the result to L,, which would 
 have given 
 
 whence 
 
 Eliminating X we get 
 
 dx, + Ad y/ = 0. 
 
 But then the point {x,, y,) is upon the straight line passing through the 
 points (x,, y,, z,), (x /7 , y// , z,,), and we have also 
 
 b d x y = ad y,, 
 whence 
 
 a = b A 
 and 
 
 ( il - - -1 - *J 
 dx A a 
 
 which shows the straight line is a normal to the curve of condition. The 
 constant a' is determined by the consideration of the second limit which is 
 given and fixed. 
 
 It would be easy to apply the preceding reasoning to three dimensions, 
 and we should arrive at similar conclusions ; we may, therefore, infer 
 generally that the shortest distance between two curves is the straight 
 line which is a normal to them. 
 
 If the shortest line required were to be traced upon a curve surface 
 whose equation is u == 0, then the equation (B) would not decompose into 
 three others. We must add to it the term X 5 u ; then regarding 6 x, d y, 
 & z as independent, we shall find the relations
 
 ANALYTICAL GEOMETRY, 
 d (T*) + : 
 
 \d S/ 
 
 IXXT 
 
 d + x ) = o. 
 
 ^d s/ d z/ 
 
 From these eliminating X, we have the two equations 
 dux , /dxv /du\ , /d 
 
 which are those of the curve required. 
 Take for example, the least distance measured upon the surface of a 
 
 sphere, whose center is at the origin of coordinates : hence 
 u = x* + y * + z * r* = 0, 
 
 (d u\ /d u\ /du\ 
 
 a x) = 2 x ' (di) = 2 * Ob) = ? ' 
 
 Our equations give, making d s constant, 
 
 z^*x = xd*z, zd z y = yd*z, 
 whence 
 
 yd*x = xd*y. 
 Integrating we have 
 zdx xdz = ads, zdy ydz = b d s, y d x xdy =: cds. 
 
 Multiplying the first of these equations by y, the second by x, the 
 third by z, and adding them, we get 
 
 ay = bx + cz 
 
 the equation of a plane passing through the origin of coordinates. Hencf 
 the curve required is a great circle "which passes through the points A' 
 (7, or which is normal to the two curves A' B and C' D which are limits 
 and are given upon the. spherical surface. 
 
 When a body moves in a fluid it encounters a resistance which ceteris
 
 Ixxvi INTRODUCTION. 
 
 paribus depends on its form (see vol. I.) : if the body be one of revolu- 
 tion and moves in the direction of its axis, we can show by mechanics 
 that the resistance is the least possible when the equation of the gener- 
 ating curve fulfils the condition 
 
 /y d d y 3 . . 
 
 d x* + d y* " ' 
 
 or 
 
 7 __ y y /3 d x 
 
 Let us determine the generating curve of the solid of least resistance 
 (see Principia, vol. II.). 
 
 Taking the variation of the above expression, we get 
 2ydy 3 dx - 2yy' 3 
 
 m O_ n * * J J n &T 
 
 2 9 M ~~ "? (XC 
 
 M - dy3 - y /3dx N - yy"(3 + y") & c - 
 
 ~dx 8 + dy 8 ~l+y' 8 ' (l + y /) >* 
 
 the second equation (D) is 
 
 M dN = 0; 
 and it follows from what we have done relatively to Z, that 
 
 d 
 
 because 
 
 M = d N. 
 Thus integrating, we have 
 
 y"y . 
 
 a (I +y")* = 2yy". 
 
 Observe that the first of the equations (D) or m d n = 0, would 
 have given the same result n = a ; so that these two equations conduct 
 to the same result. We have 
 
 substituting for y its value, this integral may easily be obtained ; it remains 
 to eliminate y 7 from these values of x and y, and we shall obtain the 
 equation of the required curve, containing two constants which we shall 
 determine from the given conditions.
 
 ANALYTICAL GEOMETRY. Ixxvii 
 
 Ex. 3. What is the curve A B M in "aJiich the area B O D M comprised 
 
 Y 
 
 between the arc B M the radii of curvature B O, D M and the arc O D 
 of the evolute, is a minimum ? 
 The element of the arc A M is 
 
 the radius of curvature M D is 
 
 and their product is the element of the proposed area, or 
 
 _ (l+y")dx __ (dx +dy') 
 y" dx dy 
 
 It is required to find the equation y =r f x, which makes yZ, a mini- 
 mum. 
 
 Take the variation B N, and consider only the second of the equations 
 (D), which is sufficient for our object, and we get 
 M = 0, N d P = 4 a, 
 
 XT dx 1 + dy 2 1 + y' 
 
 N = , ,, J . 4 d y = ,/ 4 y', 
 d x d 2 y y 
 
 But 
 
 y" * d x 
 
 = N d y' + P d y" d x 
 
 = 4 a d y + d P d y' + P d y" d x, 
 
 putting 4 a -f- P for N. Moreover y" d x = d y', changes the last 
 terms into 
 
 (y" d P -f P d y") d x = d (P y"). d x = d:( (1 + j
 
 Ixxviii INTRODUCTION. 
 
 Integrating, therefore, 
 
 finally, 
 
 On the other side we have 
 
 y = /y' d x = y' x /x d y' 
 or 
 
 y = y' x c y' 1 ^~^ d y' 
 this last term integrates by parts, and we have 
 
 y = y' x c y' (by a) tan.- 1 }" -f f. 
 Eliminating the tangent from these values of x and y, we get 
 
 - , . 
 
 ds '(by a * + g) 
 
 finally, 
 
 s = 2 V (b y a x + g) + h. 
 
 This equation shows that the curve required is a cycloid, whose four 
 constants will be determined from the same number of conditions. 
 
 Ex. 4. What is the curve of a given length s, between two faetl points, 
 for which fy d s is a maximum ? 
 We easily find 
 
 (y + v (In) = c ' whence d x = vny+ d J'-c'} ; 
 
 and it will be found that the curve required is a catenary. 
 
 is the vertical ordinate of the center of gravity of an nrc 
 
 whose length is s, we see that the center of gravity of any arc whatever of 
 the catenary is lower than that of any other curve terminated by the 
 same points. 
 
 Ex. 5. Reasoning in the same way for f y * d x = minimum, and 
 J y d x r= const, we find y * -f- X y = c, or rather y = c. We have 
 
 fv * d x 
 
 here a straight line parallel to x. Since 7^ = is the vertical ordinate 
 
 2/y d x 
 
 of the center of gravity of every plane area, that of a rectangle, whose 
 side is horizontal, is the lowest possible ; so that every mass of water
 
 ANALYTICAL GEOMETRY. Ixxix 
 
 whose upper surface is horizontal, has its center of gravity the lowest 
 possible. 
 
 FINITE DIFFERENCES. 
 
 If we have given a series a, b, c, d, . . . take each term of it from that 
 which immediately follows it, and we shall form thejirst differences, viz. 
 
 a' = b a, b' = c b, c' = d c, &c. 
 
 In the same manner we find that this series a', b', c', d 7 . . . gives the 
 second differences 
 
 a" = b' a', b" = c' b', c" = d' c', &c. 
 which again give the third differences 
 
 a'" = b" a'', b'" = c" b", c'" = d" c", &c. 
 These differences are indicated by A, and an exponent being given to 
 it will denote the order of differences. Thus A B is a ferm of the series 
 of nth differences. Moreover we give to each difference the sign which 
 belongs to it ; this is , when we take it from a decreasing series. 
 For example, the function 
 
 y = x' 9x -f 6 
 
 in making x successively equal to 0, 1, 2, 3, 4 . . . gives a series of 
 numbers of which y is the general term, and from which we get the 
 following differences, 
 
 for x = 0, 1, 2, 3, 4, 5, 6, 7 ... 
 
 series y = 6, 2, 4, 6, 34, 86, 168, 286... 
 
 first diff. A y = 8, 2, 10, 28, 52, 82, 118 ... 
 
 second diff. A * y = 6, 12, 18, 24, 30, 36 ... 
 
 third diff. A 3 y = 6, 6, 6, 6, 6, ... 
 
 We perceive that the third differences are here constant, and that the 
 second difference is an arithmetic progression : we shall always arrive at 
 constant differences, whenever y is a rational and integer function of x ; 
 which we now demonstrate. 
 
 In the monomial k x m make x = a, /3, 7, . . . 0, x, X (these numbers 
 having h for a constant difference), and we get the series 
 
 .k a m , k |8 m , . . . k 6 m , k x m , k X m . 
 
 Since x = X h, by developing k x m = k (X h) m , and designating 
 DV m, A', A" . . . the coefficients of the binomial, we find, that 
 
 k (x m K w ) = k m h x m - 1 k A' h * X ra ~ 2 + k A" 3 h. . .
 
 Ixxx INTRODUCTION. 
 
 Such is the first difference of any two terms whatever of the series 
 
 k a m , k /3 m . . . k x m , &c. 
 
 The difference which precedes it, or k (x m 6 m ) is deduced by 
 changing X into x and x into tf and since x = X h, we must put 
 A h for X in the second member: 
 
 k m h(X-h) --k A' h * (X-h m - 8 ) ...=k m h X -i-| A'+m(m-l)}kh* X m - 8 ... 
 Subtracting these differences, the two first terms will disappear, and 
 vre get for the second difference of an arbitrary rank 
 
 k m (m 1) h 2 X m - 2 + k B' h 3 X m -s + . . . 
 In like manner, changing X into X h, in this last developement, and 
 subtracting, the two first terms disappear, and we have for the third 
 difference 
 
 km (m 1) (m 2) h 3 X m - 3 -f kE'^'X 10 -*. . ., 
 and so on continually. 
 
 Each of these differences has one term at least, in its developement, 
 like the one ^bove ; the first has m terms ; the second has m 1 terms ; 
 third, m 2 terms ; and so on. From the form of the first term, which 
 ends by remaining alone in the mth difference, we see this is reduced to 
 the constant 
 
 1.2.3...mkh m . 
 
 If in the functions M and N we take for x two numbers which give the 
 results m, n ; then M + N becomes m + n. In the same manner, let 
 m', n' be the results given by two other values of x ; the first difference, 
 arising from M + N, is evidently 
 
 (m m') -f (n n'). 
 
 that is, the difference of the sum is the sum of the differences. The same 
 may be shown of the 3d and 4th . . . differences. 
 Therefore, if we make 
 
 x = , /3, y . . . 
 in 
 
 k x m + p x m - 1 + . . . 
 
 the mth difference will be the same as if these were only the first term 
 k x m , for that of p x "a- 1 , q x m ~ 2 ... is nothing. Therefore the mth 
 difference is constant, when for x we substitute numbers in arithmetic pro- 
 gression t in a rational and integer Junction of -a. 
 
 We perceive, therefore, that if it be required to substitute numbers in 
 arithmetic progression, as is the case in the resolution of numerical equa- 
 tions, according to Newton's Method of Divisors, it will suffice to find 
 the (m + 1) first results, to form the first, second, &c. differences. The
 
 ANALYTICAL GEOMETRY. IxxxL 
 
 roth difference will have but one term ; as we know it is constant and 
 = 1 . 2 . 3 . . . m k h re , we can extend the series at pleasure. That of 
 the (m l)th differences will then be extended to that of two known 
 terms, since it is an arithmetic procession ; that of the (m 2)th differ- 
 ences will, in its turn, be extended ; and so on of the rest. 
 
 This is perceptible in the preceding example, and also in this ; viz. 
 
 x = 
 
 . 1 
 
 .2. 
 
 3 
 
 3d Diff. 6 . 
 
 6. 
 
 6 
 
 . 6 
 
 . 6. 
 
 6. 
 
 6 .. . 
 
 Series 1 
 
 .-1 
 
 .1. 
 
 13 
 
 2nd . . 4 . 
 
 10. 
 
 16 
 
 . 22 
 
 .2J$. 
 
 34. 
 
 40 ... 
 
 1st ... 
 
 2 
 
 .2. 
 
 12 
 
 1st . 2 . 
 
 2. 
 
 12 
 
 . 28 
 
 . 50. 
 
 78. 
 
 112. .. 
 
 2nd . . 
 
 
 3. 
 
 10 
 
 Results 1 . 
 
 1 . 
 
 1 
 
 . 13 
 
 . 41 . 
 
 91 . 
 
 169 ... 
 
 3d ... 
 
 
 
 6 
 
 For x . 
 
 1 . 
 
 2 
 
 . 3 
 
 . 4. 
 
 5. 
 
 6 ... 
 
 These series are deduced from that which is constant 
 
 6.6.6.6... 
 
 and from the initial term already found for each of them : any term is 
 derived by adding the two terms on tlie left which immediately precede it. 
 They may also be continued in the contrary direction, in order to obtain 
 the results of x = 1, 2, 3, &c. 
 
 In resolving an equation it is not necessary to make the series of results 
 extend farther than the term where we ought only to meet with numbers 
 of the same sign, which is the case when all the terms of any column are 
 positive on the right, and alternate in the opposite direction; for the 
 additions and subtractions by which the series are extended as required, 
 preserve constantly the same signs in the results. We learn, therefore, 
 by this method, the limits of the roots of an equation, whether they be 
 positive or negative. 
 
 Let y x denote the function of x which is the general term, viz. the 
 x + 1th, of a proposed series 
 
 yo + 72 + yi + . . yx + y*+i+ 
 
 which is formed by making 
 
 x = 0, 1, 2, 3 ... 
 
 For example, y 5 will designate that x has been made = 5, or, with re- 
 gard to the place of the terms, that there are 5 before it (in the last ex- 
 ample this is 91). Then 
 
 yi yo = A yo , y 2 yi = A yi ys y = y 
 
 A yl _ Ay = A 2 y , Ay 2 A^ = A^ , A y 3 A y 2 = A 2 y 2 . . . 
 A 2 y, A 2 y = A3y , A 2 y 2 A 2 y t = A^ , A 2 y 3 A 2 y 2 = A 3 y 2 . . . 
 
 &c.
 
 Ixxxii INTRODUCTION. 
 
 and generally we have 
 
 &c. 
 
 f 
 
 Now let us form the differences of any series a, b, c, d . . . in this 
 manner. Make 
 
 b = c + a' 
 c = b + b' 
 d = c + c' 
 
 &c. 
 
 b' = a' -f- a" 
 c' = b' + b'' 
 d' = c' + c" 
 &c. 
 
 b" = a" + a'" 
 c" = b" + b'" 
 d" = c" + d" 
 
 &c. 
 
 and so on continually. Then eliminating b, b', c, c', &c. from the first 
 set of equations, we get 
 
 b = a + a' 
 
 c = a + 2 a' + a" 
 
 d = a + 3 a' + 3 a" + a'" 
 
 e = a + 4 a' + 6 a" + 4 a'" + a"" 
 
 f = a + 5 a! + 10 a" + &c. 
 
 &c. 
 Also we have 
 
 a' = b a 
 a" = c 2 b + a 
 a'" =d 3c + 3b a 
 &c. 
 
 But the letters a', a", a"', &c. are nothing else than A yp, A 2 y , A 3 y . . 
 a, b, c . . . being yo, yi, y 2 . . . , consequently 
 yi = y + A y 
 y 2 = yo + 2 Ay + A'y 
 y 3 = y -f 3 A y + 3 A 2 y -f A 3 y 
 &c.
 
 .ANALYTICAL GEOMETRY. Ixxxiii 
 
 And 
 
 A y<> = yi yo 
 
 A2 yo = ya 2 yi + y 
 
 ^ 3 yo = y s 3 y 8 + 3 y, y 
 
 A Vo = y* 4 ya +6 y* + 4 y t + y c 
 
 &c. 
 Hence, generally, we have 
 
 y 
 
 . ' - 
 
 n 1 n 1 n 2 
 
 y 
 
 . . . . 
 
 n 1 rf n 2 n 3 
 
 These equations, which are of great importance, give the general term 
 of any series, from knowing its first term and the first term of all the 
 orders of differences ; and also the first term of the series of nth differ- 
 ences, from knowing all the terms of the series y , y ls y^ . . . 
 
 To apply the former to the example in p. (81), we have 
 
 y.= i 
 
 Ay = 2 
 A*y = 4 
 A'y = 6 
 
 *y, = o 
 
 whence 
 y x= l 2x+2x(x l) + x(x l)(x 2) = x s x 2 2x+l 
 
 The equations (A), (B) will be better remembered by observing that 
 y x = (I + Ay ), 
 
 * n yo = (y D n , 
 
 provided that in the developements of these powers, we mean by the 
 exponents of A y , the orders of differences, and by those of y the place 
 in the series. 
 
 It has been shown that a, b, c, d . . . may be the values of y x , when 
 those of x are the progressional numbers 
 
 m, m + h, m + 2 h . . . m + i h 
 
 that is 
 
 a = y m , b = y m + h , c = &c. 
 
 In the equation (A), we may, therefore, put y m+ih for y x , y m fory,,. A y m 
 for A y , &c. and, finally, the coefficients of the i th power. Make i h = z, 
 and write A, A ... for A y m , A*y m . . . and we shall get 
 
 Z A zJz-h)A* . z(z-h)(z-2h)A'. 
 
 y m + -g- + - 8h . 2h 3
 
 Ixxxiv INTRODUCTION. 
 
 Tliis equation will give y x when x = m + z > z being either integer or 
 fractional. We get from the proposed series the differences of all orders, 
 and the initial terms represented by A, A 2 , &c. 
 
 But in order to apply this formula, so that it may be limited, we must 
 arrive at constant differences ; or, at least, this must be the case if we 
 would have A, A* ... decreasing in . value so as to form a converging 
 series : the developement then gives an approximate value of a term cor- 
 responding to 
 
 x = m + z; 
 
 it being understood that the factors of A do not increase so as to destroy 
 this convergency, a circumstance which prevents z from surpassing a 
 certain limit. 
 
 For example, if the radius of a circle is 1000, 
 the arc of 60 has a chord 1000,0 
 
 65 1074,6 '- _' A = 2,0 
 
 70 1 147,2 r 7 q 
 
 75 1217,5 ~ 2 ' 3 
 
 Since the difference is nearly constant from 60 to 75, to this extent 
 of the arc we may employ the equation (C); making h = 5, we get for 
 the quantity to be added to y = 1090, this 
 
 m 
 
 }. 74,6. z 3% z (z 5) = 15,12. z 0,04. z 8 
 
 So that, by taking z 1, 2, 3.. . then adding 1000, we shall obtain the 
 
 chords of 61, 62, 63 ; in the same manner, making z the necessary 
 
 fraction^ we shall get the chord of any arc whatever, that is intermediate 
 
 to those, and to the limits 60 and 75. It will be better, however, when 
 
 it is necessary thus to employ great numbers for z, to change these limits. 
 
 Let us now take 
 
 log. 3100 = y = 4913617 
 
 m A 
 
 log. 3110 =4927604 - _ , r 
 
 IQQAO n * ~~ 
 
 log. 3120 = 4941546 , r 
 
 10007 
 log. 3130 = 4955443 
 
 We shall here consider the decimal part of the logarithm as being an 
 integer. By making h = 10, we get, for the part to be added to log. 
 3100, this 
 
 1400,95 x z 0, 225 X z 2 . 
 To get the logarithms of 3101, 3102, 3103, &c. we make 
 
 z = 1, 2, 3....; 
 and in like manner, if we wish for the log. 3107, 58, we must make
 
 ANALYTICAL GEOMETRY. Ixxxv 
 
 z 7, 58, whence the quantity to be added to the logarithm of 3100 is 
 10606. Hence 
 
 log. 310768 = 5,4924223. 
 
 The preceding methods may be usefully employed to abridge the 
 labour of calculating tables of logarithms, tables of sines, chords, &c. 
 Another use which we shall now consider, is that of inserting the inter- 
 mediate terms in a given series, of which two distant terms are given. 
 This is called 
 
 INTERPOLATION. 
 
 It is completely resolved by the equation (C). 
 
 When it happens that A 2 = 0, or is very small, the series reduces to 
 
 z yA 
 
 "h 
 
 whence we learn that the results have a difference which increases propor- 
 tionally to z. 
 
 When A 2 is constant, which happens more frequently, by changing z 
 into z + 1 in (C), and subtracting, we have the general value of the first 
 difference of the new interpolated series ; viz. 
 
 First difference A' = ^ + 2 Z ~ ] ' . + 1 A. 
 li 2 h 
 
 * 2 
 
 Second difference A" == ^. 
 
 If we wish to insert u terms between those of a given series, we must 
 make 
 
 h = n + 1 ; 
 then making z = 0, we get the initial term *of the differences 
 
 A. 2 
 
 A'' = 
 
 (n+ I) 8 
 
 A . 
 
 we calculate first A", then A' ; the initial term A' will serve to compose 
 the series of first differences of the interpolated series, (A" is the constant 
 difference of it) ; and then finally the other terms are obtained by simple 
 additions. 
 
 If we wish in the preceding example to find the log. of 3101, 
 
 /3
 
 INTRODUCTION. 
 
 3102, 3103 ... we shall interpolate 9 numbers between those which are 
 given : whence 
 
 u = 9 
 
 A" = 0,45 
 
 A' = 1400,725. 
 
 We first form the arithmetical progression whose first term is A', and 
 0,45 for the constant. The first differences are 
 
 1400,725; 1400,725; 1399,375; 1398,925, &c. 
 
 Successive additions, beginning with log. 3100, will give the consecutive 
 logarithms required. 
 
 Suppose we have observed a physical phenomenon every twelve hours, 
 and that the results ascertained by such observations have been 
 
 For hours ... 78 
 
 12 ... 300 A = A 2 = 144 
 
 24 ... 666 
 
 36 ... 1176 510 H4. 
 
 &c. 
 
 If we are desirous of knowing the state corresponding to 4 h , 8 h , 12 h , 
 &c., we must interpolate two terms ; whence 
 
 u = /, A" = 16, A' = 58 
 
 composing the arithmetic progression whose first term is 58, and common 
 difference 16, we shall have the first differences of the new series, and 
 then what follow 
 
 First differences 58, 74, 90, 106, 122, 138 ... 
 
 Series 78, 136, 210, 300, 406, 528, 646 . . . 
 
 A O h , 4 h , 8 h , 16" 20 h , 24 h . 
 
 The supposition of the second differences being constant, applies almost 
 to all cases, because we may choose intervals of time which shall favour 
 such an hypothesis. This, method is of great use in astronomy; arid 
 even when observation or calculation gives results whose second differ- 
 ences are irregular, we impute the defect to errors which we correct by 
 establishing a greater degree of regularity. 
 
 Astronomical, and geodesical tables are formed on these principles. 
 We calculate directly different terms, which we take so near that their 
 first or second differences may be constant; then we interpolate to obtain 
 the intermediate numbers. 
 
 Thus, when a converging series gives the value of y by aid of that of a 
 variable x ; instead of calculating y for each known value of x, when the 
 formula is of frequent use, we determine the results y for the continually
 
 ANALYTICAL GEOMETRY. 
 
 increasing values of x, in such a manner that y shall always be nearly of 
 the same value : we then write in the form of a table every value by the 
 side of that of x, which we call the argument of this table. For the 
 numbers x which are intermediate to them, y is given by simple proposi- 
 tions, and by inspection alone we then find the results lequired. 
 
 When the series has two variables, or arguments x and z, the values 
 of y are disposed in a table by a sort of double entry ; taking for coordi- 
 nates x and z, the result is thus obtained. For example, having made 
 z =r 1, we range upon the first line all the values of y corresponding to 
 
 x = 1, V, 3 .. .; 
 
 we then put upon the second line which z = z gives ; in a third line those 
 which z = 3 gives, and so on. To obtain the result which corresponds to 
 
 x = 3, z = 5 
 
 we stop at the case which, in the third column, occupies the fifth place. 
 The intermediate values are found analogously to what has been already 
 shown. 
 
 So far we have supposed x to increase continually by the same differ- 
 ence. If this is not the case and we know the results 
 
 y = a, b, c, d . . . 
 which are due to any suppositions 
 
 x = a, ft 7, a ... 
 
 we may either use the theory which makes a parabolic curve pass through 
 a series of given points, or we may adopt the following: 
 By means of the known corresponding values 
 
 a, a; b |S; &c. 
 we form the consecutive functions 
 
 A = b ~" 
 
 /3 a 
 
 A = c ~ b 
 
 A - d ~ c 
 
 *~ o y 
 &C. 
 
 \ j X\ j - 1 \. 
 
 B,= 
 
 A, A 
 
 b~ ,3 
 
 A, A 
 
 &c.
 
 bcxxviii INTRODUCTION. 
 
 /-i __ 
 
 D _ C '~ C 
 
 v a 
 and so on. 
 
 By elimination we easily get 
 b = a + A (0 a) 
 
 c = a -f A (7 ) + B (7 ) (7 /3) 
 
 d = a + A (3 a) + B(3 a) (d /S) + C (3 a) (5 /3) (5 
 &c. 
 
 and generally 
 
 y x = a -f A(x~a) + B(x a)(x /S) + C (x a) (x /3) (x 
 We must seek therefore the first differences amongst the results 
 
 a, b, c . . . 
 and divide by the differences of 
 
 a, /3, 7 ... 
 which will give 
 
 ./\j A i) A-2) CvC. 
 
 proceeding in the same manner with these numbers, we get 
 B, Bj, 62, &c. 
 
 which in like manner give 
 
 Cr* r 1 AT,* 
 , V/}, V/^, oLC. 
 
 and, finally substituting, we get the general term required. 
 By actually multiplying, the expression assumes the form 
 
 of every rational and integer polynomial, which is the same as when we 
 neglect the superior differences. 
 The chord of 60 = rad. = 1000 
 
 62. 20' =1035 
 65. 10' = 
 
 
 A=15 
 
 A, = 14,82 
 
 A 2 = 14,61 
 
 B -0,035 
 B! = 0,031 
 
 69. 0' =1133 
 
 We have 
 
 = 0, /3 = 2i, 7 = Sff, a = 9. 
 
 We may neglect the third differences and put 
 
 y x = 100 + 15,082 x 0,035 x 2 . 
 
 Considering every function of x, y x , as being the general term of the 
 series which gives 
 
 x = m, m + h, m + 2 h, &c.
 
 ANALYTICAL GEOMETRY. Ixxxix 
 
 if we take the differences of these results, to obtain a new series, the 
 general term will be what is called the first difference of the proposed 
 function y x which is represented by A y x . Thus we obtain this difference 
 by changing x into x + h in y x and taking y x from the result ; the re- 
 mainder will give the series of first differences by making 
 
 x = m, m + h, m -f- 2 h, &c. 
 Thus if 
 
 a + x 
 (x + 
 
 J x " a + x+ h a + x* 
 
 It will remain to reduce this expression, or to develope it according to 
 the increasing powers of h. 
 
 Taylor's theorem gives generally (vol. I.) 
 d y d 2 y h 8 
 
 A y * = d x + cfx 1 T2 + 
 
 To obtain the second difference we must operate upon A y x as upon (he 
 proposed y x , and so on for the third, fourth, &c. differences. 
 
 INTEGRATION OF FINITE DIFFERENCES. 
 
 Integration here means the method of finding the quantity whose dif- 
 ference is the proposed quantity ; that is to say the general term y x of a 
 
 from knowing that of the series of a difference of any known order. This 
 operation is indicated by the symbol 2. 
 For example 
 
 2 (3 x 2 + x 2) 
 ought to indicate that here 
 
 h = 1. 
 
 A function y x generates a series by making 
 x = 0, 1, 2, 3 ... 
 the first differences which here ensue, form another series of which 
 
 3 x 2 + x 2 
 is the general term, and it is 
 
 2, 2, 12, 28 ... 
 
 By integrating we here propose to find y x such, that putting x + 1 for 
 x, and subtracting, the remainder shall be 
 3 x * + x 2.
 
 xc INTRODUCTION. 
 
 It is easy to perceive that, first the symbols 2 and A destroy one another 
 as do f and d; thus 
 
 2 A f x = f x. 
 
 Secondly, that 
 
 A (a y) = a A y 
 
 gives 
 
 2 a y = a 2 y. 
 Thirdly, that as 
 
 A (A I Bu) = AAt BA U 
 so is 
 
 2 (A t B u) = A 2 t B 2 u, 
 t and u being the functions of x. 
 
 The problem of determining y x by its first difference does not contain 
 data sufficient completely to resolve it; for in order to recompose the 
 series derived from y* in beginning with 
 
 2, 2, 12, 28, &c. 
 ive must make the first term 
 
 y<> = 
 
 and by successive additions, we shall find . 
 
 a, a 2, a + 2, a + 12, &c. 
 in which a remains arbitrary. 
 
 Kvery integral may be considered as comprised in the equation (A) 
 p. 83 ; for by taking 
 
 x = 0, 1, 2, 3 ... 
 
 in the first difference given in 'terms of x, we shall form the series of first 
 differences ; subtracting these successively, we shall have the second dif- 
 ferences ; then in like manner, we shall get the third and fourth differ- 
 ences. The initial term of these series will be 
 
 Ay , A*y ... 
 
 and these values substituted in y x will give y x . Thus, in the example 
 above, which is only that of page (81) when a = 1, we have 
 
 Ay = 2, Ay = 4, A 3 y = 6, A * y = 0, &c. ; 
 which give 
 
 y x = jo 2 x x 2 + x ' 
 
 Generally, the first term y of the equation (A) is an arbitrary constant, 
 which is to be added to the integral. If the given function is a second 
 difference, we must by a first integration reascend to the first difference 
 and thence by another step to y x ; thus we shall have two arbitrary con- 
 stants ; and in fact, the equation (A) still gives y x by finding A s , A 3 , the
 
 ANALYTICAL GEOMETRY. xci 
 
 only difference in the matter being that y and A y are arbitrary. And 
 so on for the superior orders. 
 
 Let us .now find 2 x m , the exponent m being integer and positive. 
 Represent this developement by 
 
 2x m = px-fqx b + rx c + &c. 
 
 a, b, c, &C. being decreasing exponents, which as well as the coefficients 
 p, q, &c. must be determined. Take the first difference, by suppressing 
 5 in the first member, then changing x into x + h in the second member 
 and subtracting. Limiting ourselves to the two first terms, we get 
 
 x m = pahx 3 - 1 + pa(a I)h 2 x a ~ 2 + ...qbhx b - 1 +... 
 
 But in order that the identity may be established the exponents ought 
 
 to give 
 
 a 1 = m 
 
 a 2 = b 1 
 whence 
 
 a = m + 1, b = m. 
 Moreover the coefficients give 
 
 1 = p a h, p a (a 1 ) h = q b ; 
 whence 
 
 1 
 
 = (in + 1) h ' q = 
 
 As to the other terms, it is evident, that the exponents are all integer 
 and positive ; and we may easily perceive that they fail in the alternate 
 terms. Make therefore 
 
 2X m = px ra + 1 x m + ax" 1 " 1 + /3x m - 3 -f yx m ~ 5 + . .. 
 
 and determine , $, y ... &c. 
 
 Take, as before, the first difference by putting x + h for x, and sub- 
 tracting : and first transferring 
 
 p x m+i i xM> 
 
 we find that the first member, by reason of 
 ph (m + 1) = 1, 
 reduces to 
 
 '2^ 4 ' 2.5 6 ' 2.7 
 
 To abridge the operation, we omit here the alternate terms of the deve- 
 lopement; and we designate by 
 
 1, m, A', A", &c. 
 the coefficients of the binomial. 
 
 Making the same calculations upon 
 
 ax 1 "- 1 + /3x ra - 3 + &r.
 
 xcii INTRODUCTION. 
 
 we shall have, with the same respective powers of x and of h, 
 
 n 2 m 3 ' m 2 m 4 
 
 M - - 9 o ~ j 
 
 + (m _3)/3+(m,-3).HL^...E^ /3 _ { _... 
 
 *w O 
 
 + (m 4) 7 +.. 
 
 Comparing them term by term, we easily derive 
 
 m 
 
 A" 
 ~ 2.3.4.5' 
 
 A"" 
 
 7 ~ 6.6.7 
 &c. 
 whence finally we get 
 
 m 
 
 v 
 
 2x m = r h -- g- + mahx" 1 - 1 + 
 
 + A'^'ch^^-s+A 71 dh 7 x m - 7 +...(D) 
 
 This developement has for its coefficients those of the binomial, taken 
 from two to two, multiplied by certain numerical factors a, b, c . . ., which 
 are called the numbers of Bernoulli, because James Bernoulli first deter- 
 mined them. These factors are of great and frequent use in the theory 
 of series ; we shall give an easy method of finding them presently. These 
 are their values 
 
 1 
 
 b-- 
 
 120 
 
 ^ 
 
 = 252 
 
 d = ~~240 
 
 1 
 
 = I32 
 f _ 691 
 
 1 12 
 
 32780 
 1 
 
 8160 
 
 . _ 43867 
 ~ 14364 
 &c.
 
 ANALYTICAL GEOMETRY. xciii 
 
 which it will be worth the trouble fully to commit to memory. 
 
 From the above we conclude that to obtain 2 x m , m being any number, 
 integer and positive, we must besides the two first terms 
 
 (m -f 1) h 2 
 also take the developement of 
 
 (x + h) 
 
 reject the odd terms, the first, third, fifth, &c. and multiply the retained 
 terms respectively by 
 
 a, b, c . . . 
 
 Now x and h have even exponents only tvhen m is odd and reciprocally ; 
 so that we must reject the last term h m when it falls in a useless situation ; 
 the number of terms is | m -f- 2 when m is even, and it is (m + 3) when 
 m is odd ; that is to say, it is the same for two consecutive values of m. 
 
 Required the integral ofx 10 . 
 Besides 
 
 - i- x 19 
 
 11 h 
 
 we must develope (x + h) W 9 retaining the second, fourth, sixth, &c. terms 
 and we shall have 
 
 10x 9 ah + 120x 7 bh 3 + 252x s ch 5 + &c. 
 Therefore 
 
 x u 5 5 9 
 
 In the same manner we obtain 
 x 
 
 *- - _xj* j_ \L? 3 
 ~~ 5 h 4 + 3
 
 xciv INTRODUCTION. 
 
 x 9 x 8 2hx 7 7h 3 x 5 , 2h 5 x 3 h^x 
 
 f v ' -L. -4- 
 
 " ifh 2 3 15 9 30 
 
 x 10 xj 3 h x ' __ 7 h 3 x G h s x 4 _ 3 h 7 x 
 " X ~ W""W ' 4 10 2 20 
 
 x u 
 
 2 x 10 = YV-: &c. as before, 
 
 11 h 
 
 &c. 
 
 We shall now give an easy method of determining the Number of 
 Bernoulli a, b, c . . . In the equation (D) make 
 
 x= h = 1; 
 
 2 x m is the general term of the series whose first difference is x m . We 
 shall here consider 2. x = 1, and the corresponding series which is that 
 of the natural numbers 
 
 0, 1, 2, 3 ... 
 Take zero for the first member and transpose 
 
 m -f- 1~~* 
 
 which equals 
 
 2 (m + 1) 
 "I m 
 Then we get 
 
 -p. = a m + b A" -f c A lv + d A vl + . . . + k m. 
 * (m + ' ) 
 
 By making m = 2, the second member is reduced to am, which gives 
 
 Jl 
 
 Making m = 4, we get 
 
 1 := 4 a + b A" 
 
 m 1 in 2 , 
 
 2 
 
 = 4 a + 4 b 
 
 = i + 4 b. 
 Whence 
 
 120* 
 Again, making m = 6, we get 
 
 ^ = 6 a + b h" + c A 1 * 
 = 6a+ 20 b +6c
 
 ANALYTICAL GEOMETRY. xcv 
 
 which gives 
 
 252' 
 and proceeding thus by making 
 
 m = 2, 4, 6, 8, &c. 
 
 we obtain at each step a new equation which has one term more than the 
 preceding one, which last terms, viz. 
 
 2 a, 4 b, 6 c, . . . m k 
 will hence successively be found, and consequently, 
 
 a, b, c . . . k. 
 Take the difference of the product 
 
 y x = (x h)x(x + h)(x + 2h)...(x+ih), 
 by x + h for x and subtracting ; it gives 
 
 A y x = x (x + h) (x + 2 h) . . . (x + i h) X (i + 2) h; 
 dividing by the last constant factor, integrating, and substituting for y, 
 its value, we get 
 
 2 x (x + h) (x + 2 h) . . . (x + i h) 
 
 ^(TT^yh Xx -( x + h Hx + 2h)...(x + ih) 
 
 This equation gives the integral of a product of factors in arithmetic 
 progression. 
 
 Taking the difference of the second member, we verify the equation 
 
 __ _ 1 _ _ _ j _ 
 x (x + h) (x + 2 h) . . .(x + i h) ~ i h x (x + h) . . . {x + (i 1) h} 
 which gives the integral of any inverse product 
 Required the integral of a*. 
 Let 
 
 ,. - ., X 
 
 j x _ a . 
 Then 
 
 Ay x = a *(a h 1) 
 whence 
 
 y x = 2 
 consequently 
 
 a x a h 1 = a x ; 
 
 a * 
 
 2 a x =r r + constant. 
 
 a h 1 
 
 Required the integrals of sin. x, cos. x. 
 Since 
 
 cos. B cos- A = 2 sin. (A + B). sin. | (A B) 
 A cos. x, = cos. (x + h) cos. x 
 
 , t hx . h 
 
 = 2 sin. (x + sr 1 sin. -g 
 
 \ <* / A
 
 xcvi INTRODUCTION. 
 
 Integrating and changing x + - into z, we have 
 
 2 sin. z = cos + constant. 
 
 y " 
 "2 
 
 In the same way we find 
 
 f \ 
 
 sm. (z -) 
 
 2 cos. z = - r -- \- constant. 
 2 sin.- 
 
 When we wish to integrate the powers of sines and cosines, we trans- 
 form them into sines and cosines of multiple arcs, and we get terms of 
 the form 
 
 A sin. q x, A cos. q x* 
 Making 
 
 q x = x 
 
 the integration is performed as above. 
 Required the integral of a product, viz. 
 Assume 
 
 2(uz) = u2z + t 
 
 u, z and t being all functions of x, t being the only unknown one. By 
 changing x into x + h in 
 
 u 2 z + t 
 u becomes u + A U 5 z becomes z + A z, &c. and we have 
 
 u2z+UZ + Au2(z + Az) + t+At; 
 
 substituting from this the second member 
 
 u 2 z + t, 
 we obtain the difference, or u z ; whence results the equation 
 
 = A u 2 (z + A z) + A t 
 which gives 
 
 t = 2 A u 2 (Z + A z)}. 
 
 Therefore 
 
 2 (U Z) = U 2 Z 2 JA u . 2 (z + A z)} 
 
 which is analogous to integrating by parts in differential funcvons. 
 
 There are but few functions of which we can find the finite integral ; 
 when we cannot integrate them exactly, we must have recourse to series, 
 
 Taylor's theorem gives us 
 
 dy, , d 2 y h* , 
 
 A y x = T - h + i 2 ir + &c - 
 JX dx ' dx 2 2
 
 ANALYTICAL GEOMETRY. 
 
 y" 
 
 by supposition. Hence 
 
 y x = h 2 y' + -^ 2 y" + &c. 
 
 Considering y 7 as a given function of x, viz. z, we have 
 
 y' = z 
 
 y" = z' 
 
 y'" = z" 
 
 &c. 
 and 
 
 y x =/y d x =/zdx 
 whence 
 
 h 2 
 /z d x = h 2 z + 2 z' + &c. 
 
 
 
 which gives 
 
 2 z = h~ l /z d x ~ 2 z' h 3 2 z" 8cc. 
 
 A 
 
 This equation gives 2 z, when we know z', 5 z'', &c. Take the dif- 
 ferentials of the two numbers. That of the first 2 z will give, when di- 
 vided by d x, 2 z'. Hence we get 2 z", then 2 z'", &c. ; and even without 
 making the calculations, it is easy to see, that the result of the substitution 
 of these values, will be of the form 
 
 2 z = h-'/z d x + A z + B h z' + C h 2 z" + &c. 
 It remains to determine the factors A, B, C, &c. But if 
 
 z = x m 
 we get 
 
 /z d x, z', z", &c. 
 
 and substituting, we obtain a series which should be identical with the 
 equation (D), and consequently defective of the powers m 2 t m 4, 
 so that we shall have 
 
 /^zdx z ahz' bh 3 z'" ch s z'"" dhV""" ,, 
 ~h~ 2" 1 ~T~ TT ~2^4~ 2.. .6 
 
 a, b, c, &c. being the numbers of Bernoulli. 
 For example, if 
 
 z = 1 x 
 
 yix.dx = x 1 x x 
 z' = x- 1 
 z" = &c.
 
 xcviii INTRODUCTION 
 
 consequently 
 
 2 1 x = C-fxlx x lx + ax- 1 + b x- 3 + ex-/ -f &c. 
 The series 
 
 a, b, c . . . k, 1, 
 having for first differences 
 
 a', b', c' . . . k' 
 we have 
 
 b = a + a' 
 c = b + b 
 d = c + c' 
 &c. 
 
 1 = k + k' 
 equations whose sum is 
 
 1 = a -f a' + b' + c! + . . . k'. 
 
 If the numbers a', b', c', &'c. are known, we may consider them as being 
 the first differences of another series a, b, c, &c. since it is easy to com- 
 pose the latter by means of the first, and the first term a. By definition 
 we know that any term whatever 1', taken in the given series a', b'', c', &c. 
 is nothing else tjmn A 1, for 1' = m 1 ; integrating 
 
 1' = Al 
 
 we have 
 
 21' - 1 
 
 or 
 
 2 1' = a' + b 7 + c' . . . + k', 
 
 supposing the initial a is comprised in the constant due to the integra- 
 tion. Consequently 
 
 The integral of any term whatever of a series, we obtain the sum of all 
 the terms that precede it, and have 
 
 2 yx = y + yi + 72 + y x - 1- 
 
 In order to get the sum of a series, we must add y x to the integral ; or 
 which is the same, in it must change x into x + 1 before we integrate. 
 The arbitrary constant is determined by finding the value of the sum y 
 when 
 
 x = 1. 
 
 We know therefore how to Jind the summing term of every series whose 
 general term is known in a rational and integer function of*. 
 
 Let 
 
 y x = Ax m Bx n +C 
 
 m :and n being positive and integer, and we have 
 
 A 2x m B 2x + C 2x
 
 ANALYTICAL GEOMETRY. xcix 
 
 for the sum of the terms as far as y x exclusively. This integral being 
 once found by equation D, we shall change x into x + 1, and determine 
 the constant agreeably. 
 For example, let 
 
 y,-== x(2x 1); 
 changing x into 2+1, and integrating the result, we shall find 
 
 3 _1_ Q ? L T _ 4 X 3 + 3 X* X 
 
 2.3 
 
 x + 1 4x 1 
 X '~2~ ~3~ 
 
 there being no constant, because when x = 0, the sum = 0. 
 The series 
 
 1m O m Q m 
 , G f 9 . . 
 
 of the m th powers of the natural numbers is found by taking 2 x m (equn- 
 tion D) ; but we must add afterwards the x th term which is x m ; that is to 
 say, it is sufficient to change x m , the second term of the equation 
 (D), into |- x m ; it then remains to determine the constant from the term 
 we commence from. 
 For example, to find 
 
 we find 2 x 8 , changing the sign of the second term, and we have 
 x x 2 x x + 1 2x + l 
 
 S -"3 + ~2 + ~6- X '~3~~ 'T~' 
 
 the constant is 0, because the sum is when x = 0. But if we wish to 
 find the sum 
 
 S' = (n + 1) 2 + (n + 2) 2 + . . . x J 
 S' = 0, whence x = n 1, and the constant is 
 
 n^ J 2n 1 
 
 """" n " 2 * 3 ' 
 
 which of course must be added to the former ; thus giving 
 S'= (n + 1)'+ (n + 2)'+...x 2 
 
 x + 1 2 x + 1 n 1 2 n 1 
 
 O O O O 
 
 O *w AW O 
 
 = I X {x.(x+l). (2x + 1) n.(n l)(2n 1) 
 
 = lx 2(x 3 n 3 ) + 3(x 2 + n') + x n}. 
 
 This theory applies to the summation ofjigttrate numbers, of the dif- 
 ferent orders : *
 
 c ; INTRODUCTION. 
 
 First order, 1.1.1.1.1. 1 . 1 , &c. 
 Second order, 1.2.3.4.5. 6 . 7 , &c. 
 Third order, 1.3. 6 .10.15. 21 . 28 , &c. 
 Fourth order, 1 . 4 . 10 . 20 . 35 . 56 . 84 , &c. 
 Fifth order, 1 . 5 . 15 . 35 . 70 . 126 . 210, &c. 
 
 and so on. 
 
 The law which every term follows being the sum of the one immediate 
 y over it added to the preceding one. The general terms are 
 
 First, 1 
 
 Second, x 
 
 ^ru:,.^ x. (x + 1) 
 
 J. 1111 11, 
 
 v (* -i- 
 Fniirfli ^ 
 
 1) (x + 2) 
 
 + P 2 
 
 &C. 
 n t* X.(X + 
 
 l)(x+2)...x 
 
 1.2.8...P 1 
 
 To sum the Pyramidal numbers, we nave 
 
 S = 1 + 4 + 10 + 20 + &c. 
 Now the general or x th term in this is 
 
 y x = -1 . x (x + 1) (x + 2). 
 But we find for the (x 1)> term of numbers of the next order 
 
 (x l)x(x + l)(x + 2); 
 
 finally changing x into x + 1, we have for the required form 
 S = Jj-x.(x + l)(x + 2)(x +3). 
 
 Since S = 1, when x = 1, we have 
 
 1 = 1 + constant, consequently 
 .. constant = 0. 
 
 Hence it appears that the sum of x terms of the fourth order, is the 
 x th term or general term of the fifth order, and vice versa ; and in like 
 manner, it may be shown that the x th term of the (n -f- l) th order is the 
 sum of x terms of the n th order. 
 
 Inverse Jigurate numbers are fractions which have 1 for the numerator, 
 and a figurate series for the denominator. Hence the x th term of the p th 
 order is 
 
 1.2.3...(p Jj_ 
 x(x + l)...x + p 2
 
 ANALYTICAL GEOMETRY. oi 
 
 and the integral of this is 
 
 c 1.2.S...(p 1) 
 
 Changing x into x + 1, then determining the constant by makin 
 x = 0, which gives the sum = 0, we shall have 
 
 -p_2' 
 and the sum of the x first terms of this general series is 
 
 P 1 1.2.3...'(p-l) 
 
 p 2 (p 2) (x + 1) (x + 2) . . . (x + p 2)' 
 In this formula make 
 
 p = 3, 4, 5 ... 
 and we shall get 
 
 l+'L+l ij. ! - 2 .2 2 
 
 1 " 3 " 6 " h 10 "* *x(x+l) 1 x+1 
 
 1. , V- 1 . 1 1.2.3 3 3 
 
 i ' A i 
 
 4 r 10 T 20 T "x(x + l)(x + 2) ~ 2 (x+l)(x+ 
 
 1 1 1 J 1.2.3.4 4 2.4 
 
 TiT + + TFT + o?, 
 
 5 T 10 T 35 T " x (x+ 1) (x+2) (x + 3) ~~ 3 
 1.1.1 _!_ 1.2.3.4.5 5 2.3.5 
 
 ~~ K.R i * 
 
 1 "" 6 ^ 21" 1 " 56" 1 "x(x+l)...(x + 4) ~ 4 (x+1) . . . (x + 4) 
 and so on. To obtain the whole sum of these series continued to infinity, 
 we must make 
 
 X = 00 
 
 which gives for the sum required the general value 
 
 p 2 
 
 which in the above particular cases, becomes 
 
 245 
 
 l'~2' 3'4' &C * 
 
 To sum the series 
 
 sin. a + sin. (a + h) + sin. (a + 2 h) + . . . sin. (a + x 1 h) 
 
 we have 
 
 / h\ 
 cos. ( a + h x p j 
 
 2 sin. (a + x h) = C r 
 
 2sin.- 
 
 cfoanging x into x + 1, and determining Cby the condition that x = ) 
 makes the sum = zero, we find for the summing-term. 
 
 cos. (a ^ cos. (a + h x + 
 
 2 
 
 2 sin.i
 
 cii INTRODUCTION, 
 
 or 
 
 sin. 
 
 / h x. h (x + 1) 
 iin. f a + TT x ) sln< ^ 
 
 * *6 J 
 
 ~ F 
 
 sin. -r- 
 
 In a similar manner, if we wish to sum the series 
 
 cos. a + cos. (a -f- h) -f- cos. (a -f 2 h) + . . . cos. (a + x 1 
 we easily find the summing-term to be 
 
 sin. (a -} sin. (a 4- h x + ^ ) 
 
 2sin. T 
 
 or 
 
 .A^-;X X +D 
 
 h
 
 A COMMENTARY 
 
 ON 
 
 N E W T O N'S P R I N C I P I A. 
 
 SUPPLEMENT 
 
 TO 
 
 SECTION XL 
 
 460 PROP. LVII, depends upon Cor. 4 to the Laws of Motion, 
 which is 
 
 If any number of bodies mutually attract each other,, their center of gra- 
 vity will either remain at rest or will move uniformly in a straight line. 
 . First let us prove this for two bodies. 
 Let them be referred to a fixed point by the rectangular coordinates 
 
 ^ x, y ; x', y', 
 
 and let their masses be 
 
 (ly III. 
 
 Also let their distance be f, and f (g) denote the law according to which 
 they attract each other. 
 Then 
 
 will be their respective actions, and resolving these parallel to the axes of 
 abscissas and ordinates, we have (46) 
 d 2 x 
 
 dj 
 d 
 
 VOL. II.
 
 A COMMENTARY ON [SECT. XI. 
 
 d'x' 
 
 t 
 
 Hence multiplying equations (1) by /. and those marked (2) by /' and 
 adding, &c. we get 
 
 A*d*x + /d'x' 
 
 dt ! 
 and 
 
 dt 2 
 and integrating 
 
 d x , d x' 
 
 = 0, 
 
 _ 
 
 Now if the coordinates of the center of gravity be denoted by 
 
 x, y, 
 
 we have by Statics 
 
 _ /* x + fj x 
 
 / + I*' 
 
 y = , pt- 
 
 d x _ 1 ( d x , , d x' 
 and 
 
 di ~>4V v" dt dt; - ^rV* 
 
 But 
 
 d_x d y 
 dt' dt 
 
 represent the velocity of the center of gravity resolved parallel to the axes 
 of coordinates, and these resolved parts have been shown to be constant 
 Hence it easily appears by composition of motion, that the actual velocity 
 of the center of gravity is uniform, and also that it moves in a straight 
 line, viz. in that produced which is the diagonal of the rectangular par- 
 allelogram whose two sides are d x, d y. 
 If 
 
 c = 0, c' = 
 then the center of gravity remains quiescent
 
 BOOK l.J NEWTON'S PRINCIPIA. 3 
 
 461 The general proposition is similarly demonstrated, thus. 
 Let the bodies whose masses 
 
 //' n'f i,'l f &rr- 
 ft, [A 9 fi, , &C. 
 
 be referred to three rectangular axes, issuing from a fixed point by the 
 coordinates 
 
 *', y, z' 
 
 x", y", 2." 
 x"', y'", z'" 
 
 &c. 
 Also let 
 
 f !, 2 be the distance of /*', p" 
 
 1,3 ........ ft> 
 
 .// /// 
 
 } ft 
 
 &C. &C, 
 
 and suppose the law of attraction to be denoted by 
 
 f - (&,) f (fl,3)> f (fe.s) &C. 
 
 Now resolving the attractions or forces 
 
 &c. 
 parallel to the axes, and collecting the parts we get 
 
 1, 2 PI, 3 
 
 d 2 x " = x / x // x // . 
 
 d t 2 *" 2 Si 2 i 
 
 d Z x /// / -./// x // _ 
 
 &C. 
 
 ?1, 3 3 
 
 &C. = &C. 
 
 Hence multiplying the first of the above equations by [if, the second by 
 ", and so on, and adding, we get 
 
 A* / d g x / + ft"d* x" + ^t /// d 8 x /// + &c. _ 
 
 ~dT^~ 
 
 Again, since it is a matter of perfect indifference whether we collect the 
 forces parallel to the other axes or this ; or since all the circumstances are 
 similar with regard to these independent axes, the results arising from 
 similar operations must be similar, and we therefore have also 
 
 (if d 2 z' + ft" d 2 z" + u!" d 2 T!" + &c. _ 
 
 dt 2 
 
 A2
 
 A COMMENTARY ON [SECT. XI. 
 
 Hence by integration 
 
 d x' d x" d x"' 
 
 . II A ., V.I A. ... 11 A 
 
 d v' d 
 
 '^r + ""iiT-dt 
 
 l / l ~" I" 
 
 n II 
 
 t &c ' = c ' 
 
 , y, z denoting the coordinates of the 
 we have 
 
 * ZTt -dT -d 
 
 But x, y, z denoting the coordinates of the center of gravity, by statics 
 
 - _t*'x? + I*" x" + yJ" x'" + &c. 
 tf + 0," + n'" + & c . 
 
 _ ijfy' + y." y" + ^"y"' + &c. 
 ^ + tfi + yj" + & c . 
 
 _ fify! + fi/'yf' + (*'"%'" + &c. 
 
 ^ + X' + ^" + &c. 
 
 and hence by taking the differentials, &c. we get 
 dx c 
 
 d t - nf + p 
 
 a y 
 
 " + f 
 
 + &C. 
 
 d t I*' + fj. 
 
 // 1 It/ ft 
 
 tf 
 
 + &C. 
 
 _ 
 
 d t "/*'+**" + a" 7 + &c. 
 
 that is, the velocity of the center of gravity resolved parallel to any three 
 rectangular axes is constant. Hence by composition of motion the actual 
 velocity of the center of gravity is constant and uniform, and it easily ap- 
 pears also that its path is a straight line, scil. the diagonal of the rectan- 
 gular parallelepiped whose sides are d x, d y, d z. 
 
 462. We will now give another demonstration of Prop. LXI. or that 
 Of two bodies the motion of each about the center of gravity, is the same 
 as if that center 'was the center of force, and the law of force the same as 
 that of their mutual attractions. 
 
 Supposing the coordinates of the two bodies referred to the center of 
 gravity to be 
 
 x /j y/ x // y// 
 
 we have 
 
 Hence since 
 
 X = 
 
 y = 
 
 X + > 
 
 y + : 
 
 ;;} 
 
 x' 
 
 y' 
 
 = y 
 
 dx 
 dl' 
 
 dy 
 dt 
 
 

 
 BOOK I.] NEWTON'S PRINCIPIA. 
 
 are constant as it has been shown, and therefore 
 
 d 2 X d 
 
 = 0, -rr = 
 
 dt 2 ' dt 2 
 
 we have 
 
 d'x 
 dt 2 
 
 _ 
 
 dt 2 ' 
 But by the property of the center of gravity 
 
 ft 
 being the distance of (if from the center of gravity. We also have 
 
 x x x 
 
 Hence by substitution the equations become 
 
 ill//- a 
 dt 2 * 
 
 Similarly we should find 
 
 d z x y _ /f /^ + ^,\x/ 
 dT 2 ~ ^ H ^' V 8 
 and 
 
 <*'y,_. /^ + g' 
 "" 
 
 Hence if the force represented by 
 
 were placed in the center of gravity, it would cause /// to move about it as 
 a fixed point; and if 
 
 were there residing, it would cause / to centripetate in h'ke manner. 
 Moreover if 
 
 A 3
 
 6 A COMMENTARY ON [SECT. XI 
 
 then these forces vary as 
 3', 3; 
 so that the law of force &c. &c. 
 
 ANOTHER PROOF OF PROP. LX1I. 
 
 463. Let /*, (i! denote the two bodies. Then since ^ has no motion 
 round G (G being the center of gravity), it will descend in a straight 
 line to G. In like manner y! will fall to G in a straight line. 
 
 Also since the accelerating forces on n, p are inversely as /*, /' or 
 directly as G /w, G p, the velocities will follow the same law and corre- 
 sponding portions of G ^, G p! will be described in the same times ; that 
 is, the whole will be described in the same time. Moreover after they 
 meet at G, the bodies will go on together with the same constant velocity 
 with which G moved before they met. 
 
 Since here 
 
 f w = p- 
 
 u will move towards G as if a force 
 
 or 
 
 <*" 
 
 2* 
 
 Hence by the usual methods it will be found that if a be the distance 
 at which /* begins to fall, the time to G is 
 
 ((* + /*') a 2 9 
 and if a 7 be the original distance of/*', the time is 
 
 /* 
 
 '2V2' 
 
 But 
 
 a : a' : : //,' : /* 
 
 therefore these times are equal, which has just been otherwise shown.
 
 BOOK I.] NEWTON'S PRINCIPIA. 7 
 
 ANOTHER PROOF OF PROP. LXIH. 
 
 464. We know from (461) that the center of gravity moves uniformly 
 in a straight line; and that (Prop. LVII,) p and (i! will describe about G 
 similar figures, /. moving as though actuated by the force 
 
 0* + /')'** 
 and Q as if by 
 
 ,<^ \ 
 
 GST-tV) 8 '*"' 
 
 Hence the curves described will be similar ellipses, with the center of 
 force G in the focus. Also if we knew the original velocities of p and pf 
 about G, the ellipse would easily be determined. 
 
 The velocities of p and (*/ at any time are composed of two velocities, 
 viz. the progressive one of the center of gravity and that of each round G. 
 Hence having given the "whole original velocities required to find the separate 
 parts of them, 
 
 is a problem which we will now resolve. 
 Let 
 
 V, V 
 
 be the original velocities of ^, /*.' , and suppose their directions to make 
 with the straight line /- p' the angles 
 
 a, of. 
 Also let the velocity of the center of gravity be 
 
 v 
 and the direction of its motion to make with p {i! the angle 
 
 . 
 Moreover let 
 
 v,V 
 
 be the velocities of p, fjf around G and the common inclination of their 
 directions to be 
 
 6. 
 Now V resolved parallel to & y! is 
 
 V cos. . 
 But since it is composed of v and of v it will also be 
 
 v cos. -f- v cos. & 
 .'. V cos. a = v cos. a -f- v cos. 0. 
 In like manner we get 
 
 V sin. a = v sin. a -f v sin. 6. 
 At
 
 8 A COMMENTARY ON [SECT. XI. 
 
 and also 
 
 V cos. a! = v cos. a v' cos, 6 
 
 V sin. a! = v sin. a v' sin. 0. 
 
 Hence multiplying by p, p', adding and putting 
 
 fJt, V = fJ/ v' 
 
 we get 
 
 /j, V cos. a 
 and 
 
 /A V sin. a + ft,' V sin. a' = (p -f ^/) 
 
 Squaring these and adding them, we get 
 
 ^'V 2 + X 8 V' + 2/i/ VV'cos. (a a') = 
 
 \ / 
 
 which gives 
 v 
 
 COS. a + fj,' V' COS. a' = (fjt, -\- /*') v COS. a -\ 
 sin. a + /A' V' sin. a' = (^ -j- ///) v sin. a J 
 
 / + X 
 By division we also have 
 
 ^ V sin. a + fjf V sin. a' 
 tan. a = ?T - ; rrf/ - / 
 /* V cos. a -f- ^ V cos. a 
 
 Again, from the first four equations by subtraction we also have 
 
 V cos. a V cos. a' = (v + v') cos. 6 = v . r cos. 
 
 ^ 
 
 (b -f" /U>' 
 
 V sin. a V' sin. a' = (v + v') sin. 6 =r v . ^ sin. 6 
 
 ^ 
 
 and adding the squares of these 
 
 V + V /a 2 VV'cos. (a aO= v 
 whence 
 
 V' 2 SVV'cos. (a ') 
 V /2 SVV'cos. a a' 
 
 A* + ^ 
 and by division 
 
 V sin. a V' sin. ' 
 tan. 6 VT - --ff} - / 
 V cos. a V cos. a! 
 
 Whence are known the velocity and direction of projection of /* about 
 G and (by Sect. III. or Com.) the conic section can therefore be found ; 
 and combining the motion in this orbit with that of the center of gravity, 
 which is given above, we have also that of/*. 
 
 465. Hence since the orbit of /* round /n/ is similar to the orbit of 
 round G, if A be the semi-axis of the ellipse which /A describes round
 
 BOOK I.] NEWTON'S PRINCIPIA. 9 
 
 G, and a that of the ellipse which it describes relatively to /*' which is also 
 in motion; we shall have 
 
 A : a : : &' : & + ^'. 
 
 466. Hence also since an ellipse whose serai-axis is A, is described by 
 the force 
 
 (A* + i*')* x a 2 
 
 we shall have (309) the periodic time, viz. 
 
 T~ .TV V A If 1 
 : ri = 
 
 2 it 
 
 ~ V (i* + iff) ' 
 
 467. Hence we easily get Prop. LIX. 
 
 For if i* were to revolve round /*' at rest, its semi-axis would be a, and 
 periodic time 
 
 T , _ _ 
 V n r 
 
 .-. T : T' :: V tf : V(i*+n'). 
 
 468. PROP. LX is also hence deducible. For if p revolve round A/ a* 
 rest, in an ellipse whose semi-axis is a', we have 
 
 and equating this with T in order to give it the same time about ft' at rest 
 as about /*' in motion, we have 
 
 7 1 f 
 
 V p' ~ V (A* + pf) ' 
 
 .-. a : a' : : (A& + &') * : n' $ . 
 
 ANOTHER PROOF OF PROP. LXIV. 
 
 469. Required the motions of the bodies whose masses are 
 
 & v', /", A*'", &c. 
 
 and which mutually attract each other with forces varying directly as the 
 distance. 
 
 Let the distance of any two of them as /^, /,', be g ; then the force of </ 
 on p is
 
 10 A COMMENTARY ON [SECT. XI. 
 
 and the part resolved parallel to x is 
 
 Xf.*-^ = ^'(x x'). 
 
 In like manner the force of p" on /-, resolved parallel to x, is 
 
 p" (x x") 
 
 and so on for the rest of the bodies and for their respective forces resolved 
 parallel to the other axes of coordinates. 
 
 Hence 
 
 1L^ 2 = pf (x x') + p," (x x") + &c. 
 ^ = / (x' - x) + /' (x' x") + &c. 
 
 = f. (X" - X) + p' (X" - X') + &C. 
 
 &c. = &c. 
 which give 
 
 |i - ( + X + "+ &c.) x (^x + tt x' + &c.) 
 = (^ + ^ + ^" + &c.) x' (p x + y x' + &c.) 
 
 8 - = (^ + ^ + X' + &c.) x'' (^ x + ^ x' + &c.) 
 
 &c. = &c. 
 Or since 
 
 I* X + /(*' X' + &C. = ((* + lif + &CC.) X 
 
 making the coordinates of the center of gravity 
 
 x, y, z", 
 
 we have 
 
 &c. = &c. 
 In like manner, we easily get 
 
 f = (p + p' + &c.)(y y)
 
 BOOK I.] NEWTON'S PRINCIPIA. 11 
 
 &c. = &c. 
 and also 
 
 = G* + p' + &c.) (z _ 
 
 &c. = &c. 
 
 Again, 
 
 x x, y y , z z 
 x 7 x, y' y, z' z 
 
 &c. &c. &c. 
 
 are the coordinates of /tt, ///, /*", &c. when measured from the center of 
 gravity, and it has been shown already that 
 
 d g (x - x) _d g x 
 d t 2 = d t 2 
 
 dt 2 " dt* 
 
 d 2 (z z) _ d'z 
 
 d t 1 = d t 2 
 
 and so on for the other bodies. Hence then it appears, that the motions 
 of the bodies about the center of gravity, are the same as if there were but 
 one force, scil. 
 
 (^ -f. (i! -f. &c.) X distance 
 and as if this force were placed in the center of gravity. 
 
 Hence the bodies will all describe ellipses about the center of gravity, 
 as a center ; and their periodic times will all be the same. But their 
 magnitudes, excentricities, the positions of the planes of their orbits, and 
 of the major axes, may be of all varieties. 
 
 Moreover the motion of any one body relative to any other, will be 
 governed by the same laws as the motion of a body relative to a center 
 of force, which force varies directly as the distance ; for if we take the 
 equations 
 
 d 2 x - 
 
 - = (fj, + (*' + &c.) (x x)
 
 12 A COMMENTARY ON [SECT. XL 
 
 and subtract them we get. 
 
 ^ ^ = (ft + (*' + &C.) (X X') 
 
 and similarly 
 
 -j"[7 y = (** + /*' + &c.) (y y') 
 and 
 
 Hence by composition and the general expression for force (-. \\ it 
 
 readily appears that the motion of i* about &', is such as was asserted. 
 
 470. Thus far relates merely to the motions of two bodies ; and these 
 can be accurately determined. But the operations of Nature are on a 
 grander scale, and she presents us with Systems composed of Three, and 
 even more bodies, mutually attracting each other. In these cases the 
 equations of motion cannot be integrated by any methods hitherto dis- 
 covered, and we must therefore have recourse to methods of approxi- 
 mation. 
 
 In this portion of our labours we shall endeavour to lay before the 
 reader such an exposition of the Lunar, Planetary and Cometary Theories, 
 as may afford him a complete succedaneum to the discoveries of our 
 author. 
 
 471. Since relative motions are such only as can be observed, we refer 
 the motions of the Planets and Comets, to the center of the sun, and the 
 motions of the Satellites to the center of their planets. Thus to compare 
 theory with observations, 
 
 It is required to determine the relative motion of a system of bodies, about 
 a body considered as the center of their motions. 
 
 Let M be this last body, (*, (*<', p", &c. being the other bodies of which 
 is required the relative motion about M. Also let 
 
 C,n, 7 
 be the rectangular coordinates of M ; 
 
 + x, n + y, 7 + z; 
 + x'n + y', 7 +z'; 
 
 &c. 
 
 those of /w-, (*>', &c. Then it is evident that 
 
 x, y, z; 
 
 x', y', z' 
 
 &c.
 
 BOOK I.] NEWTON'S PRINCIPIA. 13 
 
 will be the coordinates of M, ft', &c. referred to M. 
 
 Call ft f, &c. 
 
 the distances of ^, /a-', &c. from M; then we have 
 j = V(x 2 + y 2 + z 2 ) 
 ' = V (x' 2 + y' 2 -f- z' 2 ) 
 ft ^, &c. being the diagonals of rectangular parallelepipeds, whose sides 
 
 are 
 
 x, y, z 
 
 x', y', z' 
 
 &c. 
 Now the actions of /*, /', /A", &c. upon M are 
 
 and these resolved parallel to the axis of x, are 
 
 ft x /*' x' /*" x v 
 TF > -TT > "7/T" &c - 
 
 5 5 
 
 Therefore to determine , we have 
 
 s 3 
 
 the symbol 2 denoting the sum of such expressions. 
 In like manner to determine n, y we have 
 
 dt 
 
 dt 2 ' * g 3 ' 
 
 The action of M upon ft, resolved parallel to the axis of x, and in the 
 contrary direction, is 
 
 MX 
 
 " 3 ' 
 
 Also the actions of ft', ft", &c. upon ft resolved parallel to the axis of x 
 are, in like manner, 
 
 ft' (x' x) ft" (x" x) ft,'" (x'" x) 
 
 73 J 7s J 3 > <* C> 
 
 0,1 0,2 0,3 
 
 fn.ra generally denoting the distance between ft'" n and ft'" m 
 But 
 
 fo>2 = V (x"-x)* + (y'-y) 2 + (z" ) 
 &c. = &c.
 
 H A COMMENTARY ON [SECT. XI. 
 
 Sl>z =V (x" x') * + (y" - y') * + (2" z') 
 and so on. 
 
 Hence if we assume 
 
 ' " 
 
 &c. 
 
 flu 0,2 
 
 ft, 8 ft,S 
 
 &C. 
 
 and taking the Partial Difference upon the supposition that x is the only 
 variable, we have 
 
 ^ x'-x ."(x"-x) 
 
 ' 
 
 the parenthesis ( ) denoting the Partial Difference. Hence the sum of 
 all the actions of (*', /*", &c. on /* is 
 
 tf5/' 
 
 Hence then the whole action upon p parallel to x is 
 
 d. 2 ( + x) _ 1 /d X% MX 
 d t 2 = V VcTx/ "T 3 " 
 But 
 
 dt 2 g 3 
 
 d2x J_ f d x > _Mj?_-s^L? m 
 
 *'dT 2 " A* VdxJ e 3 e 3 ( j 
 
 Similarly, we have 
 
 dt 2 ' p, 
 
 d^z _ J_ /d^-x _M^_ V ^z , 3] 
 
 d t 2 " " /t* >d z/ ^ 3 " g 3 
 
 If we change successively in the equations (]), (2), (3) the quantities 
 /A, x, y, z into 
 
 (* t x > y 5 z 
 
 (*)*) y ) A > 
 
 &c. 
 
 and reciprocally ; we shall have all the equations of motion of the bodies 
 (U/, p", &c. round M.
 
 BOOK I.] NEWTON'S PRINCIP1A. 15 
 
 If we multiply the equations involving by M + 2. ^ ; that in x, by 
 . : that in x', by /<*', and so on ; and add them together, we shall have 
 
 But since 
 
 /dXx _ H6 /// (x 7 x) 
 \dx/ 3 
 
 and so on in pairs, it will easily appear that 
 
 d 2 
 
 ( M + s -")-d 
 
 whence by integrating we get 
 
 and again integrating 
 
 a arid b being arbitrary constants. 
 Similarly, it is found that 
 
 These three equations, therefore, give the absolute motion of M in 
 space, when the relative motions around it of p, pf, ft", &c. are known. 
 Again, if we multiply the equations in x and y by 
 
 and 
 
 2. /(A X 
 
 in like manner the equations in x x and y' by 
 
 '
 
 16 A COMMENTARY ON [Sacr. XL 
 
 and 
 
 r* * ji* <= * ; 
 
 M + 2 . j* ' 
 and so on. 
 
 And if we add all these results together, observing that from the nature 
 of X, (which is easily shown) 
 
 and that (as we already know) 
 
 -9 x d g y y d* x 2. At x d 2 y 
 
 "37*"" = M + 2^' 2 ^'dT 2 
 
 2. Aty ' d g x 
 * ; ' |tt * 
 
 "M + 
 and integrating, since 
 
 /(xd 2 y yd 2 x) =/xd 2 y /yd 2 x 
 = x d y /d x d y (y d x /d x d y) 
 = xdy ydx, 
 we have 
 
 xdy ydx 2./t*x dy 
 
 2 P - j + - = const - + - - 
 
 dt 
 
 v 
 
 M + s.p" 'dt 
 Hence 
 
 , , , xdy ydx xdy ydx, dx 
 
 c = M.2./t*. J J ^-2.^X2^. J , J H3.PjX2.P-f- 
 
 Q t Q I Cl t 
 
 dv 
 
 c being an arbitrary constant. 
 
 In the same mannei ;. e arrive at these two integrals, 
 
 c"=M. ,.^. 
 
 c' and c" being two other arbitrary constants.
 
 BOOK, I.] NEWTON'S PRINCIPIA. 17 
 
 Again, if we multiply the equation in x by 
 
 2/ i dx-2/,^^ d - X - ; 
 M + 2. ^ 
 the equation in y by 
 
 the equation in z by 
 
 2 . /a, d Z 
 
 2 At d z 2/4. 
 
 M + 2. ^ 
 if in like manner we multiply the equations in x', y 7 , z' by 
 
 2 "' dx '- 2 *'-irTO 
 
 ~+~ 2 . ~P 
 
 ' M + 2./^' 
 
 respectively, and so on tor the rest ; and add the several results, observ- 
 ing that 
 
 we get 
 
 dxd g x + dd g + dzd z z 22. 
 
 dt 2 M + 2^" ' dt 
 
 2 2./^dy ^d 2 y 22. ^ d z >d g z 
 h M + 2^* dt 2 h M + 2 ^ ' dt 2 
 
 2M. S.^Al + 2dX; 
 
 and integrating, we have 
 
 dx 2 + dy 2 +dz . (2. / adx) 2 + ( 
 
 2 " 
 
 + 2 M 2 + 2 X, 
 
 f 
 
 which gives 
 
 h-M ^ ^ 
 
 dt 2 ' '!\ dt 2 / 
 
 J2M 2. + 2>.j (M + 2^) .......... (7) 
 
 h being an arbitrary constant. 
 
 VOL. II.
 
 18 A COMMENTARY ON [SECT. XI. 
 
 These integrals being the only ones attainable by the present state of 
 analysis, we are obliged to have recourse to Methods of Approximation, 
 and for this object to take advantage of the facilities afforded us by the 
 constitution of the system of the World. One of the principal of these 
 is due to the fact, that the Solar System is composed of Partial Systems, 
 formed by the Planets and their Satellites : which systems are such, that 
 the distances of the Satellites from their Planet, are small in comparison 
 with the distance of the Planet from the Sun : whence it results, that the 
 action of the Sun being nearly the same upon the Planet as upon its Satel- 
 lites, these latter move nearly the same as if they obeyed no other action 
 than that of the Planet. Hence we have this remarkable property, 
 namely, 
 
 472. The motion of the Center of Gravity of a Planet and its Satellites, 
 is very nearly the same as if all the bodies formed one in that Center. 
 
 Let the mutual distances of the bodies /., /*', /*", &c. be very small 
 compared with that of their center of gravity from the body M. Let 
 also 
 
 x = x + x, ; y = f -f y /; z = z + z,. 
 
 x' = x~ -f. x/; y' = y + y/; z' = "z" + z/; 
 
 &c. 
 
 x, y, z being the coordinates of the center of gravity of the system of 
 bodies p, p', At", &c. ; the origin of these and of the coordinates x, y, z ; 
 x', y 7 , z', &c. being at the center of M. It is evident that x /5 y /5 z, ; 
 x/, y/, z/, &c. are the coordinates of /,, ,u/, &c. relatively to their center of 
 gravity ; we will suppose these, compared with x, y, z, as small quanti- 
 ties of the first order. This being done, we shall have, as we know by 
 Mechanics, the force which sollicits the center of gravity of the system paral- 
 lel to any straight line, by taking the sum of the forces which act upon the 
 bodies parallel to the given straight line, multiplied respectively by their 
 masses, and by dividing this sum by the sum of the masses. We also 
 know (by Mech.) that the mutual action of the bodies upon one another, 
 does not alter the motion of the center of gravity o the system ; nor does 
 their mutual attraction. It is sufficient, therefore, in estimating the forces 
 which animate the center of gravity of a system, merely to regard the 
 action of the body M which forms no part of the system. 
 The action of M upon /A, resolved parallel to the axis of x ie 
 MX 
 S 3 "'
 
 BOOK I.] NEWTON'S PRINCIPIA. 
 
 19 
 
 the whole force which sollicits the center of gravity parallel to this straight 
 line is, therefore, 
 
 M.I.*? 
 
 Substituting for x and g their values 
 
 x = _ J^ + x, 
 
 {(x + X/ ) 2 + (y + y,jf+- (z + z/ ) }* ' 
 
 If we neglect small quantities of the second order, scil. the squares and 
 products of 
 
 x /> y/ z/ J x/, y/, z/ ; &c. 
 and put 
 
 7 = V (x~ 2 + y* + z" 2 ) 
 
 the distance of the center of gravity from M, we have 
 ^ = J + 2*. 3x(x X/ + "y y/ + zz,) 
 
 e s 3 I 3 s 3 
 
 for omitting x ! , y * &c., w have 
 
 p = (x + x y ) X {(j) 2 + 2 (x x, + y y, + z z,)} 3 nearly 
 
 = (x+x,) X {(7) ~ 3 3 (7) ~ 5 (x x, + y y, + z"zj nearly 
 
 x + x, 3 x - - . , 
 
 = sr 1 = . (x x. + y y, + z z, ) nearly. 
 
 (f) 3 (f) 5 
 
 Again, marking successively the letters x /5 y /5 z /? with one, two, three, 
 &c. dashes or accents, we shall have the values of 
 
 -" ^cc 
 But from the nature of the center of gravity 
 
 we shall therefore have 
 
 M - 2 -7? M" 
 
 -- 1 
 
 = -= nearly. 
 
 3 
 
 Thus the center of gravity of the system is sollicited parallel to the 
 axis of x, by the action of the body M, very nearly as if all the bodies of 
 the system were collected into one at the center. The same result evi- 
 dently takes place relatively to the axes of y and z ; so that the forces, by 
 
 B2
 
 20 A COMMENTARY ON [SECT. XL 
 
 which the center of gravity of the system is animated parallel to these 
 axes, by the action of M, are respectively 
 
 6)' 
 When we consider the relative motion of the center of gravity of the 
 
 system about M, the direction of the force which sollicits M must be 
 changed. This force resulting from the action of /*, p, &c. upon M, and 
 resolved parallel to x, in the contrary direction from the origin, is 
 
 if we neglect small quantities of the second order, this function becomes, 
 after what has *been shown, equal to 
 
 1* 
 
 In like manner, the forces by which M is actuated arising from the 
 system, parallel to the axes of y, and of z, in the contrary direction, are 
 
 It is thus perceptible, that the action of the system upon the body M, 
 is very nearly the same as if all the bodies were collected at their common 
 center of gravity. Transferring to this center, and with a contrary sign, 
 the three preceding forces; this point will be sollicited parallel to the 
 axes of x, y and z, in its relative motion about M, by the three following 
 forces, scil. 
 
 These forces are the same as if all the bodies #, /', p M 9 &c. were col- 
 lected at their common center of gravity ; "which, center, therefore, moves 
 nearly (to small quantities of the second order} as if all the bodies were col- 
 lected at that center. 
 
 Hence it follows, that if there are many systems, whose centers of gra- 
 vity are very distant from each other, relatively to the respective distances 
 of the bodies of each system ; these centers will be moved very nearly, as 
 if the bodies of each system were there collected ; for the action of the 
 first system upon each body of the second system, is the same very nearly 
 as if the bodies of the first system were collected at their common center 
 of gravity ; the action of the first system upon the center of gravity of the 
 second, will be therefore, by what has preceded, the same as on this hy- 
 pothesis ; whence we may conclude generally that the reciprocal action of
 
 BOOK I.] NEWTON'S PRINCIPIA. 21 
 
 different systems upon their respective centers of gravity t is the same as if all 
 the bodies of each system were there collected, and also that these centers 
 move as on that supposition. 
 
 It is clear that this result subsists equally, whether the bodies of each 
 system be free, or connected together in any way whatever ; for their mu- 
 tual action has no influence upon the motion of their common center 
 of gravity. 
 
 The system of a planet acts, therefore, upon the other bodies of the 
 Solar system, very nearly the same as if the Planet and its Satellites, 
 were collected at their common center of gravity ; and this center itself is 
 attracted by the different bodies of the Solar system, as it would be on 
 that hypothesis. 
 
 Having given the equations of motion of a system of bodies submitted 
 to their mutual attraction, it remains to integrate them by successive 
 approximations. In the solar system, the celestial bodies move nearly as 
 if they obeyed only the principal force which actuates them, and the per- 
 turbing forces are inconsiderable ; we may, therefore, in a first approxi- 
 mation consider only the mutual action of two bodies, scil. that of a planet 
 or of a comet and of the sun, in the theory of planets and comets ; and 
 the mutual action of a satellite and of its planet, in the theory of satellites. 
 We shall begin by giving a rigorous determination of the motion of two 
 attracting bodies : this first approximation will conduct us to a second in 
 which we shall include the first powers of small quantities or the perturb- 
 ing forces ; next we shall consider the squares and products of these 
 forces; and continuing the process, we shall determine the motions of the 
 heavenly bodies with all the accuracy that observations will admit of. 
 
 FIRST APPROXIMATION. 
 
 473. We know already that a body attracted towards a fixed point, 
 by a force varying reciprocally as the square of the distance, de- 
 scribes a conic section ; or in the relative motion of the body ,"-, round 
 M, this latter body being considered as fixed, we must transfer in a di- 
 rection contrary to that of p, the action of p upon M ; so that in this re- 
 lative motion, i* is sollicited towards M, by a force equal to the sum ol 
 the masses M, and /* divided by the square of their distance. All this 
 has been ascertained already. But the importance of the subject in the 
 Theory of the system of the world, will be a sufficient excuse for repre- 
 senting it under another form, 
 
 B3
 
 A COMMENTARY ON [SECT. XI. 
 
 First transform the variables x, y, z into others more 'commodious for 
 astronomical purposes. being the distance of the centers of p and M, 
 call (v) the angle which the projection of f upon the plane of x, y makes 
 with the axis of x; and (6) the inclination of g to the same plane; we 
 shall have 
 
 x = cos. 6 cos. v ; *\ 
 
 y = f cos. 6 sin. v; > ........ (1) 
 
 z = g sin. 6. ) 
 
 Next putting 
 
 M + g _ /^(xx' + yy'+zzQ X 
 
 Q -~ T 2 -- -71- -+ 
 
 we have 
 
 J' - - 1 /u A\ 
 
 ~~\TZ)~ 
 
 I /d_X\ M Atx 
 
 # ^d x/ t 3 p 3 
 
 Similarly 
 
 Q\ = J-f^_ M _2. 
 
 P 
 
 d_Qv 1 /d ^x M_ ftz 
 dz) " fjt,\d z) ~ g 3 ' g 3 * 
 Hence equations (1), (2), (3) of number 471, become 
 d*x /dQ. dy /dQ, d'z _ /d 
 " 
 
 dt s 
 
 Now multiplying the first of these equations by cos. 6. cos. v ; the 
 second by cos. 6. sin. v ; the third by sin. 6, we get, by adding them 
 
 In like manner, multiplying the first of the above equations by f cos .0 X 
 sin. v; the second by ? cos. 6 cos, \ and adding them, &c. we have 
 
 1 /^ * dv 
 dip 2 -j cos. 
 
 b i d t 
 
 And lastly multiplying the first by f sin. 6. cos. v ; the second by 
 f sin. 6. cos. v and adding them to the third multiplied by cos. 6. we 
 have 
 
 . d*<J .dv 2 . 2d f d* 
 
 To render the equations (2), (3), (4), still better adapted for use, let 
 
 1 
 
 u = - 7 
 g COS.
 
 BOOK I.] NEWTON'S PRINCIPIA. 23 
 
 and 
 
 s = tan. 6 
 
 u being unity divided by the projection of the radius f upon the plane 
 of x, y ; and s the tangent of the latitude of ^ from that same plane. 
 If we multiply equation (3) by g* d v cos. 8 d and integrate, we get 
 
 u 2 d 
 
 h being the arbitrary constant. 
 Hence 
 
 d t = 
 
 v u 
 
 , 
 d v 
 
 (5) 
 
 If we add equation (2) multiplied by cos. 6 to equation (4) multi- 
 
 plied by - , we shall have 
 
 whence 
 
 Substituting for d t, its foregoing value, and making d v constant, we 
 shall have 
 
 /d Q\ d u /d Q\ s /d Q\ 
 
 = d'v" 2 "*~ U "^ - ft \ Ov i\ v .... (6) 
 
 In the same way making d v constant, equation (4) will become 
 
 
 
 ' 
 
 Now making M + /- = m, we have (in this case) 
 ~ m m u 
 
 Q= 7' = v(i+s 2 ) 
 
 and the equations (5), (6), (7) will become 
 
 dt = 
 
 dv 
 h.u 
 
 3T 1 
 
 d 2 s 
 dv 2 
 
 2 ' 
 
 h 2 (l 
 
 (8)
 
 24 A COMMENTARY ON [SECT. XI. 
 
 (These equations may be more simply deduced directly 124 and Wood- 
 house's Phys. Astron.) 
 
 The area described during the element of time d t, by the projection 
 
 d v 
 
 of the radius-vector is - z ; the first of equations (8) show that this area 
 
 is proportional to that element, and also that in a finite time it is propor- 
 tional to the time. 
 
 Moreover integrating the last of them (by 122) or by multiplying by 
 2 d s, we get 
 
 s = 7 sin. (v 6) (9) 
 
 7 and 6 being two arbitrary constants. 
 
 Finally, the second equation gives by integration 
 
 u = h(l+V) * VTTF " 2 + *(*)! = ^!-^; . . . (10) 
 e and -a being two new arbitraries. 
 
 Substituting for s in this expression, its value in terms of v, and then 
 this expression in the equation 
 
 d v 
 
 d t = r ; ; 
 h u^ 
 
 the integral of this equation will give t in terms of v ; thus we shall have 
 v, u and s in functions of the time. 
 
 This process may be considerably simplified, by observing that the 
 value of s indicates the orbit to lie wholly in one plane, the tangent of 
 whose inclination to a fixed plane is 7, the longitude of the node 6 being 
 reckoned from the origin of the angle v. In referring, therefore, to this 
 plane the motion of p ; we shall have 
 
 s = and 7 = 0, 
 which give 
 
 1 u, 
 
 u = ~ : = p {I + e cos. (v )}. 
 
 This equation is that of an ellipse in which the origin of g is at the 
 focus : 
 
 h* 
 
 P(l e) 
 
 is the semi-axis-major which we shall designate by a ; e is the ratio of 
 the excentricity to the semi-axis-major ; and lastly is the longitude of 
 the perihelion. The equation 
 
 d v 
 
 d t = 
 
 h u*
 
 BOOK I.] NEWTON'S PRINCIPIA. 25 
 
 hence becomes 
 
 _a^(l e 2 )? _ d v 
 
 V ft X U+ ecos. (v *r)} 2 ' 
 
 Develope the second member of this equation, in a series of the angle 
 v -a and of its multiples. For that purpose, we will commence by 
 developing 
 
 1 
 
 1 + e cos. (v zr) 
 in a similar series. If we make 
 
 A = 
 
 V (1 e 2 )' 
 we shall have 
 
 _ _ 1_ _f _ 1 __ X.C-(T- g )^- 1 \ . 
 
 ~ vi _ e 8 \ l+Xc( v ~ w ) * 1 + Xc ( T )V 1 J ' 
 
 l+ecos.(v 
 
 c being the number whose hyperbolic is unity. Developing the second 
 member of this equation, in a series; namely the first term relatively 
 to powers of c ( T w ) v '~~- 1 , and the second term relatively to powers of 
 c ( v *"") V i and then substituting, instead of imaginary exponentials, 
 their expressions in terms of sine and cosine ; we shall find 
 
 _ 
 
 I + e cos. (v r) VI e 2 
 
 . (v w ) + 2 X s cos. 2(v w) 2X 3 cos.3 (v w) + &c.J 
 Calling 9 the second member of this equation, and making q = : we 
 
 (3 
 
 shall have generally 
 
 U + ecos. (v ^)} m+ 1.2.3 ..... m. d q 
 for putting 
 
 q ' q + R 
 R being = cos. (v w) 
 
 q 
 ~ 
 
 dq "(q 
 &c. = &c.
 
 26 A COMMENTARY ON [SECT. XL 
 
 d">f-M 
 
 ^q_/ _ 2. 3 ____ m 
 
 d~q m ~ ~(q + R)" 1 * 1 
 ' m + 1 
 
 ff) "1 T 
 
 _ T _ V _____ 
 
 dq m 2.3...m~ (q 
 
 { I + e cos. (v *)} m 
 Hence it is easy to conclude that if we make 
 
 [I + e COS. (V r)} 2 ~~ ^ 
 
 [I + E<". cos. (v .) + E. cos. 2 (v tr) + &c.J 
 we shall have generally whatever be the number (i) 
 
 (1 + V 1 e 2 7~ 
 
 the signs + being used according as i is even or odd ; supposing there- 
 fore that u = a * V m, we have 
 
 ndt = dv [1 + E^cos. (v ) + Ecos.2 (v w) + &c.? 
 and integrating 
 
 n t + = v + E (1 > sin. (v *) + J E sin. 2 (v w) + &c. 
 e being an ai'bitrary constant. This expression for n t + s i fi ver y con- 
 vergent when the orbits are of small excentricity, such as are those of the 
 Planets and of the Satellites ; and by the Reversion of Series we can find 
 v in terms of t : we shall proceed to this presently. 
 
 474. When the Planet comes again to the same point of its orbit, v is 
 augmented by the circumference 2 v \ naming therefore T the time of the 
 whole revolution, we have (see also 159) 
 
 n V m 
 This could be obtained immediately from the expression 
 
 2 area of Ellipse 2 i: a b 
 
 "IT" "TT" 
 
 But by 157 
 
 h= ma (1 e 2 ) 
 
 2*a^
 
 BOOK L] 
 
 NEWTON'S PRINCIPIA. 
 
 27 
 
 If we neglect the masses of the planets relatively to that of the sun we 
 have 
 
 which will be the same for all the planets ; T is therefore proportional in 
 
 5. 
 
 that hypothesis to a 2 , and consequently the squares of the Periods are as 
 the cubes of the major axes of the orbits. We see also that the 
 same law holds with regard to the motion of the satellites around their 
 planet, provided their masses are also deemed inconsiderable compared 
 with that of the planet. 
 
 475. The equations of motion of the two bodies M and i* may also be 
 integrated in this manner. 
 
 Resuming the equations (1), (2), (3), of 471, and putting M + A*-=m, we 
 have for these two bodies 
 
 d 2 x m x" 
 
 dT 2 : + 7^ 
 
 o-^y + 
 u - dt 2 + 
 
 = 
 
 dt 
 
 z m z 
 2" "t" nr 
 
 (0) 
 
 The integrals of these equations will give in functions of the time t, the 
 three coordinates x, y, z of the body /, referred to the center of M ; we 
 shall then have (471) the coordinates , n, 7 of the body M, referred to a 
 fixed point by means of the equations 
 
 /* x 
 liT 5 
 
 = a + bt 
 
 H = a' + b' t 
 
 m 
 
 m 
 
 Lastly, we shall have the coordinates of p, referred to the same fixed 
 point, by adding x to , y to n, and z to 7 : We shall also have the rela- 
 tive motion of the bodies M and ,, and their absolute motion in space. 
 
 476. To integrate the equations (0) we shall observe that if amongst 
 
 the (n) variables x (1 >, x (2) x (n ) and the variable t, whose difference 
 
 is supposed constant, a number n of equations of tbjs following form 
 
 c (s) d 1 -- 1 '^ 
 
 = 
 
 E 
 
 H . x 
 
 dt' dt 1 - 1 dt 1 - 2 
 
 in which we suppose s successively equal to 1, 2, 3 n ; A, B H 
 
 oeing functions of the variables x (I) , x (2) , &c. and of t symmetrical
 
 28 
 
 A COMMENTARY ON 
 
 [SECT. XI. 
 
 with regard to the variables x (1) , x , &c. that is to say, such that they 
 remain the same, when we change any one of these variables to any other 
 and reciprocally ; suppose 
 
 x (1) _ a (1) x (n - i + 1) .j. b (1) x (x - i + 2) _|_ h (1 > X (n > , 
 
 x (2) _ a (2) x (n-i + l) _j_ b (2) x (n-i+2) .J. ft (2) x D> 
 
 a (1) , b (1) , h (l) ; a (2) , b (8 >, &c. being the arbitraries of which the 
 
 number is i (n i). It is clear that these values satisfy the proposed 
 system of equations : Moreover these equations are thereby reduced to i 
 
 equations involving the i variables x ("-' + 1 ) x (n) . Their integrals 
 
 will introduce i 2 new arbitraries, which together with the i (n i) pre- 
 ceding ones will form i n arbitraries which ought to give the integration 
 of the equations proposed. 
 
 477. To apply the above Theorem to equations (0) ; we have 
 
 z = a x + b y 
 
 a and b being two arbitrary constants, this equation being that of a plane 
 passing through the origin of coordinates ; also the orbit of & is wholly in 
 one plane. 
 
 The equations (0) give 
 
 >; (0' 
 
 = d 
 n d 
 
 ( 3 \ 
 
 + m d x 
 + mdy 
 + m d z 
 
 ( e > d2 y^ 
 
 = d 
 2 = x 
 
 \ ' d t 2 J 
 
 (c 3 \ 
 
 s ' dtO 
 2 +y 2 + 
 
 Also since 
 
 and 
 
 .. g d = xdx + ydy + zdz 
 and differentiating twice more, we have 
 
 g d 3 g + 3dgd 2 = xd 3 x + yd 3 y + zd 3 z 
 
 and consequently 
 
 dt 
 
 d 3 x 
 d t 2 
 
 i u 
 dx dT 
 
 d t : 
 
 d 
 
 Substituting in the second member of this equation for d 3 x, d 3 y, d : ' z
 
 BOOK I.] NEWTON'S PRINCIPIA. 29 
 
 their values given by equations ((X), and for d 2 x, d 2 y, d 2 z their values 
 given by equations (0) ; we shall find 
 
 If we compare this equation with equations (0') 5 we shall have ia virtue 
 of the preceding Theorem, by considering -5- , -^ , -p , 3-* , as so many 
 particular variables x (1) , x , x (3) , x (4) , and g as a function of the time t ; 
 
 X and 7 being constants ; and integrating 
 
 h 2 
 
 2 
 
 being a constant. This equation combined with 
 
 
 gives an equation of the second degree in terms of x, y, or in terms of 
 x, z, or of y, z ; whence it follows that the three projections of the curve 
 described by p about M, are lines of the second order, and therefore that 
 the curve itself (lying in one plane) is a line of the second order or a conic 
 section. It is easy to perceive from the nature of conic sections that, the 
 radius-vector being expressed by a linear function of x, y, the origin of 
 x, y ought to be in the focus. But the equation 
 
 h 2 
 
 g = m +Xx + 7y 
 gives by means of equations (0) 
 
 m 
 
 Multiplying this by d and integrating we get 
 
 a' being an arbitrary constant Hence 
 dt - 
 
 // e* 
 
 m J ( 2 e -r -- ) 
 V \ a' m / 
 
 which will give f in terms of t ; and since x, y, z are given above in terms 
 of , we shall have the coordinates of ^ in functions of the times. 
 
 478. We can obtain these results by the following method, which has 
 the advantage of giving the arbitrary constants in terms of the coordinates 
 x, y, z and of their first differences ; which will presently be of great use 
 to us.
 
 30 A COMMENTARY ON [SECT. XI.' 
 
 Let V = constant, be an integral of the first order of equations (0), V 
 being a function of x, y, z, , - , -j-^ , r - . Call the three last quantities 
 \' } y', z'. Then V = constant will give, by taking the differential, 
 
 dVx dx /dVx dv ,dVx dz 
 
 d xV d t " U y ; * d t " Vd z ' - a i 
 d ' 1 * * dz/ 
 
 f_\ 
 
 " \d x'/ ' d t d y' ' d t " \d z 
 But equations (0) give 
 
 d x' m x d y' m y d z' m z 
 
 dT : ~^ r9 ~d^t "' ~p ' dT = "T 5 " 
 we have therefore the equation of Partial Differences 
 
 , /d VN , /d V\ , /d V 
 
 = x (JT) + ^ (a 
 
 m /dV 
 
 It is evident that every function of x, y, z, x', y'. z' which, when sub- 
 stituted for V in this equation, satisfies it, becomes, by putting it equal to 
 an arbitrary constant, an integral of the first order of the equations (0). 
 
 Suppose 
 
 V = U + U' + U" + &c. 
 
 U being a function of x, y, z ; U' a function of x, y, z, x', y', z' but of the 
 first order relatively to x', y', z 1 ; U" a function of x, y, z, x x , y 7 , z' and of 
 the second order relatively to x', y', z', and so on. Substitute this value 
 of V in the equation (I) and compare separately 1. the terms without 
 x', y 7 , z' ; 2. those which contain their first powers ; 3. those involving their 
 squares and products, and so on ; and we shall have 
 d U\ /d U\ /d U 
 
 m f 
 
 = ^ 1 
 
 d U'\ /d U'\ , /d U' 
 
 x 
 
 ,/dU\, ,/dU'x^ ,/dU\ m 
 
 x ( dT ) +y ( err) +z (TF) = 
 
 dU 
 
 &c. 
 which four equations call (I 7 ). 
 
 The integral of the first of them is 
 
 U' = funct. Jx y' y x', x z' z x', y z' z y 7 , x, y,
 
 BOOK I.] NEWTON'S PRINCIPIA. 31 
 
 The value of U' is linear with regard to x', y', z' ; suppose it of this 
 form 
 
 U' = A (x y' y x') + B (x z' z x') + C (y z' z y') ; 
 A, B, C being arbitrary constants. Make 
 
 U'", &c. = ; 
 
 then the third of the equations (F) will become 
 
 The preceding value of U' satisfies also this equation. 
 Again, the fourth of the equations (I') becomes 
 
 of which the integral is 
 
 U" = funct. [K y' y x', x z' z x', y z' z y', x', y', z'} . 
 
 This function ought to satisfy the second of equations (F), and the first 
 member of this equation multiplied by d t is evidently equal to d U. The 
 second member ought therefore to be an exact differential of a function of 
 x, y, z ; and it is easy to perceive that we shall satisfy at once this condi- 
 tion, the nature of the function U", and the supposition that this function 
 ought to be of the second order, by making 
 
 U" = (D y' E x') . (x y' y x') + (D z' F x') (x z' z x') 
 
 + (E z' F y) (y Z ' _ z y'} + G (x' 2 + y 2 + z' 2 )^ __ 
 D, E, F, G being arbitrary constants ; and then g being = Vx 8 +y 2 +z 8 , 
 we have 
 
 
 U = -(Dx + Ey+Fz-fSG); 
 
 Thus we have the values of 
 
 U, U', U" ; 
 and the equation V = constant will become 
 
 const.= -JDx+Ey+Fz+2G? + (A + Dy' Ex 7 ) (x y' y x') 
 
 z' Fx') (xz zx') + (C+Ez' F /) (y z' z y) 
 + G(x' 2 + y"+ z' 2 ). 
 
 This equation satisfies equation (I) and consequently the equations (0) 
 whatever may be the arbitrary Constants A, B, C, D, E, F, G. Sup- 
 posing all these = 0, 1. except A, 2. except B, 3. except C, &c. and 
 putting 
 
 d x d y d z ,, 
 
 '
 
 32 A COMMENTARY ON [SECT. XT. 
 
 all have the integrals 
 
 r r _ xd y > d * r /_xd 
 
 z z d 
 
 | 
 
 x r //_y dz zd y 
 
 dt 
 
 o- f+x f m d y' + d 
 
 dt 
 z '1 , 
 
 > * 
 
 y d y . d x 
 
 dt 
 z d z . d 
 
 x 
 
 _ f , Cm dx2 + d 
 
 J ' 
 
 Z ^ 1 
 
 dt 2 
 xdx.dy 
 
 z d z . d 
 
 y 
 
 y l f d t 2 
 
 J ' 
 
 y ' -L 
 
 xdx.dz 
 
 dt 2 
 
 , y d y. d 
 
 z 
 
 - m 2 m d x 2 + d 
 
 t T 
 
 y 2 + d 
 
 z 2 
 
 dt 2 
 
 
 c, c', c 7 ', f, r, f" and a being arbitrary constants. 
 
 The equations (0) can have but six distinct integrals of the first order, 
 by means of which, if we eliminate d x, d y, d z, we shall have the three 
 variables x, y, z in functions of the time t ; we must therefore have at least 
 one of the seven integrals {P) contained in the six others. We also per- 
 ceive d priori, that two of these integrals ought to enter into the five 
 others. In fact, since it is the element only of the time which enters 
 these integrals, they cannot give the variables x, y, z in functions of the 
 time, and therefore are insufficient to determine completely the motion of 
 . about M. Let us examine how it is that these integrals make but five 
 distinct integrals. 
 
 z d v - v d z 
 If we multiply the fourth of the equations (P) by -- A t -- > and 
 
 x d / __ z ci x 
 add the product to the fifth multiplied by - 5 - , we shall have 
 
 f zdy ydz x d z z d x , x d y y d x / m 
 ~dT~ ~dT~ "dt \ S 
 
 xdy ydxf x d x . d z ydy.dz 1 
 dt ~t d! 2 ~ dt 2 J ' 
 
 dt 
 
 ,. xdy ydx xdz zdx ydz zdy ,. 
 Substituting for - ^ t - - d~ t - - "Jt - their 
 
 values given by the three first of the equations (P), we shall have 
 Pc 7 fc" /m dx 2 + d y 2 ) , xdx.dz , 
 ~ Z " " 1 ' 
 
 fc" /m dx 2 + d y 2 ) , xdx.dz , y dy.d z 
 C~t Z l7" "dl 1 j dt' dt 2 
 
 This equation enters into the sixth of the integrals P, by making 
 
 f" = f/ c/ ~" f ^ or = f c" f c' + f" c. Also the sixth of these 
 c 
 
 integrals results from the five first, and the six arbitraries c, c 7 , c", f, f , f" 
 are connected by the preceding equation.
 
 BOOK I.] NEWTON'S PRINCIPIA. 33 
 
 If we take the squares off, f, f" given by the equations (P), then add 
 them together, and make f 2 + P * + f" 2 = 1 2 , we shall have 
 18 m g- f ft dx 2 +dy 2 +dz 2 /gdgx 2 ) ^ fdx 2 +dy 2 +dz' 2ml 
 
 but if we square the values of c, c', c", given by the same equations, and 
 make c 2 + c' 2 -f- c" 2 = h 2 ; we get 
 
 the equation above thus becomes 
 
 dx 2 + dy z + dz 2 2jn m 2 I 2 
 
 . dt 2 g h 2 
 
 Comparing this equation with the last of equations (P), we shall have 
 the equation of condition, 
 
 m 2 I 2 jn 
 
 h* a ' 
 
 The last of equations (P) consequently enters the six first, which are 
 themselves equivalent only to five distinct integrals, the seven arbitrary 
 constants, c, c', c", f, P, f", and a being connected by the two preceding 
 equations of condition. Whence it results that we shall have the most 
 general expression of V, which will satisfy equation (I) by taking for this 
 expression an arbitrary function of the values of c, c 7 , c", f, and P, given 
 by the five first of the equations (P). 
 
 479. Although these integrals are insufficient for the determination of 
 x, y, z hi functions of the time ; yet they determine the nature of the 
 curve described by /* about M. In fact, if we multiply the first of the 
 equations (P) by z, the second by y, and the third by x, and add the 
 results, we shall have 
 
 = c z c' y + c/' x, 
 
 the equation to a plane whose position depends upon the constants 
 c, c', c". 
 
 If we multiply the fourth of the equations (P) by x, the fifth by y, and 
 the sixth by z, we shall have 
 
 > dx ! +dy ! +dz 2 , e 2 dg 2 
 2 r * T 
 
 but by the preceding number 
 
 , dx 2 + dy 8 + dz 8 g'dg 2 , , 
 
 dt 2 dt 2 
 
 .-. = m g h 2 + f x + f y -f- f" z. 
 This equation combined with 
 
 = c" x c' y + c z 
 VOL. II. C
 
 34 A COMMENTARY ON [SECT. XI. 
 
 and 
 
 g* = x 2 + y* -f- z* 
 
 gives the equation to conic sections, the origin of being at the focus. 
 The planets and comets describe therefore round the sun very nearly 
 conic sections, the sun being in one of the foci ; and these stars so move 
 that their radius-vectors describe areas proportional to the times. In fact, 
 it' d v denote the elemental angle included by , g + d f, we have 
 
 dx*+ dy"- r .dz 2 = f 2 dv 2 + df t 
 and the equation 
 
 dt 
 becomes 
 
 S d v* = hd 
 
 4 
 
 hdt 
 
 .-. d v = 
 
 Hence we see that the elemental area g * d v, described by g y is propor- 
 tional to the element of time d t ; and the area described in a finite time is 
 therefore also proportional to that time. We see also that the angular 
 
 motion of p about M, is at every point of the orbit, as , ; and since without 
 
 sensible error we may take very short times for those indefinitely small, we 
 shall have, by means of the above equation^ the horary motions of the planets 
 a fid comets, in the different points of their orbits. 
 
 The elements of the section described by <a, are the arbitrary constants 
 of its motion ; these are functions of the arbitraries c, c', c", f, f, f", and 
 
 . Let us determine these functions, 
 a 
 
 Let 6 be the angle which the intersection of the planes of the orbit and 
 of (x, y) makes with the axis of x, this intersection being called the line 
 of the nodes ; also let <p be the inclination of the planes. If x', y' be the 
 coordinates of ft referred to the line of the nodes as the axis of abscissas, 
 then we have 
 
 x' = x cos. 6 -}- y sin. d 
 y' = y cos. & x sin. 6. 
 Moreover 
 
 z = y' tan. <f> 
 
 .-. z = y cos. tan. p x sin. 6 tan. <p. 
 Comparing this equation with the following one 
 = c" x c' y + c /.
 
 BOOK I.] NEWTON'S PRINCIPIA. 35 
 
 we shall have 
 
 c' = c cos. 6 . tan. <p 
 c" = c sin. 6 tan. <p 
 
 whence 
 
 c 
 tan. 6 
 
 c 
 and 
 
 , 1n .,^(c" + 
 c 
 
 Thus are determined the position of the nodes and the inclination of the 
 orbit, in functions of the arbitrary constants c, c', c". 
 At the perihelion, we have 
 
 P, d P, = 0, orxdx + ydy + zdzurO. 
 
 Let X, Y, Z be the coordinates of the planet at this point ; the fourth 
 and the fifth of the equations (P) will give 
 Y P 
 "X =: 7' 
 
 But if I be called the longitude of the projection of the perihelion upon 
 the plane of x, y this longitude being reckoned from the axis of x, we have 
 
 Y 
 
 ^- = tan. I ; 
 
 ft 
 .-. tan. I = f , 
 
 which determines the position of the major axis of the conic section. 
 If from the equation 
 
 dx*+ dy 2 + dz g 2 dg 2 __ ,, 
 d t 2 dt 2 
 
 d x ^ "I" d v ^ '"I"* d z ^ 
 
 we eliminate , J . , by means of the last of the equa- 
 
 d t* 
 
 tions (P), we shall have 
 
 9 9 J 9 
 
 m P* e Q e 
 
 _ 
 
 a d t 2 
 
 but d is at the extremities of the axis major ; we therefore have at these 
 points 
 
 The sum of the two values of in this equation, is the axis major, and 
 their difference is double the excentricity ; thus a is the semi-axis major of 
 the orbit, or the mean distance of p from M ; and 
 
 JO--) 
 
 v ma/
 
 36 A COMMENTARY ON [SECT. XI. 
 
 is the ratio of the excentricity to the semi-axis major. Let 
 
 e = I (\ } 
 
 >y v ma/ 
 
 and having by the above 
 
 m m * 1 ' 
 ~a~ ~F^ J 
 we shall get 
 
 m e = 1. 
 
 Thus we know all the elements which determine the nature of the conic 
 section and its position in space. 
 
 480. The three finite equations found above between x, y, z and g give 
 x, y, z in functions of g; and to get these coordinates in functions of the 
 time it is sufficient to obtain g in a similar function ; which will require a 
 new integration. For that purpose take the equation 
 
 i* />S A 2 
 
 But we have above 
 
 mo 
 
 - = h 
 
 = -- (m 2 I 2 ) = am (1 e s ); 
 
 ...dt = 
 
 
 
 whose integral (237) is 
 
 a* 
 t + T = (u - e sin. u) ....... (S) 
 
 (1 & \ 
 --- ], and T ail arbitrary constant. 
 
 This equation gives u and therefore in terms of t; and since x, y, z 
 are given in functions of ?, we shall have the values of the coordinates for 
 any instants whatever. 
 
 We have therefore completely integrated the equations (0) of 475, and 
 thereby introduced the six arbitrary constants a, e, I, 6, p, and T. The 
 two first depend upon the nature of the orbit ; the three next depend upon 
 its position in space, and the last relates to the position of the body u. 
 at any given epoch ; or which amounts to the same, depends upon the 
 instant of its passing the perihelion. 
 
 Referring the coordinates of the body p, to such as are more commodious 
 for astronomical uses, and for that, naming v the angle which the radius-
 
 BOOK I.] NEWTON'S PRINCIPIA. 37 
 
 vector makes with the major axis setting out from the perihelion, the 
 equation to the ellipse is 
 
 a (1 e 2 ) 
 1 + e cos. v ' 
 
 The equation 
 
 g =: a (1 e cos. u) 
 
 indicates that u is at the perihelion, so that this point is the origin of two 
 angles u and v ; and it is easy hence to conclude that the angle u is formed by 
 the axis major, and by the radius drawn from its center to the point where 
 the circumference described upon the axis major as a diameter, is met by 
 the ordinate passing through the body /* at right angles to the axis major. 
 Hence as in (237) we have 
 
 v , 1 + e u 
 
 tan - =- tan - 
 
 We therefore have (making T = 0, &c.) 
 n t = u e sin. u 
 g = a (1 e cos. u) 
 
 and 
 
 (0 
 
 n t being the Mean Anomaly, 
 
 n the Eccentric Anomaly, 
 v the True Anomaly. 
 
 The first of these equations gives u in terms of t, and the two others 
 will give f and v when u shall be determined. The equation between u 
 and t is transcendental, and can only be resolved by approximation. 
 Happily the circumstances attending the motions of the heavenly bodies 
 present us with rapid approximations. In fact the orbits of the stars are 
 either nearly circular or nearly parabolical, and in both cases, we can de- 
 termine u in terms of t by series very convergent, which we now proceed 
 to develope. For this purpose we shall give some general Theorems 
 upon the reduction of functions into series, which will be found very use- 
 ful hereafter. 
 
 481. Let u be any function whatever of a, which we propose to deve- 
 lope into a series proceeding by the powers of a. Representing this 
 series by 
 
 U = U + a.^+ a 2 . q 2 + a n . q n + a n + | . q n+ " + &c. 
 
 C3
 
 38 A COMMENTARY ON [SECT. XL 
 
 K, qu q*j & c being quantities independent of a, it is evident that u is what 
 u will become when we suppose a = ; and that whatever n may be 
 
 the difference (-7 -) being taken on the supposition that every thing in 
 
 u varies with a. Hence if we suppose after the differentiations, that a = 0, 
 
 /d n u\ 
 in the expression ( j -J we have 
 
 d n u\ 
 
 X 
 
 1.2 n' 
 
 This is Maclaurin's Theorem (see 32) for one variable. 
 Again, if u be a function of two quantities a, a', let it be put 
 u = u -f a. qi >0 + a 2 . q 2 ; + &c. 
 
 + a ' * qo,2 + &c. 
 the general term being 
 
 Then if generally 
 
 / d n + n/ u x 
 
 \d a n . d a! n ) 
 
 denotes the (n + n x ) tb difference of u, the operation being performed (n) 
 times, on the supposition that a is the only variable, and then n' times on 
 that of a! being the only variable, we have 
 
 3 a . q 3) o ~T" 4 * q^ Q-f-5a .qso't" &c. 
 
 Vq 4>1 + &c. 
 a^q-? 9 + &C. 
 
 = 2 q *- 9 + 3t 2aq3>0 + 4 " 3 a2 q*. + 5 - 4 3 q5,o + &c. 
 
 + 2 q 2; , + 3. 2aaq 3>1 + 4.3a 8 <q 4>1 + 8j;c. 
 + 2 a e , 2 + 3.2aa z &c. 
 
 + 2 a q 2>2 + &c. 
 and continuing the process it will be found that
 
 BOOK I.] NEWTON'S PRINCIPIA. 39 
 
 so that when , d both equal 0, we have 
 
 / d + "' u 
 _ \d a n . d a. rn> 
 
 q "' n '~ 
 
 2. 3....n X 2. 3....n' ........ 
 
 A nil generally, if u be a function of a, a, a", &c. and in developing it 
 into a series, if the coefficient of a tt . a n/ . a" n ". &c. be denoted by q n , n ,, <-, &c . 
 we shall have, in making a , <, a", &c. all equal 0, 
 
 J n + n' + n" + &c. u 
 
 &c _ _ ... _ 
 
 qn.n<,n> = 2. 3 . . . . n X 2. 3 . . . . n' X 2. 3 . . . . n" X &c. ' 
 
 This is Maclaurin's Theorem made general. 
 
 482. Again let u be any function of t + a, t' + a, t" + a", &c. and 
 put 
 
 U = V (t + a, t' + a, t" + a", &c.) 
 then since t and a are similarly involved it is evident that 
 
 (d n + n ' + n " + &c - . u \ / d n + n/ + n " + &c - . u \ 
 d a", d <x. n/ . d //n "&c./ = \d t n . d t /n '. d t //n ". &c./ 
 and making 
 
 , a, a", &c. = 0, 
 or 
 
 u = 9 (t, t', t", &c.) 
 by (2) of the preceding article we have 
 
 .dn + n' + n".&c. . p ( t> t / ? t // ? &c> ) 
 
 v d t n . d t /n> . d t //p<< &c / _ 
 q n.n'.n-.&c. ~ 2 . 3 . . . . n X 2. 3 ---- n' X 2. 3 ---- n" X &c. ' * ' "' 
 
 which gives Taylor's Theorem in all its generality (see 32). 
 Hence when 
 
 u = <f> . (t + ) 
 
 d-.p(t) 
 ]n ~ 2.3 ____ n.dt n 
 
 and we thence get 
 
 , ( + .) = M t) + d -^ + -.*- + te ..... (i) 
 
 483. Generally, suppose that u is a function of , , a", &c. and of 
 t, t', t", &c. Then, if by the nature of the function or by an equation of 
 R. ftial Differences which represents it, we can obtain 
 
 c - .u 
 
 - .u \ 
 
 a*. da n/ . &cJ 
 
 in a function of u, and of its Differences taken with regard to t, t', &c. 
 
 C4,
 
 40 A COMMENTARY ON [SBCT. XI. 
 
 calling it F when for u we put or make a t d, a", &c. = ; it is evident 
 we have 
 
 _ F __ 
 
 qn.n'.n'.te. ~ g. 3 . . . H X 2. 3 . . . Il' X 2. 3 ... ll", X &C. 
 
 and therefore the law of the series into which u is developed. 
 
 For instance, let u, instead of being given immediately in terms of a, 
 and t, be a function of x, x itself being deducible from the equation of 
 Partial Differences 
 
 d 
 
 in which X is any function whatever of x. That is 
 
 Given 
 
 u = function (x) 
 
 to dcvelope u into a series ascending by the powers of a. 
 First, since 
 
 (k) 
 
 Hence 
 
 /d'uv /d'/Xduv. 
 
 VdaV"~ V da.dt )' 
 But by equation (k), changing u into f X d u 
 /d./Xduv _ /d./X*dux 
 
 *' (d~oV = ( dT^ ) ' 
 
 Again 
 
 (d~o"~ 3 ) = ( da.dt 8 )' 
 But by equation k, and changing u intoyX 8 d u 
 
 V d~^ ~V dt 
 
 _ 
 
 dt 
 Thus proceeding we easily conclude generally that 
 
 Now, when a = 0, let 
 
 x = function of t = T
 
 BOOK I.] NEWTON'S PRINCIPIA. 41 
 
 and substitute this value of x in X and u ; and let these then become X 
 and u respectively. Then we shall have 
 
 d*->.X. d " 
 /d n u\ ... _ d t 
 
 \d n /~ d I - 1 
 
 and 
 
 d t 
 
 ' - 2.3 ____ ndt 11 - 1 
 which gives 
 
 t 
 
 v du , a* , , v 
 
 u = + X. T - t + T .-- aT --+ - 1 .-- arti --+&c. ...(p) 
 
 which is Lagrange's Theorem. 
 
 To determine the value of x in terms of t and a, we must integrate 
 
 In order to accomplish this object, we have 
 and substituting 
 we shall have 
 
 .-. d x = 
 
 dX 
 
 which by integration, gives 
 
 x = <f> (t +,a X) ' . . (2) 
 
 <p denoting an arbitrary function. 
 
 Hence whenever we have an equation reducible to this form x = 
 <f> (t + X), the value of u will be given by the formula (p), in a series of 
 the powers of . 
 
 By an extension of the process, the Theorem may be generalized to the 
 case, when 
 
 u = function (x, x', x", &c.)
 
 43 A COMMENTARY ON [SECT. XL 
 
 and 
 
 X = <f> (t + a X) 
 X' = f (f + a' X') 
 X" = <f/' (t" + a" X") 
 &c. = &c. 
 484. Given (237) 
 
 u = n t + e sin. u 
 
 required to develope u or any function of it according to the powers ofe. 
 Comparing the above form with 
 
 x = <p (t + X) 
 x, t, a, X become respectively 
 
 11, n t, e, sin. u. 
 Hence the formula (p) 483. gives 
 
 e* d H/(nt) sin. *nt* 
 4/(u) = ^(nt) + e-4/(nt)sin.nt + -.- f^ 1 
 
 n dt 
 
 To farther develope this formula we have generally (see Woodhouse's 
 Trig.) 
 
 sin. 1 (n t) = (^ 2 7-T ~) ; C S>i < n t} = ( 'V DtV " 1 ) ;t 
 c being the hyperbolic base, and i any number whatever. Developing the 
 second members of these equations, and then substituting 
 
 cos. r n t + V I sin. r n t, and cos. r n t V 1 sin. r n t 
 for c rnt *</~ l 9 and c~ rn * v / ?, r being any number whatever, we shall 
 have the powers i of sin. n t, and of cos. n t expressed in sines and cosines 
 of n t and its multiples ; hence we find 
 
 e e 2 
 
 P =r sin. n t + sin * n t + 5-^ sin. 3 n t -f &c. 
 
 = sin. n t ^-g . {cos. 2 n t 1 } 
 
 e* 
 . f sin. 3 n t 3 sin. n t} 
 
 172 Si
 
 BOOK I.] NEWTON'S PRINCIPIA. 43- 
 
 &C. 
 
 Now multiply this function by -\J/ (n t), and differentiate each of its 
 terms relatively to t a number of times indicated by the power of e which 
 multiplies it, d t being supposed constant; and divide these differentials 
 by the corresponding power of n d t Then if P' be the sum of the 
 quotients, the formula (q) will become 
 
 4 (u) = ^ (n t) + e P'. 
 
 By this method it is easy to obtain the values of the angle u, and of 
 the sine and cosine of its multiples. Supposing for example, that 
 
 4/ u = sin. i u 
 
 we have . 
 
 vj/ (n t) = i cos. int. 
 
 Multiply therefore the preceding value of P, by i. cos. i n t, and deve- 
 lope the product into sines and cosines of n t and its multiples. The 
 terms multiplied by the even powers of e, are sines, and those multiplied 
 by the odd powers of e, are cosines. We change therefore any term of 
 the form, K e 2 r sin. s n t, into + Ke 2r s 2r sin. s n t, + or obtaining 
 according as r is even or odd. In like manner, we change any term 
 of the form, K e 2 r + l cos. s n t, into + K e 2r + 1 . s 2 r + l . sin. s n t, or 
 + obtaining according as r is even or odd. The sum of all these terms 
 will be P' and we shall have 
 
 sin. i u = sin., i n t + e P'. 
 
 But if we suppose 
 
 v|/ (u) = u ; 
 
 then 
 
 ^ (n t) = 1 
 and we find by the same process 
 
 e 2 
 
 u = n t + e sin. n t -f- 55 . 2 sin. 2 n t 
 ~. & 
 
 + & 3 >2 ,.{3sin. 3 n t ~~ 3 sin ' n l * 
 
 sin< 4 n t 4. 2 s sin. 2 n t} 
 
 a . 34. 2 
 
 3 * 
 
 e s f 5 4 1 
 
 s 4 5 a 4 ' | 34sin - 5nt 5.3 4 sin.3nt+j^2sin.nt| 
 
 &c.
 
 44 A COMMENTARY ON [SECT. XL 
 
 a formula which expresses the Excentric Anomaly in terms of the Mean 
 Anomaly. 
 
 This series is very convergent for the Planets. Having thus determin- 
 ed u for any instant, we could thence obtain by means of (237), the cor- 
 responding values of f and v. But these may be found directly as fol- 
 lows, also in convergent series. 
 
 485. Required to express g in terms of the Mean Anomaly. 
 By (237) we have 
 
 g = a (1 e cos. u). 
 Therefore if in formula (q) we put 
 
 v}/ (u) = 1 e cos. u 
 we have 
 
 \J/ (n t) = e sin. n t, 
 and consequently 
 
 1 e cos. u = 1 e cos. n t + e * sin. 8 n t + -^ . 4- + &c. 
 
 <& nut 
 
 Hence, by the above process, we shall find 
 
 P e 8 e * 
 -i = 1 -f- - e cos. n t cos. 2 n t 
 
 e 3 
 
 p-p-, .{3 cos. 3 n t 3 cos. n t} 
 
 e 4 
 3 .4 8 cos. 4 n t 4. 2 s . cos. 2 n t} 
 
 i. O. <6 
 
 - ./5 3 cos. 5 n t 5. 3 3 cos. 3 n t + ^. cos.ntl 
 
 , o. 4. \. * j 
 
 s ]6 4 cos.6nt 6. 4 4 cos. 4n t+rr^. 2 4 cos.2nt| 
 
 &c. 
 
 486. To express the True Anomaly in terms of the Mean. 
 First we have (237) 
 
 v . u 
 
 : i ii _ Qin 
 
 in - ~2 1 + e ' ' g 
 
 7" = V 1 e* ~u 
 
 COS. COS. -jr- 
 
 A & 
 
 .. substituting the imaginary expressions 
 
 c vv i -f. 1 = -v 'i e c u v i + 1* 
 and making 
 
 X= 1 + V(l-e')
 
 BOOK L] NEWTON'S PRINCIPIA. 45 
 
 we shall have 
 
 c vV-l - cV-l X 1-*C-V-1. 
 
 ' 
 
 . .C-UV- 1 ) log.( 1_X c V-i ) 
 
 ~V^n~~ 
 
 whence expanding the logarithms into series (see p. 28), and putting 
 sines and cosines for their imaginary values, we have 
 
 2 X 2 2 X 3 
 
 v = u + 2 X sin. u -\ sin. 2 u -J -- ~ sin. 3 u + &c. 
 
 A 
 
 But by the foregoing process we have u, sin. u, sin. 2 u, &c. in series 
 ordered by the powers of e, and developed into sines and cosines of n t 
 and its multiples. There is nothing else then to be done, in order to 
 express v in a similar series, but to expand X into a like series. 
 
 The equation, (putting u = 1 + V 1 e s ) 
 
 e 3 
 
 u = 2 - 
 u 
 
 will give by the formula (p) of No. (483) 
 
 -- i-v -'. i(i + S)(i + 5) e 
 
 771 ol "i 2 14.2 "" Q *2'+ 4 2. 3 * oiAfi** 
 
 2 
 and since 
 
 u = 1 + V 1 e 2 
 we have 
 
 These operations being performed we shall find 
 
 v = 
 
 1 5 1 
 
 e 3 + gg e 5 | sin. n 
 
 451 
 
 6 -. 
 1097 
 1)60 
 1223 
 
 . . 
 
 Sm ' 5nt 
 6 . 
 
 SU1 ' 6nt > 
 
 the approximation being carried on to quantities of the order e 6 in- 
 clusively.
 
 46 A COMMENTARY ON [SECT. XI. 
 
 487. The angles v and n t are here reckoned from the Perihelion ; but 
 if we wish to compute from the Aphelion, we have only to make e nega- 
 tive. It would, therefore, be sufficient to augment the angle n t by r, in 
 order to render negative the sines and cosines of the odd multiples of n t ; 
 then to make the results of these two methods identical ; we have only in 
 the expressions for g and v, to multiply the sines and cosines of odd 
 multiples of n t by odd powers of e ; and the even multiples by the even 
 powers. This is confirmed, in fact, by the process, a posteriori. 
 
 488. Suppose that instead of reckoning v from the perihelion, we fix 
 its origin at any point whatever ; then it is evident that this angle will be 
 augmented by a constant, which we shall call , and which will express 
 the Longitude of the Perihelion. If instead of fixing the origin of t at 
 the instant of the passage over the perihelion, we make it begin at any 
 point, the angle n t will be augmented by a constant which we will call 
 
 e v ; and then the foregoing expressions for and v, will become 
 
 1 3 11 
 
 = l-f--g-e s (e e 3 )cos.(nt-f-s ) ( - e 2 - e 4 )cos.2(nt+ 
 
 1 511 
 
 v = nt+ 1 + (2e e 3 )sin. (nt + w ) +( 7 e 2 ~T e 4 )sin. 2 (n t + e 
 
 where v is the true longitude of the planet and n t + s its mean longi- 
 tude, these being measured on the plane of the orbit. 
 
 Let, however, the motion of the planet be referred to a fixed plane a 
 little inclined to that of the orbit, and <p be the mutual inclination of the 
 two planes, and d the longitude of the Ascending Node of the orbit, mea- 
 sured upon the fixed plane ; also let /3 be this longitude measured upon 
 the plane of the orbit, so that 6 is the projection of /3, and lastly let v, be 
 the projection of v upon the fixed plane. Then we shall have 
 
 v, 6, v /3, 
 
 making the two sides of a right angled spherical triangle, v /3 being 
 opposite the right angle, and <p the angle included between them, arid 
 therefore by Napier's Rules 
 
 tan. (v, 6) = cos. <p tan. (v 15) (1) 
 
 This equation gives v, in terms of v and reciprocally ; but we can ex- 
 press cither of them in terms of the other by a series very convergent 
 after this manner. 
 
 By what has preceded, we have the series 
 
 11 X* X s 
 
 i v = u + X sin. u + -g- sin. 2 u + ~ sin, 3 u + &c. 
 
 A & &
 
 BOOK L] NEWTON'S PRINCIPIA 47 
 
 from 
 
 ^ 1 /I + e ^ 1 
 
 by making 
 
 . 
 
 If we change - v into \, d, and - u into v j8, and /-r - into 
 <& * ^/ 1 e 
 
 cos. f , we have 
 
 The equation between ^ v and - u will change into the equation be- 
 
 4i 
 
 tween \ t 6 and v j3, and the above series will give 
 
 v, 6 = v jS tan 2 - f. sin. 2 (v S) + - tan. 4 - p. sin. 4 (v /3) 
 
 A mm 
 
 - tan. 6 5 <p sin. 6 (v /3) + &c ....... (2) 
 
 O /i 
 
 v u 
 
 If in the equation between and - , we change r v into v /3 and 
 
 & 
 
 u into v, 6. and . / =-^ into - , we shall have 
 2 VI e cos. <f> 
 
 X = tan. 2 ;* p, .......... (3) 
 
 and 
 
 v j8 = v x d+tan. * ? s i n ' ^ (v y ^) 
 
 + tan. 4 - p. sin. 4 (v, 
 
 . 
 
 + I tan. 6 I p. sin. 6 (v y *) ..... (4) 
 
 Thus we see that the two preceding series reciprocally interchange, 
 l;y changing the sign of tan. 2 J p, and by changing v, 0, v /3 the one 
 for the other. We shall have v, 6 in terms of the sine and cosine of 
 n't and its multiples, by observing that we have, by what precedes 
 
 v = n t + + e Q, 
 
 Q being a function of the sine of the angle n t + i , and its multi- 
 ples; and that the formula (i) of number (482) gives, whatever is i, 
 sin. i (v j8) = sin. i (n t + /3 + e Q)
 
 48 A COMMENTARY ON [SECT. XI. 
 
 Lastly, s being the tangent of the latitude of the planet above the fixed 
 
 plane, we have 
 
 s = tan. <p sin. (v, 6} ; 
 
 and if we call f y the radius-vector projected upon the fixed plane, we 
 shall have 
 
 we shall therefore be able to determine v /? s and g, in converging series 
 of the sines and cosines of the angle n t and of its multiples. 
 
 489. Let us now consider very excentric orbits or such as are those of 
 the Comets. 
 
 For this purpose resume the equations of No. (237), scil. 
 
 1 + e cos. v 
 n t = u e sin. u 
 
 1 + e 
 
 tan. i v = . / v - tan. \ u. 
 VI e 
 
 In this case e differs very little from unity ; we shall therefore suppose 
 
 1 e = a 
 a being very small compared with unity. 
 
 Calling D the perihelion distance of the Comet, we shall have 
 
 D r= u(l e) = a a ; 
 and the expression for will become 
 
 (2 a) D _ D _ 
 
 g - _ - p- a 1 ) ' 
 
 2 cos. 2 - v a cos. v cos. 2 - v 1 1 + 2 __ g tan. 2 - v| 
 which gives, by reduction into a series 
 
 8 = -Jij- {l-J^tan.^ v + (^)'an.' I v- & o.} 
 
 cos. 2 v 
 
 To get the relation of v to the time t, we shall observe that the expres- 
 sion of the arc in terms of the tangent gives 
 
 u = 2 tan. i u {l ^ tan. 8 i u + * tan.* i u &c.} 
 But 
 
 tan. u =
 
 BOOK I.] NEWTON'S PRINCIPIA. 49 
 
 \ve therefore have 
 
 u =2 '-^-tan.I v { 1-i ( )tan.'ly + \ (^-) 'tan.| Y-&C. 
 /V 2 a 21 3 \2-^-a/ 2 5 \2 a/ 2 
 
 Next we have 
 
 2 tan. - - u 
 sin. u =r 
 
 = 2 tan. - u| 1 tan. 2 -| + tan. 4 ~ &c. j 
 
 1 -f- tan. 2 u 
 
 m 
 
 Whence we get 
 e sin. u = 2(l a)/-^ tan. - v. { 1 tan. * -- v 
 
 Substituting these values of u, and e sin. u in the equation n t = u 
 e sin. u, we shall have the time t in a function of the anomaly v, by a series 
 very convergent ; but before we make this substitution, we shall observe 
 that (237) 
 
 _ 5. 
 
 n = a 2 . V m, 
 and since 
 
 D = a, 
 we have 
 
 1 Pi 
 
 " "" cJ-Vm* 
 Hence we find 
 
 t= ^(2 )m tan *2 V i 1+ 2^r^ tan ' ! 2 V "r(2^ a ) 2tan<4 2 
 If the orbit is parabolic 
 
 a = 
 and consequently 
 
 D 
 
 COS. V 
 
 
 
 D^ V 2 S v 1 , 1 
 
 f^zssri^j + 3 tan - 2 
 
 which expression may also be got at once from (237). 
 
 The time t, the distance D and sum m of the masses of the sun and 
 comet, are heterogeneous quantities, to compare which, we must divide 
 each by the units of their species. We shall suppose therefore that the 
 mean distance of the sun from the Earth is the unit of distance, so that D 
 is expressed in parts of that distance. We may next observe that if T 
 
 VOL. II. D
 
 50 A COMMENTARY ON [SECT. XI. 
 
 represent the time of a sidereal revolution of the Earth, setting off' from 
 the perihelion ; we shall have in the equation 
 
 n t = u e sin. u 
 u = at the beginning of the revolution, and u = 2 it at the end of it. 
 
 Hence 
 
 n T = 2 r. 
 
 But we have 
 
 _ s. 
 n = a s V m = V m, 
 
 .-. V m = ~ . 
 
 The value of m is not exactly the same for the Earth as for the Comet, 
 for in the first case it expresses the sum of the masses of the sun and 
 earth ; whereas in the second it implies the sum of the masses of the sun 
 and comet : but the masses of the Earth and Comet being: much smaller 
 
 O 
 
 than that of the sun, we may neglect them, and suppose that m is the 
 same for, all Planets and all Comets and that it expresses the mass of the 
 
 2 f 
 sun merely. Substituting therefore for V m its value TTT in the preced- 
 
 ing expression for t ; we shall have 
 
 i i i 
 
 i 
 
 This equation contains none but quantities comparable wYth each other ; 
 it will give t very readily when v is known ; but to obtain v by means of 
 t, we must resolve a Cubic Equation, which contains only onereal root. 
 We may dispense with this resolution, by making a table of the values of 
 v corresponding to those of t, in a parabola of which the perihelion dis- 
 tance is unity, or equal to the mean distance of the earth from the sun. 
 This table will give the time corresponding to the anomaly v, in any par- 
 
 3 
 
 abola of which D is the perihelion distance, by multiplying by D 8 , the 
 time which corresponds to the same anomaly in the Table. We also gel 
 the anomaly v corresponding to the time t, by dividing t by D *, and 
 seeking in the table, the anomaly which corresponds to the quotient 
 arising from this division. 
 
 490. Let us now investigate the anomaly, corresponding to' the time t, 
 in an ellipse of great excentricity. 
 
 If we neglect quantities of the order a 2 , and put 1 e for a, the above 
 expression of t in terms of v in an ellipse, will give 
 
 D * V 2 f tan. v + $ tan. 3 | v '1 
 
 V m t + (1 e ) tea- * i v U i tan - * v ~ J tan - * 1 V H 
 Then, find by the table of the motions of the comets, the anomaly cor-
 
 BOOK I.] NEWTON'S PRINCIPIA, 5I 
 
 responding to the time t, in a parabola of which D is the perihelion dis- 
 tance. Let U be this anomaly and U + x the true anomaly in an ellipse 
 corresponding to the same time, x being a very small angle. Then if we 
 substitute in the above equation U + x for v, and then transform the 
 second member into a series of powers of x, we shall have, neglecting the 
 square of x, and the product of x by 1 e, 
 
 t = 
 
 But by supposition 
 
 D^ V 2 , 
 
 t = -. itan. 4 U 4- i tan. i \j\. 
 
 V m 
 
 Therefore, substituting for x its sine and substituting for sin. 4 i U its 
 value (1 cos. 2 U) 2 , &c. 
 
 sin. x = Jg. (1 e) tan. | U 4 3 cos. 2 U 6 cos. 4 U] . 
 
 Hence, in forming a table of logarithms of the quantity 
 ^ tan. U 4 3 cos. 2 U 6 cos. 4 U} 
 
 it will be sufficient to add the logarithm of 1 e, in order to have that of 
 sin. x ; consequently we have the correction of the anomaly U, estimated 
 from the parabola, to obtain the corresponding anomaly in a very excen- 
 tric ellipse. 
 
 491. To find the masses of such planets as have satellites. 
 
 The equation 
 
 T _ 2 *a 2 
 V m 
 
 gives a very simple method of comparing the mass of a planet, having sa- 
 tellites, with that of the sun. In fact, M representing the mass of the sun, 
 if /tt the mass of the planet be neglected, we have 
 
 f*r^ & ft C* 
 
 VM 
 
 If we next consider a satellite of any planet ft-', and call its mass p. and 
 mean distance from the center of ///, h, and Tits periodic time, we shall 
 have 
 
 T = 2frh * 
 -vV+P 
 
 . *' + P _ b _! v T2 
 
 M " a 3 X T 2 * 
 
 This equation gives the ratio of the sum of the masses of the planet /*' 
 and its satellite to that of the sun. Neglecting therefore the mass of the 
 
 D2
 
 52 A COMMENTARY ON [SECT. XI. 
 
 satellite, as small compared with that of the planet, ov supposing their ra- 
 tio known, we have the ratio of the mass of the planet to that of the sun. 
 
 492. To determine the Elements of Elliptical Motion. 
 
 After having exposed the General Theory of Elliptical Motion and 
 Method of Calculating by converging series, in the two cases of nature, 
 that of orbits almost circular, and the case of orbits greatly excentric, it 
 remains to determine the Elements of those orbits. In fact if we call V 
 the velocity of ^ in its relative motion about M, we have 
 
 V - dx g + dy* + dz' 
 
 dt 2 
 
 and the last of the equations (P) of No. 478, gives 
 
 f 2 1 ) 
 
 V 2 = m i f . 
 
 I a ) 
 
 To make m disappear from this expression, we shall designate by U 
 the velocity which /* would have, if it described about M, a circle whose 
 radius is equal to the unity of distance. In this hypothesis, we have 
 
 g = a = 1, 
 and consequently 
 
 U 2 = m. 
 Hence 
 
 V 2 = U 2 {- } . 
 If a J 
 
 This equation will give the semi-axis major a of the orbit, by means of 
 the primitive velocity of ^ and of its primitive distance from M. But a is 
 positive in the ellipse, and infinite in the parabola, and negative in the 
 
 hyperbola. Thus the orbit described by /j> is an ellipse, a parabola, or hy- 
 
 i 2 
 perbola, according as V is < = or > than U ./ - . It is remarkable 
 
 that the direction of primitive motion has no influence upon the species of 
 conic section. 
 
 To find the excentricity of the orbit, we shall observe that if t repre- 
 sents the angle made by the direction of the relative motion of //, with the 
 radius-vector, we have 
 
 f 2 1 1 
 
 Substituting for V 2 its value m -| 1 , we have 
 
 1 = m ( ) cos. ! e; 
 d t* ^ f a /
 
 BOOK I.] NEWTON'S PRINC1PIA. 
 
 But by 480 
 
 2 2 2 . 
 
 = m a ( 1 e 2 ) : 
 
 e = 
 
 whence we know the excentricity a e of the orbit. 
 To find v or the true anomaly, we have 
 a(l e 2 ) 
 1 -f- e cos. v 
 
 e ) g 
 
 COS. V = 
 
 This gives the position of the Perihelion. Equations (f ) of No. 480 will 
 then give u and by its means the instant of the Planet's passing its peri- 
 helion. 
 
 To get the position of the orbit, referred to a fixed plane passing 
 through the center of M, supposed immoveable s let <f> be the inclination of 
 the orbit to this plane, and 8 the angle which the radius makes with the 
 Line of the Nodes. Let, Moreover, z be the primitive elevation of ^ 
 above the fixed plane, supposed known. Then we 
 shall have, CAD being the fixed plane, A D the 
 line of the nodes, A B = g , &c. &c. 
 
 z = B D . sin. <p = g sin. ft sin. <p ; 
 so that the inclination of the orbit will be known 
 when we shall have determined j3. For this pur- 
 pose, let X be the known angle which the primitive 
 
 direction of the relative motion of /* makes with the fixed plane ; then if 
 we consider the triangle formed by this direction produced to meet the 
 line of the nodes, by this last line and by the radius g, calling 1 the side 
 of the triangle opposite to j8, we have 
 ] g sin. /3 
 
 " sin. (j8 + ' 
 
 Next we have 
 
 z 
 
 -r- = sin. X 
 
 consequently 
 
 z sin. s 
 
 tan. 8 = 
 
 ^ sin. X z cos. s 
 The elements of the Planetary Orbit being determined by these formu- 
 las, in terms of g and z, of the velocity of the planet, and of the direction 
 of its motion, we can find the variation of these elements corresponding 
 
 D3
 
 54 A COMMENTARY ON [SECT. XI. 
 
 to the supposed variations in the velocity and its direction ; and it will be 
 easy, by methods about to be explained, from hence to obtain the differ- 
 ential variations of the Elements, due to the action of perturbing forces. 
 Taking the equation 
 
 V s = U J {- * j. 
 If a J 
 
 In the circle a = g and .*. 
 
 v = u IL 
 
 \ f 
 
 so that the velocities of the planets in different circles are reciprocally as 
 the squares of their radii (see Prop. IV of Princip.) 
 In the parabola, a = oo , 
 
 the velocities in the different points of the orbit, are therefore in this case 
 reciprocally as the squares of the radius- vectors; and the velocity at each 
 point, is to that which the body would have if it described a circle whose 
 radius = the radius-vector g, as V 2 : 1 (see 160) 
 
 An ellipse indefinitely diminished in breadth becomes a straight line, 
 and in this case V expresses the velocity of ft, supposing it to descend in 
 a straight line towards M. Let & fall from rest, and its primitive dis- 
 tance be i ; also let its velocity at the distance j' be V ; the above expres- 
 sion will give 
 
 0=*--! 
 
 a 
 whence 
 
 V'= U 
 
 Many other results, which have already been determined after another 
 manner, may likewise be obtained from the above formula. 
 493. The equation 
 
 _ dx 2 + dy* + dz 2 __ / _ J. N 
 d t 2 vjf a / 
 
 is remarkable from its giving the velocity independently of the excentricity. 
 It is also shown from a more general equation which subsists between the 
 axis-major of the orbit, the chord of the elliptic arc, the sum of the ex- 
 treme radius- vectors, and the time of describing this arc. 
 To obtain this equation, we have 
 a(] e 2 ) 
 
 s 
 
 e cos. v
 
 BOOK I.] NEWTON'S PRINCIP1A. 53 
 
 g = a (1 e cos. u) 
 
 3 
 
 t = a * (u e sin. a) ; 
 
 in which suppose f, v, u, and t to correspond to the first extremity of the 
 elliptic arc, and that g', v', u', t' belong to the other extremity ; so that we 
 also have 
 
 Let now 
 
 1 + e cos. v 
 f' = a ( 1 e cos. u') 
 
 t' =r a^ (u' e sin. u'). 
 
 then, if we take the expression oft from that oft', and observe tliat 
 
 sin. u' sin. u = 2 sin. /3 cos. /3' 
 we shall have 
 
 5. 
 
 T = 2a 2 \j3 e sin. /3 cos. ft], 
 If we add them together taking notice that 
 
 cos. u' + cos. u = 2 cos. |8. cos. /3" 
 we shall get 
 
 R = 2 a (1 e cos. |8 cos. /S'). 
 
 Again, if c be the chord of the elliptic arc, we have 
 
 c 2 = g* + / 2 2 g g' cos. (v v') 
 
 but the two equations 
 
 a /I e a) 
 
 e = -f-~ ; e = a ( 1 e cos. u) 
 
 1 -f- e cos - v 
 
 give these 
 
 cos. u e . &V 1 e 2 . sin. u 
 
 cos. v = a ; sin. v = ; 
 
 S S 
 
 and in like manner we have 
 
 cos. u' e , a V 1 e 2 sin. u' 
 cos. v' = a . -. ; sin. v = -, ; 
 
 r f 
 
 whence, we get 
 
 g f'cos. (v v') = a 2 (e cos. u) (e cos. u') + a s (l e 2 ) sin. u sin. u ; 
 
 and consequently 
 
 c ! = 2a 2 (l e 2 )Jl sin. u sin. u' cos. u cos. u'} 
 
 + a 2 e 2 (cos. u cos. u') B ; 
 D 4
 
 56 A COMMENTARY ON [SECT. XI. 
 
 But 
 
 sin. u sin. u' + cos. u cos. u' = 2 cos. z (3 1 
 cos. u cos. u' = 2 sin. j3 sin. /3' 
 .-.c s = 4a s sin. 2 /3(l e 2 cos. 2 /3'). 
 We therefore have these three equations, scil. 
 R = 2 a {1 e cos. |8 cos. &} ; 
 T = 2 a ^ J/3 e sin. |3 cos. j3'\ , 
 c 2 = 4 a 2 sin. 2 18 (1 e*cos. 2 |8). 
 The first of them gives 
 
 2a R 
 
 e cos. p = s - 
 2 a cos. /S 
 
 and substituting this value of e cos. j6' in tlie two others, we shall have 
 
 -- T?\ 2 
 
 f ,9 a 
 
 c 2 = 4a a tan. 2 /3|cos. 2 /3 ( 
 
 These two equations do not involve the excentricity e, and if in the 
 first we substitute for |8 its value given by the second, we shall get T in a 
 function c, R, and a. Thus we see that the time T depends only on the 
 semi-axis major, the chord c and the sum R of the extreme radius- 
 vectors. 
 
 If we make 
 
 2a R + c 2a R c 
 
 rj ' rj' 
 
 2a 2a 
 
 the last of the preceding equations will give 
 
 cos. 2 $ = z z' + V (1 z 2 ). (1 z /2 ) ; 
 whence 
 
 2 /? = cos. - l z' cos. - ' z 
 
 (for cos. (A B) = cos. A cos. B + sin. A sin. B). 
 Consequently 
 
 _ sin, (cos. ~ ' z') sin, (cos. ~ ' z) t 
 
 we have also 
 
 2a R 
 
 
 Hence the expression of T will become, observing that if T is the du- 
 ration of the sidereal revolution, whose mean distance from the sun is 
 taken for unity, we have
 
 BOOK I.] NEWTON'S PRINCIPIA. 57 
 
 T = 2 r, 
 
 a 
 T = - 
 
 - {cos.- 1 z' cos.- 1 z sin. (cos.- 1 z') + sin. (cos- 1 z) } . . . (a) 
 
 Since the same cosines may belong to many arcs, this expression is 
 ambiguous, and we must take care to distinguish the arcs which corre- 
 spond to z, z'. 
 
 In the parabola, the semi-axis major is infinite, and we have 
 
 i / / /\ 1 /R + c\ I 
 
 cos.^z' sin. (cos. - ' z ) = ( - ) . 
 
 o \ a / 
 
 And making c negative we shall have the value of 
 
 cos. ~ l z sin. (cos. * z) ; 
 
 hence the formula (a) will give the time T employed to describe the arc 
 subtending the chord c, scil. 
 
 T = + f + C) * + fe + ''- C) 
 
 the sign being taken, when the two extremities of the parabolic arc are 
 situated on the same side of the axis of the parabola. 
 Now T being = 365.25638 days, we have 
 
 -5_ = 9. 688754 days. 
 
 The formula (a) gives the time of a body's descent in a straight line to- 
 wards the focus, beginning from a given distance ; for this, it is suffi- 
 cient to suppose the axis-minor of the ellipse indefinitely diminished. If 
 we suppose, for example, that the body falls from rest at the distance 2 a 
 from the focus and that it is required to find the time (T) of falling to 
 the distance c, we shall have 
 
 R = 2 a + , g = 2 a c 
 whence 
 
 / c a 
 
 z' = - 1, z = 
 
 and the formula gives 
 
 a * T f . c a J 2 ac c ) 
 
 T = -\ T cos. - l -- f- . / - 5 I 
 
 2 T I a \ a 2 
 
 There is, however, an essential difference between elliptical motion to- 
 wards the focus, and the motion in an ellipse whose breadth is indefinite- 
 ly small. In the first case, the body having arrived at the focus, passes 
 beyond it, and again returns to the same distance at which it departed ; 
 but in the second case, the body having arrived at the focus immediately 
 returns to the point of departure. A tangential velocity at the aphelion,
 
 58 A COMMENTARY ON [SECT. XL 
 
 however small, suffices to produce this difference which has no influence 
 upon the time of the body's descent to the center, nor upon the ve- 
 locity resolved parallel to the axis-major. Hence the principles of the 
 7th Section of Newton give accurately the Times and Velocities, although 
 they do not explain all the circumstances of motion. For it is clear that 
 if there be absolutely no tangential velocity, the body having reached the 
 center of force, will proceed beyond it to the same distance from which it 
 commenced its motion, and then return to the center, pass through it, 
 and proceed to its first point of departure, the whole being performed in 
 just double the time as would be required to return by moving in the in- 
 definitely small ellipse. 
 
 494. Observations not conducting us to the circumstances of the pri- 
 mitive motion of the heavenly bodies ; by the formulas of No. 492 we 
 cannot determine the elements of their orbits. It is necessary for this 
 end to compare together their respective positions observed at different 
 epochs, which is the more difficult from not observing them from the 
 center of their motions. Relatively to the planets, we can obtain, by 
 means of their oppositions and conjunctions, their Heliocentric Longitude. 
 This consideration, together with that of the smallness of the excentricity 
 and inclination of their orbits to the ecliptic, affords a very simple method 
 of determining their elements. But in the present state of astronomy, 
 the elements of these orbits need but very slight corrections ; and as the 
 variations of the distances of the planets from the earth are never so great 
 as to elude observation, we can rectify, by a great number of observations, 
 the elements of their orbits, and even the errors of which the observa- 
 tions themselves are susceptible. But with regard to the Comets, this is 
 not feasible ; we see them only near their perihelion : if the observations 
 we make on their appearance prove insufficient for the determination of 
 their elements, we have then no means of pursuing them, even by thought, 
 through the immensity of space, and when after the lapse of ages, they 
 again approach the sun, it is impossible for us to recognise them. It be- 
 comes therefore important to find a method of determining, by observa- 
 tions alone during the appearance of one Comet, the elements of its orbit. 
 But this problem considered rigorously surpasses the powers of analysis, 
 and we are obliged to have recourse to approximations, in order to obtain 
 the first values of the elements, these being afterwards to be corrected to 
 any degree of accuracy which the observations permit. 
 
 If we use observations made at remote intervals, the eliminations will 
 lead to impracticable calculations ; we must therefore be content to con-
 
 BOOK IJ NEWTON'S PRINCIPIA. 59 
 
 sider only near observations ; and with this restriction, the problem is abun- 
 dantly difficult. 
 
 It appears, that instead of directly making use of observations, it is 
 better to get from them the data which conduct to exact and simple re- 
 sults. Those in the present instance, which best fulfil that condition, are 
 the geocentric longitude and latitude of the Comet at a given instant, and 
 their first and second differences divided by the corresponding powers of 
 the element of time ; for by means of these data, we can determine rigo- 
 rously and with ease, the elements, without having recourse to a single 
 integration, and by the sole consideration of the differential equations of 
 the orbit. This way of viewing the problem, permits us moreover, to 
 employ a great number of near observations, and to comprise also a con- 
 siderable interval between the extreme observations, which will be found 
 of great use in diminishing the influence of such errors, as are due to ob- 
 servations from the nebulosity by which Comets are enveloped. Let us 
 first present the formulas necessary to obtain the first differences, of the 
 longitude and latitude of any number of near observations ; and then de- 
 termine the elements of the orbit of a Comet by means of these differences ; 
 and lastly expose the method which appears the simplest, of correcting 
 these elements by three observations made at remote intervals. 
 
 495. At a given epoch, let a be the geocentric longitude of a Comet, 
 and 6 its north geocentric latitude, the south latitudes being supposed ne- 
 gative. If we denote by s, the number of days elapsed from this epoch, 
 the longitude and latitude of the Comet, after that interval, will, by using 
 Taylor's Theorem (481), be expressed by these two series 
 
 /d 
 
 ITS T 
 
 a\ 
 
 ) 
 
 We must determine the values of 
 
 2 
 
 by means of several observed geocentric longitudes and latitudes. To do 
 this most simply, consider the infinite series which expresses the geocen- 
 tric longitude. The coefficients of the powers of s, in this series, ought to 
 be determined by the condition, that by it is represented each observed 
 longitude ; we shall thus have as many equations as observations ; and i( 
 their number is n, we shall be able to find from them, in series, the n
 
 60 A COMMENTARY ON [SECT. XI. 
 
 quantities , (T-) &c. But it ought to be observed that s being sup- 
 posed very small, we may neglect all terms multiplied by s n , s n + ', &c. 
 which will reduce the infinite series to its n first terms ; which by n ob- 
 servations we shall be able to determine. These are only approximations, 
 and their accuracy will depend upon the smallness of the terms which are 
 omitted. They will be more exact in proportion as s is more diminutive, 
 and as we employ a greater number of observations. The theory of inter- 
 polations is used therefore To find a rational and integer function qfs such, 
 that in substituting therein for s the number of days 'which correspond to each 
 observation, it shall become the observed longitude. 
 
 Let /3, /3', /3", &c. be the observed longitudes of the comet, and by 
 i, i', i", &c. the corresponding numbers of days from the given epoch, the 
 numbers of the days prior to the given epoch being supposed negative. 
 
 If we make 
 
 01 o on ___ of out on 
 
 -, = 3/3; -?r. TT- = 5/3'; -^y-. TJT = 3/3"; &c. 
 
 i i i" i' \"' i" 
 
 &c. ; 
 
 the required functions will be 
 
 /3 + ( S _i).a0 + ( S _ i)(s i / ).a 2 /3+(s i)(s i')(s i / 
 
 for it is easy to perceive that if we make successively s = i, s = i', s = i // , &c. 
 
 it will change itself into /3, (?, $", &c. 
 
 Again, if we compare the preceding function with this 
 /da x s 2 /d 2 \ , 
 
 " + s (;n) + 2 (dir) + &c - 
 
 we shall have by equating coefficients of homogeneous terms. 
 a=j8 i a 8 + i . V. 5 2 /3 i . i'. i" a 3 /3+&c. 
 
 &c. 
 
 The higher differences of a will be useless. The coefficients of these 
 expressions are alternately positive and negative ; the coefficient of 5 r /3 
 is, disregarding the sign, the product of r and r together of r quantities 
 i, i', . . . . i (l - r ' in the value of ; it is the sum of the products of the
 
 BOOK I.] NEWTON'S PRINCIPIA. 61 
 
 same quantities, r 1 together in the value of T~) ; lastly it is the sum 
 of the products of these quantities r 2, together in the value of 
 
 2 
 
 If 7, 7', /', &c. be the observed geocentric latitudes, we shall have the 
 values of 6, (y-) (T- - 2 ) > &c. by changing in the preceding expressions 
 
 for a (j^V (<r ")' &c - tne quantities /3, /3', /3" into 7, 7', 7". 
 
 These expressions are the more exact, the greater the number of ob- 
 servations and the smaller the intervals between them. We might, 
 therefore, employ all the near observations made at a given epoch, pro- 
 vided they were accurate ; but the errors of which they are always sus- 
 ceptible will conduct to imperfect results. So that, in order to lessen the 
 influence of these errors, we must augment the interval between the ex- 
 treme observations, employing in the investigation a greater number of 
 them. In this way with five observations we may include an interval of 
 thirty-five or forty degrees, which would give us very near approximations 
 to the geocentric longitude and latitude, and to their first and second 
 differences. 
 
 If the epoch selected were such, that there were an equal number of 
 observations before and after it, so that each successive longitude may 
 have a corresponding one which succeeds the epoch. This condition will 
 
 j 1 g 
 
 give values still more correct of a, (-? ) and ( , jj , and it easily appears 
 
 that new observations taken at equal distances from either side of the epoch, 
 would only add to these values, quantities which, with regard to their last 
 
 \ a 
 
 terms, would be as s 2 (^ 2 )to . This symmetrical arrangement takes 
 
 place, when all the observations being equidistant, we fix the epoch at 
 the middle of the interval which they comprise. It is therefore advanta- 
 geous to employ observations of this kind. 
 
 In general, it will be advantageous to fix the epoch near the middle of 
 this interval ; because the number of days included between the extreme 
 observations being less considerable, the approximations will be more con- 
 vergent. We can simplify the calculus still more by fixing the epoch at 
 the instant of one of the observatiqns ; which gives immediately the values 
 of , and 8.
 
 62 A COMMENTARY ON [SECT. XI 
 
 When we shall have determined as above the values of 
 
 /dx /d 2 x ,djx , /d'<K 
 Vis/' VdTV \ds/' VdsV 
 
 we shall then obtain as follows the first and second differences of a, and t> 
 divided by the corresponding powers of the elements of time. If we neg- 
 lect the masses of the planets and comets, that of the sun being the unit 
 of mass ; if, moreover, we take the distance of the sun from the earth for 
 the unit of distance ; the mean motion of the earth round the sun will 
 be the measure of the time t. Let therefore X be the number of se- 
 conds which the earth describes in a day, by reason of its mean sidereal 
 motion ; the time t corresponding to the number of days will be X s ; we 
 shall, therefore, have 
 
 d 
 
 (d \ _ 1 
 dV ~ T 
 
 /d * a\ _ l/d 2 \ 
 
 vTFv *~ x 1 \d~sV ' 
 
 Observations give by the Logarithmic Tables, 
 
 log. X = 4. 0394622 
 and also 
 
 log. X 2 = log. X + log. ~ 
 
 R being the radius of the circle reduced to seconds ; whence 
 log.X 2 = 2.2750444; 
 
 .. if we reduce to seconds, the values of IT) > and of [-, z j , we shall 
 
 1 1 o 
 
 have the logarithms of (' ") , and of (T i) by taking from the logarithms 
 of these values the logarithms of 4. 039422, and 2. 2750444. In like 
 
 manner we get the logarithms of d ) , jf-j > after subtracting the 
 
 same logarithms, from the logarithms of their values reduced to seconds. 
 On the accuracy of the values of 
 
 da\ /d 2 \ . /d 
 
 /da\ / 2 \ . / (K , / < 
 
 Mat)' (dT)' "' (art) mid (a t 
 
 depends that of the following results ; and since their formation is very 
 simple, we must select and multiply observations so as to obtain them with 
 the greatest exactness possible. We shall determine presently, by means 
 of these values, the elements of the orbit of a Comet, and to generalize 
 these results, we shall
 
 BOOK I.] NEWTON'S PRINCIPIA. fi3 
 
 496. Investigate the motion of a system of bodies sollicited by any forces 
 whatever. 
 
 Let x, y, z be the rectangular coordinates of the first body ; x', y', z' 
 those of the second body, and so on. Also let the first body be sollicited 
 parallel to the axes of x, y, z by the forces X, Y, Z, which we shall sup- 
 pose tend to diminish these variables. In like manner suppose the second 
 body sollicited parallel to the same axes by the forces X', Y', Z', and so 
 on. The motions of all the bodies will be given by differential equations 
 of the second order 
 
 = dt + x ' = JT+ Y ; = a^ + z ; 
 
 d 2 x' d 2 v' (\ 2 -r' 
 O __ _ fi i X' = 4- Y' - -I- 7' 
 
 &c. = &c. 
 
 If the number of the bodies is n, that of the equations will be 3 n ; and 
 their finite integrals will contain 6 n arbitrary constants, which will be the 
 elements of the orbits of the different bodies. 
 
 To determine these elements by observations, we shall transform the 
 coordinates of each body into others whose origin is at the place of the 
 observer. Supposing, therefore, a plane to pass through the eye of the 
 observer, and of which the situation is always parallel to itself, whilst the 
 observer moves along a given curve, call r, r' r", &c. the distances of 
 the observer from the different bodies, projected upon the plane ; 
 a, of, a", &c. the apparent longitudes of the bodies, referred to the same 
 plane, and d, ^, 0", &c. their apparent latitudes. The variables x, y, z 
 will be given in terms of r, a, 6, and of the coordinates of the observer. 
 In like mannei', x', y', z' will be given in functions of r 7 , a', 6', and of the 
 coordinates of the observer, and so on. Moreover, if we suppose that the 
 forces X, Y, Z ; X', Y', Z', &c. are due to the reciprocal action of the 
 bodies of the system, and independent of attractions ; they will be given in 
 functions of r, r', r", &c. ; a, a', a", &c. ; 6, d', &", &c. and of known quan- 
 tities. The preceding differential equations will thus involve these new 
 variables and their first and second differences. But observations make 
 known, for a given instant, the values of 
 
 d \ /d z \ . /d 0\ /d 2 0\ , /d '\ 
 
 -i 1 , &c. 
 
 There will hence of the unknown quantities only remain r, r 7 , r", &c. 
 and their first and second differences. These unknowns are in number 
 3 n, and since we have 3 n differential equations, we can determine them.
 
 64 A COMMENTARY ON [SECT. XI. 
 
 At the same time we shall have the advantage of presenting the first and 
 second differences of r, r', r", &c. under a linear form. 
 
 The quantities , 0, r, a', ^, r', &c. and their first differences divided by 
 d t, being known ; we shall have, for any given instant, the values of 
 x, y, z, x', y', z', &c. and of their first differences divided by d t. If we 
 substitute these values in the 3 n finite integrals of the preceding equa- 
 tions, and in the first differences of these integrals; we shall have 6 n 
 equations, by means of which we shall be able to determine the 6 n arbi- 
 trary constants of the integrals, or the elements of the orbits of the dif- 
 ferent bodies. 
 
 497. To apply this method to the motion of the Comets, 
 
 We first observe that the principal force which actuates them is the 
 attraction of the sun ; compared with which all other forces may be ne- 
 glected. If, however, the Comet should approach one of the greater 
 planets so as to experience a sensible perturbation, the preceding method 
 will still make known its velocity and distance from the earth ; but this 
 case happening but very seldom, in the following researches, we shall ab- 
 stain from noticing any other than the action of the sun. 
 
 If the sun's mass be the unit, and its mean distance from the earth the 
 unit of distance; if, moreover, we fix the origin of the coordinates 
 x, y, z of a Comet, whose radius-vector is g ; the equations (0) of No. 475 
 will become, neglecting the mass of the Comet, 
 
 d*x x 
 
 U ^^ j r T* ' 
 
 d2 y y 
 
 II j i ^j_ r 
 
 - dt- " h ? 3 
 
 _ d 2 Z Z 
 
 *-dt* + ? 
 
 Let the plane of x, y be the plane of the ecliptic. Also let the axis of 
 x be the line drawn from the center of the sun to the first point of aries, 
 at a given epoch ; the axis of y the line drawn from the center of the sun 
 to the first point of cancer, at the same epoch ; and finally the positive 
 values of z be on the same side as the north pole of the ecliptic. Next 
 call x', y 7 the coordinates of the earth and R its radius-vector. This be- 
 ing supposed, transfer the coordinates x, y, z to others relative to the 
 observer ; and to do this let a be the geocentric longitude, and r its dis- 
 tance from the center of the earth projected upon the ecliptic ; then we 
 shall have 
 
 x = x' + r cos. a ; y = y -f r sin, a ; z = r tan. 6.
 
 BOOK I.] NEWTON'S PRINCIPIA. 65 
 
 If we multiply the first of equations (k) by sin. , and take from the re- 
 sult the second multiplied by cos. a, we shall have 
 
 d 2 x d 2 y x sin. a y cos. a 
 
 whence we derive, by substituting for x, y their values given above, 
 
 d 2 x' d 2 y' x' sin. a y cos. a 
 
 = Sin. a . TTT COS, a . -~i- -f 
 
 d t 2 d t 2 3 
 
 dax /d 2 a 
 
 The earth being retained in its orbit like a comet, by the attraction of 
 the sun, we have 
 
 d 8 x' x' d 2 y' y' 
 
 : dt 2 H " R 3> l : TF " h ,fr ; 
 
 which give 
 
 d 2 x' d 2 y __ y' cos. a K' sin. 
 
 We shall, therefore, have 
 
 rda 
 
 Let A be the longitude of the earth seen from the sun ; we shall have 
 
 x' = R cos. A ; y' = R sin. A ; 
 therefore 
 
 y' cos. x' sin. a = R sin. (A a) ; 
 and the preceding equation will give 
 
 
 /dr x _Rsin.(A a) M 1) ' Vdt 2 / 
 
 (dt)= /da, ' lR>-"7 '" /dx 
 
 - 
 
 Now let us seek a second expression for (-T-) For this purpose we 
 
 will multiply the first of equations (k) by tan. 6 . cos. , the second by 
 tan. 6 sin. a, and take the third equation from the sum of these two pro- 
 ducts ; we shall thence obtain 
 
 ./ d z x , d 2 y) 
 
 = tan. 6 S cos. a -r -f sin. a ^ | r 
 
 l dt 2 dt z ; 
 
 x cos. a + y sin. a d 2 z z 
 + tan. 6 . - ^-^ -- -y -- r . 
 
 S 3 dt 2 f s 
 
 This equation will become by substitution for x, y, z 
 
 Vox.. II.
 
 66 A COMMENTARY ON [SECT. XI. 
 
 _ 
 
 COS. 2 COS. 2 COS. 3 
 
 But 
 
 Therefore, 
 
 R sin. 6 cos. cos. (A a) f 1 
 
 f 1 11 
 
 IP""B" 3 J ..... ( 
 
 If we take this value of ( p ) from the first and suppose 
 /d\ /d*0\ /d^\ /d 2 \ . rt /da\ /d^\ 2 , /da\ 3 . 
 
 Si) (arO- r t ) (a^) +2 (ai) (at) tan -'+(a-r) '- 
 
 _ - . - - 
 
 ,-\ sin. 6 cos. cos. (A ) + ( , . j sin. (A o) 
 
 we shall have 
 
 R 
 
 The projected distance r of the comet from the earth, being always po- 
 sitive, this equation shows that the distance g of the comet from the sun, 
 is less or greater than the distance R of the sun from the earth, according 
 as 11! is positive or negative; the two distances are equal if (if = 0. 
 
 By inspection alone of a celestial globe, we can determine the sign of 
 // ; and consequently whether the comet is nearer to or farther from the 
 Earth. For that purpose imagine a great circle which passes through 
 two Geocentric positions of the Comet infinitely near to one another. 
 Let 7 be the inclination of this circle to the ecliptic, and X the longitude 
 of its ascending node ; we shall have 
 
 tan. 7 sin. ( x) = tan. 6 ; 
 whence 
 
 d 6 sin. (a X) = d a sin. 6 cos. 6 cos. (a X).
 
 BOOK L] NEWTON'S PRINCIPIA. 67 
 
 Differentiating, we have, also 
 
 1 sin. d cos. 6; 
 
 d 2 6 t being the value of d 2 6, which would take place, if the apparent mo- 
 tion of the Comet continued in the great circle. The value of /*' thus be- 
 comes, by substituting for d d its value 
 
 d a sin. t cos. 6 cos. (a X) 
 sin. (a X) 
 
 sin. d cos. 6 sin. (A X) 
 
 The function - . ' \ - f is constantly positive : the value of a, is there- 
 sin. 6 cos. 6 
 
 fore positive or negative, according as- \ J-TS has the same or 
 
 a different sign from that of sin. (A X). But A X is equal to two 
 right angles plus the distance of the sun from the ascending node of the 
 great circle. Whence it is easy to conclude that (if will be positive or 
 negative, according as in a third geocentric position of the comet, inde- 
 finitely near to the two first, the comet departs from the great circle on 
 the same or the opposite side on which is the sun. Conceive, therefore, 
 that we make a great circle of the sphere pass through the two geocentric 
 positions of the comet ; then according as, in a third consecutive geocen- 
 tric position, the comet departs from this great circle, on the same side as 
 the sun or on the opposite one, it will be nearer to or farther from the 
 sun than the Earth. If it continues to appear in this great circle, it will 
 be equally distant from both ; so that the different deflections of its ap- 
 parent path points out to us the variations of its distance from the sun. 
 
 To eliminate g from equation (3), and to reduce this equation so as to 
 contain no other than the unknown r, we observe that f 2 = x 
 in substituting for x, y, z, their values in tei'ms of 
 
 r, a, and 6; 
 and we have 
 
 r 2 
 2 = x' 2 + y' 2 + 2rx'cos. + /sin. "l + ^T^/ 
 
 but we have 
 
 x' = R cos. A, y' = R sin. A ; 
 
 " s * = d^~* + 2Rr cos - (A ~ a) + RS; 
 
 E 2
 
 68 A COMMENTARY ON [SECT. XL 
 
 But 
 
 x' = R cos. A ; y' = R sin. A 
 
 .-. f * = ^-T. + 2 R r cos. (A a) + R 2 . 
 cos. * 6 
 
 If we square the two members of equation (3) put under this form 
 
 f'fc'R'r + 1}= R 3 
 we shall get, by substituting for g *, 
 
 (c^ + 2 R r C S * ( A - ) + R } V R2 r + 1J'= R6 - - (4) 
 an equation in which the only unknown quantity is r, and which will rise 
 to the seventh degree, because a term of the first member being equal to 
 R 6 , the whole equation is divisible by r. Having thence determined r, 
 
 we shall have (-,) by means of equations (1) and (2). Substituting, for 
 
 example, in equation (1), for -^ -, its value -^- , given by equation 
 (3) ; we shall have 
 
 The equation (4) is often susceptible of many real and positive roots ; 
 reducing it and dividing by r, its last term will be 
 
 2 R s cos. 6 6\ft' R 3 + 3 cos. (A a)}. 
 
 Hence the equation in r being of the seventh degree or of an odd de- 
 gree, it will have at least two real positive roots if /&' R 3 + 3 cos. (A a) 
 is positive ; for it ought always, by the nature of the problem, to have 
 one positive root, and it cannot then have an odd number of positive 
 roots. Each real and positive value of r gives a different conic section, 
 for the orbit of the comet ; we shall, therefore, have as many curves 
 which satisfy three near observations, as r has real and positive values; 
 and to determine the true orbit of the comet, we must have recourse to a 
 new observation. 
 
 498. The value of r, derived from equation (4) would be rigorously 
 
 exact, if 
 
 do\ /d 2 \ /d 0\ /d 2 
 
 were exactly known ; but these quantities are only approximate. In fact, 
 by the method above exposed, we can approximate more and more, mere- 
 ly by making use of a great number of observations, which presents the 
 advantage of considering intervals sufficiently great, and of making the 
 errors arising from observations compensate one another. But this
 
 BOOK I.] NEWTON'S PRINCIP1A. 69 
 
 method has the analytical inconvenience of employing more than three 
 observations, in a problem where three are sufficient. This may be 
 obviated, and the solution rendered as approximate as can be wished by 
 three observations only, after the following manner. 
 
 Let and 0, representing the geocentric longitude and latitude of the 
 intermediate; if we substitute in the equations (k) of the preceding 
 No. instead of x, y, z their values x' + r cos. a ; y' + r sin. a ; and 
 
 r tan. 6; they will give (-, 2 J , (j 1 2 ) an ^ ( j 2) m functions of r, , and 
 6, of their first differences and known quantities. If we differentiate these, 
 
 we shall have ( j-r? ) j VTTi) an ^ (iTT 3 ) m terms ^ r "' ^ an< ^ ^ their 
 
 first and second differences. Hence by equation (2) of 497 we may eli- 
 minate the second difference of r by means of its value and its first differ- 
 
 ence. Continuing to differentiate successively the values of (-r j ) > ("TTs) > 
 
 and eliminating the differences of a, and of & superior to second differences, 
 and all the differences of r, we shall have the values of 
 
 /v / < 
 
 (irr)' Carry j&c - intennsor 
 
 d 
 
 this being supposed, let 
 
 a,, a, a', 
 
 be the three geocentric observed longitudes of the Comet; /? 6, 6' its 
 three corresponding geocentric latitudes ; let i be the number of days 
 which separate the first from the second observation, and i' the interval 
 between the second and third observation ; lastly let X be the arc which 
 the earth describes in a day, by its mean sidereal motion; then by (481) 
 we have 
 
 2 22 i 3 X 3 /d 3 a\ , 
 
 - 3 + **' 
 
 , ., ,/da\ , i /2 .X 2 /d 2 ax i /3 .X 8 /d s a\ 
 
 = * + < - Hart) + ,; 2 (<nd + ii(d^) + &c "
 
 70 A COMMENTARY ON [SECT. XI. 
 
 If we substitute in these series for 
 
 tlieir values obtained above, we shall have four equations between the 
 five unknown quantities 
 
 d a\ /d 2 a\ /d 6\ d 2 
 
 These equations will be the more exact in proportion as we consider a 
 greater number of terms in the series. We shall thus have 
 
 in terms of r and known quantities ; and substituting in equation (4) of 
 the preceding No. it will contain the unknown r only. As to the rest, 
 this method, which shows how to approximate to r by employing three 
 observations only, would require in practice, laborious calculations, and 
 it is a more exact and simple process to consider a greater number of ob- 
 servations by the method of No. 495. 
 
 499. When the values of r and -TT shall be determined, we shall have 
 
 those of 
 
 /d x\ /d y\ , /d z\ 
 
 ^'Mcu)' (dl) and Vdl)' 
 
 by means of the equations 
 
 x = 11 cos. A + r cos. a 
 
 y = R sin. A + r sin. 
 
 z = r tan. 6 
 and of their differentials divided by d t, viz. 
 
 On) = (itr)"- A - R O in - A 
 A * 
 
 /d z\ /d r\ A 
 
 (,u) = (cn) '""' + 
 
 The values of (' ! , A ) and of ('-yr) are given by the Theory of the 
 
 V U w ' ** ** ' 
 
 motion of the Earth : 
 
 To facilitate the investigation let E be the excentricity of the earth's
 
 BOOK I.] NEWTON'S PRINCIPI A. 71 
 
 orbit, and H the longitude of its perihelion ; then by the nature of 
 elliptical motion we have 
 
 V(1-E 2 ). _ 1-E* 
 
 ~R r ~ - 1 -f-Ecos. (A H)' 
 
 These two equations give 
 
 /d RN E sin. (A H) 
 \dt) V (1 E 2 ) 
 
 Let R' be the radius- vector of the earth corresponding to the longitude 
 A of this planet augmented by a right angle ; we shall have 
 
 1 Esin. (A H)' 
 whence is derived 
 
 T? / A u\ R' 1 + E 2 
 E sin. ( A H) = - > R , ; 
 
 /d Rx R' + E 2 1 
 
 ' Vdt/~ R' V (1 E 2 )* 
 
 If we neglect the square of tlie excentricity of the earth's orbit, which is 
 very small, we shall have 
 
 
 dt 
 
 the preceding values of (-, ) and f Will hence become 
 
 /dx\ _ . A sin. A /d r\ /d\ 
 
 (d t) = < R - J) COS ' A - -- IT + (fft) COS ' a - r (dt) Sin ' " ; 
 
 /dy\ ._,. ,. . . cos. A , /d r\ . /da\ 
 
 (ai) =( R - 1} sm - A +-R- + (d-t) sm * a +r (d t ) cos - a; 
 
 R, R', and A being given immediately by the tables of the sun, the esti- 
 mate of the six quantities x, y, Z J(^T-)J (nt ) J (HT) w ^ ^ e eas 7 
 
 when r and (-p-) shall be known, Hence we derive the elements of the 
 
 orbit of the comet after this mode. 
 
 The indefinitely small sector, which the projection of the radius-vector 
 and the comet upon the plane of the ecliptic describes during 'the element 
 
 of time d t, is -- ^ j-- -- ; and it is evident that this sector is posi- 
 
 A 
 tive or negative, according as the motion of the comet is direct or retro- 
 
 grade. Thus in forming the quantity x ( ) 7 (rs ) it w iH indicate 
 
 by its sign, the direction of the motion of the comet. 
 
 E4,
 
 72 A COMMENTARY ON [SECT. XI. 
 
 To determine the position of the orbit, call <p its inclination to the 
 ecliptic, and I the longitude of the node, which would be ascending if the 
 motion of die comet were direct or progressive. We shall have 
 z = y cos. I tan. <p x sin. I tan. <p 
 
 These two equations give 
 
 , 
 
 tan. I = 
 
 /dz\ 
 X \T~J 
 
 f iz\ 
 
 ,d t) 
 tan. 9 = 
 
 Wherein since <p ought always to be positive and less than a right 
 angle, the sign of sin. I is known. But the tangent of I and the sign of 
 its sine being determined, the angle I is found completely. This angle 
 is the longitude of the ascending node of the orbit, if the motion is pro- 
 gressive; but to this we must add two right angles, in order to get the 
 longitude of the node when the motion is retrograde. It would be more 
 simple to consider only progressive motions, by making vary p, the in- 
 clination of the orbits, from zero to two right angles; for it is evident that 
 then the retrograde motions correspond to an inclination greater than a 
 right angle. 
 
 In this case, tan. <p has the same sign as x (-r-M y (T ) which will 
 
 determine sin. I, and consequently the angle I, which always expresses 
 the longitude of the ascending node. 
 
 If a, a e be the semi-axis major and the excentricity of the orbit, we 
 have (by 492) in making m = 1, 
 
 1 2 /dx\ / d y\ 2 /dz\' 
 a " 1 ~~V1 t/ " Wt/ Vdt/ ' 
 
 d 
 
 The first of these equations gives the semi-axis major, and the second 
 the excentricity. The sign of the function x (~^ + y ( j-J ) + z ( j-^) 
 
 shows whether the comet has already passed its perihelion ; for it ap- 
 proaches if this function is negative ; and in the contrary case, the comet 
 recedes from that point.
 
 BOOK I.] NEWTON'S PRINCIPIA. 73 
 
 Let T be the interval of time comprised between the epoch and pas- 
 sage of the comet over the perihelion ; the two first of equations (f) (480) 
 
 will give, observing that m being supposed unity we have n = a ~^ , 
 f = a (1 e cos. u) 
 
 5. 
 
 T = a 2 (u e cos. u). 
 
 The first of these equations gives the angle u, and the second T. This 
 time added to or subtracted from the epoch, according as the comet ap- 
 proaches or leaves its perihelion, will give the instant of its passage over 
 this point. The values of x, y, determine the angle which the projection 
 of the radius-vector makes with the axis of x ; and since we know the an- 
 gle I, formed by this axis and by the line of the nodes, we shall have the 
 angle which this last line forms with the projection of g ; whence we derive by 
 means of the inclination <p of the orbit, the angle formed by the line of the 
 nodes and the radius g. But the angle u being known, we shall have by 
 means of the third of the equations (f ), the angle v which this radius forms 
 with the line of the apsides ; we shall therefore have the angle comprised 
 between the two lines of the apsides and of the nodes, and consequently, 
 the position of the perihelion. All the elements of the orbit will thus be 
 determined. 
 
 500. These elements are given, by the preceding investigations, in terms 
 of r, (-r- and known quantities; and since f- is given in terms of r 
 
 by No. 497, the elements of the orbit will be functions of r and known 
 quantities. If one of them were given, we should have a new equation, 
 by means of which we might determine r ; this equation would have a 
 common divisor with equation (4) of No. 497; and seeking this di- 
 visor by the ordinary methods, we shall obtain an equation of the first 
 degree in terms of r ; we should have, moreover, an equation of condition 
 between the data of the observations, and this equation would be that 
 which ought to subsist, in order that the given element may belong to the 
 orbit of the comet. 
 
 Let us apply this consideration to the case of nature. First suppose 
 that the orbits of the comets are ellipses of great excentricity, and are 
 nearly parabolas, in the parts of their orbits in which these stars are 
 visible. We may therefore without sensible error suppose a = oo, and 
 
 consequently - = 0; the expression for - of the preceding No. will there- 
 
 il <J 
 
 fore give
 
 74 A COMMENTARY ON [SECT. XI. 
 
 2 dx* + dy 2 + dz 2 
 
 : s ~ dt 2 
 
 If we then substitute for (-, V in-M and (-riJ their values found in 
 ^d t y ^d t / ^d t/ 
 
 the same No., we shall have after all the reductions and neglecting the 
 square of R 7 1, 
 
 . 
 
 d t cos. 2 6 
 
 R 2 " g ' 
 
 Substituting in this equation for (T~) its value 
 
 found in No. 497, and then making 
 
 *(^)' B = H^) 4 +{( d aV") + '"^( A - 
 
 + \< * f d * a \ , 2 (cTt)* (dl) > 
 
 l tan - ' (dTV + "" tan ' tf sin ' < A ~ a) ^M j 
 
 and 
 
 { (drO + "' sin ' ( A ~ a ) } r sin. ( A-) 
 
 -d^- R (R-l) 
 
 dt) 
 
 we shall have 
 
 I 
 and consequently 
 
 ( 
 
 This equation rising only to the sixth degree, is in that respect, more
 
 BOOK I.] NEWTON'S PRINCIPIA. 75 
 
 simple than equation (4) of No. (497) ; but it belongs to the parabola 
 alone, whereas the equation (4) equally regards every species of conic 
 section. 
 
 501. We perceive by the foregoing investigation, that the determina- 
 tion of the parabolic orbits of the comets, leads to more equations than 
 unknown quantities; and that, therefore, in combining these equations in 
 different ways, we can form as many different methods of calculating the 
 orbits. Let us examine those which appear to give the most exact re- 
 sults, or which seem least susceptible of the errors of observations. 
 
 It is principally upon the values of the second differences f -r -^\ and 
 
 /d 2 $\ 
 
 ( -i g j, that these errors have a sensible influence. In fact, to determine 
 
 them, we must take the finite differences of the geocentric longitudes and 
 latitudes of the comet, observed during a short interval of time. But 
 these differences being less than the first differences, the errors of obser- 
 vations are a greater aliquot part of them ; besides this, the formulas of 
 No. 495 which determine, by the comparison of observations, the values 
 c , /da\ /d 6\ /d z a\ , /d 2 6\ . ., 
 
 ' ' Vdl)' Vdl) 5 VcTF) \cTtV glve grater precision the 
 
 four first of these quantities than the two last. It is, therefore, desirable 
 to rest as little as possible upon the second differences of a and 6; and 
 since we cannot reject both of them together, the method which employs 
 the greater, ought to give the more accurate results. This being granted 
 let us resume the equations found in Nos. 497, &c. 
 
 r = 
 
 cos. 2 6 
 
 + 2 R r cos. (A a) + R * 
 
 /drx R sin. (A a) /J_ jl 
 
 ~ "' " 
 
 f 
 
 tan - 
 
 j 
 
 V 
 
 R sin. 6 cos. 6 cos. (A a) M 1 
 
 d ' " 
 
 d~i
 
 76 A COMMENTARY ON [SECT. XI. 
 
 ' - 1) sin. (A - .) 
 
 If we wish to reject f -j -5) , we consider only the first, second and fourtn 
 of those equations. Eliminating (T ) from the last by means of the 
 
 second, we shall form an equation which cleared of fractions, will contain 
 a term multiplied by g 6 r 2 , and other terms affected with even and odd 
 powers of r and f. If we put into one side of the equation all the terms 
 affected with even powers of , and into the other all those which involve 
 its odd powers, and square both sides, in order to have none but even 
 powers of f, the term multiplied by g 6 r 2 will produce one multiplied by 
 g 12 r 4 . Substituting, therefore, instead of 2 , its value given by the first 
 of equations (L), we shall have a final equation of the sixteenth degree in 
 r. But instead of forming this equation in order afterwards to resolve it, 
 it will be more simple to satisfy by trial the three preceding ones. 
 
 ((I 3.\ 
 T 5J, we must consider the first, third and fourth 
 
 of equations (L). These three equations conduct us also to a final equa- 
 tion of the sixteenth degree in r ; and we can easily satisfy by trial. 
 
 The two preceding methods appear to be the most exact, which we can 
 employ in the determination of the parabolic orbits of the -comets. It is 
 at the same time necessary to have recourse to them, if the motion of the 
 comet in longitude or latitude is insensible, or too small for the errors of 
 observations sensibly to alter its second difference. In this case, we must 
 reject that of the equations (L), which contains this difference. But al- 
 though in these methods, we employ only three equations, yet the fourth 
 is useful to determine amongst all the real and positive values of r, which 
 satisfy the system of three equations, that which ought to be selected. 
 
 502. The elements of the orbit of a comet, determined by the above 
 process, would be exact, if the values of a, & and their first and second 
 differences, were rigorous ; for we have regarded, after a very simple 
 manner, the excentricity of the terrestrial orbit, by means of die radius- 
 vector R' of the earth, corresponding to its true anomaly + a right an- 
 gle ; we are therefore permitted only to neglect the square of this excen-
 
 BOOK L] NEWTON'S PRINCIPIA. 77 
 
 tricity, as too small a fraction to produce by its omission a sensible influ- 
 ence upon the results. But 6, a and their differences, are always suscep- 
 tible of any degree of inaccuracy, both because of the errors of observa- 
 tions, and because these differences are only obtained approximately. It 
 is therefore necessary to correct the elements, by means of three distant 
 observations, which can be done in many ways ; for if we know nearly, 
 two quantities relative to the motion of a comet, such that the radius-vec- 
 tor corresponding to two observations, or the position of the node, and 
 the inclination of the orbit ; calculating the observations, first with these 
 quantities and afterwards with others differing but little from them, the 
 law of the differences between the results, will easily show the necessary 
 corrections. But amongst the combinations taken two and two, of the 
 quantities relative to the motion of comets, there is one which ought to 
 produce greatest simplicity, and which for that reason should be selected. 
 It is of importance, in fact, in a problem so intricate, and complicated, to 
 spare the calculator all superfluous operations. The two elements which 
 appear to present this advantage, are the perihelion distance, and the 
 instant when the comet passes this point. They are not only easy to be 
 
 derived from the values of r and (-5) ; but it is very easy to correct them 
 
 by observations, without being obliged for every variation which they 
 undergo, to determine the other corresponding elements of the orbit. 
 Resuming the equation found in No. 492 
 
 a (1 e 2 ) is the semi-parameter of the conic section of which a is the 
 semi axis-major, and a e the excentricity. In the parabola, where a is 
 infinite, and e equal to unity, a (1 e 2 ) is double the perihelion dis- 
 tance : let D be this distance : the preceding equation becomes relatively 
 to this curve 
 
 p d P id? 2 r 2 
 
 S - is equal to -^57 ; in substituting for e 2 its value s-;+2Rrx 
 
 d t d t 2 cos. 2 
 
 cos. (A a) + R 2 , and for (-J-T) and (-, \ their values found in 
 
 No. 499, we shall have 
 
 d t cos. 2
 
 76 A COMMENTARY ON [SECT. XI. 
 
 //r ,\ N sm - (A 
 
 + r j(R' 1) cos. (A ) -- ^ 
 
 + r R l~ sin. (A o) + R (R' 1). 
 
 Let P represent this quantity ; if it is negative, the radius-vector de- 
 creases, and consequently, the comet tends towards its pei'ihelion. But 
 it goes off into the distance, if P is negative. We have then 
 
 D = f -ip; 
 
 the angular distance v of the comet from its perihelion, will be determined 
 from the polar equation to the parabola, 
 
 t * D 
 
 cor.*-v = T ; 
 
 and finally we shall have the time employed to describe the angle v, by 
 the table of the motion of the comets. This time added to or subtracted 
 from that of the epoch, according as P is negative or positive, will give 
 the instant when the comet passes its perihelion. 
 
 503. Recapitulating these different results, we shall have the following 
 method to determine the parabolic orbits of the comets. 
 
 General method of determining the orbits of the comets. 
 
 This method will be divided into two parts ; in the first, we shall give 
 the means of obtaining approximately, the perihelion distance of the comet 
 and the instant of its passage over the perihelion ; in the second, we shall 
 determine all the elements of the orbit on the supposition that the former 
 are known. 
 
 Approximate determination of the Perihelion distance of the comet , and 
 the instant of its passage over the perihelion. 
 
 We shall select three, four, five, &c. observations of the comet 
 equally distant from one another as nearly as possible ; with four obser- 
 vations we shall be able to consider an interval of 30 ; with five, an in- 
 terval of 36, or 40 and so on for the rest ; but to diminish the in- 
 fluence of their errors, the interval comprised between the observations 
 must be greater, in proportion as their number is greater. This being 
 supposed, 
 
 Let j8, j6', /3", &c. be the successive geocentric longitudes of the comet, 
 7> /> /' the corresponding latitudes, these latitudes being supposed positive 
 or negative according as they are north or south. We shall divide the dif- 
 ference /3' ft, by the number of days between the first and second ob- 
 servation ; we shall divide in like manner the difference 0" ^ by the
 
 BOOK L] NEWTON'S PRINCIPIA. 79 
 
 number of days between the second and third observation ; and so on. 
 Let 3 ft, a ft', 5 ft", &c. be these quotients. 
 
 We next divide the difference 5 & d ft by the number of days be- 
 tween the first observation and the third ; we divide, in like manner, the 
 difference d B" d ft' by the number of days between the second and 
 fourth observations ; similarly we divide the difference 8 ft'" 8 ft" by the 
 number of days between the third and fifth observation, and so on. Let 
 d z ft, a 2 ft', d z ft", &c. denote these quotients. 
 
 Again, we divide the difference 5* ft' 3* ft by the number of days 
 which separate the first observation from the fourth ; we divide in like 
 manner 3 z ft" 8 2 ft f by the number of days between the second obser- 
 vation and the fifth, and so on. Make 3 3 ft, d 3 ft', &c. these quotients. 
 Thus proceeding, we shall arrive at 3 n l ft, n being the number of obser- 
 vations employed. 
 
 This being done, we proceed to take as near as may be a mean epoch 
 between the instants of the two extreme observations, and calling i, i', i 77 , 
 &c. the number of days, distant from each observation, i, i', i", &c. ought 
 to be supposed negative for the 'observations made prior to this epoch; 
 the longitude of the comet, after a small number z of days reckoned from 
 the Epoch will be expressed by the following formula : 
 i 3 ft + i i' 3 2 ft i i' i" d 3 ft + &c. 
 
 . . (p) 
 
 &c,} 
 
 The coefficients of 3 ft, + 3 2 ft, 3 3 ft, &c. in the part independent 
 of z are 1st the numbers i and i', secondly the sum of the products two 
 and two of the three numbers i, i 7 , i" ; thirdly the sum of the products 
 three and three, of the four numbers i, i', i", i'", &c. 
 
 The coefficients of 5 3 ft, + d* ft, 8 5 ft, &c. in the part multiplied 
 by z 2 , are first, the sum of the three numbers i, i', i" ; secondly of the 
 products two and two of the four numbers i, i', i' 7 , i 777 ; thirdly the sum of 
 the products three and three of the five numbers i, V, i", i"', i"", &c. 
 
 Instead of forming these products, it is as simple to develope the func- 
 tion ft + (z i) 3 ft + (z i) (z i') 3 2 ft + (z i) (z i 7 ) (z i 77 ) 
 X 3 3 ft + &c. rejecting the powers of z superior to the square. This 
 gives the preceding formula. 
 
 If we operate in a similar manner upon the observed geocentric lati- 
 tudes of the comet ; its geocentric latitude, after the number z of days 
 from the epoch, will be expressed by the formula (p) in changing ft into 
 y. Call (q) the equation (p) thus altered. This being done,
 
 80 A COMMENTARY ON [SECT. XL 
 
 a will be the part independent of z in the formula (p) ; and 6 that in the 
 formula (q). 
 
 Reducing into seconds the coefficient of z in the formula (p), and 
 takin" from the tabular logarithm of this number of seconds, the logarithm 
 4,0394622, we shall have the logarithm of a number which we shall de- 
 note by a. 
 
 Reducing into seconds the coefficients of z 2 in the same formula, and tak- 
 ing from the logarithm of this number of seconds, the logarithm 1.9740144, 
 we shall have the logarithm of a number, which we shall denote by b. 
 
 Reducing in like manner into seconds "the coefficients of z and z 2 in 
 the formula (q) and taking away respectively from the logarithms of these 
 numbers of seconds, the logarithms, 4,0394622 and 1,9740144, we shall 
 have the logarithms of two numbers, which we shall name h and 1. 
 
 Upon the accuracy of the values of a, b, h, 1, depends that of the 
 method; and since their formation is very simple, we must select and 
 multiply observations, so as to obtain them with all the exactness which 
 the observations will admit of. It is perceptible that these values are only 
 
 /da\ /d 2 a\ /d .\ /d 2 Q\ ... 
 the quantities (-r-J , (TT^)' VH/' \TT 2 7' w ' ucn we nave ex P r ^ss- 
 
 ed more simply by the above letters. 
 
 If the number of observations is odd, we can fix the Epoch at the 
 instant of the mean observation; which will dispense with calculating the 
 parts independent of z in the two preceding formulas ; for it is evident, 
 that then these parts are respectively equal to the longitude and latitude 
 of the mean observation. 
 
 Having thus determined the values of a, a, b, 0, h, and 1, we shall de- 
 termine the longitude of the sun, at the instant we have selected for the 
 epoch, R the corresponding distance of the Earth from the sun, and R' 
 the distance which answers to E augmented by a right angle. We shall 
 have the following equations 
 
 (1) 
 R sin. (E a) | 1^ Ll_^J5 (2) 
 
 f . 1 a 2 sin. 6 . cos. 6 \ ^ 
 
 R sin. 6 cos. 6 /17 . f 1 111 
 
 + gh coME-aJjjp-pj ) 
 
 a z sin. d . cos. 
 
 sin. (E a)
 
 BOOK I.] NEWTON'S PRINCIPIA. 81 
 
 (R' 1) cos. (E a)} 2 a x {(R' 1) sin. (E a) + 
 
 cos.(E-n . L .2 m 
 
 R j + R 2 
 
 To derive from these equations the values of the unknown quantities 
 x, y, , we must consider, signs being neglected, whether b is greater or 
 less than 1. In the first case we shall make use of equation (1), (2), and 
 (4). We shall form a first hypothesis for x, supposing it for instance 
 equal to unity; and we then derive by means of equations (1), (2), the 
 values of f and of y. Next we substitute these values in the equation (4) ; 
 and if the result is 0, this will be a proof that iZ; value of x has been 
 rightly chosen. But if it be negative we must augment the value of x, 
 and diminish it if the contrary. We shall thus obtain, by means of a 
 small number of trials the values of x, y and g. But since these unknown 
 quantities may be susceptible of many real and positive values, we must 
 seek that which satisfies exacdy or nearly so the equation (3). 
 
 In the second case, that is to say, if 1 be greater than b, we shall use 
 the equations (1), (3), (4), and then equation (2) will give the verifi- 
 cation. 
 
 Having thus the values of x, y, , we shall have the quantity 
 
 p = + h x tan - R cos - E a ) 
 
 + x {25ifc?5 -(R' 1) cos. (E a)} 
 
 + R.(R' 1). 
 The Perihelion distance D of the comet will be 
 
 Rax rin 
 
 D P - - P 2 - 
 
 2 
 
 the cosine of its anomaly v will be given by the equation 
 
 , 1 D 
 cos*-v = -; 
 
 and hence we obtain, by the table of the motion of the comets, the time 
 employed to describe the angle v. To obtain the instant when the comet 
 passes the perihelion, we must add this time to, or subtract it from the 
 epoch according as P is negative or positive. For in the first case the 
 comet approaches, and in the second recedes from, the perihelion. 
 
 Having thus nearly obtained the perihelion distance of the comet, and 
 the instant of its passage over the perihelion ; we are enabled to correct 
 them by the following method, which has the advantage of being inde- 
 pendent of the approximate values of the other elements of the orbit 
 
 VOL. II. F
 
 82 A COMMENTARY ON [SECT. XI. 
 
 An exact Determination of the elements of the orbit, when we know ap- 
 proximate values of the perihelion distance of the comet ', and of the instant 
 of its passage over the perihelion. 
 
 We shall first select three distant observations of the comet; then 
 taking the perihelion distance of the comet, and the instant of its crossing 
 the perihelion, determined as above, we shall calculate the three anomalies 
 of the comet and the corresponding radius-vectors corresponding to the 
 instants of the three observations. Let v, v', v" be these anomalies, those 
 which precede the passage over the perihelion being supposed negative. 
 Also let f, g' g" be the corresponding radius-vectors of the comet; then 
 v 7 v, v" v will be the angles comprised by g and g' and by f, t>". 
 Let U be the first of these angles, U' the second. Again, call a, a' a!' the 
 three observed geocentric longitudes of the comet, referred to a fixed 
 equinox ; 6, tf, 6" its three geocentric latitudes, the south latitudes being 
 negative. Let |3, /3', |S'' be the three corresponding heliocentric longi- 
 tudes and v, &', zr", its three heliocentric latitudes. Lastly call E, E', E /; 
 the three corresponding longitudes of the sun, and R, R', R" its three 
 distances to the center of the earth, 
 
 Conceive that the letter S indicates the center of the sun, T that of the 
 earth, and C that of the comet, C' that of its projection upon the plane 
 of the ecliptic. The angle S T C' is the difference of the geocentric lon- 
 gitudes of the sun and of the comet. Adding the logarithm of the cosine 
 of this angle, to the logarithm of the cosine of the geocentric latitude of 
 the comet, we shall have the logarithm of the cosine of the angle S T C. 
 We know, therefore, in the triangle S T C, the side S T or R, the side 
 S C or f, and the angle S T C, to find the angle C S T. Next we shall 
 have the heliocentric latitude 9 of the comet, by means of the equation 
 
 sin. 6 sin. C S T 
 
 SUl. -a =: : /-i T Q 
 
 sin. CIS 
 
 The angle T S C' is the side of a spherical right angled triangle, of 
 which the hypothenuse is the angle T S C, and of which one of the sides 
 is the angle . Whence we shall easily derive the angle T S C', and con- 
 sequently the heliocentric longitude /3 of the comer. 
 
 We shall have after the same manner */, 0'; *", |3" ; and the values of 
 0, /3', j9" will show whether the motion of the comet be direct or retro- 
 grade. 
 
 If we imagine the two arcs of latitude , ~', to meet at the pole of die 
 ecliptic, they would make there an angle equal to & /3 ; and in the
 
 BOOK I.] NEWTON'S PHINCIPIA. 83 
 
 spherical triangle formed by this angle, and by the sides &, - -j 
 
 2 
 
 * being the semi-circumference, the side opposite to the angle /3' (3 
 will be the angle at the sun comprised between the radius-vectors e, and 
 
 f'. We shall easily determine this by spherical Trigonometry, or by the 
 formula 
 
 sin. 2 I V = cos. 2 ^ (a- + ~') cos 2 \ (/3' /3) cos. * cos. ', 
 
 i9 ' im 
 
 in which V represents this angle ; so that if we call A the angle of which 
 the sine squared is 
 
 cos 2 - (/3' jS) cos. . cos. *', 
 <ii 
 
 and which we shall easily find by the tables, we shall have 
 
 sin.* I V = cos. ('I * + \ ~'+ A) cos. (i . + 1 ~'-A). 
 
 If in like manner we call V the angle formed by the two radius-vectors 
 g, ", we have 
 
 in.iv=cos.(i 
 
 sn.. 
 
 A' being what A becomes, when w', /3' are changed into ", |6". 
 
 If, however, the perihelion distance and the instant of the comet's 
 crossing the perihelion, were exactly determined, and if the observations 
 were rigorously exact, we should have 
 
 V = U, V = U'; 
 
 But since that is hardly ever the case, we shall suppose 
 m = U V ; m' = U' V'. 
 
 We shall here observe that the revolution of the triangle S T C, gives 
 for the angle C S T two different values : for the most part the nature 
 of the motion of the comets, will show that which we ought to use, and 
 the more plainly if the two values are very different ; for then the one will 
 place the comet more distant from the earth, than the other, and it will 
 be easy to judge, by the apparent motion of the comet at the instant of 
 observation, which ought to be preferred. But if there remains any un- 
 certainty, we can always remove it, by selecting the value which renders 
 V and V least different from U and U'. 
 
 We next make a second hypothesis in which, retaining the same pas 
 sage over the perihelion as before, we shall suppose the perihelion dis- 
 tance to vary by a small quantity ; for instance, by the fiftieth part of 
 
 F2
 
 4 A COMMENTARY ON [SECT. XL 
 
 its value, and we shall investigate on this hypothesis, the values of U V, 
 U 7 V 7 . Let then 
 
 n = U V ; n' = U' V. 
 
 Lastly, we shall frame a third hypothesis, in which, retaining the same 
 perihelion distance as in the first, we shall suppose the instant of the pas- 
 sage over the perihelion to vary by a half-day, or a day more or less, In 
 this new hypothesis we must find the values of 
 U VandofU 7 V'; 
 which suppose to be 
 
 p = U - V, p' = U' V'. 
 
 Again, if we suppose u the number by which we ought to multiply the 
 supposed variation in the perihelion distance in order to make it the 
 true one, and t the number by which we ought to multiply the supposed 
 variation of the instant when the comet passes over the perihelion in 
 order to make it the true instant, we shall have the two following equa- 
 tions : 
 
 (m n ) u + (m p ) t = m ; 
 (m' n') u + (m 7 p') t = m'; 
 
 whence we derive u and t and consequently the perihelion distance cor- 
 rected, and the true instant of the comet's passing its perihelion. 
 
 The preceding corrections suppose the elements determined by the 
 first approximation, to be sufficiently near the truth for their errors to be 
 regarded as infinitely small. But if the second approximation should 
 not even suffice, we can have recourse to a third, by operating upon the ele- 
 ments already corrected as we did upon the first ; provided care be taken to 
 make them undergo smaller variations. It will also be sufficient to calculate 
 by these corrected elements the values of U V, and of U 7 V 7 . Call- 
 ing them M, M 7 , we shall substitute them for m, m 7 in the second mem- 
 bers of the two preceding equations. We shall thus have two new equa- 
 tions which will give the values of u and t, relative to the corrections of 
 these new elements. 
 
 Thus having obtained the true perihelion distance and the true instant 
 of the comet's passing its perihelion, we obtain the other elements of the 
 orbit in this manner. 
 
 Let j be the longitude of the node which would be ascending if the 
 motion of the comet were direct, and <f> the inclination of the orbit. We 
 shall have by comparison of the first and last observation, 
 
 tan, v sin. )3 7 tan. tS sin. /? 
 tan ' J - tan. cos. &' tan. " cos. ''
 
 BOOK I.] NEWTON'S PRINCIPIA. 85 
 
 tan. *" 
 
 tan. <p = -i 7-577 rr . . 
 
 sm. ($" j) 
 
 Since we can compare thus two and two together, the three observa- 
 tions, it will be more correct to select those which give to the above frac- 
 tions, the greatest numerators and the greatest denominators. 
 
 Since tan. j may equally belong to j and <n + j, j being the smallest of 
 the positive angles containing its value, in order to find that which we 
 ought to fix upon, we shall observe that <p is positive and less than a right 
 angle; and that sin. (j3" j) ought to have the same sign as tan. -a". 
 This condition will determine the angle j, and this will be the position 
 of the ascending node, if the motion of the comet is direct ; but if retro- 
 grade we must add two right angles to the angle j to get the position of 
 the node. 
 
 The hypothenuse of the spherical triangle whose sides are $" j and 
 v" t is the distance of the comet from its ascending node in the third ob- 
 servation; and the difference between v" and this hypothenuse is the 
 interval between the node and the perihelion computed along the orbit. 
 
 If we wish to give to the theory of a comet all the precision which ob- 
 servations will admit of, we must establish it upon an aggregate of the best 
 observations ; which may be thus done. Mark with one, two, &c. dashes 
 or strokes the letters m, n, p relative to the second observation, the third, 
 &c. all being compared with the first observation. Hence we shaH form 
 the equations 
 
 (m n ) u + (m p ) t = m 
 (m' - n' ) u + (m' p' ) t = m' 
 (m" n") u + (m" p") t = m" 
 
 &c. = &c. 
 
 Again, combining these equations so as to make it easier to determine 
 u and t, we shall have the corrections of the perihelion distance and of the 
 instant of the comet's passing its perihelion, founded upon the aggregate 
 of these observations. We shall have the values of 
 
 ft ff, 6", &c. *, x/, /', &c., 
 and obtain 
 
 . _ tan, tr (sin. B f + sin, ff" + &c.) sin. |3 (tan. ^ + tan. J' + &c.) 
 "' J ~~ tan. (cos. j3' + cos. /3" + &c.) cos. /3 (tan. ' + tan. " + &c.) 
 
 _ tan. '*' + tan. *" + &c. 
 
 J ' * ~ sin. (? j) + sin. (0" j) + &c. ' 
 
 504. There is a case, very rare indeed, in which the orbit of a comet 
 can be determined rigorously and simply ; it is that where the comet has 
 been observed in its two nodes. The straight line which joins these 
 
 F3
 
 80 A COMMENTARY ON [SECT. XL 
 
 two observed positions, passes through the center of the sun and coincides 
 with the line of the nodes. The length of this straight line is determined 
 by the time elapsed between the two observations. Calling T this time 
 reduced into decimals of a day, and denoting by c the straight line in 
 question, we shall have (No. 493) 
 
 a 
 
 _ 1 / 
 
 " 2V i 
 
 (9.688724) 2 ' 
 
 Let /3 be the heliocentric longitude of the comet, at the moment of the 
 first observation ; g its radius-vector ; r its distance from the earth ; and a 
 its geocentric longitude. Let, moreover, R be the radius of the terrestrial 
 orbit, at the same instant, and E the corresponding longitude of the sun. 
 Then we shall have 
 
 sin. /S =r r sin. a. R sin. E ; 
 cos. /3 = r cos. a R cos. E. 
 
 Now <ff + jS will be the heliocentric longitude of the comet at the in- 
 stant of the second observation ; and if we distinguish the quantities , , 
 r, R, and E relative to this instant by a dash, we shall have 
 o' sin. 3 = R' sin. E' r' sin. / ; 
 g' cos. 8 = R' cos. E' T' cos. a!. 
 These four equations give 
 
 _ r sin R sin. E _ r' sin, o! R' sin. E / 
 tan< " = rcos.a Rcos.E ~ r' cos. a' R 7 cos. E' ; 
 whence we obtain 
 
 , _ R R' sin. (E EQ R r sin. ( E') 
 
 r sin. (a! a) R sin. (' E) 
 We have also 
 
 ( _|_ g') sin. = r sin. a. r' sin. a! R sin. E + R 7 sin. E' 
 (g -|- g') cos. j3 = r cos. a r' cos. ' R cos. E + R' cos. E 7 . 
 Squaring these two equations, and adding them together, and substitut- 
 ing c for f + g', we shall have 
 
 c* = R* 2RR / cos.(E / E) + R /z 
 + 2 r { R' cos. (a _ EO R cos. (a _ E)} 
 + 2 r' R cos. (' E) R' cos. (a' E') J 
 + r 2 2rr / cos. (a' a) + r /2 . 
 
 If we substitute in this equation for r 7 its preceding value in terms of r, 
 we shall have an equation Lti r of the fourth degree, which can be resolved 
 by the usual methods. But it will be more simple to find values of r, r' 
 by trial such as will satisfy the equation. A few trials will suffice for that 
 purpose. rf .
 
 BOOK I.] NEWTON'S PRINCIPIA. 87 
 
 By means of these quantities we shall have /3, and g'. If v be the 
 angle which the radius g makes with the perihelion distance called D ; 
 it v will be the angle formed by this same distance, and by the radius g'. 
 We shall thus have by the equation to the parabola 
 
 D D 
 
 - ; e = - 
 
 cos. -- v sin. v 
 
 which give 
 
 tan. 2 -v= 
 
 -r-" 
 5 + f 
 
 We shall therefore have the anomaly v of the comet, at the instant of 
 the first observation, and its perihelion distance D, whence it is easy to 
 find the position of the perihelion, at the instant of the passage of the 
 comet over that point. Thus, of the five elements of the orbit of the co- 
 met, four are known, namely, the perihelion distance, the position of the 
 perihelion, the instant of the comet's passing the perihelion, and the posi- 
 tion of the node. It remains to learn the inclination of the orbit; but for 
 that purpose it will be necessary to have recourse to a third observation, 
 which will also serve to select from amongst the real and positive roots of 
 the equation in r, that which we ought to make use of. 
 
 505. The supposition of the parabolic motion of comets is not rigorous ; 
 it is, at the same time, not at all probable, since compared with the cases 
 that give the parabolic motion, there is an infinity of those which give the 
 elliptic or hyperbolic motions. Besides, a comet moving in either a para- 
 bolic or hyperbolic orbit, will only once be visible ; thus we may with 
 reason suppose these bodies, if ever they existed, long since to have dis- 
 appeared ; so that we shall now observe those only which, moving in or- 
 bits returning into themselves, shall, after greater or less incursions into 
 the regions of space, again approach their center the sun. By the follow- 
 ing method, we shall be able to determine, within a few years, the period 
 of their revolutions, when we have given a great number of very exact 
 observations, made before and after the passage over the perihelion. 
 
 Let us suppose we have four or a greater number of good observations, 
 which embrace all the visible part of the orbit, and that we have deter- 
 mined, by the preceding method, the parabola, which nearly satisfies these 
 observations. Let v, v', v", v'", &c. be the corresponding anomalies; 
 > ,g' "> $"* & c< tne radius-vectors. Let also 
 
 v' v = U, v" v = U', v'" v = U", &c. 
 
 F4
 
 88 A COMMENTARY ON [SECT. XI. 
 
 Then we shall estimate, by the preceding method with the parabola 
 already found, the values of U, U', V", &c., V, V, V", &c. Make 
 m = U V, m' = U' V, m" = U" V", &c. 
 
 Next, let the perihelion distance in this parabola vary by a very small 
 quantity, and on this hypothesis suppose 
 
 n = U V; n' = U' V; n" = U" V", &c. 
 We will form a third hypothesis, in which the perihelion distance re- 
 maining the same as in the first, we shall make the instant of the comet's 
 passing its perihelion vary by a very small quantity ; in this case let 
 
 p = U V; p' = U' V; p" = U" V''; &c. 
 Lastly, we shall calculate the angle v and radius g t with the perihelion 
 distance, and instant over the perihelion on the first hypothesis, supposing 
 the orbit an ellipse, and the difference 1 e between its excentricity and 
 unity a very small quantity, for instance yV- To get the angle v, in this 
 hypothesis, it will suffice (489) to add to the anomaly v, calculated in the 
 parabola of the first hypothesis, a small angle whose sine is 
 
 1 (l--e)tan. iv J4 Scos.'-i v 6cos. 4 ^ vj. 
 
 Substituting afterwards in the equation 
 
 D ( i _e , 1 
 - 1 1 -- 2~ tan - 1 
 
 cos. v 
 
 for v, this anomaly, as calculated in the ellipse, we shall have the corre- 
 sponding radius-vector g. After the same manner, we shall obtain v', g , 
 v", g", &c. Whence we shall derive the values of U, U', U", &c. and 
 (by 503) of V, V, V", &c. 
 In this case let 
 
 q = U V; q' = U' V; q" = U" V", &c. 
 Finally, call u the number by which we ought to multiply the supposed 
 variation in the perihelion distance, to make it the true one ; t the number 
 by which we ought to multiply the supposed variation in the instant over 
 the perihelion, to make it the true instant; and s that by which we should 
 multiply the supposed value of 1 e, in order to get the true one; and 
 we shall obtain these equations : 
 
 (m n) u + (m p) t + (m q; s = m ; 
 ( m ' _ n' ) u + (m' _ p' ) t + (m' q'} s = m ; 
 ( m " __ n ") u + (m" p") t + (m" q") s = m"; 
 (m'" _ n'") u + (m'" p'") t + (m"' q'") s = m'"; 
 
 &c.
 
 BOOK 1.] NEWTON'S PRINCIPIA. 89 
 
 We shall determine, by means of these equations, the values of u, t, s ; 
 whence will be derived the true perihelion distance, the true instant over 
 the perihelion, and the true value of 1 e. Let D be the perihelion 
 distance, and a the semi-axis major of the orbit; then we shall have 
 
 a = = - ; the time of a sidereal revolution of the comet, will be expressed 
 
 A - G 
 
 5. / D 5- 
 
 by a number of sidereal years equal to a 2 or to (- -) * tne mean 
 
 distance of the sun from the earth being unity. We shall then have 
 (by 503) the inclination of the orbit and the position of the node. 
 
 Whatever accuracy we may attribute to the observations, they will 
 always leave us in uncertainty as to the periodic times of the comets. To 
 determine this, the most exact method is that of comparing the observa- 
 tions of a comet in two consecutive revolutions. But this is practicable, 
 only when the lapse of time shall bring the comet back towards its peri- 
 helion. 
 
 Thus much for the motions of the planets and comets as caused by the 
 action of the principal body of the system. We now come to 
 
 506. General methods of determining by successive approximations, the 
 motions of the heavenly bodies. 
 
 In the preceding researches we have merely dwelt upon the elliptic 
 motion of the heavenly bodies, but in what follows we shall estimate them 
 as deranged by perturbing forces. The action of these forces requires only 
 to be added to the differential equations of elliptic motion, whose integrals 
 in finite terms we have already given, certain small terms. We must deter- 
 mine, however, by successive approximations, the integrals of these same 
 equations when thus augmented. For this purpose here is a general me- 
 thod, let the number and degree of the equations be what they may. 
 
 Suppose that we have between the n variables y, y', y", &c. and the 
 time t whose element d t is constant, the n differential equations 
 
 0= Ttr + 
 
 &C. = &C. 
 
 PJ Q? P' Q'J &c. being functions of t, y, y', &c. and of the differences to 
 the order i 1 inclusively, and a being a very small constant coefficient, 
 which, in the theory of celestial motions, is of the order of the perturb- 
 ing forces. Then let us suppose we have the finite integrals of those
 
 90 A COMMENTARY ON [SECT. XL 
 
 equations when Q, Q', &c. are nothing. Differentiating each i 1 
 times successively, we shall form with their differentials i n equations by 
 means of which we shall determine by elimination, the arbitrary constants 
 c, c', c", &c. in functions of t, y, y', y", &c. and of their differences to the 
 order i 1. Designating therefore by V, V, V", &c. these functions 
 we shall have 
 
 c = V; c' = V; c" = V"; &c. 
 
 These equations are the z n integrals of the (i I) 01 order, which the 
 equations ought to have, and which, by the elimination of the differences 
 of the variables, give their finite integrals. 
 
 But if we differentiate the preceding integrals of the order i 1, we 
 shall have 
 
 = dV; = d V; = d V"; &c. 
 
 and it is clear that these last equations being differentials of the order i 
 without arbitrary constants, they can only be the sums of the equations 
 d ' v 
 
 = + p 
 
 = &c. 
 
 each multiplied by proper factors, in order to make these sums exact dif- 
 ferences. Calling, therefore, F d t, F' d t', &c. the factors which ought 
 respectively to multiply them in order to make = d V ; also in like 
 manner making H d t, H' d t', &c. the factors which would make = d V, 
 and so on for the rest, we shall have 
 
 d V'=Hdt + P + H'dt + P' +&c. 
 
 &c. 
 
 F, F', &c. H, H', &c. are functions of t, y, y', y", &c. and of their dif- 
 ferences to the order i 1 . It is easy to determine them when V, V', &c. 
 
 d' 1 v 
 are known. For F is evidently the coefficient of vp in the differential 
 
 of V ; F' is the coefficient of . f in the same differential, and so on. 
 
 d * v d ' \' . i 
 
 In like manner, H, H', &c. are the coefficients of -y j , -.-5 , &c. in the 
 
 differential of V. Thus, since we may suppose V, V', &c. known, by dif-
 
 BOOK I.] NEWTON'S PRINCIPIA. 91 
 
 d i - 1 y (1 1 - 1 y' 
 
 ferentiating with regard to jjufr > ^ t i _ i > &c - we shall have the 
 factors by which we ought to multiply the differential equations 
 
 in order to make them exact differences. 
 Now resume the differential equations 
 
 =^f 4- P+ .Q; = i^-' + P' + .Q', &c. 
 
 If we multiply the first by F d t, the second by F' d t, and so on, we 
 shall have by adding the results 
 
 = dV + adt{FQ+FQ / + &c.J, 
 In the same manner, we shall have 
 
 = d V' + a d t {H Q + H' Q' + Sec.} 
 &c. 
 whence by integration 
 
 c a/d t F Q + F Q' + &c.} = V; 
 c' _ a /d t {H Q + H' Q' + &c.} = V; 
 &c. 
 
 We shall thus have i n differential equations, which will be of the same 
 form as in the case when Q, Q', &c. are nothing, with this only differ- 
 ence, that the arbitrary constants c, c', c", &c. must be changed into 
 
 c /dt iFQ + F'Q'+&c.}, c _ a /dtHQ + H'Q'+&c.}&c. 
 But if. in the supposition of Q, Q', &c. being equal to zero, we eliminate 
 from the i n integrals of the order i 1, the differences of the variables 
 y, y', &c. we shall have n finite integrals of the proposed equations. We 
 shall therefore have these same integi'als when Q, Q', &c. are not zero, by 
 changing in the first integrals, c, c', &c. into 
 
 c a/d t F Q + &c.}, c' a/d t {H Q + &c.}&c. 
 
 507. If the differentials 
 
 dt{FQ + FQ' + &c.}, dtHQ + H'Q' + &c.}&c. 
 are exact, we shall have, by the preceding method, finite integrals of the 
 proposed differentials. But this is not so, except in some particular cases, 
 of which the most extensive and interesting is that in which they are 
 linear. Thus let P, P 7 , &c. be linear functions of y, y', &c. and of their 
 differences up to the order i 1, without any term independent of these 
 variables, and let us first consider the case in which Q, Q', &c. are no- 
 thing. The differential equations being linear, their successive integrals
 
 92 A COMMENTARY ON [SECT. XL 
 
 are likewise linear, so that c = V, c' = V', &c. being the i n integrals of 
 the order i 1, of die linear differential equations 
 
 V, V', &c. may be supposed linear functions of y y', &c. and of their dif- 
 ferences to the order i 1. To make this evident, suppose that in the 
 expressions for y, y', &c. the arbitrary constant c is equal to a determinate 
 quantity plus an indeterminate 5c; the arbitrary constant c' equal to a 
 determinate quantity plus an indeterminate 8 c! &c. ; then reducing these 
 expressions according to the powers and products of d c, d c', &c. we shall 
 have by the formulas of No. 487 
 
 Sc* , 
 
 + ITS + &c - 
 
 
 * 
 
 &c. 
 
 Y, Y', f-j J , &c. being functions oft without arbitrary constants. Sub- 
 
 stituting those values, in the proposed differential equations, it is evident 
 that 8 c, d c', &c. being indeterminate, the coefficients of the first powers 
 of such of them ought to be nothing in the several equations. But these 
 equations being linear, we shall evidently have the terms affected with the 
 first powers of 8 c, d c', &c. by substituting for y, y', &c. these quantities 
 respectively 
 
 These expressions of y, y', &c. satisfy therefore separately the proposed 
 equations ; and since they contain the i n arbitraries d c, d c', &c. they are 
 complete integrals. Thus we perceive, that the arbitraries are under a 
 linear form in the expressions of y, y', &c. and consequently also in their 
 differentials. Whence it is easy to conclude that the variables y, y', &c. 
 and their differences, may be supposed to be linear in the successive inte- 
 
 grals of the proposed differential equations. 
 
 d ' v d ' y 7 
 Hence it follows, that F, F', &c. being the coefficients of -r -, -- ,
 
 BOOK L] NEWTON'S PRINCIPI A. 93 
 
 &c. in the differential of V ; H 3 H', &c. being the coefficients of the same 
 differences in the differential of V, &c. these quantities are functions ot 
 variable t only. Therefore, if we suppose Q, Q', &c. functions of t alone, 
 the differentials 
 
 d t {F Q + F Q' + Sec.} ; d t H Q + H' Q' + &c.} ; &c. 
 will be exact. 
 
 Hence there results a simple means of obtaining the integrals of any 
 number whatever n of linear differential equations of the order i, and 
 which contain any terms a Q, a Q', &c. functions of one variable t, having 
 known the integrals of the same equations in the case where Q, Q', &c. 
 are supposed nothing. For then if we differentiate their n finite integrals 
 i 1 times successively, we shall have i n equations which will give, by 
 elimination, the values of the i n arbitrary constants c, c', &c. in functions 
 of t, y, y', &c. and of their differences to the i 1 th order. We shall thus 
 form the i n equations c = V, c' = V, &c. This being done, F, F', &c. 
 
 will be the coefficients of ^4r > ^T^f &c ' in V ' H > H/ > &c - wil1 
 
 be the coefficients of the same differences in V 7 , and so on. We shall, 
 therefore, have the finite integrals of the linear differential equations 
 
 = f + P + Q; 0= + P' + aQ'; &c. 
 
 by changing, in the finite integrals of these equations deprived of their last 
 terms a Q, a Q', &c. the arbitrary constants c, c', &c. into 
 
 c /dt FQ+FQ'+&c], c' a/dt 
 Let us take, for example, the linear equation 
 
 The finite integral of the equation 
 
 is (found by multiplying by cos. a t, and then by parts getting 
 / cos. a t . -T-2. = cos. a t -^- + a / sin. a t , ^ . d t = cos. a t . ^ + 
 
 a sin. a t . y a 2 f cos. a t . y .'. c = a cos. a fc xf + a sm - a t ? & c ') 
 
 c c' 
 
 v = sin. a t + cos. a t, 
 'a a 
 
 c, c' being arbitrary constants.
 
 94 A COMMENTARY ON [SECT. XL 
 
 This integral gives by differentiation 
 
 -r-s = c cos. at c' sin. a t. 
 a t 
 
 If we combine this with the integral itself, we shall form two integrals 
 of the first order 
 
 dy 
 c = a y sin. a t + -* cos. a t ; 
 
 dy 
 
 c' = a y cos. at ~ sin. a t ; 
 
 and therefore shall have in this case 
 
 F = cos. at; H = sin. a t, 
 and the complete integral of the proposed equation will therefore be 
 
 c . c' a sin. a t r ~ , 
 
 y = - sin. a t + cos. at j Q d t cos. a t 
 
 a a a 
 
 , a. cos. a t rf> . , . . 
 H jQ d t sin. a t. 
 
 Hence it is easy to conclude that if Q is composed of terms of the form 
 
 sin 
 
 K . (m t 4- s) each of these terms will produce in the value of y the 
 cos. v 
 
 corresponding term 
 
 a K sin. 
 
 . (m t + s). 
 m z a 2 cos. v 
 
 If m be equal to a, the term K (m t + e) will produce in y, 1st. the 
 
 COi3 
 
 term -. . (a t + t) which being comprised by the two terms 
 
 4, a 2 cos. v 
 
 C C cc 1C t COS 
 
 sin. a t-l cos. at, may be neglected: 2dly. the term + . . (at4-i), 
 
 a a 2 a sm. v 
 
 -}- or being used according as the term of Q is a sine or cosine. We 
 thus perceive how the arc t produces itself in the values of y, y', &c. with- 
 out sines and cosines, by successive integrations, although the differentials 
 do not contain it in that form. It is evident this will take place when- 
 ever the functions F Q, F', Q', &c. H Q, H' Q', &c. shall contain con- 
 stant terms. 
 
 508. If the differences 
 
 d t { Q + &c.J, d t [H Q + &c.J 
 
 are not exact, the preceding analysis will not give their rigorous integrals. 
 But it affords a simple process for obtaining them more and more nearly 
 by approximation when a is very small, and when we have the values of
 
 BOOK I.j NEWTON'S PRINCIPIA. 95 
 
 y, y 7 , &c. on the supposition of a being zero. Differentiating these values, 
 i 1 times successively, we shall form the differential equations of the 
 order i 1, viz. 
 
 c = V; c' = V, &c. 
 
 The coefficients of ^-? , -r--* , &c. in the differentials of V, V, &c. 
 
 being the values of F, F', &c. H, H', &c. we shall substitute them in the 
 differential functions 
 
 d t (F Q + F' Q' + &c.) ; d t (H Q + H' Q' + &c) ; &c. 
 
 Then, we shall substitute in these functions, for y, y', &c. their first 
 approximate values, which will make these differences functions of t and of 
 the arbitrary constants c, c', &c. 
 
 Let T d t, T d t, &c. be these functions. If we change in the first 
 approximate values of y, y', &c. the arbitrary constants c, c', &c. re- 
 spectively into c y T d t, c' a f T d t, &c. we shall have the 
 second approximate values of those variables. 
 
 Again substitute these second values in the differential functions 
 d t . (F Q + &c.) ; d t (H Q + &c.) &c. 
 
 But it is evident that these functions are then what T d t, T' d t, &c. 
 become when we change the arbitrary constants c, c', &c. into c af T d t, 
 c' /T d t, c. Let therefore T /5 T,', &c. denote what T, T, &c. 
 become by these changes. We shall get the third approximate values of 
 y, y', &c. by changing in the first c, c', &c. respectively into c yT, d t, 
 c- /T; d t, &c. 
 
 Calling T y/ , T,/, in like manner, what T, T', &c. become when 
 we change c, c', &c. into c af T, d t, c' ./T/ d t, &c. we shall 
 have the fourth approximate values of y, y', &c. by changing in the first 
 approximate values of these variables into c o.J"T Jt d t, c afT,/ d t, 
 &c. and so on. 
 
 We shall see presently that the determination of the celestial motions, 
 depends almost always upon differential equations of the form 
 
 Q being a rational and integer function of y, of the sine and cosine of 
 angles increasing proportionally with the time represented by t. The 
 following is the easiest way of integrating this equation. 
 
 First suppose nothing, and we shall have by the preceding No. a first 
 value of y. 
 
 Next substitute this value in Q, which will thus become a rational and
 
 90 A COMMENTARY ON [SECT. XI. 
 
 entire function of sines and cosines of angles proportional to the time. 
 Then integrating the differential equation, we shall have a second value 
 of y approximate up to quantities of the order a inclusively. 
 
 Again substitute this value in Q, and, integrating the differential equa- 
 tion, we shall have a third approximation of y, and so on. 
 
 This way of integrating by approximation the differential equations of 
 the celestial motions, although the most simple of all, possesses the dis- 
 advantage of giving in the expressions of the variables y, y', &c. the arcs 
 of a circle (symbols sine and cosine) in the very case where these arcs 
 do not enter the rigorous values of these variables. We perceive, in 
 fact, that if these values contain sines or cosines of angles of the order a t, 
 these sines or cosines ought to present themselves in the form of series, in 
 the approximate values found by the preceding method; for these last 
 values are ordered according to the powers of a. This developement 
 into series of the sine and cosine of angles of the order a t, ceases to be 
 exact when, by lapse of time, the arc a t becomes considerable. The ap- 
 proximate values of y, y', &c. cannot extend to the case of an unlimited 
 interval of time. It being important to obtain values which include both 
 past and future ages, the reversion of arcs of a circle contained by the 
 approximate values, into functions which produce them by their develope- 
 ment into series, is a delicate and interesting problem of analysis. Here 
 follows a general and very simple method of solution. 
 
 509. Let us consider the differential equation of the order i, 
 
 d y d i 1 y 
 
 a being very small, and P and Q algebraic functions of y, -^ , . . . . , , _ x , 
 
 and of shies and cosines of angles increasing proportionally with the time. 
 Suppose we have the complete integral of this differential, in the case of 
 o = 0, and that the value of y given by this integral, does not contain the 
 arc t, without the symbols sine and cosine. Also suppose that in inte- 
 grating this equation by the preceding method of approximation, when a 
 is not nothing, we have 
 
 y = X + t Y + t 2 Z + t 3 S + &c. 
 
 X, Y, Z, &c. being periodic functions of t, which contain the i arbitraries 
 c, c', c", &c. and the powers of t in this expression of y, going on to in- 
 finity by the successive approximations. It is evident the coefficients 
 of these powers will decrease with the greater rapidity, the Jess is . 
 In the theory of the motions of the heavenly bodies, expresses the order 
 of perturbing forces, relative to the principal forces which animate them.
 
 BOOK I.] NEWTON'S PRINCIPIA. 97 
 
 d'y 
 If we substitute the preceding value of y in the function TT^ + P+aQ, 
 
 it will take the form k + k' t + k" t 2 + &c., k, k', k", &c. being perio- 
 dic functions of t ; but by the supposition, the value of y satisfies the dif- 
 ferential equation 
 
 = 
 
 we ought therefore to have identically 
 
 = k + k' t + k" t 2 + &c. 
 
 If k, k', k", &c. be not zero this equation will give by the reversion of 
 series, the arc t in functions of sines and cosines of angles proportional to 
 the time t. Supposing therefore a to be infinitely small, we shall have t 
 equal to a finite function of sines and cosines of similar angles, which is 
 impossible. Hence the functions k, k', &c. are identically nothing. 
 
 Again, if the arc t is only raised to the first power under the symbols 
 sine and cosine, since that takes place in the theory of celestial motions, 
 the arc will not be produced by the successive differences of y. Substi- 
 
 tuting, therefore, the preceding value of y, in the function j~i + P + a Q 
 
 the function of k + k' t + &c. to which it transforms, will riot contain 
 the arc t out of the symbols sine and cosine, inasmuch as it is already con- 
 tained in y. Thus changing in the expression of y, the arc t, without the 
 periodic symbols, into t 6, 6 being any constant whatever, the function 
 k + k' t + &c. will become k + k' (t 6) + &c. and since this last 
 function is identically nothing by reason of the identical equations k = 
 k' = 0, it results that the expression 
 
 y = x + (t <o Y + (t oy z + &c. 
 
 also satisfies the differential equation 
 
 Although this second value of y seems to contain i + 1 arbitrary con- 
 stants, namely, the i arbitraries c, c, c", &c. and 0, yet it can only have i 
 distinct ones. It is therefore necessary that by a proper change in the 
 constants c, c', &c. the arbitrary 6 be made to disappear, and thus the 
 second value of y will coincide with the first This consideration will fur- 
 nish us with the means of making disappear the arc of a circle out of the 
 periodic symbols. 
 
 Give the following form to the second expression for y : 
 
 y = x + (t- o).R. 
 
 Vol. T7. G
 
 98 A COMMENTARY ON [SECT. XL 
 
 Then supposing 6 to disappear from y, we have 
 
 and consequently 
 
 B = Oar) + <'- 
 
 Differentiating successively this equation we shall have 
 
 whence it is easy to obtain, by eliminating R and its differentials, from the 
 preceding expression of y, 
 
 V j_ /f a\ f \ _!_ fN , /\ , 
 
 x+ <'-'> tar) + -ITS taw + W- (drO + &c - 
 
 X is a function of t, and of the constants, c, c', c", &c. and since these 
 constants are functions of 6, X is a function of t and of 6, which we can 
 represent by <f> (t, 6). The expression of y is by Taylor's Theorem 
 the developement of the function p (t, 6 + t 0), according to the powers 
 of t 6. We have therefore y = p (t, t). Whence we shall have y by 
 changing in X, 6 into t. The problem thus reduces itself to determine 
 X in a function of t and 0, and consequently to determine c, c', c", &c. 
 in functions of 6. 
 
 To solve this problem, let us resume the equation 
 
 y = X + (t 6) . Y + (t /) 2 . Z + &c. 
 
 Since the constant 6 is supposed to disappear from this expression of y, 
 we shall have the identical equation 
 
 .. (a) 
 
 Applying to this equation the reasoning which we employed upon 
 
 = k + k 7 1 + k" t 2 + &c. 
 
 we perceive that the coefficients of the successive powers of t 6 ought 
 to be each zero. The functions X, Y, Z, &c. do not contain 0, inasmuch 
 as it is contained in c, c', &c. so that to form the partial differences 
 
 (TT) 
 
 sufficient to make 
 
 these functions, which gives 
 
 d Xx /d Xx d c ,d Xx d c' /!Xx_d^ 
 H " Vd cV"dT h \d c'V d ^
 
 BOOK L] NEWTON'S PRINCIPIA. 99 
 
 f d Y > fd Yx die /dY N d c' /d Yx d c" 
 VdT/ " VdJdd + Vdc'/oTT " \dc'V dd H 
 
 &C. rr &C. 
 
 Again, it may happen that some of the arbitrary constants c, c', c", &c. 
 multiply the arc t in the periodic functions X, Y, Z, &c. The differentia- 
 tion of these functions relatively to 6, or, which is the same thing, relatively 
 to these arbitrary constants, will develope this arc, and bring it from without 
 
 the symbols of the periodic functions. The differences ( , j\ , (-j-j) 5 
 
 tnen 
 
 &c. 
 
 X', X", Y', Y", Z', Z", &c. being periodic functions of t, and containing 
 moreover the arbitrary constants c, c', c", &c. and their first differences 
 divided by d 6, differences which enter into these functions only under a 
 linear form ; we shall have therefore 
 
 z' + * z" + (t - <o z" 
 
 &c. 
 
 Substituting these values in the equation (a) we shall have 
 = X' + 6 X" Y 
 + (t d) { Y' + e Y" + X" 2 Z} 
 + (t 6)* [Z' + Z" + Y" 3-SJ + &c.; 
 
 whence we derive, in equalling separately to zero, the coefficients of the 
 powers of t 0, 
 
 = X' + 6 X" Y 
 
 = Y 7 + 6 Y" + X" 2 Z 
 
 = Z' + 6Z" + Y" 3 S; 
 
 &c. 
 
 G2
 
 100 A COMMENTARY ON [SECT. XI. 
 
 If we differentiate the first of these equations, i 1 times successively 
 relatively to t, we shall thence derive as many equations between the 
 quantities c, c', c", &c. and their first differences divided by d 0. Then 
 integrating these new equations relatively to 6, we shall obtain the con- 
 stants in terms of 6. 
 
 Inspection alone of the first of the above equations will almost always 
 suffice to get the differential equations in c, c', c", &c. by comparing se- 
 parately the coefficients of the sines and cosines which it contains. For 
 it is evident that the values of c, c', &c. being independent of t t the dif- 
 ferential equations which determine them, ought, in like manner, to be in- 
 dependent of it. The simplicity which this consideration gives to the pro- 
 cess, is one of its principal advantages. For the most part these equations 
 will not be integrable except by successive approximations, which will 
 introduce the arc 6 out of the periodic symbols, in the values of c, c', &c. 
 at the same time that this arc does not enter the rigorous integrals. But 
 we can make it disappear by the following method. 
 
 It may happen that the first of the preceding equations, and its i 1 
 differentials in t, do not give a number i of distinct equations between the 
 quantities c, c', c", &c. and their differences. In this case we must have 
 recourse to the second and following equations. 
 
 When we shall have thus determined c, c', c", &c. in functions of d, 
 we shall substitute them in X, and changing afterwards 6 into t, we shall 
 obtain the value of y, without arcs of a circle or free from periodic symbols, 
 when that is possible. 
 
 510. Let us now consider any number n of differential equations. 
 
 o = + P' + Q' ; 
 
 &c. 
 
 P, Q, P', Q' being functions of y, y', &c. of their differentials to the order 
 j _ ij and of the sines and cosines of angles increasing proportionally 
 with the variable t, whose difference is constant. Suppose the approximate 
 integrals of these equations to be 
 
 y = X + t Y + t 2 Z + t 3 S + &c. 
 
 y' = X, + t Y, + t 2 Z, + t 3 S, + &c. 
 
 X, Y, Z, &c. X,, Y,, Z,, &c. being periodic functions of t and containing 
 i n arbitrary constants c, c', c", &c. We shall have as in the preceding 
 No.
 
 BOOK I.] NEWTON'S PRINCIPIA. 101 
 
 = X' + OX" Y; 
 
 = Y' + 6 Y" + X" 2 Z; 
 
 = Z' + d Z" + Y" 3 S ; 
 
 &c. 
 The value of y 7 will give, in like manner, equations of this form 
 
 = X/ + *X," Y,; 
 
 = Y/ 4- 6 Y/ 7 + X/ 7 2 Z,; 
 
 &c. 
 
 The values of y 7/ , y 777 , &c. will furnish similar equations. We shall 
 determine by these different equations, selecting the most simple and 
 approximable, the values of c, c', c 77 , &c. in functions of d. Substituting 
 these values in X, X 7 , &c. and then changing 6 into t, we shall have the 
 values of y, y 7 , &c. independent of arcs free from periodic symbols when 
 that is possible. 
 
 511. Let us resume the method already exposed in No. 506. It thence 
 results that, if instead of supposing the parameters c, c 7 , c 77 , &c. constant, 
 we make them vary so that we have 
 
 d c = a d t [Y Q + F 7 Q 7 + &c] ; 
 dc 7 = adtHQ + H 7 Q / + &c.} ; 
 we shall always have the * n integrals of the order i 1, 
 
 c = V; c 7 = V 7 ; c 77 = V 77 ; &c. 
 
 as in the case of a = 0. Whence it follows that not only the finite in- 
 tegrals, but also all the equations in which these enter the differences 
 inferior to the order i, will preserve the same form, in the case of 
 a = 0, aiid in that where it is any quantity whatever ; for these equations 
 may result from the comparison alone of the preceding integrals of the 
 order i 1. We can, therefore, in the two cases equally differentiate 
 i 1 times successively the finite integrals, without causing c, c 7 , &c. to 
 vary ; and since we are at liberty to make all vary together, there will 
 thence result the equations of condition between the parameters c, c 7 , &c. 
 and their differences. 
 
 In the two cases where a = 0, and a = any quantity whatever, the 
 values of y, y 7 , &c- and of their differences to the order i 1 inclusively, 
 are the same functions of t and of the parameters c, c 7 , &c. Let Y be any 
 function of the variables y, y 7 , y 77 , &c. and of their differentials inferior to 
 the order i 1, and call T the function of t, which it becomes, when we 
 substitute for these variables and their differences their values in t. We 
 can differentiate the equation Y = T, regarding the parameters c, c 7 , &c. 
 constant ; we can only, however, take the partial difference of Y relatively 
 
 G 3
 
 108 A COMMENTARY ON [SECT. XL 
 
 to one only or to many of the variables y, y', &c. provided we suppose 
 what varies with these, to vary also in T. In all these differentiations, the 
 parameters c, c', c", &c. may always be treated as constants ; since by 
 substituting for y, y', &c. and their differences, their values in t, we shall 
 have equations identically zero in the two cases of a nothing and of a any 
 quantity whatever. 
 
 When the differential equations are of the order i. 1, it is no longer 
 allowed, in differentiating them, to treat the parameters c, c', &c. as con- 
 stants, To differentiate these equations, consider the equation <p = 0, <p 
 being a differential function of the order i 1, and which contains the 
 parameters c, c', c", &c. Let d <p be the difference of this function taken 
 in regarding c, c', &c. constant, as also the differences d i - l y, d i - l y', &c. 
 
 d' v 
 Let S be the coefficient of , .^ in the entire difference of <p . Let S' 
 
 Cl L 
 
 d ' V 
 be the coefficient of-, j^j in this same difference, and so on. The e na- 
 
 tion <f> = when differentiated will give 
 
 d ' v d ' v' 
 
 Substituting for ^ r^-j its value d t {P + a Q] ; for -j -^ its value 
 
 d t [P' + aQ'} &c. we shall have 
 
 d t [S P + S' F + &c.} _adt{SQ + S'Q' + &c.] . (t) 
 In the supposition of = 0, the parameters c, c', c", &c. are constant. 
 We have thus 
 
 = 3p dt{SP+S / P / + &c.} 
 
 If we substitute in this equation for c, c', c", &c. their values V, V', V", 
 Sec. we shall have differential equations of the order i 1, without arbi- 
 traries, which is impossible, at least if this equation is to be identically 
 nothing. The function 
 
 3p dt{SP+S / F + &c.} 
 
 becoming therefore identically nothing by reason of equations c = V, 
 c' = V, &c. and since these equations hold still, when the parameters 
 c, c', c", &c. are variable, it is evident, that in this case, the preceding
 
 BOOK I.] NEWTON'S PRINCIPLE 103 
 
 function is still identically nothing. The equation (t) therefore will be- 
 come 
 
 d t [S Q + S' Q' + &c.} ....... (x) 
 
 Thus we perceive that to differentiate the equation <p = 0, it suffices to 
 vary the parameters c, c', &c. in <p and the differences d ' ~ 1 y, d l ~ 1 y^ 
 &c. and to substitute after the differentiations, for a Q, a Q', &c. the 
 
 d'y'd'y 7 
 quantities^, -^ , &c. 
 
 Let 4* = 0, be a finite equation between y, y', &c. and the variable t. If 
 we designate by 8 -vj/, d 2 -v]/, &c. the successive differences of -\J/, taken in 
 regarding c, c', &c. as constant, we shall have, by what precedes, in that 
 case where c, c', &c. are variable, these equations : 
 
 4< = 0; d 4 = 0; 5 2 -v|/ = ...... a 1 - 1 -^ = 0; 
 
 changing therefore successively in the equation (x) the function <p into -v}/, 
 d -vp, d z >4/, &c. we shall have 
 
 d. S 1 - 1 ^ /d. S 5 - 
 
 = )' dc 
 
 dc 
 
 Thus the equations -^ = 0, ^ = 0, &c. being supposed to be the n 
 finite integrals of the differential equations 
 
 d' 
 
 
 
 
 - 
 dt' 
 
 &c. 
 
 we shall have i n equations, by means of which we shall be able to de- 
 termine the parameters c, c', c", &c. without which it would be necessary 
 for that purpose to form the equations c = V, c' = V', &c. But when 
 the integrals are under this last form, the determination will be more 
 simple. 
 
 512. This method of making the parameters vary, is one of great utility 
 
 G3
 
 104 A COMMENTARY ON [SECT. XL 
 
 in analysis and in its applications. To exhibit a new use of it, let us take 
 the differential equation 
 
 P being a function of t, y, of their differences to the order i ], and of 
 the quantities q, q', &c. which are functions of t. Suppose we have the 
 finite integral of this differential equation of the supposition of q, q', &c. 
 being constant, and represent by p = 0, this integral, which shall contain 
 i arbitrages c, c', &c. Designate by 3 p, 8 2 <p, 5 3 <p, &c. the successive differ- 
 ences of p taken in regarding q, q', &c. constant, as also the parameters 
 c, c', c", &c. If we suppose all these quantities to vary, the differences of 
 <p will be 
 
 making therefore 
 
 d <p will be still the first difference of <p in the case of c, c', &c. q, q', &c. 
 being variable. If we make, in like manner, 
 
 d a p, 8 3 <p , ..... d ' <p will likewise be the second, third, &c. differences of 
 <p when c, c', &c. q, q', &c. are supposed variable. 
 
 Again in the case of c, c', &c. q, q', &c. being constant, the differential 
 equation 
 
 d ' v 
 
 = a^ + p ' 
 
 is the result of the elimination of the parameters c, c', &c. by means of 
 the equations <p = 0, 8 <p = 0, a s <p = 0, . . . . d ' <p = 0. Thus, these 
 last equations still holding good when q, q', &c. are supposed variable, the 
 equation <p = will also satisfy, in this case, the proposed differential 
 equation, provided the parameters c, c', &c. are determined by means 
 of the i preceding differential equations ; and since their integration 
 gives i arbitrary constants, the function <p will contain these arbitraries, 
 and the equation <p = will be the complete integral of the proposed 
 equation.
 
 BOOK I.] 
 
 NEWTON'S PRINCIPIA. 
 
 105 
 
 This method, the variation of parameters, may be employed with ad- 
 vantage when the quantities q, q', &c. vary very slowly. Because this 
 consideration renders the integration by approximation of the differential 
 equations which determine the variables c, c', c", &c. in general much 
 easier. 
 
 513. Second Approximation of Celestial Motions. 
 
 Let us apply the preceding method to the perturbations of celestial 
 motions, in order thence to obtain the most simple expressions of their 
 periodical and secular inequalities. For that purpose let us resume the 
 differential equations (1), (2), (3) of No. 471, which determine the relative 
 motion of p about M. If we make 
 
 R = y! (x x' + y y' + z zQ + ft" (x x" + y y" + z z") 
 ( x '* + y' 2 + z /2 ) 2. (x //2 + y" 2 + z" 2 )* 
 
 + &c. - - 
 * 
 
 X being by the No. cited equal to 
 
 j (x" x') 2 + (y" y') 2 + (z" z') * 
 If, moreover, we suppose M + P = m 
 
 V x 2 + y 2 + z 
 
 /2 
 
 we shall have 
 
 /d_Rx 
 h Vdx/ 
 
 ' dt 2 
 
 - ? 4- --^ 4- f-- 
 ~ 2 3 h 
 
 
 dt 
 
 _d 2 zmz 
 - dT 1 " ' "" 
 
 dR 
 
 (P) 
 
 The sum of these three equations multiplied respectively by d x, d y, d z 
 gives by integration 
 
 
 the differential d R being only relative to the coordinates x, y, z of the 
 body /a., and a being an arbitrary constant, which, when R = 0, becomes 
 by No. 499, the semi-axis major of the ellipse described by p about 
 M.
 
 106 A COMMENTARY ON [SECT. XL 
 
 The equations (P) multiplied respectively by x, y, z and added to the 
 integral (Q) will give 
 
 , d 8 ?* m m , _ rjT> fdRx , / d R\ , /d R\ . 
 = l 7r f- T + V +2/^R + x( aT )+y( H -~) + Z ( nT ); (R) 
 
 We may conceive, however, the perturbing masses /*', ///', &c. multi- 
 plied by a coefficient , and then the value of will be a function of the 
 time t and of . If we develope this function according to the powers of a, 
 and afterwards make a = 1, it will be ordered according to the powers 
 and products of the perturbing masses. Designate by the characteristic 
 d when placed before a quantity, this differential of it taken relatively to , 
 and divided by d a. When we shall have determined d g in a series or- 
 dered according to the powers of , we shall have the radius by multi- 
 plying this series by d , then integrating it relatively to a, and adding to 
 the integral a function of t independent of , a function which is evidently 
 the value of in the case where the perturbing forces are nothing, and 
 where the body p describes a conic section. The determination of g re- 
 duces itself, therefore, to forming and integrating the differential equation 
 which determines 3 g. 
 
 For that purpose, resume the differential equation (R) and make for the 
 greater simplicity 
 
 d 
 
 differentiating this relatively to a, we shall have 
 
 Call d v the indefinitely small arc intercepted between the two radius- 
 vectors g and f + d g ; the element of the curve described by p around M 
 will be Vd^ + gMv 2 . We shall thus have 
 
 dx 2 + dy 2 + dz 2 = dg 2 + 2 dv 2 , 
 and the equation (Q) will become 
 
 ' t ._., m 
 
 dt 2 
 
 Eliminating from this equation by means of equation (R) we shall 
 a 
 
 have 
 
 g 2 d v 2 _ g d z g m 
 
 whence we derive, by differentiating i-elatively to a, 
 
 2g 2 d d V.d8v = gd 2 a^-a f d 2 g _3j^a i+g3R/ _ R/agt
 
 BOOK I.] NEWTON'S PRINCIPIA 107 
 
 If we substitute in this equation for -j- - its value derived from equa- 
 ^ 
 tion (S), we shall have 
 
 d v ' 
 
 By means of the equations (S), (T), we can get as exactly as we wish the 
 values of 5 g and of 3 v. But we must observe that d v being the angle 
 intercepted between the radii g and g -f- d g, the integral v of these angles 
 is not wholly in one plane. To obtain the value of the angle described 
 round M, by the projection of the radius-vector g upon a fixed plane, de- 
 note by v, , this last angle, and name s the tangent of the latitude of ft above 
 
 this plane ; then g(l + s 2 ) ~ ^ will be the expression of the projected ra- 
 dius-vector, and the square of the element of the curve described by p 
 will be 
 
 g 2 dv/ g g ds* 
 
 1 + s* H h (l + s s ) 2; 
 
 But the square of this element is also g 2 dv 2 + d g 2 ; therefore we have, 
 by equating these two expressions 
 
 
 We shall thus determine d v, by means of d v, when s is known. 
 
 If we take for the fixed plane, that of the orbit of (i at a given epoch, 
 
 d s 
 s and -, - will evidently be of the order of perturbing forces. Neglecting 
 
 therefore the squares and the products of these forces, we shall have 
 v = v, . In the Theory of the planets and of the comets, we may neglect 
 these squares and products with the exception of some terms of that 
 order, which particular circumstances render of sensible magnitude, and 
 which it will be easy to determine by means of the equations (S) and (T). 
 These last equations take a very simple form, when we take into account 
 the first power only of the disturbing forces. In fact, we may then con- 
 sider 3 g and a v as the parts of g and v due to these forces ; a R, a. g R' 
 are what R and g R' become, when we substitute for the coordinates of 
 the bodies their values relative to the elliptic motion : We may designate 
 them by these last quantities when subjected to that condition. The 
 equation (S) thus becomes,
 
 108 A COMMENTARY ON [SECT. XL 
 
 The fixed plane of x, y being supposed that of the orbit of p at a given 
 epoch, z will be of the order of perturbing forces : and since we may 
 neglect the square of these forces, we can also neglect the quantity 
 
 z f-i ). Moreover, the radius g differs only from its projection by quan- 
 
 tities of the order z 2 . The angle which this radius makes with the axis 
 of x, differs only from its projection by quantities of the same order. 
 This angle may therefore be supposed equal to v and to quantities nearly 
 of the same order 
 
 x = g cos. v ; y = g sin. v ; 
 
 whence we get 
 
 d R\ /dR /d 
 
 and consequently g . R' = (, j . It is easy to perceive by differentia- 
 
 tion, that if we neglect the square of the perturbing force, the preceding 
 differential equation will become, by means of the two first equations (P) 
 
 x/ydt{2/rfR + f ( <1 d ^)}-y/xdt{2/rfR+f(^)} 
 
 POP _ * * * _ , 
 
 /x d y y d x\ 
 
 \ ~dl / 
 
 In the second member of this equation the coordinates may belong to 
 
 j (| y _ y (1 X 
 
 elliptic motion ; this gives - 3 ^~ -- constant and equal to V m a(l e ! ). 
 
 a e being the excentricity of the orbit of /. If we substitute in the ex- 
 pression of d g for x and y, their values cos. v and g sin. v, and for 
 
 y ~ y ( ^ thg q uan tity V i* a (I e 2 ) ; finally, if we observe that 
 
 (1 L 
 
 by No. (480) 
 
 m = n 2 a 3 , 
 we shall have 
 
 C acos.v/ndt. f sin.v|2/cZ R + g (^jy) } ) 
 
 ^- asin.v/ndt.gcos.v|a/dR + S ( d d ~)}) 
 
 dg = " m V le z 
 
 The equation (T) gives by integration and neglecting the square of 
 perturbing forces, 
 
 _ 
 
 d - - a 2 n d t T m JJ m j s W g 
 
 T ^ ~~~
 
 BOOK I.] NEWTON'S PRINCIPIA. 109 
 
 This expression, when the perturbations of the radius-vector are known, 
 will easily give those of the motion of/* in longitude. 
 
 It remains for us to determine the perturbations of the motion in lati- 
 tude. For that purpose let us resume the third of the equations (P) : 
 integrating this in the same manner as we have integrated the equation 
 (S), and making z = g 8 s, we shall have 
 
 r j ,. /d R\ /. j /d R\ 
 
 a cos. \j n d t . sin. v f , ) a sin. vj n d t . g cos. v( -, ) 
 
 v cl z / \a z / ,. 
 
 d s r= - - ; (j) 
 
 m v 1 e 2 
 
 5 s is the latitude of /* above the plane of its primitive orbit : if we wish 
 to refer die motion of /* to a plane somewhat inclined to this orbit, by 
 calling s its latitude, when it is supposed not to quit the plane of the 
 orbit, s + 5 s will be very nearly the latitude of /* above the proposed 
 plane. 
 
 514. The formulas (X), (Y), (Z) have the advantage of presenting the 
 perturbations under a finite form. This is very useful in the Cometary 
 Theory, in which these perturbations can only be determined by quad- 
 ratures. But the excentricity and inclination of the respective orbits of 
 the planets being small, permits a developement of their perturbations 
 into converging series of the sines and cosines of angles increasing pro- 
 portionally to the time, and thence to make tables of them to serve for 
 any times whatever. Then, instead of the preceding expressions of 8 g, 
 8 s, it is more commodious to make use of differential equations which 
 determine these variables. Ordering these equations according to the 
 powers and products of the excentricities and inclinations of the orbits, 
 we may always reduce the determination of the values of 5 & and of 5 s 
 to the integration of equations of the form 
 
 equations whose integrals we have already given in No. 509. But we 
 can immediately reduce the preceding differential equations to this simple 
 form, by the following method. 
 
 Let us resume the equation (R) of the preceding No., and abridge it 
 by making 
 
 Q = 2fd R + 
 
 It thus becomes
 
 110 A COMMENTARY ON [SECT. XL 
 
 In the case of elliptic motion, where Q = 0, g 2 is by No. (488) a func- 
 tion of e cos. (n t + * *0 a e being the excentricity of the orbit, and 
 n t + t -a the mean anomaly of the planet p. Let e cos. (n t + t -a] 
 = u, and suppose f 2 = <p (u) ; we shall have 
 
 In the case of disturbed motion, we can still suppose g 2 = <f> (u), but 
 u will no longer be equal to e cos. (n t + e -or}. It will be given by 
 the preceding differential equation augmented by a term depending upon 
 the perturbing forces. To determine this term, we shall observe that if 
 we make u = 4/ (g z ) we shall have 
 
 nu = 
 
 4/ ( 2 ) being the differential of $> (f 2 ) divided by d.g 2 and y (g 2 ) the 
 
 d 2 . e 2 
 differential of -v]/ (f 2 ) divided by d.f 2 . The equation (R/) gives ' 2 
 
 equal to a function of plus a function depending upon the perturbing 
 ibrce. If we multiply this equation by 2 d f , and then integrate it, we 
 
 shall have - , | equal to a function of plus a function depending upon 
 
 122 2 ^1 2 
 
 the perturbing force. Substituting these values of , , and of , j- in 
 
 d t 2 d t 2 
 
 d 2 u 
 the preceding expression of - + n 2 u, the function of f, which is in- 
 
 v i L 
 
 dependent of the perturbing force will disappear of itself, because it is 
 identically nothing when that force is nothing. We shall therefore have 
 
 d 2 ti d 2 . e 2 p 2 de 2 
 
 the value of -. - + n 2 u by substituting for ' 2 , and =-j |- , the parts 
 
 Cl L Cl L (1 L 
 
 of their expressions which depend upon the perturbing force. But re- 
 garding these parts only, the equation (R') and its integral give 
 
 Wherefore 
 
 If*'- = - 8/Q , d , 
 
 C\ /\ < / / 9\ f\ I // / O\ /* ^X 1 
 
 jp- + n 2 u = 2 Q -4,' (g 2 ) 8 >|/ 7 (e S )/Q . g d f. 
 
 Again, from the equation u = p (^ 2 ), we derive d u = 2 g d g 4' (? ! ) i 
 this f * = 'p (u) gives 2 f, d g = d u. p' (u) and consequently
 
 BOOK I.I NEWTON'S PRINCIPIA. Ill 
 
 Differentiating this last equation and substituting <f>' (u) for ~ * , we 
 shall have 
 
 <p" (u) being equal to \ ^ , in the same way as <f>' (u) is equal to 
 
 ', . This being done : if we make 
 d u 
 
 u = e cos. (n t + e } + ^ u, 
 the differential equation in u will become 
 
 d t 2 p' (u) 3 0' (u) ' 
 
 and if we neglect the square of the perturbing force, u may be supposed 
 equal to e cos. (n t + s "*)> i n the terms depending upon Q. 
 
 Q 
 
 The value of - found in No. (485) gives, including quantities -of the 
 a 
 
 order e 3 
 
 whence we derive 
 
 ^ = a 3 f l+2e 2 2u(l \ e 2 ) u 2 u 5 i = p(u). 
 t ^ & ' ) 
 
 If we substitute this value of <p (u) in the differential equation in d u, 
 
 / (\ T? 
 
 and restore to Q its value 2 f d R + f (q ) , and e cos. (n t -f -a) 
 
 a 
 
 for u, we shall have including quantities of the order e 3 , 
 
 2-| 1 + T e2 ecos. (nt + t *) - e 2 cos. (2 n t f 2 t 
 
 
 |/hdt[sin. (nt+i-w) 
 When we shall have determined d u by means of this differential equa-
 
 112 A COMMENTARY ON [SECT. XI. 
 
 tion, we shall have & g by differentiating the expression of g, relative to 
 the characteristic 3, which gives 
 
 . (nt + i )+ -e 4 cos.(2nt+2s 2 
 
 This value of d will give that of 8 v by means of formula (Y) of the 
 preceding number. 
 
 It remains for us to determine d s ; but if we compare the formulas (X) 
 and (Z) of the preceding No. we perceive that 8 g changes itself into 8 s 
 
 by substituting (-7 )for 2fd R + g (-g ) in its expression. Whence 
 
 it follows that to get 8 s, it suffices to make this change in the differential 
 equation in 8 u, and then to substitute the value of d u given by this equa- 
 tion, and which we shall designate by 8 u', in the expression of 8 g. Thus 
 we get 
 
 ecos.(nt + ~)+ e*cos.(2nt+2e 2 
 
 The system of equations (X'), (Y), (Z') will give, in a very simple 
 manner, the perturbed motion of ^ in taking into account only the first 
 power of the perturbing force. The consideration of terms due to this 
 power being in the Theory of Planets very nearly sufficient to determine 
 their motions, we proceed to derive from them formulas for that purpose.- 
 
 515. It is first necessary to develope the function R into a series. If 
 we disregard all other actions than that of , upon //, we shall have by (513) 
 
 R = .<* / (xx'+yy / + zz / ) x 
 
 (x' 2 + y' 2 + z")i {(x' x) + (y' y) 2 + (z' z)*p ' 
 This function is wholly independent of the position of the plane of x, 
 y ; for the radical V (x' x) 2 + (y' y) 2 + (z' z) 2 , expressing the 
 distance of /<*, ^'5 is independent of the position ; the function x 8 + y * 
 -f- z* + x' * + y' 2 + z' ! 2 x x' 2 y y' 2 z z' is in like manner in- 
 dependent of it But. the squares x 8 + y 2 + z 2 and x /2 + y' 2 + z /2 
 of the radius-vectors, do not depend upon the position ; and therefore the 
 quantity x x' -f y y' + z z' does not depend upon it, and consequently
 
 BOOK I.] NEWTON'S PRINCIPI A. 1!3 
 
 R is independent of the position of the plane of x, y. Suppose in this 
 function 
 
 x = g cos. v ; y = f sin. v ; 
 
 x' = g' cos. v' ; y' = g' sin. v' ; 
 we shall then have 
 u _ A^g j cos, (v' v) + z z'} u/ 
 
 * 
 
 os.(v'_ v)+g' 2 -f; (z' z) 2 P 
 The orbits of the planets being almost circular and but little inclined 
 to one another, we may select the plane of x, y, so that z and z' may be 
 very small. In this case g and g' are very little different from the semi- 
 axis-majors a, a' of the elliptic orbits, we will therefore suppose 
 
 g = a(l + u,); g' = a'(l + u/) ; 
 
 u, and u/ being small quantities. The angles v, v' differing but little 
 from the mean longitudes n t + , n' t + e', we shall suppose 
 
 v = n t + + v,; v' = n' t + > + v/; 
 
 v' and v/ being inconsiderable. Thus, reducing R into a series ordered 
 according to the powers and products of u /t v /5 z, u/, v/, and z', this series 
 will be very convergent. Let 
 
 a l 
 
 2 cos. (n' t n t + i' j) fa * 2 a a' cos. (of t n t + *' e)+a' 2 } J 
 
 
 
 = 5 1< + A ( cos. (n' t n t -f *' s) -f A cos. 2 (n r t n t +' ) 
 
 Sv 
 
 + A cos. 3 (n r t n t + e r ) + &c. ; 
 
 We may give to this series the form | 2 A (i) cos. i (n' t n t + *' s )> 
 the characteristic 2 of finite integrals, being relative to the number i, and 
 extending itself to aH whole numbers from i = QD to i = oc ; the value 
 i = 0, being comprised in this infinite number of values. But then we 
 must observe that A (-i) = A [i) . This form has the advantage of serving 
 to express after a very simple manner, not only the preceding series, but 
 also the product of this series, by the sine or the cosine of any angle 
 ft-\-zr', for it is perceptible that this product is equal to 
 
 C|T| 
 
 2A {i(n't nt+ ' ) +f t + w}. 
 
 C-Ob 
 
 This property will furnish us with very commodious expressions for 
 the perturbations of the planets. Let in like manner 
 
 Ja * 2 a a cos. (n' t n t -f t' e) + a' *} ~ * 
 
 = TJ 2 B ' cos. i (n t n t + s s) ; 
 
 m 
 
 B ( i} being equal to B (i} . This being done, we shall have by (483) 
 VOL. II. H-
 
 Ill A COMMENTARY ON [SECT. XL 
 
 
 R = -- . 2 A o> cos. i (n' t n t : -f ' ) 
 
 I 
 
 + U 2 ' 003 ' [ n/ t n t + .' 
 
 + "/^'(- 7 " 008 ' i (n' t n t + e' 
 
 ,' 
 ~2 ( V// v /) 2 * A (i) sin. i (n' t n t -f t' -- t) 
 
 + . u,. 2.a 2 (ijA^)cos. i (n' t n t + ,' - .) 
 
 ~ /" 2 a /2 ('^ 2 )cos. i (n' t n t + e' ) 
 4 \ d a * / 
 
 /*' /d A w \ 
 
 -g- (v/ v,) u, 2 . i a ( j - J sin. i (n' t n t + *' e) 
 
 ~ ( v / ~ v ') u / 2 " * a/ sin ' i (n' t - n t + f ' - 
 
 _ 1L ( v ; _ v/ ) 2 . 2 . i 2 A W cos. i (n 7 I n t + ' 
 
 T 1 
 
 zz' 3/i' az /2 
 
 , . 
 cos. (n' t n t + ' 
 
 / 3 --- 2a /4 
 /(Z/ ~ Z) * 2 B W cos. i (n' t n t + ' 
 
 + &c. 
 
 If we substitute in this expression of R, instead of u /5 u/, v 7 , v/, z and z', 
 their values relative to elliptic motion, values which are functions of sines 
 and cosines of the angles n t + e, n' t + ' and of their multiples, R will 
 be expressed by an infinite series of cosines of the form // k cos. (i n' t 
 i n t + A), i and i' being whole numbers. 
 
 It is evident that the action of /A", //", &c. upon p will produce in R 
 terms analogous to those which result from the action of //, and we shall 
 obtain them by changing in the preceding expression of R, all that relates 
 to //, in the same quantities relative to /*"> (*'"* &c. 
 
 Let us consider any term // k cos. (i' n' t i n t + A) of the expres- 
 sion of R. If the orbits were circular, and in one plane we should 
 have i' = i. Therefore i' cannot surpass i or be exceeded by it, except 
 by means of the sines or cosines of the expression for u /} v /5 z, u/, v/, z' 
 which combined with the sines and cosines of the angle n' t n t + ' e
 
 BOOK I.] NEWTON'S PRINCIPIA. 115 
 
 and of its multiples, produce the sines and cosines of angles in which i' 
 is different from i. 
 
 If we regard the excentricities and inclinations of the orbits as very 
 small quantities of the first order, it will result from the theorems of 
 (481) that in the expressions of u,, v,, z or g s, s being the tangent of the 
 latitude of ,u, the coefficient of the sine or of the cosine of an angle such 
 as f. (n t + g), is expressed by a series whose first term Is of the order f ; 
 second term of the order f + 2 ; third term of the order f + 4 and so 
 on. The same takes place with regard to the coefficient of the sine or of 
 the cosine of the angle f (n' t + s') in the expressions of u/, v/, z'. Hence 
 it follows that i, and i' being supposed positive and i' greater than i, the 
 coefficient k in the term m' k cos. (i 7 n 7 t i n t + A) is of the ordar 
 i' i, and that in the series which expresses it, the first term is of the 
 order V i the second of the order i' i + 2 and so on ; so that the 
 series is very convergent. If i be greater than i', the terms of the series 
 will be successively of the orders i i', i V + 2, &c. 
 
 Call -a the longitude of the perihelion of the orbit of p and 6 that of its 
 node, in like manner call ' the longitude of the perihelion of (j/, and & 
 that of its node, these longitudes being reckoned upon a plane inclined 
 to that of the orbits. It results from the Theorems of (481), that in the 
 expressions of u /s v,, and z, the angle n t + s is always accompanied by 
 or by 6 ; and that in the expressions of u/, v/, and z', the angle 
 n' t -{ t' is always accompanied by */, or by ^ ; whence it follows 
 that the term p' k cos. (i' n' t i n t + A) is of the form 
 
 /u/kcos. (in't int + V t is g w g* CT ' g"^ g"' 0> 
 g, g', g", g"' being whole positive or negative numbers, and such that 
 we have 
 
 = i' - i g g' g" g'". 
 
 It results also from this that the value of R, and its different terms are 
 independent of the position of the straight line from which the longitudes 
 are measured. Moreover in the Theorems of (No. 481) the coefficient of 
 the sine and cosine of the angle *?, has always for a factor the excentricity e 
 of the orbit of ^ ; the coefficient of the sine and of the cosine of the angle 
 2 w, has for a factor the square e 2 of this excentricity, and so on. In like 
 manner, the coefficient of the sine and cosine of the angle 6, has for its 
 factor tan. ^ <p, <p being the inclination of the orbit of p upon the fixed 
 plane. The coefficient of the sine, and of the cosine of the angle 2 0, has for 
 its factor tan. 2 1 <p, and so on. Whence it results that the coefficient k has for 
 its factor, e *. e' '. tan. g " ( p) tan. '" ( <p'} ; the numbers g, g 7 , g", g"' being 
 
 H2
 
 116 A COMMENTARY ON [SECT. XI. 
 
 taken positively in the exponents of this factor. I fall these numbers are 
 positive, this factor will be of the order V i, by virtue of the equation 
 
 = i' i gg' g"_g"'j 
 
 but if one of them such as g, is negative and equal to g, this factor 
 will be of the order i' i + 2 g. Preserving, therefore, amongst the 
 terms of R, only those which depending upon the angle i' n' t i n t are of 
 the order i' i, and rejecting all those which depending upon the same 
 angle, are of the order i' i + 2, i' i -f- 4, &c. ; the expression of 
 R will be composed of terms of the form 
 
 H e . e' *' tan. " ( ~ p) tan. g'". ( i ^) cos. (i' n' t i n t + i' t' 
 
 - i _ g . ._ g '. ' _ g//. 6 _ g///. 0'), 
 
 H being a coefficient independent of the excentricities, and inclinations 
 of the orbits, and the numbers g, g 7 , g", g /// being all positive, and such 
 that their sum is equal to i' i. 
 
 If we substitute in R, a (1 + u,), instead off, we shall have 
 d Rx /d 
 
 If in this same function, we substitute instead of u', v' and z, their values 
 given by the theorems of (481), we shall have 
 d RN /d Rx 
 
 provided that we suppose e , and s 6 constant in the differential of 
 R, taken relatively to e ; for then u,, v x and z are constant in this differ- 
 ential, and since we have v = n t + z + v /} it is evi'dent that the preced- 
 ing equation still holds. We shall, therefore, easily obtain the values 
 
 ^(~1 )' an< * ^ (~1 ) which enter into the differential equations of 
 
 the preceding numbers, when we shall have the value of R developed 
 into a series of angles increasing proportionally to the time t. The dif- 
 ferential d R it will be in like manner easy to determine, observing to vary 
 in R the angle n t, and to suppose n' t constant ; for d R is the difference 
 of R, taken in supposing constant, the coordinates of ,/, which are func- 
 tions of n' t. 
 
 516. The difficulty of the developement of R into a series, may be 
 reduced to that of forming the quantities tf*\ B (i) , and their differences 
 taken relatively to a and to a'. For that purpose consider generally the 
 function 
 
 (a 2 2 a a' cos. d + a' 2 ) ~
 
 BOOK I.] NEWTON'S PRINCIPIA. 117 
 
 and develope it according to the cosine of the angle 8 and its multiples. 
 
 m 
 
 If we make -. = , it will become 
 a 
 
 2s i 
 
 a .{] 2 a COS. 6 -}- a *} 
 Let 
 
 (1 2 a cos. 6 + a 5 ) ~ S = $ b W + b (1 > cos. + b cos. 2 j 
 
 S 8 S 
 
 + b (5 < cos. 3 <? + &c. 
 
 s 
 
 b (0) , b (I) , b (e -', &c. being functions of a and of s. If we take the logarith- 
 
 f 8 S 
 
 mic differences of the two members of this equation, relative to the vari- 
 able 0, we shall have 
 
 b M sin. 6 2 b sin. 2 6 &c. 
 2 s a sin. 6 
 
 1 2cos. d + a z '~ 
 
 Multiplying this equation crosswise, and comparing similar cosines, we 
 find generally 
 
 (i_ i) (i + -)b- (i + s 2)ab< 1 - 2 > 
 
 b 0) = - L, - . - , - ^ - . . . (a) 
 
 (I 5). $ ; 
 
 We shall thus have b (2) , b (8) , &c. when b [0) and b r ' 5 are known. 
 
 8 8 
 
 If we change s into s + 1, in the preceding expression of (1 2 a cos. 6 
 -f a 2 ) , we shall have 
 
 (12 a cos. 0+ 2 ) = | b +b (1) cos. 0+b (2 > cos.2 6+b cos.30-f &c. 
 
 8+1 S+l 8+1 8+1 
 
 Multiplying the two members of this equation, by 1 2 a cos. 6 + <*% 
 
 and substituting for (1 2 a cos. 6 + a*) its value in series, we shall 
 
 have 
 
 + b^cos. 6 + b 12 ' cos. 20 + &c. 
 
 = (1 2 a cos. 6+a z ){ b (0 > + b (1 > cos. -f b cos. 26 + &c.j 
 
 S + l S+ 1 6+1 
 
 whence by comparing homogeneous terms, we derive 
 b = (1 -f 2 )b ab- o 
 
 Tk f 1 "/ N S + J ' + 1 
 
 1 he tormula (a) gives 
 
 i(l + a)bW (i + s)ab (i - 
 b P+i) = _ . . 8 + ' T ^- - L_L . 
 s+l (l Sj.a 
 
 The preceding expression of b (i) will thus become 
 
 8 
 
 2s.ab- 8(1 + a)b 
 
 i+J . 
 S 
 H3
 
 1 18 A COMMENTARY ON [SECT. XL 
 
 Changing i into i -j- 1 in this equation we shall have 
 
 gsab^ s(l + a*)bO+ 
 
 b C' + !) = _ 1 _ _L+i 
 
 i _ s + 1 
 
 and if we substitute for b (i + 1} its preceding value, we shall have 
 
 8 + 1 
 
 8(1 + s)a(l + a e )b< l -'> + s2(i s) a* i (l+a j ) 2 jb > 
 1J __ L+J 
 
 _ 
 
 (i s) (i s + l)a 
 These two expressions of b and b (i + 1} give 
 
 s s; 
 
 b CO - - _ 5 _ I _ _ /M 
 
 s + l~ (1 2 ) 2 
 
 substituting for b (i + !) its value derived from equation (u), we shall have 
 
 an expression which may be derived from the preceding by changing i 
 into i, and observing that b (i) = b (i) . We shall therefore have by 
 means of this formula, the values of b (0) , b (1) , b (2) , &c. when those of 
 
 8+1 S+l 8+1 
 
 b<% b<, b, &c. are known. 
 
 a * 
 
 Let X, for brevity, denote the function 1 2 a cos. 6 + a 2 . If we 
 differentiate relatively to a, the equation 
 
 X - = b c> + b <> cos. 6 + b cos. 2 & + &c. 
 
 S g 8 
 
 we shall have 
 
 d b w > d b < d b ^ 
 
 2s (a cos. 6} X - s - 1 = A . ? -- 1 -- ^ cos. 6 + . cos. 2 d + &c. 
 
 da da da 
 
 But we have 
 
 1 a 2 X 
 
 a + cos. d = - - - ; 
 a 
 
 We shall, therefore, have 
 
 . dbW db< 
 
 __ _ __ 
 
 a a J d a d a 
 
 whence generally we get 
 
 d b > s b W 
 
 d- . 
 
 a a s+l a 
 
 Substituting for b (i) its value given by the formula (b), we shall have 
 
 d 
 
 8 + 1 
 
 + 2s)a g m 
 '
 
 BOOK I.] NEWTON'S PRINCIPIA. 119 
 
 If we differentiate this equation, we shall have 
 
 
 (l+a j_) 
 *) 2 J? ' 
 
 da 2 " a(l a 8 ) 'da 
 
 d h (i + ') 
 
 (i 
 
 1 a 2 da (Ia 2 ) 2 . 
 
 Again differentiating, we shall get 
 
 d 3 b (i) d 2 b (i) d b 0) 
 
 . _i-f-(i+2.s)a 2 , f(i + s)(I + 2 ) jllL 
 
 da a ' a (l_ a 2) 'da 2 ' \ (l!_a 8 ) 8 a 2 / da 
 
 d ' 
 
 , 
 '" 3 
 
 (1 a 2 ) 3 "a 3 /, 1 a 2 da 2 
 
 8(j- S+ I) g ^ 4(1 S+l)(l + 3a 2 ) fl 
 
 (1 a 2 ) 2 da (I a 2 ) 3 J 
 
 Thus we perceive that in order to determine the values of b and 01 
 
 8 
 
 its successive d inferences, it is sufficient to know those of b (0) and of b (1) . 
 
 8 8 
 
 We shall determine these two as follows : 
 
 If we call c the hyperbolic base, we can put the expression of X~ s un- 
 der this form 
 
 X- 8 = (1 a C 0v'-l)-s (! _ ac _0v-l)-s. 
 
 Developing the second member of this equation relatively to the powers of 
 c 8 V i, and c e ^ ] , it is evident the two exponentials c i e V 1 , c * e V 1 
 will have the same coefficient which we denote by k. The sum of the 
 two terms k . c ' e \ l and k c i v/ 1 is 2 k cos. i 6. This will be the 
 value of b (l) cos. i d. We have, therefore, b (i) = 2 k. Again the ex- 
 
 8 S 
 
 pression of X~ s is equal to the product of the two series 
 
 2 C 2flV-i + & c . 
 
 | 
 
 Ik <Q 
 
 1 + sac-OV-l + !_( S +Jl a c-2flV_I + &c . . 
 
 1 *, 3S 
 
 multiplying therefore these two together, we shall have when i = 
 k = l s 
 
 and in the case of i = 1, 
 
 wherefore 
 
 H i
 
 120 A COMMENTARY ON [SECT. XL 
 
 That these series may be convergent, we must have less than unity, 
 which can always be made so, unless a = a' ; a being = - , we have only 
 
 to take the greater for the denominator. 
 
 In the theory of the motion of the bodies ., ///, p", &c. we have occasion 
 to know the values of b (0) and of b a) when s = \ and s = f . In these 
 
 S 8 
 
 two cases, these values have but little convergency unless a is a small 
 fraction. 
 
 The series converge with greater rapidity when s = , and we have 
 
 1 > / 1 \ 2 
 
 o ' v o 7 
 -* 
 
 b ( = a f l 1 - 1 g8 1 . 1 ' 1 - 8 tt 4 1.3 1.1.3.5 a6 1.3.5 1.1.3 ...71 8|&c> 
 
 In the Theory of the planets and satellites, it will be sufficient to take 
 the sum of eleven or a dozen first terms, in neglecting the following 
 terms or more exactly in summing them as a geometric progression whose 
 common ratio is 1 a 2 . When we shall have thus determined b (0) and 
 
 -i 
 b 0) , we shall have b (0) in making i = 0, and s = in the formula (b). 
 
 ~a s 
 
 and we shall find 
 
 (1 + ) bW + 6 ab> 
 
 If in the formula (c) we suppose i = 1 and s = we shall have 
 2ab + 3 (1 + a 2 )b> 
 
 b) - ^i ^i . 
 
 , (1 -")' 
 
 By means of these values of b (0) and of b (1) we shall have by the pre- 
 
 i I 
 
 ceding forms the values of b (i) and of its partial differences whatever may 
 
 2 
 
 be the number i ; and thence we derive the values of b (l) and of its dif- 
 
 I 
 ferences. The values of b (0) and of b (1) may be determined very simply,
 
 BOOK L] NEWTON'S PRINCIPIA. 121 
 
 by the following formulae 
 
 b w> b W 
 
 ' " 
 
 Again to get the quantities A (0) , A (1) , &c. and their differences, we 
 must observe that by the preceding No., the series 
 
 A (0) + A W cos. 6 + A w cos. 2 d + &c. 
 results from the developement of the function 
 
 a_^L^ _ (a* _ 2 a a' cos. 6 + a' 2 ) "*, 
 
 1 
 
 into a series of cosines of the angle and of its multiples. Making 7 = a, 
 
 1 
 
 this same function becomes 
 
 2 . ~cos.2* See. 
 
 2 a i \a /2 a , / a' , 
 
 ^ 2 
 
 which gives generally 
 
 when i is zero, or greater than 1, abstraction being made of the sign. 
 In the case of i = 1, we have 
 
 A = A-.-lb"). 
 a a 'i 
 
 We have next 
 
 db (i > 
 d A W\ ! 
 
 d A W\ _!_ /^x 
 
 d a ) ~ ~ a r ' ~d a \<\ J ; 
 
 But we have -* = : ; therefore 
 da a! 
 
 db 
 d A to l 
 
 __ 
 
 V da >" "a' * d 
 and in the case of i = 1, we have 
 
 ^v J_f * 1 
 
 )- a' 2 I da J 
 
 d 
 
 Finally, we have, in the same case of i = 1 
 
 d 2 b ^ 
 /d* AW 
 
 _ __ 
 da 2 / ~ " a' 3 ' d a s
 
 122 A COMMENTARY ON [SECT. XI. 
 
 d 3 b w 
 
 /d 3 A"\ 1 __ i 
 
 V da 3 )~ a' 4 * da 3 ; 
 
 &c. 
 
 To get the differences of A (i) relative to a', we shall observe that A w 
 being a homogeneous function in a and a', of the dimension 1, we 
 have by the nature of such functions, 
 
 whence we get 
 
 ,/dAWx /dAO\ 
 
 a ( -3 r ) = A w a ( -a ) ; 
 
 v d a' / V d a / 
 
 d A W\ /d s A (i) 
 
 , 3 3 A x . m _ Q /dA t'\ , /d 2 A W N 3 /d 3 A 
 
 a -- = - 6A n - 18 a - 9a 
 
 &c. 
 
 We shall get B (i) and its differences, by observing that by the No. pre- 
 ceding, the series 
 
 -I B (0 > + B W cos. 6 + B cos. 2 6 + &c. 
 is the developement of the function 
 
 a'- 3 (1 2 COS. 6 -f- a 2 )~i" 
 
 according to the cosine of the angle 6 and its multiples. But this function 
 thus developed is equal to 
 
 a'- 3 f ib<> + b cos. 4 + b cos. 2 + &c.) 
 
 if i i J ; 
 
 therefore we have generally 
 B"> = -U< ; 
 
 Whence we derive 
 
 - 
 
 /d B MX J_ _f_; /d'B^x J. 
 V da ;~ a /4 * da V d a 2 / " a /5 ' 
 
 __ 
 d a 2 
 
 Moreover, B (i} being a homogeneous function of a and of a', of the 
 dimension 3 we have
 
 BOOK I.] NEWTON'S PRINCIPIA. 123 
 
 whence it is easy to get the partial differences of B w taken relatively to 
 a' by means of those in a. 
 
 In the theory of the Perturbations of p', by the action of /tt, the values 
 of A (i) and of B (i) , are the same as above with the exception of A (n which 
 
 in this theory becomes 2 -- , b (1) . Thus the estimate of the values of 
 a a ] 
 
 a 
 
 A (i) , B (i) , and their differences will serve also for the theories of the two 
 bodies /j> and p/. 
 
 517. After this digression upon the developement of R into series, let 
 us resume the differential equations (X 7 ), (Y), (Z') of Nos. 513, 514; and 
 find by means of them, the values of 3 , d v, and 8 s true to quantities 
 of the order of the excentricities and inclinations of orbits. 
 
 If in the elliptic orbits, we suppose 
 
 g = a(l + u,); /=a'(l + u/): 
 v = n t -f s + v, ; v' = ri t f' + v/ ; 
 we shall have by No. (488) 
 
 U, = e cos. (n t + s *) u/ = e' cos. (n' t + s' &'}, 
 v, = 2 e sin. (n t + t tr) ; v/ = 2 e' sin. (n' t + e 7 *S) ; 
 n t + , n' t + s' being the mean longitudes of ft, yf ; a, a! being the semi- 
 axis-majors of their orbits ; e, e' the ratios of the excentricity to the semi- 
 axis-major; , and lastly , ' being the longitudes of their perihelions. All 
 these longitudes may be referred indifferently to the planes of the orbits, 
 or to a plane which is but very little inclined to the orbits ; since we ne- 
 glect quantities of the order of the squares and products of the excen- 
 tricities and inclinations. Substituting the preceding values in the ex- 
 pression of R in No. 515, we shall have 
 
 R = 2 A cos. i (n' t n t + ' 
 
 -{^{(TrV"*'}* 
 
 e COS.U (n' t n t + t' t) + n t + **} 
 
 e' cos.[i (n" t nt+t/ g) + nt + e */}; 
 
 the symbol 2 of finite integrals, extending to all the whole positive and 
 negative values of i, not omitting the value i = 0. 
 Hence we obtain 
 
 d 
 
 2
 
 121- A COMMENTARY ON [SECT. XI. 
 
 d 
 
 + n t + s ^; 
 
 the integral sign 2 extending, as in what follows, to all integer positive 
 and negative values of i, the value i = being alone excepted, because 
 we have brought from without this symbol, the terms in which i = : /*' g 
 is a constant added to the integral/" d R. Making therefore 
 
 ,/y . ,/ . ,/x 
 
 D = i a 2 a ( -j T-/ ) + a ( i - ) + a a 7 1 , , ) + 2 a A 
 ^dadaV ^da/ \da/ 
 
 ,. 
 2li (n n ) n} I V d a 
 
 i (n n x ) n t ^ d a 
 
 /H 2 A (-i)x /d A 0-1)*, 
 
 D W = ia'a-( d , A . , )- (i - 1) W2~ - ) 
 
 \ da da ' J Vda/ 
 
 (i l)n f ,/dA( } - 5 \ 1 
 
 + nr - 7V - i a a ( - i/ ) 2 ( l 1 ) a A (1 - '' r ; 
 i (n n') n I \ d a / r 
 
 taking then for unity the sum of the masses M + ^ an ^ observing that 
 (237) M ^ = n 2 , the equation (X') will become
 
 BOOK I.] NEWTON'S PRINCIPIA. 125 
 
 + n z (jJ C e cos. (n t + =r) 
 + n * p' D e' cos. (n t + t *') 
 
 + n V' 5 C W e cos. i (n 7 t n t + e 7 i) + n t + wj 
 + n V 2 D (i) e' cos.{i (n' t n t -f ' ) + n t + c J\\ 
 and integrating 
 
 > v \ u n / n IL j 'tit. i \ 
 
 ~ n 2 2 . r^ 7vr . s - cos. i (n 7 t n t + e f ) 
 
 2 i" (n n 7 ) 8 n 2 
 
 fjf f, e cos. (n t + e w) + A/ f/ e 7 sin. (n t + *>'} 
 C . n t . e sin. (n t + ) -^- D . n t. e 7 sin. (n t + 
 
 ' |i (n )~i}VHir' 
 T) 0) n 2 
 
 n t 
 
 f y and f/ being two arbitraries. The expression of & g in terms 3 u, found 
 in No. 514 will give 
 >S a , f' 
 
 -' 
 
 _ / . 1 
 
 ; "-JL__}cos.i(n' t - 
 
 i 2 (n n 7 ) 2 n * 
 
 nt + .'- 
 
 /t 7 f e cos. (n t + *r) yJ f 7 e 7 cos. (n t + CT/ ) 
 + \ y! C n t e sin. (n t + s w) + 1 1*! D n t e 7 sin. (n t + s 
 
 2 n . r 
 
 ; - t a A fl) r (i) 
 
 if n * 
 
 i 
 da/ 
 
 1 2 ' < ^ i 2 (n n') 2 n 2 ]i (n n') n} 2 n 
 
 X ecos. [i (n't n t+ 7 s) + nt + s . 
 
 D (i) 
 [jf . n 2 2 . ,. g e" cos. Ji(n 7 1 n t+ e 7 )+n t+s s/j, 
 
 f and f 7 being arbitrary constants independent of f /5 f/. 
 
 This value of 5 , substituted in the formula (Y) of No. 513 will give 3 v 
 or the perturbations of the planet in longitude. But we must observe that 
 n t expressing the mean motion of ^, the term proportional to the time, 
 ought to disappear from the expression of 8 v. This condition determines 
 the constant (g) and we find 
 
 1 
 3
 
 126 A COMMENTARY ON- [SECT. XI. 
 
 We might have dispensed with introducing into the value of 5 the 
 arbitraries f y f/, for they may be considered as comprised in the elements 
 e and * of elliptic motion. But then the expression of 8 v would include 
 terms depending upon the mean anomaly, and which would not have 
 been comprised in those which the elliptic motion gives : that is, it is more 
 commodious to make these terms in the expression of the longitude dis- 
 appear in order to introduce them into the expression of the radius-vector > 
 we shall thus determine f, and f/ so as to fulfil this condition. Then if we 
 
 /( jA Ci - 1) \. /d A G J) \ 
 
 substitute for a( 55 jits value A^~ l) a( -j- -'), we shall 
 V d a' / \ d a / 
 
 have 
 
 AC1 , 
 
 - a 
 
 ro = (I-l)-l)" aA(i - 
 
 n i (n n') \ d a 
 
 2 d 
 = 
 
 f = ; 
 Moreover let 
 
 3n nA ^ i 
 ' aA 
 
 [n + i (n n')} 3 n 2 
 i 2 (n n') 2 n 2 
 
 2n 2 E 
 
 ( i_])(2i_l)naA^-^+(i-l)na 2 (44^ - 
 PI a) 
 
 2 Jn i (n n')} 
 
 2 n 2 D w . 
 
 n 2 Jn i (n n')] 25
 
 BOOK I.] NEWTON'S PRINCIPIA. 127 
 
 and we shall have 
 
 *!_*' *(<LA ( \ . ^ /n % 
 7 "" 6 a V. d a ) 4 ~2 * 
 
 . 2n 
 
 l " + n n /a 
 
 i 2 (n n') z n 2 
 cos. i (n' t n t + t' s) 
 
 (i! f e cos. (n t + + w) (if f' e' cos. (n t + w/ ) 
 + \ fjt' C . n t e sin. (n t + s w) + 1 y! D n t e' sin. (n t + ') 
 
 E (i) 
 r-^ 7-pj, e cos. Ji (n r t n t + 1' + n t+ e 
 
 - ;/ .N^e^os.^i^t nt + t 7 + nt+t 
 
 f 2n 
 
 _(jf } n 2 . m \an n sin. i 
 
 V ~2 2 (i(n n') fa ' i (n n') . U 2 . (n n') 2 n 2 f ) 
 
 (n' t n t + e 7 ) 
 
 + X . C . n t . e cos. (n t + e r) + (if D . n t . e' cos. (n t + s */) 
 r F (i) ->. 
 
 L _ i (n _ nO C Si ' ^ ( n/ 1 n t + 6 7 t ) +n t+* } } 
 + n / 2 ^ ^ (i) ^ ; 
 
 In i (n n / ) 6/sin ^ i(n/t ~' nt + t/ ~ g) + nt + ~ g/ U 
 the integral sign 2 extending in these expressions to all the whole positive 
 and negative values of i, with the value i = alone excepted. 
 
 Here we may observe, that even in the case where the series represent- 
 ed by 
 
 2. A W cos. 5 (n r t n t + ^ e} 
 
 5 
 
 is but little convergent, these expressions of -- and of d v, become con- 
 
 3, 
 
 vergent by the divisors which they acquire. This remark is the more 
 important, because, did this not take place, it would have been impossible 
 to express analytically the mutual perturbations of the planets, of which 
 the ratios of their distances from the sun are nearly unity. 
 
 These expressions may take the following form, which will be useful to 
 us hereafter. Let 
 
 h rr e sin. v\ h' = e' sin. / ; 
 1 = e cos. =r ; 1' = e' cos. a/ ; 
 then we shall have 
 
 2 /dA (i >\ 2n 
 
 *= a'(i^) +<, { a bir)+n^ aA " | co , i(n , t _ nt+ ,_, ) 
 
 a 6 Vda/ n 2 \ i ( n n'J 2 n 1 ) 
 
 h'P) cos.(n t + /'(! f -f V f) sin. (n t + f)
 
 128 A COMMENTARY ON [SECT. XI. 
 
 jntsin.(nt+i) ' {hC+h'D}ntcos. (nt + s) 
 
 . ,., . _ . 
 
 li(n n') 1 i(n n') U 2 . (n n') * n 
 
 sin. i (n' t n t + t f H) 
 
 {h C+h' D}. n t . sin. (n t +)+**' H - C+l x . D} n t . cos. (n t+ 
 
 - 7 . 
 
 " .(..' t-.t-K-O 
 
 Connecting these expressions of 5 g and 3 v with the values of and v 
 relative to elliptic motion, we shall have the entire values of the radius- 
 vector of [*, and of its motion in longitude. 
 
 518. Now let us consider the motion of p in latitude. For that pur- 
 pose let us resume the formula (Z') of No. 514. If we neglect the pro- 
 duct of the inclinations by the excentricities of the orbits it will become 
 
 d 2 3 u' 1 /d Rx 
 
 -- nT - + n *3u'--- 2 ( dz -); 
 
 the expression of R of No. 515 gives, in taking for the fixed plane that 
 of the primitive orbit of /A, 
 
 the value of i belonging to all whole positive and negative numbers in- 
 cluding also i = 0. Let y be the tangent of the inclination of the orbit 
 of ^', to the primitive orbit of ^ and n the longitude of the ascending 
 node of the first of these orbits upon the second ; we shall have very 
 nearly 
 
 T! = a' y sin. (n' t + ' II) ; 
 which gives 
 
 X?) = ~i y. sin. (n' t+s' n) -^ . a' B (i > y sin.(n t+n) 
 Q z / a /& 
 
 ~ a' s B o-^y sin. {i (n' t n t + g 7 *) + n t + s n} 
 
 < 
 
 the value here, as in what follows, extending to all whole positive and 
 negative numbers, i = being alone excepted. The differential equation
 
 BOOK I.] NEWTON'S PRINCIPIA. 129 
 
 in 8 a' will become, therefore, when the value of (, \ is multiplied by 
 n 2 a 3 , which is equal to unity } 
 
 r !2S n / j 
 
 = "_-"- + n 2 a u' ft' n 2 . ~ 7 sin. (n' t + ' n) 
 at 2 a ' - 
 
 + ?-- a &' B (i > 7 sin. (n t + e U) 
 
 " 1 ' {i (n' t nt+s' + n t+s n)} ; 
 
 whence by integrating and observing that by 514 
 8s = a 3 u', 
 
 / 2 2 
 
 3 s = g z . -j- z 7 sin. (n' t + ' n) 
 
 tjf a E a' 
 '-g B (i > . n t . 7 cos. (n t + s n) 
 
 ///n 2 a 2 a/ TJ (' J ) 
 
 ,U> 11 . d. d, JJ 
 
 To find the latitude of p above a fixed plane a little inclined to that of 
 its primitive orbit, by naming the inclination of this orbit to the fixed 
 plane, and 6 the longitude of its ascending node upon the same plane ; it 
 will suffice to add to d s the quantity tan. <p sin. (v 6\ or tan. <p sin. (n t 
 + e 0), neglecting the excentricity of the orbit. Call p/ and ^ what <p 
 and 6 become relatively to /t*'. If p were in motion upon the primitive 
 orbits of fi/ 9 the tangent of its latitude would be tan. <p' sin. (n t + s ^) ; 
 this tangent would be tan. <p sin. (n t + e 6}, if p continued to move in 
 its own primitive orbit. The difference of these two tangents is very 
 nearly the tangent of the latitude of ^, above the plane of its primitive 
 orbit, supposing it moved upon the primitive orbit of (jf ; we have there- 
 fore 
 
 tan. p'sin. (nt+s ^) tan. f sin. (n t+e ^ = 7 sin. (nt + e n). 
 
 Let 
 
 tan. <p sin. 6 = p ; tan. <f> sin. (f p' ; 
 tan. 9 cos. 6 = q ; tan. p' cos. ^ = q' ; 
 we shall have 
 
 7 sin. n = p' p ; 7 cos. n = q' q 
 
 and consequently if we denote by s the latitude of /. above the fixed plane, 
 we shall very nearly have 
 s = q sin. (nt + s) p cos. (nt + s) 
 
 __ (p , _ p) B p, n t sin> (n t 
 VOL II I
 
 130 A COMMENTARY ON [SECT. XI. 
 
 i- q) B n t cos. (n t + s) 
 
 u! a 2 a' 
 
 cos ' 
 
 
 
 519. Now let us recapitulate. Call (g) and (v) the parts of the radius- 
 vector and longitude v upon the orbit, which depend upon the elliptic 
 motion, we shall have 
 
 s = ( f ) + 3g; v = (v) + 3v. 
 
 The preceding value of s, will be the latitude of p above the fixed plane. 
 But it will be more exact to employ, instead of its two first terms, which 
 are independent of^j the value of the latitude, which takes place in the 
 case where p quits not the plane of its primitive orbit. These expressions 
 contain all the theory of the planets, when we neglect the squares and the 
 products of the excentricities and inclinations of the orbits, which is in 
 most cases allowable. They moreover possess the advantage of being 
 under a very simple form, and which shows the law of their different 
 terms. 
 
 Sometimes we shall have occasion to recur to terms depending on the 
 squares and products of the excentricities and inclinations, and even to 
 the superior powers and products. We can find these terms by the pre- 
 ceding analysis, the consideration which renders them necessary will al- 
 ways facilitate their determination. The approximations in which we 
 must notice them, would introduce new terms which would depend upon 
 new arguments. They would reproduce again the arguments, which the 
 preceding approximations afford, but with coefficients still smaller and 
 .smaller, following that law which it is easy to perceive from the deve- 
 lopement of R into a series, which was given in No. 515 ; an argument 
 ivhich, in the successive approximations, is found for thejirst time among the 
 quantities of any order 'whatever r, and is reproduced only by quantities oj 
 the orders r+2, r+4, &c. 
 
 Hence it follows that the coefficients of the terms of the form 
 
 01 i 
 
 t . * . (n t + )j which enter into the expressions of , v, and s, are ap- 
 
 proximated up to quantities of the third order, that is to say, that the 
 approximation in which we should have regard to the squares and pro-
 
 BOOK I.] NEWTON'S PRINCIPIA. 131 
 
 ducts of the excentricities and inclinations of the orbits would add nothing 
 
 O 
 
 to their values ; they have therefore ah 1 the exactness that can be desired. 
 This it is the more essential to observe, because the secular variations of 
 the orbits depend upon these same coefficients. 
 
 The several terms of the perturbations off, v, s are comprised in the 
 form 
 
 sin 
 
 k . [i (n' t n t + s' g) + r n t + r g?, 
 cos. 
 
 r being a whole positive number or zero, and k being a function of the 
 excentricities and inclinations of the orbits of the order r, or of a superior 
 order. Hence we may judge of what order is a term depending upon a 
 given angle. 
 
 It is evident that the motion of the bodies (jJ\ yJ"^ &c. make it neces- 
 sary to add to the preceding values of f, v, and s, terms analogous to 
 those which result from the action of fjf \ and that neglecting the square of 
 the perturbing force, the sums of all these terms will give the whole va- 
 lues of ^ v and s. This follows from the nature of the formulas (X'), 
 (Y), (Z'), which are linear relatively to quantities depending on the dis- 
 turbing force. 
 
 Lastly, we shall have the perturbations of X produced by the action of 
 fi by changing in the preceding formulas, a, n, h, 1, e, , p, q, and p! into 
 a', n', h', Y, s', v, p', q', and p and reciprocally. 
 
 THE SECULAR INEQUALITIES OF THE CELESTIAL MOTIONS. 
 
 520. The perturbing forces of elliptical motion introduce into the expres- 
 sions of g, , - , and s of the preceding Nos. the tune t free from the sym- 
 
 Cl. L 
 
 bols sine and cosine, or under the form of arcs of a circle, which by in- 
 creasing indefinitely, must at length render the expressions defective. It 
 is therefore essential to make these arcs disappear, and to obtain the 
 functions which produce them by their developement into series. We 
 have already given, for this purpose, a general method, from which it re- 
 sults that these arcs arise from the variations of elliptic motion, which are 
 then functions of the time. These variations taking place very slowly 
 have been denominated Secular Inequalities. Their theory is one of the 
 most interesting subjects of the system of the world. We now proceed to 
 expound it to the extent which its importance demands. 
 
 12
 
 132 A COMMENTARY ON [SECT. XI. 
 
 By what has preceded we have 
 
 <-l h sin. (n t + s) 1 cos. (n t + &c.- 
 
 1 + !L [I . C + 1'. D] . n t . sin. (n t + s) 
 
 [_ {h . C + h' . DJ . n t . cos. (n t + s) + p! S. J 
 
 d v 
 
 -T-- = n + 2 n h sin. (n t + ) + 2 n 1 cos. (n t + f) + &c. 
 
 p {\ C + 1' D} n t sin. (n t + s) 
 
 + ft! [h C + h' D| n 2 1 cos. (n t + i) + / T ; 
 
 s = q sin. (n t + P cos. (n t + *) + &c. 
 
 / 
 
 a 2 a' (p' p) B U) . n t . sin. (n t + t) 
 
 t 
 
 !~ a 2 a! (q' q) B (1) . n t. cos. (n t + s) +/">'% 5 
 
 ~fr 
 
 S, T, ^ being periodic functions of the time t. Consider first the expres- 
 sion of -. , and compare it with the expression of y in 510. The arbi- 
 trary n multiplying the arc t, under the periodic symbols, in the expres- 
 
 d v 
 sion of -, ; we ought then to make use of the following equations found 
 
 in No. 510, 
 
 = X' + 6. X" Y; 
 = Y' + 6. Y" +X"~ 2 Z; 
 Let us see what these X, X', X", Y, &c. become. By comparing the ex- 
 
 d v 
 pression of ^ with that of y cited above, we find 
 
 X = n + 2 n h sin. (n t + e) + 2 n 1 cos. (n t + t) + (i! T 
 
 Y = ft' n 2 {h C+h' D} cos. (n t+i) /n {1 C+l' D} sin. (n t+i). 
 
 If we neglect the product of the partial differences of the constants by 
 
 the perturbing masses, which is allowed, since these differences are of the 
 
 order of the masses, we shall have by No. 510, 
 
 X' = (~) [I + 2 h sin. (n t + i) + 2 1 cos. (n t + t}} 
 
 /d *\ 
 + 2 nf-r-^) [h cos. (n t + s) 1 sin. (n t + s)} 
 
 \ Q O/ 
 
 + 2 n (;n-) sm - (n t + i) + 2 n(-,-)cos. (n t + ) ; 
 
 X" = 2 n(r) ft cos. (n t + ) 1 sin. (n t +
 
 BOOK I.] NEWTON'S PRINCIPIA. 133 
 
 The equation = X' + 6 X" Y will thus become 
 
 = (^JJ) {1 + 2 h sin. (n t + ) + 2 1 cos. (n t + i)} 
 \C1 Q * 
 
 + 2n(jj)sin. (n t + ) + 2 n(^)cos. (n t + g) 
 
 /n 2 [h C+h'D] cos. (nt+0+^' ! U C+1'D} sin.(n t+). 
 Equating separately to zero, the coefficients of like sines and cosines, we 
 shall have 
 
 = 
 
 If we integrate these equations, and if in their integrals we change d 
 into t, we shall have by No. 510, the values of the arbitrages in functions 
 of t, and we shall be able to efface the circular .arcs from the expressions 
 
 d v 
 
 of -5 and of g. But instead of this change, we can immediately change 
 
 U L 
 
 6 into t in these differential equations. The first of the equations shows 
 us that n is constant, and since the arbitrary a of the expression for j de- 
 
 pends upon it, by reason of n 2 = - 3 , a is likewise constant. The two 
 
 tl 
 
 other equations do not suffice to determine h, 1, s. We shall have a new 
 equation in observing that the expression of -^ , gives, in integrating, 
 
 y*n d t for the value of the mean longitude of /i. But we have supposed 
 this longitude equal to n t + e ; we therefore have nt+* =yndt, which 
 
 gives 
 
 and as we have ^ = 0, we have in like manner -5 = 0. Thus the two 
 d t d t 
 
 arbitraries n and i are constants ; the arbitraries h, 1, will consequently be 
 determined by means of the differential equations, 
 
 = -UC + l'D !; (I) 
 
 13
 
 134 A COMMENTARY ON [SECT. XI. 
 
 The consideration of the expression of -t having enabled us to deter- 
 
 ' '- I 
 
 mine the values of n, a, h, 1, and s, we perceive a priori, that the differen- 
 tial equations between the same quantities, which result from the expres- 
 sion of f, ought to coincide with those preceding. This may easily be 
 shown a posterior^ by applying to this expression the method of 510. 
 
 Now let us consider the expression of s. Comparing it with that of y 
 cited' above, we shall have 
 
 X = q sin. (n t + p cos. (n t + + & % 
 
 Y = ^ a 8 a' B> (p p') sin. (n t + ) 
 
 ^P 
 
 + ~^. a 8 a' B> (q q') cos. (n t + i), 
 n and , by what precedes, being constants; we shall have by No. 510, 
 
 X" = 0. 
 The equation = X' + Q X" Y hence becomes 
 
 COS * 
 
 _ ^ a* a' B ) (p p') sin. (n t + ) 
 
 T? 
 
 ^ a 2 a' B > (q q') cos. (n t + e) ; 
 
 9 
 
 whence we derive, by comparing the coefficients of the like sines and co- 
 sines, and changing 6 into t, in order to obtain directly p and q in 
 functions of t, 
 
 (q_q') ; (3) 
 
 P p'); (4) 
 
 When we shall have determined p and q by these equations, we shall 
 substitute them in the preceding expression of s, effacing the terms which 
 contain circular arcs, and we shall have 
 
 s = q sin. (n t + e) p cos. (n t + e) + p! %. 
 
 521. The equation -? ? = 0, found above, is one of great importance 
 
 in the theory of the system of the world, inasmuch as it shows that the 
 mean motions of the celestial bodies and the major-axes of their orbits are 
 unalterable. But this equation is approximate to quantities of the order
 
 BOOK L] NEWTON'S PRINCIPIA. 135 
 
 /*' h inclusively. If quantities of the order (i! h % and following orders, 
 
 d v 
 
 produce in -5 , a term of the form 2 k t, k being a function of the ele- 
 ments of the orbits of /* and (i!\ there will thence result in the expression of 
 v, the term k t z , which by altering the longitude of /x, proportionally to 
 the time, must at length become extremely sensible. We shall then no 
 longer have 
 
 1? - 0- 
 dt * 
 
 6ut instead of this equation we shall have by the preceding No. 
 
 dn - 2k- 
 dl ' 
 
 It is therefore very important to know whether there are terms of the 
 form k . t 2 in the expression of v. We now demonstrate, that if 
 We retain only the j^rst power of the perturbing masses, however far may pro- 
 ceed the approximation, relatively to the powers of the excentricities and 
 inclinations of the orbits, the expression v will not contain such terms. 
 
 For this object we will resume the formula (X) of No. 513, 
 
 acos.v/hdt.f sin.v j 2/SR+f (-,-) f -asin.v/hdt^cos.v \ 2fdR+^(^~ ) ? 
 
 dp= * , 1 
 
 m */ l_e 2 
 
 Let us consider that part of d which contains the terms multiplied by t 2 , 
 or for the greater generality, the terms which being multiplied by the sine 
 or cosine of an angle a t + (3, in which a is very small, have at the same 
 time a 2 for a divisor. It is clear that in supposing a = 0, there will re- 
 sult a term multiplied by t 2 , so that the second case shall include the first. 
 The terms which have the divisor a 2 , can evidently only result from a 
 double integration ; they can only therefore be produced by that part of 
 d which contains the double integral signf. Examine first the term 
 2 a cos. vy"n d t (^ sin. \fd R) 
 
 m V (1 e 2 ) 
 
 If we fix the origin of the angle v at the perihelion, we have 
 a(l e 2 ) 
 
 p x L- 
 
 1 + e cos. v ' 
 and consequently 
 
 a (1 _e 2 ) P 
 
 cos. v = S * ; 
 
 eg 
 
 whence we derive by differentiating, 
 
 a(l e 8 ) 
 j z d v . sin. v = * d ^ ; 
 
 G 
 
 14
 
 136 A COMMENTARY ON [SECT. XI. 
 
 but we have, 
 
 g d v = d t V m a (1 e 2 ) =a l . n d t V 1 c f ; 
 we shall, therefore, have 
 
 a n d t sin. v _ g d g 
 V 1 e ~e~~ 
 
 The term 
 
 2 a cos. v./n d t . {g sin. 
 
 m V 1 e 
 will therefore become 
 
 R), or fc/rf R 
 
 me me 
 
 It is evident, this last function, no longer containing double integrals, 
 there cannot result from it any term having the divisor a 2 . 
 Now let us consider the term 
 
 _ 2 a sin, vyn d t {g cos. vfd R} 
 
 m V I e* 
 
 of the expression of -3 g. Substituting for cos. v, its preceding value in g, 
 this term becomes 
 
 2 a sin, v/n d t. { s a(l e 2 )j ./<? R 
 
 me V 1 e* 
 We have 
 
 ff = aU+*e + e;rt, 
 
 tf being an infinite series of cosines of the angle n t + , and of its multi- 
 ples; we shall therefore have 
 
 JJlAl [ s _ a (1 e*)}fd R = a/n d t f| e + tflfd R. 
 
 Call %" the integral fy n d t ; we shall have 
 a/n d t . [ | e + # . } /d R = f a e/n d t/d R + a ^' / rf R a/;/' . d R . 
 
 These two last terms not containing a double integral sign, there can- 
 not thence result any term having a 1 for a divisor; reckoning only terms 
 of this kind, we shall have 
 
 _ 2 a sin, v/n d t {g cos, vfd Rj _ 3 a a e sin, v/n d tfd R 
 m V 1 e 2 m V 1 e* 
 
 " n d t ' 
 and the radius g will become 
 
 (e) + (--./ V /ndt./rfR; 
 r \n d t/ m J J
 
 BOOK L] NEWTON'S PRINCIPIA. 137 
 
 (?) and f /-} being the expressions of e and of -~- , relative to the el- 
 ^n d t/ n d t 
 
 liptic motion. Thus, to estimate in the expression of the radius-vector, 
 that part of the perturbations, which is divided by 2 , it is sufficient to 
 
 3 a 
 
 augment by the quantity . x^ndt.y^R, the mean longitude 
 
 n t + s, of this expression relative to the elliptic motion. 
 
 Let us see how we ought to estimate this part of the perturbations in 
 the expression of the longitude v. The formula (Y) of No. 516 gives by 
 
 substituting . - . /n d t /"d R for 5 P and retaining only the terms 
 3 m n d t ^ 
 
 divided by a 2 , 
 
 {~a?n^dt d gt + l } 3 a 
 V I e ~ * m 
 
 But we have by what precedes 
 
 a e . n d t . sin. v , .-= 
 
 d e = ===== ; e 2 d v = a * n d t V I e 2 ; 
 
 V 1 e 2 
 
 whence it is easy to obtain, by substituting for cos. v its preceding value 
 a*n 8 d t 2 *" 1 . d v _ 
 
 V 1 e z ndt> 
 
 in estimating therefore only that part of the perturbations, which has the 
 divisor a 2 , the longitude v will become 
 
 / d v \ 3 a 
 
 ( v ) + ( 3-1 ) /"D d t fd R ; 
 \n d t/ m J 
 
 (v) andf ^J being the parts of v and -T- , relative to the elliptic mo- 
 tion. Thus, in order to estimate that part of the perturbations in the ex- 
 pression of the longitude of /4, we ought to follow the same rule which we 
 have given with regard to the same in the expression of the radius-vector, 
 that is to say, we must augment in the elliptic expression of the true 
 
 3 a 
 
 longitude, the mean longitude n t + e by the quantity -/*n d ifd R. 
 
 The constant part of the expression of ( j ) developed into a series 
 
 of cosines of the angle n t + s and of its multiples, being reduced (see 
 488) to unity, there thence results, in the expression of the longi-
 
 138 A COMMENTARY ON [SECT. XI. 
 
 3 a 
 tude, the term -yndty^R. If d R contain a constant term 
 
 k ,u' . n d t, this term will produce in the expression of the longitude v, 
 the following one, -^- . k n * t 2 . To ascertain the existence of such 
 
 /* 111 
 
 terms in this expression, we must therefore find whether d R contains a 
 constant term. 
 
 When the orbits are but little excentric and little inclined to one ano- 
 ther, we have seen, No. 518, that R can always be developed into an in- 
 finite series of sines and cosines of angles increasing proportionally to the 
 time. We can represent them generally by the term 
 
 k iif . cos. [i f nf t + i n t + A}, 
 
 i and i' being whole positive or negative numbers or zero. The differen- 
 tial of this term, taken solely relatively to the mean motion of ^, is 
 
 i k . (if . n d t . sin. [V n' t + i n t + AJ; 
 
 this cannot be constant unless we have = i' n' + i n, which supposes 
 the mean motions of the bodies p and /*' to be parts of one another ; and 
 since that does not take place in the solar system, we ought thence to con- 
 clude that the value of d R does not contain constant terms, and that in 
 considering only the first power of the perturbing masses, the mean mo- 
 tions of the heavenly bodies, are uniform, or which comes to the same thing, 
 
 - = 0. The value of a being connected to n by means of the equation 
 
 n * = - T , it thence results that if we neglect the periodical quanthies, the 
 
 major-axes of the orbits are constant. 
 
 If the mean motions of the bodies (* and (if, without being exactly com- 
 mensurable, approach, however, very nearly to that condition, there will 
 exist in the theory of their motions, inequalities of a long period, and 
 which, by reason of the smallness of the divisor a 2 , will become very sen- 
 sible. We shall see hereafter this is the case with regard to Jupiter and 
 Saturn. The preceding analysis will give, in a very simple manner, that 
 part of the perturbations which depend upon this divisor. It hence re- 
 sults that in this case it is sufficient to vary the mean longitude n t -f- i 
 
 3 a 
 ory*n d t by the quantity fn d tfd R; or, which is the same, to aug- 
 
 3 a n 
 ment n in the integral/n d t by the quantity fd R; but consider-
 
 BOOK L] NEWTON'S PRINCJPIA. 139 
 
 ing the orbit of /. as a variable ellipse, we have n = 3 ; the preceding 
 
 variation of n introduces, therefore, in the semi-axis-major a of the orbit, 
 
 2 a*fd R 
 
 the variation * . 
 
 m 
 
 d v 
 
 If we carry the approximation of the value -r , to quantities of the 
 
 Cl L 
 
 order of the squares of the perturbing masses, we shall find terms propor- 
 tional to the time ; but considering attentively the differential equations of 
 the motion of the bodies /A, ft/, &c. we shall easily perceive that these terms 
 are at the same time of the order of the squares and products of the ex- 
 centricities and inclinations of the orbits. Since, however, every thing 
 which affects the mean motion, may at length become very sensible, we 
 shall now notice these terms, and perceive that they produce the secular 
 equations observed in the motion of the moon. 
 
 522. Let us resume the equations (1) and (2) of No. 520, and suppose 
 
 (0, 1) = 
 
 they will become 
 
 dh 
 
 dl j : 
 
 d t 
 
 The expression of (0, 1) and of |0, 1] may be very simply determined in 
 this way. Substituting, instead of C and D, their values determined in 
 No. 517, we shall have 
 
 AW, 
 
 /d 2 
 
 (IT 
 
 
 
 A( - a 
 
 We have by No. 516, 
 
 and we shall easily obtain, by the same No. * and ^ ?, in functions 
 
 of b (0 > and b w ; and these quantities are given in linear functions of b (0 > 
 

 
 140 A COMMENTARY ON [SECT. XL 
 
 and of b (1) ; this being done, we shall find 
 
 ~~ 2 
 
 3a 2 b< ! > 
 2 /dA\ , l 3 /d 2 A>\ -J 
 
 a 'hnr) + * ft (-air-) = 8 (i - V 
 
 wherefore 
 
 3 X. n . a . b (1 ? 
 
 4 (1 a 2 ) 2 ' ' 
 
 Let 
 
 (a 2 2 a a' cos. 6 + a' 2 ) * = (a, a') + (a, a')' cos. * + (a, a')" cos. 2 <J+&c. 
 we shall have by No. 516. 
 
 (a, a ) = a', b W> ; (a, a')' = a', b <, &c. 
 
 We shall, therefore, have 
 _ 
 
 Next we have, by 516, 
 
 _ Sg'.na'a'. (a,a')' 
 
 4 (a' 2 a 2 ) 8 
 
 f ( ^ i e ^ 1 
 
 = a-( b^ a.-^ -- 1 a g . * * , V. 
 (j da da 2 J 
 
 T - -, 
 
 a. a / \ d a 
 
 2 
 
 Substituting for b (1) and its differences, their values in b (0) and b (1) , we 
 
 I -i ~\ 
 
 shall find the preceding function equal to 
 
 therefore 
 
 3 . (if n ( (1 + a 2 ) b W + I a . b () | 
 
 2(1 a 2 ) 2 
 or 
 
 3 /*'. a n|(a g + a' 2 ) (a, a')' + a a' (a, a')j 
 
 2 (a' 2 a 2 ) 2 
 We shall, therefore, thus obtain very simple expressions of (0, 1) and 
 
 of JO, Ij, and it is easy to perceive from the values in the series of b (0) and 
 
 ~ e 
 of b (1) , given in the No. 516, that these expressions are positive, if n is 
 
 ~ a 
 positive, and negative if n is negative. 
 
 Call (0, 2) and |0, 2|, what (0, 1) and |0, Ij become, when we change a'
 
 BOOK I.] 
 
 NEWTON'S PRINCIPIA. 
 
 141 
 
 and til into a" and /&". In like manner let (0, 3), and (0, 3) be what the 
 same quantities become, when we change a' and fif into a!" and p'" ; and 
 so on. Moreover let h", 1" ; h"', 1"', &c. denote the values of h and 1 
 relative to the bodies p", p'", &c. Then, in virtue of the united actions of 
 the different bodies /,', /&", /&"', &c. upon p, we shall have 
 
 + (0, 2) + (0, 3) + &C.J1 [QTl|.r |(V2|.l" &c. ; 
 
 dl 
 
 |0,_lj.h' + |0,2|.h"+&c. 
 
 Ji = {(0, 1) + (0,2) + (0,3) 
 
 , d h' d 1' d h" d 1" 
 
 It is evident that -5 , , -- ; - T , -3 ; &c. will be determined by 
 dt at dt dt 
 
 expressions similar to those of-? and of -j ; and they are easily obtain- 
 
 ed by changing successively what is relative to /& into that which relates 
 to (jf 9 (jf' 9 &c. and reciprocally. Let therefore 
 
 (1,0),|MJ1; (1,2), [H2J; &c. 
 be what 
 
 (0,1), JoTy; (0,2), joTf; & c . 
 
 become, when we change that which is relative to , into what is relative 
 to /' and reciprocally. Let moreover 
 
 (2,0), gToj; (2,1), glj; &c. 
 be what 
 
 (0,2),JOT2|; (0,1),J5T|; &c. 
 
 become, when we change what is relative to p into what is relative to it!' 
 and reciprocally; and so on. The preceding differential equations re- 
 ferred successively to the bodies /*, /-', ,'x", &c. will give for determining 
 h, 1, h', 1', h'', 1", &c. the following system of equations, 
 
 ~ = f(0, 1) + (0,2) + &C.J1 |0, 1|. 1' |0,2|.l" &c. 
 d t 
 
 - (0, 2) + &c.} h + f~ 
 
 rl = -. 
 
 h' + |0, 2| 
 
 &c. 
 
 dh' 
 
 dl' 
 
 = {(1, 0) + (1, 2) + &c.} V |1,0|. 1 [T72| 1" &c. 
 
 ^ = {(1, 0) + (1, 2) + &c.]h' + 11,01. h + |l,2|.h" 
 ^'=J(2,0) + (2, !) + &< 
 
 - 20]. i - . r - &c. 
 
 dl 
 
 . = (2, 0) + (2, 1) + &c.J.h"+ |2,0|h+|2,l|h'+&c. 
 
 (A) 
 
 &c.
 
 142 A COMMENTARY ON [SECT. XI. 
 
 The quantities (0, 1) and (1, 0), |0, 1| and |I, 0| have remarkable rela- 
 tions, which facilitate the operations, and will be useful hereafter. By 
 what precedes we have 
 
 (0 u - ._ 3A'.na.a'(a,a')' 
 4 ( a " a 2 ) 2 
 
 If in this expression of (0, 1) we change y! into /., n into n', a into a' 
 and reciprocally, we shall have the expression of (1, 0), which will con- 
 
 sequently be 
 
 _ 8g.n'a".a (a/ a/ . 
 
 4 (a' 2 a 2 ) 2 
 
 but we have (a, a')' = (a', a)', since both these quantities result from th 
 developement of the function (a e 2 a a' cos 6 + a' s ) * into a series or- 
 dered according to the cosine of 6 and of its multiples. We shall, there- 
 fore, have 
 
 (0, 1). p. n' a' = (1, 0). /*'. n a. 
 But, neglecting the masses /,, /&', Sec. in comparison t with M, we have 
 
 M M , 
 
 n 2 = ; n" = ^- 3 ; &c. 
 
 Therefore 
 
 (0, I)/* A/ a = (1,0)^' Va'; 
 
 an equation from which we easily derive (1, 0) when (0, 1) is determined. 
 In the same manner we shall find, 
 
 JOTT| /* V a = |I70| tif V a'. 
 
 These two equations will also subsist in the case where n and n' have 
 different signs ; that is to say, if the two bodies /-, ft' circulated in different 
 directions ; but then we must give the sign of n to the radical V a, and 
 the sign of n' to the radical V a'. 
 
 From the two preceding equations evidently result these 
 (0, 2) ft V a = (2, 0) p" V a"; \(^2\ ft V a = pM)|. ft" V a"; &c. 
 (I, 2) in V a'= (2, 1) it!' V a"; [I72| fif V a' = gjSJ. ft"- V a"; &c. 
 523. To integrate the equations (A) of the preceding No. } we shall 
 make 
 
 h = N. sin. (g t + /S) ; 1 = N . cos. (g t + |8) ; 
 h' = N'. sin. (g t + /3) ; 1' = N' cos. (g t + /3) ; 
 &c. 
 
 Then substituting these values in the equations (A), we shall have 
 N g = {(0, 1) + (0, 2) + &c.JN 
 N'g =f(l, 0) + (1 
 N x/ g = f(2, 0) + (2, 
 
 , 2) + &c.JN joTl|. N' [O^l N" &c.-\ 
 
 , 2) + &c.}N' [g. N |]72J N" &c.V s (B) 
 
 , 1) + &C.JN" 'go]. N |27l| N' &cj
 
 BOOK I.] NEWTON'S PRINCIPIA. 143 
 
 If we suppose the number of the bodies /-&, /.', //', &c. equal to i ; these 
 equations will be in number i, and eliminating from them the constants 
 N, N', &c., we shall have a final equation in g, of the degree i, which we 
 easily obtain as follows : 
 
 Let <p be the function 
 
 N 2 .^ V ajg (0,.]) (0,2) &c.J 
 + N' 2 p! V a {g (1, 0) (1, 2) &c.} 
 + &c. 
 
 + 2 N ft V a E|07T| N' + |072| N" + &c.} 
 + 2 N fit V a {|]72f N" + \1^3\ N'"+ &c.} 
 + 2 N>" V a" E[273| N'" + &c.$ 
 + &c. 
 
 The equations (B) are reducible from the relations given in the pre- 
 ceding No. to these 
 
 Considering therefore, N, N', N", &c. as so many variables, <p will be 
 a maximum. Moreover, <p being a homogeneous function of these varia- 
 bles, of the second dimension ; we have 
 
 we have, therefore, <p =. 0, in virtue of the preceding equations. 
 
 Thus we can determine the maximum of the function <p. We shall first 
 differentiate this function relatively to N, and then substitute in p, for N, 
 
 its value derived from the equation (-j-vf) = 0, a value which will be a 
 
 linear function of the quantities N', N", &c. In this manner we shall 
 have a rational function whole and homogeneous of the second dimension 
 in terms of N', N", &c. : let p (1) be this function. We shall differentiate 
 <p (1) relatively to N', and we shall substitute in p (1 > for N 7 its value derived 
 
 from the equation (, ^ J/ ) = : we shall have a homogeneous function 
 
 of the second dimension in N", N w , &c. : let <p be this function. Con- 
 tinuing thus, we shall arrive at a function <p (i "~ l ~> of the second dimension, 
 in N (i ~ ] ) and which will consequently be of the form (N (i ~ 1) ) 2 . k, k being 
 a function of g and constants. If we equal to zero, the differential of 
 p (i-1) taken relatively to N 11 - 1 ', we shall have k = 0; which will give 
 an equation in g of the degree i, and whose different roots will give as 
 many different systems for the indeterminates N, N', N", &c. : the inde-
 
 144 A COMMENTARY ON [SECT. XL 
 
 terminate N (i - J ) will be the arbitrary of each system; and we shall im- 
 mediately obtain, the relation of the other indeterminates N, N', &c. of 
 the same system, to this one, by means of the preceding equations taken 
 in an inverse order, viz., 
 
 d N- 2 V " ' d N^ 
 
 Let g, gi, g2, &c. be the i roots of the equation in g : let N, N', N", &c. 
 be the system of indeterminates, relative to the rootg: letN /5 N/, N/', &c. 
 be the system of indeterminates relative to the root gi, and so on : by the 
 known theory of linear differential equations, we shall have 
 h = N sin. (g t + |3) + N! sin. ( gl t + ft) + N 2 (g 2 t + ft) + &c. ; 
 h' = N' sin. (g t + /3) + N!' sin. ( gl t + ft) + N 2 ' (g 2 t + &) + &c. ; 
 h"= N"sin. (g t + jS) + N/'sin. ( gl t + ft) + N 2 "(g 2 1 + &) + &c. ; 
 
 &c. 
 
 ft ft 5 /Sg, Sec. being arbitrary constants. Changing in these values of 
 h, h', h", &c. the sines into cosines ; we shall have the values of 1, 1', 1", &c. 
 These different values contain twice as many arbitraries as there are roots 
 g> gu g-2) &c. ; for each system of indeterminates contains an arbitrary, 
 and moreover, it has i arbitraries |3, ft, /3 2 , &c. ; these values are conse- 
 quently the complete integrals of the equations (A) of the preceding 
 No. 
 
 It is necessary, however, to determine only the constants N, N 1} &c. ; 
 N,' N/, &c. ; ft ft, &c. Observations will not give immediately the con- 
 stants, but they make known at a given epoch, the excentricities e, e', &c. 
 of the orbits, and the longitudes &, =/, &c. of their perihelions, and conse- 
 quently, the values of h, h', &c., 1, 1', &c. : we shall thus derive the values 
 of the preceding constants. For that purpose, we shall observe that if 
 we multiply the first, third, fifth, &c. of the differential equations (A) of 
 the preceding No., respectively by N. /-*. V a, N'. /*'. V a', &c. ; we 
 shall have in virtue of equations (B), and the relations found in the pre- 
 ceding No. between (0, 1) and (1, 0), (0, 2), and (2, 0), &c. 
 
 N ' 7 ^ V a + N ' TF "' V a/ + N " TF "" V a " + &C> 
 = g {N. 1 . p. V a + N'. 1'. iJJ. V a' + N". 1". p". V a." + &c.} 
 If we substitute in this equation for h, h', &c. 1, 1', &c. their preceding 
 values ; we shall have by comparing the coefficients of the same cosines 
 = N . N! . ^ V a + N'. NI'. & V a + N". N/'. /,". V a" + &c. ; 
 = N. N g . i* V a + N'. N 8 '. /' V a' + N". N 2 ". /*". V a" + &c.
 
 BOOK L] NEWTON'S PRINCIPIA. 145 
 
 Again, if we multiply the preceding values of h, h', &c. respectively by 
 
 N./z. V a, N'. tit. V a', &c. 
 
 we shall have, in virtue of these last equations, 
 
 N . I* h . V a + NV. h'. V a' + N". p," h". V a" + &c. 
 
 = N 2 . p.Va + W. tit. V a' + N" 2 . /". V a" + &c.} sin (g t + |8) 
 
 In like manner, we have 
 
 N . (i, 1 . V a + N'. ^ 1'. V a! + N". //' 1". V a" + &c. 
 = {N 2 . ^ . V a + N' 2 . pi. V a' + N //2 . ///'. V a" + &c.| cos. (g t -f |8). 
 
 By fixing the origin of the time t at the epoch for which the values of 
 h, 1, h', 1', &c. are supposed known ; the two preceding equations give 
 
 B - N ' h ft V a + N/> h/ ^ V a ' + N " h " ^"' V a " + &C ' 
 = N. 1 ft,. V a + N'. IV. V a' + N". 1" X'. V a" + &c. ' 
 
 This expression of tan. /3 contains no indeterminate ; for although the 
 constants N, N', N", &c. depend upon the indeterminate N (i ~ % yet, as 
 their relations to this indeterminate are known by what precedes, it will 
 disappear from the expression of tan. (3. Having thus determined 8, we 
 shall have N (i ~ l \ by means of one of the two equations which give tan. (3 ; 
 and we thence obtain the system of indeterminates, N, N', N", &c. rela- 
 tive to the root g. Changing, in the preceding expressions, this root into 
 gi> g-25 gaj &c. we shall have the values of the arbitraries relative to each 
 of these roots. 
 
 If we substitute these values in the expressions of h, 1, h', 1', &c. ; we 
 hence derive the values of the excentricities e, e', &c. of the orbits, and 
 the longitudes of their perihelions, by means of the equations 
 e 2 = h 2 + l 2 ; e' 2 = h /2 + 1 /2 ; &c. 
 
 tan. w = -p- 5 tan - '** Y > & c - 
 
 we shall thus have 
 
 e 2 = N 2 + Nj 2 + N 2 2 + &c. + 2 N N; cos. J( gl g) t + fa B\ 
 
 + 2 N N 2 cos. {(g^g) t + &-) } + 2 NX N 2 cos. Kgr-gi) t + &-,} +&c. 
 
 This quantity is always less than (N + NI + N 2 + &c.) 2 , when the 
 roots g, g 1$ Sec. are all real and unequal, by taking positively the quanti- 
 ties N, N l5 &c. In like manner, we shall have 
 
 tan v _ N sin, (g t + B) + N t sin. ( gl t + ft) + N 2 sin. (g 2 1 + ft) + &c. 
 " N cos. (g t + j8) + N! cos. ( gl t + ft) + N 2 cos. (g 2 1 + &) + &c. 
 
 whence it is easy to get, 
 
 n , ox N t sin, {- t + ft-^ + N 2 sin, 
 
 * ^ 
 VOL. II.
 
 146 A COMMENTARY ON [SECT. XI. 
 
 Whilst the sum NI + No + &c. of the coefficients of the cosines of 
 the denominator, all taken positively, is less than N, tan. ( g t /3) 
 can never become infinite ; the angle w g t /3 can never reach the 
 quarter of the circumference ; so that in this case the true mean motion 
 of the perihelion is equal to g t. 
 
 524. From what has been shown it follows, that the excentricities of 
 the orbits and the positions of their axis-majors, are subject to considera- 
 ble variations, which at length change the nature of the orbits, and whose 
 periods depending on the roots g, gj, g 2 , &c., embrace with regard to the 
 planets, a great number of ages. We may thus consider the excentrici- 
 ties as variably elliptic, and the motions of the perihelions as not uniform. 
 These variations are very sensible in the satellites of Jupiter, and we shall 
 see hereafter, that they explain the singular inequalities, observed in the 
 motion of the third satellite. 
 
 But it is of importance to examine whether the variations of the excen- 
 tricities have limits, and whether the orbits are constantly almost circular. 
 We know that if the roots of the equation in g are all real and unequal, 
 the excentricity e of the orbit of t* is always less than the sum N + NI 
 + N 2 + &c. of the coefficients of the sines of the expression of h taken 
 positively ; and since the coefficients are supposed very small, the value 
 of e will always be inconsiderable. By taking notice, therefore, of the 
 secular variations only, we see that the orbits of the bodies p, //, /*", &c. 
 will only flatten more or less in departing a little from the circular form ; 
 but the positions of their axis-majors will undergo considerable variations. 
 These axes will be constantly of the same length, and the mean motions 
 which depend upon them will always be uniform, as we have seen in No. 
 521. The preceding results, founded upon the smallness of the excentricity 
 of the orbits, will subsist without ceasing, and will extend to all ages past 
 and future ; so that we may affirm that at any time, the orbits of the 
 planets and satellites have never been nor ever will be very excentric, at 
 least whilst we only consider their mutual actions. But it would not be 
 the same if any of the roots g, g l5 g 2 , &c. were equal or imaginary : the 
 sines and cosines of the expressions of h, 1, h', 1', &c. corresponding to 
 these roots, would then change into circular arcs or exponentials, and 
 since these quantities increase indefinitely with the time, the orbits would 
 at length become very excentric ; the stability of the planetary system 
 would then be destroyed, and the results found above would cease to 
 take place. It is therefore highly important to show that g, gu gg> &c. 
 are all real and unequal. This we will now demonstrate in a very simple
 
 BOOK L] NEWTON'S PRINCIPIA. 147 
 
 manner, for the case of nature, in which the bodies p, (if, p", &c. of the 
 system, all circulate in the same direction. 
 
 Let us resume the equations (A) of No. 528. If we multiply the first 
 by At. . V a . h ; the second, by p . V a . 1 ; the third by /u/. V a', h' ; the 
 fourth by p'. V a'. 1', &c. and afterwards add the results together ; the 
 coefficients of h 1, h' V, h" 1", &c. will be nothing in this sum, the coeffi- 
 cients of h' 1 h T will be jo7T|. v> . V a '\l^Q\. P f . V a', and this will 
 be nothing in virtue of the equation |0, 1|. /x. V a = [1, 0{. //.'. V a' found 
 in No. 522. The coefficients of h"fl- h 1", h" 1' h' I", &c. will be 
 nothing for the same reason ; the sum of the equations (A) thus prepared 
 will therefore be reduced to 
 
 . d . 
 
 and consequently to 
 
 = e d e . fj, . V a + e' d e'. (i!. V a' + &c. 
 
 Integrating this equation and observing that (No. 521) the semi-axis- 
 majors are constant, we shall have 
 
 e 2 . & V a + e' 2 . p!. V of + e" 2 . //'. V a" + &c. constant ; (a) 
 
 The bodies /-, <ti', AV" &c. however being supposed to circulate in the 
 same direction, the radicals V a, V a', V a", &c. ought to be taken po- 
 sitively in the preceding equation, as we have seen in No. 522; all the 
 terms of the first member of this equation are therefore positive, and con- 
 sequently, each of them is less than the constant of the second member. 
 But by supposing at any epoch the eccentricities to be very small, this 
 constant will be very small ; each of the terms of the equation will, there- 
 fore, remain always very small and cannot increase indefinitely ; the orbits 
 will always be very nearly circular. , . 
 
 The case which we have thus examined, is that of the planets and 
 satellites of the solar system ; since all these bodies circulate in the same 
 direction, and at the present epoch their orbits have little excentricity. 
 That no doubt may exist as to a result so important, we shall observe 
 that if the equation which determines g, contained imaginary roots, some 
 of the sines and cosines of the expressions of h, 1, h', 1', &c, would trans- 
 form into exponentials ; thus the expression of h would contain a finite 
 number of terms of the form P . c ft , c being the number of which the 
 hyperbolic logarithm is unity, and P being a real quantity, because h or 
 e sin. & is a real quantity. Let 
 
 Q . c f S P'. c f ', Q'. c f S P". c f ', &c. 
 
 be the corresponding terms of 1, h', 1', h", &c. ; Q, P r , Q', P", &c. being 
 
 K2
 
 U8 A COMMENTARY ON [SECT. XI. 
 
 also real quantities : the expression of e ~ will contain the term (P 2 + Q ) 
 c * f l ; the expression of e' 2 will contain the term (P' 2 -f. Q' z ) c 2 f r , and 
 so on ; the first member of the equation (u) will therefore contain the 
 term 
 
 If, therefore, we suppose c f c to be the greatest of the exponentials 
 which contain h, 1, h', 1', &c. that is to say, that in which f is the most 
 considerable, c 2ft will be the greatest of the exponentials which contain 
 the first member of the preceding equation : the preceding term cannot 
 therefore be destroyed by any other term of this first member ; so that for 
 this member to be reduced to a constant, the coefficient of c 2ft must be 
 nothing, which gives 
 
 =(P 8 +QV Va+(P' 8 +Q' 8 )^Va' + (P" 2 + Q" 2 ) /' V a" + &c. 
 When V a, V a', V a", &c. have the same sign, or which is tantamount, 
 when the bodies p, &', p", &c. circulate in the same direction, this equa- 
 tion is impossible, provided we do not suppose P = 0, Q = 0, P 7 = 0, &c.; 
 whence it follows that the quantities h, 1, h' F, &c. do not contain expo- 
 nentials, and that the equation in g does not contain imaginary roots. 
 
 If this equation had equal roots, the expressions of h, 1, h', 1', &c. would 
 contain as we know, circular arcs and in the expression of h, we should 
 have a finite number of terms of the form P t r . Let Q t r , P' t r , Q' t r , &c. 
 be the corresponding terms of 1, h', V, &c. P, Q, P 7 , Q', &c. being real 
 quantities; the first member of the equation (u) will contain the term 
 j(P * + Q *) ft, -/ a +(P' 2 +Q' 2 ) (t'V&'+ (P //2 + Q" 2 ) v" V a" + &c.}. t 2r . 
 
 If t r is the highest power of t, contained by the values of h, 1, h' F, &c. ; 
 t lr will be the highest power of t contained in the first member of the 
 equation (u) ; thus, that this member may be reduced to a constant, we 
 must have 
 
 = (P 2 +Q 2 )/* Va + (P' 2 + Q' ') /*' V a' + &c. . 
 which gives 
 
 P = 0, Q = 0, F = 0, Q' = 0, &c. 
 
 The expressions of h, 1, h', F, &c. contain therefore, neither exponen- 
 tials nor circular arcs, and consequently all the roots of the equation in g 
 are real and unequal. 
 
 The system of the orbits of /,, ft', p", &c. is therefore perfectly stable 
 relatively to their excentricities ; these orbits merely oscillate about a 
 mean state of ellipticity, which they depart from but little by preserving 
 the same major-axis : their excentricities are always subject to this condi-
 
 BOOK L] NEWTON'S PRINCIPIA. 149 
 
 tion, viz. that the sum of their squares multiplied respectively by the masses 
 of the bodies and by the square roots of the major-axes is always the same. 
 
 525. When we shall have determined, by what precedes, the values of 
 e and of ; we shall substitute in all the terms of the expressions of ^ 
 
 and -T- 5 given in the preceding Nos., effacing the terms which contain 
 
 Cl L 
 
 the time t without the symbols sine and cosine. The elliptic part of these 
 expressions will be the same as in the case of an orbit not disturbed, with 
 this only difference, that the excentricity and the position of the perihe- 
 lion are variable ; but the periods of these variations being very long, by 
 reason of the smallness of the masses ^ (*', (*>", &c. relatively to M ; we 
 may suppose these variations proportional to the time, during a great 
 interval, which, for the planets, may extend to many ages before and 
 after the given epoch. 
 
 It is useful, for astronomical purposes, to obtain under this form, the 
 secular variations of the excentricities and perihelions of the orbits : we 
 may easily get them from the preceding formulae. In fact, the equation 
 e 2 = h* + l 2 , gives ede = hdh+ 1 d 1 ; but in considering only the 
 action of /<*', we have by No. 522, 
 
 -[Mi 7 ; 
 
 wherefore 
 
 ede 
 
 = 10, II. {h'l - h 1'J; 
 
 dt 
 
 but we have h' 1 hi' = e e' sin. (=/ ) ; we, therefore, have 
 
 ~ = |QTi[. e' sin. (J w) ; 
 
 thus, with regard to the reciprocal action of the different bodies ///, /^", &c. 
 we shall have 
 
 ~ = joTl). e' sin. (*/*) + [072|. e" sin. (^' ~) 4- &c. 
 
 Cl L " 
 
 ~ = II, 01 e sin. ( ~') + II, 21 e" sin. (=/' ./) + &c. 
 d t 
 
 i = J2TO| e sin. ( .") + \zJ\ e' sin. (t/ V 7 ) + &c. 
 ci t 
 
 &c. 
 
 K3
 
 150 A COMMENTARY ON [SECT. XL 
 
 The equation tan. = y , gives by differentiating 
 
 e 2 d = 1 d h h d 1. 
 
 With respect only to the action of /*', by substituting for d h and d 1 
 their values, we shall have 
 
 = (0, 1) (h + 1) - M- Hi ^ + 1 1'J; 
 which gives 
 
 Y= (0,1) _|(jg^ COS. (.>--.); 
 
 we shall, therefore, have, through the reciprocal actions of the bodies 
 At A*' A*" &c. 
 
 = (0,l) + (0,2)+&c. (Ml. -cos.(*r' w)_ (o72].-cos.( w " .) & c . 
 t - e - e 
 
 d/' 
 
 &c. 
 
 If we multiply these values of-, , -5 - , &c. -y , ^~ , &c. by the time t ; 
 
 we shall have the differential expressions of the secular variations of the 
 excentricities and of the perihelions, and these expressions which are only 
 rigorous whilst t is indefinitely small, will however serve for a long in- 
 terval relatively to the planets. Their comparison with precise and distant 
 observations, affords the most exact mode of determining the masses of the 
 planets which have no satellites. For any time t we have the excentricity 
 e, equal to 
 
 djj t 2 d 2 e 
 
 d e d 2 e > 
 
 e, -T , -| 3 , &c. being relative to the origin of the time t or to the given 
 
 Cl L U L 
 
 d e 
 epoch. The preceding value of -. - will give, by differentiating it, and 
 
 d e e d 3 e 
 observing that a, a', &c. are constant, the values of -T - z , ^3 , &c. ; we 
 
 can, therefore, thus continue as far as we wish, the preceding series, and 
 by the same process, the series also relative to -a : but relatively to the 
 planets, it will be sufficient, in comparing the most ancient observations
 
 BOOK L] NEWTON'S PRINCIPLE. 151 
 
 which have come down to us, to take into account the square of the time, 
 in the expressions of the series of e, e', &c. w, ', &c. 
 
 526. We will now consider the equations relative to the position of the 
 orbits. For this purpose let us resume the equations (3) and (4) of 
 No. 520, 
 
 By No. 516, we have 
 
 a 2 a'. B (1 > = a*. b n >; 
 
 and by the same No., 
 
 Sb'w 
 
 We shall therefore have 
 
 3 11! . n . a. z b < 
 
 4 4(1 a 2 )* ' 
 
 The second member of this equation is what we have denoted by (0, 1) 
 in 522 ; we shall hence have 
 
 jjJ? = (0, 1) (q' - q) ; 
 
 Hence, it is easy to conclude that the values of q, p, q', p', &c. will be 
 determined by the following system of differential equations : 
 
 jj-9 = f(0, 1) + (0, 2) + &c.} . p - (0, 1) p' (0, 2) p" ~ &c. 
 
 1 n 
 
 :.} . q + (0, 1) q' + (0, 2) q" + &c. 
 
 = {(1, 0) + (1, 2) + &c.} . p'- (1, 0) p (1, 2) p"-&c. 
 
 = - {(1, 0) + (1, 2) + &c.} . q' + (1, 0) q + (1, 2) q" + &c. 
 = 1(2,0) + (2, 1) + &c.} . p" (2, 0) p (2, 1) p' &c. 
 = {(. 0) + (2, 1J + &C.J .q" +(2, 0) q + (2, 1) q' + &c. 
 
 K 4
 
 153 A COMMENTARY ON [SECT. XI. 
 
 This system of equations is similar to that of the equations (A) of No. 
 522: it would entirely coincide with it, if in the equations of (A) we were 
 to change h, 1, h', 1', c. into q, p, q', p', &c. and if we were to suppose 
 
 IM = (0,1); 
 
 |1,0| = (1,0); 
 
 &c. 
 
 Hence, the process which we have used in No. 528 to integrate the 
 equations (A) applies also to the equations (C). We shall therefore 
 suppose 
 
 q =N cos.(gt+/3) + N!Cos. (git+ftJ + Ns cos. (g 2 t+/3 2 )+&c. 
 
 p =N sin. (gt+) + N! sin. ( gl t+ft) + N 8 sin. (g s t+&)+&c. 
 
 q' = N' cos. (g t + /3) + N/ cos. ( gl t+ ft )+ N 2 ' cos. (g 2 t+ &) + &c. 
 
 p'=N'sin. (g t+^+N/ sin. (g, t-f ft) + N 2 'sin. (g 8 t+&) + &c. 
 
 &c. 
 
 and by No. 523, we shall have an equation in g of the degree i, and whose 
 different roots will be g, g 1} g 2 , &c. It is easy to perceive that one of 
 these roots is nothing ; for it is clear we satisfy the equations (C) by sup- 
 posing p, p', p", &c. equal and constant, as also q, q', q", &c. This 
 requires one of the roots of the equation in g to be zero, and we can 
 thence depress the equation to the degree i 1. The arbitraries 
 N, NI, N', &c. /3, j8 l5 &c. will be determined by the method exposed in 
 No. 523. Finally, we shall find by the process employed in No. 524. 
 
 const. = (p 2 + q 2 ) p, V a + (p /2 + q' 2 ) p' V a' + &c. 
 
 Whence we conclude, as in the No. cited, that the expressions of p, q, 
 p', q', &c. contain neither circular arcs nor exponentials, when the bodies 
 p, p', p", &c. circulate in the same direction : and that therefore the equa- 
 tion in g has all its roots real and unequal. 
 
 We may obtain two other integrals of the equations (C). In fact, if 
 we multiply the first of these equations by p V a, the third by // V a', 
 the fifth by //," V a", &c. we shall have, because of the relations found in 
 No. 522, 
 
 = ^Wa + ^V Va' + &c.; 
 
 which by integration gives 
 
 constant = q ^ V a + q' /*' V a! + &c. . . . % . (1) 
 In the same manner we find 
 
 constant = p p V a + p' fi/ V a' + &c (2) 
 
 Call <f> the inclinatior of the orbit of> to the fixed plane, and 6 the Ion-
 
 BOOK I.] NEWTON'S- PRINCIPIA. 153 
 
 gitude of the ascending node of this orbit upon the same plane ; the lati- 
 tude of /* will be very nearly tan. <p sin. (n t + s 6): Comparing this 
 value with q sin. (n t + t) p cos. (n t + ), we shall have 
 
 p = tan. <p sin. 6 ; q = tan. p cos. S ; 
 whence we obtain 
 
 tan. p = V (p s + q 2 ) ; tan. 6 = ; 
 
 We shall, therefore, have the inclination of the orbit of [t, and tne po- 
 sition of its node, by means of the values of p and q. By marking suc- 
 cessively with one dash, two dashes, &c. relatively to ^', C>", &c. the values 
 of tan. p, tan. 0, we shall have the inclinations of the orbits of // /*", &c. 
 and the positions of their nodes by means of p', q', p", q", &c. 
 
 The quantity V p z + q 2 is less than the sum N + NX + Ng + &c. of 
 the coefficients of the sines in the expression of q ; thus, the coefficients 
 being very small since the orbit is supposed but little inclined to the fixed 
 plane, its inclination will always be inconsiderable ; whence it follows, that 
 the 'system of orbits is also stable, relatively to their inclinations as also to 
 their excentricities. We may therefore consider the inclinations of the 
 orbits, as variable quantities comprised within determinate limits, and the 
 motion of the nodes as not uniform. These variations are very sensible 
 in the satellites of Jupiter, and we shall see hereafter, that they explain 
 the singular phenomena observed in the inclination of the orbit of the 
 fourth satellite. 
 
 From the preceding expressions of p and q results this theorem : 
 
 Let us imagine a circle whose inclination to a fixed plane is N, and of 
 which the longitude of the ascending node is g t + J3 , also let us imagine 
 upon this first circle, a second circle inclined by the angle N! , the longitude 
 of whose intersection with the former circle is g! t + & ,- upon this second 
 circle let there be a third inclined to it by the angle N 2 , the longitude of 
 whose intersection with the second circle is g& t + /3 2 , and so on ,- the po- 
 sition of the last circle will be that of the orbit of p. 
 
 Applying the same construction to the expressions of h and 1 of No. 
 523, we see that the tangent of the inclination of the last circle upon the 
 fixed plane, is equal to the excentricity of /i's orbit, and that the longitude 
 of the intersection of this circle with the same plane, is equal to that of 
 the perihelion of //s orbit. 
 
 527. It is useful for astronomical purposes, to have the differential va- 
 riations of the nodes and inclinations of the orbits. For this purpose, let 
 us resume the equations of the preceding No.
 
 154 A COMMENTARY ON [SECT. XI 
 
 tan. ? = V (p + q 2 ), tan. = - . 
 
 Differentiating these, we shall have 
 
 dp = dp sin. 6 -f- d q cos. ; 
 
 d p cos. 6 d q sin. 6 
 d 6 = - 3 - . 
 
 tan. 9 
 
 If we substitute for d p and d q, their values given by the equations (C) 
 of the preceding No. we shall have 
 
 =(0, 1) tan. <f> sin. (6 ff) + (0, 2) tan <f>" . sin. (6 0" 
 
 cos. 
 
 In like manner, we shall have 
 
 d 7 
 
 jT=(l, 0) tan. <f> sin. (tf d) + (l, 2) tan. p" sin. (<T 0" 
 
 &c. 
 
 Astronomers refer the celestial motions to the moveable orbit of the 
 earth ; it is in fact from the plane of this orbit that we observe them ; it is 
 therefore important to know the variations of the nodes and the inclina- 
 tions of the orbits, relatively to the orbit of one of the bodies /*, fjf, p", &c. 
 for example to the orbit of p. It is clear that 
 
 q sin. (n t + e') p cos. (n' t + t) 
 
 would be the latitude of p above the fixed plane if it were in motion upon 
 the orbit of p. The latitude of this moveable plane above the same 
 plane is 
 
 q' sin. (n't + P' cos- ( n ' l + *')' 
 
 but the difference of these two latitudes is very nearly the latitude of p 
 above the orbit of /u.; calling therefore p>/ the inclination, and 6/ the lon- 
 gitude of the node of p' upon the orbit of p, we shall have, by what 
 precedes, v 
 
 tan. f / = V (p p) 2 + (q' q) 2 5 tan. 6f - P __ 
 If we take for the fixed plane, that of p's orbit at a given epoch ; we
 
 BOOK I.] NEWTON'S PRINCIPIA. 155 
 
 shall have at that epoch p = 0, q = ; but the differentials d p and d q 
 will not be zero ; thus we shall have. 
 
 d <p; = (dp' dp) sin. 3 + (d q' d q) cos. & \ 
 
 Ad 1 ^ p' d p) cos. & (d q' d q) sin. & 
 
 tan. <p 
 
 Substituting for d p, d q, d p', d q', &c. their values given by the equa- 
 tions (C) of the preceding No., we shall have 
 
 i^ = J(l, 2) (0, 2)} tan. ?' sin. (9 6") 
 
 + {(I, 3) (0, 3)J tan.V" sin. (9 &'"} + Sec. 
 d = f(l,0) + (1,2) + (1,3) + &C.J (0,1) 
 
 + J(l, 3) - (0, 3)] . - cos. ff - 6'"} + &c. 
 
 It is easy to obtain from these expressions the variations of the nodes, 
 and inclinations of the orbits of the other bodies p", /S", &c. upon the 
 moveable orbit of ^. 
 
 528. The integrals found above, of the differential equations which deter- 
 mine the variations of the elements of the orbits, are only approximate, and 
 the relations which they give among the elements, only take place on the 
 supposition that the excentricities of the orbits and their inclinations are 
 very small. But the integrals (4), (5), (6), (7), which are given in No. 
 471, give the same relations, whatever may be the excentricities and in- 
 
 clinations. For this, we shall observe that - , - - is double the 
 
 d t 
 
 area described during the instant d t, by the projection of the radius- 
 vector of the planet p upon the plane of x, y. In the elliptic motion, if 
 we neglect the mass of the planet as nothing compared with that of the 
 sun, taken for unity, we shall have, by the Nos. 157, 237, relatively to the 
 plane of /t's orbit, 
 
 x d y y d x . T= - -^ 
 dt - = ^a(l-e). 
 
 In order to refer the area upon the orbit to the fixed plane, we must 
 multiply by the cosine of the inclination <p of the orbit to this plane ; we 
 shall, therefore, have, with reference to this plane, 
 
 x d y y d x 
 - y , / - = cos. 
 d t 
 
 , ^ - rr a . ( 1 e *) 
 
 Va(l e 2 ) = . / . , v , -. 
 -V 1 + tan. * <f>
 
 156 A COMMENTARY ON [SECT. XL 
 
 In like manner 
 
 x'dy' y'dx' _ la' (I e' 2 ) . 
 d t V 1 + tan. 2 <f>' ' 
 
 &c. 
 
 These values of x d y y d x, x' d y' y' d x', &c. may be used, 
 abstraction being made of the inequalities of the motion of the planets, 
 provided we consider the elements e, e', &c. f, p', &c. as variables, in 
 virtue of the secular inequalities; the equation (4) of No. 471 will there- 
 fore give in that case, 
 
 , /a' 
 " A/1 
 
 a(l e 2 ) , , /a' (1 e' 8 ) 
 C := ^ 
 
 +tan. 
 
 Neglecting this last term, which always remains of the order ^ /x/, we 
 shall have 
 
 a. (1 e 2 ) , ' ' 2 
 
 = 
 
 Thus, whatever may be the changes which the lapse of time produces 
 hi the values of e, e', &c. ?>, >', &c. by reason of the secular variations, 
 these values ought always to satisfy the preceding equation. 
 
 If we neglect the small quantities of the order e 4 , or e 2 p 2 , this equa- 
 tion will give 
 
 c = (j, V a + (if V o! + &c. p V a {e 2 + tan. 2 <p] 
 
 i At' V a' [e 2 + tan * <?'} &c. ; 
 
 and consequently, if we neglect the squares of e, e', p, &c. we shall have 
 /a V a + /' V a' + &c. constant. We have seen above, that if we only 
 retain the first power of the perturbing force, a, a', &c. will be separately 
 constant; the preceding equation will therefore give, neglecting small 
 quantities of the order e 4 or e 2 <p *, 
 
 const. = i* V a {e 2 + tan. 2 ?} + // V a' e /2 + tan. 2 pj + &c. 
 
 On the supposition that -the orbits are nearly circular, and but little 
 inclined to one another, the secular variations are determined (No. 522) 
 by means of differential equations independent of the inclinations, and 
 which consequently are the same as though the orbits were in one plane. 
 But in this hypothesis we have 
 
 9 = 0, <p' = 0, &c. 
 the preceding equation thus becoming 
 
 constant = e 2 A* V a + e' V' V a' + e //2 y ; V a" + &c. 
 an equation already given in No. 524.
 
 BOOK L] NEWTON'S PRINCIPIA. 157 
 
 In like manner the secular variations of the inclinations of the orbits, 
 are (No. 526) determined by means of differential equations, independent 
 of excentricities, and which consequently are the same as though the or- 
 bits were circular. But in this hypothesis we have e = 0, e' = 0, &c. 
 Wherefore 
 
 const,=,uVa . tan 2 <f>+ p'Va' . tan. 2 <p'+i*" Va" . tan. 2 p"-f&c. 
 an equation which has already been given in No. 526. 
 
 If we suppose, as in the last No. 
 
 p = tan. <p sin. 6; q = tan. <p cos. 6; 
 
 it is easy to prove that, the inclination of the orbit of p to the plane of 
 x, y being <p, and the longitude of its ascending node reckoned from the 
 axis of x being 6, the cosine of the inclination of this orbit to the plane of 
 x, z, will be 
 
 q 
 
 V (1 + tan. 2 <p}' 
 
 "V fi V ^^" V f 1 "5C 
 
 Multiplying this quantity by j j or by its value Va (I e 2 ), 
 
 Cl L- 
 
 X {I 2 Z Cl X 
 
 we shall have the value of -, ; the equation (5) of No. 471, 
 
 will therefore give us, neglecting quantities of the order /, 2 , 
 
 </ = . / a ( 1 e2 ) -.a/ /iiILn!I + &c. 
 
 * 'V 1 -f- tan. z . <p * *V 1 + tan. 2 f' 
 
 We shall find, in like manner, that the equation (6) of No. 471, gives 
 
 (1e 2 ) 
 
 C =*-P 
 
 If in these two equations we neglect quantities of the order e 3 or e 8 <p ; 
 they will become 
 
 const. = fJt> q . V a + ^' q' V a' + &c. 
 const. = pt, p V a + ^ p r V a' + &c. 
 equations already found in No. 526. 
 
 Finally, the equation (7) of No. 471, will give, observing that by 478, 
 m_ _ 2m dx* + dy 2 + dz 2 
 
 "a" S d t 1 
 
 and neglecting quantities of the order p //, 
 
 Ui u/ u/ r 
 
 const. = ^ + -, + p> + &c. 
 
 These different equations subsist, when we regard inequalities due to 
 very long periods, which affect the elements of the orbits of ^, /u/, &c. 
 We have observed in No. 521, that the relation of the mean motions of 
 these bodies may introduce into the expressions of the axis-majors of the
 
 158 A COMMENTARY ON [SECT. XI. 
 
 orbits considered variable, inequalities whose arguments proportional to 
 the time increase very slowly, and which having for divisors the coeffi" 
 cients of the time t, in these arguments, may become sensible. But it is 
 evident that, retaining the terms only which have like divisors, and consi- 
 dering the orbits as ellipses whose elements vary by reason of those terms, 
 the integrals (4), (5), (6)> (7), of No. 471, will always give the relations 
 between these elements already found; because the terms of the order 
 (j, /jf which have been neglected in these integrals, to obtain the relations, 
 have not for divisors the very small coefficients above mentioned, or at 
 least they contain them only when multiplied by a power of the perturb- 
 ing forces superior to that which we are considering. 
 
 529. We have observed already, that in the motion of a system of 
 bodies, there exists an invariable plane, or such as always is of a 
 parallel situation, which it is easy to find at all times by this condition, that 
 the sum of the masses of the system, multiplied respectively by the pro- 
 jections of the areas described by the radius-vectors in a given time is a 
 maximum. It is principally in the theory of the solar system, that the re- 
 search of this plane is important, when viewed with reference to the proper 
 motions of the stars and of the ecliptic, which make it so difficult to astro- 
 nomers to determine precisely the celestial motions. If we call 7 the 
 inclination of this invariable plane to that of x, y, and n the longitude of 
 its ascending node, it is easily found that 
 
 c" c' 
 
 tan. /sin. H=. ; tan. 7 cos. n = -- ; 
 
 C C 
 
 and consequently that 
 
 _(j.Va(l e 2 ) sin. <p sin. 6+p'</a'(l e' 2 ) sin, p' sin. 
 
 f a? (\ e' 2 )cos. 
 
 e 2 ). sin. pcos. 64-!*' Vof(\ e' 2 ) sin. <t> cos.0'+&c. 
 tan. 7 . cos. n= v ' ' 
 
 (1 e a ) . cos. <p+p'V a'(l e' 2 ) .cos. 
 We shall determine very easily, by means of these values, the angles 7 
 and IL We see that to determine the invariable plane we ought to know 
 the masses of the comets, and the elements of their orbits ; fortunately 
 these masses appear to be so very small that we may, without sensible 
 error, neglect their action upon the planets ; but time alone can clear up 
 this point to us. We may observe here, that relatively to this invariable 
 plane the values of p, q, p', q', &c. contain no constant terms ; for it is 
 evident by the equations (C) of No. 526, that these terms are the same for 
 p, p', p", &c. and that they are also the same for q, q', q", &c. ; and since re- 
 latively to the invariable plane, the constants of the first members of the
 
 BOOK L] NEWTON'S PRINCIPIA. 159 
 
 equations (1) and (2) of No. 526 are nothing: the constant terms disap- 
 pear, by reason of these equations, from the expressions p, p', &c. 
 q, q', &c. 
 
 . Let us consider the motion of the two orbits, supposing them inclined 
 to one another, by any angle whatever : we shall have by No. 528, 
 
 c' = sin. <p cos. d . p V a ( 1 e 2 ) +sin. <f>' . cos. 6' . /*' V a' ( 1 e' 2 ) ; 
 
 c" = sin. <p sin. 6 . p V a (1 e 2 ) + sin. p' . sin. 6' . i*' V a' (1 e' 2 ). 
 
 Let us suppose that the fixed plane to which we refer the motion of the 
 orbits, is the invariable plane of which we have spoken, and by reference 
 to which the constants of the first members of these equations, are no- 
 thing, as may easily be shown. The angles <p and <p f being positive, the 
 preceding equations give the following ones : 
 
 p V a(l e 2 ) .sin. p =p'V a' (1 e /2 ). sin. p'; 
 sin. 6 = sin. 6' ; cos. d = cos. tf ; 
 
 whence we derive tf = 6 -f- the semi circumference ; the nodes of the or- 
 bits are consequently upon the same line ; but the ascending node of the 
 one coincides with the decending node of the other ; so that the mutual 
 inclination of the two orbits is equal to f + ?' 
 
 We have by No. 528, 
 
 c = /A V a (1 e 2 ) . cos. <p + /*' V a! (1 e /2 ) cos. tf ; 
 by combining this equation with the preceding one between sin. <p and 
 sin. p', we shall have 
 
 os. p. V a(l e 2 )=c 8 +iK*a(l e 2 ) n fz . a'(l e /2 ). 
 
 If we suppose the orbits circular, or at least having excentricity so small 
 that we may neglect the squares of their excentricities, the preceding 
 equation will give <p constant : for the same reason <p' will be constant ; the 
 inclinations of the planes of the orbits to the fixed plane, and to one ano- 
 ther, will therefore be constant, and these three planes will always have a 
 common intersection. It thence results that the mean instantaneous va- 
 riation of this intersection is always the same ; because it can only be a 
 function of these inclinations. When they are very small, we shall easily 
 find by No. 528, and in virtue of the preceding relation between sin. f 
 and sin. p',.that for the time t, the motion of this intersection is 
 [(0,1) + (1,0)}. t. 
 
 The position of the invariable plane to which we refer the motion of 
 the orbits, may easily be determined for any instant whatever ; for we 
 have only to divide the angle of the mutual inclination of the orbits into 
 two angles, p and f', such as that we have in the preceding equation be-
 
 A COMMENTARY ON [SECT. XI. 
 
 Uveen sin. p and sin. <p'. Designating, therefore, this mutual inclination 
 by m, we shall have 
 
 lif V a' (1 e' 2 ). sin. 
 tan. = 
 
 a (I e 2 ) + (if V a' (1 e /2 ) . cos. 
 
 SECOND METHOD OF APPROXIMATION OF THE CELESTIAL MOTIONS. 
 
 530. We have already seen that the coordinates of the celestial bodies, 
 referred to the foci of the principal forces which animate them, are deter- 
 mined by differential equations of the second order. We have integrated 
 these equations in retaining only the principal forces, and we have shown 
 that in this case, the orbits are conic sections whose elements are the 
 arbitrary constants introduced by integration. 
 
 The perturbing forces adding only small inequalities to the elliptic mo- 
 tion, it is natural to seek to reduce to the laws of this motion the troubled 
 motion of the celestial bodies. If we apply to the differential equations 
 of elliptic motion, augmented by the small terms due to the perturbing 
 forces, the method exposed in No. 512, we can also consider the celestial 
 motions in orbits which turn into themselves, as being elliptic; but the 
 elements of this motion will be variable, and by this method we shall ob- 
 tain their variations. Hence it results that the equations of motion, being 
 differentials of the second order, not only their finite integrals, but also 
 their infinitely small integrals of the first order, are the same as in the 
 case of invariable ellipses ; so that we may differentiate the finite equa- 
 tions of elliptic motion, in treating the elements of this motion as con- 
 stant. It also results from the same method that the differential equa- 
 tions of the first order may be differentiated, by making vary only the 
 elements of the orbits, and the first differences of the coordinates ; pro- 
 vided that instead of the second differences of these coordinates, we sub- 
 stitute only that part of their values which is due to their perturbing 
 forces. These results can be derived immediately from the consideration 
 of elliptic motion. 
 
 For that purpose, conceive an ellipse passing through a planet, and 
 through the element of the curve which it describes, and whose focus is 
 occupied by the sun. This ellipse is that which the planet would invari- 
 ably describe, if the perturbing forces were to cease to act upon it. Its 
 elements are constant during the instant d t; but they vary from one 
 instant to another. Let therefore V = 0, be a finite equation to an in- 
 variable ellipse, V being a function of the rectangular coordinates x, y, z
 
 BOOK I.] NEWTON'S PRINCIPIA. 161 
 
 and the parameters c, c', &c. which are functions of the elements of ellip- 
 tic motion. Since, however, this ellipse belongs to the element of the 
 curve described by the planet during the instant d t ; the equation V = 
 will still hold good for the first and last point of this element, by regard- 
 ing c, c', &c. as constant. We may, therefore, differentiate this equation 
 once in only supposing x, y, z, to vary, which gives 
 
 ^ ; (0 
 
 We also see the reason why the finite equations of the invariable el- 
 lipse, may, in the case of the variable ellipse, be differentiated once in 
 treating the parameters as constant. For the same reason, every differ- 
 ential equation of the first order relative to the invariable ellipse, equally 
 holds good for the variable ellipse ; for let V = be an equation of this 
 
 order, V being a function of x, y, z, -: , , y- , and the parameters 
 
 Cl L ( i L Cl L 
 
 c, c', &c. It is clear that all these quantities are the same for the varia- 
 ble ellipse as well as for the invariable ellipse, which for the instant d t 
 coincides with it. 
 
 Now if we consider the planet, at the end of the instant d t, or at the 
 commencement of the following one ; the function V will vary from the 
 ellipse relative to the instant d t to the consecutive ellipse only by the 
 variation of the parameters, since the coordinates x, y, z, relative to the 
 end of the first instant are the same for the two ellipses ; thus the function 
 V being nothing, we have 
 
 This equation may be deduced from the equation V =: 0, by making 
 x, y, z, c, c', &c. vary together ; for if we take the differential equation 
 (i) from this differential, we shall have the equation (i'). 
 
 Differentiating the equation (i), we shall have a new equation in d c, 
 d c', &c. which with the. equation (i') will serve to determine the parame- 
 ters c, c', &c. Thus it is that the geometers, who were first occupied in 
 the theory of celestial perturbations, have determined the variations of 
 the nodes and the inclinations of the orbits : but we may simplify this 
 differentiation in the following manner. 
 
 Consider generally the differential equation of the first order V = 0, 
 an equation which belongs equally to the variable ellipse, and to the in- 
 variable ellipse which, in the instant d t, coincides with it. In the follow- 
 ing instant, this equation belongs also to the two ellipses, but with this 
 
 VOL. II. L
 
 162 A COMMENTARY ON [SECT. XI. 
 
 difference, that c, c', &c. remain the same in the case of the invariable 
 ellipse, but vary with the variable ellipse. Let V' 7 be what V becomes, 
 when the ellipse is supposed invariable, and V/ what this same function 
 becomes in the case of the variable ellipse. It is clear that in order to 
 have V we must change in V, the coordinates x, y, z, which are rela- 
 tive to the commencement of the first instant d t, in those which are rela- 
 tive to the commencement of the second instant; we must then augment 
 the first differences d x, d y, d z respectively by the quantities d 2 x, d 2 y> 
 d 2 z, relative to the invariable ellipse, the element d t of the time, being 
 supposed constant. 
 
 In like manner, to get V/, we must change in V 7 , the coordinates 
 x, y, z, in those which are relative to the commencement of the second 
 instant, and which are also the same in the two ellipses ; we must then 
 augment d x, d y, d z respectively by the quantities d 2 x, d 2 y, d 2 z ; finally, 
 we must change the parameters c, c', &c. into c + d c, c' + d c' ; &c. 
 
 The values of d 2 x, d 2 y, d 2 z are not the same in the two ellipses ; 
 they are augmented, in the case of the variable ellipse, by the quantities 
 due to the perturbing forces. We see also that the two functions V" 
 and V/ differing only in this that in the second the parameters c, c', &c. 
 increase by d c, d c', &c. ; and the values of d 2 x, d 2 y, d 2 z relative to 
 the invariable ellipse, are augmented by quantities due to the perturbing 
 forces. We shall, therefore, form V/ V", by differentiating V in the 
 supposition that x, y, z are constant, and that d x, d y, d z, c, c', &c. 
 are variable, provided that in this differential we substitute for d 2 x, d 2 y, 
 d 2 z, &c. the parts of their values due solely to the disturbing forces. 
 
 If, however, in the function V" V' we substitute for d 2 x, d 2 y, d 2 z 
 their values relative to elliptic motion, we shall have a function of x, y, z, 
 
 d x d y d z 
 
 -j , -r*- , -? , c, c , &c., which in the case ot the invariable ellipse, is 
 
 ci t ci t d t 
 
 nothing; this function is therefore also nothing in the case of the variable 
 ellipse. We evidently have in this last case, V/ V =r 0, since this 
 equation is the differential of the equation V = : taking it from the 
 equation V/ V = 0, we have V/ V" = 0. Thus, we may, in this 
 case, differentiate the equation V = 0, supposing d x, d y, d z, c, c', &c. 
 alone to vary, provided that we substitute for d 2 x, d 2 y, d 2 z, the parts 
 of their values relative to the disturbing forces. These results are exactly 
 the same as those which we obtained in No. 512, by considerations purely 
 analytical ; but as is due to their importance, we shall here again present 
 them, deduced from the consideration of elliptic motion.
 
 BOOK I.] 
 
 NEWTON'S PRINCIPIA. 
 
 163 
 
 531. Let us resume the equations (P) of No. 513, 
 
 d z x m x /d 
 = TTT + -73- T 
 
 = 
 
 dt 2 " s 3 
 d 2 z m z 
 
 dT 2 + 7^ 
 
 If we suppose R = 0, we shall have the equations of elliptic motion, 
 which we have integrated in (478)- We have there obtained the seven 
 following integrals 
 x d y y d x 
 
 dt 
 x d z z d x 
 
 dt 
 , ydz z d y 
 
 c' = 
 
 c" = 
 
 z d z . d x -i 
 
 = 
 
 fm 
 
 = f'+zi m - 
 
 = 
 
 m 
 
 2m 
 
 + 
 
 dt 2 
 
 d x 2 -f- d z 2 
 
 J 
 
 \ 1 X 
 
 dt 2 
 d x.d y 
 
 dt 2 
 z d z. d y 
 
 dt 2 
 d x 2 + d y* 
 
 J T 
 
 \ \ X 
 
 dt 4 
 d x .d z j 
 
 dt 2 
 y d y . d z i 
 
 dt* 
 d x 2 + d yH 
 
 J T 
 
 - d z 2 
 
 dt 2 
 
 dt 2 ' 
 
 (P) 
 
 a d t 2 
 
 These integrals give the arbitraries in functions of their first differences; 
 they are under a very commodious form for determining the variations of 
 these arbitraries. The three first integrals give, by differentiating them, 
 and making vary by the preceding No. the parameters c, c/, c", and the 
 first differences of the coordinates, 
 
 x d 2 y y d 2 x 
 
 Hp J J 
 
 u ^ i 
 
 dc' = 
 
 dc"= 
 
 x d - z z d 2 x 
 
 dt 
 
 z d 2 v 
 
 dt 
 
 Substituting for d * x, d 2 y, d 2 z, the parts of their values due to the 
 perturbing forces, and which by the differential equations (P) are
 
 164 A COMMENTARY ON [SECT. XL 
 
 we shall have 
 
 <.-.{,<")_,([)}, 
 
 We know from 478, 479 that the parameters c, c', c" determine three 
 elements of the elliptic orbit, viz., the inclination <p of the orbit to the 
 plane of x, y, and the longitude 6 of its ascending node, by means of the 
 equations 
 
 V (c' 2 + c" 2 ) c" 
 
 tan. <p = - ki__lLJ ; tan. 6 = ~ fl 
 
 w C 
 
 and the semi-parameter a (1 e 2 ) of the ellipse by means of the equa- 
 tion 
 
 m a (1 e 2 ) = c 2 + c' 2 + c"*. 
 
 The same equations subsist also in the case of the variable ellipse, 
 provided we determine c, c', c" by means of the preceding differential 
 equations. We shall thus have the parameter of the variable ellipse, its 
 inclination to the fixed plane of x, y and the position of its node. 
 
 The three first of the equations (p) have given us in No. (479) the 
 finite integral 
 
 = c" x c' y + c z : 
 
 this equation subsists in the case of the troubled ellipse, as also its first 
 difference 
 
 = c" d x c'dy-fcdz 
 taken in considering c, c', c" constant. 
 
 If we differentiate the fourth, the fifth and the sixth of the integrals 
 (p), making only the parameters f, P, f", and the differences d x, d y, d z 
 vary ; if moreover, we substitute then for d 2 x, d 2 y, d 2 z, the quantities 
 
 - d t 2 ( C 15V - d t (2-5), - d t (15), we shall have 
 \d x / \d / Vd z / 
 
 df =
 
 BOOK. I.] NEWTON'S PRINCIPIA. 165 
 
 1f/ , , / /d Rx /dR\i , f /d Rx 
 
 df = dx i x (dT)- z (dT)} + d nndv) 
 
 +(ydz_zdy)(). 
 
 Finally, the seventh of the integrals (p), differentiated in the same 
 manner, will give the variation of the semi-axis-major a, by means of the 
 equation 
 
 d. - = 2dR, 
 
 the differential d R being taken relatively to the coordinates x, y, z alone 
 of the body /*. 
 
 The values of f, P, f" determine the longitude of the projection of the 
 perihelion of the orbit, upon the fixed plane, and the relation of the ex- 
 centricity to the semi-axis-major; for I being the longitude of this projec- 
 tion by (479) we have 
 
 P 
 tan. I = ->-; 
 
 and e being the ratio of the excentricity to the semi-axis-major, we have 
 
 me = V(f* + f /2 + f" 2 ). 
 
 This ratio may also be determined by dividing the semi-parameter 
 a (1 e 2 ), by the semi-axis-major a : the quotient taken from unity will 
 give the value of e 2 . 
 
 The integrals (p) have given by elimination (479) the finite integral 
 
 = m $ h 2 + f x + f ' y + f " z : 
 
 this equation subsists in the case of the troubled ellipse, and it determines 
 at each instant, the nature of the variable ellipse. We may differentiate 
 it, considering f, P, i" as constant ; which gives 
 
 = m d g + f d x + P d y + f' d z. 
 
 The semi- axis-major a gives the mean motion of p, or more exactly, 
 that which in the troubled orbit, corresponds to the mean motion in the 
 
 invariable orbit ; for we have (479) n = a ~~ 2 V m ; moreover, if we de- 
 note by the mean motion of /-, we have in the invariable elliptic orbit 
 <i = n d t : this equation equally holds good in the variable ellipse, 
 since it is a differential of the first order. Differentiating we shall have 
 d * = d n . d t ; but we have 
 
 therefore 
 
 J29 
 
 d 2 I = 
 
 dn 
 
 185 
 
 3 
 
 a n 
 
 .d. 
 dt. 
 
 m 
 
 R 
 
 3 a n d R 
 
 ' 2 
 3 
 
 m 
 a n 
 
 a 
 d 
 
 m 
 
 m 
 
 L3
 
 166 A COMMENTARY ON [SECT. XT. 
 
 and integrating 
 
 = - ./Yandt.dR. 
 m JJ 
 
 Finally we have seen in (No. 4-73) that the integrals (p) are equivalent 
 to but five distinct integrals, and that they give between the seven para- 
 meters c, c', c", f, f ', f" e, the two equations of condition 
 
 = fc" f c' + PC; 
 
 rn p + f' + f"g m 2 
 : a c 2 + c' 2 + c" 2 
 
 these equations subsist therefore in the case of the variable ellipse provid- 
 ed that the parameters are determined as above. 
 We can easily verify these statements a posteriori. 
 We have determined five elements of the variable orbit, viz., its inclin- 
 ation, position of the nodes, its semi-axis-major which gives its mean mo- 
 tion, its excentricity and the position of the perihelion. It remains for us 
 to find the sixth element of elliptic motion, that which in the invariable 
 ellipse corresponds to the position of ft at a given epoch. For this pur- 
 pose let us resume the expression of d t (473) 
 
 d t V m d v(l e 2 )^ 
 
 a f = U + ecos.(v .)}' 
 
 This equation developed into series gives (473) 
 n d t = d v + E W cos. (v ) + E <> cos. 2 (v ~) + &c.}. 
 
 Integrating this equation on the supposition of e and w being con- 
 stant, we shall have 
 
 /n d t + e =: V + E U> sin. (v w) -f ~- sin. 2. (v w) + &c. 
 
 A 
 
 t being an arbitrary. This integral is relative to the invariable ellipse : 
 to extend it to the variable ellipse, in making every thing vary even to 
 the arbitraries, t, e, -a which it contains, its differential must coincide with 
 the preceding one ; which gives 
 
 s. (v ~) + Ecos.2(v w) + &c.} 
 v -a, being the true anomaly of /* measured upon the orbit, and * the 
 longitude of the perihelion also measured upon the orbit. We have de- 
 termined above, the longitude I of the projection of the perihelion upon 
 a fixed plane. But by (488) we have, in changing v into w and v 7 into I 
 in the expression of v (3 of this No. 
 
 * 3 = I 6 + tan. * \ f sin. 2 (I 6) + &c.
 
 BOOK I.] NEWTON'S PRINCIPIA. 167 
 
 Supposing next that v, v /? are zero in this same expression, we have 
 
 |3 = 6 + tan. 2 <f> sin. 2 6 + &c. 
 wherefore, 
 
 = I + tan. 2 p. {sin. 2 + sin. 2 (I 6) + &c.} 
 which gives 
 
 d* = dI.l + 2 tan. 2 p cos. 2 (I 6) + &c.} 
 
 + 2 d 6 tan. 2 <f> {cos. 20 cos. 2 (I '0) + ixc.} 
 
 dptan.ip ^ n< 2 6 ^^ 2 j _ ^ &c 
 cos. 2 1 <p 
 
 Thus the values of d I, d 6, and d f> being determined by the above, we 
 shall have that of d -a ; whence we shall obtain the value of d e. 
 
 It follows from thence that the expressions in series, of the radius-vec- 
 tor, of its projection upon the fixed plane, of the longitude whether re- 
 ferred to the fixed plane or to the orbit, and of the latitude which we 
 have given in (No. 488) for the case of the invariable ellipse, subsist equal- 
 ly in the case of the troubled ellipse, provided we change n t intoyn d t, 
 and we determine the elements of the variable ellipse by the preceding 
 formulas. For since the finite equations between g, v, s, x, y, z, and 
 y n d t, are the same in the two cases, and because the series of No. 488 
 result from these equations, by analytical operations entirely independent 
 of the constancy or variability of the elements, it is evident these expres- 
 sions subsist in the case of variable elements. 
 
 When the ellipses are very excentric, as is the case with the orbits of 
 the comets, we must make a slight change in the preceding analysis. The 
 inclination <p of the orbit to the fixed plane, the longitude 6 of its ascend- 
 ing node, the semi-axis-major a, the semi-parameter a (1 e 2 ), the ex- 
 centricity e, and the longitude I of the perihelion upon the fixed plane 
 may be determined by what precedes. But the values of & and of d w 
 being given in series ordered according to the powers of tan. | p, in order 
 to render them convergent, we must choose the fixed plane, so as to make 
 tan. p inconsiderable ; and to effect this most simply is to take, for the 
 fixed plane, that of the orbit of /a- at a given epoch. 
 
 The preceding value of d s is expressed by a series which is convergent 
 only in the case where the excentricity of the orbit is inconsiderable, we 
 cannot therefore make use of it in this case. Instead, let us resume the 
 equation 
 
 dtVm d v (1 e 2 )* 
 
 a | * 11 + ecos. (v w)~}' 
 
 L4
 
 168 A COMMENTARY ON [SECT. XI. 
 
 If we make 1 e = a, we have by (489) in the case of the invariable 
 ellipse, 
 
 T being an arbitrary. To extend this equation to the variable ellipse, 
 we must differentiate it by making vary T, the semi parameter a ( 1 e 2 ), 
 , and v. We shall thence obtain a differential equation which will de- 
 termine T, and the finite equations which subsist in the case of the in- 
 variable ellipse, will still hold good in that of the variable ellipse. 
 
 532. Let us consider more particularly the variations of the elements 
 of p's orbit, in the case of the orbits being of small excentricity and but 
 little inclined to one another. We have given in No. 515. the manner of 
 developing R in a series of sines and cosines of the form 
 
 (jf k cos. (i' n' t i n t + A) 
 
 k and A being functions of the excentricity and inclinations of the orbits, 
 the positions of their nodes and perihelions, the longitudes of the bodies 
 at a given epoch, and the major-axes. When the ellipses are variable 
 all these quantities must be supposed to vary conformably to what pre- 
 cedes. We must moreover change in the preceding term, the angte 
 i' n' t i n t into i' fn! d t \ J n d t, or which is tantamount, into 
 
 y ?' i . 
 
 However, by the preceding No., we have 
 - = 2fdR; 
 
 =/n d t = .//a n d t . d R. 
 
 The difference d R being taken relatively to the coordinates x, y, z, 
 of the body /&, we must only make vary, in the term 
 
 fjf k cos, (V ' i + A) 
 
 of the expression of R developed into a series, what depends upon the 
 motion of this body ; moreover, R being a finite function of x, y, z, x', y', 2' 
 we may by No." 530, suppose the elements of the orbit constant in the 
 difference d R ; it suffices therefore to make vary in the preceding term, 
 and since the difference of is n d t, we have 
 
 i (if. k n d t. sin. (V % i + A) 
 
 for the term of d R which corresponds to the preceding term of R. Thus, 
 with respect to this term only, we have 
 
 119 i n 1 
 m & L fM - , - / t f/t 3 A \ 
 
 = ^/M n d t . sin. (r ' i + A) ; 
 
 in
 
 BOOK I.] NEWTON'S PRINCIPIA. 169 
 
 = -'//a k n 2 d t 2 sin. (i' ? - i 5 + A). 
 
 If we neglect the squares and products of the perturbing masses, we 
 may, in the integrals of the above terms, suppose the elements of elliptic 
 motion constant. Hence becomes n t and ', n' t ; whence we get 
 
 1 2 i [jf n k 
 
 = TV - f - = r cos. (i n' t i n t + A) 
 a m (i' n' i n) 
 
 3 i ft a n 2 k . ,., . 
 
 = -- -^-7 - -. r-r sm. (i' n' t i n t + A). 
 m (i' n' in) 2 
 
 Hence we perceive that if i' n' i n is not zero, the quantities a and 
 only contain periodic inequalities, retaining only the first power of the 
 perturbing force ; but i and i' being whole numbers, the equation in' in 
 = cannot subsist when the mean motions of /u. and fjf are incommen- 
 surable, which is the case with the planets, and which can be admitted 
 generally, since n and n' being arbitrary constants susceptible of all possi- 
 ble values, their exact relation of number to number is not at all probable. 
 
 We are, therefore, conducted to this remarkable result, viz., that tlie 
 principal axes of the planets, and their mean motions, are only subject to 
 periodic inequalities depending on their configuration, and that thus in ne- 
 glecting these quantities, their principal axes are constant and their mean 
 motions uniform, a result agreeing with what has otherwise been found by 
 No. 521. 
 
 If the mean motions n t and n' t, without being exactly commensurable, 
 approach very nearly to the ratio i' : i ; the divisor i' n' in is very 
 small, and there may result in and % inequalities, which increasing very 
 slowly, may give reason for observers to suppose that the mean motions 
 of the two bodies p, yl are not uniform. We shall see, in the theory of 
 Jupiter and Saturn, that this is actually the case with regard to these two 
 planets : their mean motions are such that twice that of Jupiter is very nearly 
 equal to five times that of Saturn ; so that 5 n' 2 n is hardly the sixty- 
 fourth part of n. The smallness of this divisor, renders very sensible the 
 term of the expression for , depending upon the angle 5, n' t 2 n t, 
 although it is of the order i' i, or of the third order, relatively to the 
 excentricities and inclinations of the orbits, as we have seen in No. 515. 
 The preceding analysis gives the most sensible part of these inequalities ; 
 for the variation of the mean longitude depends on two integrations, whilst 
 the variations of the other elements of elliptic motion depend only on 
 one integration ; only terms of the expression of the mean longitude can 
 therefore have the divisor (i' n' in) 2 ; consequently with regard only
 
 170 A COMMENTARY ON [SECT. XI. 
 
 to these terms, which, considering the smallness of the divisor ought to 
 
 O D 
 
 be the more considerable, it will suffice, in the expressions of the radius- 
 vector, the longitude and latitude, to derive from these terms, the mean 
 longitude. 
 
 When we have inequalities of this kind, which the action of (if produces 
 in the mean motion of p t it is easy thence to get the corresponding ine- 
 qualities which the action of y> produces in the mean motion of (il In 
 fact, if we have regard only to the mutual action of three bodies M, (* and 
 /u/; the formula (7) of (471) gives 
 
 dx 2 + dy 2 + dz s . d x' 2 + d/ 2 + d z' 2 
 const. = - -JL - + jf m - H_J_ - 
 
 (ft d x + y! d x') 2 + fc d y + / d y') + (g d z + A/ d zQ 2 . 
 
 (M +^ + ^')*dt 8 
 
 2 M 
 
 V x 2 +y +z 2 Vx' 2 +y' 2 +z' 2 V(x' x) 2 +(y' ?)*+(* z)*' 
 The last of the integrals (p) of the preceding No. gives,* by substituting 
 
 for the integral 2fd R, 
 
 B 
 
 dx + d y 2 + dz 2 2 (M + M) _ 
 
 d t 
 
 z 
 
 2 fd R 
 
 If we then call R', what R becomes when we consider the action of /* 
 upon ft,', we shall have 
 
 R / _ ^(x^ + yy^ + zzQ _ t* 
 
 * 2 * " V>'-x) 2 + (y"-y) 2 + (z'- 
 
 Cl t 'v jw "T _y "T ** 
 
 the differential characteristic d' only belonging to the coordinates of the 
 
 dx a +dy 2 + dz 2 , dx /2 + dy' 2 -fdz' 2 
 body &'. Substituting for . J z and j-p 
 
 the values in the equation (a), we shall have 
 R' = const. 
 
 2 (M + p + tif) d t 2 
 , /' 
 
 "T" . /"_. / o , _./ e i /2* 
 
 It is evident that the second member of this equation contains no terms 
 oi the order of squares and products of the /-, /*', which have the divisor 
 i' n' in; relative, therefore, only to these terms, we shall have 
 
 + v/fd'W = 0;
 
 BOOK I.] NEWTON'S PRINCIPIA. 171 
 
 thus, by only considering the terms which have the divisor (i' n' in) 2 , 
 we shall have 
 
 _ / ct(M+^).a / n / Sffandt.dR 
 i>! (M + iif) a n * ' M + y. 
 
 Sffandt.dR _ Sffafn'dt.d' R' 
 
 M + ft M + tf 
 
 we therefore get 
 
 pf (M + ^) a n ' + jE6 (M + ^) a' n' % = 0. 
 Again, we have 
 
 V (M. + A*) V(M + /O 
 
 n = - 3 - ; n = - 5 - ~ ; 
 
 a* a' 2 
 
 neglecting therefore ,., /', in comparison with M, we shall have 
 
 I* V a . + v/ V a'. % = ; 
 or 
 
 V a. 
 
 lit V a!' ' 
 
 Thus the inequalities of , which have the divisor (i' n' in) 2 , give 
 us those of ', which have the same divisor. These inequalities are, as 
 we see, affected with the contrary sign, if n and n' have the same sign, or 
 which amounts to the same, if the two bodies p and ft/ circulate in the 
 same direction ; they are, moreover, in a constant ratio ; whence it follows 
 that if they seem to accelerate the mean motion of p, they appear to re- 
 tard that of p' according to the same law, and the apparent acceleration 
 of jti, will be to the apparent retardation of /u-', as /A' V a' is to / V a. The 
 acceleration of the mean motion of Jupiter and the retardation of that of 
 Saturn, which the comparison of modern with ancient observations made 
 known to Halley, being very nearly in this ratio ; it may be concluded 
 from the preceding theorem, that they are due to the mutual action of the 
 two planets; and, since it is constant, that this action cannot produce in 
 the mean motions any alteration independent of the configuration of the 
 planets, it is very probable that there exists in the theory of Jupiter and 
 Saturn a great periodic inequality, of a very long period. Next, consider- 
 ing that five times the mean motion of Saturn, minus twice that of Jupi 
 ter is very nearly equal to nothing, it seems very probable that the phe- 
 nomenon observed by Halley, was due to an inequality depending upon 
 this argument The determination of this inequality will verify the con- 
 jecture. 
 
 The period of the argument i' n' t i n t being supposed very long,
 
 172 A COMMENTARY ON [SECT. XI. 
 
 the elements of the orbits of /*, and (*' undergo, in this interval sensible 
 variations, which must be taken into account in the double integral 
 ffa k n d t 2 sin. (V n' t i n t + A). 
 
 For that purpose we shall give to the function k sin. (i' n' t i n t + A), 
 the form 
 
 Q sin. (i' n t i n t + i' t' i ) + Q' cos. (i' n' t i n t -j- i' i i ) 
 Q and Q' being functions of the elements of the orbits : thus we shall 
 have 
 
 ffa k n d t 2 sin. (i' n' t i n t + A) = 
 
 n'a sin. (i'n' t int+i't is) ( Q 2 d Q' 3d 2 Q 1 
 
 (i'n' in) 2 *\ W (i'n' in)dt (i'n' in) 2 dt a " i " C 'J 
 
 n* a cos.(i'n't i n t+i' e i ) f o/ 2d Q 3d 2 Q' 
 
 (i'n' in) 2 't w (iV-in)dt (i'n' in) 2 dt 2+ C * 
 
 The terms of these two series decreasing very rapidly, with regard to 
 the slowness of the secular variations of the elliptic elements, we may, in 
 each series, stop at the two first terms. Then substituting for the ele- 
 ments of the orbits their values ordered according to the powers of the 
 time, and only retaining the first power, the double integral above may 
 be transformed in one term to the form 
 
 (F + E t) sin. (V n' t i n t + A + H t). 
 
 Relatively to Jupiter and Saturn, this expression may serve for many 
 ages before and after the instant from which we date the given epoch. 
 
 The great inequalities above referred to, become sensible amongst the 
 terms depending upon the second power of the perturbing forces. In 
 fact, if in the formula 
 
 | = ^f'//a k n 2 . d t 2 . sin. (i' % i + A), 
 
 we substitute for , ' their values 
 
 3 i (*' a n 2 k . . . . . . 
 
 n t -- TT-, : \i sm ' (i n' t i n t + A) ; 
 m (I'n' in)* 
 
 there will result among the terms of the order ^ 2 , the following 
 
 9iV 2 a 2 n 4 k 2 i ^ V a' + \' p, V a . 
 
 5 5Tv ; ^-^4- - r-5 - ' - sin. 2 (i' n't int + A). 
 8 m 2 (r n' i n) 4 p' V a 
 
 The value of ' contains the corresponding term, which is to the one 
 preceding in the ratio ^ V a : (*/ V a', viz. 
 
 8 9 m)' 8S^+1> * ! ^7 - 2 (i'n' t-i n t + A). 
 533. It may happen that the inequalities of the mean motion which are the
 
 BOOK I.] NEWTON'S PRINCIPIA. 173 
 
 most sensible, are only to be found among terms of the order of the 
 squares of the perturbing masses. If we consider three bodies, /., AO, //," 
 circulating around M, the expression of d R relative to terms of this or- 
 der, will contain inequalities of the form 
 
 k sin. (i n t i' n' t + i" n" t + A) 
 
 but if we suppose the mean motions n t, n' t, n" t such that in i' n' 
 + i"n"is an extremely small fraction of n, there will result a very sensible 
 inequality in the value of . This inequality may render rigorously equal 
 to zero, the quantity in i' n' + i" n", and thus establish an equation of 
 condition between the mean motions and the mean longitudes of the three 
 bodies ^, /t*', /*". This very singular case exists in the system of Jupiter's 
 satellites. We will give the analysis of it. 
 
 If we take M for the mass-unit, and neglect /tt, At', V>" in comparison with 
 it, we shall have 
 
 2 -, 1. >2 - JL "2 - _L 
 
 n ~~ a 3 ' n ~~a' 3 ' n ~ a" 3 ' 
 we have then 
 
 d = n d t; d ' = n' d t; d " = n" d t ; 
 wherefore 
 
 d 2 g 
 
 3 
 
 1 
 
 da 
 
 ~d~T 
 
 ~~ ~2 
 
 n . 
 
 T" 
 
 d 2 ' 
 
 3 
 
 n /.V 
 
 da' 
 
 dt 
 
 2 
 
 
 a' 2 
 
 d 2 " 
 
 3 
 
 n"* 
 
 d a" 
 
 dt 
 
 ~~ 2 
 
 
 a" 2 ' 
 
 We have seen in No. 528, that if we neglect the squares of the excen- 
 tricities and inclinations of the orbits, we have 
 
 const. = /* V a + tf. V a' + /*" V a!' ; 
 which gives 
 
 da' , d a' d a" 
 
 From these several equations, it is easy to get 
 
 dt 
 
 d'r 
 
 dt 
 
 dt 
 
 3 
 
 2' 
 
 3 
 
 2 
 
 ft 
 
 . n 
 '. n' 
 
 i. 
 1 
 
 da 
 
 a 2 
 
 n n" d 
 
 a 
 
 a 
 
 3 
 
 &'. n 
 /tt.n" 
 
 n' n"' a 
 ^ n n' 
 
 
 d
 
 174 A COMMENTARY ON [SECT. XL 
 
 Finally the equation 
 
 m = 2fd R 
 a 
 
 of No. 531, gives 
 
 a 2 
 
 We have therefore only to determine d R. 
 
 By No. 513, neglecting the squares and products of the inclinations of 
 the orbits, we have 
 
 / _j 
 
 R = T|- COS. (V 7 V) (*' (f 2 2 / COS. (v 7 v) + ' 2 ) 
 
 /i" P - 
 
 + ~ cos. (v" v) (* f (e 2 2 P ft" cos. (v" v) 4- /' ^). a 
 
 p//2 \ / vs * s v xib/ 
 
 If we develope this function in a series ordered according to the cosines 
 of v' v, v" v and their multiples ; we shall have an expression of 
 this form 
 
 R = (g, ft W + ^ (ft ft (i) cos . (V _ V ) + /,' (?, f O P) cos. 2 (v' - v) 
 + <"' (e e') (3) cos. 3 (v' v) + &c. 
 
 \a' 9 * 
 
 r o *>5 5 / i r^ ^5' 9 * * V ^ i * \* s s * * 
 
 + ^"(ft f'O (3) cos. 3 (v" v) + &c. , 
 whence we derive 
 
 dH 
 
 cos. 2 (v' v) + &c. 
 
 _cos. 2 (v" v) + &c. 
 
 f t*'(s> f') (1) sin. (v ; v) + 2 /*' (?, f') (2) sin. 2(\ f -^v) + &c. "i 
 - + V 1 + A^(f, g") (1 ^sin.(v" v) + 2 ( a"(, /') Wsin.2(v 7/ v) + &c. J - 
 Suppose, conformably to what observations indicate in the system of 
 the three first satellites of Jupiter, that n 2 n' and n' 2 n" are 
 very small fractions of n, and that their difference n 2 n 7 (n' 2 n' x ) 
 or n 3 n' + 2 n" is incomparably smaller than each of them. 
 
 <; 
 
 It results from the expressions of - - s , and of d v of No. 517, that the 
 
 action of// produces in the radius-vector and in the longitude oOs a very 
 sensible inequality depending on the argument 2 (n' t n t + e' e). 
 The terms relative to this inequality have the divisor 4 (n' n) 2 n 2 ,
 
 BOOK I.] NEWTON'S PRINCIPIA. 175 
 
 or (n 2 n') (3 n 2 n'), and this divisor is very small, because of the 
 smallness of the factor n 2 n'. We also perceive, by the consideration 
 of the same expressions, that the action of ft produces in the radius- 
 vector, and in the longitude of /', an inequality depending on the argu- 
 ment (n' t n t + s' E), and which having the divisor (n' n) 2 n' 2 , 
 or n (n 2 n'), is very sensible. We see, in like manner, that the action 
 of ft" upon ft' produces in the same quantities a considerable inequality 
 depending upon the argument 2 (n" t n' t + s" i'). Finally, we 
 perceive that the action of ft f produces in the radius-vector and in the 
 longitude of ft" a considerable inequality depending upon the argument 
 n" t n' t + *" - These inequalities were first recognised by obser- 
 vations ; we shall develope them at length in the Theory of Jupiter's Sa- 
 tellites. In the present question we may neglect them, relatively to other 
 inequalities. We shall suppose, therefore, 
 d g = (j! E' cos. 2 (n' I n t + f' g) ; 
 a v = ft' F sin. 2 (n' t - n t + ' ; 
 
 a g' = it" E" cos. 2 (n" t n' t + s" g') +(t G cos. (n t n t+ < t) 
 3 v' = p" F" sin. 2 (n" t n' t + e" s 7 ) +(t H sin. (n' t n t+ ' e) 
 d t" = ^ G cos. (n" t n' t + *" g') 
 d v" = it" H' sin. (n" t n' t + " s). 
 
 We must, however, substitute in the preceding expression of d R for 
 fj v > f' v/ > S"i v "> the values of a 3 g, n t + + d v, a' + ^ ', n t+ e' + <5 v', 
 a" + ^ f"> n" t + e" + 8 v", and retain only the terms which depend upon 
 the argument n t 3 n' t + 2 n" t + e 3 t' + 2 t". But it is easy to see 
 that the substitution of the values of 8 g, 8 v, 3 ?", 3 v" cannot produce any 
 such term. This is not the case with the substitution of the values of 
 8 and 5 v' : the term fif (P, fl W d v sin. (v 7 v) of the expression of 
 d R, produces the following, 
 
 
 sin. (n t 3 n' t + 2 n" t + g' 3 s' + 2 *"). 
 This is the only expression of the kind which the expression of d R 
 
 contains. The expressions of , and of 3 v of No. 5 17, applied to the 
 
 A 
 
 action of ft" upon ft', give, retaining only the terms which have the divisor 
 n' 2 n", and observing that n" is very nearly equal to ^ n', 
 
 _^ a ' (a ', a") '*> 
 "' 
 
 E " i 'i v da 
 
 ' 
 
 n' 2 n") (3 n' 2 n")
 
 176 A COMMENTARY ON [SECT. XI- 
 
 F'- 2E ". 
 
 F -"7"' 
 
 we therefore have 
 
 /.V'ndt f2(a,a')) /d . (a, a') <*\ 1 
 ~~2~ ' \ a 7 " "^ dV J f 
 
 Xsin. (n t 3 n' t + 2 n" t + s 3 s' + 2 ") = I ? 
 
 da d 2 d 2 <' d 2 t" 
 
 Substituting this value of - in the values of p - , -r- 2 - , p- , and 
 
 making for brevity's sake 
 
 we shall have, sin.ce n is very nearly equal to 2 n', and n' to 2 n", 
 
 ^15 3. jI + 2. i^V = /Sn'sin. (n t Sn' t + 2n" t + i 8 f + 2 "); 
 
 or more exactly 
 
 so that if we suppose 
 
 V = 3 + 2 T + . - 3 t + 2 a", 
 we shall have 
 
 The mean distances a, a', a", varying but little as also the quantity n, 
 we may in this equation consider /3 n % as a constant quantity. Integrat- 
 ing, we have 
 
 V c 2 |8 n 2 cos. V 
 
 c being an arbitrary constant. The different values of which this con- 
 stant is susceptible, give rise to the three following cases. 
 
 If c is positive and greater than + 2 & n 2 , the angle V will increase 
 continually, and this ought to take place, if at the origin of the motion, 
 (n 3 n' + 2 n") * is greater than + 2 /S n 2 (1 + cos. V), the upper or 
 lower signs being taken according as /3 is positive or negative. It is easy 
 to assure ourselves of this, and we shall see particularly in the theory of 
 the satellites of Jupiter, that (3 is a positive quantity relatively to the three 
 first satellites. Supposing therefore + w = T V, it being the semi cir- 
 cumference, we shall have 
 
 a* 
 
 d t = - . 
 
 V c + 2 n 2 cos.
 
 BOOK I.] NEWTON'S PRINCIPIA. 177 
 
 In the interval from = to -a = , the radical V c + 2 /3 n 2 cos. 
 
 is greater than V 2 ft n 2 , when c is equal or greater than 2 /3 n z ; we 
 have therefore in this interval -a > n t V 2 |3. Thus, the time t which the 
 
 i cr 
 
 angle & employs in arriving from zero to a right angle is less than - ;,- . 
 
 The value of /3 depends upon the masses, a, ///, p" ; the inequalities ob- 
 served in the three first satellites of Jupiter, and of which we spoke above, 
 give, between their masses and that of Jupiter, relations from whence it 
 
 results that - - ^- . is under two years, as we shall see in the theory 
 
 of these satellites ; thus the angle -a would employ less than two years to 
 increase from zero to a right angle ; but the observations made upon Ju- 
 piter's satellites, give since their discovery, -a constantly 'nothing or insen- 
 sible; the case. which we are examining is not therefore that of the three 
 first satellites of Jupiter. 
 
 If the constant c is less than + 2 /3 n 2 , the angle V will not oscillate ; 
 it will never reach two right angles, if (3 is negative, because then the 
 radical V c 2 (3 n * cos. V, becomes imaginary ; it will never be no- 
 thing if /3 is positive. In the first case its value will be alternately greater 
 and less than zero ; in the second case it will be alternately greater and 
 less than two right angles. All observations of the three first satellites of 
 Jupiter, prove to us that this second case belongs to these stars ; thus the 
 value of (3 ought to be positive relatively to them ; and since the theory 
 of gravitation gives j3 positive, we may regard the phenomenon as a new 
 confirmation of that theory. 
 
 Let us resume the equation 
 
 V c + 2/3n 2 cos. ~* 
 
 The angle -a being always very small, according to the observations, 
 we may suppose cos. -a = 1 -a 2 ; the preceding equation will give by 
 integration 
 
 TT = X sin. (n t V /3 + y) 
 
 X and y being two arbitrary constants which observation alone can deter- 
 mine. Hitherto, it has not been recognised, a circumstance which proves 
 it to be very small. 
 
 From the preceding analysis result the following consequences. Since 
 the angle n t + 3 n' t -f 2 n" t + e 3 t' + It" oscillates being some- 
 times less and sometimes greater than two right angles, its mean value is 
 
 VOL. II. M
 
 178 A COMMENTARY ON [SECT. XI. 
 
 equal to two right angles ; we shall therefore have, regarding only mean 
 quantities 
 
 n 3 iv + 2 n" = 
 
 that is to say, that che mean motion of the Jirst satellite, minus three times 
 that of the second, plus twice that of the third, is exactly and constantly 
 equal to zero. It is not necessary that this equality should subsist exactly 
 at the origin, which would not in the least be probable ; it is sufficient 
 that it did very nearly so, and that n 3 n' + 2 n" has been less, ab- 
 straction being made of the sign, than X n V (3; and then that the mutual 
 attraction has rendered the equality rigorous. 
 
 We have next 3 s -f- 2 i" equal to two right angles ; thus the mean 
 longitude of the Jirst satellite, minus three times that of the second, plus twice 
 that of the third, is exactly and constantly equal to two right angles. 
 
 From this theorem, the preceding values of 5 /, and of 3 v' are reduci- 
 ble to the two following 
 
 a / = (^ G ft," E") cos. (n t n t + ' i) 
 5 v' = (ft H ft" F") sin. (n' t n t + *' ) 
 
 The two inequalities of the motion of fi' due to the actions of p and of 
 /*'', merge consequently into one, and constantly remain so. 
 
 It also results from this theorem, that the three first satellites can never 
 be eclipsed at the same time. They cannot be seen together from Jupi- 
 ter neither in opposition nor in conjunction with the sun ; for the preced- 
 ing theorems subsist equally relative to the synodic mean motions, and to 
 the synodic mean longitudes of the three satellites, as we may easily 
 satisfy ourselves. These two theorems subsist, notwithstanding the alter- 
 ations which the mean motions of the satellites undergo, whether they 
 arise from a cause similar to that which alters the mean motion of the 
 moon, or whether from the resistance of a very rare medium. It is evi- 
 dent that these several causes only require that there should be added to 
 
 the value of -| r , a quantity of the form of . "\ , and which shall only 
 
 become sensible by integrations ; supposing therefore V = it -a, and -a 
 very small, the differential equation in V will become 
 
 The period of the angle n t V /3 being a very small number of years, 
 
 1 2 1 
 
 whilst the quantities contained in -: - are, either constant, or embrace 
 
 Cl L 
 
 many ages; by integrating the above equation we shall have
 
 BOOK L] NEWTON'S PRINCIPIA. 179 
 
 ~ = X sin. (n t V ft + 7) fl *.'* t .. 
 
 Thus the value of -a will always be very small, and the secular equa- 
 tions of the mean motions of the three first satellites will always be order- 
 ed by the mutual action of these stars, so, that the secular equation of the 
 first, plus twice that of the third, may be equal to three times that of the 
 second. 
 
 The preceding theorems give between the six constants n, n', n", 
 i, t' t i" two equations of condition which reduce these arbitraries to four ; 
 but the two arbitraries X and 7 of the value of =r replace them. This 
 value is distributed among the three satellites, so, that calling p, p', p" the 
 coefficients of sin. (n t V /3 -f- /) in the expressions of v, v', v", these 
 
 d 2 T d z T d 2 t" 
 coefficients are as the preceding values of -T -j- , ^-~ , -rfg- > and more- 
 
 over we have p 3 p' + 2 p" = X. Hence results, in the mean mo- 
 tions of the three first satellites of Jupiter, an inequality which differs for 
 each only by its coefficients, and which forms in these motions a sort of 
 libration whose extent k arbitrary. Observations show it to be insen- 
 sible. 
 
 534*. Let us now consider the variations of the excentricities and of the 
 perihelions of the orbits. For this purpose, resume the expressions of 
 d f, d f ', d i" found in 53 ? : calling g the radius- vector of ^ projected 
 upon the plane of x, y ; v the angle which this projection makes with the 
 axis of x ; and s the tangent of the latitude of u above the same plane, we 
 shall have 
 
 x = g cos. v ; y = f sin. v ; z = j s 
 whence it is easy to obtain 
 
 dx 
 d R x /d R x /d Rx /d 
 
 x ~ z = J 8 cos - v s cos - v 
 
 d 
 
 + s sin. v 
 
 R\ /d R 
 
 8 sin - v 
 
 / \ / 
 
 (-37) - ' s sin - v (-5 
 
 dR 
 
 S COS. V 
 
 By 531, we also have 
 
 xdy ydx = cdt; xdz zdxrrc'dt; ydz zdy = c dt; 
 
 M2
 
 ISO A COMMENTARY ON [SECT. XI. 
 
 the differential equations in f, f ' , f " will thus become 
 
 df = - d 
 
 + s sin. 
 
 /d Rx \ 
 in. v ( a ) ) 
 
 d s 
 
 /dRx) 
 
 - scos - v (dv)/ 
 
 d R\ sin. v/d R\ s. sin. v/dR\) c"dt/d 
 
 i T/' /i 2\ /d R\ . . /d R 
 
 = dx (1 + s 2 ) cos. v- S s cos. v + s sin. 
 
 .. 
 d y (l + s 2 ) sin. v(^~ s s. sin. v(^)- s 
 
 d R\ sin. v/d R 
 
 s. cos. v /d R\ 1 
 
 - -Y (ar)l 
 
 . d t {sin. v (ii l ) + 2il ( d -5) - i^LI . (dB) i 
 I \d/ \dv/ \ds/J 
 
 The quantities c', c" depend, as we have seen in No. 531, upon the in- 
 clination of the orbit of # to the fixed plane, in such a manner that they 
 become zero when the inclination = ; moreover it is easy to see by the 
 
 nature of R that [-5 \ is of the order of the inclinations of the orbits ; 
 \d s/ 
 
 neglecting therefore the squares and products of these inclinations, the 
 preceding expressions of d f and of d f ', will become 
 
 , f 
 
 df= - 
 
 , 
 
 d f : = d x 
 
 but we have 
 
 d x = d (^ cos. v) ; d y = d (g sin. v); cdt=xdy ydx = g ! dv, 
 
 we therefore get 
 
 d f = Jd s sin. v + 2 g d v cos. v} ( ) f 2 d v sin. v 
 
 d R\ / /d R\ sin. v /d R\ ) 
 
 dv) + c d i l cos - v CT?) - ~r (ar) J ; 
 
 d f' = [d g cos. v 2 f d v sin. v} (-3) + * d v cos. v (-j ) 
 
 These equations are more exact, if we take for the fixed plane of x, y,
 
 BOOK L] NEWTON'S PR1NC1P1A. 181 
 
 that of the orbit of p, at a given epoch ; for then c', c" and s are of the 
 order of the perturbing forces ; thus the quantities which we neglect, are 
 of the order of the squares of the perturbing forces, multiplied by the 
 square of the respective inclination of the two orbits of (J> and of p'. 
 
 The values of e, d P, d v, (-3 j. [-3 \ remain clearly die same what- 
 V d g J \d v/ 
 
 ever is the position of the point from which we reckon the longitudes ; 
 but in diminishing v by a right angle, sin. v becomes cos. v, and cos. v 
 becomes sin. v ; the expression of d f changes consequently to that of 
 d f 7 ; whence it follows that having developed, into a series of shies and 
 cosines of angles increasing proportionally with the times, the value of 
 d f, we shall have the value of d f ', by diminishing in the first the angles 
 , ', , -a', 6 and ^ by a right angle. 
 
 The quantities f and f ' determine the position of the perihelion, and 
 the excentricity of the orbit ; in fact we learn from 531, that 
 
 V 
 
 tan. 1 = ; 
 
 I being the longitude of the perihelion referred to the fixed plane. When 
 this plane is that of the primitive orbit of p, we have up to quantities of 
 the order of the squares of the perturbing forces multiplied by the square 
 of the respective inclinations of the orbits, 1 = *r, -a being the longitude of 
 the perihelion upon the orbit ; we shall therefore then have 
 
 P 
 tan. -a = -7.- ; 
 
 which gives 
 
 f _f 
 
 sm. = ; cos. = .^_-_ ^ . 
 
 By 531. we then set 
 
 f/ c / f c " 
 
 me = V f* + f' 2 + i" 2 ,f" = - 
 
 C 
 
 thus c' and c" being in the preceding supposition of the order of the 
 perturbing forces, f " is of the same order, and neglecting the terms of the 
 square of these forces, we have 
 
 m e = V f 2 + f /2 . 
 
 If we substitute for V f * + f *, its value m e, in the expressions of 
 sin. , and of cos. *r, we shall have 
 
 m e sin. * = f; m e cos. = f ; 
 
 these two equations will determine the excentricity and the position of the 
 perihelion, and we thence easily obtain 
 
 m s . ede = f d f + f'df; m'e'dw = fdf f ' d f. 
 
 MS
 
 182 A COMMENTARY ON [SECT. XL 
 
 Taking for the plane of x, y that of the orbit of /x ; we have for the 
 cases of the invariable ellipses, 
 
 a ( 1 e g ) , g * d v . e . sin, (v v) 
 
 ~ I + e cos. (v w) ' s ~ a(l e*) 
 
 g d v = a 2 n d t V 1 e 2 ; 
 
 and by No. 530, these equations also subsist in the case of the variable 
 ellipses ; the expressions of d f and of d f will thus become 
 
 d f = -- ==|. [2 cos. v+f e cos. T*+\ e cos ' ( 2 v *}} 
 
 a 1 n d t V } e 2 . sin. 
 
 , 
 
 \ d 
 
 a n d t (n . , m \ 
 
 d f = . [2 sin. v-f-| e sin. + e sin. (2 v *r) 
 
 + a s ndtv / l e *. cos. 
 
 wherefore 
 
 a n d t . . , v , /d R< 
 
 e d v = , sin. (v ) [2 + e cos. (v 
 
 a z . n d t V 1 e* . . /d K\ 
 
 + S - (v - -) (-37) 
 
 andt , ./ \7/"^ 
 
 d e = 8 . {2 cos. (v *) + e + e cos. (v )j (-j 
 
 T-r - ; . x / \ 
 
 V 1 e 2 . sin. (v *). (-T ). 
 
 V- d o ) 
 
 ra 
 
 This expression of d e may be put into a more commodious form in 
 some circumstances. For that purpose, we shall observe that 
 
 A 
 
 = d R " d v 
 
 substituting for g and d g their preceding values, we shall have 
 
 but we have _ 
 
 g 2 d v = a 2 ndt V 1 e*; 
 
 n d t{l + e cos. (v )}* 
 d v = - - -- 5 - *-> 
 
 (1 e) 
 
 wherefore 
 
 _ /d R 
 
 a 2 n d t V 1 e 2 . sin. (v *).
 
 BOOK I.] NEWTON'S PRINCIPIA. 183 
 
 the preceding expression of d e, will thus give 
 
 a n d t V 1 e 2 /d Rx a (1 e ) , 
 
 e d e = . f -j ) \ d R. 
 
 m \d v/ m 
 
 We can arrive very simply at this formula, in the following manner 
 We have by No. 531, 
 
 <*c_/dJU x fdRx_ (d 
 dT ~ y Vdx; Vd v/ " " \~dv 
 
 but by the same No. c = V m a (I e 2 ) which gives 
 
 daVma(l 
 
 dc = 
 
 andtVl e 2 /d R\ 9 \da 
 
 e d e = ( -3 ) -f a ( 1 e *) - v ; 
 
 m \ d v/ x 2 a 2 
 
 and then we have by No. 53 1 
 
 ^ = - dU - 
 
 We thus obtain for e d e the same expression as before. 
 
 535. We have seen in 532, that if we neglect the squares of the per- 
 turbing forces, the variations of the principal axis and of the mean mo- 
 tion contain only periodic quantities, depending on the configuration of 
 the bodies p, p', p", &c. This is not the case with respect to the varia- 
 tions of the excentricities aricl inclinations : their differential expressions 
 contain terms independent of this configuration and which, if they were 
 rigorously constant, would produce by integration, terms proportional to 
 the time, which at length would render the orbits very excentric and 
 greatly inclined to one another ; thus the preceding approximations, found- 
 ed upon the smallness of the excentricity and inclination of the orbits, 
 would become insufficient and even faulty. But the terms apparently 
 constant, which enter the differential expressions of the excentricities and 
 inclinations, are functions of the elements of the orbits ; so that they vary 
 with an extreme slowness, because of the changes they there introduce. 
 We conceive there ought to result in these elements, considerable inequa- 
 lities independent of the mutual configuration of the bodies of the system, 
 and whose periods depend upon the ratios of the masses /a, /u/, &c. to the 
 mass M. These inequalities are those which we have named secular in- 
 equalities, and which have been considered in (520). To determine them 
 by this method we resume the value of d f of the preceding No. 
 
 1 , /d R\ 
 
 d f = ===== [2 cos v 4- I e cos. 4- i e cos. (2v w)} {-j ) 
 
 V 1 e 2 Vriv/
 
 184 A COMMENTARY ON [SECT. XL 
 
 a* n d t V 1 e 2 .sin 
 
 d 
 
 We shall neglect in the developement of this equation the square and 
 products of the excentricities and inclinations of the orbits ; and amongst 
 the terms depending upon the excentricities and inclinations, we shall re- 
 tain those only which are constant : we shall then suppose, as in No. 515. 
 g = a(l + U/ ); f = a'(l + u/) ; 
 v = n t + t + v, ; v' = n' t + s' + v/. 
 
 Again, if we substitute for R, its value found in 515; if we next con- 
 sider that by the same No. we have, 
 
 di v i I V * "// V J 
 
 / g * d a/ V d a 
 
 and lastly if we substitute for u /5 u/, v,, v/ their values e cos. (n t+ < **) 
 e' cos. (n' t + t' *'), 2 e sin. (n t -f t W), 2 e 7 sin. (n' t + *' ') 
 given in No. 484, &c. by retaining only the constant terms of those which 
 depend upon the first power of the excentricities of the orbits, and ne- 
 glecting the squares of the excentricities and inclinations, we shall find 
 that 
 
 , .. a At' n d t f /d A (0) > 
 
 d t = 
 
 a AI' n d t. 2 1 i A + 1- a (rjj-.) } sin.{i(n' t n t + e 7 0+n t + t\; 
 
 the integral sign belonging as in the value of R of 515, to all the whole 
 positive and negative values of i, including also the value of i = 0. 
 
 We shall have by the preceding No. the value of d f, by diminishing 
 in that of d f the angles e, *', , iS by a right angle ; whence we get 
 
 A (0 \ . /dA<>i 
 
 + a y! n d t. 2 1 i A + J a (-^-*) 1 cos '^ ^ n ' t- ~ n t + t/ ~ + n 
 
 Let X, for the greater brevity, denote that part of d f, which is con- 
 tained under the sign 2, and Y the corresponding part of d f . Make also, 
 as in No. 522, 
 
 <" /n / 
 
 = r |
 
 BOOK L] NEWTON'S PRINCIPIA. 185 
 
 then observe that the coefficient of e' d t sin. v, in the expression of d f 
 is reducible to [0, 1[ when we substitute for the partial differences in a', 
 their values in partial differences relative to a; finally suppose, as in 517, 
 that 
 
 e sin. w =h; e' sin. -a = h' 
 
 e cos. zr = 1 ; e' cos. -a = 1' 
 
 which gives by the preceding No. f = m 1, f = m h or simply f = 1, 
 f = hj by taking M for the mass-unit, and neglecting i* with regard to 
 M ; we shall obtain 
 
 = (0, l).l- 
 
 
 t = - (0, 1). h + |OTT|. h' - a*' n. X. 
 
 Hence, it is easy to conclude that if we name (Y) the sum of the terms 
 analogous to a (if n Y, due to the motion of each of the bodies /a', /A", &c. 
 upon ^ ; that if we name in like manner (X) the sum of the terms analo- 
 gous to a (i! n X due to the same actions, and finally if we mark suc- 
 cessively with one dash, two dashes, &c. what the quantities (X), (Y), h, 
 and 1 become relatively to the bodies p, /u", &c. ; we shall have the fol- 
 lowing differential equations, 
 
 ~ = [(0, 1) + (0, 2) + Sec.} 1 _ gj 1' _ joT2| 1" - &c. + (Y); 
 
 = {(0, 1) + (0, 2) + &c.| h + OH] h' + |0,J8 h" + &c+ (X); 
 3J- = i(l, 0) + (1, 2) + &c.J 1' - [M>| 1 - [53 1" - &c. + (Y') 
 
 = - {(1, 0) + (1, 2) + &c.} h' + [TOI h + 
 
 &c. 
 
 To integrate these equations, we shall observe that each of the quanti- 
 ties h, 1, h', T, &c. consists of two parts ; the one depending upon the 
 mutual configuration of the bodies -, //, &c. ; the other independent of 
 this configuration, and which contains the secular variations of these quan- 
 tities. We shall obtain the first part by considering that if we regard 
 hat alone, h, 1, h', 1', &c. are of the order of the perturbing masses, and 
 consequently, (0, 1). h, (0, 1). 1, &c. are of the order of the squares of
 
 186 A COMMENTARY ON [SECT. XI. 
 
 these masses. By neglecting therefore quantities of this order, we shall 
 nave 
 
 wherefore, 
 
 h =/(Y) d t; 1 =/(X) d t; h' =/(') d t; &c. 
 If we take these integrals, not considering the variability of the ele- 
 ments of the orbits and name Q what/ (Y) d t becomes ; by calling d Q 
 the variation of Q due to that of the elements we shall have 
 
 /(Y)dt = Q-/5Q; 
 
 but Q being of the order of the perturbing masses, and the variations of 
 the elements of the orbits being of the same order, 3 Q is of the order of 
 the squares of the masses ; thus, neglecting quantities of this order, we 
 shall have 
 
 /(Y) d t = Q. 
 
 We may, therefore, take the integrals/ (Y) d t, / (X) d t, / (Y') d t, 
 &c. by supposing the elements of the orbits constant, and afterwards con- 
 sider the elements variable in the integrals ; we shall after a very simple 
 method, obtain the periodic portions of the expressions of h, 1, h', &c. 
 
 To get those parts of the expressions which contain the secular inequa- 
 lities, we observe that they are given by the integration of the preceding 
 differential equations deprived of their last terms, (Y), (X), &c. ; for it is 
 clear that the substitution of the periodic parts of h, 1, h', &c. will cause 
 these terms to disappear. But in taking away from the equations their 
 last terms, they will become the same as those of (A) of No. 522, which 
 we have already considered at great length. 
 
 536. We have observed in No. 532, that if the mean motions n t and 
 n' t of the two bodies p and /.', are very nearly in the ratio of i' to i so 
 that i' n' in may be a very small quantity ; there may result in the 
 mean motions of these bodies very sensible inequalities. This relation of 
 the mean motions may also produce sensible variations in the excentrici- 
 ties of the orbits, and in the positions of their perihelions. To determine 
 them, we shall resume the equation found in 534, 
 
 a n d t . vT 
 e d e = 
 
 1 1 I* JLl* 
 
 m \ d v / m 
 
 It results from what has been asserted in 515, that if we take for the 
 fixed plane, that of the orbit of ^, at a given epoch, which allows ns to
 
 BOOK L] NEWTON'S PRINCIPIA. 187 
 
 neglect in R the inclination <f> of the orbit of & to this plane; all the terms 
 of the expression of R depending upon the angle i' n' t i n t, will be 
 comprised in the following form, 
 
 I*' k cos. (i' n' t i n t + i' f i e g * g */ g // tf\ 
 i, i', g, g', g" being whole numbers and such that we have = i'-i-g-g'-g". 
 The coefficient k has the factor e s . e /g/ (tan. <p') g "; g, g 7 , g' being taken 
 positively in the exponents : moreover, if we suppose i and i' positive, and 
 i' greater than i ; we have seen in No. 515, that the terms of R which 
 depend upon the angle i' n' t i n t are of the order i' i, or of a su- 
 perior order of two, of four, &c. units ; taking into account therefore only 
 terms of the order i' i, k will be of the form e g . e' g ' (tan. |- p') *". Q, 
 Q being a function independent of the excentricities and the inclination 
 of the orbits. The numbers g, g', g" comprehended under the symbol 
 cos., are then positive ; for if one of them, g for instance, be negative and 
 equal to f, k will be of the order f + g' + g' ; but the equation = i' 
 i g gf g" gives f + g 7 + g" = i' i + 2 f ; thus k will be 
 of an order superior to i' i, which is contrary to the supposition. Hence 
 
 by No. 515, we have ( -^ ) = f-, ) provided that in this last partial 
 
 difference, we make t -a constant; the term of (-, \ corresponding 
 
 to the preceding term of R, is therefore 
 
 v,' (i + g) k sin. (i' n' t i n t + iV i e g * g' */ g" 6'}. 
 
 The corresponding term of d R is 
 u! # i n k d t sin. (i' n' t i n t + i' e' \t g a- g' ' g" 0- 
 
 Hence only regarding these terms and neglecting e 2 in comparison with 
 unity, the preceding expression of e d e, will give 
 
 A/ a n d t e k . ... 
 d e = - - . s_ sln . (i' n ' t i n t + i' s i t g g' ' g" f) , 
 
 but we have 
 
 = g e-'. e'*. (tan. J f/) 8 "- Q = ; 
 
 integrating therefore we get 
 
 e = -- -^. r ( ) cos. (i' n' t i n t + i' s' i i g of */ g" ^). 
 m (in in) \d e/ 
 
 The sum of all the terms of R, however, which depend on the angle 
 i' n' t i n t, being represented by the following quantity 
 I*'. P sin. (i' n' t i n t + i' ' i e)+ "' P' cos. (i' n' t i n t + i' ' i i) 
 the corresponding part of e will be
 
 188 A COMMENTARY ON [SECT. XL 
 
 This inequality may become very sensible, if the coefficient i' n' i n 
 is very small, for it actually takes place in the theory of Jupiter and Sa- 
 turn. In fact, it has for a divisor only the first power of i' n' i n, whilst 
 the corresponding inequality of the mean motion, has for a divisor the se- 
 
 cond power of this quantity, as we see in No. 532; butf -, ) and (-1) 
 
 being of an order inferior to P and P', the inequality of the excentricity 
 may be considerable, and even sui'pass that of the mean motion, if the 
 excentricities e and e' are very small; this will be exemplified in the 
 theory of Jupiter's satellites. 
 
 Let us now determine this corresponding inequality of the motion of 
 the perihelion. For that purpose, resume the two equations 
 
 fdf + f df fdf' f'df 
 
 e d e = - , e* d w = - 5 -; 
 
 m 2 m 2 
 
 which we found in No. 534. These equations give 
 
 d f = m d e cos. -a m e d w. sin. r ; 
 thus with regard only to the angle 
 
 i' n' t i n t + rV i * g *r g */ g" ', 
 we shall have 
 
 d f = IL'. a n d t cos. sin. (i' n' t i n t + i 7 ' i * g tr g' ~' g '*') 
 
 m e d -a. sin. w. 
 Representing by 
 
 y!. a n d t { (^) +k' j cos. (i' n' t i n t + i' ' i * g ~ g' -' g" 0, 
 the part of m e d w, which depends upon the same angle, we shall have 
 d f = pf. a n d t { (^) + k'} sin.(i' n' t-i n t + i' '-i (g-l).-gV-g"*') 
 
 sin. (i' n't int + iV ia (g + l) 
 
 It is easy to see by the last of the expressions of d f, given in the No. 
 534, that the coefficient of this last sine has the factor e + l . e' B/ (tan. J >)*" ; 
 
 j i 
 
 k' is therefore of an order superior to that of (-y- ) "by two units ; thus, 
 
 in neglecting it in comparison to (-7) t we sna ^ ^ ave 
 
 __^andt /dkx cos (i , n , t _ int+iv _ ig _ g . -- g' w '_g"0 
 
 m ' \d e/ 
 for the term of e d *, which corresponds to the term 
 
 /*' k cos. (i' n' t i n t + i' t' i g g 7 w' g // O>
 
 BOOK I.] NEWTON'S PRINCIPIA. 189 
 
 of the expression of R. Hence it follows that the part of w, which cor- 
 responds to the part of R expressed by 
 
 Ak'Psin. (i'n't int+ i's' i e) + tf Fcos. (i' n't in t + V t' i ), 
 is equal to 
 
 L = r- . \ ( -, ^ cos. (i' n' t-int + i' i'-\ i]- (, ^ sin. (i' n't-int + i' e'-i t} f ; 
 m(i'n'-in)e I \d e / \ d e / 
 
 we shall therefore, thus, after a very simple manner, find the variations 
 of the excentricity and of the perihelion, depending upon the angle 
 i' n' t i n t + i' e' i e. They are connected with the variation of 
 the corresponding mean motion, in such a way that the variation of the 
 excentricity is 
 
 Sin' de.dt 
 j-.nd the variation of the longitude of the perihelion is 
 i' n' in /d_|\ 
 
 Sine \cTe/" 
 
 The corresponding variation of the excentricity of the orbit of /*', due 
 to the action of /*, will be 
 
 __ 
 Si'n'. e'' 
 
 and the variation of the longitude of its perihelion, will be 
 i' n' in /d 
 
 3 i' n' e' 
 
 and since by No. 532, ' = . ; / . <?, the variations will be 
 
 y! V a 
 
 t* V a / d z \ , (i'ri i n) ^ V a d_ 
 Si'.n'./i' V a' \de'.di) 9 * Si' n'.eV V a ' d e' * 
 
 When the quantity i' n' i n is very small, the inequality depending 
 upon the angle i' n' t i n t, produces a sensible one in the expression 
 of the mean motion, amongst the terms depending on the squares of the 
 perturbing masses ; we have given the analysis of this in No. 532. This 
 same inequality produces in the expression of d e and of d -a, terms of 
 the order of the squares of the masses, and which, being only functions of 
 the elements of the orbits, have a sensible influence upon the secular 
 variations of these elements. Let us consider, in fact, the expression of 
 d e, depending on the angle i' n' t int. 
 
 By what precedes, we have 
 
 de = 
 
 At', a n . d 
 
 - \ (-r -) cos. (i' n' t i n t -f i' t' i g) 
 
 m 
 (- - V sin. (i' n' t i n t + i' t' i
 
 190 A COMMENTARY ON [SECT. XI. 
 
 By No. 532 the mean m< tion n t, ought to be augmented by 
 
 ,? ?' a "! ' 1 . { Pcos. (i'n't int+iV i) Fsin.(i' n' t-i n t+ i' i' i i ) \ 
 (in in)*.m I 
 
 and the mean motion n' t, ought to be augmented by 
 
 3 u! a n 2 . i A V a CT > >'_*! , / / \ 
 SB ^-TZ / / /$? cos. (i' n' t i n t + i' ' i j) 
 (i' n' i n) 2 . m /*' v a 7 
 
 F sin. (i' n' t i n t + i' *' i t)}. 
 
 In virtue of these augments, the value of d e will be augmented by the 
 function 
 
 3/i'a 2 . in 8 , dt f . ,,,,-, / 
 
 and the value of d w will be augmented by the function 
 
 3/a 2 . in 3 , dt . , f /d P N , , /d P\ ) 
 
 7 v , - . , .{ii*'Vaf+ r> Va}. 1 P. (^ -) + Ff-j ) \ . 
 z Vai'n' in 2 , e I \d e / vde/J 
 
 - v , - . , . 
 
 2m z Va(i'n' in) 2 , e 
 
 In like manner we find that the value of d e 7 will be augmented by the 
 function 
 
 3/*a 2 .Va.in 3 . dt c . . / /d P 
 
 . / /d P\ . /d P\ 1 
 
 + X ' A* V 9} iP-fj-/- )-?(-]) (' 
 
 \d e' / \de/J 
 
 5- e / /v / \2' ' 
 
 2 m 2 . a', (i' n x -i n) 2 
 and that the value of d e' will be augmented by the function 
 
 These different terms are sensible in the theory of Jupiter and Saturn, and 
 in that of Jupiter's satellites. The variations of e, e', &, v' relative to the 
 angle i' n' t i n t may also introduce some constant terms of the order of 
 the square of the perturbing masses in the differentials d e, d e', dw, and d*/, 
 and depending on the variations of e, e', r, -a' relative to the same angle. 
 This may easily be discussed by the preceding analysis. Finally it will 
 be easy,, by our analysis, to determine the terms of the expressions of 
 e, *, e', -s/ which depending upon the angle i' n' t i n t -f- i' ' is 
 have not i' n' in for a divisor, and those which, depending on the same 
 angle and the double of this angle, are of the order of the square of the 
 perturbing forces. These different terms are sufficiently considerable in 
 the theory of Jupiter and Saturn, for us to notice them : we shall deve- 
 lope them to the extent they merit when we come to that theory. 
 
 537. Let us determine the variations of the nodes and inclinations of 
 the orbits, and for that purpose resume the equations of 531, 
 
 dc = d
 
 BOOK I.] NEWTON'S PRINCIPIA. 191 
 
 f /d RN ,d R 
 
 dc = 
 
 If we only notice the action of A*', the value of R of No. 5 I 3, gives 
 
 1 1 } . 
 
 x ( d U \ ' ' _ xz ") 
 
 { l - 1 1 }; 
 
 * (x' 2 + y' * + z' 2 ) s {(*.' - x) 2 + (y' - y) 2 + (z' - z) 2 } 8 j 
 d R\ /d R 
 
 i L l 
 
 ' 2 4. y' 2 a. 2' 2 ) * f , 3 r * 
 
 Let however, 
 
 c^ a c^ __ 
 c c 
 
 the two variables p and q will determine, by No. 531, the tangent of the 
 inclination <p of the orbit of /> and the longitude 6 of its node by means of 
 the equations 
 
 tan. <p = V p 2 + q - \ tan. d = . 
 
 Call p', q', p", q", &c. what p and q become relatively to the bodies 
 /a', p", &c. : we shall have by 531, 
 
 z = q y p x ; z' = q' y' p' x', &c. 
 The preceding value of p differentiated gives 
 dp 1 d c" P d c 
 eft = : T' "" d t 
 substituting for d c, and d c" their values we get 
 
 j| = f- Kq q') yy'+ (P' P) x/ y* x 
 
 { x .. + v! + z - ) i- S(x >_ x) . + ( ^ y) . + ( .-^ l i} ' 
 
 In like manner we find 
 
 7TT = T" *(P' P) x x' + (q q') x y'} X
 
 192 A COMMENTARY ON SECT. XI. 
 
 / + y" +z") 8 ~ f( x '_x)*+(y'_ y)'+(z' z)*} 
 If we substitute for x, y, x', y' their values cos. v, e sin. v, ^ cos. v', 
 $' sin. v', we shall have 
 
 (q q') y / + (P' P) x/ y = ^~^- s 2'- * cos - ( v '+ v ) cos. (\ / \}\ 
 
 P' P / e 
 
 + 2 * *' ^ Sm ' ^ V +V ^ ~~ Sm * ( v ~~ v ^ ; 
 (p' p) x x' + (q q') x y 7 = ~- J ^- S s'- Jcos. (v'+v) + cos. (v' v)} 
 
 + - 5"^ S S;- ^ sm - ( v/ + v ) + sin. (v' v)}. 
 Neglecting the excentricities and inclinations of the orbits, v. e have 
 
 I = a ; v = n t + t ; = a' ; v' = n 7 1 + ' ; 
 which give 
 1 1 _ 1_ 
 
 S / 3. a /3 
 
 1 
 
 fa 2 2 a of cos. (n' t n t + t' ) + a'*} * 
 moreover by No. 516, 
 
 - - - 1 = I 2. B fo. cos. i (n' t n t+t' e) 
 Ja 8 2 a a' cos. (n' t n t-H' ) +a' 2 } 
 
 the integral sign 2 belonging to all whole positive and negative values of 
 i, including the value i = ; we shall thus have, neglecting terms of the 
 order of the squares and products of the excentricities and inclinations of 
 the orbits, 
 
 p = qlpq^ ^_| 4 j cos> (n , t+ n t + v + ) _ cos . (n / t _n t + a'_0} 
 
 t / C & 
 
 + HlpE . ^. jsin. (n t + n t + t' + t) sin. (n t n t + t 7 t}} 
 
 . ^ a a '. 2. B P> Jcos.[(i+ 1) (n t n t +')] 
 
 - cos.[(i+l) (n t n t+a' i) + 2nt+2*]| 
 . /'. a a'. 2. B W Jsin.[(i + l) (n' t n t+e' e)J 
 
 sin.[(i+l) (n't n t+t' 0+ 2nt+2e]j. 
 ' ?' ^ cos ' (n' t + n t +.* + ) + cos.(n' t-n t + / )l
 
 BOOK L] NEWTON'S PRINCIPIA. 193 
 
 T * sin - (n r t + n t + / + ) + sin. (n' t nt+i' .)} 
 
 < C & 
 
 =E! . p*. a a '. 2. B W.fcos. [(i+ 1) (n' t n t+s' )] 
 
 *i C 
 
 + cos. [(i+1) (n't n t-H' ) + 2n t+20I 
 . a a /. 2 . B W. $sin. [(i+ 1) (n' t n t+ ' s)] 
 
 Tf C 
 
 + sin. [(i+1) (n't n t+s' ) + 2 n t+2 0}- 
 The value i = 1 gives in the expression of -. , the constant quan- 
 
 tity ^ ~ 3 . p/ t a of B (- 1) . a n t ne other terms of the expression of T- 
 4 c d t 
 
 are periodic : denoting their sum by P, and observing that B ( ~ 15 = B W 
 by 516, we shall have 
 
 .. . 
 
 at 4 c 
 
 By the same process we shall find, that if we denote by Q the sum of 
 all the periodic terms of the expression of -j , we shall have 
 
 U L 
 
 ... . 
 
 at 4 c 
 
 If we neglect the squares of the excentricities and inclinations of the 
 orbits, by 531, we have c = V m a, and then supposing m rr 1, we 
 
 have ii 2 a 3 = 1 which gives c = ; the quantity - - thus be- 
 
 comes - -^ - which by 526, is equal to (0, 1 ) ; hence we get 
 
 j = (0, 1). (q'-q)+P; 
 
 ^ = (0, 1). (p - p') + Q. 
 
 Hence it follows that, if we denote by (P) and (Q) the sum of all the 
 functions P and Q relative to the action of the different bodies p', p", &c. 
 "upon (L ; if in like manner we denote by (P'), (Q'), (P") (Q")> &c. what 
 (P) and (Q) become when we change successively the quantities relative 
 to p into those w"hich are relative to /u/, /*", &c. and reciprocally ; we shall 
 have for determining the variables p, q, p', q r , p", q", &c. the following 
 system of differential equations, 
 
 |P- = {(0, 1) + (0, 2) + c.} q + (0, 1). q' + (0, 2) q"+ &c.+ (P) ; 
 VOL. II. N
 
 194 A COMMENTARY ON [SECT. XI. 
 
 IS. = J(0, 1) + (0, 2) + &c.J p (0, 1) p' (0, 2) p" &c. + (Q) ; 
 ^ = {(1, 0) + (1, 2) + &c.} q' + (1, 0) q + (1, 2) q"+ &c. 
 =1(1,0) + (1,2) + &c.}.p' (1,0) p (1, 2)p" 
 
 &c. 
 
 The analysis of 535, gives for the periodic parts of p, q, p', q', &c. 
 p =/(P).dt; q =/(Q).dt; 
 p'=/(F).dt; q'=/(Q').dt; 
 &c. 
 
 We shall then have the secular parts of the same quantities, by inte- 
 grating the preceding differential equations deprived of their last terms 
 (P), (Q), (P'), &c. ; and then we shall again hit upon the equations (C) 
 of No. 526, which have been sufficiently treated of already to render it un- 
 necessary again to discuss them. 
 
 538. Let us resume the equations of No. 531, 
 
 Vc* +c" c" 
 
 tan. <p = - - - ; tan. 6 = 
 c c 
 
 whence result these 
 
 c' c" 
 
 = tan. a cos. 6 : - = tan. sin. 6. 
 c c 
 
 Differentiating, we shall have 
 
 d tan. <p =r {d c' cos. 6 + d c" sin. d d c tan. <p\ 
 
 C 
 
 d 6 tan. p = - {d c" cos. & d c' sin. 6}. 
 c 
 
 d c d c' d c" 
 
 If we substitute in these equations for ^ , -, , -j , their values 
 
 d t d t d t 
 
 /d , f 
 
 -' andfor 
 
 these last quantities their values given in 534; if moreover we observe 
 that s = tan. <p sin. (v d), we shall have 
 
 d t tan. <p cos. (v - 6} f /d Rx . . . /d R 
 - *- ' - - -- 
 
 . tan. f = 
 
 (1 + s 2 )dt , /d 
 
 _^ -- 
 
 d t tan. sin. (v - 6} ( /d R\ . . ., , /d R 
 - - - - 
 
 (1 + s 2 ) dt . 
 - n ' v - 
 
 d R
 
 BOOK I.] NEWTON'S PRINCIPIA. 195 
 
 These two differential equations will determine directly the inclination 
 of the orbit and the motion of the nodes. 
 They give 
 
 gin. (v 6} d tan. <p d 6 cos. (v 6) tan. <p = ; 
 
 an equation which may be deduced from this 
 
 s = tan. <p sin. (v 6) ; 
 
 in fact, this last equation being finite, we may (530) differentiate it whe- 
 ther we consider p and 6 constant or variable ; so that its differential, 
 taken by only making <p and 6 vary, is nothing ; whence results the pre- 
 ceding differential equation. 
 
 Suppose, however, that the fixed plane is inclined extremely little to the 
 orbit of /ij so that we may neglect the squares of s and tan. <p, we shall 
 have 
 
 , d t , N /d R\ 
 
 d . tan. CD = -- cos. (v 6) . [ ) : 
 c \ ds / 
 
 d t . ., /d R 
 
 d 6 tan. p = sin. (v 6) (-^ 
 
 by making therefore as before 
 
 p = tan. <f> sin. 6 ; q = tan. p cos. d ; 
 
 we shall have, instead of the preceding differential equations, the follow- 
 ing ones, 
 
 d t /d Rx 
 
 d q = -- cos. v . ( -5 ) ; 
 c \ d s / 
 
 d t . /d R 
 
 But we have also 
 
 s = q sin. v p cos. v 
 which gives 
 
 /dRx _ 1 /dRx /djlx _ _ 1 /d_Rx 
 Vds ) ~~ sin. v* \d q/' \ds/~ cos. v\dp/' 
 wherefore 
 
 dt/d Rx 
 d q = (-1 ); 
 c \d p/ 
 
 d t/d Rx 
 d p = (T 1. 
 c Vd q / 
 
 We have seen in 515 that the function R is independent of the po- 
 sition of the fixed plane of x, y ; supposing, therefore, all the angles of 
 that function referred to the orbit of p, it is evident that R will be a 
 function of these angles and the respective inclination of two orbits, an 
 
 N2
 
 196 A COMMENTARY ON [SECT. XL 
 
 inclination we denote by p/. Let OJ be the longitude of the node of the 
 orbit of (jJ upon the orbit of p ; and supposing that 
 
 ti! k (tan. p/) cos. (i f n' t i n t + A g */) 
 
 is a term of R depending on the angle i' n' t i n t, we shall have, by 
 527, 
 
 tan. <f>/ . sin. */ = p' . p ; tan. <pf cos. Of = q' q ; 
 whence we get 
 
 (tan. t;) . sin. g /= jq'-q + (p'-p) V-H '-W -q-(p'-p) V-lj ' 
 
 (tan. f /) cos. g /= iq'-.| + (p-- P ) ^-IS'+W-q- (p'-p) V-H ' . 
 With respect to the preceding term of R, we shall have 
 
 ) = ~ g (tan * ^ g ~' * k ' Sin< *'' "' t ~~ i n t+ A ~" (g ~ ] * *'* ' 
 
 ) = g (tan. p/)-y k cos. Ji' n' t i n t + A (g 1 ) 6f\. 
 If we substitute these values in the preceding expressions of d p and 
 d q, and observe that very nearly c = , we shall have 
 
 / ' 
 
 ( l=^ir=riT) ?** ?8 ~ cos ' n 1 - n 
 
 Substituting these values in the equation 
 
 s =r q sin. v p cos. v 
 we shall have 
 
 s= g ;./*, k '*! I \(tan. p/)^- 1 sin. tf n' t in t v + A (g-1) 9f\. 
 m (i' n i n) v 
 
 This expression of s is the variation of the latitude corresponding to 
 the preceding term of R : it is evident that it is the same whatever may 
 be the fixed plane to which we refer the motions of ^ and /*', provided that 
 it is but little inclined to the plane of the orbits ; we shall therefore thus 
 have that part of the expression of the latitude, which the smallness of the 
 divisor i' n' -in may make sensible. Indeed the inequality of the lati- 
 tude, containing only the first power of this divisor, is in that degree 
 jess sensible than the corresponding inequality of the mean longitude, 
 which contains the square of the same divisor ; but, on the other hand, 
 tan. tpf is then raised to a power less by one ; a remark analogous to that 
 which was made in No. 536, upon the corresponding inequality of the 
 excentricities of the orbits. We thus see that all these inequalities are
 
 BOOK I.] NEWTON'S PRINCIPIA. 197 
 
 connected with one another, and with the corresponding part of R, by 
 very simple relations. 
 
 If we differentiate the preceding expressions of p and q, and if in the 
 
 values of -T - and -^ - we augment the angles n t and n' t by the inequa- 
 
 d t Cl L 
 
 lilies of the mean motions, depending on the angle i' n' t i n t, there 
 will result in these differentials, quantities which are functions only of the 
 elements of the orbits, and which may influence, in a sensible manner, the 
 secular variations of the inclinations and nodes although of the order of 
 the squares of the masses. This is analogous to what was advanced in 
 No. 536 upon the secular variations of the excentricities and aphelions. 
 
 539. It remains to consider the variation of the longitude e of the epoch. 
 By No. 531 we have 
 
 d ~ [E < cos. (v ~) + E ' cos. 2 (v r) + &c.} ; 
 substituting for E (l \ E ;?) , &c. their values in series ordered according to 
 the powers of e, series which it is easy to form from the general expres- 
 sion of E ^ (473) we shall have 
 d s = 2de sin. (v ^) + 2 e d v cos. (v v) 
 
 + ede SS + J e 2 +&c.} sin. 2 (v ) e 2 d~ f + i e*+&c.}cos.2(v w) 
 
 e*de U 4- &c.J sin. 3(v ) + e'dU + &c.} cos. 3 (v - v ) 
 
 + &c. 
 
 If we substitute for d e and e d their values given in 534, we shall 
 find, carrying the approximation to quantities of the order e * inclusively, 
 
 j a 2 .ndt ,- - r~ , , , , , v /d R 
 
 as = - VI e 2 [2 fecos. (v &)+ e 2 cos. 2(v-m 
 
 a n d t . ., v Cl \, /d R 
 
 -- ^ . e.sm.(v >)U +iecos. (v ^)i (-r- 
 
 m V 1 e 2 v a? 
 
 The general expression of d t contains terms of the form 
 
 (*' k . n d t . cos. (i' n' t i n t + A) 
 and consequently the expression of s contains terms of the form 
 
 -. ; - : sin. (i' n' t i n t + A) : 
 i 7 n in 
 
 but it is easy to be convinced that the coefficient k in these terms is of 
 the order i' i, and that therefore these terms are of the same order as 
 those of the mean longitude, which depend upon the same angle. These 
 having the divisor (V n' in) *, we see that we may neglect the corre- 
 sponding terms of s, when i' n' i n is a very small quantity. 
 
 N3
 
 193 A COMMENTARY ON [SECT. XI. 
 
 If in the terms of the expression of d e, which are solely functions of the 
 elements of the orbits, we substitute for these elements the secular parts 
 of their values ; it is evident that there will result constant terms, and 
 others affected with the sines and cosines of angles, upon which depend 
 the secular variations of the excentricities and inclinations of the orbits. 
 The constant terms will produce, in the expression of e, terms propor- 
 tional to the time, and which will merge into the mean motion /*. As to 
 the terms affected with sines and cosines, they will acquire by integration, 
 iii the expression of t, very small divisors of the same order as the per- 
 turbing forces ; so that these terms being at the same time multiplied and 
 divided by the forces, may become sensible, although of the order of the 
 squares and products of the excentricities and inclinations. We shall see 
 in the theory of the planets, that these terms are there insensible; but in 
 the theory of the moon and of the satellites of Jupiter, they are very sen- 
 sible, and upon them depend the secular equations. 
 
 We have seen in No. 532, that the mean motion of ^, is expressed by 
 
 A//a n d t . d R, 
 
 and that if we retain only the first power of the perturbing masses, d R 
 will contain none but periodic quantities. But if we consider the squares 
 arid products of the masses, this differential may contain terms which are 
 functions only of the elements of the orbits. Substituting for the elements 
 the secular parts of their values, there will thence result terms affected with 
 sines and cosines of angles depending upon the secular variations of the 
 orbits. These terms will acquire, by the double integration, in the ex- 
 pression of the mean motion, small divisors, which will be of the order of 
 the squares and products of the perturbing masses; so that being both 
 multiplied and divided by the squares and products of the masses, they 
 become sensible, although of the order of the squares and products of the 
 excentricities and inclinations of the orbits. We shall see that these terms 
 are insensible in the theory of the planets. 
 
 540. The elements of (t's orbit being determined by what precedes, by 
 substituting them in the expressions of the radius-vector, of the longitude 
 and latitude which we have given in 484, we shall get the values of these 
 three variables, by means of which astronomers determine the position of 
 the celestial bodies. Then reducing them into series of sines and cosines, 
 we shall have a series of inequalities, whence tables being formed, we may 
 easily calculate the position of p at any given instant. 
 
 This method, founded on the variation of the parameters, is very useful
 
 BOOK I.] NEWTON'S PKINCIPIA. 199 
 
 in the research of inequalities, which, by the relations of the mean motions 
 of the bodies of the system, will acquire great divisors, and thence become 
 very sensible. This sort of inequality principally affects the elliptic ele- 
 ments of the orbits ; determining, therefore, the variations which result 
 in these elements, and substituting them in the expression of elliptic mo- 
 tion, we shall obtain, in the simplest manner, all the inequalities made 
 sensible by these divisors. 
 
 The preceding method is moreover useful in the theory of the comet:. 
 We perceive these stars in but a very small part of their courses, and ob- 
 servations only give that part of the ellipse which coincides with the arc 
 of the orbit described during their apparitions ; thus, in determining the 
 nature of the orbit considered a variable ellipse, we shall see the changes 
 undergone by this ellipse in the interval between two consecutive appari- 
 tions of the same comet. We may therefore announce its return, and 
 when it reappears, compare theory with observation. 
 
 Having given the methods and formulas for determining, by successive 
 approximations, the motions of the centers of gravity of the celestial bo- 
 dies, we have yet to apply them to the different bodies of the solar system : 
 but the ellipticity of these bodies having a sensible influence upon the 
 motions of many of them, before we come to numerical applications, we 
 must treat of the figure of the celestial bodies, the consideration of which 
 is as interesting in itself as that of their motions. 
 
 SUPPLEMENT 
 
 TO 
 
 SECTIONS XII. AND XIII. 
 
 ON ATTRACTIONS AND THE FIGURE OF THE CELESTIAL BODIES. 
 
 541. The figure of the celestial bodies depends upon the law of gravi 
 tation at their surface, and the gravitation itself being the result of the at- 
 tractions of all their parts, depends upon their figure; the law of gravi- 
 ty at the surface of the celestial bodies, and their figure have, therefore, a 
 reciprocal connexion, which renders the knowledge of the one necessary 
 to the determination of the other. The research is thus very intricate, 
 
 Nl
 
 200 A COMMENTARY ON [SECT. XII. & XIII. 
 
 and seems to require a very particular sort of analysis. If the planets were 
 entirely solid, they might have any figure whatever ; but if, like the earth, 
 they are covered with a fluid, all the parts of this fluid ought to be dis- 
 posed so as to be in equilibrium, and the figure of its exterior surface de- 
 pends upon that of. the fluid which covers it, and the forces which act 
 upon it. We shall suppose generally that the celestial bodies are covered 
 with a fluid, and on that hypothesis, which subsists in the case of the earth, 
 and which it seems natural to extend to the other bodies of the system of 
 the world, we shall determine their figure and the law of gravity at their 
 surface. The analysis which we propose to use is a singular application 
 of the Calculus of Partial Differences, which by simple differentiation, will 
 conduct us to very extensive results, and which with difficulty we should 
 obtain by the method of integrations. 
 
 THE ATTRACTIONS OF HOMOGENEOUS SPHEROIDS BOUNDED BY SURFACES 
 OF THE SECOND ORDER. 
 
 542. The different bodies of the solar system may be considered as 
 formed of shells very nearly spherical, of a density varying according to 
 any law whatever ; and we shall show that the action of a spherical shell 
 upon a body exterior to it, is the same as if its mass were collected at its 
 center. For that purpose we shall establish upon the attractions of sphe- 
 roids, some general propositions which will be of great use hereafter. 
 
 Let x, y, z be the three coordinates of the point attracted which we 
 call p ; let also d M be the element or molecule of the spheroid, and 
 x', y', z' the coordinates of this element ; if we call g its density, f being a 
 function of x', y 7 , z' independent of x, y, z, we shall have 
 d M = g.dx'.dy'.dz'. 
 
 The action of d M upon ^ decomposed parallel to the axis of x and 
 directed towards their origin, will be 
 
 g d x' . d y' . d z' (x x') 
 
 j( X x') 2 + (y y') 2 + (z z'H l 
 
 and consequently it will be equal to 
 
 I ^(x-xr + (y-y') 
 
 cnlling therefore V the integral 
 
 / _ g d x' . d y' . d z' 
 J Vx x' 
 
 V(x x') 2 + (y - y') 8 + (z z') 2 
 extended to the entire mass of the spheroid, we shall have (-: J
 
 BOOK I.] NEWTON'S PRINCIPIA. 201 
 
 for the total action of the spheroid upon the point /*, resolved parallel to 
 the axis of x and directed towards its origin. 
 
 V is the sum of the elements of the spheroid, divided by their respec- 
 tive distances from the point attracted ; to get the attraction of the sphe- 
 roid upon this point, parallel to any straight line, we must consider V as 
 a function of three rectangular coordinates, one of which is parallel to this 
 straight line, and differentiate this function relatively to this coordinate ; 
 the coefficient of this differential taken with a contrary sign, will be the 
 expression of the attraction of the spheroid, parallel to the given straight 
 line, and directed towards the origin of the coordinate which is parallel to 
 it. 
 
 If we represent by |3, the function (x x') 2 +(y y'Y + (z zj}~* ; 
 we shall have 
 
 V = //3. f .dx'dy'dz'. 
 
 The integration being only relative to the variables x', y', z', it is evi- 
 dent that we shall have 
 
 But we have 
 
 /d'/3v xd'jSv 
 
 - teJ + (dyO + 
 
 in like manner we get 
 d 2 V 
 
 This remarkable equation will be of the greatest use in the theory of the fi- 
 gure of the celestial bodies. We may present it under more commodious 
 forms in different circumstances ; conceive, for example, from the origin 
 of coordinates we draw to the point attracted a radius which we call ; 
 let 6 be the angle which this radius makes with the axis of x, and the 
 angle which the plane formed by f and this axis makes with the plane of 
 x, y ; we shall have 
 
 x = i cos. 6 ; y = g sin. 6 cos. -or ; z = g sin. 6 sin. & ; 
 whence we derive 
 
 : Vx'+yM-z* ; : 7 ; 
 
 thus we can obtain the partial differences of g, 0, ^, relative to the varia- 
 
 /d 2 V\ /d 2 V\ /d 
 bles x, y, z, and thence get the values of
 
 202 A COMMENTARY ON [SECT. XII. & XIII. 
 
 in partial differences of V relative to the variables , 6, -a. Since we shall 
 often use these transformations of partial differences, it is useful here to 
 lay down the principle of it. Considering V as a function of the variables 
 x, y, z, and then of the variables g, 0, 9, we have 
 
 dV\ . /dVx /djx /cTV\ /djx ,dV 
 " " 
 
 To get the partial differences (j-)> (T~)> (H~~) we must 
 
 x alone vary in the preceding expressions of j, cos. 6, tan. w ; differentiat- 
 ing therefore these expressions, we shall have 
 
 /d f\ /d 0. sin. /d tsr\ 
 
 ( -> ) = cos. ; ( j ) = --- ; (,--)= ; 
 
 Vdx/ \dx/ 5 \dx/ 
 
 which gives 
 
 d V\ sin. 6 d V 
 
 = cos. 
 
 
 Thus we therefore get the partial difference ( -r -^ , in partial differ- 
 ences of the function V, taken relatively to the variables g, 6, **. Differ- 
 
 entiating again this value of (--r J , we shall have the partial difference 
 
 r\ 2 V 
 (^ 2 Jin partial differences of V taken relatively to the variables g, 6, -a. 
 
 
 By the same process the values of (-^ jrj and (-, ~\ may be found. 
 In this way we shall transform equation (A) into the following one: 
 
 And if we make cos. 6 = m, this last equation will become 
 
 543. Suppose, however, that the spheroid is a spherical shell whose 
 origin of coordinates is at the center ; it is evident that V will only de- 
 pend upon g, and contain neither m nor w, the equation (C) will therefore 
 give 
 
 whence by integration we get
 
 BOOK I.] NEWTON'S PRINC1PIA. 203 
 
 A and B being two arbitrary constants. We therefore have 
 d Vx 
 
 d V 
 
 ~j expresses, by what precedes, the action of the spherical shell upon 
 
 the point /&, decomposed along the radius and directed towards the 
 center of the shell ; but it is evident that the total action of the shell 
 
 ought to be directed along this radius; ( -. J expresses therefore 
 
 the total action of the spherical shell upon the point /A. 
 
 First suppose this point placed within the shell. If it were at the center 
 itself, the action of the shell would be nothing ; we have therefore, 
 
 -(^)=o.-F = ' 
 
 
 densed at this center; calling, therefore Mthe mass of the shell, (-5- 
 
 when = 0, which gives B = 0, and consequently (--. ) = 0, what- 
 
 ever may be; whence it follows that a point placed in the interior of the 
 shell, suffers no action, or \vhich comes to the same thing, it is equally at- 
 tracted on all sides. 
 
 If the point //. is situated without the spherical shell, it is evident, sup- 
 posing it infinitely distant from the center, that the action of the shell 
 upon the point will be the same, as if all the mass of the shell were con- 
 
 - 
 
 or will become in this case equal to - , which gives B = M; we have 
 
 therefore generally relatively to exterior points, 
 /d Vx JV1 
 
 Yd!/ :: g* 
 
 that is to say, the shell attracts them as if all its mass were collected at 
 its center. 
 
 A sphere being a spherical shell, the radius of whose interior surface is 
 nothing, we see that its attraction, upon a point placed at or above its 
 surface, is the same as if its mass were collected at its center. 
 
 This result obtains for globes formed of concentric shells, varying in 
 density from the center to the circumference according to any law what- 
 ever, for it is true for each of the shells : thus since the sun, the planets, 
 and satellites may be considered nearly as globes of this nature, they at- 
 tract exterior bodiejs very nearly as if their masses were collected into 
 their centers of gravity. This is conformable with what has been found by
 
 204 A COMMENTARY ON [SECT. XII. & XIII. 
 
 observations. Indeed the figure of the celestial bodies departs a lit- 
 tle from the sphere, but the difference is very little, and the error which 
 results from the preceding supposition is of the same order as this sup- 
 position relatively to points near the surface; and relatively to distant 
 points, the error is of the same order as the product of this difference by 
 the square of the ratio of the radii of the attracting bodies to their 
 distances from the points attracted; for we know that the considera- 
 tion alone of the distance of the points attracted, renders the error of 
 the preceding supposition of the same order as tne square of this ratio. 
 The celestial bodies, therefore, attract one another very nearly as if their 
 masses were collected at their centers of gravity, not only because they 
 are very distant from one another relatively to their respective dimensions, 
 but also because their figures differ very little from the sphere. 
 
 The property of spheres, by the law of Nature, of attracting as if their 
 masses were condensed into their centers, is very remarkable, and we may 
 be curious to learn whether it also obtains in other laws of attraction. 
 For that purpose we shall observe, that if the law of gravity is such, that 
 a homogeneous sphere attracts a point placed without it as if all its mass 
 were collected at its center, the same result ought to obtain for a spherical 
 shell of a constant thickness ; for if we take from a sphere a spherical 
 shell of a constant thickness, we form a new sphere of a smaller radius 
 with the remainder, but which, like the former, shall have the property of 
 attracting as if all its mass were collected at its center ; but it is evident, 
 that these two spheres can only have this common property, unless it also 
 belongs to the spherical shell which forms their difference. The problem, 
 therefore, is reduced to determine the laws of attraction according to which 
 a spherical shell, of an infinitely small and constant thickness, attracts an 
 exterior point as if all its mass were condensed into its center. 
 
 Let i be the distance of the point attracted to the center of the spherical 
 shell, u the radius of the shell, and d u its thickness. Let 6 be the angle 
 which the radius u makes with the straight line f, v the angle which the 
 plane passing through the straight lines f, u, makes with a fixed plane 
 passing through f, the element of the spherical shell will be u 2 d u . d w . 
 d 6 sin. 6. If we then call f the distance of this element from the point at- 
 tracted, we shall have 
 
 f 2 = s z 2 u cos - * + u 5 - 
 
 Represent by p'(f) the law of attraction to the distance f; the action of 
 the shell's element upon the point attracted, decomposed parallel to g and 
 directed towards the center of the shell, will be
 
 BOOK I.] NEWTON'S PRINCIPIA. 205 
 
 1 " J A S U COS ' * ,rt 
 
 u s d u . d -a sin. d 7= <f> (f) ; 
 
 but we have 
 
 g_ u cos. 6 _ /d f \ 
 
 which gives to the preceding quantity this form 
 u 2 d u . d w sin. 6 f-r - ) <p (f) j 
 
 wherefore if we denote yd f <p (f) by fy (f) we shall have the whole action 
 of the spherical shell upon the point attracted, by means of the integral 
 u 2 d uyd -a d 6 sin. d. <p t (f ), differentiated relatively to g, and divided by 
 d;. 
 
 This integral ought to be taken relatively to w, from v = Q io & equal 
 to the circumference, and after this integration it becomes 
 
 2cr u 2 /d0sin. 6 <p t (f ) ; 
 If we differentiate the value of f relatively to 0, we shall have 
 
 fdf 
 d t> sin. 6 = - ; 
 
 and consequently 
 
 11 t\ 11 
 
 9. (f )- 
 
 The integral relative to 6 ought to be taken from 6 = to 6 = K, and 
 at these two limits we have f = j u, and f =r g + u ; thus the integral 
 relative to f must be taken from f = utof=g + u; let therefore 
 /f d f. 9, (f) = -4, (f ), we shaU have 
 
 rr j ,. , c . 2T.udu f , v .-, 
 
 /f d ft, (f) = - - - ty (g+u) -vKf u)}. 
 
 The coefficient of d g, in the differential of the second member of this 
 equation, taken relatively to g, will give the attraction of the spherical 
 shell upon the point attracted ; and it is easy thence to conclude that in 
 
 nature where <p (f ) = -^ this attraction is equal to 
 
 4 it . u 2 d u 
 
 ~ 
 
 That is to say, that it is the same as if all the mass of the spherical 
 shell were collected at its center. This furnishes a new demonstration of 
 the property already established of the attraction of spheres. 
 
 Let us determine <p (f ) on the condition that the attraction of the shell 
 is the same as if its mass were condensed into its center. This mass 
 is equal to 4 T . u 8 d u, and if it were condensed into its center, its action
 
 206 A COMMENTARY ON [SECT. XII. & XIII. 
 
 upon the point attracted would be 4 . u 2 d u . $ (f) ; we shall therefore 
 have 
 
 d 
 integrating relatively to g, we shall get 
 
 4- (S + ") 4 (s u ) = 2 f u/d f. f> (f) + g U, 
 U being a function of u and constants, added to the integral 2 u/d p(g) 
 If we represent -^ (j + u) -vf/ (g u) by R, we shall have, by differen 
 tiating the preceding equation 
 
 
 But we have, by the nature of the function R, 
 d*R 
 
 du 
 wherefore 
 
 or 
 
 2 p (g) d . p (g) 1 /d 2 U\ 
 
 dg = auVdu*/' 
 
 9 3 
 
 Thus the first member of this equation being independent of u and the 
 functions of g, each of its members must be equal to an arbitrary which we 
 shall designate by 3 A ; we therefore have 
 
 whence in integrating we derive 
 
 B 
 = A? + -,- 
 
 B being a new arbitrary constant. All the laws of attraction in which a 
 sphere acts upon an exterior point placed at the distance from its center, 
 as if all the mass were condensed into its center, are therefore comprised 
 in the general formula 
 
 it is easy to see in fact that this value satisfies equation (D) whatever may 
 be A and B. 
 
 If we suppose A = 0, we shall have the law of nature, and we see that
 
 BOOK I.] NEWTON'S PRINCIPIA. 207 
 
 in the infinity of laws which render attraction very small at great dis- 
 tances, that of nature is the only one in which spheres have the property 
 of acting as if their masses were condensed into their centers. 
 
 This law is also the only one in which a body placed within a spherical 
 shell, every where of an equal thickness, is equally attracted on all sides. 
 It results from the preceding analysis that the attraction of the spherical 
 shell, whose thickness is d u, upon a point placed in its interior, has the 
 expression 
 
 2*ru e du 
 To make this function nothing, we must have 
 
 4 (u + e) ^ (u s) = s u, 
 
 U being a function of u independent of g, and it is easy to see that this 
 
 T> 
 
 obtains in the law of nature, where p (f ) = 5 . But to show that it 
 
 takes place only in this law, we shall denote by ty (f ) the difference of 4- 
 (f ) divided by d f, we shall also denote by y (f) the difference of -\}/ (f) 
 divided by d f, and so on ; thus we shall get, by differentiating twice suc- 
 cessively, the preceding equation relatively to f, 
 
 V (" + ) -V' (u f) = 0. 
 
 This equation obtaining whatever may be u and f, it thence results 
 that -4/' (f ) ought to be equal to a constant whatever f may be, and that 
 therefore 4-'" (f ) = 0. But, by what precedes, 
 
 4/(f)=f. f/ (f), 
 whence we get 
 
 4/"(f) = 2f>(f) + f?(f); 
 
 we therefore have 
 
 o = 2p(f) +ff'(f); 
 
 which gives by integration 
 
 and consequently the law of nature. 
 
 554. Let us resume the equation (C) of No. 541. If this equation 
 could generally be integrated, we should have an expression of V, which 
 would contain two arbitrary functions, which we should determine by 
 finding the attraction of a spheroid, upon a point situated so ^s to facili- 
 tate this research, and by comparing this attraction with its general ex- 
 pression. But the integration of the equation (C) is possible only in some 
 particular cases, such as that where the attracting spheroid is a sphere, 
 which reduces this equation to ordinary differences; it is also possible in
 
 208 A COMMENTARY ON [SECT. XII. & XIII. 
 
 the case where the attracting body is a cylinder whose base is an oval or 
 curve returning into itself, and whose length is infinite. This particular 
 case contains the theory of Saturn's ring. 
 
 Fix the origin of g upon the same axis of the cylinder, which we shall 
 suppose of an infinite length on each side of the origin. Naming g the 
 distance of the point attracted from the axis; we shall have 
 
 g ' = g V 1 m 2 . 
 
 It is evident that V only depends on g' and *, since it is the same for 
 all the points relatively to which these two variations are the same ; it 
 contains therefore only m inasmuch as f' is a function of this variable. 
 This gives 
 
 <LV\ = /dV\ /dj\ j_m /dJV\. 
 
 d m/~ " \d g / " vdm/ VI _ m * VEJT/' 
 
 d*V\ g'm 2 /d'Vx _ $__ /d 
 
 m 
 
 the equation (C) hence becomes 
 
 , dV /dV 
 
 = 
 
 whence by integrating we get 
 
 V = y>[j> cos. -a + | V 1 sin. w} + ^[g cos. r (>' V 1 sin. J ; 
 <f (/) and 4/ (f ) being arbitrary functions of g', which we can determine 
 by seeking the attraction of the cylinder when tr is nothing and when it 
 is a right angle. 
 
 If the base of the cylinder is a circle, V will be evidently a function of 
 f independent of v, the preceding equation of partial differences will 
 thus become 
 
 which gives by integrating, 
 
 H 
 
 H being a constant. To determine it, we shall suppose f relatively to 
 the radius of the base of the cylinder extremely great, which supposition 
 permits us to consider the cylinder as an infinite straight line. Let A be 
 this base, and z the distance of any point whatever of the axis of the cy- 
 linder, to the point where this axis is met by gf ; the action of the cylin- 
 der considered as concentrated or condensed upon its axis, will be, paral 
 
 Jel to ff, equal to 
 
 C A g / . d z 
 
 '
 
 BOOK I.] NEWTON'S PRINCIPIA. 209 
 
 the integral being taken from z = oo to z = <x> ; this reduces the in- 
 tegral to - , ; which is the expression of (;r~7~) when ^ is very con- 
 
 siderable. Comparing this with the preceding one we have H = 2 A, 
 and we see that whatever is /, the action of the cylinder upon an exterior 
 
 . 2 A 
 
 point, is - . 
 
 t 
 
 If the attracted point is within a circular cylindrical shell, of a constant 
 
 thickness, and infinite length, we shall have ( , } =r' 9 and since 
 
 vug/ j 
 
 the attraction is nothing when the point attracted is upon the axis of the 
 shell, we have H = 0, and consequently, a point placed in the interior of 
 the shell is equally attracted on all sides. 
 
 545. We have thus determined the attraction of a sphere and of a 
 spherical shell : let us now consider the attraction of spheroids terminated 
 by surfaces of the second order. 
 
 Let x, y, z be the three rectangular coordinates of an element of the 
 spheroid ; designating d M this element, and taking for unity the density 
 of the spheroid which we shall suppose homogeneous, we shall have 
 dM = dx.dy.dz. 
 
 Let a, b, c be the rectangular coordinates of the point attracted by the 
 spheroid, and denote by A, B, C the attractions of the spheroid upon 
 this point resolved parallel to the axes of x, y, z and directed to the origin 
 of the coordinates. 
 
 It is easy to show that we have 
 
 \ _ rrr _ (a - x) d x . d y . d z __ ^ 
 {(a x) 2 + (b y; 2 + (c z) 2 } 1 ' 
 
 B = ' ( b y) dx - d y- dz 
 
 a-x) 2 + (b y) 2 + <c z) 2 }* 
 
 n rrr (c z) dx.dy.dz 
 
 ^ JJJ ~ " 
 
 ( a x) 2 + (b y)* + (c z) 2 }* 
 
 All these triple integrals ought to be extended to the entire mass of the 
 spheroid. The integrations under this form present great difficulties, 
 which we can often in part remove by transforming the differentials into 
 others more convenient. This is the general principle of such trans- 
 formations. 
 
 Let us consider the differential function Pdx.dy.dz, P being any 
 function whatever of x, y, z. We may suppose x a function of y and z 
 and of a new variable p : let p (y, z, p) denote this function ; in this case, 
 
 VOL. II. O
 
 210 A COMMENTARY ON [SECT. XII. & XIII. 
 
 we shall have, making y and z constant, d x = /3 . d p, /3 being a function 
 of y, z and p. The preceding differential will thus become ft . P . d p . 
 d y . d v z ; and to integrate it, we must substitute in P, for x, its value 
 
 9 (y z > p)- 
 
 In like manner we may suppose in this new differential, y = <p' (z, p, q), 
 q being a new variable, and <p' (z, p, q) being any function of the three 
 variables z, p and q. We shall have, considering z and p constant, 
 d y = ft' d q, ft' being a function of z, p, q ; the preceding differential 
 will thus take this new form ft ft' P. d p . d q . d z, and to integrate it, we 
 must substitute in $ P for y its value <p' (z, p, q). 
 
 Lastly we may suppose z equal to <p" (p, q, r), r being a new variable, 
 and <p" (p, q, r) being any function whatever of p, q, r. We shall have, 
 considering p and q constant, d z = ft" d r, ft" being a function of p, q, r ; 
 the preceding differential will thus become (3. ft'. ft". P . d p . d q . d r 
 and to integrate it, we must substitute in ft.ft'.P for z its value 0" (p, q, r). 
 The proposed differential function is thence transformed to another rela- 
 tive to the three new variables p, q, r, which are connected with the pre- 
 ceding by the equations 
 
 x = <p (y, z, p) ; y = <f>' (z, p, q) ; z = p" (p, q, r). 
 
 It only remains to derive from these equations the values of /3, /?, ft". 
 For that purpose we shall observe that they give x, y, z, in functions of 
 the variables p, q and r ; let us consider therefore the three first variables 
 as functions of the three last. Since ft" is the coefficient of d r in the dif- 
 ferential of z, taken by considering p and q constant, we have 
 
 ft' is the coefficient of d q, in the differential of y taken on the supposi- 
 tion that p and z are constant ; we shall therefore have ft', by differen- 
 tiating y on the supposition that p is constant, and by eliminating d r by 
 means of the differential of z taken on the supposition that p is constant, 
 and equating it to zero. Thus we shall have the two equations 
 
 which give
 
 Boox I.] NEWTON'S PRINCIPIA. 211 
 
 wherefore 
 
 _ d q/ \d r ~ ~ \dr 
 
 di' 
 
 Finally, /3 is the coefficient of d p, in the differential of x taken on the 
 supposition that y and z are constant. This gives the three following 
 equations 
 
 /d z 
 
 =(crj 
 If we make 
 
 - (l5 . 
 
 ' \dp \dq \dr ~\dp \dr 
 
 we shall have 
 
 dx = ~d 
 
 V<TqV Vd~r 
 
 which gives 
 
 4. dz 
 
 dr 
 
 d_yx /d^x /d_yx /dz 
 
 _yx /^x /_yx /_zx 
 dqJ \drJ ~~ \dr) \d q 
 
 wherefore /3 . $'. ft" = s and the differential P. d x . d y . d z is transform- 
 ed into e. P. dp. dq. dr; P being here what P becomes when we 
 substitute for x, y, z their values in p, q, r. The whole is therefore re- 
 duced to finding the variables p, q, r such that the integrations may be- 
 come possible. 
 
 Let us transform the coordinates x, y, z into the radius drawn from 
 the point attracted to the molecule, and into the angles which this ra- 
 dius makes with given straight lines or with given planes. Let r be 
 this radius, p the angle which it forms with a straight line drawn through 
 the attracted point parallel to the axis of x, and let q be the angle which 
 
 O2
 
 212 A COMMENTARY ON [SECT. XII. & XIII. 
 
 its projection makes on the plane of y, z with the axis of y ; we shall 
 have 
 
 x = a T cos. p ; y = b r sin. p cos. q ; z = c r sin. p sin. q. 
 We shall then find e = r * sin. p, and the differential d x . d y . d z will 
 thus be transformed into r 2 sin. p . d p . d q. d r : this is the expres- 
 sion of the element d M, and since this expression ought to be positive 
 in considering sin. p, d p, d q, d r as positive, we must change its sign, 
 which amounts to changing that of e, and to making i = r * sin. p. 
 
 The expressions of A, B, C will thus become 
 
 A =fffd r d p d q . sin. p cos. p ; 
 B = fffA r dp d q. sin. 2 p cos. p; 
 C = fff& r dp d q . sin. ! p sin. q. 
 
 It is easy to arrive by another way at these expressions, by observing 
 that the element d M may be supposed equal to a rectangular parallele- 
 piped, whose dimensions are d r, r d p and r d q sin. p, and by then observing 
 that the attraction of the element, parallel to the three axes of x, y, z is 
 
 dM d M dM , 
 
 -^- cos. p ; ,- sin. p cos. q ; ^- sin. p sin. q. 
 
 The triple integrals of the expressions of A, B, C must extend to the 
 entire mass of the spheroid : the integrations relative to r are easy, but 
 they are different according as the point attracted is within or without the 
 spheroid ; in the first case, the straight line which passing through the 
 point attracted, traverses the spheroid, is divided into two parts by this 
 point ; and if we call r and r 7 these parts, we shall have 
 
 A =ff(r + r 7 ) d p d q. sin. p cos. p; 
 
 B= > /y(r + r / )dp dq. sin. 2 p cos. p ; 
 
 C ff(r + r') d p d q . sin. 2 p sin. q ; 
 
 the integrals relative to p and q ought to be taken from p and q equal to 
 zero, to p and q equal to two right angles. 
 
 In the second case, if we call r, the radius at its entering the spheroid, 
 and r 7 the radius at its farther surface, we shall have 
 
 A =ff(r' r) d p d q . sin. p cos. p ; 
 B = Jf(? r) d p d q . sin. 2 p cos. q ; 
 C =ff(r' r) d P d q . sin. 2 p sin. q. 
 
 The limits of the integrals relative to p and to q, must be fixed at the 
 points where r' r = 0, that is to say, where the radius r is a tangent 
 to the surface of the spheroid. 
 
 546. Let us apply these results to spheroids bounded by surfaces of the
 
 BOOK L] NEWTON'S PRINCIPIA. 213 
 
 second order. The general equation of these surfaces, referred to the 
 
 three orthogonal coordinates x, y, z is 
 
 0= A+B.x + C.y+E. z+F. x 2 + H. x y+L. y 2 +M. x z+N. y z+O. z 2 . 
 
 The change of the origin of coordinates introduces three arbitraries, 
 since the position of this new origin relating to the first depends upon 
 three arbitrary coordinates. The changing the position of the coordi- 
 nates around their origin introduces three arbitrary angles ; supposing, 
 therefore, the coordinates of the origin and position in the preceding 
 equation to change at the same time, we shall have a new equation of the 
 second degree whose coefficients will be functions of the preceding coeffi- 
 cients and of the six arbitraries. If we then equate to zero the first 
 powers of the coordinates, and their products two and two, we shall de- 
 termine these arbitraries, and the general equation of the surfaces of the 
 second order, will take this very simple form 
 
 x 2 + m y 2 + n z 8 = k 2 ; 
 it is under this form that we shall discuss it. 
 
 In these researches we shall only consider solids terminated by finite 
 surfaces, which supposes m and n positive. In this case, the solid is an 
 ellipsoid whose three semi-axes are what the variables x, y, z become 
 
 when we suppose two of them equal to zero ; we shall thus have k, - f , 
 
 k 
 , for the three semi-axes respectively parallel to x, to y and to z. The 
 
 solid content of the ellipsoid will be 
 
 3 V m n 
 
 If, however, in the preceding equation we substitute for x, y, z their 
 values in p, q, r given by the preceding No., we shall have 
 
 r z (cos. 2 p + m sin.*p cos. 2 q + n sin. z p sin. 2 q) 
 2 r (a cos. p + m b sin. p cos. q + n c sm - P sm q) =k 2 -a 2 -m b ? -n c ? ; 
 so that if we suppose 
 
 I = a cos. p + m b sin. p cos. q -}- n c sin. p sin. q ; 
 L = cos. 2 p + m sin. 2 p cos. 2 q + n sin. 2 p sin. 2 q ; 
 R = I 2 + (k 2 a 2 mb 2 nc 1 ). L 
 we shall have 
 
 I + V R 
 
 TT 
 whence we obtain r' by taking -f, and r by taking ; we shall theie- 
 
 fore have 
 
 21 , 2 V R 
 
 r + , = _ ; r'-r^. 
 
 * 03
 
 214 A COMMENTARY ON [SECT. XII, & XIII. 
 
 Hence relatively to the interior points of the spheroid, we get 
 . r r d p . d q . I . sin, p . cos, p 
 
 *JJ Tf~ 
 
 r r d p . d q . I . sin. 2 p . cos, q 
 
 * JJ ~L~ 
 
 r> o /'^.dp.dq.I. sin. 2 p . sin. q 
 
 2 ff ~L~ 
 
 and relatively to the exterior points 
 
 A o / / d p d q . sm - P cos< P ^ ^ 
 JJ L 
 
 d p . d q . sin. 2 p cos. q V R 
 
 y J 
 
 d p . d q . sin. 2 p sin, q V R 
 
 J 3 
 
 the three last integrals being to be taken between the two limits which 
 correspond to R = 0. 
 
 547. The expressions relative to the interior points being the most 
 simple, we shall begin with them. First, we shall observe that the semi- 
 axis k of the spheroid does not enter the values of I and L ; the values of 
 A, B, C are consequently independent ; whence it follows that we may 
 augment at pleasure, the shells of the spheroid which are above the point 
 attracted, without changing the attraction of the spheroid upon this point, 
 provided the values of m and n are constant. Thence results the follow- 
 ing theorem. 
 
 A point placed within an elliptic shell whose interior and exterior sur- 
 faces are similar and similarly situated, is equally attracted on all sides. 
 
 This theorem is an extension of that which we have demonstrated in 
 542, relative to a spherical shell. 
 
 Let us resume the value of A. If we substitute for I and L their va- 
 lues, it will become 
 
 A / >dp.dq.sin.p.cos.p.(acos.p -f- mbsin.pcos.q + ncsin.psin.q) 
 
 J J cos. 2 p + in sin. * p cos. 2 q -f n sin. 2 p sin. 2 q 
 
 Since the integrals relative to p and q, must be taken from p and q 
 equal to zero, to p and q equal to two right angles, it is clear we have 
 generally J P d p . cos. p = 0, P being a rational function of sin. p and 
 of cos. 2 p ; because the value of p being taken at equal distances greater 
 and less than the right angle, the corresponding values of P . cos. p are 
 equal and have contrary signs ; thus we have 
 
 A = 2 a //* d p.dq.sin. p cos. * p ^ 
 
 ' J J cos. 2 p + m sin. 2 p cos 2 q -\> n sin 7 p sin. 2 q *
 
 BOOK L] NEWTON'S PRINCIPIA. 215 
 
 If we integrate relatively to q from q = to q = two right angles, we 
 shall find 
 
 * _ 2 a * r dp. sin. p cos. * p 
 
 V mn / 1 m \ /, 1 n T' 
 
 S ' P)- + -IT C S ' P) 
 
 n 
 
 COS ' 
 
 an integral which must be taken from cos. p = 1 to cos. p = 1. Let 
 cos. p = x, and call M the entire mass of the spheroid ; we shall have 
 
 ,'..,,, 4 r . k 3 , 4*- 3M 
 
 by 545, M = and consequently -= = -r-r- : we shall there- 
 Vmn ^Vmn k 3 
 
 fore have 
 
 3 a M f x 2 d x 
 
 A = 
 
 which must be taken from x = 0, to x = 1. 
 
 Integrating in the same manner the expressions of B, C we shall reduce 
 them to simple integrals ; but it is easier to get these integrals from the 
 preceding expression of A. For that purpose, we shall observe that this 
 expression may be considered as a function of a and of the squares k 2 , 
 
 k 2 k 2 
 , of the semi-axes of the spheroid, parallel to the coordinates a, b, c 
 
 of the point attracted ; calling therefore k' 2 the square of the semi-axis 
 parallel to b, and consequently k' 2 . m, and k' 2 n the squares of the two 
 
 other semi-axes, B will be a similar function of b, k'*, k' 2 m, k' 2 ; thus 
 
 to get B we must change in the expression of A, a into b, k into k' or 
 
 k 1 . n , . , . 
 
 , m into , and n into , which gives 
 ' 
 
 m m m 
 
 3 b M f m^. x 2 dx 
 
 B = 
 
 Let 
 
 t 
 
 ' V m + (1 m). t 2 ' 
 
 we shall have 
 
 3bM 
 
 Jt> = r-= 
 
 m 
 
 an integral relative to t which must be taken, like the integral relative to x 
 
 o 4
 
 216 A COMMENTARY ON [SECT. XII. & XIII. 
 
 from t = to t = 1, because x = gives t = and x = 1, gives t = 1 
 Hence it follows that if we suppose 
 
 1 - m x ' dx 
 
 m n V(l + X 2 x 2 ). (1 + X' 2 x*) 
 
 we shall have 
 
 _ 3_b M /d.x Fx 
 
 k 3 V dx )' 
 
 If we change in this expression, b into c, X into X' and reciprocally, we 
 shall have the value of C. The attractions A, B, C of the spheroid, par- 
 allel to its three axes are thus given by the following formulas 
 
 _ 3aM _ 3bM d . X Fx 3 c M d . X F 
 
 *" ~~ 
 
 We may observe that these expressions obtaining for all the interior 
 points, and consequently for those infinitely near to the surface, they also 
 hold good for the points of the surface. 
 
 The determination of the attractions of a spheroid thus depends only 
 on the value of F ; but although this value is only a definite integral, it 
 has, however, all the difficulty of indefinite integrals when X and X' are 
 indeterminate, for if we represent this definite integral, taken from x = 
 to x = 1, by f (X*, X' *), it is easy to see that the indefinite integral will 
 be x 3 <p (X x *, X' 2 x f ), so that the first being given, the second is likewise 
 given. The indefinite integral is only possible in itself when one of the 
 quantities X, X' is nothing, or when they are equal : in these two cases, 
 the spheroid is an ellipsoid of revolution, and k will be its semi-axis of 
 revolution if X and X' are equal. In this last case we have 
 > x * d x 1 c , 
 
 = /l+x- = Dfr- 
 
 To get the partial differences ( *. - j, ( ^ ; ), which enter the 
 
 expressions of B, C, we shall observe that 
 
 dx/d.xFx dx;/d.^Fx __ /d_x <U\ 
 
 X\dxJ H X'VdX'J 'Vx X' ) 
 
 but when X = X', we have 
 
 /d . X Fx _ jd . X' Fx, d_x _ d_>. 
 
 \ d X )~ \ dx' / ; X >/ ; 
 
 wherefore , 
 
 x = ^.x d F+Fdx = Ld. x F. 
 
 Substituting for F its value, we shall have
 
 BOOK I.] NEWTON'S PRINCIPIA. ' 217 
 
 we shall therefore have relatively to ellipsoids of revolution, whose semi- 
 axis of revolution is k, 
 
 3 b . M 
 
 3 c . M 
 
 = 
 
 548. Now let us consider the attraction of spheroids upon an exterior 
 point. This research presents greater difficulties than the preceding be- 
 cause of the radical V R which enters the differential expressions, and 
 which under this form renders the integrations impossible. We may ren- 
 der them possible by a suitable transformation of the variables of which 
 they are functions ; but instead of that method, let us use the following 
 one, founded solely upon the differentiation of functions. 
 
 If we designate by V the sum of all the elements of the spheroid divided 
 by their respective distances from the point attracted, and x, y, z the co- 
 ordinates of the element d M of the spheroid, and a, b, c those of the 
 point attracted, we shall have 
 
 V = f dM 
 
 J V (a x) 2 + (b y) 2 + (c z) 2 ' 
 
 Then designating, as above, by A, B, C the attractions of the spheroid 
 parallel to the axes of x, y, z, and directed towards their origin, we shall 
 have 
 
 / (a x). d M /d 
 
 A = / * 
 
 J(a x) 2 + (b y) * + ( c 
 In like manner we get 
 
 B = -( ),C = -(- V> 
 
 whence it follows that if we know V, it will be easy thence to obtain by 
 differentiation alone, the attraction of a spheroid parallel to any straight 
 line whatever, by considering this straight line as one of the rectangular 
 coordinates of the point attracted ; a remark we have already made in 
 541. 
 
 The preceding value of V, reduced into a series, becomes 
 
 2 a x+ 2 b y+2cz x 2 y 2 z ! 
 
 v =/ 
 
 This series is ascending relatively to the dimensions of the spheroid
 
 218 A COMMENTARY ON [SECT. XII. & XI11. 
 
 and descending relatively to the coordinates of the point attracted. If we 
 only retain the first term, which is sufficient when the attracted point is 
 at a very great distance, we shall have 
 
 y 
 
 V & 2 + b 2 + c s> 
 
 M being the entire mass of the spheroid. This expression will be still 
 more exact, if we place the origin of coordinates at the center of gravity 
 of the sphere ; for by the property of this center we have 
 
 / x. d M = ; / y. d M = ; / z. d M = ; 
 
 so that if we consider a very small quantity of the first order, the ratio 
 of the dimensions of the spheroid to its distance from the point attracted, 
 the equation 
 
 y _ M 
 
 V a 2 + b 2 + c 2 
 will be exact to quantities nearly of the third order. 
 
 We shall now investigate a rigorous expression of V relatively to ellip- 
 tic spheroids. 
 
 549. If we adopt the denominations of 544, we shall have 
 
 V =/ =fffr d r d p d q sin. p = //(r" - r 2 ) d p dq. sin. p. 
 
 Substituting for r and r 7 their values found in 544, we shall have 
 v - o rr d p . d q sin, p. I . V R 
 
 *JJ L 2 
 
 Let us resume the values of A B, C relative to the exterior points, and 
 given in 546, 
 
 A o / /* d P d , sm> P cos - p V R 
 * ~~~~ 
 
 o r r d P . d q sin. g p cos, q V R 
 
 5 - *JJ - ~TT 
 
 c = 2 /ydp-dqsin.*psin. q v R . 
 
 Since at the limits of the integrals, we have V R = 0, it h easy to see 
 that by taking the first differences of V, A, B, C relatively to any of the 
 six quantities a, b, c, k, m, n, we may dispense with regarding the varia- 
 tions of the limits ; so that we have, for example, 
 
 for the integral 
 
 r d P sin, p I V R 
 
 J ~17~~
 
 BOOK L] NEWTON'S PRINCIPIA. 219 
 
 z 
 is towards these limits, very nearly proportional to R 2 , which renders 
 
 equal to zero, its differential at these limits. Hence it is easy to see by 
 differentiation that if for brevity we make 
 
 aA + bB + cC = F; 
 
 we shall have between the four quantities B, C, F, and V the following 
 equation of partial differences, 
 
 We may eliminate from this equation, the quantities B, C, F by means 
 of their values 
 
 dVx , /dVx ,/dV 
 
 We shall thus get an equation of partial differences in V alone. Let 
 therefore 
 
 , T 4 v. k 3 ,, 
 
 V zr - . y = M . v, 
 
 3 V m n 
 
 M being by 545, the mass of the elliptic spheroid ; and for the variables 
 m and n let us here introduce 6 and w which shall be such that we have 
 
 m n 
 
 6 will be the difference of the square of the axis of the spheroid parallel 
 to y and the square of the axis parallel to x ; -a will be the difference of 
 the square of the axis of z and the square of the axis of x ; so that if we 
 take for the axis of x, the smallest of the three axes of the spheroid, V d 
 and *S -a will be its two excentricities. Thus we shall have 
 
 TU/ k V d V> \ j V 1 
 
 31 HrTAdJ" 1 " 2lij : 
 
 V being considered in the first members of those equations as a function 
 of a, b, c, k, m, n ; and v being considered in their second members as a 
 function of a, b, c, 6 t -a, k.
 
 220 A COMMENTARY ON [SECT. XII. & XIII. 
 
 If we make 
 
 >ve shall have F = M Q, and we shall get the values of k 
 
 (ars) ' (s~n~) by chan in g in the P recedin s valu e* of k 
 
 -T -jt v into Q. Moreover V and F are homogeneous functions in 
 
 a, b, c, k, */ 6, V v of the second dimension, for V being the sum of the 
 elements of the spheroid, divided by their distances from the point at- 
 tracted, and each element being of three dimensions, V is necessarily of 
 two dimensions, as also F which has the same number of dimensions as 
 V ; v and Q are therefore homogeneous functions of the same quantities 
 and of the dimension 1; thus we shall have by the nature of homo- 
 geneous functions, 
 
 (d v\ , ,/d v\ . /d v\ , _ t /d v\ , 
 y-J +b -(db) + c -te) +2 Hdr) +2 
 
 an equation which may be put under this form 
 
 We shall have in like manner 
 
 then, if in equation (1) we substitute for V, F and their partial differences; 
 
 k 2 k 2 
 
 if moreover we substitute ,-s - . for m and ,- - for n, we shall have 
 
 k 8 + ^ k 2 -f- -a 
 
 550. Conceive the function v expanded into a series ascending rela- 
 tively to the dimensions k, V 6, V -a of the spheroid, and consequently 
 descending relatively to the quantities a, b, c : this series will be of the 
 following form : 
 
 v = U (0 >+ U('' + tM 2 >+ UW+&C.; 
 
 U (0) , U (1) , &c. being homogeneous functions of a, b, c, k, V 6, V *, and 
 separately homogeneous relatively to the three first and to the three last
 
 BOOK I.] 
 
 NEWTON'S PRINCIPIA. 
 
 221 
 
 of these six quantities ; the dimensions relative to the three first always 
 decreasing, and the dimensions relative to the three last increasing con- 
 tinually. These functions being of the same dimension as v, are all of the 
 dimension 1. 
 
 If we substitute in equation (2) for v its preceding expanded value ; if 
 we call s the dimension of U (i) in k, V 6, V &, and consequently s 1 
 its dimension in a, b, c; if in like manner we name s' the dimension of 
 UC + i) in k, V 6, V &, and consequently s' 1 its dimension in a, b, 
 c ; if we then consider that by the nature of homogeneous functions we 
 have 
 
 we shall have, by rejecting the terms of a dimension superior in k, V 
 V v to that of the terms which we retain, 
 
 . . . (3) 
 
 s. 
 
 This equation gives the value of U (i + 1 \ by means of U (i) and of its 
 partial differences ; but we have 
 
 since, retaining only the first term of the series, we have found in 548, that 
 
 v = _ M _ .. 
 
 Substituting therefore this value of U (0) in the preceding formula, we 
 shall get that of U (1 > ; by means of that of U (1 > we shall have that of U (2 > 
 and so on. But it is remarkable that none of these quantities contains k: 
 for it is evident by the formula (3) that U (0) , not containing U (1) , does 
 not contain it ; that U (1) not containing it, U ( " 2) will not contain it, and so 
 on ; so that the entire series U (0) + U (lj + &c. is independent of k, or 
 
 which is the same thing (-TT) = 0. The values of v, (T y (nT
 
 A COMMENTARY ON [SECT. XII. & XIIL 
 
 (T~) are therefore the same for all elliptic spheroids similarly si- 
 tuated, and which have the same excentricities V 6, V -a ; but M f-r-^ 
 
 *d a/ 
 
 ^\d~b) ' ^(d /' ex P ress ty 548, the attractions of the spheroid 
 parallel to its three axes; therefore the attractions of different elliptic 
 spheroids which have the same center, the same position of the axes and 
 the same excentricities, upon an exterior point, are to one another as their 
 masses. 
 
 It is easy to see by formula (3) that the dimensions of U^, U (1) , U (2) , 
 &c. in Vdand V =?, increase two units at a time, so that s = 2i, s' = 2i + 2; 
 moreover we have by the nature of homogeneous functions 
 
 ,( d J^l\-i u ,f du "'y 
 
 I "* V 2T^i J J 
 
 \dw/ \ d 6 J 
 
 this formula will therefore become 
 
 By means of this equation, we shall have the value of v in a series very 
 convergent, whenever the excentricities V 6, V -a are very small, or when 
 the distance V a 2 + b 2 + c 2 of the point attracted from the center of 
 the spheroid is very great relatively to the dimensions of the spheroid. , 
 
 If the spheroid is a sphere, we shall have 6 = 0, and = 0, which 
 give U (1 > = 0, U (2 > = 0, &c. ; wherefore 
 
 v = U W = l : 
 
 V a 2 + b 2 -f- c 2 
 
 and 
 
 y- M 
 
 V a 2 + b 2 + c 2 ' 
 
 whence it follows that the value of V is the same as if all the mass of .the 
 sphere were condensed into its center, and that thus, a sphere attracts any 
 exterior point, as if all its whole mass were condensed into its center ; a 
 result already obtained in 542. 
 
 551. The property of the function of v being independent of k, fur- 
 nishes the means of reducing its value to the most simple form of which it 
 is susceptible ; for since we can make k vary at pleasure without changing 
 this value, provided the spheroid retain the same excentricities, V 6 and
 
 BooKl.] NEWTON'S PRINCIPIA. 223 
 
 V *, we may suppose k such that the spheroid shall be infinitely flatten- 
 ed, or so contrived that its surface pass through the point attracted. In 
 these two cases, the research of the attractions of the spheroid is rendered 
 more simple; but since we have already determined the attractions of elliptic 
 spheroids, upon points at the surface, we shall now suppose k such that 
 the surface of the spheroid passes through the point of attraction. 
 
 If we call k', m', n' relatively to this new spheroid what in 545, we 
 named k, m, n relatively to the spheroid we there considered ; the condi- 
 tion that the point attracted is at the surface, and that also a, b, c are the 
 coordinates of a point of the surface, will give 
 
 a* + m'b* + n'c 2 = k 2 ; 
 
 and since we suppose the excentricities V d and V *r to remain the same, 
 we shall have 
 
 ] m ],/_*. 1 ~ n/ k' 2 - 
 
 ~isr~'* ' ~^~- 
 
 whence we obtain 
 
 = F r +iJ ~k /2 + ~ ; 
 
 we shall therefore have to determine k 7 , the equation 
 
 \r' 2 V 2 
 
 n 2 J. _ h 2 J. r 2 k' 2 f ^ 
 
 "i" V'2 i 4 u T C7Fj~Z ~~ * * ' V ' 
 
 K. -p P Jti -J- 
 
 It is easy hence to conclude that there is only one spheroid whose sur- 
 face passes through the point attracted, d and -a remaining the same. For 
 if we suppose, which we always may do, that 6 and are positive, it is 
 clear that augmenting in the preceding equation, k' 2 by any quantity which 
 we may consider an aliquot part of k /2 , each of the terms of the first 
 member of this equation, will increase in a less ratio than k' 2 ; therefore 
 if in the first state of k' 2 , there subsist an equality between the two mem- 
 bers of this equation, this equality will no longer obtain in the second 
 state ; whence it follows that k' 2 is only susceptible of one real and posi- 
 tive value. 
 
 Let M' be the mass of the new spheroid, and A', B', C' its attractions 
 parallel to the axes of a, b, c ; if we make 
 1 __ m / 1 _ n' 
 
 m n 
 
 x 2 dx 
 
 F =/ 
 
 V(l + X 2 . x 2 ). (1 + X' 2 
 by 547, we shall have 
 
 _ 3 a M' F , 3 b M' /d . X Fx c/ _
 
 22 1 A COMMENTARY ON [SECT. XII. & X11I. 
 
 Changing in these values of A 7 , B', C', M' into M, we shall have \iy 
 the preceding No., the values of A, B, C relatively to the first spheroid 
 but the equations 
 
 " - 
 
 m' n' 
 
 give 
 
 X' = 4-; X"= ; 
 k' * being given by equation (5) which we may put under this form 
 
 we shall therefore have 
 
 A _ 3 a M 3 b M /d . X F\ _ 3 c M ^d . X' F^ 
 
 lr' 3 * ' ~ k' 3 \ H > / ' k' 3 V H >/ y 
 
 A. 14. II A * K. x Cl A / 
 
 These values obtain relatively to all points exterior to the spheroid, and 
 to extend them to those of the surface, and even to the interior points 
 we have only to change k' to k. 
 
 If the spheroid is one of revolution, so that 6 = *r, the formula (5) 
 will give 
 
 2 k /2 = a 2 +b 2 +c 2 6 + V(a 2 +b 2 +c 2 0) 2 +4 a 2 . 0; 
 and by 547 we shall have 
 
 3aM .. 
 
 A = f-.T- -, (X tan.- 1 X) 
 
 I. r o \ S \ ' 
 
 3 b M / X x 
 
 B =. YI 1 tan. ~- l X ) 
 
 C - 3 c M 
 
 Thus we have terminated the complete theory of the attractions of el- 
 liptic spheroids ; for all that remains to be done is the integration of the 
 differential expression of F, and this integration in the general sense is 
 impossible, not only by known methods, but also in itself. The value of F 
 cannot be expressed in finite terms by algebraic, logarithmic or circular 
 quantities ; or which it tantamount, by any algebraic function of quantities 
 whose exponents are constant, nothing or variable. Functions of this kind 
 being the only ones which can be expressed independently of the symbol 
 Jl all the integrals which cannot be reduced to such functions, are impos- 
 sible in finite terms. 
 
 If the elliptic spheroid is not homogeneous, and if it is composed of 
 elliptic shells varying in position, excentricity and density according to 
 any law whatever, we shall have the attraction of one of its shells, by de-
 
 BOOK I.] NEWTON'S PRINCIPIA. 225 
 
 termining as above the difference of the attractions of two homogeneous 
 elliptic spheroids, having the same density as the shell, one of which shall 
 have for its surface the exterior surface of the shell, and the other the in- 
 terior surface of the shell. Then summing this differential attraction, we 
 shall have the attraction of the whole spheroid. 
 
 THE DEVELOPEMENT INTO SERIES, OF THE ATTRACTIONS OF ANY 
 SPHEROIDS WHATEVER. 
 
 552. Let us consider generally the attractions of any spheroids what- 
 ever. We have seen in No. 547, that the expression V of the sum of the 
 elements of the spheroid, divided by their distances from the attracted 
 points, possesses the advantage of giving by its differentiation, the attrac- 
 tion of this spheroid parallel to any straight line whatever. We shall see 
 moreover, when treating of the figure of the planets, that the attraction of 
 their elements presents itself under this form in the equation of their equi- 
 librium ; thus we proceed particularly to investigate V. 
 
 Let us resume the equation of No. 548, 
 
 V= /_ dM 
 
 J v (a x) 2 + (b y) 2 + (c z) *' 
 
 a, b, c being the coordinates of the point attracted ; x, y, z those of the 
 element d M of the spheroid ; the origin of coordinates being in the in- 
 terior of the spheroid. This integral must be taken relatively to the va- 
 riables x, y, z, and its limits are independent of a, b, c; hence we shall 
 find by differentiation, 
 
 an equation already obtained in 541, 
 
 Let us transform the coordinates to others more commodious. For 
 that purpose, let r be the distance of the point attracted from the origin 
 of coordinates ; & the angle which the radius r makes with the axis of a ; 
 a the angle which the plane formed by the radius and this axis, makes 
 with the plane of the axis of a, and of b ; we shall have 
 
 a = r cos. 6 ; b = r sin. 6 cos. d ; c = r sin. 6 sin. -a. 
 
 If in like manner we name R, ^, / what r, 6, -a become relatively to 
 the element d M of the spheroid ; we shall have 
 
 x = R cos. ff ; y = R sin. (f cos. -a' ; z = R sin. ^. sin. -a'. 
 
 Moreover, the element d M of the spheroid is equal to a rectangular 
 parallelepiped whose dimensions are d R, R d 5', R d */ sin. tf, and con- 
 
 VOL. II. P
 
 226 A COMMENTARY ON [SECT. XII. & XIII. 
 
 sequently it is equal to . R-. d U. d if. d '. sin. <K, being its density; we 
 shall thus have 
 
 _ 
 
 ~ 
 
 g R.d R.d^. d Vsin. 
 
 ' r * - 2 r RrjcOS. 0. COS. ^+Sin. Sin. 6' COS. (ar' - w)}-f-R 2 ' 
 
 the integral relative to R must be taken from R = to the value of R at 
 the surface of the spheroid ; the integral relative to */ must be taken from 
 v = to -a/ equal to the circumference ; and the integral relative to 6' 
 must be taken from 6' to 6' equal to the semi-circumference. Differ- 
 entiating this expression of V, we shall find 
 
 dl") 
 
 an equation which is only equation (1) in another form. 
 If we make cos. 8 = m, we may give it this form 
 
 We have already arrived at these several equations in 541. 
 
 553. First, let us suppose the point attracted to be exterior to the sphe- 
 roid. If we wish to expand V into a series, it ought in this case, to de- 
 scend relatively to powers of r, and consequently to be of this form 
 
 U <o> 
 
 V = - - + - + -3- + &c. 
 r r 2 T 3 
 
 Substituting this value of V in equation (3) of the preceding No., the 
 comparison of the same powers of r will give, whatever i may be 
 
 It is evident from the integral expression alone of V that U U) is a ra- 
 tional and entire function of m, V \ m 2 . sin. ^ and V 1 m 2 . cos. *r, 
 depending upon the nature of the spheroid. "When i = 0, this function 
 becomes a constant ; and in the case of i = 1, it assumes the form 
 
 Hm + H'Vl m 2 . sin. * + H" V 1 m 2 . cos. 
 
 H, H', H" being constants. 
 
 To determine generally U (i) call T the radical 
 
 1 
 
 Vr 2 2 R r Jcos.dcos.tf'+sin.
 
 BOOK I.] NEWTON'S PRINCIP1A. 
 
 we shall have 
 
 This equation will still subsist if we change 6 into ^, into w', and re- 
 ciprocally ; because T is a similar function of S ', ' and of 0, *. 
 
 If we expand T, in a series descending relatively to r, we shall have 
 
 O (0) P T? 2 
 
 T = 
 
 Q (i) being, whatever i maybe, subject to the condition that 
 
 and moreover it is evident, that Q (i) is a rational and entire function of m, 
 and V 1 m 2 . cos. (or w) : Q (i) being known, we shall have U (i) by 
 means of the equation 
 
 U W =fg R (i + 2) . d R. d '. d ff . sin. &' . Q >. 
 
 Now suppose the point attracted in the interior of the spheroid : we 
 must then develope the integral expression of V, in a series ascending re- 
 latively to r, which gives for V a series of the form 
 
 V = v <> + r . v (1 > + r 2 . v ^ + r 3 . v + &c. 
 
 v (l) being a rational and whole function of m, V 1 m 2 . sin. v and 
 VI m z cos. , which satisfies the same equation of partial differences 
 that U (i) does ; so that we have 
 
 0= . 
 
 dm / 1 m 
 
 To determine v (i) , we shall expand the radical T into a series ascending 
 according to r, and we shall have 
 
 O W) r r 2 
 
 the quantities Q (0) , Q (1) , Q (2) , &c. behig the same as above; we shall 
 therefore get 
 
 r e. d R. d~'. d*. sin. (f . QW 
 v( ' )= / 1 -R^~ 
 
 But since the preceding expression of T is only convergent so long as 
 R is equal to or greater than r, the preceding value of V only relates to the 
 shells of the spheroid, which envelope the point attracted. This point 
 being exterior, relatively to the other shells, we shall determine that part 
 
 of V which is relative to them by the first series of V. 
 
 P2
 
 A COMMENTARY ON [SECT. XII. & XIII. 
 
 554. First let us consider those spheroids which differ but very little 
 from the sphere, and determine the functions U ( % U (1) , U (2) , &c. v , 
 v (1 >, v W, & c . relatively to these spheroids. There exists a differential 
 equation in V, which holds good at their surface, and which is remarkable 
 because it gives the means of determining those functions without any in- 
 tegration. 
 
 Let us suppose generally, that gravity is proportional to a power n of 
 the distance ; let d M be an element of the spheroid, and f its distance 
 from the point attracted; call V the integral ff n + l d M, which shall ex- 
 tend to the entire mass of the spheroid. In nature we have 11 = 2, 
 
 it becomes JT. > and we have expressed it in like manner by V in the 
 
 preceding Nos. The function V possesses the advantage of giving, by its 
 differentiation, the attraction of the spheroid, parallel to any straight line 
 whatever ; lor considering f as a function of the three coordinates of the 
 point attracted perpendicular to one another, and one of which is parallel 
 to this straight line. Call r this coordinate, the attraction of the spheroid 
 
 1 /* 
 
 along r and directed towards its origin, will bey\ f n . (~v -) d M. Con- 
 sequently it will be equal to . (-= J , which, in the case of nature, 
 
 becomes ( , ) , conformably with what has been already shown. 
 
 , 
 
 Suppose, however, that the spheroid differs very little from a sphere of 
 the radius a, whose center is upon the radius r perpendicular to the sur- 
 face of the spheroid, the origin of the radius being supposed to be arbi- 
 trary, but very near to the center of gravity of the spheroid ; suppose, 
 moreover, that the sphere touches the spheroid, and that the point at- 
 tracted is at the point of contact of the two surfaces. The spheroid is 
 equal to the sphere plus the excess of the spheroid above the sphere ; but 
 we may conceive this excess as being formed of an infinite number of 
 molecules spread over the surface of the sphere, these molecules being 
 supposed negative wherever the sphere exceeds the spheroid; we shall 
 therefore have the value of V by determining this value, 1st, relatively to 
 the sphere ; 2dly, relatively to the different molecules. 
 
 Relatively to the sphere, V is a function of a, which we denote by A ; 
 if we name d m one of the molecules of the excess of the spheroid above 
 the sphere, and f its distance from the point attracted ; the value of V rela-
 
 BOOK L] NEWTON'S PRINCIPIA. 229 
 
 tive to this excess will bey*. f n + 1 . d m; we shall therefore have, for the 
 entire value of V, relative to the spheroid, 
 
 V = A+y-.f^ + ^dm. 
 
 Conceive that the point attracted is elevated by an infinitely small 
 quantity d r, above the surface of the spheroid arid the sphere upon r or a 
 produced ; the value of V, relative to this new position of the attracted 
 point, will become 
 
 A will increase by a quantity proportional to d r, and which we shall re- 
 present by A' . d r. Moreover, if we name 7 the angle formed by the two 
 radii drawn from the center of the sphere to the point attracted, and to 
 the molecule d m, the distance f of this element or molecule from the point 
 attracted, will be in the first position of the point, equal to 
 
 V 2 a 2 (1 cos. 7); 
 in the second position it will be 
 
 V (a -f- d r) 2 - 2 a (a + d r) cos. 7 + a 2 , 
 or 
 
 the integraiy. f n + 1 dm, will thus become 
 
 d - ~ , , . 
 ._ /.fn .dm; 
 
 we shall therefore have 
 
 /d V\ , A/ , n+ldr,.,, 
 
 ( aT ).dr==A'.dr+ -g--. /.f '.dm; 
 
 substituting for/, f n + 1 . d m, its value V A, we shall have 
 
 In the case of nature, the equation (1) becomes 
 
 fl (** A \ - _ a A' i A 4- i V 
 
 d I -, I rt \. a ^1 -ir o T . 
 
 \dr J 
 The value of V relative to the sphere of radius a, is, by 550, equal to 
 
 4ira s 4fi-a 3 4<ra in^ir 
 
 , which gives A = - ; A' = 5 ; we shall therefore 
 3 r 
 
 get 
 
 We must here observe that this equation obtains, whatever may be the 
 position of the straight line r, and even in the case where it is not perpen- 
 
 P3
 
 230 A COMMENTARY ON [SECT. XII. & XIII. 
 
 dicular to the surface of the spheroid, provided that it passes very near its 
 center of gravity, for it is easy to see that the attraction of the spheroid, 
 resolved parallel to these straight lines, and which, as we have seen, is 
 
 equal to (, J , is, whatever may be their position, always the same, to 
 
 quantities nearly of the order of the square of the excentricity of the 
 spheroid. 
 
 555. Let us resume the general expression of V of 553, relative to a 
 point attracted exterior to the spheroid, 
 V = U un, U* 
 r r 2 r 3 
 
 the function U (i) being, whatever i may be, subject to the equation of par- 
 tial differences 
 
 By differentiating the value of V relatively to r, we have 
 d \\ U<> . 2LHD 8U(*> 
 
 ~^ 4 
 
 Let us represent by a (1 + a y) the radius drawn from the origin of 
 r to the surface of the spheroid, a being a very small constant coefficient, 
 whose square and higher powers we shall neglect, and y being a function 
 of m and depending on the nature of the spheroid. We shall have to 
 
 4 T a 3 
 
 quantities nearly of the order a, V = -^ - ; whence it follows that in the 
 
 o r 
 
 preceding expression of V, 1st, the quantity U (0 ' is equal to - plus a very 
 
 8 
 
 small quantity of the order a, and which we shall denote by U' (0) ; 
 2dly, that the quantities U (1) , U (2) , &c. are small quantities of the order a. 
 Substituting a (1 + a-y) for r in the preceding expressions of V and of 
 
 (-1-7) > and neglecting quantities of the order a 2 , we shall have rela- 
 
 tively to an attracted point placed at the surface 
 
 U U) U (2) 
 
 ^- ! + i7 i + &c. ; 
 
 2 
 
 If we substitute these values in equation (2) of the preceding No. we 
 
 shall have 
 
 U'<> , 3.U> , 5U^ 2 ' 7U, a 
 4a*a 2 .y = - -+ - + -TT- + -- +&c -
 
 BOOK I.] NEWTON'S PRINCIPIA. 231 
 
 It thence follows that the function y is of this form 
 
 y _ Y co) + yw + Y< 2 ) + &c. 
 
 the quantities Y^, Y>, Y, &c. as well as U ( % U, &c. being subject 
 to the equation of partial differences 
 
 = 
 
 l-m- 
 
 this expression of y is not therefore arbitrary, but it is derived from the 
 developement of the attractions of spheroids. We shall see in the follow- 
 ing No. that y cannot be thus developed except in one manner only ; we 
 shall therefore have generally, by comparing similar functions, 
 
 uw = 271TT ai+3 - Y(i); 
 
 whence, whatever r may be, we derive 
 
 _ Y U), Y 
 
 8r r I- Tr ^5r 
 
 To get V, therefore, it remains only to reduce y to the form above de- 
 scribed ; for which object we shall give, in what follows, a very simple 
 method. 
 
 If we had y = Y (i) , the part of V relative to the excess of the spheroid 
 above the sphere whose radius is a, or which is the same thing, relative to 
 a spherical shell whose radius is a, and thickness a a y, would be 
 
 4cra i + 3 .Y .. 
 
 -. . .. j-qry- ; this value would consequently be proportional to y, 
 
 and it is evident that it is only in this case that the proportionality can 
 subsist. 
 
 556. We may simplify the expression Y<> + Y C1 > + Y + &c. of y, 
 and cause to disappear the two first terms, by taking for a, the radius of a 
 sphere equal in solidity to the spheroid, and by fixing the arbitrary origin 
 of r at the center of gravity of the spheroid. To show this, we shall ob- 
 serve that the mass M of the spheroid supposed homogeneous, and of a 
 density represented by unity, is by 552, equal to^/R 2 d R d m d w, or to 
 ^f R /3 d m d w, R' being the radius R produced to the surface of the 
 spheroid. Substituting for R' its value a (1 -f a y) we shall have 
 
 M = ^ a + a a 3 /y d m d ~. 
 
 O 
 
 All that remains to be done, therefore, is to substitute for y its value 
 Y W) + Y (1) + & c and then to make the integrations. For this purpose 
 here is a general theorem, highly useful also in this analysis. 
 
 P4
 
 232 A COMMENTARY ON [SECT. XII. & XIII. 
 
 " If Y w and Z (;; be rational and entire functions of m, V 1 m * . sin. -a 
 " and V 1 m 2 . cos. , which satisfy the following equations : 
 /i n / dY J 
 
 i 
 
 \ dm y 1 m 2 
 
 " we shall have generally 
 
 "/Y (i) . Z.dmd=r = 0, 
 
 " whilst i and i' are whole positive numbers differing from one another . 
 " the integrals being taken from m = 1 to m = 1, and from w = 
 " to tr = 2 T." 
 
 To demonstrate this theorem, we shall observe that in virtue of the first 
 of the two preceding equations of partial differences, we have 
 
 1 m 2 
 But integrating by parts relatively to m we have 
 
 , 
 dm 
 
 * a 
 
 d m . d 
 
 _. 
 dm y f \ ami ' 
 
 dm 
 
 'dm; 
 
 and it is clear that if we take the integral from m = 1 to m = 1, the 
 second member of this equation will be reduced to its last term. In like 
 manner, integrating by parts relatively to , we get 
 
 and this second member also reduces to its last term, when the integral
 
 BOOK I.] NEWTON'S PRINCIPIA. 233 
 
 i ^7 ''* 
 
 is taken from *r = to = 2 ir y because the values of Y (i) , ( . 
 
 \ d -a 
 i rj (jf\ 
 
 Z (l/) , (- -\ are the same at these two limits; thus we shall have 
 
 ii w / 
 
 . 
 dm / h 1 m 
 
 whence we derive, in virtue of the second of the two preceding equations 
 of partial differences, 
 
 / Y 0). Z . d m . d >='. ||'+ 1} ./ Y w. Z W. d m . d . , 
 
 we therefore have 
 
 =/Y. Z dm.d, 
 when i is different from i'. 
 
 Hence it is easy to conclude that y can be developed into a series of 
 the form Y (0 > + Y^ + Y C2 > + &c. in one way only; for we have 
 generally 
 
 fy . Z d m d w =/Y W. Z W d m . d ~; 
 
 If>we could develope y into another series of the same form, Y, (0) + 
 Y/i) + Y/ 2 > + &c. we should have 
 
 /y.Z =//. Zdm.d; 
 wherefore 
 
 /Y, >. Z ). d m d . =/Y . Z ) d m . d ~. 
 
 But it is easy to perceive that if we take for Z w the most general 
 function of its kind, the preceding equation can only subsist in the case 
 wherein Y, (i) = Y w ; the function y can therefore be developed thus in 
 only one manner. 
 
 If in the integraiyy d m . d w, we substitute for y its value Y (0) + Y (1) 
 + Y (2) + & c -j we shall have generally =. J Y (i) d m . d *>, i being 
 equal to or greater than unity ; for the unity which multiplies d m . d * 
 is comprised in the form Z (0) , which extends to every constant and quan- 
 tity independent of m and ^. The integraiyy d m. d w reduces there- 
 fore to/Y (0) d m. d , and consequently to 4 cr Y (0) ; we have there- 
 fore 
 
 M = |ca 3 + 4ffa 3 . YW; 
 
 thus, by taking for a, the radius of the sphere equal in solidity to the sphe- 
 roid, we shall have Y (0} = 0, and the term Y (0) will disappear from the 
 expression of y.
 
 834 A COMMENTARY ON [SECT. XII. & XIII. 
 
 The distance of the element d M, or R 2 .dRdm.dw, from the 
 plane of the meridian from whence we measure the angle r, is equal to 
 R V 1 m 2 . sin. w; the distance of the center of gravity of the sphe- 
 roid from this plane, will be therefore,/ R 3 d R d m . d VI m 2 . sin. r, 
 and integrating relatively to R, it will be \f R' 4 d m . d VI m 2 sin. -a-, 
 R' being the radius R produced to the surface of the spheroid. In like 
 manner the distance of the element d M from the plane of the meridian 
 perpendicular to the preceding, being R V 1 m 2 . cos. w, the distance 
 of the center of gravity of the spheroid from this plane will be ^ f R' 4 
 dm.dw. V I m 2 . cos. -a. Finally, the distance of the element d M 
 from the plane of the equator being m, the distance of the center of gra- 
 vity of the spheroid from this plane will be \J R' 4 m . d m . d . These 
 functions m, V 1 m 2 . sin. *, V 1 m 2 . cos. w, are of the form Z (1) , 
 Z (l) being subiect to the equation of partial differences 
 
 = 
 
 dm / \ m 2 
 
 If we conceive R' * developed into the series N (0) -f N (1) -f N (2) + &c. 
 N U) being a rational and entire function of m, VI m 2 . sin. *, 
 cos. *r, subject to the equation of partial differences. 
 
 - . 
 
 dm / 1 m 2 
 
 the distances of the center of gravity of the spheroid, from the three 
 preceding planes, will be, in virtue of the general theorem above demon- 
 strated, 
 
 < l > . d m . d w . VT^nT 2 . sin. 
 4/N (1) . d m. d . VI m 2 . cos. 
 
 N (1) is, by No. 553, of the form A m + B V 1 m 2 . sin. * + 
 C -y/ i nT 2 . cos. w, A, B, C being constants ; the preceding distances 
 
 will thus become - . B, -J- . C, -J- . A. The position of the center of 
 
 o o rf 
 
 gravity of the spheroid, thus depends only on the function N (1) . This 
 gives a very simple way of determining it. If the origin of the radius R' 
 is at the center; this origin being upon the three preceding planes, the 
 distances of the center of gravity from these planes will be nothing. This 
 gives A = 0, B = 0, C = 0; therefore N = 0.
 
 BOOK I.] NEWTON'S PRINCIPIA 235 
 
 These results obtain whatever may be the spheroid : when it is very 
 little different from a sphere, we have R' = a (I + y), and R' 4 = 
 a 4 (1 + 4 a y) ; thus, y being equal to Y (0 > + Y (1 > + Y + &c., we 
 have N C1) = 4 a * Y (1) , the function Y (1 - disappears, therefore, from the 
 expression of y, when we fix the origin of R' at the center of gravity of 
 the spheroid. 
 
 557. Now let the point attracted be in the interior of the spheroid, we 
 shall have by 553 
 
 V = v () + r . v "> + r 2 . v (2 > + r 3 v (3 > -f &c. 
 
 /d R. d*/. d ft . sin. tf . Q*> 
 v W ; / - _ - _r 
 
 J K 1 -' 
 
 Suppose that this value of V is relative to a shell whose interior surface is 
 spherical and of the radius a, and the radius of whose exterior surface is 
 a (1 y) ; the thickness of the shell is a a y. If we denote by y' what 
 y becomes when we change 0, & into <J', &', we may, neglecting quantities 
 of the order a 2 , change r into a, and d R into a a y', in the integral ex- 
 pression of v W ; thus we shall have 
 
 v = -i/y' d r/ . d V . sin. tf. Q w. 
 
 Si 
 
 Relatively to a point placed without the spheroid, we have, by 553, 
 U co) 
 
 R.d^.d^. sin.*. Q . 
 
 If we suppose this value of V relative to a shell, whose interior and ex- 
 terior radii are respectively a, a (1 + a y), we shall have 
 
 U = a. a' + '.yy. d J. d tf. sin. tf. Q ; 
 wherefore 
 
 We have by 555 
 
 TT (i) _ 4 < ra i + 3 . 
 2i + 1 
 therefore 
 
 4 a *r Y 
 
 (i) 
 
 (2i + 1) a 1 - 
 which gives 
 
 V = 4a<ra 8 4 Y()+^-.Y^ + 9 .Y (2) +&c. 
 t 3 a 5 a 
 
 To this value of V we must add that which is relative to the spherical 
 shell of the thickness a r which envelopes the attracted point plus that 
 which is relative to the sphere of radius r, and which is below the same
 
 236 A COMMENTARY ON [SECT. XII. & XIII. 
 
 point If we make cos. tf = m', we shall have, relatively to the first of 
 the two parts of V, 
 
 r d R . d / . d m' . Q W 
 = /- -Rl^I- 
 
 an integral which, relative to m', must be taken from in' = 1 to m = 1 
 Integrating relative to R, from R = r to R = a, we shall have 
 1 
 
 But we have generally, by the theorem of the preceding No., 
 . dm'. Q (i) = when i is equal to or greater than unity; when 
 i = 0, we have, by 553, Q w =: 1 ; moreover the integration relative to 
 tr' must be taken from &' = to -a' =. 2 it ; we shall therefore have 
 
 v> = 2-r (a 2 r 2 ). 
 
 This value of v (0) is that part of* V which is relative to the spherical shell 
 whose thickness is a r. 
 
 The part of V which is relative to the sphere whose radius is r is equal 
 to the mass of this sphere, divided by the distance of the attracted point from 
 
 4 CT 1* 
 
 its center : it is consequently equal to - . Collecting the different 
 
 D 
 
 parts of V,we shall have its whole value 
 
 ; . (4) 
 
 o a 5 a x 
 
 Suppose the point attracted, placed within a shell very nearly spherical, 
 whose interior radius is 
 
 a + a JYW + Y^ + Y + &c.J 
 and whose exterior radius is 
 
 a' + a a? {' + Y x + Y' + &c.J 
 
 The quantities a a Y (0) and a af Y' (0) may be comprised in the quanti- 
 ties a, a'. Moreover, by fixing the origin of coordinates at the center of 
 gravity of the spheroid whose radius is 
 
 a+ a a JYW -f Y) + &c.i, 
 
 we may cause Y (I - to disappear from the expression of this radius ; and 
 then the interior radius of the shell will be of this form, 
 
 a + a a {Y + Y< 3 > + &c.}, 
 and the exterior radius will be of the form, 
 
 a' + a a' JY'W + Y'< 2 > + &c.J. 
 
 We shall have the value of V relative to this shell, by taking the differ- 
 ence of the values of V relative to two spheroids, the smaller of which 
 shall have for the radius of its surface the first quantity, and the greater
 
 BOOK I.] NEWTON'S PRINCIPIA. 
 
 237 
 
 the second quantity for the radius of its surface; calling therefore A. V, 
 what V becomes relatively to this shell, we shall have 
 
 {r a? r z r 3 /Y'( s ) V(3) 1 
 
 ^Y'w+^Y'n_ Y ^ + ^ (~r~) +&c.} 
 
 If we wish that the point placed in the interior of the shell, should be 
 equally attracted on all sides, A . V must be reduced to a constant inde- 
 pendent of r, 0, r ; for we have seen that the partial differences of A . V, 
 taken relatively to these variables, express the partial attractions of the 
 shell upon the point attracted ; we therefore, in this case have Y' (1) = 0, 
 and generally 
 
 Y' P) = (JLy-2. Y (i >- 
 ^ a / 
 
 so that the radius of the interior surface being given, that of the exterior 
 surface will be found. 
 
 When the interior surface is elliptic, we have Y (3) = 0, Y w = 0, &c. 
 and consequently Y 7 = 0, Y' (4) = ; the radii of the two surfaces, in- 
 terior and exterior, are therefore 
 
 a[l + aY}; a' 1 + a Y}; 
 
 thus we see that these two surfaces are similar and similarly situated, 
 which agrees with what we found in 547. 
 
 558. The formulas (3), (4) of Nos. 555, and 557, comprehend all the 
 theory of the attractions of homogeneous spheroids, differing but little from 
 the sphere; whence it is easy to obtain that of heterogeneous spheroids, 
 whatever may be the law of the variation of the figure and density of their 
 shells. For that purpose let a (1 + a y) be the radius of one of the shells 
 of a heterogeneous spheroid, and suppose y to be of this form 
 
 the coefficients which enter the quantities Y ( % Y (1) , &c. being functions 
 of a, and consequently variable from one shell to another. If we differ- 
 entiate relatively to a, the value of V given by the form (3) of No. 555 ; 
 and call e the density of the shell whose radius is a (1 + a y) being a 
 function of a only ; the value of V corresponding to this shell will be, for 
 an exterior attracted point, 
 
 1? g d a + 4 " T ' g d |a Y + . Y< + f^. Y + &c.l ; 
 3 r s r 3 r 5 r* 
 
 this value will be, therefore, relatively to the whole spheroid, 
 
 . . (5) 
 
 the integrals being taken from a = to that value of a which subsists at 
 the surface of the spheroid, and which we denote by a.
 
 238 A COMMENTARY ON [SECT. XII. & Xlil. 
 
 To get the part of V relative to an attracted point in the interior of the 
 spheroid, we shall determine first the part of this value relative to all the 
 shells to which this point is exterior. This first part is given by formula 
 (5) by taking the integral from a = to a = a, a being relative to the 
 shell in which is the point attracted. We shall find the second part of V 
 relative to all the shells in the interior of which is placed the point attract- 
 ed, by differentiating the formula (4) of the preceding No. relatively to a; 
 then multiplying this differential by P, and taking the integral from a = a, 
 to a = a, the sum of the two parts of V will be its entire value relative to 
 an interior point, which sum will be 
 
 a 2 +4 a */g d . {a 2 Y W+~ Y "> + ~ Y <+ &c.} . (6) 
 
 the two first integrals being taken from a = to a = a, and the two last 
 being taken from a = a to a = a ; after the integrations, moreover, we 
 
 I __ fa -y 
 
 must substitute a for r in the terms multiplied by a, and - J - for 
 
 fl 
 
 1 .i. ^ v , 
 
 - in the term jr / P d . a 3 . 
 
 r 3 r- 7 
 
 559. Now let us consider any spheroids whatever. The research of 
 their attraction is reduced, by 553, to forming the quantities U (i) and v (i) , 
 by that No. we have 
 
 U =/R i + 2 .d Rdm'dV. QW; 
 
 in which the integrals must be taken from R = to its value at the sur- 
 face, from m' = 1 to m' = 1, and from */ = to -a' = 2 x. 
 
 To determine this integral, Q W must be known. This quantity may 
 be developed into a finite function of cosines of the angle -a &', and of 
 its multiples. Let /3 cos. n (* -a'} be the term of Q (i > depending on 
 cos. n (a '), /3 being a function m, m'. If we substitute for Q (i) its 
 value in the equation of partial differences in Q (l) of No. 553, we shall 
 have, by comparing the terms multiplied by cos. n (& '), this equation 
 of ordinary differences, 
 
 _ _ 
 
 dm 1 m 2 
 
 R (i > 
 Q (i ' being the coefficient of - . + t , in the developement of the radical 
 
 1 
 
 V r 2 2RrImm'+VT m' 2 . V 1 m*. cos.(w
 
 BOOK L] NEWTON'S PRINCIPIA. 339 
 
 The term depending on cos. n (* '), in the developement of this 
 radical, can only result from the powers of cos. ( */), equal to n, n + 2, 
 n + 4, &c. ; thus cos. (a /) having the factor V 1 m 2 , /3 must have 
 
 the factor (1 m 2 ) ^. It is easy to see, by the consideration of the de- 
 velopement of the radical, that jS is of this form 
 
 m. . m 
 
 If we substitute this value in the differential equation in /3, the compari- 
 son of like powers of m will give 
 
 A- (i~n-2s+2).(i-n-2s+ 1) 
 
 2 s (2 i 2 s + 1) 
 
 whence we derive, by successively putting s = 1, s = 2, &c. the values of 
 A (1) , A (2) , and consequently, 
 
 ( m i-n (i-")(i-"-D m i-n- 2 . (i-n)(i-n-l)(i-n-2)(i-n-3) . 
 
 aW ^2U=T) ra 2. M 2i-l)(2i_3) r i 
 
 j . 1 (i-n)(i-n-l)(i-n-2)(i-n-3)(i-n-4)(i-n-5) f 
 * 2.4.6(2i l)(2i 3) (2 i 5) 
 
 A is a function of m' independent of m ; but m and m' entering alike into 
 the preceding radical, they ought to enter similarly into the expression of 
 (3; we have therefore 
 
 7 being a coefficient independent of m and m' ; therefore 
 
 |8= 7 (1 m' r~ m*- '^' m ' i ~"~ 2 + &c - (1 m 2 )^ X 
 
 i-n _ (i n) (i n 1) ,__ 
 2(2i 1) 
 
 Thus we see that jS is split into three factors, the first independent of 
 m and m' ; the second a function of m' alone ; and the third a like function 
 of m. We have only now to determine 7. 
 
 For that purpose, we shall observe, that if i n be even ; we have, 
 supposing m = 0, and m' = 0, 
 
 , Jl j nl 2 
 
 a y. jl. ^ i n{ 
 
 ' {2. 4. ... (i n). (2 i 1). (2 i 3). . . . (i + n 
 
 7. U- 3. 5....(i n 1). 1.3.5. ...(i + n Ijj* 
 [1.3. 5.... (2i I)} 2
 
 240 A COMMENTARY ON [SECT. XII. & XIII. 
 
 Ifi n is odd, we shall have, in retaining only the first power of m, 
 and m', 
 
 o _ _ y.m.nV fl. 2.. . . (i n)| 2 _ 
 " [2. 4.... (i n 1) (2i l)(2i 3) . . . (i + n + 2)J 
 y.m. m' [I. 3. 5.... (i n) . 1.8. 5.... (i + n)} z 
 
 [I. 3. 5 ---- (2i ])}* 
 The preceding radical becomes, neglecting the squares of m, m', 
 
 {r*-2 R r cos.(*r-*r')+ R 2 }~* + R r. m m' {i*-2r R cos. (-') + R 2 }~* 5 (0 
 If we substitute for cos. (v w'), its value in imaginary exponentials, 
 and if we call c the number whose hyperbolic logarithm is unity, the part 
 independent of m m', becomes 
 
 IT R . c (O V p . {r R . c -( O^f. 
 The coefficient of 
 
 R 1 c n (* ')v i + c n (') v-i Ri 
 
 77+7-- 3 -,orof iTT1 .cos.n( *') 
 
 in the developement of this function is 
 
 2. 1. 3. 5 ---- (i + n 1). 1. 3. 5 ____ (i n 1) 
 
 2. 4. 6 ---- (i + n) 2. 4. 6 ____ (i n) 
 
 This is the value of ]8 when i n is even. Comparing it with that 
 which in the same case we have already found, we shall have 
 
 _ /I. 3. 5.... (2 i IV i(i-I)....(i-n+ 1) 
 \ 1.2.3 ____ i / ^(i+l)(i + 2) ---- (i + n) 
 When n = 0, we must take only half this coefficient, and then we 
 
 have 
 
 _ /I. 3. 5 ____ 2i 1\ 2 
 7 == \ 1.2.3 ---- i / ' 
 
 R 1 
 
 In like manner, the coefficient of . , , m . m' cos. n (-a ') in the 
 
 r i + i 
 
 function (f) is 
 
 2. 1. 3. 5 ____ (i + n) . 1. 3. 5 ____ (i n) 
 2. 4. 6. (i + n 1) . 2. 4. 6 ..... (i n 1) ' 
 
 this is the coefficient of m m 7 in the value of /3, when we neglect the 
 squares of m, m', and when i n is odd. Comparing this with the va- 
 lue already found, we shall have 
 
 , /I. 3. 5. ...(2i IK' i(i 1) ..... (i n + 1. 
 '\ 1.2.3 ---- i ; ' (i+1) (i + 2) ---- (i + n)' 
 an expression which is the same as in the case of i n being even. 
 If n = 0, we also have 
 
 /I. 3. 5. ... (2 i l)x 
 7 ~ \ 1. 2. 3. . 71 /
 
 BOOK I.] NEWTON'S PRINCIPIA. 241 
 
 560. From what precedes, we may obtain the general form of functions 
 Y > of m, V 1 m z . sin. , and V 1 m 2 . cos. w, which satisfy the 
 equation of partial differences 
 
 Designating by /3, the coefficient of sin. n -a, or of cos. n ^ y in the 
 function Y (i) , we shall have 
 
 d {(!_,.)(")} 
 l v Ml m/ ) 
 
 n * p 
 
 - - - - 
 
 3 is equal to (I m z ) ^ multiplied by a rational and entire function of m, 
 and in this case, by the preceding No., we have 
 
 ^"^ + &c. 
 
 j, ' 
 
 A (n) being an arbitrary constant ; thus the part of Y (i > depending on the 
 angle n -a, is 
 
 + B (n > cos. n *\ ; 
 
 A (n) and B (n) being two arbitrages. If we make successively in this func- 
 tion, n = 0, n = 1, n =r 2 . . . n = i ; the sum of all the functions which 
 thence result, will be the general expression of Y (i) , and this expression 
 will contain 2 i + 1 arbitraries B <>, A (1 >, B , A , B , &c. 
 
 Let us now consider a rational and entire function S of the order s, 
 of the three rectangular coordinates x, y, z. If we represent by R the 
 distance of the point determined by these coordinates from their origin ; 
 by 6 the angle formed by R and the axis of x ; and by -a the angle which 
 the plane of x, y forms with the plane passing through R and the axis of 
 x ; we shall have 
 
 x = Rm;y = R. VI m 2 . cos. ; z = RV 1 in 2 , sin. &. 
 
 Substituting these values in S, and developing this function into sines 
 and cosines of the angle -a and its multiples, if S is the most general func- 
 tion of the order s, then sin. n w 9 and cos. n -sr t will be multiplied by func- 
 tions of the form 
 
 n 
 
 (1 m 2 ) T fA .m s - n + B.m 8 - 11 - 1 + C.m s ~ n - 2 + &c.} ; 
 
 thus the part of S, depending on the angle n &, will contain 2 (s n -f- 1 ) 
 
 indeterminate constants. The part of S depending on the angle -a and its 
 
 .multiples will contain therefore s (s -f- 1) in determinates ; the part inde- 
 
 VOL. II. Q
 
 A COMMENTARY ON [SECT. XII. & XIII. 
 
 pendent of r will contain s + 1, and S will therefore contain (s + 1) * 
 indeterminate constants. 
 
 The function Y (0 > + Y (1 > + &c. Y (s > contains in like manner (s + 1) 2 
 indeterminate constants, since the function Y (i) contains 2 i + 1 ; we may 
 therefore put S into a function of this form, and this may be effected as 
 follows : 
 
 From what precedes we shall learn the most general expression of Y (s) , 
 we shall take it from S and determine the arbitrages of Y (s) so that the 
 powers and products of m and V 1 m 2 of the order s shall disappear 
 from the difference S Y w ; this difference will thus become a function 
 of the order s 1 which we shall denote by S'. We shall take the most 
 general expression of Y (s ~ !) ; we shall subtract it from S', and determine 
 the arbitraries of Y (i 1) so that the powers and products of m and 
 V 1 m 2 of the order s 1 may disappear from the difference 
 S' Y (s 1) . Thus proceeding we shall determine the functions Y (s) , 
 Y*- , <-*>, &c. of which the sum is S. 
 
 561. Resume now, the equation of No. 559, 
 
 U =/.R i + 2 dR.dm'. d*r'. Q . 
 
 Suppose R a function of m', -a' and of a parameter a, constant for all 
 shells of the same density, and variable from one shell to another. The 
 difference d R being taken on the supposition that m', *' are constant we 
 shall have 
 
 therefore 
 
 U <" = -St d a . d m'. d ,'. Q 
 
 Let R l + 3 be developed into a series of the form 
 
 Z'W + Z'W + Z'< + &c,, 
 
 Z' (i) being whatever i may be, a rational and entire function of m', 
 ^ \ _ m ^2. s i n . .', and V 1 m 7 2 . cos. a-', which satisfies the equation 
 of partial differences 
 
 "din 7 " / 1 m' 2 
 
 The difference of 7J (i) taken relatively to a, satisfies also this equation, 
 and consequently it is of the same form ; by the general theorem of 556, 
 we ought therefore only to consider the term Z /(i) in the developement of 
 R ' + 3 , and then we have
 
 BOOK L] NEWTON'S PRINCIPIA. 243 
 
 When the spheroid is homogeneous and differing but little from a 
 sphere, we may suppose g = 1, and R = a (1 + a y') ; then we have, by 
 integrating relatively to a 
 
 U m = j-1^ / Z' W. d m' . d ->. Q . 
 Moreover, if we suppose y' developed into a series of the form 
 
 Y e) satisfying the same equation of partial difference as Z' (i) ; we shall have, 
 neglecting quantities of the order a 2 , Z' (1) = (i + 3). a. a' + 3 Y /(i) ; we 
 shall therefore have 
 
 U W = a . a 1 + 3 ./Y' W. d m'. d w'. Q < ! >. 
 
 If we denote by Y (i) what Y' (i) becomes when we change m' and -a' into 
 m and -a ; we shall have by No. 554, 
 
 2i + 1 
 we therefore have this remarkable result, 
 
 .dm\d^Q=f- ...... (1) 
 
 This equation subsisting whatever may be Y' w we may conclude ge- 
 nerally that the double integration of the function fTJ w dm', d *r'. Q W 
 taken from m' = 1 to m' = 1, and from / = to ' = 2 *, only 
 
 transforms Z' (i) into ^ =-; Z w being what Z' (i) becomes when we 
 
 /w 1 "J~ X 
 
 change m' and -a' into m and -a ; we therefore have 
 
 TT m 4 * /- /d Z (i) \ i 
 
 = (i + s)(8i+i)M-cnr)- da; 
 
 and the triple integration upon which U w depends, reduces to one in- 
 tegration only taken relatively to a, from a = to its value at the surface 
 of the spheroid. 
 
 The equation (1) presents a very simply method of integrating the func- 
 tion f Y (i) . Z w . dm. d -sf, from m = 1 to m = 1, and from * = 
 to * = 2 v. In fact, the part of Y (i) depending on the angle n w, is by 
 what precedes, of the form X {A (n) sin. n -a + B (n) cos. n }, X being 
 equal to 
 
 &c. 
 
 } ; 
 
 we shall have therefore 
 
 Y' W = x' A {n > sin. n J + B W cos. n ^} ; 
 
 X' bemg what X becomes when m is changed into m'. The part of Q (l) 
 depending on the angle n ~, is by the preceding No., 7 X X' cos. n ( 
 
 Q2
 
 244, A COMMENTARY ON [SECT. XII. & XIII. 
 
 or y X'. X.^cos. n . cos. n w' + sin. n v. sin. n w'| ; thus that part of the 
 integral/ Y (i) . d m. d w'. Q which depends on the angle n -, will be 
 7 A. sin. n ./X' 2 . d m'. d *'. sin. n */ [A < n > sin. n */ + B W cos. n i*/} 
 7 X. cos. n vf X' 2 . d m'. d &'. cos. n -a' [A (n) sin. n &' + B (n) cos. n '}. 
 Executing the integrations relative to ', that part becomes 
 
 7 X w {A tn} sin. n + B (n ^ cos. n wj./x' 2 . d m' ; 
 but in virtue of equation (1), the same part is equal to 
 
 ^: . *. jA (n > sin. n v + B w cos. ri ? 
 2 i + 1 
 
 we therefore have 
 
 /X". d m' = /CT . f .. . 
 
 (2i + 1} 7 
 
 Now represent by X {A /(n) sin. n + B /(n) cos. n w| that part of Z (i) 
 which depends on the angle n -a. This part ought to be combined with 
 the corresponding part of Y (i) ; because the terms depending on the sines 
 and cosines of the angle and its multiples, disappear by integration, in 
 the function/ Y (i) Z (i) d m . d , integrated from w = Otow = 2T;we 
 shall thus obtain, in regarding only that part of Y w which depends on 
 the angle n w, 
 
 /Y . Z W d m d . = 
 /X 2 . d m . d *{ A (n) sin. n + B ^ cos. n H A' (n) sin. n * + B' (n > cos. n J 
 
 = J A w. A' ^ + B w. B' wj ./x ! d m = . 4 ff . . {A w. A' w + B ^).B' w j . 
 
 Supposing therefore successively in the last member n = 0, n = 1, 
 n = 2 . . . n = i ; the sum of all the terms, will be the value of the in- 
 tegral/ Y W Z d m . d v . 
 
 If the spheroid is one of revolution, so that the axis with which the ra- 
 dius R forms the angle w, may be the axis of revolution ; the angle w will 
 disappear from the expression of Z (i >, which then takes the following 
 form : 
 
 A W being a function of a. Call X W the coefficient of A w , in this func- 
 tion : the product 
 
 1.8.5...(2i-lK| Mi-l). i &c V 
 
 -- 1.2.3. ..i - )'\ l - 2 .(2i-l) 4 C '/ 5 
 
 R' 
 
 is by the preceding No., the coefficient of 7 r^ r - l in the developement of 
 
 the radical 
 
 2Rr{mm'+V 1 m 2 . V 1 m /2 cos. (*
 
 BOOK I.] NEWTON'S FRINCIPIA. 245 
 
 when we therein suppose m and m' equal to unity. This coefficient is 
 then equal to 1 ; we have therefore 
 
 1.3 5...(2i-l) i(j-l) 
 
 1.8.3. ..i \ 2(2i 
 
 that is to say, X W reduces to unity, when m = 1. We have then 
 
 4 cr X G) r /a A 
 
 = ( i + 3).(2i + i)- /g ("ai 
 
 Relatively to the axis of revolution, m = 1, and consequently, 
 TT m 4 * d A (i 
 
 = - 
 
 therefore if we suppose that relatively to a point placed upon this axis 
 produced, we have 
 
 B (0 > B (1 > B <*> 
 V = + ^ v - + r+ &c.; 
 
 I > % y. 5 
 
 we shall have the value of V relative to another point placed at the mean 
 distance from the origin of coordinates, but upon a radius which makes 
 with the axis of revolution, an angle whose cosine is m ; by multiplying 
 the terms of this value respectively by X c % X (1 >, X , &c. 
 
 In the case when the spheroid is not of revolution, this method will 
 give the part of V independent of the angle : we shall determine the 
 other part in this manner. Suppose for the sake of simplicity, the sphe- 
 roid such that it is divided into two equal and similar parts by the equa- 
 tor, whether by the meridian where we fix the origin of the angle &, or 
 by the meridian which is perpendicular to the former. Then V will be 
 a function of m 2 , sin. z , and cos. 2 &, or which comes to the same, it will 
 be a function of m 2 , and of the cosine of the angle 2 and its multiples ; 
 U (i) will therefore be nothing, when i is odd, and in the case when it is 
 even, the term which depends on the angle 2 n -a, will be of the form 
 
 Relatively to an attracted point placed in the plane of the equator, 
 where m = 0, that part of V which depends on this term becomes 
 
 -r T +^* 2 (i + 2 n + 1) (i + 2 n + 2) . . . (2 i - 1) * S ' 2 
 whence it follows that having developed V into a series ordered according 
 to the cosines of the angle 2 -a and its multiples, when the point attracted 
 is situated in the plane of the equator : to extend this value to any attract- 
 ed point whatever, it will be sufficient to multiply the terms which depend 
 
 cos. 2 n rr 
 on ^ j by the function 
 
 Q3
 
 246 A COMMENTARY ON [SECT. XII. & XIII. 
 
 - 1. 3. 5 . . . (i 2 n 1 ) * ^ 2 (2 i 1) 
 
 m i-2n-2 + &C.J; 
 
 we shall hence obtain, therefore, the entire value of V, when this value 
 shall be determined in a series, for the two cases where the part attracted 
 is situated upon the polar axis produced, and where it is situated in the 
 plane of the equator ; this greatly simplifies the research of this value. 
 
 The spheroid which we are considering comprehends the ellipsoid. 
 Relatively to an attracted point situated upon the polar axis, which we 
 shall suppose to be the axis of x, by 546, we have b = 0, c = 0, and 
 then the expression of V of No. 549, is integrable relatively to p. Rela- 
 tively to a point situated in the plane of the equator, we have a = 0, and 
 the same expression of V still becomes, by known methods, integrable re- 
 latively to q, by making tan. q = t. In the two cases, the integral being 
 taken relatively to one of these variables in its limits, it then becomes 
 possible relatively to the other, and we find that M being the mass of 
 
 y 
 the spheroid, the value of ^ is independent of the semi-axis k of the 
 
 spheroid perpendicular to the equator, and depends only on the ex- 
 centricities of the ellipsoid. Multiplying therefore the different terms 
 
 V 
 
 of the values of j-* relative to these two cases, and reduced into se- 
 ries proceeding according to the powers of - , by the factors above men- 
 tioned, to get the value of -^ relative to any attracted point whatever; the 
 
 function which thence results will be independent of k, and only depend 
 on the excentricities ; this furnishes a new demonstration of the theorem 
 already proved in 550. 
 
 If the point attracted is placed in the interior of the spheroid, the at- 
 traction which it undergoes, depends, as we have seen in No. 553, on the 
 function v (i; , and by the No. cited, we have 
 /. p d R d m' d -m. Q w 
 
 = 
 
 an equation which we can put under this form 
 
 v " = szn -ft (T^) d a . d m'. d '. Q 
 Suppose R 2 - 1 developed into a series of the form 
 
 z 
 
 z ' U) + 2! + &c.
 
 BOOK L] NEWTON'S PR1NCIPIA. 247 
 
 z' W satisfying the equation of partial differences, 
 
 if moreover we call z (i) what z (i > becomes when we change m' into m, and 
 or' into w, we shall have by what precedes, 
 4 T /d z 
 
 s 
 
 thus therefore we shall get the expression of V relative to all the shells of 
 the spheroid which envelope the point attracted. The value of V relative 
 to shells to which it is interior, we have already shown how to deter- 
 mine. 
 
 ON THE FIGURE OF A FLUID HOMOGENEOUS MASS IN EQUILIBRIUM, 
 AND ENDOWED WITH A ROTATORY MOTION. 
 
 562. Having exposed in the preceding Nos. the theory of the attrac- 
 tions of spheroids, we now proceed to consider the figure which they 
 must assume in virtue of the mutual action of their parts, and the other 
 forces which act upon them. We shall first seek the figure which satis- 
 fies the equilibrium of a fluid homogeneous mass endowed with a rotatory 
 motion, and of that problem we shall give a rigorous solution. 
 
 Let a, b, c be the rectangular coordinates of any point of the surface of 
 the mass, and P, Q, R the forces which solicit it parallel to the coordi- 
 nates, the forces being supposed as tending to diminish them. We know 
 that when the mass is in equilibrium, we have 
 
 = P. d a + Q. d b + R. d c; 
 
 provided that in estimating the forces P, Q, R, we reckon the centrifugal 
 force due to the motion of rotation. 
 
 To estimate these forces, we shall suppose that the figure of the fluid 
 mass, is that of the ellipsoid of revolution, whose axis of rotation, is the axis 
 itself of revolution. If the forces P, Q, R which result from this hypothe- 
 sis, substituted in the preceding equation of equilibrium give the differen- 
 tial equation of the surface of the ellipsoid ; the preceding hypothesis is 
 legitimate, and the elliptic figure satisfies the equilibrium of the fluid 
 mass. 
 
 Suppose that the axis of a is that also of revolution ; the equation of 
 the surface of the ellipsoid will be of this form 
 a s + m (b 2 + c 2 ) =k 2 ; 
 Q4,
 
 248 A COMMENTARY ON [SECT. XII. & XIII. 
 
 the origin of the coordinates a, b, c being at the center of the ellipsoid, 
 k will be the semi-axis of revolution, and if we call M the mass of the el- 
 lipsoid, by 546, we shall have 
 
 AT 4*?k 3 
 M = 
 
 3 m 
 
 1 --m 
 
 m 
 
 f being the density of the fluid. If we make as in 547, - = X s ,-we 
 shall have m = . 8 , and consequently 
 
 1 -f- A 
 
 an equation which will give the semi-axis k, when X is known. 
 Let 
 
 B' = ^| 5(1+^) tan. -'X X)}; 
 
 we shall have by 547, regarding only the attraction of the fluid mass 
 
 P = A' a; Q = B'b; R = B' c. 
 
 If we call g, the centrifugal force at the distance 1, from the axis of 
 rotation ; this force at the distance V b 2 + c 2 from the same axis, will 
 be g V b 2 -f- c ! : resolving this parallel to the coordinates b, c there will 
 result in Q the term g b, and in R the term g c; thus we shall have, 
 reckoning all the forces which animate the molecules of the surface, 
 
 P = A'a; Q = (B'-g)b; R = (B'-g).c; 
 the preceding equation of equilibrium, will therefore become 
 
 TV _ 
 
 = a d a H -- jy-Q (b d b + c d c). 
 
 A 
 
 The differential equation of the surface of the ellipsoid is by substitut- 
 
 ing for m its value = 5 
 I + >. 
 
 . b d b + c d c 
 5 = ada+~- i-f^T- ; 
 
 by comparing this with the preceding one, we shall have 
 
 (1 + >- 2 )(B'-g) = A'; ........ (1) 
 
 if we substitute for A', B' their values, and if we make J^ q; we shall 
 
 -s* 
 
 have
 
 BOOK I.J NEWTON'S PRINCIPIA. 249 
 
 determining therefore X by this equation which is independent of the co- 
 ordinates a, b, c, the equation of equilibrium will coincide with the equa- 
 tion of the surface of the ellipsoid ; whence it follows, that the elliptic fi- 
 gure satisfies the equilibrium, at least, when the motion of rotation is such 
 that the value of X 2 is not imaginary, or when being negative, it is neither 
 equal to nor greater than unity. The case where X 2 is imaginary would 
 give an imaginary solid; that where X 2 = 1, would give a paraboloid, 
 and that where X 2 is negative and greater than unity, would give a hy- 
 perboloid. 
 
 563. If we call p the gravity at the surface of the ellipsoid, we shall 
 have 
 
 p = V P 2 + Q a + R 2 . 
 
 In the interior of the ellipsoid, the forces P, Q, R, are proportional to 
 the coordinates a, b, c ; for we have seen in No. 547, that the attractions 
 of the ellipsoid, parallel to these coordinates, are respectively proportional 
 to them, which equally takes place for the centrifugal force resolved pa- 
 rallel to the same coordinates. Hence it follows, that the gravities at dif- 
 ferent points of a radius drawn from the center of the ellipsoid to its sur- 
 face, have parallel directions, and are proportional to the distances from 
 the center ; so that if we know the gravity at its surface, we shall have 
 the gravity in the interior of the spheroid. 
 
 If in the expression of p, we substitute for P, Q, R, their values given 
 in the preceding No., we shall have 
 
 p = V A /2 a 2 -f- (B' g) . (b 8 + c); 
 whence we derive, in virtue of equation (1) of the preceding No. 
 
 p = 
 
 b 2 + c 2 
 but the equation of the surface of the ellipsoid gives -j , X8 = k 2 a 2 ; 
 
 we shall therefore have 
 
 1 + x 2 
 
 a is equal to k at the pole, and it is nothing at the equator ; whence it fol- 
 lows, that flie gravity at the pole is to the gravity at the equator, as 
 V 1 4- X 2 is to unity, and consequently, as the diameter of the equator 
 is to the polar axis. 
 
 Call t the perpendicular at the surface of the ellipsoid, produced to 
 meet the axis of revolution, we shall have 
 
 t = V (I + X 2 ) (k 2 + X-a 2 ) ;
 
 250 A COMMENTARY ON [SECT. XII. & XIII. 
 
 wherefore 
 
 A't 
 P = 
 
 1 + X s ' 
 thus gravity is proportional to t. 
 
 Let \j/ be the complement of the angle which t makes with the axis of 
 revolution ; -\}/ will be the latitude of the point of the surface, which we 
 are considering, and by the nature of the ellipse, we shall have 
 
 
 V 1 + X 2 cos. 2 4' 
 we therefore shall have 
 
 - A'k 
 
 V I + X 2 . cos. 2 V 
 and substituting for A' its value, we shall get 
 
 - *<rg.k.(l + X').(X tan.-'x) . 
 
 X 3 V 1 + X 2 . cos.'-v}/ 
 
 this equation gives the relation between gravity and the latitude ; but we 
 must determine the constants which it contains. 
 
 Let T be the number of seconds in which the fluid mass will effect a 
 revolution ; the centrifugal force at the distance 1 from the axis of revo- 
 
 4 cr 2 
 lution, will be equal to -rpY ; we therefore have 
 
 g 12ff 2 
 
 1= 4r.,- 4* f T 2; 
 which gives 
 
 12. cr 2 
 
 The radius of curvature of the elliptic meridian is 
 
 (1 + X 2 COS. 2 -4/) 
 
 calling therefore c the length of a degree at the latitude -sj/, we shall have 
 
 = 200 c. 
 
 This equation combined with the preceding one, gives 
 
 thus we shall have 
 
 onn ft a i \ ^ - tan. 1 X 12 
 
 p = 200 c(l + X 2 cos. 2 -4.) - - . 
 
 q 
 
 Let 1 be the length of the simple pendulum which oscillates seconds ;
 
 BOOK I.J NEWTON'S PRINC1PIA. 251 
 
 from dynamics it results that p = -r 2 1 (seeX.) ; comparing these two 
 expressions of p, we get 
 
 - 2400 c (X tan.- 1 X) (1 + X g cos. 2 4) t 
 
 T 1 1 2 X 3 
 
 this equation and equation (2) of the preceding No. will give the values 
 of q and X by means of the length 1 of the seconds' pendulum, and the 
 length c of the degree of the meridian, both being observed at the lati- 
 tude -^. 
 
 Suppose -^ = 50, these equations will give 
 
 800 c l /800 c 
 q ~ rlT 2 ~~* WTT 1 
 
 observations give, as we shall see hereafter, 
 c = 100000;!= 0.741608; 
 
 moreover we have T = 99727 ; we shall thus obtain 
 q = 0.00344957; X 2 = 0.00868767. 
 
 The ratio of the axis of the equator to the polar axis, being V 1 + X 2 , 
 it becomes in this case 1.00433441 ; these two axes are very nearly in 
 the ratio of 231.7 to 230.7, and by what precedes, the gravities at the 
 pole and at the equator are in the same ratio. 
 
 We shall have the semi polar axis k, by means of the equation 
 
 which gives 
 
 k = 6352534. 
 
 To get the attraction of a sphere, whose radius is k, and density any 
 whatever ; we shall observe that a sphere, having the radius k and density 
 g, acts upon a point placed at its surface, with a force equal to f it . k, 
 
 X 3 p V 1 + X s 
 and consequently, in virtue of equation (3) equal to 4,% g\ />- " -*xl ' 
 
 or to p (l ^ X 2 + &c.V or finally to 0.998697. p, p being the gravi- 
 ty upon the parallel of 50. Hence it is easy to obtain the attractive force 
 of a sphere of any radius and density whatever, upon a point placed with- 
 in or without it. 
 
 564. If the equation (2) of No. 562, were susceptible of many real 
 roots, many figures of equilibrium might result from the same motion of 
 rotation ; let us examine therefore whether this equation has several real
 
 252 A COMMENTARY ON [SECT. XII. & XIII. 
 
 Q X 1 2 O X J 
 
 roots. For that purpose, call p the function ' -- tan.- l X, 
 
 y -j- o A 
 
 which being equated to zero, produces the equation (2). It is easy to see, 
 that by making X increase from zero to infinity, the expression of <p begins 
 and ends by being positive ; thus, by imagining a curve whose abscissa is 
 X and ordinate p, this curve will cut its axis when X, = ; the ordinates 
 will afterwards be positive and increasing ; when arrived at their maxi- 
 mum, they will decrease; the curve will cut the axis a second time at a 
 point which will determine the value of X corresponding to the state of 
 equilibrium of the fluid mass ; the ordinates will then be negative, and 
 since they are positive when X = oo ; the curve necessarily cuts the axis 
 a third time, which determines a second value of X which satisfies the 
 equilibrium. Thus we see, that for one and the same value of q, or for 
 one given motion of rotation, there are several figures for which the 
 equilibrium may subsist. 
 
 To determine the number of these figures, we shall observe, that we 
 have 
 
 , 6 X g d X {q X 4 + (10 q 6) X*+ 9 q} 
 
 (3X 2 +9) 2 . (1 + X 2 ) 
 
 The supposition of d <p = 0, gives 
 
 = q X 4 + (10 q 6) X 2 + 9 q; 
 whence we derive, considering only the positive values of X 
 
 These values of X determine the maxima and minima of the ordinate <p ; 
 there being only two similar ordinates on the side of positive abscissas, on 
 that side the curve cuts its axis in three points, one of them being the 
 origin ; thus, the number of figures which satisfy the equilibrium is reduc- 
 ed to two. 
 
 The curve on the side of negative abscissas being exactly the same as 
 on the side of positive abscissas; it cuts its axis on each side in corre- 
 sponding points equidistant from the origin of coordinates ; the negative 
 values of X which satisfy the equilibrium, are therefore, as to the sign 
 taken, the same as the positive values ; which gives the same elliptic fi- 
 gures, since the square of X only enters the determination of these figures ; 
 it is useless therefore to consider the curve on the side of negative ab- 
 scissas. 
 
 If we suppose q very small, which takes place for the earth, we may 
 satisfy equation (2) of 562, in the two hypotheses of X s being very small,
 
 BOOK L] NEWTON'S PRINCIPIA. 253 
 
 and of X 2 being very great. In the first, by the preceding No., we 
 have 
 
 K IJti 
 
 a *J O|O 
 
 To get the value of X 2 in the second hypothesis, we shall observe that 
 then tan.- z X differs very little from \ n, so that if we suppose X = - , 
 
 a will be a very small angle of which the tangent is - ; we shall there- 
 
 A 
 
 fore have, p. 27. Vol. I. 
 
 and consequently 
 
 . - 3 - 5 . 
 
 equation (2) of No. 562, will thus become 
 
 9X+2q.XS__ i,J_ &c . 
 
 9 + 3X 2 "2 X^3X 3 
 whence by the reversion of series we get 
 
 3 v 8 4 q / , 64 N 
 
 X = + - 3-(l _ ; -) + &C. 
 
 4q * * \ 3cr 2 / 
 
 = 2.356195. -L 2.546479 1.478885 q + &c. 
 
 We have seen in the preceding No., that relatively to the earth, 
 q = 0.00344957 ; this value of q substituted in the preceding expression, 
 gives X = 680.49. Thus the ratio of the two axes equatorial and polar, 
 a ratio which is equal to V 1 + X 2 , is in the case of a very thin spheroid, 
 equal to 680.49. 
 
 The value of q has a limit beyond which the equilibrium is impossible, 
 the figure being elliptic. Suppose, in fact, that the jurve cuts its axis 
 only at its origin, and that in the other points it only touches; at the 
 point of contact we shall have <p =r 0, and d <p = ; the value of <p will 
 never therefore be negative on the side of positive abscissas, which are 
 the only ones we shall here consider. The value of q determined by the 
 two equations p = 0, d p = 0, will therefore be the limit of those with which 
 the equilibrium can take place, so that a greater value will render the 
 equilibrium impossible ; for q being supposed to increase by f, the func- 
 
 2 f X 3 
 tion <p increases by the term 2 ; thus, the value of <p correspond- 
 
 y "y- O A 
 
 ing to q, being never negative, whatever X may be, the same function cor- 
 responding to q + f, is constantly positive, and can never become no-
 
 254 A COMMENTARY ON [SECT. XII. & XIII. 
 
 thing; the equilibrium is then therefore impossible. It results also from 
 this analysis, that there is only one real and positive value of q, which 
 would satisfy the two equations <p = 0, and d <p = 0. These equations 
 give 
 
 q = 
 
 7 X 5 + 30 X 3 + 27 X _ 1 
 
 = (i + x*)(3 + X')(9 + X')- 
 
 The value of X which satisfies this last equation is X = 2.5292 ; whence 
 we get q = 0.337007 ; the quantity V 1 + X 2 , which expresses the ra- 
 tio of the equatorial axis to the polar axis, is in this case equal to 2.7197. 
 
 The value of q relatively to the earth is equal to 0.00344957. This 
 value corresponds to a tune of rotation of 0.99727 days ; but we have 
 
 generally q = ^ , so that relatively to masses of the same density, q is 
 
 J b 
 
 proportional to the centrifugal force g of the rotatory motion, and conse- 
 quently in the inverse ratio of the square of the time of rotation; whence 
 it follows, that relatively to a mass of the same density as the earth, the 
 time of rotation which answers to q = 0.337007, is 0.10090 days. Whence 
 .result these two theorems. 
 
 " Every homogeneous fluid mass of a density equal to the mean density 
 of the earth, cannot be in equilibrium having an elliptic figure, if the time 
 of its rotation is less than 0.10090 days. If this time be greater, there 
 will be always two elliptic figures and no more which satisfy the equili- 
 brium." 
 
 " If the density of the fluid mass is different from that of the earth ; we 
 shall have the time of rotation in which the equilibrium ceases to be pos- 
 sible under an elliptic figure, by multiplying 0.10090 days by the square 
 root of the ratio of the mean density of the earth to that of the fluid 
 mass." 
 
 This relatively to a fluid mass, whose density is only a fourth part of 
 that of the earth, which nearly is the case with the sun, this time will be 
 0.20184 days; and if the density of the earth supposed fluid and homo- 
 geneous were about 98 times less than its actual density, the figure which 
 it ought to take to satisfy its actual motion of rotation, would be the limit 
 of all the elliptic figures with which the equilibrium can subsist. The 
 density of Jupiter being about five times less than that of the eai'th. and 
 the time of its rotation being 0.41377 days; we see that this duration is 
 in the limits of those of equilibrium.
 
 BOOK I.] NEWTON'S PRINCIPIA. 255 
 
 It may be thought that the limit of q, is that where the fluid would be- 
 gin to fly off by reason of a too rapid motion of rotation ; but it is easy to 
 be convinced of the contrary, by observing that by 563, the gravity at the 
 equator of the ellipsoid is to that at the pole in the ratio of the polar axis 
 to that of the equator, a ratio which in this case, is that of 1 to 2.7197 ; 
 the equilibrium ceases therefore to be possible, because with a motion of 
 rotation more rapid, it is impossible to give to the fluid mass, an elliptic 
 figure such that the resultant of its attraction and of the centrifugal force, 
 may be perpendicular to the surface. 
 
 Hitherto we have supposed X 2 positive, which gives the spheroids flat- 
 tened towards the poles ; let us now examine whether the equilibrium can 
 subsist with a figure lengthened towards the poles, or with a prolate sphe- 
 roid. Let X * = X' 2 ; X' 2 must be positive and less than unity, otherwise, 
 the ellipsoid will be changed into a hyperboloid. The preceding value 
 of d f gives 
 
 /.X 2 d X fq X 4 + (10 q 6) X 2 + 9 q} . 
 ~ J ' (1 + X 2 ) (9 + 3 X 2 ) 2 
 
 the integral being taken from X = 0. Substituting for X its value + X' V - 1, 
 we shall have 
 
 _ --T f 
 
 l 'J ' (i _x /2 ) (93 X' 2 ) 
 
 but it is evident that the elements of this last integral are all of the same 
 sign from X' 2 = 0, to X' 2 = 1 ; the function <p can therefore never be- 
 come nothing in this interval. Thus then the equilibrium cannot subsist 
 in the case of the prolate spheroid. 
 
 565. If the motion of rotation primitively impressed upon the fluid 
 mass, is more rapid than that which belongs to the limit of q, we must 
 not thence infer that it cannot be in equilibrium with an elliptic figure ; 
 for we may conceive, that by flattening it more and more, it will take a 
 rotatory motion less and less rapid ; supposing therefore that there exists, 
 as in the case of all known fluids, a force of tenacity between its mole- 
 cules, this mass, after a great number of oscillations, may at length arrive 
 at a rotatory motion, comprised within the limits of equilibrium, and may 
 continue in that state. But this possibility it would also be interesting to 
 verify ; and it would be equally interesting to know whether there would 
 not be many possible states of equilibrium ; for what we have already de- 
 monstrated upon the possibility of two states of equilibrium, correspond- 
 ing to one motion of rotation, does not infer the possibility of two states 
 of equilibrium corresponding to one primitive force; because the two
 
 256 A COMMENTARY ON [SECT. XII. & XIII. 
 
 states of equilibrium relative to one motion of rotation, require two pri- 
 mitive forces, either different in quantity or differently applied. 
 
 Consider therefore a fluid mass agitated primitively by any forces what- 
 ever, and then left to itself, and to the mutual attractions of all its parts. 
 If through the center of gravity of this mass supposed immoveable, we 
 conceive a plane relatively to which the sum of the areas described upon 
 this plane, by each molecule, multiplied respectively by the correspond- 
 ing molecules, is a maximum at the origin of motion; this plane will 
 always have this property, whatever may be the manner in which the 
 molecules act upon one another, whether by their tenacity, by their attrac- 
 tion, and their mutual collision, even in the very case where there is finite 
 loss of motion in an instant of time ; thus, when after a great number of 
 oscillations, the fluid mass shall take a uniform rotatory motion about a 
 fixed axis, this axis shall be perpendicular to the plane above-mentioned, 
 which will be that of the equator, and the motion of rotation will be such 
 that the sum of the areas described during the instant d t, by the mole- 
 cules projected upon this plane, will be the same as at the origin of mo- 
 tion ; we shall denote by E d t this last sum. 
 
 We shall here observe, that the axis in question, is that relatively to 
 which the sum of the moments of the primitive forces of the system was a 
 maximum. It retains this property during the motion of the system, and 
 finally becomes the axis of rotation ; for what is above asserted as to the 
 plane of the maximum of projected areas, equally applies to the axis of the 
 greatest moment of forces ,- since the elementary area described by the pro- 
 jection of the radius- vector of a body upon a plane, and multiplied by its 
 mass, is evidently proportional to the moment of the finite force of this 
 body relatively to the axis perpendicular to this plane. 
 
 Let, as above, g be the centrifugal force due to the rotatory motion at 
 the distance 1 from the axis; V g will be the angular velocity of rotation 
 (p. 166. Vol. I.).; then call k the semi-axis of rotation of the fluid mass, 
 and k V 1 + X * the semi-axis of its equator. It is easy to show that 
 the sum of the areas described during the instant d t, by all the molecules 
 projected upon the plane of the equator and multiplied respectively by the 
 corresponding molecules, is 
 
 1^(1 +>.*)*. k'dt Vg 
 we shall therefore have 
 
 lll(l +AT.fc s /g = E.
 
 BOOK I.] NEWTON'S PR1NCIPIA. 257 
 
 Then calling M, the fluid mass, we shall have 
 
 the quantity *-* -r which we have called q, in No. 562, thus becomes 
 
 ? 25 E 2 (^ cr P) * 
 q' (1 + X ) 3 , denoting by q' the function ivnT" ^ ne e( l ua ~ 
 
 tion of the same No. becomes 
 
 _, 9X + 8q'X3(l+XV 
 
 9 + 3X 2 
 
 This equation will determine X ; we shall then have k by means of the 
 preceding expression of M. 
 Call <p the function 
 
 9 + 3X 2 
 
 which, by the condition of equilibrium, ought to be equal to zero : this 
 equation begins by being positive, when X is very small, and ends by being 
 negative, when X is infinite ; there exists therefore between X = 0, and 
 X = infinity, a value of X which renders this function nothing, and conse- 
 quently, there is always, whatever q' may be, an elliptic figure, with which 
 the fluid mass may be in equilibrium. 
 
 The value of <p may be put under this integral form 
 
 18q'-{q'X 2 + 18(1 + X 2 ) 
 
 When it becomes nothing the function 
 
 (9 + 3X"-) 2 (1 + x 
 181 
 
 X 
 
 has already passed through zero to become negative ; but from the in- 
 stant when this function begins to be negative, it continues to be so as X 
 
 27 q' 
 increases, because the positive part + 18 q' decreases whilst the ne- 
 
 A 
 
 gative part q' X 8 + 18 (1 + X 2 ) 3 ] increases ; the function f cannot 
 therefore twice become nothing ; whence it follows, that there is but one 
 real and positive value of X which satisfies the equation of equilibrium, 
 and consequently, the fluid can be in equilibrium with one elliptic figure 
 only. 
 
 VOL. II. U
 
 253 A COMMENTARY ON [SECT. XII. & XIII. 
 
 ON THE FIGURE OF A SPHEROID DIFFERING VERY LITTLE FROM A SPHERE, 
 AND COVERED WITH A SHELL OF FLUID IN EQUILIBRIUM. 
 
 566. We have already discussed the equilibrium of a homogeneous 
 fluid mass, and we have found that the elliptic figure satisfies this equili- 
 brium; but in order to get a complete solution of the problem, we must 
 determine a priori all the figures of equilibrium, or we must be certified 
 that the elliptic is the only figure which will fulfil these conditions; be- 
 sides, it is very probable that the celestial bodies have not homogeneous 
 masses, and that they are denser towards the center than at the surface. 
 In the research, therefore, of their figure, we must not rest satisfied with 
 the case of homogeneity ; but then this research presents great difficul- 
 ties. Happily it is simplified by the consideration of the little difference 
 which exists between the spherical figure and those of the planets and 
 satellites ; by which we are permitted to neglect the square of this differ- 
 ence, and of the quantities depending on it. Notwithstanding, the research 
 of the figure of the planets is still very complex. To treat it with the 
 greatest generality, we proceed to consider the equilibrium of a fluid mass 
 which covers a body formed of shells of variable density, endowed with 
 n rotatory motion, and sollicited by the attraction of other bodies. For 
 that purpose, we proceed to recapitulate the laws of equilibrium of fluids, 
 as laid down in works upon hydrostatics. 
 
 If we name the density of a fluid molecule, II the pressure it sustains, 
 F, F', F", Sec. the forces which act upon it, and d f, d f ', d f " the ele- 
 ments of the directions of these forces; then the general equation of the 
 equilibrium of the fluid mass will be 
 
 - = F d f + F' d f ' + F" d f " + &c. 
 S 
 
 Suppose that the second member of this equation is an exact difference ; 
 designating by d <f> this difference, will necessarily be a function of II and 
 of <p: the integral of this equation will give p in a function of n; we may 
 therefore reduce to a function of n only, from which we can obtain n in 
 a function of p; thus, relatively to shells of a given constant density, we 
 shall have d n r= 0, and consequently 
 
 = F d f + F' d f ' + F" d f " + &c. ; 
 
 an equation which indicates the tangential force at the surface of those 
 shells is nothing, and consequently, that the resultant of all the forces 
 F, F', F", &c. is perpendicular to this surface ; so that the shells are 
 pheriesL
 
 BOOK I.] NEWTON'S PRINCIPIA. 259 
 
 The pressure n being nothing at the exterior surface, must there be 
 constant, and the resultant of all the forces which animate each molecule 
 of the surface is perpendicular to it. This resultant is \vnat we call gravi- 
 ty. The conditions of equilibrium of a fluid mass, are therefore 1 st, that 
 the direction of gravity be perpendicular to each point of the exterior sur- 
 face : 2dly, that in the interior of the mass the directions of the gravity of 
 each molecule be perpendicular to the surface of the shells of a constant 
 density. Since we may take, in the interior of a homogeneous mass, such 
 shells as we wish for shells of a constant density, the second of two pre- 
 ceding conditions of equilibrium, is always satisfied, and it is sufficient for 
 the equilibrium that the first should be fulfilled ; that is to say, that the 
 resultant of all the forces which animate each molecule of the exterior 
 surface should be perpendicular to the surface. 
 
 567. In the theory of the figure of the celestial bodies, the forces F, F', 
 F", &c. are produced by the attraction of their molecules, by the centrifu- 
 gal force due to their motion of rotation, and by the attraction of distant 
 bodies. It is easy to be certified that the difference F d f + F d f ' +&c. 
 is there exact ; but we shall clearly perceive that, by the analysis which 
 we are about to make of these different forces, in determining that part of 
 the integraiy(F d f + F' d f ' + &c.) which is relative to each of them. 
 
 If we call d M any molecule of the spheroid, and f its distance from the 
 
 point attracted, its action upon this latter will be ^- . Multiplying this 
 
 action by the element of its direction, which is d f, since it tends to 
 diminish f, we shall have, relatively to the action of the molecule d M, 
 
 /F d f = ^- ; whence it follows that that part of the integral /(F d f 
 
 + F d f ' + &c.), which depends on the attraction of the molecules of 
 the spheroid, is equal to the sum of all these molecules divided by their 
 respective distances from the molecule attracted. We shall represent this 
 sum by V, as we have already done. 
 
 We propose, in the theory of the figure of the planets, to determine 
 the laws of the equilibrium of all their parts, about their common center of 
 gravity ; we must, therefore, transfer into a contrary direction to the mole- 
 cule attracted, all the forces by which this center is animated in virtue of 
 the reciprocal action of all the parts of the spheroid; but we know 
 that, by the property of this center, the resultant of all the actions upon 
 this point is nothing. To get, therefore, the total effect of the attraction 
 
 R 2
 
 260 A COMMENTARY ON [SECT. XII. & XIII. 
 
 of the spheroid upon the molecules attracted, we have nothing to add 
 toV. 
 
 To determine the effect of the centrifugal force, we shall suppose the 
 position of the molecule determined by the three rectangular coordinates 
 x', y', z', whose origin we fix at the center of gravity of the spheroid. 
 We shall then suppose that the axis of x 7 is the axis of rotation, and that 
 g expresses the centrifugal force due to the velocity of rotation at the dis- 
 tance I from the axis. This force will be nothing in the direction of x' 
 and equal to g y' and g z' in the direction of y' and of z' ; multiplying, 
 therefore, these two last forces respectively by the elements d y', d z' of 
 their directions, we shall have ^ g (y' s + z /2 ) for that part of the integral 
 f(}? d f + F' d f ' + &c.), which is due to the centrifugal force of the 
 rotatory motion. 
 
 If we call, as above, r the distance of the molecule attracted from the 
 center of gravity of the spheroid, 6 the angle which the radius r forms with 
 the axis of x', and the angle the plane which passes through the axis 
 of x', and through the molecule, forms with the plane of x',y'; finally, if 
 we make cos. 6 = m, we shall have 
 
 x' = r m ; y' = rVl m 2 . cos. ; z' = r V 1 m * . sin. -a ; 
 whence we get 
 
 g(y' 2 + z' 2 ) = *gr(l m). 
 
 We shall put this last quantity under the following form : 
 
 igr- : |gr*<m--i) 
 
 to assimilate its terms to those of the expression V which are given in No. 
 559; that is to say, to give them the property of satisfying the equation of 
 partial differences 
 
 Y <\ \ /d Y 
 
 in which Y (i) is a rational and entire function of m, V 1 m 2 . cos. 
 and V 1 m 2 sin. * of the degree i ; for it is clear that each of the two 
 terms gr* and igr 2 (m 2 i) satisfies for Y , the preceding 
 equation. 
 
 It remains now for us to determine that part of the integral 
 /"(F d f + F' d f' + &c.) which results from the action of distant bodies. 
 Let S be the mass of one of these bodies, f its distance from the molecule 
 attracted, and s its distance from the center of gravity of the spheroid. 
 Multiplying its action by the element d f of its direction, and then inte-
 
 BOOK I.] NEWTON'S PRINCIPIA. 261 
 
 o 
 
 grating we shall have r- . This is not the entire part of the integral 
 
 /(F d f + F' d f ' + &c.) which is due to the action of S; we have still 
 to transfer, in a contrary direction to the molecule, the action of this body 
 upon the center of gravity of the spheroid. For that purpose, call v the 
 angle which s forms with the axis of x', and -vj/ the angle which the plane 
 passing through this star and through the body S, makes with the plane of 
 
 S 
 x', y'. The action of T of this body upon the center of gravity of the 
 
 & 
 
 spheroid, resolved parallel to the axes of x', y', z', will produce the three 
 
 following forms : 
 
 S 
 
 2 cos. v; sin. v cos. ; ^ sin. v sin. %/. 
 
 S S S 
 
 Transferring them in a contrary direction to the molecule attracted, 
 which amountstoprefixingto them the sign , then multiplying them by 
 the elements d x', d y', d z', of their directions, and integrating them, the 
 
 sum of the integrals will be 
 
 c 
 -- ^ .x' cos. v + y' sin. v. cos. %{/ + z' sin. v sin. -vj/J + const. ; 
 
 B 
 
 the entire part of the integral y (F d f -f- F' d f + &c.), due to the ac- 
 tion of the body S, will therefore be 
 
 S S 
 F --- jjx' cos. v + y' sin. v cos. -\-\-T! sin. v sin. -^} -f- const. ; 
 
 and since this quantity ought to be nothing relatively to the center of gra- 
 vity of the spheroid, which we suppose immoveable, and that relatively to 
 this point, f becomes s, and x', y', z', are nothing, we shall have 
 
 const. = . 
 s 
 
 However, f is equal to 
 
 {(s cos. v x') 2 + (s. sin. v cos. %}/ y') 2 + (s sin. v sin. -^ z') ? ] ? j 
 which gives, by substituting for x', y', z', their preceding values 
 S__ _ S 
 
 f Vs* 2s rjcos. v cos. 6 + sin. v sin. 6 cos. (r >}/) + r 2 }' 
 If we reduce this function into a series descending relatively to powers 
 of s, and if we thus represent the series, 
 
 - JPW + - P + 
 
 si s s 
 
 we shall have generally by 561 and 562, 
 P n? _ 1.3.5.. (21-1) | a i i(i-D ^ . i(i-D(i-2)(l-3) 
 -1.2.3 ..... i I 2(2i l) d h 2.4(2i l)(2i 3) d 
 
 &c.| ; 
 )
 
 262 A COMMENTARY ON [SECT. XII. & XIII. 
 
 i being equal to cos. v cos. 6 -f- sin. v sin. 6 . cos. (^ -4/) ; it is evident 
 that by 553, we have 
 
 = I ' " '" ' ) I 4. 1_ '4- i (i + 1) r W ; 
 
 V dm / ]_m 2 
 
 so that the terms of the preceding have this property, common with those 
 of V. This being shown, we have 
 
 S S S 
 rr j(x' cos. v + y' sin. v cos. -^ + z sin. v sin. -^) 
 
 S r * T r r - 
 
 = - 1 P (2) -f- P (3) + P (4) 4- &c 
 s 3 t s s 2 
 
 If there were other bodies S', S", &c.; denoting by s', v', -4/, P' (i >; s'', 
 v", -4/", P" W, &c. what we have called s, v, -vf/, P (i \ relatively to the body 
 S, we shall have the parts of the integral /(F d f + F' d P + &c.) due 
 to their action, by marking with one, two, &c. dashes, the letters s, v, -^, 
 and P in the preceding expression of that part of this integral, which is 
 due to the action of S. 
 
 If we collect all the parts of this integral, and make 
 
 J-=aZ; 
 
 _ P (*> +p(3) +&c.- m--= 
 
 & c . = 
 
 s * 
 
 &c. 
 
 a being a very small coefficient, because the condition that the spheroid is 
 very little different from a sphere, requires that the forces which produce 
 this difference should themselves be very small; we shall have 
 /(Fdf + F'df -f &c.) = V + ar * [Z^+ Z {8 >+ rZ+ r Z^ + &c.j 
 Z w satisfying, whatever i maybe, in the equation of partial differences 
 
 . dZW 1 /d 2 Z (i 
 
 m * -,- > \ (-1 
 
 dm > V d 
 
 = m > | ~ + i (i + J) Z<. 
 
 \ dm / 1 m 2 
 
 The general equation of equilibrium will therefore be 
 
 /-i?JL = V + a r s [Z + Z + r Z r 2 Z^ + &c.} . (1) 
 J s 
 
 If the extraneous bodies are very distant from the spheroid, we may ne- 
 glect the quantities r 3 Z (3) , r 4 Z (4 ), &c., because the different terms of these 
 quantities being divided respectively by s 4 , s 3 , &c. s'*, s", &c. these terms 
 become very small when s, s', &c. are very great compared with r. 'I his
 
 BOOK I.] NEWTON'S PRINC1PIA. 263 
 
 case subsists for the planets and satellites with the exception of S.iturn, 
 whose ring is too near his surface for us to neglect the preceding terms. 
 In the theory of the figure of that planet, we must therefore prolong the 
 second member of equation (1), which possesses the ad vantage of forming 
 a series always convergent; and since then the number of corpuscles ex- 
 terior to the spheroid is infinite, the values of Z (0) , Z (2) , &c.' are given in 
 definite integrals, depending on the figure and interior constitution of the 
 ring of Saturn. 
 
 568. The spheroid may be entirely fluid ; it may be formed of a solid 
 nucleus covered by a fluid. In both cases the equation (1) of the preced- 
 ing No. will determine the figure of the shells of the fluid part, by con- 
 sidering, that since n must be a function of f, the second member of this 
 equation must be constant for the exterior surface, and for that of the 
 shells in equilibrium, and can only vary from one shell to another. 
 
 The two preceding cases reduce to one when the spheroid is homoge- 
 neous ; for it is indifferent as to the equilibrium whether it is entirely 
 fluid, or contains an interior solid nucleus. It is sufficient by No. 556, that 
 at the exterior surface we have 
 
 constant = V + a r 2 {Z<>+ Z< 9 >+ r Z+ &c.}. 
 
 If we substitute in this equation for V its value given by formula (3) of 
 No. 555, and if we observe that by No. 556,Y (0) disappears by taking for 
 a the radius of a sphere of the same volume as the spheroid, and that 
 Y (c; is nothing when we fix the origin of coordinates at the center of the 
 spheroid ; we shall have 
 
 4 cr a 3 4 <K a a s J 1 v ,, a v (31 a ! v ,.. fl ) 
 constant =__+-_-- |_-\ W + _. Y*> + ^J (4 ' + &c.j 
 
 + a r z {Z <> + Z + r Z + r e Z W 4. & c .} 
 
 Substituting in the equation of the surface of the spheroid for r its value 
 at the surface 1 + K y, or 
 
 a + a a {< + Y< 3 > + Y' 1 ' + &c.J 
 which gives 
 
 const. =-^a* ^P 1 ! 5 Y +-f Y(3) +4 YW) + &c '} 
 
 + a a* {Z< -f Z^ + a Z + a 8 Z + c.} 
 
 We shall determine the arbitrary constant of the first member of this 
 equation, by means of this equation, 
 
 const. =~^a 2 + aa* Z>; 
 
 O 
 
 we shall then have by comparing like functions, that is to say, such as are 
 subject to the same equation of partial differences, 
 
 R4
 
 A COMMENTARY ON [SECT. XII. & XIII. 
 
 i being greater than unity. The preceding equation may be put under the 
 form 
 
 the integral being taken from r = to r = a. The radius a (1 ay) 
 of the surface of the spheroid will hence become 
 
 1 + |_?_jz<*> + aZ + a 2 
 
 + y> = *< V; 
 
 + TFVdr{Z> + 
 
 We may put this equation under a finite form, by considering that we 
 have by the preceding No. 
 
 ., _ S S' 
 
 J_ - =^^==== == _ _ &C 
 
 r r s Vs 2 2srS + r 2 s r r 2 
 
 so that the integraiyd r [Z'^ + r Z ;3) + &c.J is easily found by known 
 methods. 
 
 569. The equation (1) of 567 not only has the advantage of showing the 
 figure of the spheroid, but also that of giving by differentiation the law of 
 gravity at its surface ; for it is evident that the second member of this 
 equation being the integral of the sum of all the forces with which each 
 molecule is animated, multiplied by the. elements of their respective direc- 
 tions, we shall have that part of the resultant which acts along the radius 
 r, by differentiating the second member relatively to r; thus calling p 
 the force by which a molecule of the surface is sollicited towards the center 
 of gravity of the spheroid, we shall have 
 
 p = (^) ~ d {r Z<> + r 2 Z + r 3 Z + r 4 Z<*> + &:.}. 
 
 If we substitute in this equation for (-? V its value at the surface 
 
 2 V 
 
 * a + , given by equation (2) of No. 554, and for V, its value given 
 
 O / it 
 
 by equation (1) of No. 567; we shall have 
 
 p = *-* a \ a a [Z + a Z a 2 Z <*> + &c.J 
 o 
 
 -L . d . {r Z<> + r 2 Z' 2 > -f r 3 Z> + r 4 Z<*> + &c,} (3)
 
 BOOK L] NEWTON'S PRINCIPIA. 263 
 
 r must be changed into a after the differentiations in the second mem- 
 ber of this equation, which by the preceding No. may always be reduced 
 to a finite function. 
 
 p does not represent exactly gravity, but only that part of it which is 
 directed towards the center of gravity of the spheroid, by supposing it re- 
 solved into two forces, one of which is perpendicular to the radius r, and 
 the other p is directed along this radius. The first of these two forces is 
 evidently a small quantity of the order a ; denoting it therefore by a 7, 
 gravity will be equal to Vp* + 2 7 a ? a quantity which, neglecting the 
 terms of the order a , reduces to p. We may thus consider p as express- 
 ing gravity at the surface of the spheroid, so that the equations (2) and 
 (3) of the preceding No. and of this, determine both the figure of ho- 
 mogeneous spheroids in equilibrium, and the law of gravity at their 
 surfaces ; they contain the complete theory of the equilibrium of these 
 spheroids, on the supposition that they differ very little from the sphere. 
 
 If the extraneous bodies S, S', &c. are nothing, and therefore the 
 spheroid is only sollicited by the attraction of its molecules, and the cen- 
 trifugal force of its rotatory motion, which is the case of the Earth and 
 primary planets with the exception of Saturn, when we only regard the 
 permanent state of thetf figures ; then designating by a <p , the ratio of 
 the centrifugal force to gravity at the equator, a ratio which is very nearly 
 
 equal to -^-, the density of the spheroid being taken for unity; we shall 
 
 I* 
 find, 
 
 p = 
 
 the spheroid is then therefore an ellipsoid of revolution, upon which in- 
 crements of gravity, and decrements of the radii, from the equator to 
 the poles, are very nearly proportional to the square of the sine of the 
 latitude, m being to quantities of the order a, equal to this sine. 
 
 a, by what precedes, is the radius of a sphere, equal in solidity to the 
 spheroid ; gravity at the surface of this sphere will be it a ; thus we shall 
 have the point of the surface of the spheroid, where gravity is the same as 
 at the surface of the sphere, by determining m by the equation 
 
 = f + f (m 2 i); 
 which gives 
 
 IS
 
 266 A COMMENTARY ON [SECT. XII. & XIII. 
 
 570. The preceding analysis conducts us to the figure of a homoge- 
 neous fluid mass in equilibrium, without employing other hypotheses than 
 that of a figure differing very little from the sphere: it also shows that 
 the elliptic figure which satisfies this equilibrium, is the only figure 
 which does it. But as the expansion of the radius of the spheroid into 
 a series of the form a 1 + a Y w + a Y y) + &c.} may give rise to some 
 difficulties, we proceed to demonstrate directly, and independently of this 
 expansion, that the elliptic figure is the only figure of the equilibrium of 
 a homogeneous fluid mass endowed with a rotatory motion ; which by con- 
 firming the results of the preceding analysis, will at the same time serve 
 to remove any doubts we may entertain against the generality of this ana- 
 lysis. 
 
 First suppose the spheroid one of revolution, and that its radius is a 
 ( 1 + y), y being a function of m, or of the cosine of the angle 6 which this 
 radius makes with the axis of revolution. If we call f any straight line 
 drawn from the extremity of this radius in the interior of the spheroid; p 
 the complement of the angle which this straight line makes with the plane 
 which passes through the radius a ( 1 + ay) and through the axis of revolu- 
 tion ; q the angle made by the projection of f upon this plane and by the 
 radius ; finally, if we call V the sum of all the molecules of the spheroid, 
 divided by their distances from the molecules placed at the extremity of 
 the radius a (1 + a y) ; each molecule being equal to f 2 d f. d p. d q . 
 sin. p, we shall have 
 
 V = i/f' 2 dp.dq.sin. p, 
 
 f being what f becomes at its quitting the spheroid. We must now de- 
 termine f ' in terms of p and q. 
 
 For that purpose, we shall observe that if we call 6', the value of rela- 
 tive to this point of exit, and a (1 + ay'), the corresponding radius of the 
 spheroid, y' being a similar function of cos. tf or of m' that y is of m ; it 
 is easily seen that the cosine of the angle formed by the two straight lines 
 f and a ( 1 + a y) is equal to sin. p . cos. q ; and therefore that in the 
 triangle formed by the three straight lines f, a (1 + ay) and a (1 + ay') 
 we have 
 
 a* (l + a y') * = f /s 2af'(l + y) sin. p . cos. q + a 2 (l + y) 8 . 
 
 This equation gives for f ' a two values ; but one of them being of the 
 order a 2 is nothing when we neglect the quantities of that order; the 
 other becomes 
 
 f' 2 = 4 a 8 sin. s p cos. 2 q (1 + 2 ay) + 4 a a " (y' y) ; 
 which gives
 
 BOOK L] NEWTON'S PRINCIPIA 267 
 
 V = 2 a "f d p d q sin. p {(1 -f 2 a y) sin. 2 p cos. 2 q + a (y y) j. 
 It is evident that the integrals must be taken from p rr 0, to p = -T, and 
 from q = <r to q = <r; we shall therefore have 
 
 V = f r a 5 f a c a 2 y + 2 a a -fd p . d q . y'sin. p . 
 y' being a function of cos. 0', we must determine this cosine in a function 
 of p and q; we may therefore in this determination neglect the quantities 
 of the order a, since y' is already multiplied by a ; hence we easily find 
 
 a cos. 6 f =. (g, P sin. p cos. q) cos. 6 -f- f ' sin. p . sin. q . sin. 6 ; 
 whence we derive, substituting for f its value 2 a sin. p cos. q, 
 
 in' =r m cos. 2 p sin. 2 p cos. (2 q -f- 6). 
 
 Here we must observe, relatively to the integral J y' d p . d q. sin. p, 
 taken relatively to q from 2 q = w to 2 q = r, that the result would 
 be the same, if this integral were taken from 2q = to 2 q = 2a 0, 
 because the values of m', and consequently of y' are the same from 2 q 
 tf to 2 q = das from 2q = r to 2 q = 2ff 6; supposing there- 
 fore 2 q + 6 = q', which gives 
 
 m' = m cos. 2 p sin. 2 p cos. q' ; 
 we shall have 
 
 V = f c a 2 f a cr a 2 y + aa 2 /y' d p d q' sin. p ; 
 the integrals being taken from p = to p = * and from q' = to q 7 = 
 2*. 
 
 Now if we denote by a 2 N the integral of all the forces extrinsic to the 
 attraction of the spheroid, and multiplied by the elements of their direc- 
 tions ; by 568 we shall have in the case of equilibrium 
 
 constant = V + a 2 N, 
 and substituting for V its value, we shall have 
 
 const. = ^ * . y fy' d p . d q' sin. p N ; 
 
 an equation which is evidently but the equation of equilibrium of No. 568, 
 presented under another form. This equation being linear, it thence results 
 that if any number i of radii a (1 + ay), a (1 + v), and satisfy it; the 
 
 radius a{+ (y + v + &c.)} will also satisfy it. 
 
 Suppose that the extraneous forces are reduced to the centrifugal force 
 due to the rotation, and call g this force at the distance 1 from the axis of 
 rotation; we shall have, by 567, N = g (1 m j ) ; the equation of 
 equilibrium will therefore be 
 
 const. = f a it y /y d p d q' sin. p g (1 ni 2 ). 
 
 Differentiating three times successively, relatively to m, and observing 
 
 that ( \ = cos. 2 p, in virtue of the equation 
 vd m /
 
 268 A COMMENTARY ON [SECT. XII. & XIII. 
 
 m' = m cos. * p sin. 2 p cos. q' ; 
 
 we shall have 
 
 J * 
 = i v 
 
 but we have,/ d p d q' sin. p cos. 6 p = - ^; we may therefore put the 
 preceding equation under this form, 
 
 = /"d p d q' sin. p cos. 6 p < I (^ ^A (-5 \ \ . 
 
 (. Vd my \d m'V ) 
 
 This equation subsists, whatever m may be; but it is evident, that 
 amongst all the values between m = 1 and m = 1, there is one which 
 we shah 1 designate by h, and which is such that, abstraction being made 
 
 1 3 
 
 of the si<ni, each of the values of ( -, *^\ will not exceed that which is re- 
 
 Vd m 3 / 
 
 lative to h ; denoting therefore by H, this latter value, we shall have 
 
 /" J 3 */ \ 
 
 = f d p d q' sin. p cos. 6 p -j | H (-, ^ V . 
 
 /d 3 y' \ 
 The quantity H f -^ ^- 3 J has evidently the same sign as H, and 
 
 -, 
 
 the factor sin. p . cos. 6 p, is constantly positive in the whole extent of the 
 integral; the elements of this integral have, therefore, all of them the 
 same sign as H ; whence it follows that the entire integral cannot be no- 
 thing, at least H cannot be so, which requires that we have generally 
 
 = (T ^V whence by integrating we get 
 
 y = 1+ m. m -j- n ni 2 ; 
 ], ;#, n, being arbitrary constants. 
 
 If we fix the origin of the radii in the middle of the axis of revolution, 
 and take for a the half of this axis, y will be nothing when m = 1 and 
 when m =: ], which gives m = and n = 1 ; the value of y thus 
 becomes, 1 (1 m 2 ); substituting in the equation of equilibrium, 
 const = | a c y a fy' d p d q' sin. p g (1 m *) ; 
 
 we shall find a 1 = =-= = a p, a p being the ratio of the centrifugal 
 16 it 4> 
 
 force to the equatorial gravity, a ratio which is very nearly equal to r~ ; 
 the radius of the spheroid will therefore be 
 
 whence it follows that the spheroid is an ellipsoid of revolution, which is 
 conformable to what precedes.
 
 BOOK I.] NEWTON'S PRINCIPLE 269 
 
 Thus we have determined directly and independently of series, the 
 figure of a homogeneous spheroid of revolution, which turns round its 
 axis, and we have shown that it can only be that of an ellipsoid which 
 becomes a sphere when <p = ; so that the sphere is the only figure of 
 revolution which would satisfy the equilibrium of an immoveable homo- 
 geneous fluid mass. 
 
 Hence we may conclude generally, that if the fluid mass is sollicited 
 by any very small forces, there is only one possible figure of equilibrium . 
 or, which comes to the same, there is only one radius a (1 + a y) which 
 can satisfy the equation of equilibrium, 
 
 const. = | a <r . y af y' d p . d q' sin. p N ; 
 
 y being a function of & and of the longitude ?, and y' being what y be- 
 comes when we change 6 and r into (f and w. Suppose, in fact, that 
 there are two different rays a ( 1 + ay) and a(l + y + av ) which 
 satisfy this equation; we shall have 
 
 const. = | a v (y + v) a f(f + v/ ) d P d q' sm - P N. 
 
 Taking the preceding equation from this, we shall have 
 
 const. = f v y v' d p d q' sin. p. 
 
 This equation is evidently that of a homogeneous spheroid in equili- 
 brium, whose radius is a (1 + a v), and which is not sollicited by any 
 force extraneous to the attraction of its molecules. The angle disappear- 
 ing in this equation, the radius a (1 + a v) will still satisfy it if -a be suc- 
 cessively changed to -a + d , + 2 d 9 &c., whence it follows, that if 
 we call v l5 v 2 , &c. what v becomes in virtue of these changes; the 
 radius 
 
 a { 1 + avdw+avidw + av 2 dw + &c.}, 
 or 
 
 a (1 + /v d w), 
 
 will satisfy the preceding equation. If we take the integral fv d from 
 zj to -a = 2 T, the radius a (1 + ufv d *r) becomes that of a sphe- 
 roid of revolution, which, by what precedes, can only be a sphere : see 
 the condition which results for v. 
 
 Suppose that a is the shortest distance of the center of gravity of the 
 spheroid whose radius is a (1 + a v), to the surface, and fix the pole or 
 origin of the angle 6 at the extremity of a ; v will be nothing at the pole, 
 and positive every where else; it will be the same for the integraiyvd -a. 
 But, since the center of gravity of the spheroid whose radius is a (l+ v), 
 is at the center of the sphere whose radius is a, this point will, in like 
 manner, be the center of gravity of the spheroid whose radius is
 
 270 A COMMENTARY ON [SECT. XII. & XIII. 
 
 a (1 + a ./ v d w ) > * ne different radii drawn from this center to the sur- 
 face of this last spheroid are therefore unequal to one another, if v is not 
 nothing ; there can only therefore be a sphere in the case of v = ; thus we 
 learn for a certainty, that a homogeneous spheroid, sollicited by any small 
 forces whatever, can only be in equilibrium in one manner. 
 
 571. We have supposed that N is independent of the figure oi 
 the spheroid; which is what very nearly takes place when the forces, 
 extraneous to the action of the fluid molecules,* are due to the centri- 
 fugal force of rotatory motion, and to the attraction of bodies exterior 
 to the spheroid. But if we conceive at the center of the spheroid a finite 
 force depending on the distance r, its action upon the molecules placed at 
 the surface of the fluid, will depend on the nature of this surface, and 
 consequently N will depend upon y. This is the case of a homogeneous 
 fluid mass which covers a sphere of a density different from that of the 
 fluid ; for we may consider this sphere as of the same density as the fluid, 
 and may place at its center a force reciprocal to the square of the dis- 
 tances; so that, if we call c the radius of the sphere, andf its density, that 
 of the fluid being taken for unity, this force at the distance r will be equal 
 
 C 3 f p l\ 
 
 to f cr . - 2__ . Multiplying by the element d r of its direction 
 
 the integral of the product will be it. - -, a quantity which we 
 
 must add to a* N; and since at the surface we have r = a (1 + a y), in 
 the equation of equilibrium of the preceding No., we must add to N, 
 
 3, .<i^'l- 3 . (,_,.). 
 
 This equation will become 
 
 4 a cr f c 3 1 
 
 const. = -3- 1 1 + (s 1) ;p j y // d p . d q sin. p N. 
 
 If we denote by a (1 + ay + a v), a new expression of the radius of 
 the spheroid in equilibrium, we shall have to determine v, the equation 
 
 f c 3 ) 
 
 const. = | T 1 1 + ( s 1) . 5 j- / v' d p d q' sin. p ; 
 
 an equation which is that of the equilibrium of the spheroid, supposing it 
 immoveable, and abstracting every external force. 
 
 If the spheroid is of revolution, v will be a function of cos. 6 or m only; 
 but in this case we may determine it by the analysis of the preceding No. ; 
 for if we differentiate this equation i + 1 times successively relatively to 
 m, we shall have 
 
 0=| ,
 
 BOOK I.] NEWTON'S PRINCIP1A. 271 
 
 but we have 
 
 yd p d q' sin. p cos. J ' + 2 p = . - ; 
 the preceding equation may therefore be put under this form, 
 
 We may take i such that, abstraction being made of the sign, we have 
 
 Supposing, therefore, that i is the smallest positive whole number which 
 renders this quantity greater than unity, we may see, as in the preceding No., 
 
 ((] 1 + 1 y 
 - : - 1 ] = 0, 
 
 which gives 
 
 v = m 1 + Am'- 1 -f Bm *-* + &c. 
 
 Substituting in the preceding equation of equilibrium for v, this value, 
 and for v' 
 
 m' 1 + A m' 1 - 1 + Bin' l ~ 2 -f &c. 
 
 m' being by the preceding No. equal to m cos. 2 p sin. 2 p cos. q', first 
 we shall find 
 
 '+<j-i>p=jfiT-i ; 
 
 which supposes o equal to or less than unity ; thus, whenever a, c, and g 
 are not such as to satisfy this equation, i being a positive whole number, 
 the fluid can be in equilibrium only in one manner. Then we shall have 
 
 so that 
 
 - 
 
 - m .-..--- m i-4 fcc .. 
 -2(2i-l)' ] + 2.4.(2i 1) (2i-3) ni 
 
 there are, therefore, generally two figures of equilibrium, since a v is sus- 
 ceptible of two values, one of which is given by the supposition of a = 0, 
 and the other is given by the supposition of v being equal to the preced- 
 ing function of m. 
 
 If the spheroid has no rotatory motion, and is not sollicited by any ex- 
 traneous force, the first of these two figures is a sphere, and the second 
 has for its meridian a curve of the order i. These two curves coincide in 
 the case of i = 1, because the radius a (1 + am) is that of a sphere in 
 which the origin of the radii is at the distance from its center; but then 
 
 O 
 
 it is easy to see that = 1, that is, the spheroid is homogeneous, a result 
 agreeing with that of the preceding No.
 
 272 A COMMENTARY ON [SECT. XII. & X11I. 
 
 572. When we have figures of revolution which satisfy the equilibrium, 
 it is easy to obtain those which are not of revolution by the following 
 method. Instead of fixing the origin of the angle 6 at the extremity of 
 the axis of revolution, suppose it at the distance y from this extremity, and 
 call $ the distance from this same extremity of the point of the surface 
 whose distance from the new origin of the angle & is d. Call, moreover, 
 v & the angle comprised between the two arcs 6 and y ; we shall 
 
 have 
 
 cos. ^ = cos. y cos. 6 + sin. y sin. 6 . cos, (v j3) ; 
 
 designating therefore by r. (cos. tf] the function 
 
 COS ' ' ' - 
 
 the radius of the immoveable spheroid in equilibrium, which we have seen 
 is equal to a [I + a r. (cos. ^)}, will be 
 
 a + a a r. [cos. y . cos. 6 + sin. y . sin. 6 cos. (*r j3}] ; 
 and although it is a function of the angle *r t it belongs to a solid of revo- 
 lution, in which the angle 6 is not at the extremity of the axis of revo- 
 lution. 
 
 Since this radius satisfies the equation of equilibrium, whatever may be 
 a, /3, and y, it will also satisfy in changing these quantities into ', /3', y', 
 "> "> y", &c. whence it follows that this equation being linear, the radius 
 a + a r . cos. y cos. 6 + sin. y sin. 6 cos. (*r /3 )} 
 + a' a r . cos. / cos. 6 + sin. / sin. 6 cos. (& /3')} 
 + &c. 
 
 will likewise satisfy it. The spheroid to which this radius belongs is no 
 longer one of revolution ; it is formed of a. sphere of the radius a, and ot 
 any number of shells similar to the excess of the spheroid of revolution 
 whose radius is a -f a r . (m) above the sphere whose radius is a, these 
 shells being placed arbitrarily one over another. 
 
 If we compare the expression of r. (cos. 6'} with that of P (i) of No. 567, 
 we shall see that these two functions are similar, and that they differ only 
 by the quantities y and /3, which in P (i > are v and -^, and by a factor in* 
 dependent of m and r ; we have, therefore, 
 
 It is easy hence to conclude, that if we represent by a Y (i) the function 
 
 a . r . [cos. y cos. 6 -f- sin. y sin. 6 . cos. (& /3 )} 
 -}- a' . r . [cos. y' cos. 6 + sin. y' sin. . cos. (t* /3')}
 
 BOOK I.] NEWTON'S PRINCIP1A. 273 
 
 Y [i) will be a rational and entire function of m, VI rn~* cos. &, 
 
 VI m 2 sin. , which will satisfy the equation of partial differences, 
 
 d, 
 
 choosing for Y W, therefore, the most general function of that nature, the 
 function a ( 1 + Y (i) ) will be the most general expression of the equili- 
 brium of an immoveable spheroid. 
 
 We may arrive at the same result by means of the series for V in 555 ; 
 for the equation of equilibrium being, by the preceding No., 
 
 const. = V + a 2 N ; 
 if we suppose that all the forces extraneous to the reciprocal action of the fluid 
 
 tp _ 1) c 3 
 molecules, are reducible to a single attractive force equal to f it. - p , 
 
 placed at the center of the spheroid, by multiplying this force by the ele- 
 ment d r of its direction, and then integrating, we shall have 
 
 and since at the surface r = a(l + y) the preceding equation of equi- 
 librium will become 
 
 c 3 
 const. = V + | r . (1 f) jr. 
 
 cl 
 
 Substituting in this equation for V its value given by formula (3) of 
 No. 555, in which we shall put for r its value a (1 + ay), and by sub- 
 stituting for y its value 
 
 YW + YW + Y + &c.; 
 we shall have 
 
 o= 
 
 the constant a being supposed such, that const. = it a 8 . This equation 
 gives Y W = 0, Y (I > = 0, Y = 0, &c. unless the coefficient of one of these 
 quantities, of Y (i) for example, is nothing, which gives 
 
 c 3 _ 2i 2 
 ~ S ' a 3 ~ 2 i + 1 ' 
 
 i being a positive whole number, and in this case all these quantities ex- 
 cept Y (i) are nothing ; we shall therefore have y = Y (i) , which agrees 
 with what is found above. 
 
 Thus we see, that the results obtained by the expansion of V into a se- 
 
 VOL. II. S
 
 274 A COMMENTARY ON [SECT. XII. & XIII. 
 
 ries, have all possible generality, and that no figure of equilibrium has 
 escaped the analysis founded upon this expansion ; which confirms what 
 we have seen a priori, by the analysis of 555, in which we have proved 
 that the form which we have given to the radius of spheroids, is not arbi- 
 trary but depends upon the nature itself of their attractions. 
 
 573. Let us now resume equation (]) of No. 567. If we therein sub- 
 stitute for V its value given by formula (6) of No. 558, we shall have rela- 
 tively to the different fluid shells 
 
 {a* Y^+y 
 
 or w 
 
 + r 2 {Z<> + Z + r Z + r 8 Z< 4 > + &c.J ;....(!) 
 the differentials and integrals being relative to the variable a ; the two first 
 integrals of the second member of this equation must be taken from a = a to 
 a = 1, a being the value of a, relative to the leveled fluid shell, which we are 
 considering, and this value at the surface being taken for unity : the two last 
 integrals ought to be taken from a = to a = a : finally, the radius r 
 ought to be changed into a ( 1 + a y) after all the differentiations and in- 
 tegrations. In the terms multiplied by a it will suffice to change r into 
 
 a ; but in the term -_ ~fg d . a 3 we must substitute a (1 + y) for r ; 
 o r 
 
 which changes it into this 
 
 and consequently, into the following 
 
 {I _ a Y ( ) Y> a Y (3 > &c.}./g d a 3 . 
 o a 
 
 Hence if in equation (1) we compare like functions, we shall have 
 / d n = 2 cr/g d a 2 + 4 a / d (a 2 Y>) +4^/f da 3 
 
 a 
 
 the two first integrals of the second member of this equation being taken 
 from a = a to a = 1, the three other integrals must be taken from a 
 = to a = a. This equation determining neither a nor Y (0) , but only a 
 relation between them, we see that the value of Y (0) is arbitrary, and may 
 be determined at pleasure. We shall have then, i being equal to, or 
 greater than unity,
 
 BOOK I.] NEWTON'S PRINCIPIA. 275 
 
 i fed fY^x _4^ , 
 l J S a> Va'-V 3a * J f 
 
 the first integral being taken from a = a, to a = ], and the two others 
 being taken from a = to a = a. This equation will give the value of 
 Y (i) relative to each fluid shell, when the law of the densities g shall be 
 known. 
 
 To reduce these different integrals within the same limits, let 
 4*-,,/Y\ 4 * 
 
 '< d = + z (1} = z * 
 
 the integral being taken from a = to a = 1 ; Z' (i) will be a quantity in- 
 dependent of a, and the equation (2) will become 
 
 3/g d (a ! + 3 Y )) 3 a 2 '*+ l 7J ; 
 all the integrals being taken from a = to a = a. 
 
 We may make the signs of integration disappear by differentiating re- 
 latively to a, and we shall have the differential equation of the second 
 order, 
 
 /d 2 Y (i \ _ fi ( i + 1) _ 6 g a ) y (i) _ 6 g a g /d Y (i >\ 
 V da 2 / : I a^~ ~/f da 3 ) ~~f^d.a 3 \da)' 
 
 The integral of this equation will give the value of Y w with two arbi- 
 trary constants ; these constants are rational and entire functions of the 
 order i, of m, VI m 2 . sin. &, and VI m - . cos. ^-, such, that re- 
 presenting them by U (i) , they satisfy the equation of partial differences, 
 
 ( d ; u 2 (i) ) 
 
 V (J. v J 
 
 o = i x um ' > ) + ^ gr + i (i + i) . u . 
 
 \ dm / 1 m 2 
 
 One of these functions will be determined by means of the function 
 Z' (i) which disappears by differentiation, and it is evident that it will be a 
 multiple of this function. As to the other function, if we suppose that 
 the fluid covers a solid nucleus, it will be determined by means of the 
 equation of the surface of the nucleus, by observing that the value of 
 Y ^ relative to the fluid shell contiguous to this surface, is the same as 
 that of the surface. Thus the figure of the spheroid depends upon the 
 figure of the internal nucleus, and upon the forces which sollicit the 
 fluid. 
 
 574. If the mass is entirely fluid, nothing then determining one of the 
 arbitrary constants, it would seem that there ought to be an infinity of 
 
 S2
 
 276 A COMMENTARY ON SECT. XII. & XIII. 
 
 figures of equilibrium. Let us examine this case particularly, which is 
 the more interesting inasmuch as it appears to have subsisted primi- 
 tively for the celestial bodies. 
 
 First, we shall observe that the shells of the spheroid ought to decrease 
 in density from the center to the surface ; for it is clear that if a denser 
 shell were placed above a shell of less density, its molecules would pene- 
 trate into the other in the same manner that a ponderous body sinks into 
 a fluid of less density ; the spheroid will not therefore be in equilibrium. 
 But whatever may be its density at the center, it can only be finite ; re- 
 ducing therefore the expression of into a series ascending relatively to 
 the powers of a, this series will be of the form & 7 . a n &c. /3, y and 
 n being positive ; we shall thus have 
 
 ,* + 3)/3 
 
 and the differential equation in Y w will become 
 
 . 6 n 7 . 
 
 _ &c 
 
 H C ' 
 
 _ 
 
 (n+3)|8 '' \ da 
 
 To integrate this equation, suppose that Y (y is developed into a series 
 ascending according to the powers of a, of this form 
 
 YW = a. U< + a*'. U' + &c.; 
 the preceding differential equation will give 
 
 s / i + 2)a s '- 2 U' (i > + &c. 
 
 '-&c.J . (e) 
 
 Comparing like powers of a, we have (s + i + 3) (s i + 2) = 0, 
 which gives s = i 2, and s = i 3. To each of these values of 
 s, belongs a particular series, which, being multiplied by an arbitrary, will 
 be an integral of the differential equation in Y fl) ; the sum of these two in- 
 tegrals will be its complete integral. In the present case, the series which 
 answers to s = i 3 must be rejected ; for there thence results for a 
 Y (i) , an infinite value, when a shall be infinitely small, which would render 
 infinite the radii of the shells which are infinitely near to the center. Thus 
 of the two particular integrals of the expression of Y (i) , that which answers 
 to s = i 2 ought alone to be admitted. This expression then, contains 
 no more than one arbitrary which will be determined by the function Z (i) . 
 
 Z (1) being nothing by No. 567, Y (1) is likewise nothing, so that the 
 center of gravity of each shell, is at the center of gravity of the entire
 
 BOOK I.] NEWTON'S PRINCIPIA. 
 
 277 
 
 spheroid. In fact the differential equation in Y W of the preceding No. 
 gives 
 
 6ga^ 6ga ,dY('\ 
 
 f s d. a 3 ' /g d. a 3 ' V~d~a~V 
 
 * 
 We satisfy this equation by making Y ^ = - , U (I) being indepen- 
 
 dent of a. This value of Y (1J is that which answers to the equation 
 s = i 2 ; it is, consequently, the only one which we ought to admit. 
 Substituting it in the equation (2) of the preceding No., and supposing 
 Z (1) = 0, the function U (1) disappears, and consequently remains arbitrary; 
 but the condition that the origin of the radius r is at the center of gravity 
 of the terrestrial spheroid, renders it nothing; for we shall see in the follow- 
 ing No. that then Y (1) is nothing at the surface of every spheroid covered 
 over with a shell of fluid in equilibrium ; we shall have, therefore, in the 
 present ease U w = ; thus, Y (1) is nothing relatively to all the fluid shells 
 which form the spheroid. 
 
 Now consider the general equation, 
 
 Y = a s . U + a s '.U' + &c.; 
 
 s being, as we have seen, equal to i 2, s is nothing or positive, when i 
 is equal to or greater than 2; moreover, the functions U' (i) , U" w , &c. are 
 given in U w , by the equation (e) of this No. ; so that we have 
 
 = h. U; 
 
 h being a function of a, and U (i) being independent of it. If we substi- 
 tute this value of Y w in the differential equation in Y (l) , we shall have 
 
 ^h__ c 
 
 a 2 " V ' 
 
 da 2 "V ' /gd.a 3 /* a 2 "/gd.a 3 'da' 
 
 The product i (i + 1) is greater than -^4 r , when i is equal to or 
 
 e a s 
 greater than 2, for the fraction -j-^\ 3 l * ^ ess ^ an ^^S * n ^ act lis 
 
 denominator / g d . a 3 is equal to ga 3 /a 3 d g, and the quantity 
 /a 3 d g is positive, since g decreases from the center to the surface. 
 
 Hence it follows that h and T are constantly positive, from the 
 
 center to the surface. To show this, suppose that both these quantities are 
 positive in going from the center; d h ought to become negative before h, 
 and it is clear that in order to do this it must pass through zero ; but 
 from the instant it is nothing, d 2 h becomes positive in virtue of the pre- 
 ceding equation, and consequently d h begins to increase ; it can never 
 therefore become negative. Whence it follows that h and d h always pre- 
 
 S3
 
 278 A COMMENTARY ON [SECT. XII. & XIII. 
 
 serve the same sign from the center to the surface. Now both of these 
 quantities are positive in going from the center ; for we have in virtue of 
 equation (e), s' 2 = s + n 2, which gives s' = i + n 2 ; hence 
 we have 
 
 (S ' + i + 3) <s' - i + 2) u- = iii_ib^! ; 
 
 whence we derive 
 
 U0)_ 6(i-l) y .UCO 
 
 ~(n + 3)(2i + n + l).|S' 
 we shall therefore get 
 
 -'+n- 2 
 
 _/i g )3 i- 3 , - n-y.a- 
 
 da~ (1 " (n+3)(2i + n+l)/3 
 
 7, /3, n, being positive, we see that at the center h and d h are positive, 
 when i is equal to or greater than 2 ; they are therefore constantly positive 
 from the center to the surface. 
 
 Relatively to the Earth, to the Moon, to Jupiter, &c. Z (i) is nothing or 
 insensible, when i is equal to or greater than 3; the equation (2) of the 
 preceding No. then becomes 
 
 0= 
 
 the first integral being taken from a = a, to a = 1, and the two others 
 being taken from a = 0, to a = a. At the surface where a = 1, this equa- 
 tion becomes 
 
 = { (2i+ l)h/d. a 3 +3/gd(a i + 3 h)j. U ; 
 an equation which we can put under this form 
 
 = (2i 2) f h + (2i+l)h/a 3 d 3/a 5 + 3 h.d^ U W. 
 
 d is negative from the center to the surface, and h increases in the 
 same interval; the function (2 i + 1) hy a 3 d 3/a * + 3 h d g is therefore 
 negative in the same interval ; thus in the preceding equation the coeffi- 
 cient of U (i) is negative and cannot be nothing at the surface ; U (i) ought 
 therefore to be nothing, which gives Y (i) =: ; the expression of the ra- 
 dius of the spheroid thus reduces to a + a a JY (0) + Y <*)] ; that is to say, 
 that the surface of each leveled shell of the spheroid is elliptic, and conse- 
 quently its exterior surface is elliptic. 
 
 Z (2) , relatively to the Earth is, by No. 567, equal to ^ (m "~ ) ; 
 the equation (2) of the preceding No. gives therefore
 
 BOOK I.] NEWTON'S PRINCIPIA. 279 
 
 At the surface, the first integraiy^ dh is nothing; we have tlierefore at 
 this surface where a = 1, 
 
 TT (2) 
 
 - 
 
 .h./gd.a 3 f*/ f d(a 5 h)' 
 Let a <p f be the ratio of the centrifugal force to the equatorial gravity ; 
 the expression of gravity to quantities of the order , being equal to 
 I xft d . a 3 ; we shall have g = f it a <pfg d . a 3 ; wherefore 
 
 TT (2) - ~ 
 
 - 
 
 2h 2y g .d(a*b). 
 
 comprising therefore in the arbitrary constant a, what we have taken for 
 unity, the function 
 
 *YW) ah? 
 
 3 y g d(a*h)' 
 
 -SVf-a'da 
 
 the radius of the terrestrial spheroid at the surface will be 
 
 i h 9 (1 m 2 ) 
 
 
 2h 
 
 * ii ~~ 
 
 "^* . 7; 
 
 5 ./& da 
 
 The figure of the earth supposed fluid, can therefore only be that of an 
 ellipsoid of revolution ; all of whose shells of constant density are elliptic, 
 and of revolution, and in which the ellipticities increase, and the densities 
 decrease from the center to the surface. The relation between the ellip- 
 ticities and densities is given by the differential equation of the second 
 order, 
 
 d z h _ 6_h / g a 3 x 2ga 8 d_h 
 
 da 2 : a 2 \ 3/ga 2 da/ /g.a*da*da" 
 
 This equation is not integrable by known methods except in some par- 
 ticular suppositions of the densities g ; but if the law of the ellipticities 
 were given, we should easily obtain that of the corresponding densities. 
 We have seen that the expression of h given by the integral of this equa- 
 tion contains, in the present question, only one arbitrary, which disappears 
 from the preceding value of the radius of the spheroid ; there is therefore 
 only one figure of equilibrium differing but little from a sphere, which is 
 possible, and it is easy to see that the limits of the flattening of this figure 
 
 are and f a p, the former of which corresponds to the case where all 
 
 9 
 
 SI
 
 280 A COMMENTARY ON [SECT. XII. & XIII. 
 
 the mass of the spheroid is collected at its center, and the second to the 
 case where this mass is homogeneous. 
 
 The directions of gravity from any point of the surface to the center do 
 not form a straight line, but a curve whose elements are perpendicular to 
 the leveled shells which they traverse : this curve is the orthogonal tra- 
 jectory of all the ellipses which by their revolution form these shells. To 
 determine its nature, take for the axis, the radius drawn from the center 
 to a point of the surface, Q being the angle which this radius forms with 
 the axis of revolution. We have just seen that the general expression of 
 any shell of the spheroid is a + k . a h . (1 m 2 ), k being independent 
 of a : whence it is easy to conclude that if we call a y', the ordinate let 
 fall from any point of the curve upon its axis, we shall have 
 
 ay' = a a k . sin. 2 d j c /" r , 
 
 c being the entire value of the integral f - , taken from the center to 
 
 the surface. 
 
 575. Now consider the general case in which the spheroid always fluid 
 at its surface, may contain a solid nucleus of any figure whatever, but dif- 
 fering but little from the sphere. The radius drawn from the center of 
 gravity of the spheroid to its surface, and the law of gravity at this sur- 
 face have some general properties, which it is the more essential to con- 
 sider, inasmuch as these properties are independent of every hypothesis. 
 
 The first of these properties is, that in the state of equilibrium the 
 fluid part of the spheroid must always be disposed so, that the function 
 Y (1) may disappear from the expression of the radius drawn from the cen- 
 ter of gravity of the whole spheroid to its surface ; so that the center of 
 gravity of this surface coincides with that of the spheroid. 
 
 To show this, we shall observe that R being supposed to represent the 
 radius drawn from the center of gravity of the spheroid to any one of its 
 molecules, the expression of this molecule will be R 2 . d R . d m . d *, 
 and we shall have by 556, in virtue of the properties of the center of 
 
 gravity, 
 
 =f s R 3 . d R.dm.d w.m; 
 
 = R 8 . d R.d m.d *. V 1 m 2 . sin. w; 
 
 =/g R 3 . d R . d m . d *r . V 1 m 2 . cos. ~. 
 
 Conceive the integral fg R 3 . d R taken relatively to R from the origin 
 of R to the surface of the spheroid, and then developed into a series of 
 the form
 
 BOOK I.] NEWTON'S PRINCIPIA. 281 
 
 N (i) being whatever i may be, subject to the equation of partial differ- 
 ences, 
 
 we shall have by No. 556, when i is different from unity, 
 
 =/N (i >. m d m . d ; =/N . d m . d . V 1 m 2 . sin. 
 and 
 
 =/N Q -'. dm. d. V 1 m 2 .cos. w. 
 
 The three preceding equations given by the nature of the center of 
 gravity, will become 
 
 =/N (1 >mdm.dw; =/N< 1 >dm.dw. V 1 m 2 .sin.*; 
 
 =/N to d m . d * . V I m 2 . cos. -a. 
 N to is of the form 
 
 H m + H'. V 1 m 2 . sin. -a + H". V 1 m 2 . cos. *,. 
 Substituting this value, in these three equations, we shall have 
 
 H = 0; H' = 0; H" = 0; 
 
 where N (1) = ; this is the condition necessary that the origin of R is at 
 the center of gravity of the spheroid. 
 
 Now let us see, what N (1) becomes relatively to the spheroids differing 
 little from the sphere, and covered over with a fluid in equilibrium. In 
 this case we have R = a (1 + a y), and the integral f% . R 3 . d R, be- 
 comes \ J l d.a*(l + 4a y)}, the differential and integral being rela- 
 tive to the variable a, of which is a function. Substituting for y its va- 
 lue Y + Y + Y + &c., we shall have 
 N<" = /d (a 4 Y>). 
 
 The equation (2) of No. 573 gives, at the surface where a = 1, and 
 observing that Z (1) is nothing 
 
 the value of Y (1) in the second member of this equation, being relative to 
 the surface ; thus, N (1) being nothing, when the origin of R is at the cen- 
 ter of gravity of the spheroid, we have in like manner Y (1) = 0. 
 
 576. The permanent state of equilibrium of the celestial bodies, makes 
 known also some properties of their radii. If the planets did not turn ex- 
 actly, or at least if they turned not nearly, round one of their three principal 
 axes of rotation, there would result in the position of their axes of rota- 
 tion, changes which for the earth above all would be sensible; and since 
 the most exact observations have not led to the discovery of any, we may 
 conclude that long since, all the parts of the celestial bodies, and princi-
 
 282 A COMMENTARY ON [SECT. XII. & XIII. 
 
 pally the fluid parts of their surfaces, are so disposed as to render stable 
 their state of equilibrium, and consequently their axes of rotation. It is 
 in fact very natural to suppose that after a great number of oscillations, 
 they must settle in this state, in virtue of the resistances which they suffer. 
 Let us see, however, the conditions which thence result in the expression 
 of the radii of the celestial bodies. 
 
 If we name x, y, z the rectangular coordinates of a molecule d M of 
 the spheroid, referred to three principal axes, the axis of x being the axis 
 of rotation of the spheroid ; by the properties of these axes as shown in 
 dynamics, we have 
 
 0=/xy.dMj 0=/xz.dM; 0=/yz.dM; 
 the integrals ought to be extended to the entire mass of the spheroid, 
 R being the radius drawn from the origin of coordinates to the molecule 
 d M ; 6 being the angle formed by R and by the axis of rotation ; and 
 a being the angle which the plane formed by this axis and by R, makes 
 with the plane formed by this axis and by that of the principal axes, which 
 is the axis of y ; we shall have 
 
 x = Rm; y = R V 1 m *. cos. ; z = R V 1 m 2 . sin. -a ; 
 
 dM = gR J dRdm.d*r. 
 
 The three equations given by the nature of the principal axes of rota- 
 tion, will thus become 
 
 = f g .R 4 . dR.dm.dw.m VI m 2 . cos. 
 
 = y^ . R 4 . d R . d m . d w . m VI m 2 . sin. -a ; 
 
 = /.R 4 . d R.dm.dw.(l m 2 ) sin. 2 *. 
 
 Conceive the integral fg R 4 d R taken relatively to R, from R = 0, 
 to the value of R at the surface of the spheroid, and developed into a 
 series of the form U W + U (1 > + U (2) + &c. ; U (i > being, whatever i may 
 be, subject to the equation of partial differences, 
 
 d m 
 
 We shall have by the theorem of No. 556, where i is different from 2, 
 a^l by observing that the functions m V 1 m 2 . cos. w> m V I m 2 . sin. w, 
 and (1 m 8 ) sin. 2 *, are comprised in the form U (2) 5 
 
 = / U 0) . d m . d . m . VI m z . cos. ; 
 0=/U^. dm.dw.m. V I m 2 . sin. ; 
 =/U W. dm.d*r.(l m s )sin. 2 .
 
 BOOK I.] NEWTON'S PRINCIPIA. 283 
 
 The three equations relative to the nature of the axes of rotation, will 
 thus become 
 
 =/U . dm.dw.m.Vl m 2 . cos. ; 
 = /U (2) . dm.d . m. V I m 2 . sin. ; 
 =/U. dm.dw. (1 m 2 ) sin. 2 . 
 
 These equations therefore depend only on the value of U : this value 
 is of the form 
 
 H (m 2 |) + H' m V 1 m 2 . sin. *r + H" m V 1 m 2 . cos. * + 
 
 H"' (1 m 2 ) sin. 2 * + H"" (1 m 2 ) cos. 2 : 
 substituting it in the three preceding equations, we shall have 
 
 H' = 0; H" = 0; H'" = 0. 
 
 It is to these three conditions that the conditions necessary to make the 
 three axes of x, y, z the true axes of rotation are reduced, and then U (2) 
 will be of the form 
 
 H (m 2 i) + H"" (1 m 2 ) cos. 2 ~. 
 
 When the spheroid is a solid differing but little from the sphere, and 
 covered with a fluid in equilibrium, we have R = a ( 1 + a y), and con- 
 sequently 
 
 /fR'.dR =ifed.{a s .(l+5ay)}. 
 
 If we substitute for y, its value Y (0 > + Y (1) + Y (2 > + &c. ; we shall - 
 have 
 
 U = afg d(a 5 Y). 
 The equation (2) of No. 573, gives for the surface of the spheroid, 
 
 Y (2) and Z (2) in the second member of this equation being relative to the 
 surface j we have therefore, 
 
 = ja 
 
 The value of Z & is of the form 
 
 -- O ( m 2 - J) + g' m ^ 1 - m 2 ' SU1 ' * + g" m ^ 1 - m 2 ' COS< 
 
 B 
 
 + g w (l_m 2 )sin. 2 W + g w/ (l m 2 ) cos. 2^; 
 and that of Y is of the form 
 
 h (m 2 |) + h' m V 1 m 2 . sin. * + h" m V 1 m 2 . cos. 
 + h w . (1 m 2 ) sin. 2 * + h w/ (1 m ) cos. 2 . 
 Substituting in the preceding equation, these values, and H (m 2 
 + H x/// (1 m 2 ) cos 2 , for U (2 > ; we shall have 
 
 h' o h" ^ . },"/ & 
 
 ~4<r/g.ada' "4r/g.a s da' "4r/g.a 8 da
 
 284 A COMMENTARY ON [SECT. XII. & XIII. 
 
 Such are the conditions which result from the supposition that the sphe- 
 roid turns round one of its principal axes of rotation. This supposition 
 determines the constants h', h", h'" by means of the values g', g", g'" ; 
 but it leaves indeterminate the quantities h and h"" as also the functions 
 Y< 3 >, Y<, & c . 
 
 If the forces extraneous to the attraction of the molecules of the sphe- 
 roid are reduced to the centrifugal force due to its rotatory motion ; we 
 shall have g' = 0, g" = 0, g"' = ; wherefore h' = 0, h" = 0, h"' = 0, 
 and the expression of Y (8) , will be of the form 
 
 h (m 2 J) + h"" (1 m 2 ) cos. 2 *. 
 
 577. Let us consider the expression of "gravity at the surface of the 
 spheroid. Call p this force ; it is easy to see by No. 569, that we shall 
 have its value by differentiating the second member of the equation (1) of 
 573 relatively to r, and by dividing its differential by d r ; which gives 
 at the surface 
 
 r [2Z + 2Z< 2 > +3r.Z+ 4r*. Z<*> + &c.J ; 
 these integrals being taken from a = 0, to a = 1. The radius r at the 
 surface is equal to 1 + a y, or equal to 
 
 1 + a jytfl) + Y < + Y + &c.] ; 
 we shall hence obtain 
 
 ~-Y + See.} 
 
 {2 Z ( ) + 2 Z t2 > + 3 Z + 4 Z + &c.j. 
 
 The integrals of this expression may be made to disappear by means of 
 equation (2) of No. 573, which becomes at the surface, 
 
 supposing therefore 
 
 P = !*r/ d - a3 
 
 B 
 
 we shall have 
 
 p = P + a R JY< 2 ) + 
 
 J5 Z +7Z + 9Z<*> + .. . + (2i+l)Z+&c.J. 
 
 By observations of the lengths of the seconds' pendulum, has been re- 
 
 cognised the variation of gravity at the surface of the earth. By dy- 
 
 namics it appears that these lengths are proportional to gravity; let
 
 BOOK I.] NEWTON'S PRINCIPIA. 285 
 
 therefore 1, L be the lengths of the pendulum corresponding to the gravi- 
 ties p, P ; the preceding equation will give 
 
 Relatively to the earth a Z< 2 > reduces by 567, to -| (m 2 ), or, 
 
 JO 
 
 which comes to the same, to ^-?. P. (m 2 ^), a <p being the ratio of 
 
 the centrifugal force to the equatorial gravity; moreover, Z^, Z^, &c. 
 are nothing ; we have therefore 
 
 1 = L + L. [Y<v + 2 Y( 3 > + 3 Y< 4 > + . . . + (i 1) Y| 
 
 + f ap.L.(m 8 i). 
 
 The radius of curvature of the meridian of a spheroid which has for its 
 radius 1 + a > is 
 
 + 
 
 n 
 
 dm / 
 
 designating therefore by c, the magnitude of the degree of a circle whose 
 radius is what we have taken for unity ; the expression of the degree of 
 the spheroid's meridian, will be 
 
 C / / 
 
 c -! i ft f d ' m y\ j ( 
 
 I 1 + I i 1 T a \ j 
 
 (^ \ d m / dm 
 
 y is equal to Y^ 4. Y< ! > 4. Y ( *> + &c. We may cause Y<> to disap- 
 pear, by comprising it in the arbitrary constant which we have taken for 
 the unit j and Y (1) by fixing the origin of the radius at the center of gravity 
 of the entire spheroid. This radius thus becomes, 
 1 + a [Y(V + Y< 3 ) + Y< 4 > + &c.J. 
 
 If we then observe that 
 
 dm 
 
 the expression of the degree of the meridian will become 
 
 a c . -- = - 
 1 m
 
 286 A COMMENTARY ON [SECT. XII. & XIII. 
 
 If we compare these expressions of the terrestrial radius with the length 
 of the pendulum, and the magnitude of the degree of the meridian, we 
 see that the term a Y (i - of the expression of the radius is multiplied by 
 i 1, in the expression of the length of the pendulum, and by i 2 -fi 1 
 in that of the degree ; whence it follows, that whilst i 1 is considerable, 
 this term will be more sensible in the observations of the length of the 
 pendulum than in that of the horizontal parallax of the moon which is 
 proportional to the terrestrial radius ; it will be still more sensible in the 
 measures of degrees than in the lengths of the pendulum. The reason of 
 it is, that the terms of the expression of the terrestrial radius undergo two 
 variations in the expression of the degree of the meridian ; and each dif- 
 ferentiation multiplies these terms by the corresponding exponent of m, 
 and this renders them the more considerable. In the expression of the 
 variation of two consecutive degrees of the meridian, the terms of the ter- 
 restrial radius undergo three consecutive differentiations; those which 
 disturb the figure of the earth from that of an ellipsoid, may thence be- 
 come very sensible, and the ellipticity obtained by this variation may be 
 very different from that which the observed lengths of the pendulum give. 
 These three expressions have the advantage of being independent of the 
 interior constitution of the earth, that is to say, of the figure and density 
 of its shells ; so that if we are going to determine the functions Y (2) , Y (3) , 
 &c. by measures of degrees of meridians and parallaxes, we shall have 
 immediately the length of the pendulum; we may therefore thus ascertain 
 whether the law of universal gravity accords with the figure of the earth, 
 and with the observed variations of gravity at its surface. These remark- 
 able relations between the expressions of the degrees of the meridian and 
 of the lengths of the pendulum may also serve to verify the hypotheses 
 proper to represent the measures of degrees of this meridian : this will be 
 perceptible from the application we now proceed to make to the hypothe- 
 sis proposed by Bouguer, to represent the degrees measured northward 
 in France and at the equator. 
 
 Suppose that the expression of the terrestrial radius is 1 + a Y (2) + 
 a Y (4) , and that we have 
 
 <*> = A(m 2 ); Y> = B (m 4 f m 2 + A) ; 
 
 it is easy to see that these functions of m satisfy the equations of partial 
 differences which Y (2) and Y (4) ought to satisfy. The variation of the de- 
 grees of the meridian will be, by what precedes, 
 
 J3 A ^B 
 
 a C
 
 BOOK I.] NEWTON'S PRINCIPIA. 287 
 
 Bouguer supposes this variation proportional to the fourth power of the 
 sine of the latitude, or, which nearly comes to the same, to m 4 ; the term 
 multiplied by m 2 , therefore, being made to disappear from the preceding 
 function, we shall have 
 
 thus in this case the radius drawn from the center of gravity of the earth 
 at its surface, will be in taking that of the equator for unity, 
 
 The expression of the length 1 of the pendulum, will become, denoting 
 by L, its value at the equator, 
 
 L+ fap.Lm 2 ^^(16 m z + 21m 4 ). 
 
 o4 
 
 Finally, the expression of the degree of the meridian, will be, calling c 
 its length at the equator, 
 
 105 
 c + -p-.A.c.m 4 . 
 
 We shall observe here, that agreeably to what we have just said, the 
 term multiplied by m 4 is three times more sensible in the expression of 
 the length of the pendulum than in that of the terrestrial radius, and five 
 times more sensible in the expression of the length of a degree, than in 
 that of the length of the pendulum ; finally, upon the mean parallel it 
 would be four times more sensible in the expression of the variation of 
 consecutive degrees, than in that of the same degree. According to Bou- 
 
 959 
 guer, the difference of the degrees at the pole and equator is r ; it is 
 
 OO I Oo 
 
 the ratio which, on his hypothesis, the measures of degrees at Pello, Paris 
 
 105 
 
 and the equator, require. This ratio is equal to -^j- . a A ; we have 
 
 o4 
 
 therefore 
 
 a A = 0. 0054717. 
 
 Taking for unity the length of the pendulum at the equator, the va- 
 riation of this length, in any place whatever, will be 
 
 _0. 0054717 . ilflm . + 81m1 + |. y ... 
 
 O A* 
 
 By No. 563, we have p = 0. 00345113, which gives f ap = 0. 0086278, 
 and the preceding formula becomes 
 
 0. 0060529. m 2 0. 0033796. m \
 
 288 A COMMENTARY ON [SECT. XII. & XIII. 
 
 At Pello, where m = sin. 74. 22', this formula gives 0. 0027016 for 
 the variation of the length of the pendulum. According to the observa- 
 tions, this variation is 0. 0044625, and consequently much greater ; thus, 
 since the hypothesis of Bouguer cannot be reconciled with the observations 
 made on the length of the pendulum, it is inadmissible. 
 
 578. Let us apply the general results which we have just found, to the 
 case where the spheroid is not sollicited by any extraneous forces, and 
 where it is composed of elliptic shells, whose center is at the center of 
 gravity of the spheroid. We have seen that this case is that of the earth 
 supposed to be originally fluid : it is also that of the earth in the hypo- 
 thesis where the figures of the shells are similar. In fact, the equation 
 (2) of No. 573 becomes at the surface where a = 1, 
 
 The shells being supposed similar, the value of Y (i) is, for each of 
 them, the same as at the surface ; it is consequently independent of a, and 
 we have 
 
 When i is equal to or greater than 3, Z (l) is nothing relatively to the 
 
 i 4- 3 
 
 earth; besides the factor 1 . . a ' is always positive ; therefore Y ' 
 
 *4 1 *J~ J. 
 
 is then nothing. Y (1) is also nothing by No. 575, when we fix the origin 
 of the radii at the center of gravity of the spheroid. Finally, by No. 577, 
 we have Z (2) equal to 
 
 4)4*A.a 2 da; Y > = -. (/*' i)/g a d a ; 
 we have therefore 
 
 Y w = _ -- _ _ 
 fg a*da(l a 2 ) 
 
 Thus the earth is then an ellipsoid of revolution. Let us consider there- 
 fore generally the case where the figure of the earth is elliptic and of re- 
 volution. 
 
 In this case, by fixing the origin of terrestrial radii at the center of 
 gravity of the earth, we have 
 
 yd) = 0; Y<*> = 0; Y< 4 > = ; &c.
 
 ROOK I.] NEWTON'S PRINCIPIA. 
 
 h being a function of a ; moreover we have 
 
 Z> = 0; Z = 0; Z<*> = ; &c. 
 
 the equation (2) of No. 573 will therefore give at the surface 
 
 = 6./ f d(a 5 h) + 5. (?>-2h)/,d.a 8 . . . (1) 
 
 This equation contains the law which ought to exist to sustain the 
 equilibrium between the densities of the shells of the spheroid and their 
 ellipticities ; for the radius of a shell being a { I +a Y (0) a h (/A 2 ^)] 5 
 if we suppose, as we may, that Y w = ^ h, this radius becomes 
 a (1 h . ,a 2 ), and a h is the elh'pticity of the shell. 
 
 At the surface, the radius is 1 a h . /a, * ; whence we see that the de- 
 crements of the radii, from the equator to the poles, are proportional to 
 p 2 , and consequently to the square of the sines of the latitude. 
 
 The increment of the degrees of the meridian from the equator to the 
 poles is, by the preceding No., equal to 3 a h c . t* *, c being the degree 
 of the equator ; it is therefore also proportional to the square of the sine 
 of the latitude. 
 
 The equation (1) shows us that the densities being supposed to decrease 
 from the center to the surface, the ellipticity of the spheroid is less than 
 in the case of homogeneity, at least whilst the ellipticities do not increase 
 from the surface to the center in a greater ratio than the inverse ratio of 
 
 the square of the distances to this center. In fact, if we suppose h = z , 
 we shall have 
 
 If the ellipticities increase in a less ratio than =-, u increases from the 
 
 a 8 
 
 center to the surface, and consequently d u is positive ; besides, d g is ne- 
 gative by the supposition that the densities decrease from the center to the 
 surface; thus J( /( d ufa 3 d g) is a negative quantity, and making at the 
 surface 
 
 / f d(a 5 h) = (h_f)/gd.a 3 , 
 f will be a positive quantity. Hence equation (1) will give 
 
 a h will therefore be less than - ~ , and consequently it will be less than 
 
 T? 
 
 Vot. II. T
 
 290 A COMMENTARY ON [SECT. XII. & XIIl. 
 
 in the case of homogeneity, where d being equal to nothing f is also equal 
 to zero. 
 
 Hence It follows, that in the most probable hypotheses, the flattening of 
 
 the spheroid is less than - ; for it is natural to suppose that the shells 
 
 T* 
 
 of the spheroid are denser towards the center, and that the ellipticities 
 increase from the surface to the center in a less ratio than , this ratio 
 
 cl 
 
 giving an infinite radius for shells infinitely near to the center, which is 
 absurd. These suppositions are the more probable, inasmuch as they 
 become necessary in the case where the fluid is originally fluid ; then the 
 denser shells are, as we have seen, the nearer to the center, and the ellip- 
 ticities so far from increasing from the surface to the center, on the con- 
 trary, decrease. 
 
 If we suppose that the spheroid is an ellipsoid of revolution, covered 
 with a homogeneous fluid mass of any depth whatever, by calling a' the 
 semi-minor axis of the solid ellipsoid, and a h' its ellipticity, we shall have 
 at the surface of the fluid, 
 
 /fd(a 5 h) = h a' s h'+/fd(aMi); 
 
 the integral of the second member of this equation being taken relatively 
 to the interior ellipsoid, from its center to its surface, and the density of 
 the fluid which covers it being taken for unity. The equation (1) will 
 give for the expression of the ellipticity h, of the terrestrial spheroid, 
 5a<p{l a /3 +/gda 3 j 6 h' . a' 5 + 6a/gd (a 5 h) . 
 
 4 10 a' 3 + 10 ./gd. a 3 
 the integrals being taken from a = to a = a'. 
 
 Let us now consider the law of gravity, or which comes to the same, 
 that of the length of the pendulum at the elliptic surface in equilibrium. 
 The value of 1, found in the preceding No., becomes in this case 
 
 1 = L + L f i p h}(m 2 i) 5 
 
 making, therefore, L' = L a L (| <p h), we shall have, in neglecting 
 quantities of the order a 2 , 
 
 1 = L' + aL'(| p h)/* 2 ; 
 
 an equation from which it results that L' is the length of the seconds' 
 pendulum at the equator, and that this length increases from the equator 
 to the poles, proportionally to the square of the sine of the latitude. 
 
 If we call t the excess of the length of the pendulum at the pole above 
 its length at the equator, divided by the latter, we shall have 
 a E = a ( <p h);
 
 BOOK I.] NEWTON'S PRINCIPIA. 291 
 
 and consequently 
 
 a-f-ah = ->ap; 
 
 a remarkable equation between the ellipticity of the earth and the varia- 
 tion of the length of the pendulum from the equator to the poles. In the 
 case of homogeneity ah = a p ; hence in this case = h ; but if 
 the spheroid is heterogeneous, as much as a h is above or below f a <p, so 
 much is a i above or below the same quantity. 
 
 579. The planets being supposed covered with a fluid in equilibrium, it 
 is necessary, in the estimate of their attractions, to know the attraction of 
 spheroids whose surface is fluid and in equilibrium : we may express it 
 very simply in this way. Resume the equation (5) of No. 558 ; the signs 
 of integration may be made to disappear by means of equation (2) of No. 
 573, which gives at the surface of the spheroid, 
 
 thus fixing the origin of the radii r at the center of gravity of the spheroid 
 which makes Y W disappear; then observing that Z w is nothing, and that Y (0) 
 
 being arbitrary, we may suppose -^ Y W Z w = 0, the equation (5) 
 
 A 
 
 of 558, will give 
 
 A. of 
 
 an expression in which we ought to observe that -$-fi d . a 3 expresses the 
 
 9 
 
 mass of the spheroid, since, in the case of r being infinite, the value of V 
 is equal to the mass of the spheroid divided by r. Hence the attraction 
 
 of the spheroid parallel to r will be f -j \ ; the attraction perpendicu- 
 
 lar to this radius, in the plane of the meridian will be -- 
 
 r r 
 
 -T - ; finally, the attraction perpendicular to this same radius in the 
 
 -T 
 direction of the parallel will be 
 
 r V 1 m 2 
 The expression of V, relatively to the earth supposed elliptic, becomes 
 
 M being the mass of the earth. 
 
 T2
 
 292 A COMMENTARY ON [SECT. XII. XIII. 
 
 580. Although the law of attraction in the inverse ratio of the square 
 of the distance is the only one that interests us, yet equation (1) of 554, 
 affords a determination so simple of the gravity at the surface of homoo-e- 
 neous spheroids in equilibrium, whatever is the exponent of the power of 
 the distance to which the attraction is proportional, that we cannot here 
 omit it. The attraction being as any power n of the distance, if we de- 
 note by d m a molecule of the spheroid, and by f its distance from the 
 point attracted, the action of d m upon this point multiplied by the element 
 d f of its direction, will be d ^ f n . d f. The integral of this quantity, 
 
 d fjt> f n + 1 
 taken relatively to f, is , and the sum of these integrals ex- 
 
 V 
 
 tended to the entire spheroid is ^ ; supposing, as in 554, that V = 
 
 If the spheroid be fluid, homogeneous, and endowed with rotatory mo- 
 tion, and not sollicited by any extraneous force, we shall have at the sur- 
 face, in the case of equilibrium, by No. 567, 
 
 const. = + i g r 2 (1 m ), 
 
 r being the radius drawn from the center of gravity of the spheroid at its 
 surface, and g the centrifugal force at the distance 1 from the axis of ro- 
 tation. 
 
 The gravity p at the surface of the spheroid is equal to the differential 
 of the second member of this equation taken relatively to r, and divided 
 by d r, which gives 
 
 i (dVx r(l __ m2> 
 
 Let us now resume equation (1) of 554, which is relative to the sur- 
 face, 
 
 /d Vx A , (n + 1) A (n + 1)V. 
 VH7J = ~2~a~~ 2 a 
 
 this equation, combined with the preceding ones, gives 
 
 p = const. + { ( "+ a 1)r -- l} g r (1 - m*). 
 
 At the surface, r is very nearly equal to a ; by making them entirely so, 
 for the sake of simplicity, we shall have 
 
 p = const. + -. g (1 m 2 ) 
 Let P be the gravity at the equator of the spheroid, and
 
 JBooK I.] NEWTON'S PRINCIPIA. 293 
 
 the ratio of the centrifugal force to gravity at the equator; we shall 
 have 
 
 whence it follows that, from the equator to the poles, gravity varies as the 
 square of the sine of the latitude. In the case of nature, where n = 2, 
 we have 
 
 p = P [I + p.m 2 }; 
 which agrees with what we have before found. 
 
 But it is remarkable that if n = 3, we have p = P, that is to say, that 
 if the attraction varies as the cube of the distance, the gravity at the sur- 
 face of homogeneous spheroids is every where the same, whatever may be 
 the motion of rotation. 
 
 581. We have only retained, in the research of the figure of the celestial 
 bodies, quantities of the order a ; but it is easy, by the preceding analysis, 
 to extend the approximations to quantities of the order a 2 , and to superior 
 orders. For thai purpose, consider the figure of a homogeneous fluid 
 mass in equilibrium, covering a spheroid differing but little from a sphere, 
 and endowed with a rotatory motion ; which is the case of the earth and 
 planets. The condition of equilibrium at the surface gives, by No. 557, 
 the equation 
 
 const. = V -f- r s (m 8 i). 
 
 m 
 
 The value of V is composed, 1st, of the attraction of the spheroid co- 
 vered by the fluid upon the molecule of the surface, determined by the 
 coordinates r, 6, and ; 2dly, of the attraction of the fluid mass upon this 
 molecule. But the sum of these two attractions is the same as the sum of 
 the attractions, 1st, of a spheroid supposing the density of each of its shells 
 diminished by the density of the fluid; 2dly,of a spheroid of the same density 
 as the fluid, and whose exterior surface is the same as that of the fluid. 
 Let V be the first of these attractions and V" the second, so that 
 V = V+V" ; we shall have, supposing g of the order a and equal to g', 
 
 const. = V' + V" -f . r * . (m 2 - ). 
 
 9 
 
 W T e have seen in 553 that V may be developed into a series of the form 
 
 V + -^ + TT- + &C. 
 
 U (i) being subject to the equation of partial differences, 
 / d {(1 m- % /d UWx 1 X /d 2 U> 
 
 = \ ^ m 
 
 T3
 
 291 A COMMENTARY ON [SECT. XII. & XIII. 
 
 and by the analysis of 561, we may determine U (i) , with all the accuracy 
 that may be wished for, when the figure of the spheroid is known. 
 In like manner V" may be developed into a series of the form 
 
 U (0) U < f > U ( 2 > 
 < + i^. + ^ + & c . 
 r r* r 3 
 
 U, (i) being subject to the same equation of partial differences as U (i >. If 
 we take for the unit of density that of the fluid, we have, by 561, 
 
 U fli - 4 * z ). 
 
 - (i + 3) (2 i + 1 ' 
 
 r ! + 3 being supposed developed into the series 
 
 Z + ZW + Z< 2 > +&c. 
 
 in which Z is subject to the same equation of partial differences, as U w . 
 The equation of equilibrium will therefore become 
 
 i being equal to greater than unity. 
 
 If the distance r from the molecule attracted to the center of the sphe- 
 roid were infinite, V would be equal to the sum of the masses of the sphe- 
 roid and fluid divided by r ; calling, therefore, m this mass, we have 
 U<) + U/ 0) = m. Carrying the approximation only to quantities of the 
 order a 2 , we may suppose 
 
 r = 1 + y + 2 y'; 
 which gives 
 
 Suppose 
 
 y = Y (1 > + Y (2 > + Y + &c. 
 
 Y' &c. 
 
 y"= 
 
 Y fl, Y 7 {i) , and M W being subject to the same equation of partial differ- 
 ences as U (i) ; we shall have 
 
 Z = (i + 3) a Y (> + (i + ^ ( ^ + 8) s M + (i + 3) Y'. 
 
 Then observe that U (i) is a quantity of the order a, since it would be 
 nothing if the spheroid were a sphere ; thus carrying the approximation 
 only to terms of the order a 2 , U will be of this form U' + 2 U /y . 
 Substituting therefore these values in the preceding equation of equili- 
 brium, and there changing r into 1 + a y + 2 /, we shall have to quan- 
 tities of the order a 3 ,
 
 BOOK I.] NEWTON'S PRINCIPIA. 295 
 
 const. = (Jt, [1 y + 2 y 2 2 y'} 
 
 'a. U' W + 2 U" ) (i + 1) a* y U' 
 
 + 2 
 
 T a g Y (i) ag. 
 
 h 2i + I 1 2i+l y f 2i + l 3 
 
 !' *(i + 2) M(i) 
 
 Equating separately to zero the terms of the order , and those of the 
 order a 2 , we shall have the two equations, 
 
 Y c " = 2 u/ "' - 
 
 C' being an arbitrary constant. The first of these equations detects Y (i > 
 and consequently the value of y. Substituting in the second member of 
 the second equation, we shall develope by the method of No. 560. in a 
 series of the form 
 
 N (i) being subject to the same equation of partial differences as U (i \ and 
 we shall determine the constant C' in such a manner that N w is nothing; 
 thus we shall have 
 
 y ffl= NO 
 
 2i + 1 
 
 and consequently 
 
 _ 
 
 The expression of the radius r of the surface of the fluid will thus be 
 determined to quantities of the order 3 , and we may, by the same process, 
 carry the approximation as far as we wish. We shall not dwell any longer 
 upon this object, which has no other difficulty than the length of calcula- 
 tions; but we shall derive from, the preceding analysis this important con- 
 clusion, namely, that we may affirm that the equilibrium is rigorously pos- 
 sible, although we cannot assign the rigorous figure which satisfies it ; for 
 we may find a series of figures, which, being substituted in the equation of 
 equilibrium, leave remainders successively smaller and smaller, and which 
 
 become less than any given quantity. 
 
 T4
 
 296 A COMMENTARY ON [SECT. XII. & XIII. 
 
 COMPARISON OF THE PRECEDING THEORY WITH OBSERVATIONS. 
 
 582. To compare with observations the theory we have above laid down, 
 we must know the curve of the terrestrial meridians, and those which we 
 trace by a series of geodesic operations. If through the axis of rotation 
 of the earth, and through the zenith of a plane at its surface we imagine 
 a plane to pass produced to the heavens ; this plane will trace a great cir- 
 cle which will be the meridian of the plane : all points of the surface of 
 the earth which have their zenith upon this circumference, will lie under 
 the same celestial meridian, and they will form, upon this surface, a curve 
 which will be the corresponding terrestrial meridian. 
 
 To determine this curve, represent by u = the equation of the surface 
 of the earth j u being a function of three rectangular coordinates x, y, z. 
 Let x', y', z', be the three coordinates of the vertical which passes through 
 the place on the earth's surface determined by the coordinates x, y, z ; we 
 shall have by the theory of curved surfaces, the two following equations, 
 
 d 
 
 Adding the first multiplied by the indeterminate X to the second, we 
 get 
 
 d z' =jll _ . dx'_ X d/. 
 
 This equation is that of any plane parallel to the said vertical : this ver- 
 tical produced to infinity coinciding with the celestial meridian, whilst its 
 foot is only distant by a finite quantity from the plane of this meridian, 
 may be deemed parallel to that plane. The differential equation of this 
 plane may therefore be made to coincide with the preceding one by suita- 
 bly determining the indeterminate X. 
 
 Let 
 
 d z' = a d x' + b d y', 
 
 be the equation of the plane of the celestial meridian ; comparing it with 
 the preceding one, we shall get 
 
 /d u\ /d u\ . /du\ , v 
 
 IT) al-j ) b (y ) = 0; (a) 
 
 \d z/ Vd x/ \d y/ 
 
 To get the constants a, b, we shall suppose known the coordinates of
 
 BOOK I.] NEWTON'S PRINCIPIA. 297 
 
 the foot of the vertical parallel to the axes of rotation of the earth and that 
 of a given place on its surface. Substituting successively these coordi- 
 nates in the preceding equation, we shall have two equations, by means of 
 which we shall determine a and b. The preceding equation combined 
 with that of the surface u = 0, will give the curve of the terrestrial meri- 
 dian which passes through the given plane. 
 
 If the earth were any ellipsoid whatever, n would be a rational and 
 entire function of the second degree in x, y, zj the equation (a) would 
 therefore then be that of a plane whose intersection with the surface of the 
 earth, would form the terrestrial meridian : in the general case, this me- 
 ridian is a curve of double curvature. 
 
 In this case the line determined by geodesic measures, is not that of 
 the terrestrial meridian. To trace this line, we form a first horizontal 
 triangle of which one of the angles has its summit at the origin of 
 this curve, and whose two other summits are any visible objects. We de- 
 termine the direction of the first side of the curve, relatively to two sides 
 of the triangle, and to its length from the point where it meets the side 
 which joins the two objects. We then form a second horizontal triangle 
 with these objects, and a third one still farther from the origin of the 
 curve. This second triangle is not in the plane of the first; it has nothing 
 in common with the former, but the side formed by the two first objects ; 
 thus the first side of the curve being produced, lies above the plane of 
 this second triangle ; but we bend it down upon the plane so as always to 
 form the same angles with the side common to the two triangles, and it is 
 easy to see that for this purpose it must be bent along a vertical to this 
 plane. Such is therefore the characteristic property of the curve traced 
 by geodesic operations. Its first side, of which the direction may be 
 supposed any whatever, touches the earth's surface; its second side is this 
 tangent produced and bent vertically ; its third is the tangent of the se- 
 cond side bent vertically, and so on. 
 
 If through the point where the two sides meet, we draw in the tangent 
 plane at the surface of the spheroid, a line perpendicular to one of the 
 sides, it is clear that it will be perpendicular to the other ; whence it follows* 
 that the sum of the sides is the shortest line which can be drawn upon the 
 surface between their extreme points. Thus the lines traced by geodesic 
 operations, have the property of being the shortest we can draw upon the 
 surface of the spheroid between any two of their points; and p. 2 94, Vol. I. 
 they would be described by a body moving uniformly in this surface.
 
 298 A COMMENTARY ON [SECT. XII. & XIII. 
 
 Let x, y, z be the rectangular coordinates of any part whatever of the 
 curve ; x + cl x, y + d y, z -J- d z will be those of points infinitely near to 
 it. Call d s the element of the curve, and suppose this element produced 
 by a quantity equal to d s ; x + 2 d x, y + 2 d y, z + 2 d z will be the 
 coordinates of extremity of the curve thus produced. By bending it ver- 
 tically, the coordinates of this extremity will become x4-2dx + d*x, 
 y + 2 d y + d 2 y, z + 2 d z -f- d 2 z; thus d 2 x, d 2 y, d 2 z 
 will be the coordinates of the vertical, taken from its foot ; we shall there- 
 fore have by the nature of the vertical, and by supposing that u = is 
 the equation of the earth's surface, 
 
 d_u 
 
 equations which are different from those of the terrestrial meridian. In these 
 equations d s must be constant ; for it is clear that the production of 
 d s meets the foot of the vertical at an infinitely small quantity of the fourth 
 order nearly. 
 
 Let us see what light is thrown upon the subject of the figure of the earth 
 by geodesic measures, whether made in the directions of the meridians, or in 
 directions perpendicular to the meridians. W"e may always conceive an ellip- 
 soid touching the terrestrial surface at every point of it, and upon which, the 
 geodesic measures of the longitudes and latitudes from the point of contact, 
 for a small extent, would be the same as at the surface itself. If the entire 
 surface were that of an ellipsoid, the tangent ellipsoid would every where 
 be the same ; but if, as it is reasonable to suppose, the figure of the meri- 
 dians is not elliptic, then the tangent ellipsoid varies from one country to 
 another, and can only be determined by geodesic measures, made in diffe- 
 rent directions. It would be very interesting to know the osculating ellip- 
 soids at a great number of places on the earth's surface. 
 
 Let u = x 2 + y s + z 2 1 2 a u', be the equation to the surface 
 of the spheroid, which we shall suppose very little different from a sphere 
 whose radius is unity, so that a is a very small quantity whose square may 
 be neglected. We may always consider u' as a function of two variables 
 x, y ; for by supposing it a function of x, y, z, we may eliminate z by 
 means of the equation z = V 1 x 2 - y *. Hence, the three equa- 
 tions found above, relatively to the shortest line upon the earth's surface, 
 become
 
 BOOK I.] NEWTON'S PRINCIPIA. 299 
 
 xd 2 z zd 2 x = a - -- d 2 z ; 
 
 u 
 
 (O) 
 
 This line we shall call the Geodesic line. 
 
 Call r the radius drawn from the center of the earth to its surface, 6 the 
 angle which this radius makes with the axis of rotation, which we shall 
 suppose to be that of z, and p the angle which the plane formed by this 
 axis and by r makes with the plane of x, y ; we shall have 
 
 x = r sin. 6. cos. 9 ; y = r sin. 6 sin. p ; z = r cos. 6 ; 
 whence we derive 
 
 r 2 sin. * 6. dp = xdy ydx; 
 
 r 2 d = (xdz zdx) cos. p + (y d z zdy) sin. p 
 d s 2 = dx 2 -fdy 2 +dz 2 =dr*+r 2 d0 2 +r 2 dp 2 sin. 2 0. 
 Considering then u', as a function of x, y, and designating by 4 the lati- 
 tude ; we may suppose in this function r= 1, and 4= 100 0, which gives 
 
 x = cos. 4 cos. p ; y = cos. 4 sin. p ; 
 thus we shall have 
 
 rdu' 
 
 but we have 
 
 x 2 + y 2 = cos. 2 -vj/ ; - = tan. <p ; 
 
 whence we derive 
 
 xdx + ydy , xdy ydx 
 
 d ^ = -- : - r 1 -^ f~; d o = - * J - . C os. 2 f. 
 sin. 4 cos. -vf- x* 
 
 Substituting these values of d 4 1 and of d p in the preceding differential 
 equation in u', and comparing separately the coefficients of d x and d y ; 
 
 we shall have 
 
 /d u\ _ _ cos, p /d u'\ sin. <p /d u\ 
 
 VoTx/ = ~ sin. ^ ' VoSf/ ~ cos. ^ ' \d )' 
 
 /d u\ _ ^ sin. <? /d u\ cos, p /d u\ 
 \d y / sin. -^ " Vd -^ / cos. 4- \d f / ' 
 
 which give 
 
 ( d -^ 
 
 . (x d 2 y - y d x) 
 v * 
 
 sin. / cos. 
 
 COS.
 
 300 A COMMENTARY ON [SECT. XII. & XIII. 
 
 But neglecting quantities of the order , we have xd 2 y y d 2 x = ; 
 and the two equations 
 
 xd 2 z zd 2 x = 0, yd 8 z zd 2 y = 0, 
 give 
 
 and 
 
 x* + y + z 2 = 1 
 gives 
 
 xd ! x + yd 2 y + zd s z + ds 2 = 0; 
 substituting for z d 2 z its preceding value, we shall have 
 
 xd 2 x + yd 2 y = (x 2 + y s )ds 2 = d s 2 cos. 
 wherefore 
 
 d u'\ , /d 
 
 The first of equations (O), will thus give by integration, 
 
 r 2 d p sin. 2 d = c d s + a d syd s(^ -); ..... (p) 
 
 c being the arbitrary constant. 
 
 The second of equations (O) gives 
 
 d.(xdz zdx) = a 
 
 but it is easy to see by what precedes, that we have 
 
 d 2 z = ds 2 . sin. 4 1 > 
 we have therefore 
 
 d(xdz zdx) = ads *(- 7) sin. >{/; 
 
 * Cl A. * 
 
 in like manner we have 
 
 d(ydz zdy) = ads 2 ( , 7) sin. 4/; 
 
 we shall therefore have 
 
 r 2 d^ = c'ds sin. f> + c" d s cos. <p 
 
 ads cos. <pf d s 1 (T r ) cos - 9 + (T ) si n - 9 ton- 4* r 
 
 a d s sin. p/d s| (~)sin. p (^)cos. <p tan. -4/|; . (q) 
 
 First consider the case in which the first side of the Geodesic line is 
 parallel to the corresponding plane of the celestial meridian. In this case 
 d f is of the order , as also d r ; we have, therefore, neglecting quantities 
 of the order a 2 , ds = rdd, the arc s being supposed to increase from
 
 BOOK I.] NEWTON'S PRINCIPIA. 301 
 
 the equator to the poles. -\J/ expressing the latitude, it is easy to see that 
 we have 6 = 100 ^ \ , which gives 
 
 we have therefore 
 
 Thus naming t the difference in latitude of the two extreme points of 
 the arc s, we shall have 
 
 u/ being here the value of u' at the origin of s. 
 
 If the earth were a solid of revolution, the geodesic line would be al- 
 ways in the plane of the same meridian ; it departs from it if the parallels 
 are not circles ; the observations of this deflection may therefore clear up 
 this important point of the theory of the earth. Resume the equation (p) 
 and observe that in the present case, d p and the constant c of this equa- 
 tion are of the order a, and that we may there suppose r = 1, d s = d 4*, 
 6 55 100 -vj>; we shall thus get 
 
 -ty ( - 
 
 cos. = -, 
 
 However, if we call V the angle which the plane of the celestial meri- 
 dian makes with that of x, y, whence we compute the origin of the angle 
 p; we shall have d x' = tan. V = d y'; x', y', 2! being the coordinates 
 of that meridian whose differential equation, as we have seen in the pre- 
 ceding No., is 
 
 d z' = a d x' + b d y'. 
 
 Comparing it with the preceding one, we see that a, b are infinite and 
 
 such that -- r- = tan. V, the equation (a) of the preceding No. thus 
 
 gives 
 
 _ /d u\ ,, /d u\ 
 
 = t-j ). tan. V (-. ), 
 \d x/ \d y/ 
 
 whence we derive 
 
 We may suppose V = <p, in the terms multiplied by ; moreover 
 
 * =r tan. p : w have therefore 
 x
 
 A COMMENTARY ON [SECT. XII. & XIII. 
 
 /du\ 
 
 vH / 
 
 cos. 4 cos. <t> Uan, <p tan. V\ = . 
 
 cos. 4 cos. f> 
 
 which gives 
 
 cos. 2 4 ' 
 
 The first side of the Geodesic line, being supposed parallel to the plane 
 of the celestial meridian, the differentials of the angle V, and of the dis- 
 tance (<f> V) cos. 4 from the origin of the curve to the plane of the 
 celestial meridian ought to be nothing at this origin ; we have therefore 
 at this point 
 
 d u 
 
 and consequently, the equation (p) gives 
 
 u. and 4-, being referred to the origin of the arc s. 
 
 At the extremity of the measured arc, the side of the curve makes with 
 the plane of the corresponding celestial meridian an angle very nearly 
 equal to the differential of (p V) cos. 4j divided by d 4> V being sup- 
 posed constant in the differentiation ; by denoting therefore this angle by 
 a, we shall have 
 
 d <? TT\ 
 
 w = - cos. 4 (P V) sin. 4- 
 
 If we substitute for -v-y- its value obtained from the equation (p), and for 
 
 p V, its preceding value, we shall have 
 
 a C /d u/\ /d u'\ . ., , , /d u'\ ") 
 
 " = -- ! (-j-^)tan. 4, (-J Jtan. 4 +J d4(- r -J V; 
 cos. pt\dp/ * d / T Vdp/J' 
 
 the integral being taken from the origin of the measured arc, to its extre- 
 mity. Call the difference in latitude of its two extreme points ; i being 
 supposed sufficiently small for i 2 to be rejected, we shall have 
 
 d 2 
 
 in which the values of 4, (, V and(-, 3 r^ must be referred, for the 
 \ d <p 1 \a <f> d 4' 
 
 greater exactness, to the middle of the measured arc. The angle -a must be
 
 BOOK I.] NEWTON'S PRINCIPIA. 303 
 
 supposed positive, when it quits the meridian, in the direction of the in- 
 crements off. 
 
 To obtain the difference in longitude of the two meridians correspond- 
 ing to the extremities of the arc, we shall observe, that u/, V,, >}/,, and 
 9,t being the values of u', V, %J/, and p, at the first extremity, we have 
 
 _ --- _ V 
 
 ' '~ cos.'-v},/ : cos. 2 ^ 
 
 but we have very nearly, neglecting the square of t, 
 
 c i /d u 
 
 / u\ 
 
 = a 1 -j - 1 
 \d < / 
 
 cos. v 
 we shall have, therefore, 
 
 v v <** f /d u/ 
 
 - 
 
 tan. 
 
 \ , , / "u 
 ) tan. 4>. + { j j-V 
 - 
 
 cos. -/, \fy 
 
 whence results this very simple equation, 
 (V V,) sin. ^ = ,; 
 
 thus we may, by observation alone, and independently of the knowledge 
 of the figure, determine the difference in longitude of the meridians cor- 
 responding to the extremities of the measured arc ; and if the value of the 
 angle -a is such that we cannot attribute it to errors of observations, we 
 shall be certain that the earth is not a spheroid of revolution. 
 
 Let us now consider the case where the first side of the Geodesic line 
 is perpendicular to the corresponding plane of the celestial meridian. If 
 we take this plane for that of x, y, the cosine of the angle formed by this 
 
 y 7 d x g - 1- d z g 
 side upon the plane, will be - r^- - ; thus this cosine being no- 
 
 thing at the origin, we have d x = 0, d z = 0, which gives 
 
 d . r sin. 6 cos. <f> = ; d . r cos. 6 = ; 
 and consequently 
 
 r d 6 = r d <p sin. 6 . cos. 6 . tan. f ; 
 
 but we have, to quantities of the order a% ds = rdp sin. 6; we shall 
 have, therefore, at the origin, 
 
 d 6 _ tan. <p . cos. 6 
 d s r 
 
 The constant c", of the equation (q), is equal to the value of x d z 
 z d x, at the origin ; it is therefore nothing, and the equation (q) gives at 
 the origin, 
 
 d0 c'
 
 304 A COMMENTARY ON [SECT. XII. & XIII. 
 
 we have, therefore, observing that p is here of the order a, and that thus 
 neglecting quantities of the order a 2 , we have sin. <p = tan. p , 
 
 c' = r, cos. O^ 
 
 the quantities r, and 6, being relative to the origin ; therefore, if we con 
 sider that at this origin the angle p is what we have before called it, 
 
 <p, V,, and whose value we have found equal to - r~r* we 
 
 COS. y / 
 
 have at this point 
 
 d0, /d u/\ sin. 4v 
 
 ds~ a \~d~? ) cos. z -fy,' 
 
 The equation (q) then gives 
 
 dM, _ cos.O, d p, (<^f\ . 
 
 ds 2 " r y 'ds " '\d^J' 
 
 but we have 
 
 ^ = -4; r, = !+/; tf^lOOo-^-afe'); 
 ds ^sm. O/ \d-v}// 
 
 we shall get therefore 
 
 ^ = (1 - 2 . u/) tan. 4, + (^)tan.= 4-, 
 Observing that at the origin, 
 
 T -- -- 
 
 ds r/ffO*^ cos. 
 the equation (p) gives 
 
 c = r, sin. ^ ; 
 whence we get 
 
 d u/ _ d 0, /d u/ 
 
 Sa.-j-^- 2.T 'cos. ^ l- 
 d 8 p, _ d s d s ' \ 
 
 d s 2 " " r, sin. ^ r 7 sin. 8 ^ cos. 
 
 and consequently 
 
 d 2 ^ _ /d u/\ 2 cos. -4/, 
 
 ds 2 " vdp/' cos. 4 -4/ 7 
 
 The equation 
 
 gives, by retabing amongst the terms of the order s 2 , only those which are 
 independent of a, 
 
 7 as / d'll/x 
 
 ds 2 cos. ^/\dpd-4/r 

 
 BOOK I.] NEWTON'S PRINCIPIA. 305 
 
 wherefore 
 
 The difference of latitudes at the two extremities of the measured arc, 
 will therefore give 
 
 as f /d u/\ / d 2 u/ \ \ 
 
 -- r 1 ( T-^ ) tan- 4/ + ( T rrV ) f J 
 cos. 4, I Vdf / \d p d 4/ J 
 
 It is remarkable, that for the same arc, measured in the direction of the 
 
 meridian, this function, by what precedes, is equal to rr ; it may thus 
 
 tan. -y, 
 
 be determined in two ways, and we shall be able to judge whether the 
 values thus found of the difference of latitudes, or of the azimuthal 
 angle rr t are due to the errors of observations, or to the excentricity of the 
 terrestrial parallels. 
 
 Retaining only the first power of s, we have 
 
 p- f/ = *.** = -^ .{l -u/+ (^' 
 d s cos. 4> | \d s/ 
 
 f p, is not the difference in longitude of the two extremities of the arc 
 s ; this difference is equal to V V, ; but we have, by what precedes, 
 
 du 
 
 which gives 
 
 / d 2 u 
 a s ( j - 3- I S 
 
 v / v\ - Vd f d 
 
 9 - -(^--v,)~- 2 
 
 wherefore 
 
 For greater exactness, we must add to this value of V V, the term 
 depending on s 3 , and independent of , which we obtain in the hypothesis 
 
 tan. 2 -\J/ 
 of the earth being a sphere. This term is equal to i s 3 . - ^-' ; 
 
 thus we have 
 
 It remains to determine the azimuthal angle at the extremity of the 
 arc s. For that purpose, call x', and y', the coordinates x, y, referred to 
 VOL. II. U
 
 306 A COMMENTARY ON [SECT. XII. & XIII. 
 
 the meridian of the last extremity of the arc s ; it is easy to see that the 
 
 V d x' 2 + d z 2 
 cosine ot the azunuthal angle is equal to - -, - . If we refer 
 
 the coordinates x, y, to the plane of the meridian corresponding to the 
 first extremity of the arc ; its first side being supposed perpendicular to 
 the plane of this meridian, we shall have 
 
 d s d s d s 
 
 wherefore, retaining only the first power of s, 
 
 d x d 2 x, d z d * z, 
 
 d~s~ : ' Ts^'' dl = s> d7 ; 
 
 but we have 
 
 x' = x cos. (V V,) + y sin. (V V,) ; 
 thus V V, being, by what precedes, of the order a, we shall have 
 
 ._.. 
 
 d s ds 2 ' ds 
 
 Again, we have 
 
 x = r sin. 6 cos. p ; z = r cos. d ; 
 we therefore shall obtain, rejecting quantities of the order a \ and observ- 
 
 ing that p,, T ^rj and , ' are quantities of the order a, 
 
 d 2 X/ d 2 u/ . d 2 ^ " d p, 
 
 {. = a . j \ sin. 6 + r. . T I cos. 0. r. sin. 0, . -.- ' 
 ds 2 ds 2 y ds 2 ds 
 
 Thence we have 
 du/_ 
 
 moreover, d s = r, sin. d, . d ^ ; we shall, therefore, have by substituting 
 for r /? 6 f9 -j ^ , and -, 2 -, their preceding values, 
 
 d 2 x. sin. 8 4>. /d u/\ A . , 
 
 ' = (1 c6 u/) . - P + [ T-^f ) tan. s 4, sin. ^ 
 cl s s cos. -4^ \d -v}/ / 
 
 1 / 1 / rd <> , 1 1 
 
 r V* au. + a( -r~ ) tan. ^ f H 
 cos. 4 1 / ' Vd 4- / cos. 
 
 Neglecting the superior powers of s, we have, as we have seen, 
 
 -V / = ~ /du\ 
 
 cos. ^ 1 1 _ u / + (^) 
 
 tan, - 
 
 and -t- 1 - 1 = 1 5 vv 'e therefore have 
 
 cl s
 
 BOOK I.] NEWTON'S PRINCIPIA. 307 
 
 d x, ., /x sin. 2 -4/. 
 
 <if= s < l "''> ^+ 
 
 in like manner we shall find 
 
 the cosine of the azimuthal angle, at the extremity of the arc s, will thus 
 be 
 
 s tan. -4> ,d UN \ d 
 
 This cosine being very small, it may be taken for the complement of 
 the azimuthal angle, which consequently is equal to 
 
 100 C 
 
 f K f d2 "/\) 
 
 s tan. 4# ., , , /d u/x . V d p V >. 
 
 T/ ) 1 a u/+o ( ^r-f ) tan. -4/ y i, f 
 
 \. \d vj/ / cos. 2 -vj/, J 
 
 For the greater exactness, we must add to this angle that part depend- 
 ing on s 3 , and independent of , which we obtain in the hypothesis of the 
 earth's sphericity. This part is equal to s 3 (| -f- tan. 2 -4/J tan. ^ Thus 
 the azimuthal angle at the extremity of the arc s is equal to 
 
 f 
 
 .-4vJ 
 
 1 
 
 100-stan. , . ... 
 
 -* s 2 (Jtan. 
 
 The radius of curvature of the Geodesic line, forming any angle what- 
 ever with the plane of the meridian, is equal to 
 
 ds a 
 
 V (d 2 x) 2 + (d 2 y) 2 + (d 2 z) 2 ' 
 
 d s being supposed constant; let R be this radius. The equation 
 x 8 + y st -{-z 2 = 1 + 2au' gives 
 
 xd 2 x + yd s y + zd 2 z = ds 2 +d ! u / ; 
 
 if we add the square of this equation to the squares of equations (O), we 
 shall have, rejecting terms of the order a 2 , 
 
 ( x * +y * + z2 ) (d 2 x) 2 + (d 2 y) 2 + (d 2 z) 2 J=ds 4 2ads'tl s u' 
 whence we derive 
 
 T 2 / 
 
 R= 1 + u' + a -J^T- 
 
 In the direction of the meridian, we have 
 
 d 2 u' 
 
 wherefore 
 
 R= I +au' 
 U2
 
 308 A COMMENTARY ON [SECT. XII. & XIII. 
 
 In the direction perpendicular to the meridian, we have by what pre- 
 cedes, 
 
 ton ' 
 
 d s 2 cos. 2 *, 
 wherefore 
 
 fd 2 u 
 
 R = 1 + a u/ a 
 
 cos. 2 4v 
 If in the preceding expression of V V /} we make -4- = s', it takes 
 
 Jtv 
 
 this very simple form relative to a sphere of the radius R, 
 
 v v, = -. (i 4- s ' 2 - tan - 2 4 I 
 
 cos. 4"/ A 3 ' J 
 
 The expression of the azimuthal angle becomes 
 
 100 s' tan. ^ {1 _ i s ' 2 (^ + tan. 2 vf/,)}. 
 
 Call X, the angle which the first side of the Geodesic line forms with the 
 plane corresponding to the celestial meridian, we shall have 
 
 d'u' /du\ d* /du\ d 2 ^ /d 2 u\dp 2 /d 2 u / N d^>d^ , /du 
 
 s ds + 
 
 But supposing the earth a sphere, we have 
 
 d p, sin. X d 2 <p, 2 sin. X cos. X , 
 
 -r^- = p.; -3-4-== i tan.*,; 
 
 d s cos. */ d s cos Y, 
 
 d -vl/ d 2 *. 
 
 - T/ = cos. X; i ^-=r sin. X tan. * y ; 
 
 wherefore, 
 
 d^ = sin. X cos. X r ,duA ^ xdX\ I _ sin<2 x tan> /du/ 
 
 d s 2 cos. */ I ^ a p / \df>d*/ J ?.'T 
 
 t z '. cin 2 -i rd 2 u ' 
 
 the radius of curvature R, in the direction of this Geodesic line, is there- 
 fore 
 
 To abridge this, let 
 
 fd 2 U/ 
 
 K = 1 + /-* tan. ^ + ' .
 
 BOOK I.] NEWTON'S PRINCIPIA. 309 
 
 A= a 
 
 cos. 
 
 we shall have 
 
 R = K + A sin. 2 X + B cos. 2 X. 
 
 The observations of azimuthal angles, and of the difference of the lati- 
 tudes at the extremities of the two geodesic lines, one measured in the 
 direction of the meridian, and the other in the direction perpendicular to 
 the meridian, will give, by what precedes, the values of A, B and K ; for 
 the observations give the radii of curvature in these two directions. Let 
 R, and R' be these radii ; we shall have 
 
 R' + R" 
 
 B = 
 
 2 
 R' R" 
 
 2 
 
 and the value of A will be determined, either by the azimuth of the ex- 
 tremity of the arc measured in the direction of the meridian, or by the 
 difference in latitude of the two extremities of the arc measured in a di- 
 rection perpendicular to the meridian. We shall thus get the radius of 
 curvature of the geodesic line, whose first side forms any angle whatever 
 with the meridian. 
 
 If we call 2 E, an angle whose tangent is-n-, we shall have 
 
 R = K + VA* + B 2 . cos. (2 X 2 E) ; 
 
 the greatest radius of curvature corresponds with X = E ; the correspond- 
 ing geodesic line forms therefore the angle E, with the plane of the me- 
 ridian. The least radius of curvature corresponds with X = 100+ E; 
 let r be the least radius, and r' the greatest, we shall have 
 
 R = r + (r r) cos. 2 (X E), 
 
 X E being the angle which the geodesic line corresponding to R, forms 
 with that which corresponds with r. 
 
 We have already observed, that at each point of the earth's surface, 
 we may conceive an osculatory ellipsoid upon which the degrees, in all 
 directions, are sensibly the same to a small extent around the point of os- 
 culation. Express the radius of this ellipsoid by the function 
 
 1 sin. 2 -^ \1 + h cos. 2 (<f> + jS)}, 
 
 riie longitudes <p being reckoned from a given meridian. The expression 
 
 U3
 
 310 A COMMENTARY ON [SKCT. XII. & XIII. 
 
 of the terrestrial meridian measured in the direction of the meridian, 
 will be, by what precedes, 
 
 % H + h cos - 2 (p + /3)J . Jl + 3 cos. 2 ^ 3 e sin. 2 >}/}. 
 
 - 
 
 If the measured arc is considerable, and if we have observed, as in 
 France, the latitudes of some points intermediate between the extremity; 
 we shall have by these measures, both the length of the radius taken for 
 unity, and the value of a. [I + h cos. 2 (p -f- &)} We then have, by 
 what precedes, 
 
 tan. 8 -4, (1 + cos. 2 4) 
 
 w = 2.b. t . ^7^ r- sln - 2 to + $5 
 
 the observation of the azimuthal angles at the two extremities of the arc 
 will give a h sin. 2 (<p + /3). Finally, the degree measured in the direc- 
 tion perpendicular to the meridian, is 
 
 1 + 1. a{l + h cos. 2 (<p + /3)}sin. 2 -^+.4. ah tan. 2 -4, cos. 2 (<p + /5); 
 the measure of this degree will therefore give the value of a h sin. 2 (p+/3). 
 Thus the oscillatory ellipsoid will be determined by these several mea- 
 sures: it would be necessary for an arc so great, to retain the square of 
 in the expression of the angle w, and the more so, if, as it has been ob- 
 served in France, the azimuthal angle does not vary proportionally to 
 the measured arc : at the same time we must add a term of the form 
 k sin. -^ cos. 4> sin. (<p -f- |S'), to get the most general expression of this 
 radius. 
 
 583. The elliptic figure is the most simple after that of the sphere : we 
 have seen above that this ought to be the figure of the earth and planets, 
 on the supposition of their being originally fluid, if besides they have 
 retained their primitive figure. It was natural therefore to compare 
 with this figure the measured degrees of the meridian ; but this compari- 
 son has given for the figure of the meridians different ellipses, and which 
 disagree too much with observations to be admissible. However, before we 
 renounce entirely the elliptic, we must determine that in which the greatest 
 defect of the measured degrees, is smaller than in every other elliptic 
 figure, and see whether it be within the limits of the errors of observations. 
 We arrive at this by the following method. 
 
 Let a (1) , a (2) , a (3) , &c. be the measured degrees of the meridians ; p (t) , 
 P (z \ P (3) j Sec. the squares of the sines of the corresponding latitudes : 
 suppose that in the ellipse required, the degree of the meridian is expressed 
 by the formula z + p y ; calling x (1) , x (2) , x < 3 ), &c. the errors of observation, 
 we shall have the following equations, in which we shall suppose that p (1) 
 pftj pW ? & c . f orm an- increasing progression,
 
 BOOK I.] NEWTON'S PHINCIPIA. 311 
 
 a (1 > z pWy = x> 
 
 a (2)_Z p^'y = X< 2 ) (A) 
 
 a W z p (n) y = x (n > 
 n being the number of measured degrees. 
 
 We shall eliminate from these equations the unknown quantities z and y, 
 and we shall have n 2 equations of condition, between the n errors 
 
 x (1) , x (2) , x (n l We must, however, determine that system of errors, 
 
 in which the greatest, abstraction being made of the signs, is less than in 
 every other system. 
 
 First suppose that we have only one equation of condition, which may 
 be represented by 
 
 a = m x W -f- n x (2 > + p x '^ + &c. 
 
 a being positive. We shall have the system of the values of x (l) , x , &c. 
 which gives, not regarding signs, the least value to the greatest of them ; 
 supposing them all nearly equal, and to the quotient of a divided by the 
 sum of the coefficients, m, n, p, &c. taken positively. As to the sign 
 which each quantity ought to have, it must be the same as that of its co- 
 efficient in the proposed equation. 
 
 If we have two equations of condition between the errors, the system 
 which will give the smallest value possible to the greatest of them will be 
 such that, signs being abstracted, all the errors will be equal to one ano- 
 ther, with the exception of one only which will be smaller than the rest, 
 or at least not greater. Supposing therefore that x (1) is this error, we 
 shall determine it in function x (2) , x (3) , &c. by means of one of the proposed 
 equations of condition ; then substituting this value of x (1) in the other 
 equation of condition, we shall form one between x (2) , x (3) , &c. ; which re- 
 present by the following 
 
 a = mxW + nx + &c. 
 
 a being positive ; we shall have, as above, the values of x (2 \ x , &c. by 
 dividing a by the sum of the coefficients m, n, &c. taken positively, and by 
 giving successively to the quotient the signs of m, n, &c. These values sub- 
 stituted in the expression of x (1) in terms of x (2 \ x (3) , &c. will give the value 
 of x a) ; and if this value, abstracting signs, is not greater than that of x (2 >, 
 this system of values will be that which we must adopt; but if greater, then 
 the supposition that x M is the least error, is not legitimate, and we must 
 successively make the same supposition as to x (2 >, x (3) , &c. until we arrive 
 at an error which is in this respect satisfactory. 
 
 If we have three equations of condition between the errors ; the system 
 
 which will give the smallest value possible to the greatest of them, will be 
 
 U4
 
 312 A COMMENTARY ON [SECT. XII. & XIII. 
 
 such, that, abstracting signs, all the errors will be equal, with exception of 
 two, which will be less than the others. 
 
 Supposing therefore that x (1 >, x ^ are these two errors, we shall elimi- 
 nate them from the third of the equations of condition by means of the 
 other two, and we shall have an equation between the errors x (3) , x (1) , &c. : 
 represent it by 
 
 a = m x + n x (4) + &c. 
 
 a being positive. We shall have the values of x (3) , x ( % &c. by dividing 
 a by the sum of the coefficients m, n, &c. taken positively, and by giving 
 successively to the quotient, the signs of m, n, &c. These values substi- 
 tuted in the expressions of x (1) , and of x in terms of x (3 ), x W, &c. will 
 give the values of x (1) , and of x ( -\ and if these last values, abstracting 
 signs, do not surpass x (3) , we shall have the system of errors, which we 
 ought to adopt ; but if one of these values exceed x (3) , the supposition that 
 x (1 >, and x & are the smallest errors is not legitimate, and we must use 
 the same supposition upon another combination of errors x (1) , x (2) , &c. 
 taken two and two, until we arrive at a combination in which this suppo- 
 sition is legitimate. It is easy to extend this method to the case where 
 we should have four or more equations of condition, between the errors x (1) , 
 x (2) , &c. These errors being thus known, it will be easy to obtain the 
 values of z and y. 
 
 The method just exposed, applies to all questions of the same nature ; 
 thus, having the number n of observations upon a comet, we may by this 
 means determine that parabolic orbit, in which the greatest error is, ab- 
 stracting signs, less than in any other parabolic orbit, and thence recog- 
 nise whether the parabolic hypothesis can represent these observations. 
 But when the number of observations is considerable, this method be- 
 comes too tedious, and we may in the present problem, easily arrive at 
 the required system of errors, by the following method. 
 
 Conceive that x (i) , abstracting signs, is the greatest of the errors 
 x (1) , x (2) , &c. ; we shall first observe, that therein must exist another error 
 x (i<) , equal, and having a contrary sign to x (i) ; otherwise we might, by 
 making z to vary properly in the equation 
 
 a W z p (i) . y = x , 
 
 dimmish the error x w , retaining to it the property of being the extreme 
 error, which is against the hypothesis. Next we shall observe that x w 
 and x (i/) being the two extreme errors, one positive, and the others nega- 
 tive, and equal to one another, there ought to exist a third error x (i % 
 equal, abstracting signs, to x (i) . In fact, if we take the equation corre-
 
 BOOK I.] NEWTON'S PRINCIPIA. 313 
 
 spending to x W, from the equation corresponding to x W, we shall 
 have 
 
 a ao _ a w { p o __ p }. y x uo __ x . 
 
 The second member of this equation is, abstracting signs, the sum of 
 the extreme errors, and it is clear, that in varying y suitably, we may di- 
 minish it, preserving to it the property of being the greatest of the sums 
 which we can obtain by adding or subtracting the errors x (1) , x (2 >, &c. 
 taken two and two ; provided there is no third error x (i *> equal, abstract- 
 ing signs, to x W ; but the sum of the extreme errors being diminished, 
 and these errors being made equal, by means of the value of z, each of 
 these errors will be diminished, which is contrary to the hypothesis. 
 There exists therefore three errors x ( % x (i \ x (i//) equal to one another, 
 abstracting signs, arid of different signs the one from the other two. 
 
 Suppose that this one is x ^ ; then the number V will fall between the 
 two numbers i and i". To show this, let us imagine that it is not the 
 case, and that i' is below or above both the numbers i, i". Taking the 
 equation corresponding to i', successively from the two equations corre- 
 sponding to i and to i", we shall have 
 
 a W _ a ao _ ( p ro _ p OO) y = x > x^j 
 a ") _ a a 1 ) _ ( p d'O _ p (0) y = x (i "> x M. 
 
 The second members are equal and have the same sign ; these are also, 
 abstracting signs, the sum of the extreme errors ; but it is evident, that 
 varying y suitably, we may diminish each of these sums, since the coeffi- 
 cient of y, has the same sign in the two first members : moreover, we may, 
 by varying z properly, preserve to x (i/) the same value; x w and x (i ") will 
 therefore then be, abstracting signs, less than x (i/) which will become the 
 greatest of the errors without having an equal; and in this case, we may, 
 as we have seen, diminish the extreme error; which is contrary to the hy- 
 pothesis. Thus the number i' ought to fall between i and i". 
 
 Let us now determine which of the errors x (1) , x (2) , &c. are the extreme 
 errors. For that purpose, take the first of the equations (A) successively 
 from the following ones, and we shall have this series of equations, 
 a w __ a > (p p >) y = x x >, 
 
 a (3) _ a U) _ ( p (3) _ pU)) y - x (3) __ x (1) . . . . . (B) 
 
 &c. 
 
 Suppose y infinite ; the first members of these equations will be nega- 
 tive, and then the value of x (1> will be greater than x (2) , x (3) , &c. : dimin- 
 ishing y continually, we shall at length arrive at a value that will render 
 positive one of the first members, which, before arriving at this state, will
 
 314 A COMMENTARY ON [SECT. XII. & XIII. 
 
 be nothing. To know which of these members first becomes equal to zero, 
 we shall form the quantities, 
 
 a (g) aU) . a(3) a(l) . a(4 ' aU) . & r 
 
 p U) p (1) p (3) p (1) ' p (4) p (1) 
 
 a W a C1) 
 
 Call |3 (1) the greatest of these quantities, and suppose it to be ^ ^ : 
 
 if there are many values equal to /3 (1) , we shall consider that which cor- 
 responds to the number r the greatest, substituting /3 (1) for y, in the 
 (r l) th of the equations (B), x (r) will be equal to x (1) , and diminishing 
 y, it will be equal to x (1) , the first member of this equation then becoming 
 positive. By the successive diminutions of y, this member will increase 
 more rapidly than the first members of the equations which precede it ; 
 thus, since it becomes nothing when the preceding ones are still nega- 
 tive, it is clear that, in the successive diminutions of y, it will always be 
 the greatest which proves that x (r) will be constantly greater than x (1) , 
 x (2) , . . . x^" 1 ), when y is less than /3 (1) . 
 
 The first members of the equations (B) which follow the (r l) th will 
 be at first negative, and whilst that is the case, x (r + 1} , x (r + 2 \ &c. will be 
 less than x (1) , and consequently less than x (r) , which becomes the greatest 
 of all the errors x a) , x (2) , . . . x (n >, when y begins to be less than /3 (1 >. But 
 continuing to diminish y, we shall get a value of it, such that some of the 
 errors x (r + 1J , x (r + 2) , &c. begin to exceed x (r) . 
 
 To determine this value of y, we shall take the r th of equations (A) suc- 
 cessively from the following ones, and we shall have 
 
 a (r + 1) _ a W _ p (r + 1) __ p (r)| y _ x (r + 1) __ x (r) . 
 a (r + 2)__ a (r)_ J p Cr + S)_ p (r)Jy _ x (r + 2) _ x W< 
 
 Then we shall form the quantities 
 
 a (r+l) a (r) o(r + 2) a (-' 
 
 tl C* ' 1 1 ^^ C* 
 
 p(r)' 
 
 &C. 
 
 Call /3, the greatest of these quantities, and suppose that it is 
 a . . a ' ; if many of these quantities are equal to /3 , we shall suppose 
 
 p (O p (t) 
 
 that r 7 is the greatest of the numbers to which they correspond. Then x W 
 will be the greatest of the errors x <", x t2) , &c. . . . x (n > so long as y is com- 
 prised between /3 (n , and /3 (2 > ; but when by diminishing y, we shall arrive at 
 8 (2) ; then x (l/) will begin to exceed x (r) , and to become the greatest of the 
 errors. 
 
 To determine within what limits we shall form the quantities 
 
 a (r/ > 
 
 ptr'+l) p(r'J ptf + 2) pirV 
 
 Let /3 be the greatest of these quantities, and suppose that it is
 
 BOOK I.] NEWTON'S PRINCIPIA. 315 
 
 & (f+l^ a (rO 
 
 (t , j' - (pj : if several of the quantities are equal to ft ( % we shall sup- 
 pose that r"is the greatest of the numbers to which they correspond, x^ 
 will be the greatest of all the errors from y = /3(% to y = /3( 3 >. When 
 y = jSW, then x^ begins to be this greatest error. Thus preceding, we 
 shall form the two series, 
 
 X 0); X Wj X (r0. X (");... X (n) 
 
 oo; j3CD; j8; j8< ; . . . |30 ;-<x ; (C) 
 
 The first indicates the errors x^, x (r >, x (t/ >, &c. which become succes- 
 sively the greatest : the second series formed of decreasing quantities, in- 
 dicates the limits of y, between which these errors are the greatest ; thus, 
 x (1) is the greatest error from y = oo, to y = W; x^ is the greatest er- 
 ror from y = (3^, to y = j8< 2 >; X (O is the greatest error from y = j3 (2 \ 
 to y = j8 ( 3 >, and so on. 
 
 Resume now the equations (B) and suppose y negative and infinite. 
 The first members of these equations will be positive, x (1) will therefore then 
 be the least of the errors x (1 \ x (2) , &c. : augmenting y continually, some 
 of these members will become negative, and then x (1) will cease to be the 
 least of the errors. If we apply here the reasoning just used in the case 
 of the greatest errors, we shall see that if we call X (1) the least of the 
 quantities 
 
 a (2) a (l) a (3)_ a (l) a (4) 
 
 p 
 
 (2) 
 
 &c. 
 
 a (s) a (1) 
 
 and if we suppose that it is , ^ , s being the greatest of the num- 
 
 pV;-vP ' 
 
 bers to which X^ corresponds, if several of these quantities are equal to 
 X fl) , x (I) will be the least of the errors from y = oo, to y = X (1 \ In 
 like manner if we call X ( 2 > the least of the quantities 
 
 " 7^ i T\ 7^\ 9 ~ TT/Vi. i*\ 7^\ Otv/ 
 
 a (s') a (s) 
 
 and suppose it to be r^ , s' being the greatest of the numbers to 
 
 which X (2) corresponds, if several of these quantities are equal to X W ; x <"' 
 will be the smallest of the errors from y = X 1 ^, to y = X w ; and so forth. 
 In this manner we shall form the two series 
 
 ...X^>; oo ; (D) 
 
 The first indicates the errors x (1) , x (s) , x ( s<) , &c. which are successively 
 the least as we augment y : the second series formed of increasing terms, 
 indicates the limits of the values of y between which each of these errors
 
 316 A COMMENTARY ON [SECT. XII. & XIII. 
 
 is the least; thus x fl > is the least of the errors fromy = o>, to y = X^ 
 x w is the least of the errors, from y = X^, to y = X (2) , and thus of the 
 rest. 
 
 Hence the value of y which, to the required ellipse, will be one of the 
 quantities 0W, 0, 0(3). &c. X^, x<*>, &c. ; it will be in the first series, 
 if the two extreme errors of the same sign are positive. In fact, these 
 two errors being then the greatest, they are in the series x (1) , x (r) , x (r/ >, 
 &c. ; and since one and the same value of y renders them equal they 
 ought to be consecutive, and the value of y which suits them, can only 
 be one of the quantities j8 (1) , ( % &c.; because two of these errors cannot 
 at the same tune be made equal and the greatest, except by one only of 
 these quantities. Here, however, is a method of determining that of the 
 quantities (l) , W , &c. which ought to be taken for y. 
 
 Conceive, for example, that /3( 3 > is this value; then there ought to be 
 found by what precedes between x^, and x^"- 1 , an error which will be the 
 minimum of all the errors, since x^, and x (l "<> will be the maxima of these 
 errors; thus in the series x^, x< 8 >, x^, &c. some one of the numbers 
 s, s', &c. will be comprised between r and r'. Suppose it to be s. That 
 x (s) may be the last of the value of y, it ought to be comprised between 
 X ll) and X (2 > ; therefore if C3) is comprised by these limits, it will be the 
 value sought of y, and it will be useless to seek others. In fact, suppose 
 we take that of the equations (A), which answers to x (s) successively from 
 the two equations which respond to x Cr/) and to x (r ' ;) ; we shall have 
 a(0 a W [p W p J y = x (r/ > x (s > ; 
 
 a M_ a (s)_ p (r)_p(^ y = X ^ X. 
 
 All the members of these equations being positive, by supposing 
 y = 0, it is clear, that if we augment y, the quantity x (r/ > - x will 
 increase ; the sum of the extreme errors, taken positively, will be there- 
 fore augmented. If we diminish y, the quantity x tr "> x will be aug- 
 mented, and consequently also the sum of their extremes ; is therefore 
 the value of y, which gives the least of these sums; whence it follows that 
 it is the only one which satisfies the problem. 
 
 We shall try in this way the values of j3 (1 >, ( % (5, &c., which is easily 
 done by inspection ; and if we arrive at a value which fulfils the preced- 
 ing conditions, we shall be assured of the value required of y. 
 
 If any of these values of does not fulfil these conditions, then this 
 value of y will be some one of the terms of the series X W, X <, &c. Con- 
 ceive, for example, that it is X, the two extreme errors x (B > and x (s/) will 
 then be negative, and it will have, by what precedes, an intermediate error.
 
 BOOK L] NEWTON'S PRINCIPIA. 317 
 
 which will be a maximum, and which will fall consequently in the series 
 x to, x W, x M, &c. Suppose that this is x W, r being then necessarily 
 comprised between s and s'; X^ ought, therefore, to be comprised be- 
 tween ft > and ft M. If that is the case, this will be a proof that X < 2 > is the 
 value required of y. We shall try thus all the terms of the series X \ X , 
 X W, & c . U p to that which fulfils the preceding conditions. 
 
 When we shall have thus determined the value of y, we shall easily ob- 
 tain that of z. For this, suppose that J3 is the value of y, and that the 
 three extreme errors are x to, x (0, x W . we shall have x (s > = x to, an d 
 consequently 
 
 a to _ z _ p W. y = x w . 
 
 whence we get 
 
 a to _{. a (s) p w _j_ p ( S ) 
 
 _ . 
 
 2 2 '' 
 
 then we shall have the greatest error x (r) , by means of the equation 
 
 a W _ a (s) p (s) _ p (r) 
 
 xto=-_^- - + p 2 P y. 
 
 584. The ellipse determined in the preceding No. serves to recognise 
 whether the hypothesis of an elliptic figure is in the limits of the errors of 
 observations ; but it is not that which the measured degrees indicate witli 
 the greatest probability. This last ellipse, it seems, should fulfil the 
 following conditions, viz. 1st, that the sum of the errors committed in the 
 measures of the entire measured arcs be nothing : 2dly, that the sum of 
 these errors, all taken positively, may be a minimum. Thus considering 
 the entire ones instead of the degrees which have thence been deduced, 
 we give to each of the degrees by so much the more influence upon the 
 ellipticity which thence results for the earth, as the corresponding arc is 
 considerable, as it ought to be. The following is a very simple method 
 of determining the ellipse which satisfies these two conditions. 
 
 Resume the equations (A) of 589, and multiply them respectively 
 by the numbers which express how many degrees the measured arcs 
 contain, and which we will denote by i (1 >, i (2 >, i , &c. Let A be the sum 
 of the quantities i (1) . a (1) , i (2) . a (2) , &c. divided by the sum of the numbers 
 i toj i , &c. ; let, in like manner, P denote the sum of the quantities 
 i (1 \ p to, i < 2 ). p , &c. divided by the sum of the numbers i to, i (S, & c . ; 
 the condition that the sum of the errors i (1 >. x (1) , i (2) . x (2) , &c. is nothing, 
 gives 
 
 = A z P.y.
 
 318 A COMMENTARY ON [SECT. XII. & XIII. 
 
 If we take this equation from each of the equations A of the preceding 
 
 No., we shall have equations of the following form : 
 
 q (i)^ y _ x ( 
 
 a (2) v = x o 
 
 _ q q y = X <^ () 
 
 &c. 
 Form the series of quotients ^ , ^y, &c. and dispose them according 
 
 to their order of magnitude, beginning with the greatest ; then multiply 
 the equations O, to which they respond, by the corresponding numbers 
 i (1) , i (2) , &c. ; finally, dispose these thus multiplied in the same order as 
 the quotients. 
 
 The first members of the equations disposed in this way, will form a 
 series of terms of the form 
 
 in which we shall suppose h (1) , h (2 > positive, by changing the sign of the 
 terms when y has a negative coefficient. These terms are the errors of 
 the measured arcs, taken positively or negatively. 
 
 Then it is evident, that in making y infinite, each term of this series 
 becomes infinite ; but they decrease as we diminish y, and end by being 
 negative at first, the first, then the second, and so on. Diminishing y 
 continually, the terms once become negative continue to be so, and de- 
 crease without ceasing. To get the value y, which renders the sum- of 
 these terms all taken positively a minimum, we shall add the quantities 
 h (1) , h (2) , &c. as far as when their sum begins to surpass the semi-sum of 
 all these quantities ; thus calling F this sum, we shall determine r such 
 
 that 
 
 + h + h< + .... + h>| F; 
 + h(*> + h< 3 > + . . + hf-'J-O F. 
 
 C 
 
 We shall then have y = T , so that the error will be nothing rela- 
 
 tively to the same degree which corresponds to that of the equations (O), 
 of which the first member equated to zero, gives this value of y. 
 
 To show this, suppose that we augment y by the quantity d y, so that 
 
 c (r ~ l > c fr) 
 3 y may be comprised between (r _ l} and i . The (r 1) first 
 
 c 
 terms of the series (P) will be negative, as in the case of y = TT(7]> ^ ut in 
 
 taking them with the sign +, their sum will decrease by the quantity 
 
 h( r -J ay.
 
 BOOK I.] NEWTON'S PRINCIPIA. 319 
 
 c (r) 
 
 The first term of this series, which is nothing when y = -r- , will be- 
 
 come positive and equal to h^ 8 y ; the sum of this term and the follow- 
 ing, which are positive, will increase by the quantity 
 
 {h+ h( r + J > + &c.} 3y; 
 but by supposition we have 
 
 hd) + hW ---- M r - ) < h + h (' + i> + & c . ; 
 
 the entire sum of the terms of the series (P), all taken positively, will 
 therefore be augmented, and as it is equal to the sum of the errors 
 i(i). xW -j- K 2 ). x(% 8cc. of the entire measured arcs, all taken with the 
 
 c( r ) 
 sign +, this last sum will be augmented by the supposition of y=r-/7)-f dy. 
 
 It is easy to prove, in the same way, that by augmenting y, so as to be 
 
 . ,. c^- 1 ) , c ( r -' z) c ( r - 2 > , c< r ~ 3 > 
 
 comprised between j^rn and h ^- a; or between j^^ and ^3} , &c. 
 
 the sum of the errors taken with the sign + will be greater than when 
 
 C 
 = HW' 
 
 c (r) 
 
 Now diminish y by the quantity 5 y so that j j 8 y may be comprised 
 
 c (r) C (r + l) 
 
 between r-^ and , . ^ , the sum of the negative terms of the series (P) 
 will increase, in changing their sign, by the quantity 
 
 and the sum of the positive terms of the same series will decrease by the 
 quantity 
 
 Jh^ 1 ) + h< r + 2 > + &c.J dy; 
 and since we have 
 
 h) + h + ---- h > h^ r + J ) + h( r + 2 ) + &c., 
 
 it is clear that the entire sum of the errors, taken with the sign +, will be 
 augmented. In the same manner we shall see that, by diminishing y, so 
 
 c (r+l) C (r + 2) C fr + 2) C( r -f 3 ) 
 
 that it should be between j-^ and ^-^ , or between ^^ and j^^^, 
 &c. the sum of the errors taken with the sign + is greater than when 
 
 c (r) 
 
 y = i ; this value of y is therefore that which renders this sum a 
 a w 
 
 minimum.
 
 320 
 
 A COMMENTARY ON [SECT. XII. & XIII. 
 
 The value of y gives that of z by means of the equation 
 z = A-P.y. 
 
 The preceding analysis being founded on the variation of the degrees 
 from the equator to the poles, being proportional to the square of the sine 
 of the latitude, and this law of variation subsisting equally for gravity, it 
 is clear that it applies also to observations upon the length of the seconds' 
 pendulum. 
 
 The practical application of the preceding theory will fully establish its 
 soundness and utility. For this purpose, ample scope is afforded by the 
 actual admeasurements of arcs on the earth's surface, which have been 
 made at different times and in different countries. Tabulated below you 
 have such results as are most to be depended on for care in the observa- 
 tions, and for accuracy in the calculations. 
 
 Latitudes. 
 
 Lengths of Degrees. 
 \ 
 
 Where made. 
 
 By whom made. 
 
 o.oooo 
 
 37 .0093 
 43 .5556 
 47 .7963 
 51 .3327 
 53 .0926 
 73 .7037 
 
 25538 R .85 
 25666.65 
 25599 .60 
 25640.55 
 25658 .28 
 25683 .30 
 25832 .25 
 
 Peru. 
 Cape of Good Hope 
 Pennsylvania. 
 Italy. 
 France. 
 Austria. 
 Laponia. 
 
 Bouguer. 
 La Caille. 
 Mason & Dixon. 
 Boscovich & le Maire. 
 Delambre & Mechain. 
 Liesganig. 
 Clairaut, &c.
 
 
 SUPPLEMENT 
 
 TO 
 
 BOOK III. 
 
 FIGURE OF THE EARTH. 
 
 585. IF a fluid body had no motion about its axis, and all its pai'ts were 
 at rest, it would put on the form of a sphere ; for the pressures on all the 
 columns of fluid upon the central particle would not be equal unless they 
 were of the same length. If the earth be supposed to be a fluid body, 
 and to revolve round its axis, each particle, besides its gravity, will be 
 urged by a centrifugal force, by which it will have a tendency to recede 
 from the axis. On this account, Sir Isaac Newton concluded that the 
 earth must put on a spheroidical form, the polar diameter being the 
 - 
 
 shortest. Let P E Q represent a section of the earth, P p the axis, E Q 
 the equator, (b m) the centrifugal force of a part revolving at (b). This 
 force is resolved into (b n), (n m), of which (b n) draws fluid from (b) 
 to Q, and therefore tends to diminish P O, and increases E Q. 
 
 It must first be considered what will be the form of the curve P E p, 
 and then the ratio of P O : G O may be obtained. 
 VOL. II. X
 
 322 
 
 A COMMENTARY ON 
 
 [BOOK III. 
 
 586. LEMMA, Let E A Q, e a q, be similar and concentric ellipses, of 
 which the interior is touched at the extremity of the minor axis by P a L ; 
 draw a f, a g, making any equal angle with a C ; draw P F and P G re- 
 spectively parallel to a f, a g ; then will PF+PG = af+ag. 
 For draw P K, F k perpendicular to E Q, and F H, k r perpendicular to 
 PK, .-. FE = EK, .-. HD = Brand P D = D K, .-. PH=,Kr; 
 also F H = K r, .-. if K k be joined, K k = P F ; draw the diameter 
 M C z bisecting K k, G P, a g, in (m), (s), (z). 
 Then 
 
 Km: Kn:: Ps: Pn::az:aC::ag:ab. 
 .-. K m + Ps:Kn + Pn::ag:ab 
 
 but 
 
 K n + n P=K P=2 P D = 2 a C=ab .-. Km+Ps=a g. 
 .-. 2 Km+ 2Ps = 2ag, or P F+P G = a g + a f. 
 COR. P H + P I = 2 a i. For 
 
 PF:PH::PG:PI::ag:ai. 
 .-. PF + PG:PH+ PI::ag:ai::2ag:2ai. 
 
 but 
 
 = 2ag, .-. PH+PI = 2ai.
 
 BOOK III.] NEWTON'S PRINCIPIA. 323 
 
 587. The attraction of a particle A towards any pyramid, the area of 
 whose base is indefinitely small, oc length, the angle A being given, and 
 
 the attraction to each particle varying as j r - 1 . 
 
 For let 
 
 a = area (v x z w) 
 
 m = (A z) 
 
 x = (A a) 
 
 Then section a b = section v x z w . (A a) ' 
 
 " 
 
 .'. p'. attraction = 
 .*. attraction = 
 
 (A z) " m 
 
 a x 2 x' ax' 
 
 m z x z m x 
 a x 
 m~** 
 .'. attractions of particles at vertices of similar pyramids oc lengths. 
 
 588. If two particles be similarly situated in respect to two similar solids, 
 the attraction to the solids cc lengths of solids. 
 
 For if the two solids be divided into similar pyramids, having the par- 
 ticles in the vertices, the attractions to all the corresponding pyramids 
 their lengths o: lengths of solids, since the pyramids being similarly 
 situated in the two similar solids, their lengths must be as the lengths of 
 the solids : .*. whole attractions a lengths of the solids, or as any two 
 lines similarly situated in them. 
 
 COR. 1. Attraction of (a) to the spheroid a qf: attraction of A to 
 A Q F : : a C : A C. 
 
 COR. 2. The gravitation of two particles P and p in one diameter P C are 
 proportional to their distances from the center. For the gravitation of (p) 
 is the same as if all the matter between the surfaces A Q E, a q e, were 
 taken away (Sect. XIII. Prop. XCI. Cor. 3.) .'. P and p are similarly si- 
 tuated in similar solids, .-. attractions on P and p are proportional to 
 P C and p C, lines similarly situated in similar solids. 
 
 589. All particles equally distant from E Q gravitate towards E Q with 
 
 equal forces. 
 
 X2
 
 324 
 
 A COMMENTARY ON 
 
 [Book III. 
 
 For P G and P F may be considered as the axes of two very slender 
 pyramids, contained between the plane of the figure and another plane, 
 making a very small angle with it In the same manner we may conceive 
 of (a f ) and (a g). Now the gravity of P to these pyramids is as 
 P F + P G ; and in the direction P d is as P H + P I. Again, the 
 gravity of (a) to the pyramids (a f ), (a g) is as (a f + a g), or in the di- 
 rection (a i) as 2 a i ; but PH+PI = 2ai:.*. gravity of P in the di- 
 rection P d = gravity of (a) in the same direction. 
 
 It is evident, by carrying the ordinate (f g) along the diameter from (b) 
 to (a) ; the lines (a f ), (a g) will diverge from (a b), and the pyramids of 
 which these lines are the axes, will compose the whole surface of the in- 
 terior ellipse. The pyramids, of which P F, P G are the axes, will, in 
 like manner, compose the surface of the exterior ellipse, and this is true 
 for every section of the spheroid passing through P m. Hence the at- 
 traction of P to the spheroid P A Q in the direction P d equals the at- 
 traction of (a) to the spheroid (p a q) in the same direction, 
 
 590. Attraction of P in the direction P D : attraction of A in the same 
 direction : : P D : A C. 
 
 For the attraction of (a) in the direction P D : attraction of A in the 
 same direction : : P D : A C, and the attraction of (a) = attraction of P. 
 .-. attraction of P : attraction of A : : P D : A C. 
 
 Similarly, the attraction of P in the direction E C : attraction of A in 
 the direction E C : : P a : E C. 
 
 591. Draw M G perpendicular to the ellipse at M, and with the radius 
 O P describe the arc P n. 
 Then Q G : Q M : : Q M : Q T
 
 BOOK III.] 
 And 
 
 NEWTON'S PRINCIPIA. 
 
 325 
 
 but 
 
 c 
 
 r 
 
 Q 
 > c 
 
 : 
 
 i 
 
 P :: 
 OP ! 
 
 C 
 
 >P: 
 
 
 
 T 
 
 V- 
 
 ' ^ 
 
 
 OT 
 O 
 
 Q 
 
 M z 
 
 C 
 
 >P* QM 2 .OT 
 
 
 
 
 v/ . * 
 
 Q 
 
 rr\ 
 
 c 
 
 >T ' QT 
 
 
 
 T 
 
 : 
 
 Q:: 
 
 
 
 P 2 
 
 Q 
 
 O 2 
 
 
 
 T 
 
 : T 
 
 Q:: 
 
 O 
 
 P 2 
 
 
 
 p_OQ 2 
 
 
 
 
 i : 
 
 O 
 
 P 2 
 
 n 
 
 Q 2 
 
 
 
 ' 
 
 : : 
 
 
 
 P z : 
 
 P 
 
 Q.Qp:: OE 2 : 
 
 o 
 
 T 
 
 C 
 
 )E 2 
 
 
 
 
 
 :OP 
 
 QM 
 
 'TQ - QM 2 ' 
 /.QG: QO:: OE 2 : O P 2 
 
 . nr OE 2 
 .-. Q G = Q-J^ . Q O. 
 
 592. A fluid body will preserve its figure if the direction of its gravity, at 
 every point, be perpendicular to its surface ; for then gravity cannot put its 
 surface in motion. 
 
 593. If the particles of a homogeneous fluid attract each other with forces 
 
 varying as ^ , and it revolve round an axis, it will put on the form 
 
 of a spheroid. 
 
 For if P E p P be a fluid, P p the axis round which it revolves, then 
 may the spheroid revolve in such a time that the centrifugal force of any 
 particle M combined with its gravity, may make this whole force act per- 
 pendicularly to the surface. For let E = attraction at the equator, 
 P = attraction at the pole, F = centrifugal force at the equator. 
 
 X3
 
 326 A COMMENTARY ON [BOOK III 
 
 Then (590), 
 
 attraction of M in the direction M R : P : : Q O : P O 
 
 .'. attraction of M in the direction M R = . Q Q . 
 
 Similarly, the attraction of M in the direction M Q = E ' Q R . 
 
 O E 
 
 But the centrifugal force of bodies revolving in equal times a radii. 
 
 V* c 2 
 
 F QC X 
 
 r r. P 
 
 GC 
 
 P* 
 
 (and P being given) a r 
 
 F. O R 
 
 .*. centrifugal force of M = 
 
 .-. whole force of M in the direction M O = 
 
 OE 
 
 (E_: 
 
 __. 
 O Jti 
 
 Take Mr = -p%- , M g = (E ~J^ R , complete the paral- 
 
 lelogram, and M q will be the compound force; O E and O P .*. must 
 have such a ratio to each other that M q may be always perpendicular to 
 the curve. Suppose M q perpendicular to the curve, then, by similar 
 triangles, qgorMr:Mg::QG:QM. 
 
 . P.QO.(E-F)OR..OE 2 00 . OR 
 
 ~P~(T" OE -oT" 2 '^ 1 
 
 . P.QO.OR OR OE 2 
 
 PO >>-U~E'OP"' Q 
 
 .-. P : E F : : O E : O P, 
 
 in which no lines are concerned except the two axes ; .*. lo a spheroid 
 having two axes in such a ratio, the whole force will, at every point, be 
 perpendicular to the surface, and .*. the fluid will be at rest. 
 
 P.MR 
 594. The attraction of any point M in the direction M R = ' ^ - ; 
 
 .*. if P be represented by P O, M R will represent the attraction of M in 
 the direction M R, and M v will represent the whole attraction acting 
 perpendicularly to the surface.
 
 BOOK III.] 
 
 Draw (v c) perpendicular to M O. 
 Then 
 
 MO:Ma::M v.Mc: '.attraction in the direction Mv : MO. 
 
 M v. M a OP 2 1 
 .'. attraction in the direction M O = it/To = 'M O tt M O " 
 
 By similar triangles T O y, M v R, (the angle T O y being equal to the 
 angle v M R.) 
 
 T O : O y : : v M : M R 
 
 595. Required the attraction of an oblong spheroid on a particle placed 
 at the extremity of the major axis, the excentricity being very small. 
 
 Let axis major : axis minor : : 1 : 1 n. Attraction of the circle 
 N n (Prop XC.) 
 
 _EL . x 
 
 E N c ~ Vn 2 + (1 n) 2 (2 n n 2 ) 
 
 a 1 x {2x n. (4x fcn'jj i 
 
 a 1 x J(2x)~* + l(2x)~*n. (4 n 2n*)} 
 
 a 1-^5 5=-. (4 V^_2x*) 
 
 V 2 4 V 2 ^ 
 
 .-.A' a x' 
 
 
 4 V 2 
 
 V 2 
 /.A a x -r . x 
 
 n 
 
 4 V 2 N 3 
 X4
 
 328 A COMMENTARY ON [BOOK III. 
 
 Let x = 2 E O = 2, 
 
 A a 2 - n /16 V~2 16 V~ 
 
 " 3 ""4 V"T V 3 ~5 
 
 2 8 n 4 n 
 
 CC - rr 1 
 
 3 15 5 ' 
 
 .'. attraction of the oblong spheroid on E : attraction of a circum- 
 scribed sphere on E : : (since in the sphere n = 0.) 
 
 596. Required the attraction of an oblate spheroid on a particle placed 
 at the extremity of the minor axis. 
 
 Let axis minor : axis major : : 1 : 1 + n. 
 
 .-. A' 
 
 a x' { 
 
 , 
 + rip. (2x 
 
 x 
 
 } 
 
 V 2 x -f- 4nx 2 nx 2 / 
 
 (2x)~^4nx 2nx 2 )} 
 
 a x' , n x * 
 
 x n x 
 ax 
 
 .-.A ax 
 
 V 2 V 2 2 V 2 
 
 V~2. x* . V~2. n x* n x 
 
 3 3 
 
 .*. whole attraction 
 
 4 4n 4n 2,8n 4n 
 
 a 2 4- a 4- a 1 4- 
 
 3 3 5 3 15 5 
 
 .'. attraction of the oblate sphere on P : attraction of the sphere in- 
 scribed on P : : 1 -f - - : I. 
 o 
 
 Since these spheroids, by hypothesis, approximate to spheres, they may, 
 without sensible error, be assumed for spheres, and their attractions will be 
 nearly proportional to their quantities of matter. But oblong sphere 
 : oblate : : oblate : circumscribed sphere. .*. A of oblong sphere on E : A' 
 of oblate on E : : A' : A" of circumscribed sphere on E. 
 
 .-.A': A":: As A':: VA: v'A 
 
 / 4. n O r> 
 
 "::^/! ^:1::1 ^-:
 
 BOOK IILJ NEWTON'S PRINCIPIA. 329 
 
 Also 
 
 A. r\ 
 
 att n . of oblate sph. on P : att B . of insc d . sph. on P : : 1 + : 1 
 
 o 
 
 att n . of insc d . sph. on P : att n .of circumsc d . sph. on E : : 1 : 1 -f n 
 att a . of circumsc d . sph. onE : attr n . of oblate sph. on E : : 1 : 1 
 
 O 
 
 .. attraction of the oblate sphere on P : attraction of the oblate sphere 
 
 on E : : 1 H : 1 + n . 1 
 
 :: 1 + : 1 + _ :: 1 + - 
 
 : 1 nearly. 
 
 '.IT 
 
 n^ 
 
 n_ 3n g 
 5 25 
 _3_n 2 
 25 
 
 .-. P : E : : 1 + ^- : 1 T 
 
 but (593), P:E F::OE:OP 
 
 ::l+n:l::P + F:E nearly 
 
 ... 1 + n .E F nF = P 
 /. F+IT.E nF= P+ F 
 and since (n) is very small, as also F compared with E, 
 
 .-. r+ir E = P + F 
 
 .-. 1 + n : 1 : : P + F : E 
 
 5F 
 
 ' n = 4E 
 .-. 4 E : 5 F : : 1 : n
 
 380 A COMMENTARY ON [Boon III. 
 
 or " four times the primitive gravity at the equator : five times the centri- 
 fugal force at the equator : : one half polar axis : excentricity." 
 
 597. The centrifugal force opposed to gravity oc cos. 2 latitude. 
 
 m n 
 
 Q 
 
 o 
 
 E 
 
 Let (m n) = centrifugal force at (m), F = centrifugal force at E. 
 .*. (n r) is that part of the centrifugal force at (m) which is opposed to 
 gravity. 
 
 Now 
 
 F:mn:: O E: Km 
 and 
 
 m n : n r : : Om: K m 
 
 .'. m r a cos. 2 lat. 
 
 598. From the equator to the pole, the increase of the length of a de- 
 gree of the meridian a sin. * lat. 
 
 .).-. F: n r: : Om 8 : Km 2 
 f : : r z : cos. ! lat. 
 
 nr:Ms::nG:MG::CP:CR::l n : 1. 
 .. n r = 1 n . M S = 1 n. p' sin. 6 = 1 n . cos. 
 
 m r = s t = <b'. cos. 6 = sin. 6 . (f 
 
 i 
 
 /. m r 2 = sin. 2 6 . 6 * 
 
 .-.mn 2 = nr 2 + mr 2 = ^'.sin. 2 *? + (1 n) 1 . cos. s *. 
 = /2 . (sin. 2 6 + 1 2 n . cos. 2 6) 
 = 6' * (sin. 2 6 + cos. 2 6 2 n . cos. 2 6) 
 = ^2. (1 _2n. cos.* 6) 
 .-. mn = d. (1 n. cos. 8 0) 
 .. at the equator, since 
 
 6 = 0; m'n' = (1 n)
 
 BOOK III.] NEWTON'S PRINCIPIA. 331 
 
 .. increase = (f ( 1 n . cos. 2 6 1 + n) = 6'. n ( I cos. * 6) 
 
 = V. n sin. * 6, 
 .*. increase n ff. sin. 8 
 
 sin. 2 0, oc sin. * latitude. 
 
 599. Given the lengths of a degree at two given latitudes, required the 
 ratio between the polar and equatorial diameters. 
 
 Let P and p be the lengths of a degree at the pole and equator, m and 
 n the lengths in latitudes whose sines are S and s, and cosines C and c. 
 Then as length of a degree oo radius of curvature, (for the arc of the me- 
 ridian intercepted between an angle of one degree, which is called the 
 length of a degree, may be supposed to coincide with the circle of curva- 
 ture for that degree, and will .'. <x radius of curvature.) 
 
 . 
 
 PF ' 
 
 Now at the pole C D * becomes AC 2 , and P F becomes B C 
 
 A f 2 2 
 
 .'. length of a degree , a ~ ; 
 
 ij (j b 
 
 similarly the length of a degree at the equator 
 
 AC ' 
 
 Now 
 
 m _ p : n p. (598) : : S z : s 2 , 
 
 .. m n : n p : : S 8 s 2 : S 2 , 
 
 _ m n.S 
 but 
 
 -P, m n.S* 
 
 P_ p : n p :: i 2 : s 2 :: P p : ,,, _. , 
 
 S 2 s
 
 332 
 
 A COMMENTARY ON 
 
 n S 8 n s 2 m 
 
 S 2 s 2 
 
 [BOOK III. 
 
 n S 8 ms- 
 
 S 2 s 8 ' 
 
 p 4. m n __ n S * ms 8 -f-m n 
 
 = p -h g 8 _ s 2 = S 2 s 2 ~ 
 
 m.(l s 8 ) n.(l S 2 ) 
 
 P : p 
 
 S 2 - 
 
 m c 2 n C 
 
 S 2 s 2 
 
 me* n C 2 . n S 2 m s 2 
 
 S 2 s 8 
 
 S 1 s 2 
 
 : : m c 2 n C 2 : n S s m s 2 : : 1 : (1 n') 3 
 .-. (m c 8 n C 2 ) I : (n S 2 m s*)| : : 1 : 1 n'. 
 
 600. The variation in the length of a pendulum a sin. 2 latitude. 
 Let 1 = length of a pendulum vibrating seconds at the equator. 
 
 L = length of one vibrating seconds at latitude 6. 
 The force of gravity at the pole = 1, .'. the force of gravity at the equator 
 = 1 F, and the force of gravity in latitude 6 (603) = 1 F. cos. * 6, 
 
 .-. L : 1 : : 1 F. cos. * 6 : l F (since a 3 F) 
 .-. L 1 : 1 : : F. (1 cos. 2 6} : 1 F : : F. sin. 2 6 : 1 F, 
 
 1 F. sin 6 
 
 .: L 1 = ~ a sin. * 6. 
 
 i Jb 
 
 From the poles to the equator, the decrease of the length of a pendu- 
 lum always vibrating in the same time, a cos. 2 latitude. 
 
 Let L' = length of a pendulum vibrating seconds at the pole, 
 .-. L 7 : L :: 1 : 1 F. cos 2 0, 
 .. L' : L' L :: 1 : F. cos 8 *, 
 /. L 7 L <x cos. * d. 
 
 601, The increase of attraction from the equator to the pole sin. 8 lat. 
 
 Let 
 
 O E : O P : : 1 : 1 n. p 
 
 Let 
 
 M O = a, the angle M O E = 
 PO 2 
 
 M 
 
 .-. M R* = 
 
 OE 
 
 . {OE 2 OR 2 }, 
 
 or 
 
 a*.sin.0 = (1 n) 2 . (1 a'cos.'O) 
 = 1 _2n. (1 a 1 cos. 2 *)) 
 .-. a 2 . { sin. 6 + 1 2 n. cos. * 6} = 1 2 n
 
 BOOK III.] NEWTON'S PRINCIPIA. 333 
 
 1 2 n _ 1 2 n 
 
 sin. z 6 + cos. 2 6 2 n . cos. 2 6 ~ 1 2 n . cos. 2 6 ' 
 
 1 n - - 1 + n. cos. Z 6 
 
 . . .. , , ^ 
 
 .-. a = = - r-. = 1 n . YY^ - ; - =1 n.(l+n cos. 2 0), 
 1 n.cos. 2 d I 2 fr*.c0&*4 
 
 = 1 n (1 cos. 2 0) = 1 n-. sin. 2 0, 
 = 1 + n . sin. 2 = 
 
 , - o . J. r- A* oiii- v *w 7=^ * 
 
 a 1 n . sin. 2 6 MO 
 
 but (594) the attraction in the direction M O oc ^, 
 /. attraction in the direction M O (A) : attraction at E (A 7 ) 
 
 : : 1 + n sin. 2 6 : 1, 
 .-. A A' : A' : : n . sin. 2 6 : 1, 
 .-. A A' A', n . sin. 2 6 9 
 .'. increase of attraction oc sin. 2 d a sin. 2 latitude. 
 
 602. Given the lengths of two pendulums vibrating seconds in two 
 known latitudes ; find the lengths of pendulums that will vibrate seconds 
 at the equator and pole. 
 
 Let L, 1 be the lengths of pendulums vibrating seconds at the equator 
 and pole. 
 
 L', 1' be the lengths in given latitudes whose sines are S, s, cosines C, c. 
 
 .-. L' L : 1' L :: S 2 : s 2 
 .-. I/s 2 Ls 2 = 1' S 2 Ls 2 
 .-. L. (S 2 s 2 ) = V S 2 I/s 2 , 
 l'S s I/s 2 
 S 2 s 2 ' 
 Again 
 
 L'- L : 1 L :: S 2 : 1, 
 .-. L' L = IS 2 LS 2 , 
 
 L 7 L. (1 S 8 ) 
 
 S z 
 
 _!/ (l / S 2 ~L / s i )(l S 2 ) 
 
 S 2 S 2 . (S 2 s 2 ) 
 
 L / S g L' s 2 V S 2 + V S * + L' s 2 L 7 S 8 s * 
 
 S 2 . (S 2 s 2 ) 
 
 i/ s 2 r. s g + v. s 4 - L'. s 2 s 2 
 
 S 2 . (S 2 s 2 ) 
 ^.(1 s 2 ) F.(l S 2 ) __ L'c 2 I'C 2 
 
 S 2 s 2 S 2 s
 
 334 A COMMENTARY ON [BOOK III. 
 
 603. Given the lengths of two pendulums vibrating seconds in two 
 known latitudes ; required the ratio between the equatorial and polar 
 diameters. 
 
 Since the lengths oc forces, the times being the same, 
 .*. L : 1 : : force at the equator : force at the pole 
 
 : : ( 10 ) T : T^T : ' 1 n : 1 : : O P : O E, 
 
 JL i j n 
 
 .*. O P : O E : : polar diameter : equatorial diameter 
 
 : : L : 1 : : 1' S 2 L' s 2 : L' c 2 1' C 2 . 
 
 604. To compare the space described in one second by the force of gra- 
 vity in any given latitude, with that which would be described in the same 
 time, if the earth did not revolve round its axis. 
 
 The space which would be described by a body, if the rotatory motion 
 of the earth were to cease, equals the space actually described by a 
 body at the pole in the same time; and if the force at the pole equal 1, 
 the force at the latitude 6 (597) equal 1 F . cos. 2 0, and since S = m F T *, 
 and T is the same, .. S ex F. 
 
 .. space actually described when the earth revolves : space which 
 would be described if the earth were at rest : : 1 F. cos. 2 6 : 1. 
 
 605. Let the earth be supposed a sphere of a given magnitude, and to re- 
 volve round its axis in a given time ; to compare the weight of a body 
 at the equator, with its weight in a given latitude. 
 
 V 2 4 <ff~. r 
 The centrifugal force = - - = p^ = F equal a given quantity, 
 
 since (r) and P are known. Now the force at the equator =1 F, 
 and the force at latitude 6 = 1 F . cos. 2 6, and the weight attractive 
 force 
 
 .-. W : W :: 1 F : 1 F. cos. 2 6. 
 
 606. Find the ratio of the times of oscillation of a pendulum at the 
 equator and at the pole, supposing the earth to be a sphere, and to re- 
 volve round its axis in a given time. 
 
 L oc F T 2 but L is constant, .-. T 2 - F , 
 
 /. T. oscillation at the pole : T. oscillation at the equator 
 : : \Mbrce at the equator : V force at the pole 
 
 ;; V J F : 1.
 
 BOOK III.] 
 
 NEWTON'S PRINCIPIA. 
 
 335 
 
 ON THE THEORY OF THE TIDES. 
 
 607. If a spherical body at rest be acted upon by some other body, it 
 may put on the form of a spheroid. 
 
 Let P E p be the earth at rest; (S) a body acting upon it; (O) its cen- 
 ter; (M) a particle on its surface. 
 
 Let P = polar. 1 
 
 n -if attraction on the earth. 
 
 ill = equatorial, J 
 
 E OR, 
 
 Then the attraction on M is parallel to M Q = ' ., - . 
 
 O L 
 
 Similarly the attraction on M is parallel to M R = ' p . 
 
 Let (m) = mean addititious force of S on P. 
 (n) = mean addititious force of S on E. 
 Now since the addititious force (Sect. XI.) cc distance, 
 
 .*. the whole addititious force of S on M = 
 
 and 
 
 m. M O 
 PO 
 
 PO 
 
 : addititious force in the direction M R : : M O : MR, 
 
 , ,. . . r . . ,. . ,. , m.MR m.OQ 
 .*. addititious force in the direction M R = ^ 
 
 PO 
 
 PO 
 
 Again, since 
 
 m : n : : P O : E O, 
 
 m n 
 
 '"'TO ~^~O' 
 
 .*. whole addititious force of S on M = 
 
 n. MO 
 E O
 
 330 A COMMENTARY ON [BOOK III. 
 
 .-. addititious force in the direction M Q = "*.?? ^ = " ' ^ ^ , 
 
 & U .hi U 
 
 .-. whole disturbing force of S on M in the direction M Q = twice the 
 nddititious force in that direction, and is negative = ff~v 
 
 .'. whole attraction of M in the direction M Q = [E 2 n}. -pn<' , 
 and the whole attraction of M in the direction M R = {P + m}. ~ . 
 
 Take M g = {E 2 n} . ^-? 
 
 M r = IP + m} . g-|; 
 
 complete the parallelogram (m q), and produce M q to meet P p in G. 
 
 Now if the surface be at rest, M G will be perpendicular to the sur- 
 face. 
 
 .-. M r : M g : : g q : g M : : G Q : Q M, 
 or 
 
 .'. P + m : E 2 n : : OE: OP, 
 
 .*. figure may be an ellipse. 
 
 608. Suppose the Moon to move in the equator ; to find the greatest ele- 
 vation of tide. 
 
 Let A B C D be the undisturbed 
 sphere; M P m K a spheroid 
 formed by the attraction of the 
 Moon; M the place to which the 
 Moon is vertical. 
 
 Let 
 
 CA E = i > 
 
 <EM = 1 +> 
 
 (E F = i ft 
 
 Then since the sphere and spheroid have the same solid content, 
 
 4 g . (A E) 3 _ 4*-.EM.(FE) t 
 3 3
 
 BOOK III.] NEWTON'S PRINCIPIA. 337 
 
 .-.1 = 1 + a _2/3 2a/3 + j8 2 + /3 2 
 
 1 -f- a 2/3 nearly, (a) and (/3) being very small, 
 .. a = 2 /3 or greatest elevation = 2 X greatest depression. 
 614. To find the greatest height of the tide at any place, as (n) . 
 Let 
 
 E P = 4 z. P E M = a + /3 = = EM EF = M, 
 
 75 : 4~9 by actual division (all the terms of two or more dimen- 
 (1 + a) 8 
 
 sions being neglected) = 1 2 . (a + /3) = 1 2M, 
 
 .-. PN 2 = g 2 .sin. 2 = (1 2 M). Jl + 2 a g 2 . cos. 2 0j 
 
 (since2a = * i =-|-M) = (1 2M) {1 + ^ g 2 . cos. 2 ^. 
 
 / 4< 
 
 (1 2M)cos. 2 ^ = (1 2 M). (l +- 
 
 2M 2M 
 
 r~ i s~ 
 
 sin. 2 tf -f cos. * d 2 M . cos. 2 6 1 2 M . cos. 2 tf, 
 
 M 
 = 1 + M . cos. 2 tf ^ , 
 
 o 
 
 M 
 
 .-. g 1 = M . cos. * 6 -- Q -= EP En = Pn= elevation re- 
 
 o , 
 
 quired. 
 
 M 
 
 615. Similarly if the angle M E p = f, .-. E p = 1 + M cos. 2 ff -- --, 
 
 M 
 
 ..I E p = p n' = depression = -^ -- M . cos. 2 6 
 
 3 
 
 = M M.cos. 2 ^ ^ = Msin. 2 ^ ^. 
 
 o J 
 
 616. B M= a = i~, 
 
 .-. BM-Pn =+M.sin. 2 ^- 
 
 = M . sin. 2 ^ oc sin. 9, 
 VOL. II. Y
 
 338 A COMMENTARY ON [BOOK III. 
 
 .*. greatest elevation oc sin. 2 horizontal angle from the time of high tide. 
 617- At (O) P n = 0, 
 
 .-. M . cos. " 6 = 0, 
 
 9 
 
 .-. M . cos. * 6 = , 
 o 
 
 1 
 
 .'. cos. 6 = ^=. 
 V3 
 
 .-. 6 = 54 , 44'. 
 
 Hitherto we have considered the moon only as acting on the spheroid. 
 Now let the sun also act, and let the elevation be considered as that pro- 
 duced by the joint action of the sun and moon in their different positions. 
 
 Let us suppose a spheroid to be formed by the action of the sun, whose 
 semi-axis major = (1 + a), axis minor = (1 b). 
 618. Let (a + b) = S, (<p) = the angular distance of any place from the 
 point to which the sun is vertical. It may be shown in the same manner 
 as was proved in the case of the moon, that 
 
 and 
 
 o 
 
 S . cos. 2 <p 5- = elevation due to the sun, 
 
 o 
 
 2 S 
 S . sin. 2 <f>' ~ = depression due to the sun, 
 
 3 
 
 (tp') being the angular distance of the place of low water from the point to 
 which the sun is vertical, 
 
 .. M . cos. 2 6 + S . cos. 2 <p ~ = compound elevation. , 
 
 Similarly M . sin. 2 / + S . sin. 2 <p' f M + S = compound depres- 
 sion. 
 
 619. Let the sun and moon be both vertical to the same place, 
 
 .-. 6 = <p = 0, 
 
 ]VT i O o 
 
 .-. M + S - ^ = M + S = compound elevation, 
 3 > 
 
 and 
 
 tf = <ff = 90, 
 
 /. M + S .M+S = ^M+S = compound depression, 
 .*. compound elevation + compound depression = M + S = height ol 
 spring tide. 
 
 620. Let the moon be in the quadratures with the sun, then at a place 
 under the moon, 
 
 (6) =. 0, and (?) = 90,
 
 BOOK III.] 
 
 NEWTON'S PRINCIPIA. 
 
 339 
 
 
 .*. compound elevation = M 
 
 a 
 
 also (tf) 90, and (?) = 0, 
 
 .-. compound depression = M f . M + S, 
 .. height of the tide at the place under the moon = 2 M M + S 
 
 = M + S = height of neap tide. 
 
 Similarly at a place under the sun, height of tide = S M. 
 
 621. Given the elongation of the sun and moon, to find the place of com- 
 pound high tide. 
 
 Compound elevation = M cos. 2 6 -f. S 
 
 cos. * <p 
 
 M 
 
 i -, 
 - = maximum at high 
 
 water. 
 
 .*. 2 M cos. 6 sin. d (f 2 S 
 cos. p sin. <p <f> r= 0, 
 but 
 
 (6' -f- <p) = elongation = E 
 
 = constant quantity, 
 .-. ff + ? = 
 
 .. 2 M cos. Q sin. & = 2 S cos. <p sin. p', 
 .-. M sin. 2 6 = S sin. 2 <p, 
 
 .-. M : S : : sin. 2 f : sin. 2 0, 
 .- M + S : M S : : sin. 2 <J> + sin. 2 6 : sin. 2 <p - sin. 2 0, 
 
 -* : : tan. (<p + 6) : tan. (<p 6), 
 
 and since (<p + 6) is known, .-. (<p 6) is obtained, and .-. (<p) and (6) are 
 found, i. e. the distance of the sun and moon from the place of compound 
 high tide is determined. 
 
 622. Let P be the place of high tide, 
 
 P' the place of low water, 90 distant from P, 
 
 P m = <J P m 1 = 90 + = / Ps = p P's 
 
 = 90 <p = ?'. 
 
 Now the greatest depression = M sin. 2 ^ + S sin. * <p' f M + S, 
 but 
 
 sin. 8 Q' = sin. 2 (90 + 6) = sin. * supplemental angle (90 6) = cos. 8 0, 
 and 
 
 sin. * <p' = sin. * (90 p) = cos. z <p t 
 
 .-. the greatest depression = M cos. * 6 + S cos. e <p f M + S, 
 and the greatest elevation = M cos. * 6 + S cos. e <p M + S, 
 
 .-. the greatest whole tide = the greatest elevation + greatest depression 
 
 Y2
 
 340 
 
 A COMMENTARY ON 
 
 [BOOK III, 
 
 = 2 M cos. * 6 + 2 S cos. * 9 M + S, 
 = M [2 cos. Q 1} + S (2 cos. 1 <f> 1) 
 = M cos. 2 6 + S cos. 2 <f>. 
 623. Hence Robison's construction. 
 
 A 
 
 Let A B D S be a great circle, S and M the places to which the sun 
 and moon are vertical ; on S C, as diameter, describe a circle, bisect S C 
 in (d); and take S d : d a : : M : S. Take the angle S C M = (p+0), 
 and let C M cut the inner circle in (m), join (m a) and draw (h d) par- 
 allel to it; through (h) draw C h H meeting the outer circle hi H; then 
 will H be the place of high water. 
 
 For draw (d p) perpendicular to (m a) and join (m d). 
 Let the angle S C H = <p, and the angle M C H = d. 
 
 Since M : S S d : d a 
 .-. M+S : M S Sd + da:Sd da 
 dm+ da:dm da 
 
 dam + dma dam dma 
 tan.- _^ -:tan. ^ 
 
 Sdm dam dma 
 tan. ~ : tan. ^ 
 
 tan. S C M : tan. 
 
 S d h m d h 
 
 tan. S C M : tan. (S C H H C M) 
 tan. (<p + 6) : tan. (<f> 6) 
 /. H is the place of high water 621. 
 Also (m a) equals the height of the whole tide. For (a p) = a d. cos. pad 
 
 = S. cos. S d h = S. cos. 2 <f> 
 and 
 
 (p m) = m d. cos. p m d = M. cos. m d h = M. cos. 2 6
 
 BodK III.] 
 
 NEWTON'S PRINCIPIA. 
 
 341 
 
 .*. a m = a p + p m = M. cos. 2 6 -f- S. cos. 2 p = height of the title. 
 
 At new moon, = <p = 1 . , , , , . , 
 
 . lono > .-. tide = M + S = spring tide. 
 
 At full moon, d = 0, <p = 180 J 
 
 When the moon is in quadratures, (m a) coincides with C A, 
 
 .-. 6 = 0, <p = 90, 
 .. tide = M S = neap tide. 
 
 624. The fluxion of the tide, i. e. the increase or decrease in the height 
 of the tide a yf. (m a) /. [M. cos. 2 6 + S. cos. 2 <p\. But the sun 
 for any place is considered as constant, 
 
 .-. /. (m a) oc M. sin. 2 6. 2 P, 
 .*. <(/. (m a) is a maximum at the octants of the tide with the moon 
 
 M. sin. 2 6 
 since at the octants, 26 = 90. 
 
 The fluxion of the tide is represented in the figure by (d p). 
 
 For let (m u) be a given arc of the moon's synodical motion, draw (n v) 
 perpendicular on (m a), .'. (m v) is the difference of the tides. 
 
 Now m u : m v : : m d : d p and m u and m d are constant, .*. 
 m v a d p and d p is a maximum, when it coincides with (d a), i. e. when 
 the tide is in octants ; for then 2 (ra a d) = 90. 
 
 625. At the new and full moon, it is high water when the sun and 
 
 moon are on the meridian ; i. e. at noon and midnight. At the quadra- 
 tures of the moon, it is high water when the moon is on the meridian, 
 because then (m) coincides with C. 
 
 For let M. cos. * 6 + S. cos. 2 <f> 
 
 M + S _ 
 
 = maximum; then since 
 
 in quadratures (<f> + 6) = 90, .-. <f> = 90 6, 
 
 .: M. cos. 2 6 + S. sin. 8 d J'M + S = maximum, 
 
 .'. 2 M. cos. 6. sin. 6. S = 2 S. sin. 6. cos. 6. 0', 
 .-. M S . 2 . sin. *. cos. 6 = M S . sin. 20=0, /. sin. 26 = 
 .'. 6 = 0, that is, the moon is on the meridian. 
 Y3
 
 34-2 
 
 A COMMENTARY ON 
 
 [BOOK III. 
 
 626. From the new moon to the quadratures, the place of 
 
 tide follows the moon, i.e. is westward of it; since the moon moves 
 from west to east, from the quadratures to the full moon, the place of 
 high tide is before the moon. There is therefore some place at which its 
 distance from the moon (0) equals a maximum. 
 Now (621) M : S : : sin. 2 p : sin. 2 Q 
 
 .-. M. sin. 2 6 = S. sin. 2 p 
 .. M. 2 6 f . cos. 2 6 = S. 2 p'. cos. 2 p = < 
 .-. cos. 2 p = 0, .. p = 45. 
 
 627. By (621) M. sin. 2 6 = S. sin. 2 p 
 
 but 
 
 .-. 6f. M . cos. 2 6 = p'. S . cos. 2 p 
 
 = e 
 
 p + = e, .-. p' + <K = e', 
 .% (e' pO M . cos. 2 6 = p'. S . cos. 2 p 
 
 .. e'. M. cos. 2 = p'. S. cos. 2 p + M. cos. 2 0} 
 
 e'. M . cos. 2 d 
 M . cos. 2 + S . cos. 2 p * 
 
 Next, considering the moon to be out of the equator, its action on the 
 tides will be affected by its declination, and the action of the sun will not 
 be considered. 
 
 M 
 
 By Art (614) the elevation = M cos. s 6 -^ 
 
 M 
 .*. elevation above low water mark = M . cos. 2 =- + b 
 
 now 
 
 M 
 3 
 
 .. elevation above low water = M . cos. * d 
 
 = magnitude of the tide. 
 
 Let the angle Z P M which measures the time from the moon's pass- 
 ing the meridian equal t. 
 
 Let the latitude of the place 
 = 90 _ P Z = 1 
 
 Let the declination 
 = 90 P M = d 
 
 cos. ZM-cos. Z Pcos. P M 
 
 E 
 
 cos.ZPM = 
 
 or 
 
 cos. t = 
 
 sin. Z P sin. Z M 
 cos. 6 sin. 1 sin. d 
 
 cos. 1 cos. d 
 . cos. 6 = cos. t cos. 1 cos. d -f- sin. 1 sin. d 
 
 Q
 
 BOOK III.] NEWTON'S PRINCIPIA. 343 
 
 .*. magnitude of the tide = M. {cos. t cos. 1 cos d + sin. 1 sin. d} z 
 .-. for the same place and the same declination of the moon, the magni- 
 tude of the tide depends upon the value of (cos. t). Now the greatest 
 and least values of (cos. t) are (+1) and ( 1), and since the moon only 
 acts, it is high water when the moon is on the meridian, and the mean 
 greatest + least 
 
 ode = 8 _ _, 
 
 greatest = M. {sin. 1 sin. d + cos. 1 cos. d} 2 
 least = M. {sin. 1 sin. d cos. 1 cos. d} 2 
 
 .-. g> ' eateSt a + leaSt = M. {sin. 2 1 sin. d + cos. * 1 cos. 2 d} 
 
 15 
 
 2 sin. 2 1 = 1 cos. 2 1 
 2 sin. 2 d = 1 cos. 2 d 
 
 .-. 4. sin. 2 1 sin. 2 d = 1 {cos. 2 1 + cos. 2 d} + cos. 2 1 cos. 2 d 
 2. cos. 2 1 = cos. 21 + 1 
 2. cos. 2 d = cos. 2 d + 1 
 
 .-. 4. cos. 2 1 cos. 2 d = 1 + (cos. 2 1 + cos. 2 d) + cos. 2 1 cos. 2 d 
 .-. 4. {sin. 2 1 sin. 2 d + cos. 2 1 cos. 2 d} = 2 + 2. cos. 2 1 cos. 2 d 
 .-. mean tide = M. sin. 2 1 sin. 2 d + cos. 2 1 cos. 2 d 
 M 1 + cos. 2 1 cos. 2 d 
 
 ~~2~ 
 
 It is low water at that place from whose meridian the moon is distant 
 90, .*. cos. 4 z= 0, .'. for low water 
 
 sin. 1 sin. d , 
 
 cos. t = ? -T = tan. 1 tan. d. 
 
 cos. 1 cos. cl 
 
 When (1 + d) = 90, .'. tan. 1 tan. d = tan. 1 tan. (90 1) 
 
 tan. 1 
 
 = tan. 1 cot. 1 = ; = 1 
 
 tan. 1 
 
 . cos. t = 1, /. t = 180, .'. time from the moon's passing the meri- 
 dian in this case equals twelve hours, .-. under these circumstances there 
 is but one tide in twenty-four hours. 
 
 When 1 = d, .-. cos. t = tan. 2 1 
 and the greatest elevation = M {cos. t cos. 1 cos. d -f sin. 1 sin. d} a 
 
 (since cos. t = 1) = M. {cos. 1 1 + sin. 8 1} = M. 
 When d = 0, .-. greatest elevation = M cos. 2 1. 
 When 1 = 0, .-. greatest elevation = M cos. 2 d. 
 
 At high water t = 0, .'. greatest elevation when the moon is in the 
 meridian above the horizon, or, the superior tide = M {cos. 1 cos. d + 
 sin. 1 sin. d} 8 = M cos. * (1 _ d) = T. 
 
 For the inferior tide t = 180, ,', cos. t = 1, 
 
 Y4
 
 344 
 
 A COMMENTARY ON 
 
 [BOOK III. 
 
 .-. inferior tide = M [sin. 1 sin. d cos. 1 cos. dj 2 
 
 = M { I (cos. 1 cos. d sin. 1 sin. d)} * 
 = M cos. 2 (1 + d) = T. 
 Hence Robison's construction. 
 
 With C P = M, as a radius, describe a circle P Q p E representing 
 
 P 
 
 M 
 
 N 
 
 a terrestrial meridian ; P, p, the poles of the earth ; E Q the equator ; 
 (Z) the zenith; (N) the nadir of a place on this meridian; M the place 
 of the moon. Then 
 
 Z Q latitude of the place = 1 \ ,, , , . 
 
 TIT ^ j i- , f .'. Z M the zenith distance = 1 d. 
 
 M Q declination = d ) 
 
 Join C M, cutting the inner circle in A ; draw A T parallel to E Q. 
 Join C T and produce it to M' ; then M' is the place of the moon after 
 half a revolution, .*. M' N = nadir distance 
 
 = ME+EN=MQ + ZQ = l-f-d. 
 
 Join C Z cutting the inner circle in B; join B with the center O 
 and produce it to D ; join A D, B T, A B, D T; and draw T K, A F 
 perpendiculars on B D. 
 L. ADB = z.BC A=ZQ M Q=l d 
 
 DA 
 
 B T Z are right angles 
 BD:DA::DA:DF= 
 
 BD~ BD 
 
 = M cos.* (1 d) = height of the sup r . tide.
 
 BOOK III.] NEWTON'S PRINCIPIA. 345 
 
 Again 
 
 r>T..nT nir DT * B D'.cos. BDT _ P1 ^ _ r -^- 5 
 rJJl : : 1) 1 : U K = < p = 5~Ti = 151) cos. 1 + d 
 
 D U JJ U 
 
 = M cos. 1 + d = point of the inferior tide. 
 
 If the moon be in the zenith, the superior tide equals the maximum. 
 
 For then 1 d = 0, .*. cos. 1 d = maximum, and B D = D F. 
 
 If the moon be in the equator, d = 0, .'. D F = D K. 
 
 The superior tide = M cos. z (I d) = T 
 
 The inferior tide = M cos. 8 (1 + d) = T. 
 
 Now T > T 7 , if (d) be positive, i. e. if the moon and place be both on 
 the same side of the equator. 
 
 T < T' if (d) be negative, i. e. if the moon and place be on different 
 sides of the equator. 
 
 If (d) = 90 1, .-. D K = Mcos. 2 (1 + 90 1) = M cos. 8 90 = 0. 
 
 If (d) = 90 + 1, and in this case (d) be positive, and (1) negative, 
 .. D F = cos. 2 (d 1). M = Mcos. 2 (90 + 1 1) = M cos. ! 90 = 0.
 
 FOR 
 
 VOLUME III. 
 
 PROB. I. The altitude P R of the 
 pole is equal to the latitude of the place. 
 
 For Z E measures the latitude. 
 
 = P R by taking Z P from E P and 
 Z R. g 
 
 PROB. 2. One half the sum of the 
 greatest and least altitudes of a cir- 
 cumpolar star is equal to the altitude of 
 the pole. 
 
 The greatest and least altitudes are at 
 x, y on the meridian. 
 
 Also 
 
 xR + y R = xy + 2y R = 2 (Py-f Ry) = 2. altitude of the pole. 
 
 PROB. 3. One half the difference of the sun's greatest and least meridian 
 altitudes is equal to the inclination of the ecliptic to the equator. 
 
 The sun's declination is greatest at L, at which time it describes the 
 parallel L r. 
 
 .*. L H is the greatest altitude, 
 
 The sun's declination is least at C, when it describes the parallel 
 s C. 
 
 .*. s H is the least altitude, 
 and 
 
 4 . (L H s H) = L s = L E. 
 
 PROB. 4>. One half the sum of the sun's greatest and least meridian al- 
 titudes is equal to the colatitude of the place. 
 
 = i (2 H E) = H E.
 
 348 
 
 PROBLEMS 
 
 PROS. 5. The angle which the equator makes with the horizon is equal to 
 the colatitude = E H. 
 
 PROS. 6. When the sun describes 
 b a in twelve hours, he will describe c a 
 in six ; if on the meridian at a it be 
 noon, at c it will be six o'clock. Also 
 at d he will be due east. He travels 15 
 in one hour. The angle a P x, mea- 
 sured by the number of degrees con- 
 tained in a x (supposing x equals the 
 sun's place), converted into the time at 
 the rate of 15 for one hour, gives the 
 time from apparent noon, or from the 
 sun's arrival at a. 
 
 PROB. 7. Given the sun's declination, and latitude of the place ; Jind the 
 time of rising, and azimuth at that time. 
 
 Given Z E, .'. Z P = colat. given. 
 
 Given be, .. P b = codec, given. 
 
 Given b Z = 90 9 . 
 
 Required the angle Z P b, measuring 
 a b, which measures the time from sun 
 rise to noon. 
 
 Take the angles adjacent to the side 
 90, and complements of the other three 
 parts, for the circular parts. 
 
 .-. r . cos. Z P b = cot. Z P cot P b 
 or 
 
 r . cos. hour .=tan. lat. tan. dec. 
 
 .. log. tan. lat. + log. tan. dec. 10 = log. cos. hour L. required. 
 
 Also the angle P Z b measures b R, the azimuth referred to the north* 
 
 and 
 
 r . cos. P b = cos. P Z . cos. Z 
 
 .*. cos. Z = 
 
 r . cos, p 
 sin. L 
 
 PROS. 7. (a) r. cos. hour L = tan. latitude tan. declination,/or sun rise. 
 
 2 . tan. lat. tan. dec. 
 Hence the length of the day = 2 . cos. hour . =
 
 FOR VOLUME III. 
 
 349 
 
 h may be found thus, from A Z P b cos. h = 
 = (since Zb= 90,) 
 
 cos. Z b Z cos. P. cos.P b 
 
 or snce 
 ' 
 
 sin. Z P . sin. P b 
 >90, 
 
 cos. L sin. p 
 
 cos. h = tan. L . cot. p, or cos. h =r tan. L . cot. p. 
 and the angle P Z b may be similarly found, 
 
 n cos. P b cos. Z P . cos. Z b 
 
 r. COS. L = - : - jy-^ - : - ^-p - 
 
 sm. Z P . sin. Z b 
 cos. p 
 cos. L * 
 
 PROB. 8. Find the sun's altitude at six o'clock in terms of the latitude 
 and declination. 
 
 The sun is at d at six o'clock. The angle Z P d = right angle. 
 Z p = colat. P d = codec. Required Z d (= coalt.) 
 
 r . cos. Z d = cos. Z P . cos. d P 
 or 
 
 r. sin. altitude = sin. latitude X sin. declination. 
 
 PROS. 9. Find the time 'when the sun comes to the prime vertical (that 
 
 vertical whose plane is perpendicular to the meridian as 'well as to the. hori- 
 
 zon J, and his altitude at that time, in terms of the latitude and declination. 
 
 Z P = colatitude. Pg = codeclination. The angle P Zg= right angle. 
 
 Required the angle Z P g. 
 
 .-. r . cos. Z P g = tan. Z P . cot. P g. 
 
 = cot. latitude tan. declination. 
 Also required Z g equal to the coaltitude, 
 
 r . cos. P g = cos. P Z . cos. Z g. 
 
 r . sin. declination . , . , 
 .*. - ; , ; T - = sin. altitude. 
 sin. latitude 
 
 PROS. 10. Given the latitude, declina- 
 tion^ and altitude of the sun ; Jlnd the 
 hour and azimuth. 
 
 Let s be the place. 
 
 Given Z P, Z s, P s. Find the angle 
 ZPs. 
 
 Let Z P, Z s, P s = a, b, c, be given, 
 
 to find B. 
 
 2r 
 
 E 
 
 sin. B = 
 
 sm. a . sin. c 
 
 X 
 
 V s . (s a) . (s b) . (s 
 
 a + b + c 
 where s = ^ 
 
 c)
 
 350 
 
 Also find C . V 
 
 PROBLEMS 
 
 -. (Or by Nap. 1st and 2d AnaL) 
 
 sin. C = 
 
 2r 
 sin. a . sin. b * 
 
 2r 
 
 Similarly, sin. A = sin. L. of position = . : : 
 
 " sin. b . sin. c 
 
 PROB. 11. Given the error in the altitude- Find the error in the time 
 in terms of latitude and azimuth. 
 
 Let ra n be parallel to H, and n x be 
 the error in the altitude. 
 .*. L. m P x = error in the time = y z. 
 
 y z : m x 
 
 : rad. : cos. m y 
 
 mx : x n 
 
 : rad. : sin. n m x 
 
 .*. y z : x n 
 
 : r 2 : cos. my. sin. n m x 
 
 or 
 
 
 M 7. 
 
 r 2 . n x 
 
 cos. m y . sin. n m x 
 
 n x 
 
 but 
 
 cos. m y . sin. Z x P ' 
 
 sin. Z x P sin. Z P 
 
 Q 
 
 sin. x Z P " 
 .-. sin. Z x P = 
 
 l ~ 
 
 sin. P x 
 
 sin. P Z . sin, x Z P 
 
 cos. m y 
 
 r 2 . n x 
 
 cos. L. sin. azimuth * 
 
 COR. Sin. of the azimuth is greatest when a z = 90, or when the sun 
 is on the prime vertical, .*. y z is then least. 
 
 Also, the perpendicular ascent of a body is quickest on the prime 
 vertical, for if y z and the latitude be given, n x cc azimuth, which 
 is the greatest. 
 
 PROB. 12. Given the latitude and 
 declination. Find the time when twilight 
 begins. 
 
 (Twilight begins when the sun is 18 
 below the horizon.) 
 
 h k is parallel to H R and 18 below 
 HR. 
 
 .*. Twilight begins when the sun is in 
 hk. 
 
 .-. Z s = 90 +18, Ps = D, Z Prrcolat. 
 Find the angle Z P s.
 
 FOR VOLUME III. 
 
 351 
 
 E 
 
 PROS. 13. Find the time when the 
 apparent diurnal motion of a Jtxed star 
 is perpendicular to the horizon in terms of 
 the latitude and declination. 
 
 Let a b be the parallel described by 
 the star. 
 
 Draw a vertical circle touching it at 
 s. 
 
 .-. s is the place where the motion ap- 
 pears perpendicular to H R. 
 
 .-. Z P, P s, and A Z S P=90 is given. 
 Find Z P s. 
 
 PROB. 14. Find the time of the shortest twilight, in terms of the latitude 
 and declination- 
 
 ab is parallel to H R 18 below H R. 
 
 The parallels of declination c d, h k, 
 are indefinitely near each other. 
 
 The angles v P w, s P t, measure 
 the durations of twilight for c d, h k. 
 
 Since twilight is shortest, the incre- 
 ment of duration is nothing. 
 
 .'. V P W = S P t 
 
 .. v r = w z 
 
 and r s = t z 
 and the angle v r s = right angle 
 
 = w z t. 
 .-. L r v s = z w t, and L. Z w c = 90 z w t = 90 Z w P. 
 
 .-. A z w t = Z w P. 
 Similarly, 
 
 /Lrvs = ZvP 
 .-. Z w P = Z a P. 
 
 Take v e = 90. Join P e. Draw P y perpendicular to Z c. 
 In the triangles Z P w, P v e, Z w = e v, P w = P v, and the angles 
 contained are equal, < .'. Z P = P e. 
 
 .-. In the triangles ZPy, Pey, Z P = P e, Py com ; and the 
 angles at y are right angles. 
 .*. Z e is bisected in y. 
 
 r . cos. P v = cos. P y . cos. v y 
 r . cos. P e = cos. P y . cos. y e.
 
 852 
 
 PROBLEMS 
 
 /. cos. P v : cos. P e : : cos. v y : cos. y e 
 (but v y is greater than 90, .*. therefore cos. v y is negative.) 
 
 : : cos. ( compl. y e) : cos. y e 
 : : sin. y e : cos. y e 
 : : tan. ye: r- 
 
 . T 18 
 
 sin. L- tan. y e sin ' L ' ten> "g" sin. L. tan. 9 
 
 .*. COS. p = Z =r _ = 
 
 r r r 
 
 P Z is never greater than 90, Z y is equal to 9, .*. P y is never greater 
 than 90, .*. cos. Py is always positive; v y is always greater than 90, 
 .. cos. v y is always negative, .. cos. P v is negative, .'. the sun's decli- 
 nation is south. 
 
 Also, if instead of R b = 18, we take it equal to 2 s equal the sun's 
 
 ,. . x . _. sin. L. tan. s . 
 diameter, we get from the expression sin. D = the time 
 
 when the sun is the shortest time in bringing his body over the horizon. 
 PROB. 15. Find the duration of the shortest twilight' 
 
 ^ w P Z = v P e, .-. z. Z P e = v P w. 
 .'. 2 Z P e is equal to the duration of the shortest twilight. 
 
 r . sin. Z y = sin- Z P . sin. Z P y 
 or 
 
 . _ sin. 90 . r 
 
 sin. Z P y = f , 
 
 cos. L. 
 
 which doubled is equal to the duration required. 
 
 PROB. (A). Given the sun's azimuth at six, and also the time when 
 due east. Find the latitude. 
 
 From the triangle Z P c, 
 
 r . cos. L = tan. P c . cot P Z c. 
 
 From the triangle Z P d, 
 
 r . cos. h = cot. L . cot. P d. 
 cos. L 
 
 .-. tan. P c = 
 
 cot. P d = 
 
 .-. tan. P d = 
 
 cos. L 
 ' cbt7 Z " 
 
 .-. sin. L = 
 
 cot. Z 
 cos. h 
 cot. L 
 cot. L 
 
 cos. h '
 
 FOR VOLUME III. 
 
 353 
 
 PROB. 16. Find the declination when 
 it is just twilight all night. 
 
 Dec. bQ=QR bR 
 
 = colat 18 
 
 = 90 L 18 
 = 72 L 
 
 PROB. 17. Given the declination, 
 find the latitude, the sun being due east, 
 when one half the time has elapsed be- 
 tween his rising and noon. 
 
 Given JL Z PC, and Z P d = Z PC. 
 
 Given also P d = p, 
 and A P Z d right angle. 
 
 v by Nap. 
 
 r . cos. h = tan. Z P . cot- p 
 
 T r . cos. h 
 
 v cot L = . 
 
 cot. p 
 
 If the angle Z P c be not given. 
 From the triangle Z P d, 
 
 . cos. Z P d = tan. Z P . cot. p. 
 
 From the triangle Z P c, 
 
 r cos- Z P c = cot. Z P cot- p, 
 
 or cos. h = cot X. cot- pj 
 
 cos- 2 h = tan. X. cot. p j 
 
 = 2 cos. 2 h 1 = 2 cot 2 X. cot 2 p 1 = ' 
 
 E 
 
 1 
 
 tan. * X 
 
 tan- 3 x + 
 
 2 cot p = 0, 
 
 .. tan- 3 X. cot. p = 2 cot- 2 p tan- 2 X 
 tan- - X 
 cot. p 
 from the solution of which cubic equation, tan- X is found. 
 
 PROB. 1 8. Given the angle between 
 two and three o'clock in the horizontal 
 dial equal to a. Find the longitude. 
 From the triangle P R n, 
 r . sin. P R=tan. R n . cot- 30 
 
 = tan. Rn. VS. 
 From the triangle P R p, 
 
 r . sin. P R = tan. R p . cot- 45 
 
 = tan R p. 
 VOL. II. 7
 
 354 
 
 PROBLEMS 
 
 .'. tan. n p = tan. a = tan. 11 p 11 n 
 
 tan. R p tan. R n 
 1 + tan. R p . tan. R n 
 
 sin. X ( I 
 
 1 + 
 
 sin. 
 
 sin.X. (V 3 I) 
 V 3 + sin. ~ x 
 
 V3 
 
 PROB. 19. In what longitude is the 
 angle between the hour lines of twelve 
 and one on the horizontal dial equal 
 to twice the angle between the same 
 hour lines of the vertical sun dial ? 
 
 From the triangle P R n, 
 sin. X = cot. 15 . tan. R n 
 
 From the triangle p N m, 
 
 sin. p M = cot. 15 . tan- N m 
 
 R n 
 
 =r cos. X = cot- 15 tun. 
 sin. X 
 
 2 
 tan. R n 
 
 cos. X 
 
 tan. R n 
 ~2 
 
 Rn , 
 tan. -- f- 
 
 = tan. X 
 
 Rn 
 
 1 tan. 
 
 Rn 
 
 . Rn ' 
 1 tan. 8 
 
 tan. 
 
 Rn 
 2 
 
 PROB. 20. Given the altitude, latitude, and declination of the sun, Jind 
 the time. 
 
 cos. h = - 
 
 cos 
 
 . Z S cos. Z P . cos. P S 
 
 .'. 1 
 
 cos 
 
 sin. Z P . sin. P S 
 sin. A sin. L . cos, p 
 cos. L . sin. p 
 
 cos. L. sin. p + sin. A sin. L. cos. p 
 
 . n is ^ = . 
 
 cos. -L . sin. p 
 
 sin. (p L)+sin. A 
 cos. L . sin. p 
 
 or 
 
 2 cos. * = 
 
 A + p L 
 
 L p> 
 
 cos. L sin. p
 
 .*. COS 
 
 t h /cos. ( ). sin. ( 
 
 ' 2 - V ' 
 
 FOR VOLUME III. 355 
 
 )the form adapted to the Lo- 
 
 garithmic computation, or, see Prob. ( 1 8). 
 
 PROB. 21. Given a star's right ascen- 
 sion and declination. Find the latitude 
 and longitude of the star. 
 Given 
 
 y b, b S, L. S b y right angle 
 
 .*. find L. S y b and S y. 
 
 .-. ^ S y a = S y b Obi. 
 
 .*. S y is known, L* S y a is known 
 
 and S a y is a right angle, 
 .*. find S a = latitude 
 
 y a = longitude. 
 
 Given the sun's right ascension and 
 declination. Find the obliquity of the 
 ecliptic. 
 
 P S being known P y = 90, L. S P y 
 = R A, 
 
 .-. in the ASPy, . S y P is known. 
 
 .-. obliquity = 90 S y P is 
 known. 
 
 PROB. 22. In what latitude does the 
 twilight last all night ? Declination 
 given. 
 
 (Twilight begins when the sun is 18 
 below the horizon in his ascent, and 
 ends when he is there in his descent, 
 lasting in each case as long as he is in 
 travelling 18.) 
 R Q = H E = colat. = b Q + b R 
 
 = D + 18. 
 ... 90 18 D = L 
 
 = 72 1 D. 
 (See Prob. 16.) 
 
 n 
 
 Z 2
 
 356 PROBLEMS 
 
 Find the general equation for the hour at which the twilight begins. 
 
 Z 
 
 Let the sides P Z, P S, Z S, be a b c. 
 
 Then sin. 2 ~ = 
 
 if 
 
 /a + b + c \ . /a -4- b -4* c 
 sin. ( ^ a ^ a) sin. (-^-^ 
 
 sin. 
 
 sin. a. sin. b 
 colat. + p + 108 
 
 sin, cotan. + p + 108 
 
 or 
 
 sin. 1 = T : 
 
 2 cos. L . sin. p 
 
 PROB. 24. Given the difference be- 
 tween the times of rising of the stars, 
 and their declinations : required the lati- 
 tude of the place. 
 
 Given P m, P n, and the A m P n 
 included. 
 
 From Napier's first and second ana- 
 logies, the L. P m n is known, 
 .'. P m C = complement of P m n is 
 known, 
 
 .-. P C = 90, P m is given, and the 
 Z. P m C is found, 
 .-. P R = latitude is known. 
 
 * PROB. 25. Given the sun in the equa- 
 tor, also latitude and altitude: find the 
 time. 
 
 Given 
 Z P, Z S, P S = 90 find the ^ Z P S. 
 
 colat \
 
 FOR VOLUME III. 
 
 357 
 
 PROS. 26. The sun's declination = 8 
 south, required the latitude, when he 
 rises in the south-east point of the 
 horizon, and also the time of rising. 
 P S = 90 + 8, Z S = 90, L S Z P 
 = 45 + 90. 
 
 Find Z P, and the A Z P S. 
 
 PROB. 27. Determine a point in E Q, 
 that the sum of the arcs drawn from it 
 to two given places on the earth's sur- 
 face shall be minimum. 
 
 Let A, B, be the spectator's situations, 
 whereof the latitude and longitude are 
 known. 
 
 Let E Q be the equator, p the point 
 required ; a b = difference of the lon- 
 gitudes is known. Let a p = x. 
 .-. p b = a x. Let L, L' be the la- 
 titudes. 
 
 In A A a p, r . cos. A p = 
 cos. L'. cos. x. 
 
 In A B b p, r. cos. B p = 
 cos. L'. cos. a x, 
 .*. cos. L . cos. x + cos. L'. cos. (a x) 
 = max. 
 
 .*. cos. L . ( sin. x) . d x -f- cos. L'. x 
 sin. (a x). ( d x) = 0, 
 ... cos. L . sin. x = cos. L'. sin. a. cos. x cos. L'. cos. a. sin. x. 
 
 Let sin. x = y 
 
 .-. cos. L . y = cos. L'. sin. a. V I y 2 cos. L'. cos. a. y 
 .. transposing and squaring 
 
 cos. * L. y 8 2. cos. L. cos. L'. cos. 2 y 2 + cos. * L'. cos. * a. y 2 
 
 = cos. * L'. sin. 8 a cos. 2 L'. sin. * a y 2 , 
 /. y * = &c. = n. and y = V n. 
 
 PROB. 28. To a spectator situated within the tropics, the sun's azi- 
 muth will admit of a maximum twice every day, from the time of his leav- 
 ing the solstice till his declination equal the latitude of the place. Re- 
 quired proof. 
 
 a b the parallel of declination passing through Capricorn. 
 
 Z3
 
 358 
 
 PROBLEMS 
 
 From Z a circle may be drawn touch- 
 ing the parallel of the declination till 
 this parallel coincides with Z. .. every 
 day till that time the sun will have a 
 maximum azimuth twice a day, and at 
 that time he will have it only once at Z. 
 
 (Also the sun will have the same azi- 
 muth twice a day, i. e. he will be twice 
 at f.) 
 
 PROD. 29. The true zenith distance 
 of the polar star when it first passes the 
 meridian is equal to m, and at the se- 
 cond passage is equal to n. Required 
 the latitude. 
 
 Given b Z = m, a Z = n, 
 
 Z P = colat. = . m + n. 
 
 PROB. 30. If the sun's declination 
 E e, is greater than E Z, draw the cir- 
 cle Z m touching the parallel of the de- 
 clination, 
 .'. R m is the greatest azimuth that day 
 
 If Z v be a straight line drawn per- 
 pendicular to the horizon, the shadow 
 of this line being always opposite the 
 sun, will, in the morning as the sun 
 rises from f, recede from the south point 
 H, till the sun reaches his greatest azi- 
 muth, and then will approach H; also 
 twice in the day the shadow will be upon 
 every particular point, because the sun 
 has the same azimuth twice a day, in 
 this situation. .. shadow will go back- 
 wards upon the horizon. 
 
 But if we consider P p a straight line or the earth's axis produced, the 
 sun will revolve about it, .*. the shadow will not go backwards. 
 
 r. cot. Z P q = tan. P q. cot. P Z, 
 or 
 cot. (time of the greatest azimuth) = tan. p. tan. L. 
 
 All the bodies in our system are elevated by refraction 33', and depress- 
 ed by parallax.
 
 FOR VOLUME III. 
 
 359 
 
 .'. at their rise they will be distant from Z, 90 1 + 33' horizontal pa- 
 rallax. 
 
 A fix d star has no parallax, .*. distance from Z = 90 1 -f 33'. 
 
 PKOB. 31. Given two altitudes and 
 the time between them, and the decli- 
 nation. Find the latitude of the place. 
 
 Given Z c, Z d, P c, P d, L. c P d. 
 
 From A c P d, find c d, and A P d c. 
 
 From A Z c d, find L. Z d c, 
 
 .-. Zdp = cdP cdZ, 
 .-. From A Z P d, find Z P = colat. 
 
 PROB. 32. To find the time in which 
 the sun passes the meridian or the hori- 
 zontal wire of a telescope. 
 
 Let m n equal the diameter of the sun 
 equal d" in space. 
 
 V v : m n : : r : cosine declination, 
 m n 
 
 .-. V v = 
 
 radius 1, 
 
 cosine declination 
 = d". second declination in se- 
 conds of space, 
 
 .. 15" in space : I" in time 
 
 d" second dec. 
 
 : : d" second dec. : 
 
 15" 
 
 = time in seconds of passing the merid 
 
 Hence the sun's diameter in R A = V v = d". second declination. 
 
 (n x = d" = sun's diameter) 
 
 V v : m n : : r : sin. P n 
 
 m n : n x : : r : sin. x n P 
 
 ~V Y : n x : : r 2 : sin. P n . sin. Z n P, 
 
 /. V v = 
 
 n x 
 
 n x 
 
 sin. P n . sin. Z n P 
 
 r 8 . d" 
 cos. X. sin. azimuth 
 
 sin. Z P . sin. P Z n 
 
 r s . d" 
 
 .*. time of describing V v = = 77 r~ -= = IT 
 
 15. cos. X. sin. azimuth 
 
 which also gives the time of the sun's rising above the horizon. 
 
 24
 
 360 
 
 PROBLEMS. 
 
 PROB. 33. Flamstcad's method of determining the right ascension of a 
 star. 
 
 LEMMA. The right ascension of stars 
 passing the meridian at different times, 
 differs as the difference of the times of 
 their passing. 
 
 For the angle a P b measures the dif- / " X \ 
 
 ference of the times of passing, which is 
 measured byab = ay by. 
 
 Hence, as the interval of the times 
 of the succeeding passages of any fixed 
 star : 360 (the difference of its right 
 ascensions between those times) : : the 
 
 interval between the passages of any two fixed stars : to the difference of 
 their right ascensions. 
 
 Let A G c be the equator, ABC 
 the ecliptic, S the place of a star, S m 
 a secondary to the equator. Let the sun 
 be near the equinox at P, when on the 
 meridian. 
 
 Take C T = P A, .-. the sun's de- 
 clination at T = that at P. Draw P L, 
 T Z, perpendicular to A G c. 
 
 .-. Z L parallel to A C. 
 
 Observe the meridian altitude of the 
 sun at P, and the time of the passage 
 of his center over the meridian. 
 
 Observe what time the star passes over the meridian, thence find the 
 apparent difference of their right ascensions. 
 
 When the sun approaches T, observe his meridian altitude on one day, 
 when he is close to T, and the next day when he has passed through T, 
 so that at t it may be greater, and at e less than the meridian altitude at 
 P. Draw t b, and e s, perpendiculars. 
 
 Observe on the two days before mentioned, the differences b m, s m, of 
 the sun's right ascension, and that of the star. 
 
 Draw s v parallel to A C. 
 
 Considering the variation of the right ascension and declination to be uni- 
 form for a short time, v b (change of the meridian altitudes in one day) : o b 
 difference of the declinations) ::sb (=sm bm):Zb. Whence Z b. 
 Add or substract Z b to or from T m. Whence Z m. Add, or take the
 
 FOR VOLUME III. 
 
 361 
 
 difference of, (according to circumstances), Z m, L ra, whence Z L, 
 
 I on 2 L 
 
 .-. gives A L, the sun's right ascension at the time of the first 
 
 Q 
 
 observation. 
 
 .-. A L + L m = the star's right ascension. Whence the right ascen- 
 sion of all the stars. 
 
 PROB. 34. Given the altitudes of two known stars. Find X. 
 Right ascensions being known, .. a b 
 =s the difference of right ascensions, is 
 known, 
 
 .-. L a P b is known. 
 .-. From A s P <?, L suPis known, 
 
 and e s, 
 
 From AZs<7, /isffZis known, 
 .-. L Z o P is known, 
 from A Z a P, Z P is known. 
 
 PROS. 35. Given the apparent diameter of a planet, at the nearest and 
 most distant points of the earth's orbit. Required the radius of the planet's 
 orbit. 
 
 D oc -p - ; D greatest, D' nearest diameter. 
 
 Qistciricc 
 
 .-. D : D' : : E P : E' P 
 
 ::EP EC:CP4EC, 
 .-. D C P + D E C = D' C P D' E C, 
 
 P - E C 
 -r.*r Jb.
 
 362 
 
 PROBLEMS. 
 
 PROB. 36. Given the sun's greatest apparent diameter, and least, as 101 
 and 100. Find the excentricity of the earth's orbit. 
 
 D a r ; the sun at S, P P' the earth's orbit, 
 rad. 
 
 100 : 101 : : S P : S P' : : C P C S : C P + C S 
 .-. 100 C P + 100 C S = 101 C P 101 C S 
 .-. 201 C S = C P 
 
 .-. C S = 
 
 CJP 
 
 201 
 
 , on the same scale of notation. 
 
 PROB. 37. Two places are on the same meridian. 
 
 Find the hour on a given day, when 
 the sun will have the same altitude at 
 each place. 
 
 Z Z', two zeniths of places, .. Z Z' is 
 known, S the place of the sun in the 
 parallel a b, Z S = S Z'. 
 
 From S draw perpendicular S D, 
 .-. Z D = Z' D, 
 
 .-. P Z + 
 
 ZZ' 
 
 = P D, is known, 
 
 P S is known, L. S D P right L, 
 .*. L D P S = hour is known. 
 
 PROB. 38. Find the time in which 
 the sun passes the vertical wire of a te- 
 lescope. 
 
 Meridian = the vertical wire, 
 .. the time of passing the meridian = 
 the time of passing the vertical wire. 
 
 Take m n = the sun's diameter = d. 
 
 V v : m n : r : cos. declination, 
 
 . Vv =- d r 
 
 cos. dec. ' 
 
 .. V v converted into the time at the 
 rate of 15' for 1 = the time required. 
 
 PROB. 40. If a man be in the arctic circle, the longest day = 24 hours, 
 the shortest = 0.
 
 FOR VOLUME III. 
 
 363 
 
 P Z = obliquity = Q R, 
 .-. Z R = P Q = 90 
 Z H = P Q = 90 
 /. H R is the horizon, and the 
 nearest parallel touches at R, 
 
 .. the day = 24 hours, and the far- 
 thest parallel touches at H, 
 .. the day = hours. 
 
 PROB. 41. Given the sun's meridian 
 altitude = 62, midnight depression 
 = 22. Find the longitude and declina- 
 tion. 
 
 Qa = bQ 
 
 or Ha HQ = RQ Rb 
 = H Q R b, 
 
 Ha+ Rb ~ 
 .-. ^ = H Q = cos. X 
 
 m 
 
 = 42, .-. X = 48, 
 ... D = 62 42 = 20. 
 
 PROB. 42.Given the sun's declination, 
 apparent diameter, altitude, and longi- 
 tude. Find the time of passing the 
 horizontal wire of a telescope. 
 
 s = the place of the sun. 
 
 Take s n in a vertical circle = the 
 sun's diameter = d. 
 
 Draw n a parallel to the horizon, 
 V v : o s : : r : cos. dec. 
 a s : n s : : r : sin. n s P, 
 
 .. V v : d : : r * : sin. P s sin. n s P 
 : : r 2 : sin. Z P sin. P Z s, 
 d 
 
 Vu r 
 v = -. : : j- con- 
 sin, cos. X sin. azimuth, 
 
 verted into the time, gives the time re- 
 quired
 
 364 
 
 PROBLEMS 
 
 PROB. 43. Given the longitude, 
 right ascension, and declination of two 
 stars; find the time when both are 
 on the same azimuthal circle, and also 
 of the azimuth. 
 
 Given P S, P S', and L S P S' = 
 difference of right ascension. 
 L. P S S' is known, 
 L. P S Z is known, 
 and Z P given, and P S given, 
 .*. L. P Z S, is known = azimuth, 
 and Z P S = time for the first star, 
 or (Z P S + S P S') = time for the 
 second star. 
 
 PROB. 44- Given the longitude and 
 declination. Find the time when the 
 sun ascends perpendicular to H R. 
 
 D must be greater than X, or a Q 
 greater than Z Q. 
 
 Draw the vertical circle tangent to 
 the parallel of declination, at d. 
 
 P Z given, P d given, L P d Z is a 
 right L, 
 
 .. L. Z P d is known. 
 
 Q 
 
 PROB. 45. Find the length of the 
 longest day in longitude = 45. 
 
 Q d = obliquity, 
 .. P d = 90 obliquity = P c, 
 
 Z P = 45, 
 
 Z c = 90, 
 . 2 hours is known- 
 
 O 
 
 Q
 
 FOR VOLUME III 
 
 3G5 
 
 PROB- 46- Find the right ascension 
 and decimation of a star, when in a 
 line with two known stars, and also in 
 another line with two other known 
 stars. 
 
 The star is in the same line with S, S', 
 and in the same line with s, a, 
 .-. in the intersection s 
 
 PROB. 47. The least error in the time due to the given error in altitude 
 = b". Find the longitude, 
 n x is the given error in altitude, 
 V v : m n : : r : cos. declination, 
 m n : n x : : r : sin. x n P. 
 V v : n x : : r 2 : sin. P n sin. Z n P, 
 
 IT n x r 2 
 
 V v ~" 
 
 sin. P n sin. Z n P 
 
 n x r * 
 = sin. Z P sin. P Z n 
 
 _ n x r 2 
 
 cos. X sin. azimuth ' 
 .. V v is least when the sin. azimuth 
 is greatest, or the azimuth = 90, i. e. the prime vertical, 
 n x r* 
 
 .-. b = 
 
 .-. cos. x = 
 
 COS. X 
 
 n x r 3 
 
 PROB. 48. Given two altitudes, and 
 two azimuths of the sun. Find the longi- 
 tude. 
 
 Z S is known, Z S' is known, z. S Z S' 
 = difference of the azimuth, 
 
 . L. Z S S' is known, 
 
 .-. L Z S P = Z S S' 90 is known, 
 
 .-. Z S P, Z S, S Z P, known, 
 find Z P.
 
 366 
 
 PROBLEMS 
 
 since D 
 
 PROB. 49. Near the solstice, tho declination a longitude, nearly. 
 
 r sin. D = sin. L sin. 7, 
 .- r d (D) cos. D = sin. y d (L) cos. L 
 
 d (D) _ sin. 7 cos. L 
 r d~(L) "' cos. D 
 sin, y cos. 90 d (L) 
 
 cos- 7 
 
 may be considered the measure of 7, 
 = tan. 7 sin. d (L) 
 = tan. 7 d (L), since d (L) small, 
 
 d(D) tan. 7 
 , ^ T L =: -- - = constant quantity, 
 
 . d (D) ad (L) nearly. 
 
 PROB. 50. Given the apparent time T of the revolution of a spot on 
 the sun's surface, find the real time. 
 
 Considering the spot as the inferior planet in inferior conjunction, 
 
 T = p where P equals the earth's periodic time, p equals the planet's, 
 
 .-. T P T p = P p 
 
 PROB. 51. The sun's declination equal 8 south, find the latitude of the 
 place where he rises in the south east point, and also the time of his 
 rising. 
 
 E 
 
 Z c = 90, P c = 98, . c Z S = 135, 
 whence Z P, and . h
 
 FOR VOLUME III. 
 
 3<?7 
 
 O 
 
 Q 
 
 PROS. 52. How high must a man be raised to see the sun at mid- 
 night? 
 
 Z P = R Q. Take P d = Q b 
 
 .-. b d = 90. 
 
 Draw x d to the tangent at d, 
 /. if the person be raised to Z x, he will 
 see the sun at b, 
 
 u d C b = 90 = x C R, 
 .. x C d = R C b measured by R b 
 given. 
 .*. in the rectilinear AxdC, . x d C 
 
 = right angle, 
 
 L. x C d being known from the dec. 
 C d = radius of the earth. 
 
 .*. C x being known, 
 .-. C x 90, or Z x is known. 
 
 PROB. 53. Given the latitudes and 
 longitudes of two places, find the straight 
 line which joins them. They lie in the 
 same declination of the circle. 
 
 V v : A B : : 1 : cosine declination, 
 
 .. A B is known, 
 and the straight line joining A, B, is the 
 
 chord of A, B, = 2 sin. ^ . 
 
 
 
 PROB. 54. A clock being properly adjusted to keep the sidereal time, 
 required to find when y is on the meridian. 
 
 P 
 
 Observe the sun's center on the meridian, when the declination = x y, 
 is known,
 
 368 
 
 PROBLEMS FOR VOL. III. 
 
 ~ x y 7 = right angle 
 x 7 y = I, being known, 
 x y is known. 
 
 Whence y y = time from noon to y being on the meridian, or from 7 
 being on the meridian to noon, whence two values of 7 y are found. 
 If the declination north and before solstice the value gives the time, 
 
 after 
 
 If the declination south and befor 
 after 
 
 PROB. 55. Given the sun's declina- 
 tion and longitude, find his right ascen- 
 sion, his oblique ascension, his azimuth 
 and amplitude, and the time of his rising, 
 and the length of the day. 
 7 C is given, from A c C d, c d is given ; 
 I. and right angle, find c d. 
 .'. C 7 = R A, C d = oblique asc n . 
 and C d measures L. C P c, 
 
 . . 90 + C P c = time of rising, 
 2 (90 + C P c) = length of the day. 
 
 (Thelwall.) 
 
 O
 
 369 
 
 NOTES. 
 
 To show that (see p. 16.) 
 
 xdy ydx, dx dy 
 
 2 ./* X 2. /* - l-ft* -- |-2.^yX2.At dt - 2 .MX X 2 ft -^ 
 
 , iT(^-x)(d/-dy)-(/-y)(d^-dx)-j 
 \ d t J ' 
 
 Not considering the common factor -T-, we have 
 2 . yu. X 2 . / (x d y ydx) 
 
 = (i* + P' + ?*" + fr ( x d y yd x) 
 
 + p,' (*' d y' y' d x') + A*" (x'' dy" y" d x") + &c.} 
 = p* (x dy y d x) + n'* (x' d y' y' d x') 
 
 + ^' /2 (x" dy" y" d x") + &c. 
 + //,,/ (xdy y d x + x' d y' - y' d x') 
 
 + ft P" (x d y y d x + x" d y" y'' d x") + &c. 
 + n,' ft" (x' d y' y d x') + ( x " d y" y" d x") 
 
 + ft,' f'" (x' d y' y' d x' + x'" d y'" y'" d x'") + &c 
 + ijf' tf" (x" d y" y" d x" k + x' 7/ d y'" y'" d x"') + &c. 
 
 &c. 
 
 the law of which is evident 
 Again, 
 2./*yX2.(.dx 2. /ax X 2 . ,<i d y 
 
 = (^y + ft? y' + [L ff y" + ....) (ft d X + ft' d x' -f I*" & *" + > &C.) 
 
 = ft 1 (x d y y d x) /*' l (x' d y' y' d x') &c. 
 4- ft, fjf (y d x' x d y' + y' d x x' d y) 
 
 + fi.fi!' (ydx" xdy" + y"d x x"d y)+& c. 
 + (S it," (y 7 d x" x' d y" 4- y" d x' x" d y') 
 
 + /*' /"'" (y' d x'" x' d y'" + y'" d x' x'" d y') -f &c. 
 
 + &C. 2 A 
 
 VOL. ii.
 
 370 NOTES. 
 
 * 
 Hence by adding together these results the aggregate is 
 
 PI*' (x d y y d x + x' d y' y' d x' + y d x' x d y' + y f d x x' d y) 
 
 + p p." (x d y y d x + &c.) + &c. 
 pf ft," (x d y' y' d x' + x" d y" y" d x'' + y' d x" x' d y" 
 
 + y" d x' x" d y') + &c. 
 &c. 
 But 
 
 x d y y d x + x' d y' y' d x' + y d x' x d y' + y' d x x' d y 
 = dy (x x') + d x (y y) + d f (x' x) + d x' (y y') 
 = (x' - x) d y' - d y) - (/ y) (d x' d x) ; 
 
 and in like manner the coefficients of p p'', ,<* (if" /*' /z", (i! //", 
 
 &c. are found to be respectively 
 
 (x' 7 x) (d y" d y) (y" y) (d x" d x), 
 (x'" x) (d y"' d y) (y'" y) (d x'" d x), 
 
 (x" _ x) d y" d y') (y" y') (d x" d x'), 
 (x'" x') (d y'" d y') (y'" y') (d x'" d x') 
 &C, 
 
 Hence then the sura of all the terms in PIL', pp" i>! ^'', /*' p." 
 
 //' jU/", (j! [,"" is briefly expressed by 
 
 2 .tkuf J(x' x) (d y' d y) (y' y) (d x' d x)} 
 and the suppressed coefficient -, being restored, the only difficulty of p. 
 1 6 will be fully explained. 
 
 That 2 . ( j-'\ = 0, &c. has been shown. 
 \d x/ 
 
 2. To show that /( 2 s . /" d x X 2 . /* d * x) = (2 . ,<* d x) * 
 page 17. 
 
 2.ict d'x = ^d 2 x + ^d* x' + &c. 
 
 = d . At d x -f d . pf d x' + &c. 
 = d (p d x + p' d x' + &c.) 
 = d . 2 . p d x. 
 Hence 
 
 /(2s ./*dx X 2 ./-id'x) =/2.2^dx X d%2./* 
 
 = (2 . A* d x; * 
 being of the form y 2 n d u = u *.
 
 NOTES. 37 1 
 
 3. To show that (page 17). 
 
 2 mf t x 2 .p (dx* + dy a -f dz 2 ) 
 
 _ {(2. ( *dx) 2 + (2. My) 8 + (s.^dz)'} 
 = 2.p/ K dx ' dx ) 2 + ( d y' d y) 8 + (dz' dz) J. 
 Since the quantities are similarly involved, for brevity, let us find the 
 value of 2 . p X 2 . A& d x * (2 . p d x) *. 
 
 It = (M. + ft' + P" 4- ..) (/ d x* + it! d x' ! -f p." dx" 2 + ....) 
 (it, d x + (tf dx? + ^ dx" + . . . .) z ; 
 
 Consequently when the expression is developed, the terms ^ 8 dx z , 
 (i f * d x' 2 , /*" z d x'' 2 , &c. will be destroyed, and the remaining ones will 
 be 
 
 It, fjf (d x 2 + d x' * ,2 d x d x') = ft iif (d x' u x) 
 f/ft"(d x ! -f dx"' 2 d x dx") = /*^"(dx" dx) 2 
 
 /*V (d x' 2 + d x" z 2 d x' d x") = p* p" (d x" d x') a 
 fifftf" (d x' 2 + d x'" 2 2 d x' d x 7 ") = /' /6 W (d x"> d x x ) s 
 
 ^ X" (d x" * + d x'" * 2 d x'' d x"') = v." tif" (d x /;/ d x") * 
 &c. 
 
 Hence, of the partial expression 
 
 2 . /A X 2 . p, d x * (2 . (i d x) * = 2 . p- /*' (d x' d x) . 
 
 In like manner 
 
 2.^ x 2,/tdy 2 (2./tdy) t = 2.At/t* / (dy / dy)* 
 2./ X 2. /idz 8 (2 . /A d z) 8 = 2 . /" /^ (d z' d z) 2 
 
 and the aggregate of these three, whose first members amount to the pro- 
 posed form, is 
 
 2 . fkfif {(d x' d x) + (d y' d y) * + (d z' d z) S J 
 
 4. To show that (p. 19.) 
 
 nearly. 
 
 It is shown already in page 19 that 
 
 3 \ 2
 
 372 N Q T E S. 
 
 X X o X . . . . . 
 
 -1 = fry, 777. (*' x, + y y, + z^ z,). 
 
 b \ / \s / 
 
 But since x, = x x\ y, = y y\ z, = z z\ by substitution 
 and multiplying both members by ,, we get 
 
 ^ x .. x 3 x v , v . . . 3 x v 
 
 T~ = P""(?r l x y ^ y " tt z) + ~(>p" " tt 
 
 nearly. 
 
 Similarly 
 
 /*' x' ^' x' 3 x x . , , , , v i 
 
 7-, - = (jy> - ^y* ( x < a x + y tt > 
 
 nearly. 
 
 8cc. 
 
 Hence 
 /iX2.-x3x N . N . . 
 
 T 1 " : IFF "VST** 2 " a x y ^ y + 2 ' 
 
 But by the property of the center of gravity, 
 
 2 . fj, x = 0, 2 . ,u y = 0, 2 . p z = 0. 
 
 Hence 
 
 //. x 3 x x 
 
 5. To show that (p. 22.) 
 
 X V Z 
 
 - d 2 x + J - d 2 y + d 2 z = 
 
 x /d Qv y /d Q\ z /d 
 
 H ~ H 
 
 and that 
 
 First, we have 
 
 xd 2 x + yd*y + zd 2 z 
 
 = d (x d x + y d y + z d z) (d x 2 + d y 2 + d z). 
 But 
 
 x 2 + y* + z 2 = r, 
 xdx + ydy + zdz = fdj 
 and because 
 
 X = o COS. i) X COS. V 
 
 y = g cos. 6 X sin. v 
 z = sin. <>.
 
 NOTES. 373 
 
 .. d x 1 + dy * = [d (g cos. 6} . cos. v g cos. d X d v sin. \\ * 
 
 + J(g cos. d) sin. v + g cos. d v cos. v} 2 
 = (d . g cos. 6) 2 + g * d v * cos. * 0, 
 
 .-. dx z + dy' + dz 2 = (d.gsin. 0) 2 + (d.gcos. 0) 2 + e 2 d v* cos,'* 
 = d| 2 + g 2 d0 8 + g j d v 2 cos. 2 0. 
 
 Hence, since also 
 
 d . g d g = d g 2 ^- g d * g, 
 *d'x + ydy + zd 2 z = d , d v cos . * 4 __ g d d\ 
 
 t 
 
 Secondly, since g is evidently independent of the angles 6 and v, the 
 
 three equations (1), give us 
 
 /d x\ x 
 
 ( - r - ] = cos. 6 cos. v = , 
 Vdg/ g 
 
 d y\ . y 
 
 j-f-l = cos. sin. v = - --, 
 d^/ g 
 
 d z\ . z 
 
 1 = sin. d = 
 
 g/ s 
 
 Hence 
 
 /d QN /dxx /d Qs /d yx /d Qv /d 
 
 ' " ' ' 
 
 / N /xx / s / y 
 
 vary va g^ ' \"dy; Uv ' ' 
 
 But since Q is a function of g (observe the equations 1), and g is a func- 
 tion of x, y, z, viz. Vx 2 + y 2 + z *, 
 
 = d (I?) (S 
 
 But 
 
 x/ \ x / ' \ g / x 
 
 and like transformations may be effected in the other two terms. Conse- 
 quently we have 
 
 d 
 
 , 
 
 Hence and from what was before proved, we get 
 
 , A , /d x\ /d Q\ , /d y\ /d Q\ , /d z 
 
 d Q = d *' (a) (arr) + d l (r ) ( -3) + d Qr
 
 271 NOTE S. 
 
 6. To show that x d* y y d* x = d (0 2 d v cos.* 6), and that 
 
 ' see P- 22 - 
 
 First, since 
 
 xd s y = d.xdy dxdy 
 yd 2 x = d.ydx dxdy, 
 .. x d 2 y y d 2 x = d . (x d y y d x). 
 
 But from equations (1), p. 22, 
 
 d y = sin. v . d (* cos. 6) -f. o cos. 6 . cos. v d v 
 d x = sin. v . d (3 cos. 6) cos. d sin. v d v, 
 
 /. x d y r= sin. v cos. v . * - + ? 2 cos. " 6 cos. 2 v d v 
 
 z 
 
 d (f~ cos. 2 &) 
 
 v d x. = sin. v cos. v . : f 2 cos. 2 6 sin. * v d v 
 
 J z 
 
 the difference of which is 
 
 g 2 cos. 2 d X d v. 
 Consequently 
 
 xd 2 y yd*x = d.(o 2 d v cos. s 6}. 
 
 Secondly by equations (1) p. 22, we have 
 
 /d y\ 
 
 ( -r-*- ) = P cos. y cos. v = x 
 
 ^d v/ 
 
 (d x\ . . 
 
 -1 - 1 = s cos. s sin. v = y, 
 d v/ 
 
 w 
 But since dividing the two first of the equations (1) p. 22, we have "- 
 
 r= tan. v, v is a function of x, y only. Consequently, as in the note pre- 
 ceding this it may be shown that
 
 NOTES. 375 
 
 . . ( d y\ ( d 9\ 4. (<L?\ (*Q 
 
 \dv)(dy) Vd'vM-dx 
 
 lQ). 
 
 dx/ 
 
 Hence 
 
 dT 
 
 7. To find the value of r)" 1 terms off, v, 6, (see the last line but 
 * cl S / 
 
 two of p. 22) 
 
 Since & is a function of x, y, z, we have 
 
 ,d Qv /d Qx /dxx /d Qx /dj^N /dQx /d zv 
 VHTJ ' ' \d^) VtHJ ' ^"dT/ Vd d) \dz) Vd I/' 
 
 But from equations (1) p. 22, we get 
 
 . ^) s sin. 6 sin. v 
 d 6 J 
 
 .d 
 
 Hence multiplying the values of 
 
 /d_Qx /dQv /dQx 
 
 Vdx/' Uy/' VdzJ' 
 
 d 2 x d 2 y d 2 z 
 
 by the partial differences we get 
 
 ( Y~T j = -i z [d z z S cos - ^ d " y . sin. sin. v d 2 x j sin. 6 cos. vj 
 
 Now the first term gives 
 
 f cos. 0.d 2 zz={d 2 | sin. ^ cos. <> + 2 d d cos. 2 tf 
 
 + f cos. S ^d 2 t) pd^ 2 sin. d cos. 0j, 
 
 and the two other terms gives when added, by means of the equations ( 1 ) 
 p. 22, 
 
 ff\c. A * * f*f^o A * * ' / '\ ' J^ /i 
 
 Cvo 9 COS. tr
 
 S76 NOTE S. 
 
 But 
 
 d(ydy + xdx) = |d.(d.x*H- y 2 ) = d* (** cos.' 4) 
 = d [f cos. d d (g cos. 6)} 
 = (d . * cos. 6) * + g cos. 6 d * (f cos. 4) 
 and 
 
 dx* + dy s = (d.g cos. ) * + j ! cos. * d . d v *. 
 
 Hence 
 
 sin. 6 . . 
 
 -- -, y d 2 y + xd e x) 
 cos. 6 w 
 
 = -- ^-j [p cos. d d 2 (e cos. 0) P 2 cos. 2 ^ d v *l 
 cos. ^ 
 
 = g sin. ^ Jd 2 (o cos. 0) cos. 6 d v Z J 
 
 = sin. ^ d 2 cos. ^ 2 d d d sin. d d * 6 sin. 
 
 d 6 Z + d v 2 gcos. I}. 
 Adding this value to the preceding one of the first term, we have 
 
 (ITT) = dV x LP c]2 * + 2 d * d d + s d y2 sin> tf cos * 'i 
 
 , d*^ ,dv* . , 2 ? d ? dO 
 
 = -dT + ar^ sin * ' cos * 6 + ' d t * 
 
 the value required. 
 
 8. To develope = - A in terms of the cosines of 6 and of itsmul- 
 1 + e cos. 6 
 
 tiples, see p. 25. 
 
 If c be the member whose hyperbolic logarithm is unity, we know that 
 
 cos. = 
 
 which value of cos. 6 being substituted in the proposed expression, we 
 have 
 
 1 2 c '- 
 
 1 + e cos. 6 ec* 8 *- 1 + 2c a *- 1 + 
 2 c B ~^~ l 1 
 
 c*'V-i + - c 
 
 But since 
 
 _ c o v-i 
 
 6
 
 NOTES. 377 
 
 gives 
 
 c .v-i - l + I l ~ "71-- Vi- 
 
 e V e* VT+~ ~ 
 
 and since, if we make 
 
 1 
 
 -(1 VI e^ = X which also = , , . . ., 
 
 e v 1 + VI e 2 ' 
 
 we also have 
 
 the expression proposed becomes 
 
 1 2 c v 
 
 e COS - ^ e 
 
 e (1 + Xc*-)(l + 
 2* / 1 
 
 2 (1 A 2 ) Vl 4- 
 
 Xc V ~ J 
 
 -/ i* * * 
 
 But 
 
 X 
 
 e 
 and 
 
 e 
 
 2 
 
 1 1 / 1 Xc- ev "" 
 
 1 + e cos. d V(l e 2 ) \l + Xc fl yr - 1 1 + X c ~ ' 
 
 which when & = v w is the same expression as that in page 25. 
 Again by division 
 1 
 
 1 + Xc v- 
 ftiid 
 
 X c 9 V 1 
 , - = -- XC-0V- 1 _ X*c 2 0v"-li& r 
 
 ' ' 
 
 Taking the latter from the former, we get 
 VI e 2 
 J -j- e cos. 6 
 
 e 2 _ T _ x fl /-i 
 
 = 1 2 X cos. 6 -f 2 X* cos. 2 d 2 X 3 cos. 3 6 + &c.
 
 STS NOTE S. 
 
 and substituting for 6 the value v w, we get the expression in page 
 25. 
 
 9. To demonstrate the Theorem of page 28. , 
 
 Let us take the case of three variables x, y, z. Than our system ol 
 differential equations is 
 
 in which F, G, H, are symmetrical functions of x, y, z ; that is such as 
 would not be altered by substituting x for y, and y for x ; and so on for 
 the other variables taken in pairs; for instance, functions of this kind 
 
 Vx + y*+ z* + I*, (xy 4 xz+ xt + yz+ y t + z t)^-, 
 
 (x y z + x z t + y z t), 
 log. (x y z t) and so on. 
 
 Multiply the first of the equations by the arbitrary , the second by /3, 
 and the third by y and add them together; the result is ' 
 
 0=(x+/3y+yz) + 
 
 N6w since a, |S, 7, are arbitrary, -we may assume 
 
 ax+/3y + 7Z=0, 
 which gives 
 
 d x . d y d z 
 
 a dr + /3 dt + Mt = " 
 
 d x d * y d / 
 
 ' 
 
 and substituting for x, ^ , - y , their values hence derived in the first 
 
 Cl L (.It 
 
 f>f the proposed equations, we have
 
 NOTES. 379 
 
 S. x 7 - X = 0. 
 
 a, a 
 
 In the same it will appear that 
 
 verifies each of the other two equations. It is therefore the integral of 
 each of them, and may be put under the form 
 
 z = a x + b >' 
 
 in valuing only two arbitraries a and b, which are sufficient, two arbitra- 
 ries only being required to complete the integral of an equation of the 
 second order. 
 
 In the equations (0) p. 27. 
 
 
 = H, G = and F = 1 
 
 and g 3 being = (x 2 + y * + z 2 ) 3 is symmetrical with regard to x, y, z. 
 Hence the theorem here applies and gives for the integral of any of the 
 equations 
 
 z = a x -{- b y, 
 see page 28. 
 
 Again, let us now take four variables x, y, z, u ; then the theorem pro- 
 poses the integration of 
 
 r\ v f 1 ^ V 
 
 
 
 d t d t 
 
 d z d 2 z 
 
 Multiplying these by the arbitraries , ft 7, 8 and adding them we get, 
 as before 
 
 = H (x-j-/3y-f. 7 z-f5u) 
 
 ~, / d x _ d y d z . d u\ 
 
 + G ( <i v + <* a t + y di + a ar) 
 
 d 2 x -,d I v, d s z..du
 
 380 NOTES. 
 
 Assume 
 
 ax + /3y4-7 2 -r-3u = 0. 
 
 and upon trial it will be found as before, that this equation satisfies each 
 of the four proposed equations, or it is their integrals supposing them 
 to subsist simultaneously. As before, however, there are more arbitraries 
 than are necessary for the integral of each, two only being required. 
 
 Hence the interal of each will be of the form 
 
 
 This form might have been obtained at once, by adding the two last of 
 the proposed equations multiplied by 7 and 5 to the two first of them, and 
 assuming the coefficient of H = 0, as before. 
 
 In the same manner if we have (n) differential equations of the i-th order, 
 the order involving the n variables x (l) , x (2) . .. . x< n) , and of the general 
 form 
 
 d x W d 2 x W d ' l x W d ' x M 
 
 = Hx<-> + G^L. + F %- + .... A^r + lji_ 
 
 we shall find by multiplying i of them (for instance the i wherein first 
 s = 1, 2 . . . . i) by the arbitraries a C>, a.(\ ..... a Wj adding these results 
 together and their aggregate to the sum of the other equations ; and as- 
 suming the coefficient of H = 0, that 
 
 a (i) X W 4. x () -f .... a CO x W -f. X i + 1 + x4 2 + ..... X n = 
 
 v ill satisfy each of the proposed differential equations subsisting simulta- 
 neously ; and since it has an arbitrary for every integration, it must be 
 the complete integi'al of any one of them. 
 
 This result is the same in substance as that enunciated in the theorem 
 of p. 28, t inasmuch as it is obtained by adding together the equations 
 whose first members are x W, x W t &c. and making such arrangements as 
 are permitted by a change of the arbitraries. In short if we had multi- 
 plied the i last equations instead of the i first by the arbitraries, and 
 added the results to the n i first equations, our assumption would have 
 been 
 x (i) _|_ x W + ..... x ( - J) + a ^ x< ~ ! + J) -f a W x (n ~ i+2) + .... a f X n = 0.... (a) 
 
 which is derived at once by adding together the n i equations in pnge 
 28.
 
 NOTES. 381 
 
 If we wish to obtain these n i equations from the equatk n (a), it 
 may be effected by making assumptions of the required form, provided by 
 
 so doing we do not destroy the arbitrary nature a W, a^\ a (i) . The 
 
 \iecessary assumptions do, however, evidently still leave them arbitrary.. 
 Those assumptions are therefore legitimate, and will give the forms of 
 Laplace. 
 
 END OF S'OLUME SECOND.
 
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