361 
 
 
 Burney 
 
 Commentary on the Systems which 
 
 have been advanced for explaining 
 
 the planetary Motions
 
 THE LIBRARY 
 
 OF 
 
 THE UNIVERSITY 
 OF CALIFORNIA 
 
 LOS ANGELES 
 
 'A 'N '
 
 A COMMENTARY 
 
 THE SYSTEMS 
 
 WHICH HAVE BEEN ADVANCED FOR EXPLAINING THE 
 
 PLANETARY MOTIONS. 
 
 BY JAMES BURNEY, ESQ. 
 
 OF THE BOYAL NAVY, AND F.R.S. 
 
 HonHon : 
 
 Printed by Luke Hansard % Sons, near Lincoln's-Inn Fieldt, 
 
 1819.
 
 QB 
 3GI 
 B.93c 
 
 TO THE RIGHT HONOURABLE 
 
 SIR JOSEPH BANKS, BART. K.B. 
 
 SIR, 
 
 TN so hazardous an undertaking as deliver- 
 ing unreserved opinions upon the Systems 
 of the Planetary Motions which have been 
 proposed by the greatest astronomers, it is 
 impossible not to have hesitated between the 
 fear of committing mistakes, and the hope 
 of furnishing ideas that might assist the 
 advancement of Astronomy. How far it will 
 be allowed excusable to have ventured with 
 such uncertain prospect, will much depend 
 upon the degree in which I shall have avoided 
 the one, or succeeded in the other. 
 
 Though I do not flatter myself that this 
 
 Essay will prove an offering deserving your 
 
 A 2 acceptance 
 

 
 IV DEDICATION. 
 
 acceptance, I could not refuse myself the 
 satisfaction of dedicating it to you, as the 
 distinguished Promoter of Science, in all its 
 branches, and President of the most learned 
 Society in this kingdom ; allow me to add 
 also, in acknowledgment of the many years 
 friendship with which you have honoured me, 
 as a small token of the sincere respect and 
 esteem with which I am, and have the honour 
 to subscribe myself, 
 
 Sir, 
 
 Your much obliged 
 and most obedient 
 humble servant, 
 
 JAMES BURNEY.
 
 A COMMENTARY 
 
 On tfie Systems which have been advanced for 
 explaining the PLANETARY MOTIONS. 
 
 T T will doubtless appear to the Reader of the 
 following treatise, as it has done to the Author 
 himself, a species of disloyalty like the with- 
 holding a due allegiance, to controvert opinions 
 which have been long generally established, and 
 which were originally received upon authorities 
 the most highly respected of those which have 
 given support to any system that has been 
 imagined for explaining the economy and the 
 processes of Nature, in the disposition of the 
 heavenly bodies. The Author therefore does 
 not publish it without considerable apprehen- 
 sion, as he must be aware that a failure on 
 his part will incur a heavy charge of rashness 
 and presumption. Wherein he differs from such 
 authorities is not in mathematical deductions j 
 it is purely in the contemplation and conception 
 AS of
 
 of the subject. His reasonings and opinions are 
 of the plainest kind, and might have been 
 entertained without the necessity of mathe- 
 matical knowledge beyond the first four rules 
 of arithmetic. 
 
 Investigation intended to trace generally the 
 natural course of uninformed matter, is apt to 
 bring into discussion the divine superintendence. 
 Arguing the government of the Universe by a 
 chain of consequences has a degree of affinity 
 to the doctrines of necessity, on which account 
 the Author chooses here to declare his dissent 
 from the opinions of the fatalists, and especially 
 from those which question the interposition of 
 Providence, or which suppose the Divinity to 
 predetermine irreversibly his own will. Such pre- 
 determination, he conceives, would be a restraint 
 incompatible with infinite power and goodness, 
 and a belief of this nature is calculated to lessen 
 exertion and our reliance upon Providence. It 
 is not the Author's intention to engage in any 
 subject evidently unsearchable; but to reason 
 on what may be conceived discoverable, which 
 affords a prospect of profit, and which is field 
 large enough to exercise curiosity, or to satisfy 
 ambition. 
 
 It
 
 It is right rather than necessary, that the 
 general reader should be advertised (what the 
 more learned would not otherwise fail to discover) 
 that the work here presented is the fruit of 
 meditation rather than of study, which may 
 have caused, but will not excuse, his deliver- 
 ing sometimes as new, opinions which have 
 been already advanced by other writers. In 
 an attempt like the present, apology must 
 be ineffectual. If the public shall judge his 
 astronomical ideas more worthy to have, been 
 suppressed than to have been submitted to their 
 notice, he must be resigned to their decision. 
 
 It should be understood that this essay origi- 
 nated in accident, and quite unpremeditatedly. 
 The Author, reflecting on the disposition in the 
 balanced needle to place itself in a North and 
 South direction, was not satisfied with ascribing 
 it wholly to the influence of the magnet. A 
 paper written by him of his conjectures on this 
 subject was read at the last of the summer 
 meetings of the Royal Society. A farther pur- 
 suit of this inquiry led to the contemplation of 
 the motions of the heavenly bodies, and of the 
 manner in which they have been accounted for, 
 and gave rise to the ensuing commentary. 
 
 A 4 In
 
 8 
 
 In treating of the motions of the heavenly 
 bodies, the Universe is generally considered as 
 composed of Space and Matter. 
 
 Motion is a circumstance to which matter is 
 subject. There is motion which is also a quality 
 in nature, or a property in matter, from which, 
 as will be explained, the most inert matter is not 
 exempt. Motion is defined to be change of 
 place, which indeed is a convertible proposition ; 
 but as rotation is a motion whereby a spherical 
 body can so move, as that the whole shall con- 
 tinue to occupy one and the same place in space, 
 to prevent perplexity, and also as more con- 
 venient for the purpose of this treatise, it is 
 proposed to distinguish motion into two kinds j 
 progressive and rotary. 
 
 Of locomotive powers inherent in matter, the 
 reciprocal attraction or tendency towards each 
 other of every particle of matter (which we de- 
 nominate gravity) is allowed to be universal ; to 
 be the most regular and independent of acci- 
 dents, and at the same time the most compre- 
 hensible in its effects, and the most incompre- 
 hensible in its cause, of all the powers or quali- 
 ties of motion. It is the source of all other 
 regular motion. Without the influence and 
 
 interposition
 
 interposition of other causes, this sociable dispo- 
 sition in matter would make it seem possible for 
 matter to accumulate into one mass, and for the 
 rest of space to be a void. 
 
 But matter, that of which our planet is com- 
 posed, consists of many apparently different sub- 
 stances, some of which are endued with active 
 principles not to be repressed, as of vegetation, 
 which may be regarded as the elementary living 
 principle of matter, and others equally powerful, 
 whence arises a circle of tendencies, as to grow, 
 (which is to increase by attraction and assimi- 
 lation of other matter,) to ameliorate or purify (as 
 by insensible fermentation,) to mature, to decay, 
 to corrupt, and from corruption again to join 
 with other substances pursuing the same round *. 
 Supposing therefore all matter accumulated and 
 at rest, these active elements in its composition 
 would naturally produce a renewal of motion. 
 
 We find matter partly collected in masses, and 
 partly diffused in space; and as there cannot 
 have been a beginning, which our conceptions 
 sufficiently confirm to us, it is most natural to 
 conclude that such has ever been the disposition 
 
 of 
 
 * Similar to this in the moral system, our old almanacks 
 instruct us, that war begets poverty, poverty peace, &c.
 
 10 
 
 of matter, and that it never did exist in a state 
 of perfect rest. 
 
 It is also reasonable to believe that the hea- 
 venly bodies which we see, will not endure for 
 ever ; and that they began to exist in their 
 present state at different periods, and in various 
 ways, each with motions originating in the 
 circumstances of its first formation. The pre- 
 vailing opinion respecting the planetary motions 
 is, that they were produced by a projectile im- 
 pulse given ; the motion imparted by which, 
 combining with the motion resulting from the 
 mutual attraction betwixt them and the Sun, 
 makes them revolve in orbits round the Sun. 
 If a body so set in motion travels in void space 
 the projectile force will continue unimpaired. 
 If it travels through space occupied by matter, 
 howsoever thin and weak the matter or medium 
 may be, the projectile force must continually be 
 weakened and diminished till none remains, and 
 it will then be, as to motion, in the same cir- 
 cumstances as if its original formation had been 
 by a gradual accumulation of matter ; that is to 
 say, that it would be quiescent, except in its 
 tendency to gravitate towards other bodies, and 
 the motions consequent thereon. 
 
 A con-
 
 A consequence generally attending progressive or Rotary 
 
 Motion. 
 
 motion is rotation, or the turning of a body 
 round an axis of its own. This effect is caused 
 by want of conformity in the shape, or want of 
 uniformity in the density, of the proceeding 
 body, whereby some part or parts offer greater 
 obstruction to the progress than other parts. 
 The adherence of additional matter to the side 
 of a boat advancing through water causes im- 
 pediment to the advance on that side, and if 
 there is not the like impediment on the other, 
 the two sides will advance at unequal rates, which 
 will cause the line of progress to be continually 
 deviating. Inequalities or irregularities less 
 affect the direction of the general progress in a 
 spherical body projected, than in a lengthened 
 body ; but will have increased effect in causing 
 rotary motion. If the repulsive effect of an 
 impediment is not strong enough to cause the 
 body to turn quite round, the body jH*BT only 
 oscillate ; but if strong enough to cause a com- 
 plete revolution, the rotation will necessarily 
 proceed. Two conditions accordingly are re- 
 quired for the progress of a body to produce 
 rotation ; one, that it shall be through a medium 
 which gives resistance ; the other, that the body 
 
 shall
 
 shall not be of an exact uniform shape, and 
 throughout of equal density. A void can have 
 no action on bodies whether at rest or passing 
 through it ; and there appears no cause why a 
 body perfectly spherical and of equal density in 
 all its parts, shall, by the action on it of any 
 medium through which it passes, contract a 
 rotary motion. The heavenly bodies within 
 our solar system, concerning whose shape or 
 qualities we have any knowledge, are constituted, 
 the Sun not excepted, with varieties and in- 
 equalities in their parts, sufficiently visible to us 
 to account for their capability, in this respect-, 
 of acquiring rotary motion. The other con- 
 dition necessary will be remarked on in the 
 sequel. 
 
 One of the immediate effects of motion on a 
 mass of matter like our Earth is a dispersion 
 of the lighter and loose particles of the surface 
 of the body moved. If the motion is con- 
 tinued, these particles being incessantly fol- 
 lowed by others, (including effluvia and exha- 
 lations,) all that does not gravitate back, diffuses 
 itself. If a body at rest was first set in motion 
 in .space which was otherwise void, this disper- 
 sion from itself might form sufficient medium for 
 
 the
 
 13 
 
 the commencement of a rotary motion. Rotary 
 motion would increase the quantity of matter 
 thrown off, and of course the density of medium, 
 which in its turn would increase the velocity of 
 the rotation, till both corresponded with the 
 velocity of the progress, and the moving body 
 would thus be enveloped with a light medium 
 furnished from itself. 
 
 In this view I consider Air to be a substance 
 proceeding, and continually supplied, from the 
 body in motion, having been part thereof; that 
 the grosser matter contained in it gravitates 
 back, and the more subtile and fine part ascends ; 
 and that an unremitted continuance of this 
 process, by means of rotary motion, diffuses a 
 thin fluid, and forms the atmospheres by which 
 the Sun and the planets are surrounded. In fur- 
 therance of this view of the origin of air, I con- 
 ceive that if the motions of all the masses of 
 accumulated matter were made to cease, the 
 atmospheres and the fluid matter now floating 
 in space would subside and be collected to the 
 nearest masses ; or if rotation ceased, and pro- 
 gressive motion was continued, all collateral 
 floating substances would be in continual ap- 
 proximation to the masses. 
 
 That
 
 That the rotation of a body round its axis is 
 the cause, or one of the principal causes of a 
 surrounding atmosphere, is strongly corroborated 
 by the circumstance of the Moon, which revolves 
 round its axis only once in a lunation, having so 
 little of atmosphere visible to us that her having 
 any has been disputed. The defect has been 
 attributed by some to a want of moisture in her 
 composition ; but moisture seems a necessary 
 ingredient, if not one indispensably requisite to 
 the adhesion of matter. It seems more probable 
 that it is by the means of an atmosphere that 
 from the moon's enlightened part, the rays of 
 the Sun are so clearly reflected to us ; but that 
 this atmosphere, from the slowness of the moon's 
 rotation, has little activity imparted to it, and 
 does not extend to be visible far from the sur- 
 face of the moon's body. 
 
 It is sometimes said, in arguing against the 
 possibility of a vacuum, that air fills space. I 
 would prefer to say occupies, not fills, as by its 
 compressibility, it easily makes room for, and in 
 a very small degree resists, the admission of other 
 matter, except when already compressed. It 
 has been made a question whether any other 
 substance is in such perfect contact, or of so 
 
 continuous
 
 15 
 
 continuous a consistence as air. The movement 
 of any part causes sensible movement in the parts 
 contiguous, and a smaU concussion affects to a 
 great distance. That air admits no void has 
 probably been a general belief longer than his- 
 tory can trace. Shakespeare makes one of his 
 characters say, " the very air, but for vacancy, 
 " had gone to gaze on Cleopatra, and made a 
 " gap in nature." 
 
 The atmosphere of the earth s reckoned to 
 be an heterogeneous mixture of every kind of 
 substance which composes the body of the 
 earth ; certainly of all kinds of effluvia and ex- 
 halation. The qualities of this mixture are 
 extreme elasticity and tenacity, which seems to 
 render it incapable of being separated except by 
 intervenient matter ; compressibility ; extensi- 
 bility or capability of diffusion believed to be 
 illimitable j susceptibility to be disturbed by dif- 
 ference of temperature ; and a promptitude of 
 disposition to regain its equilibrium. The purer 
 essences of matter embodied in air possibly 
 give it this springiness and activity. Thus en- 
 dued, atmosphere seems not so properly an aggre- 
 gation of particles or atoms, as one united and 
 
 consistent
 
 16 
 
 consistent body ; as in liquids, two or more 
 small drops put together will so unite as to 
 become one drop. 
 
 In assigning a boundary (which has been done) 
 to the atmosphere of the earth, limiting its ex- 
 tension to a certain number of miles, no other 
 meaning can have been intended, than that be- 
 yond that distance the air is too unsubstantial 
 for the support of animal existence, or for effects 
 within our perception. A real, positive, limita- 
 tion at a specified distance no one can have 
 imagined : the most that can be conceived to- 
 wards a void, is a gradual diminution of density 
 of medium as the distance from the body whence 
 it proceeds increases ; and astronomers have so 
 settled the point, in a kind of logarithmic pro- 
 portion, it being admitted that * as the eleva- 
 tion or distance above the surface increases in 
 an arithmetical progression, the density of atmos- 
 phere decreases in a geometrical progression.* 
 
 The superior air has been described under the 
 name of aether, as a distinct matter from atmos- 
 phere, because more fine and rare ; but this 
 constitutes no essential difference. I esteem 
 their opinion the most orthodox who deny aether 
 
 as
 
 17 
 
 as a specific matter, and think the air capable of 
 diffusing itself through space. 
 
 Air is mostly fluid, effluvia and exhalation 
 being among the lightest parts of matter; 
 impregnated with essences, but exceedingly 
 diluted from the greater quantity of exhalation 
 purely aqueous. That air is a gravitating sub- 
 stance, is evidently true of part ; but can it 
 be said of that which ascends ? To the ques- 
 tion, What is weight ? I should answer, down- 
 ward pressure. In a thicker or thinner medium, 
 substances will be more or less compressed j 
 and it is the strength of this compression, I 
 conceive, rather than the weight of air, which 
 the quicksilver shows ; and the continual exuda- 
 tion from the earth I imagine contributes no less 
 to density and strength of compression, than 
 pressure from above. 
 
 A tenacious atmosphere rising from a spherical 
 body in rotation, is naturally drawn into a stream. 
 Dr. Pemberton, in his View of Sir Isaac Newton's 
 Philosophy, speaking of the opinion of Des Cartes, 
 that the planets were kept in their courses 
 by the means of a fluid matter, which conti- 
 nually circulating round carried the planets 
 along with it, remarks, ' that if the whole 
 B ' planetary
 
 18 
 
 * planetary region were filled with a fluid matter, 
 ' the Sun by turning round on its axis might 
 
 * communicate motion first to that part of the 
 'fluid which was contiguous, and by degrees 
 * propagate the like motion to the parts more 
 
 * remote *.' If atmosphere is considered as 
 rising from the surface of the body, it has ab 
 initio the motion of the surface ; and by the 
 common effect of adhesion, and more by its 
 peculiar tenacity, a continuance of this parent 
 motion is preserved to it. The lower air is drawn 
 after the surface of the body in rotation, and the 
 upper air is drawn by the lower to which it is 
 a cause of some detention, making of the whole 
 atmosphere at the equatorial parts, a regular cir- 
 culating and circular stream. I have said regular 
 at the equatorial parts, for there the atmosphere 
 rises or is thrown off without obliquity, and in 
 the direction of the plane of the rotary motion, 
 and it may be imagined with greater force than 
 at the parts towards the poles, both on account 
 of the greater velocity of the rotary motion, and 
 the action of a vertical Sun causing increase of 
 evaporation. 
 
 It has been said of our atmosphere, that by 
 
 moving 
 
 * Pemberton, book 2, chap. i.
 
 19 
 
 moving continually round with the Earth it has 
 acquired the same degree of motion. With 
 respect to the quantity of motion, this is some- 
 thing beyond the fact. The surface of the 
 Earth is, comparatively with the atmosphere, a 
 fixed, and the atmosphere an adhesive but yield- 
 ing, substance, which follows, not accompanies. 
 It is thus argued, 'The motion of the Earth 
 
 * being common to all bodies on its surface, and 
 i to the fluids which cover it, their relative mo- 
 ' tions are the same as if the Earth was at rest.' 
 This position is illustrated by a supposition, 
 which I question whether experiment would con- 
 firm ; which is, that ' in a vessel having uniform 
 ' motion, a projectile thrown directly upwards 
 ' falls on the same spot from which it was pro- 
 
 * jected, and seems to describe a vertical line to 
 
 * those in the vessel ; but seen from the shore, 
 ' it really describes a parabolic curve.' This is 
 carrying similitude to identity. I will propose 
 a case for consideration. Let a vessel in calm 
 weather and quiet water, be towed at an equable 
 rate, suppose of five miles per hour ; and let a 
 ball be fired from a gun pointed directly to the 
 zenith. The direction given to the ball is from 
 the composition of two impulses, the one vertical, 
 
 B 2 the
 
 20 
 
 the other horizontal, at the rate of the vessel's 
 advance. If during the ascent and descent of 
 the ball the horizontal impulse suffers no abate- 
 ment by the resistance of the air, the ball might 
 return to the gun ; but I apprehend that the ho- 
 rizontal advance communicated to the ball by 
 the motion of the vessel, would almost immedi- 
 ately after being discharged from the gun begin 
 to experience diminution, and that the ball would 
 descend more perpendicularly than it ascended. 
 As I have not heard of any experiment proving 
 the contrary, I think it possible that a plumb 
 line at the equator, would be found on nice exa- 
 mination, to have a measurable quantity of ad- 
 vance eastward at the upper and fixed part. 
 
 The connection of our atmosphere with the 
 surface of the Earth, is similar to that of the 
 waters of the sea with the coasts and shores of the 
 land, or of the waters of a river with its shores. 
 Points of land which project will interrupt and 
 divert the general course of the stream, and 
 produce eddies, whirlpools, and currents in other 
 directions; but this interruption is limited to 
 the neighbourhood of the shores, whilst the dis- 
 tant main stream pursues its natural direction 
 undisturbed. In the like manner the alternations 
 
 of
 
 21 
 
 of continent and ocean on the surface of the 
 Earth occasion interruptions to the streams of 
 air, which on a clear smooth surface would natu- 
 rally prevail ; and this may take place without 
 disturbing the order of the superior atmosphere. 
 
 At and near the Equator is found the General Of th 
 
 Trade 
 
 Easterly Wind, which we call the General Trade Wind - 
 Wind. It blows regular and constant, with small 
 exception, in the South Sea, where there is a 
 large expanse of surface clear of lands, small 
 islands excepted ; also in the middle of the equa- 
 torial part of the Atlantic the Trade Wind is no 
 less regular and constant. 
 
 The rotary motion of the Earth carrying the 
 surface continually from West to East, and its 
 greatest velocity being at the equator, the equa- 
 torial stream of atmosphere (where not inter- 
 rupted) is drawn after the Earth's surface in the 
 same direction ; that is to say, from West to 
 East ; but its not keeping pace with the surface, 
 makes the relative current of air experienced in 
 our tropical regions, to be from the East towards 
 the West. The rapid progress of the Earth's 
 surface to the East at the Equator, seems as 
 natural and obvious a cause for producing the 
 General Easterly Wind, as a flag or streamer 
 B 3 carried
 
 22 
 
 carried swiftly through the air in a calm would 
 be for producing an effect on the streamer similar 
 and equivalent to a current of the air ; yet this 
 cause seems to have been overlooked or rejected, 
 and another assigned, the effect of which I appre- 
 hend to have been erroneously applied. But be- 
 fore discussing this point, I will just remark, that 
 out of the tropical latitudes, the circles of rotation 
 becoming smaller, ,and the atmosphere rising in 
 oblique direction to the plane of the diurnal 
 motion, more and more as the poles are ap- 
 proached, the velocity and force of the rotary 
 motion is there diminished, and obliquity given 
 to the direction of the superincumbent atmos- 
 phere. Accordingly, the motions of the atmos- 
 phere have not the like regular and orderly 
 direction near or towards the poles, as in the 
 tropical latitudes. 
 
 It is argued that the Easterly Trade Wind is 
 caused by the temperature, and the alterations 
 to which it is subject, travelling from the East 
 towards the West, occasioned by the Sun passing 
 over the meridians from East to West. 
 
 The effect of temperature upon the direction 
 of the winds, is most regularly instanced in the 
 Trade Wind being from the Northward of East 
 
 in
 
 23 
 
 in North latitude, and from the Southward of 
 East in South latitude ; in the periodical winds 
 called Monsoons j and in the alternate sea and 
 land breezes near coasts of any considerable ex- 
 tent in the warm latitudes. 
 
 The argument which has been held respecting 
 the cause of the General Easterly Trade Wind, 
 is, " that where the air is heated and expanded, 
 " as by a vertical Sun, the more cool and dense 
 " air from parts of the atmosphere not so heated, 
 " continually flows in to supply the place of what 
 " the heat throws upward j and as the Sun in 
 " moving from East to West heats the air im- 
 " mediately under him, and thereby expands it, 
 " the air to the Eastward is constantly rushing 
 " towards the West to restore the equilibrium or 
 " natural state of the atmosphere, which occasions 
 " a perpetual East Wind in these (tropical) 
 " limits.*' Here a difficulty is created which 
 controverts the main argument. In the warm 
 latitudes, indeed in every latitude, the general 
 temperature between midnight and noon is cooler 
 than the temperature from noon to midnight. 
 Towards sunrise is a cooler time than any other 
 part of the 24 hours ; and taking equal distances 
 from the meridian over which the Sun is passing, 
 B 4 the
 
 m 
 
 the parts over which the Sun has passed are more 
 heated than those towards which the Sun is 
 travelling, and from which it has been longest 
 absent. If to restore the equilibrium where the 
 air is rarified by heat, the supply is to come from 
 where the air is most cool and dense, the meri- 
 dian where it is early morning should certainly 
 contribute. The greatest rush should be from 
 the unheated West, to meet the Sun, rather than 
 from the heated East. Noon should travel 
 against the Western breeze, and be followed by 
 the Eastern. This, throughout the tropical lati- 
 tudes, would probably be the case, and the cur- 
 rent of the air in the course of every 24 hours, 
 would be, on every meridian, at one time towards 
 the East, and at another towards the West, in the 
 manner of sea and land breezes, if the winds were 
 governed only by the state of the temperature. 
 
 I find I am not singular in dissenting from the 
 mode in which the General Easterly Trade Wind 
 has been accounted for. Professor Playfair has 
 remarked, " The cause usually assigned for the 
 " Trade Wind is the constant motion toward 
 " the West, of the spot to which the Sun is ver- 
 " tical, and where of course the rarefaction is 
 " greatest. This it is supposed draws along 
 
 " with
 
 25 
 
 *' with it the air from the East. This however, 
 " is by no means a satisfactory explanation*." 
 
 Dr. Henry Robertson, in his Natural History 
 of the Atmosphere, remarks that the Trade 
 Winds are never so strong during the night as 
 in the day. Something might be learnt on this 
 head by examining a great number of sea 
 journals of navigations through the trade lati- 
 tudes, whether the greater coolness of the hemis- 
 phere westward of the sun causes temporary 
 abatement in the strength of the Trade Wind, 
 which effect might be expected to be most 
 evident towards sun-rise. The motion of the 
 Earth in orbit must undoubtedly affect the at- 
 mosphere ; in what degree or manner I believe 
 has not hitherto been conjectured. 
 
 The rotation of the Sun was discovered by 
 Galileo many years before Sir Isaac Newton 
 demonstrated the universality of gravity. The 
 consequences resulting from these two causes, 
 the combined reflection of mankind is slowly 
 and gradually developing. Many important 
 truths have been strongly conceived, and some 
 have been adopted by Faith, long before their 
 demonstration .became evident, without which 
 
 the 
 
 * Outlines of Natural Philosophy, vol. I. p. 307.
 
 the discovery is not compleated. Des Cartes, 
 enlarging upon an opinion first held by Kepler, 
 imagined a system of vortices, as forming the 
 plan of our solar system. This was susceptible 
 of being supported by the discoveries of Galileo 
 and Sir Isaac Newton, but in the infant state of 
 the question, satisfactory demonstration did not 
 unfold itself. Sir Isaac Newton objected to the 
 system of motion communicated by the rotation 
 of the Sun to a fluid element enveloping and 
 carrying with it the planets ; first, * that the 
 ' velocities with which the parts of such a fluid 
 ' would move in different distances from the 
 ' centre of motion, will not agree with the 
 ' motion observed in different planets ; for 
 ' instance that the time of one entire circulation 
 ' of the fluid wherein Jupiter should swim, 
 
 * would bear a greater proportion to the time of 
 one entire circulation of the fluid in which the 
 
 * Earth is, than the period of Jupiter bears to 
 the period of the Earth/ Secondly, that the 
 ' planet cannot circulate in such a fluid so as to 
 ' keep in the same course with it, unless the 
 ' planet and the contiguous fluid are of the same 
 
 * density, and the planet be carried along with 
 
 * the same motion as the fluid.' And thirdly, 
 
 that
 
 27 
 
 that some vivifying force will be continually 
 
 * necessary at the centre of motion or attrac- 
 
 * tion ;' for that otherwise, ' the Sun, by com- 
 
 * municating motion to the ambient fluid, will 
 lose from itself as much motion as it imparts 
 
 * to the fluid, unless some acting principle reside 
 ' in the Sun to renew the motion continually *.' 
 
 A fourth objection stated by Dr. Pemberton 
 is, that ' planets and comets whilst in the same 
 
 * region or parts of the heavens, are seen to 
 
 * move with different velocities, and in different 
 
 * directions ; and if they are both carried round 
 ' by vortices, there must be one direction and 
 
 * velocity to allow for the motion of planets, 
 and another to account for the motion of 
 ' comets.' He adds, * as this cannot be ex- 
 
 * plained, either the hypothesis of vortices must 
 be given up, or its advocates must say that it 
 ' is not a single vortex, but several vortices 
 
 * distinct from each other, that impel these 
 bodies in their course ; and must suppose 
 ' several vortices to be contained in the same 
 ' space mutually penetrating each other.' 
 
 For these reasons, the system of vortices was 
 thought not to correspond so well with the 
 
 wus>.,.(u motions 
 
 * Pemberton's View of Sir Isaac Newton's Philosophy, p. 167-8.
 
 28 
 
 motions of the heavenly bodies as projectile 
 force combined with gravity or attraction. 
 
 Respecting the first objection j if the velocity 
 of Jupiter in his orbit is not in the same pro- 
 portion to distance from the Sun as that of the 
 other planets, it is to be regarded as anomalous 
 to the general order of the solar system. There 
 are many cases in astronomy that appear to us 
 not to correspond with principles which are evi- 
 dently just, and of which we are so well con- 
 vinced that although the fact disappoints us, we 
 do not nevertheless distrust the principle, but 
 reasonably content ourselves with acknowledg- 
 ing an undiscovered cause. The attraction of 
 the Sun on the planets is allowed to be as much 
 on the secondary (the satellites) as on the pri- 
 mary at the same distances ; and Pemberton 
 remarks that 'the secondary being sometimes 
 nearer to the Sun than the primary, and some- 
 times more remote, they are not at those times 
 * acted upon in the same degree with their pri- 
 mary ; but when nearer the Sun are attracted 
 ' more, and when farther distant are attracted 
 ' less, whence arise various inequalities in the 
 motion of the secondary planets.' This rea- 
 soning no one will dispute ; yet the result of 
 observation does not correspond with it so well 
 
 as
 
 29 
 
 as might have been expected. The body nearest 
 to the Sun being most attracted, is a cause why 
 the Earth and Moon should be more apart at the 
 times of the full and of the new moon, than at 
 any other time of the moon's revolution ; but 
 such is not the case. Also, from the full moon 
 to the time of new- moon, the effect of the 
 attraction of the Sun must be supposed to acce- 
 lerate the motion of the moon in orbit; and 
 from the new moon to the full to retard ; never- 
 theless we find the moon is frequently a longer 
 time in performing the half revolution from the 
 full to the new than in performing the other half. 
 It is said, * the moon goes round the earth in an 
 
 * elliptic orbit, and therefore in every lunar 
 ' month she approaches nearer to the Earth than 
 ' her mean distance, and recedes farther from it. 
 
 * When she is nearest, she attracts strongest and 
 
 * so raises the tides most. The contrary hap- 
 
 * pens when she is farthest*.' The common 
 and well known fact is that the greatest or 
 spring tides constantly fall out at, or imme- 
 diately after, the full moon and new moon j 
 but the nearest approach of the moon to the 
 Earth, and its farthest recession, does not hap- 
 pen at constant fixed times of the lunar revolu- 
 tion, 
 
 * Encyclopedia Britannica.
 
 30 
 
 tion, but at times continually varying. The 
 satellites however, constituting the inferior link 
 of a chain of dependencies to the origin of which 
 we cannot penetrate, are more subject to intri- 
 cacies of movement than the other heavenly 
 bodies. 
 
 The difference or discordance of Jupiter's re- 
 volution in orbit from the general order observ- 
 able in the motions of the planets, or from the 
 law laid down by Kepler, that the squares of the 
 times of revolution are in the proportion of the 
 cubes of the mean distances from the Sun, will be 
 found by the following table of the periods of 
 revolution and distances, to be less than the 
 remark above cited from Pemberton may give 
 reason to expect : 
 
 p 
 
 sriods of Revolution 
 Years, Days, Hours. 
 
 in Distance from the 
 Sun in Miles. 
 
 Mercury - > a>) 
 
 O. 
 
 87. 
 
 23. 
 
 ;ii i 36,000,000. 
 
 Venus - ,n g t 
 
 0. 
 
 224. 
 
 
 - - 68,000,000. 
 
 The Earth ^ - 
 
 1. 
 
 0. 
 
 0. 
 
 eTfo 4 "; 95> 000 > 000 - 
 
 Mars - - . - 
 
 1. 
 
 321. 
 
 23. 
 
 - - 144,000,000. 
 
 Vesta- - - - 
 
 3- 
 
 240. 
 
 0. 
 
 - - 225,000,000. 
 
 Juno - - - - 
 
 4- 
 
 91. 
 
 9- 
 
 - - 253,000,000. 
 
 Ceres - - .-M 
 
 4- 
 
 213. 
 
 0. 
 
 - - 253,000,000. 
 
 Pallas - - - 
 
 4. 
 
 213. 
 
 0. 
 
 - - 263,000,000. 
 
 Jupiter - ... . 
 
 11. 
 
 315. 
 
 0. 
 
 - - 495,000,000. 
 
 Saturn - - - 
 
 29. 
 
 183. 
 
 0. 
 
 - - 900,000,000. 
 
 Georgium Sidus, 
 
 83. 
 
 183. 
 
 o. 
 
 - 1,800,000,000. 
 
 ^*o:i '' 
 
 
 
 
 On
 
 31 
 
 On the subject of the second objection there 
 is occasion to speak at some length. It is not to 
 be imagined that the density of the Earth or other 
 planet does not much exceed the density of the 
 fluid by which they are severally surrounded, and 
 however the force of the Sun's attraction is 
 weakened by distance, it continues to act in pro- 
 portion to density. The hypothesis of progres- 
 sive motion being communicated to the planets 
 by projectile impulse, which combining with the 
 attraction of the Sun causes the planets to revolve 
 in a circular orbit round the Sun, supposes a 
 medium so rare as not to offer a sensible resist- 
 ance, nor to occasion a sensible diminution of 
 the force of the original impulse given. And 
 doubtless, a medium may be imagined so rare, 
 that a projectile force acting in it, shall endure 
 till the body projected falls into decay. 
 
 By projectile force is understood, an impulse 
 proceeding from no known general cause, not 
 continued nor renewed after being communi- 
 cated. One author remarks, as matter is of itself 
 inactive, and as the planets are solid bodies, it is 
 reasonable to conclude that a propelling force 
 was given to them.* It is explained by M. le 
 
 Comte 
 
 * Lectures on Astronomy. By W. Phillips, p. 32. London. 
 1817.
 
 32 
 
 Comte de la Place, in the following manner : 
 
 * Un point en repos ne pent se donner aucun 
 ' mouvement, puisqu'il ne renferme pas en soi, de 
 
 * raison pour se mouvoir dans un sens plutot que 
 ' dans un autre. Lorsqu'il est sollicite par une 
 'force quelconque, etensuite abandonne a lui-meme, 
 ' il se meut constamment d'une maniere uniforme 
 ' dans la direction de cette force, s'il n'eprouve 
 ' aucune resistance ; c'est a dire, qu y a chaque in- 
 ( stant sa force et la direction de son mowvement 
 sont les memes" * 
 
 From the above, as well as from what is said 
 in Maclaurin and Pemberton on the laws of 
 projectiles, we are led to conceive a projectile 
 impulse in the nature of event ; yet if we see 
 a r number of events of exactly the same kind, 
 occur in one peculiar manner, without a single 
 instance appearing of opposite tendency, we are 
 led to seek a cause in some general principle. 
 In this view, the supposition of such a perfect 
 coincidence as is found in the motions of all 
 the bodies of the solar system being commu- 
 nicated by means of a projectile force which 
 immediately abandons the body projected, is far 
 from satisfactory, as the mechanical effect of a 
 
 single 
 * Exposition du Systems du Monde. Liv. Ill, ch. 2.
 
 S3 
 
 single impulse cannot be demonstrated to be 
 sufficient for the purpose. 
 
 In investigating the laws of Nature and of 
 Creation, we properly, indeed by necessity, refer 
 to supernatural agency those matters which 
 demonstrably could not happen by means we 
 esteem natural, or which are evidently beyond 
 the -reach of the mental attainments of mankind 
 to conceive. The existence of matter, its pro- 
 perties, that especially of gravity, which may 
 be called the primum mobile, are of this superior 
 class. What can be traced, or can be conceived 
 to flow, from those original sources, it may be 
 allowed to us to assume as the course of Nature. 
 
 Gravity is considered as direct or rectilinear 
 attraction, and from its acknowledged univer- 
 sality is inferred, that all the heavenly bodies 
 are in progressive motion. M. de la Place ob- 
 serves, " Toutes les etoiles en vertu de la pesan- 
 " teur universelle doivent graviter les unes vers 
 " les autres*." The action and influence of 
 two bodies on each other, supposing them to be 
 so distant from all other bodies as not to be 
 sensibly affected by them, we may conjecture 
 would be, that by the direct attraction of gravity 
 C they 
 
 * Systeme du Monde, Liv. IV. ch. 16.
 
 34 
 
 they would at first advance in the shortest line 
 towards contact; but in their progress, they 
 might each acquire rotation and become pro- 
 vided with atmosphere. The atmospheres, 
 emanating from themselves and by the rotation, 
 would be each with respect to the orb whence 
 it proceeded, an ascending and circulating 
 stream ; by the former of which qualities, they 
 would be repellents, mutually opposing nearer 
 approach ; and by the latter, each orb would 
 be involved in the vortex of the other, and their 
 future progress would be in orbits. If they 
 were of equal magnitude and their rotative 
 motion in accordance, they would pursue the 
 same path always opposed to each other. If 
 they were of unequal magnitude, the smaller 
 would move in the larger orbit. 
 
 The mutual and reciprocal attraction of many 
 bodies whose positions and distances from each 
 other are continually varying, must occasion pro- 
 gressions through space continually varying in 
 direction, and infinitely complicated. Such, in 
 fact, must be conceived universally the condition 
 of the motion of all the heavenly bodies. If 
 any one could be between attractions so equally 
 balancing as to retain it in some one place, such 
 
 a situation
 
 35 
 
 a situation could be but of short continuance, 
 unless the rest also were stationary. 
 
 Kepler was the first who supposed the planets 
 were caused to revolve round the Sun by means 
 of the Sun's atmosphere. I have copied the fol- 
 lowing account of his discoveries and opinions 
 respecting the motion of the Sun and the planets 
 from M. de la Place's Systeme du Monde: " Apres 
 " avoir determine la courbe que les plane tes decri- 
 " ventautour du Soleil, et decouvert les lots de leur 
 " mouvemens ; Kepler etait trop pres du principe 
 " dont ces lots derivent, pour ne pas le pressentir. 
 " La recherche de ce principe exerga souvent son 
 " imagination active ; Kepler supposait au Soleil 
 " un mouvement de rotation sur un axe perpendi- 
 " culaire d Vecliptique : des especes immaterielks 
 " emanees de cet astre dans le plan de son equateur, 
 " donees d'une activite decroissante en raison de 
 " distances, et conservant leur mouvement primitif 
 " de revolution, faisaient participer chaque planete, 
 " a ce mouvement circulaire. En meme temps, la 
 " planete par une sor'te d 9 instinct ou de magne- 
 " tisme, s'approchait et s'eloignait alternativement 
 " du Soleil, s'elevait au dessus de V equateur solaire, 
 " et s'abaissait au dessous, de maniere a decrire 
 " une ellipse toujours situee dans un meme plan 
 c 2 "passant
 
 36 
 
 " passant par le centre du Soleil. " * This was ad- 
 mirably and comprehensively conceived, and is 
 the basis of the system of vortices afterwards 
 advanced by Descartes. If in addition, Kepler 
 had inferred the progressive motion of the Sun 
 from its rotary motion, it is probable that his 
 system would have prevailed from that time to 
 the present, in preference to every other. Kep- 
 ler's hypothesis was however esteemed visionary 
 and was neglected, until revived by Descartes, 
 with the addition of attributing to each of the 
 planets a distinct and separate vortex. 
 
 Repetitions cannot well be avoided in argu- 
 ment, which the author must hope will be 
 
 admitted 
 
 * " After having determined the curve which the planets 
 
 " describe round the Sun, and discovered the laws of their 
 
 " motions, Kepler approached too near to the principle whence 
 
 " these laws were derived, not to acquire some insight into it. 
 
 " Attempts to discover this principle often exercised his active 
 
 " imagination. Kepler supposed the Sun to have a rotary 
 
 " motion round an axis perpendicular to the ecliptic; that 
 
 *' ethereal particles (des especes immatericlles) emanated from 
 
 " this star in the plane of its equator, endued with an activity 
 
 " decreasing in a proportion to [increase of] the distances, and 
 
 " preserving their primitive movement of revolution, made 
 
 " each of the planets participate in this circular motion. At 
 
 " the same time, the planet by a kind of instinct or magnetism, 
 
 " alternately approached to, and receded from, the Sun, ascended 
 
 " above the solar equator and declined below, so as to describe 
 
 " an ellipse always situate in the same plane, passing through 
 
 " the centre of the Sun."
 
 37 
 
 admitted as their excuse. Air, capable of com- 
 pression but prone to expansion, the lighter will 
 not gravitate or descend in space occupied by 
 medium more dense ; and the whole is continually 
 impelled upward by fresh emanations. 
 
 It is obvious that matter proceeding continually 
 from a body, of which only a part gravitates 
 back, must cause its continual impoverishment 
 and diminution, unless it receives supplies. 
 Concerning this I shall speak hereafter. 
 
 In supposing the equatorial stream of the 
 Sun's atmosphere to carry the planets and their 
 atmospheres round the Sun, and to diffuse itself 
 to the remote parts of the system, that is to say, 
 till it arrives at space where it mixes with ele- 
 ment under the influence of other stars, the 
 ascending of the stream must act to the sustain- 
 ment of the planets within that range against 
 the power of gravity ; towards which end the 
 atmospheres of the planets assist by their con- 
 nection, tenacity, and being largely extended, 
 which last mentioned particular acts as an expan- 
 sion (equal to a diminution) of the planet's 
 density. The effect of this in rendering theplanets 
 buoyant in the solar fluid must appear great, if 
 it is believed that the moon is carried round in 
 its orbit by the atmosphere of the Earth. 
 
 c 3 That
 
 58 
 
 That the atmosphere of the Sun and the 
 atmospheres of the planets may mutually pene- 
 trate each other, appears possible from what is 
 experienced in the seas of our Earth where cur- 
 rents which have different directions meet. In 
 many instances they coalesce, or neutralize each 
 other ; but there are instances of currents mutu- 
 ally penetrating through each other, and after 
 crossing, continuing each in its original direction 
 undisturbed. The capability of fluids to pene- 
 trate and expand in each other is well evinced 
 by the visible effect which a single drop of red 
 wine has on a comparatively large quantity of 
 water. Effluvia will pervade and be diffused 
 against a moderate stream of air, which however 
 gradually abates its progress. Chymists I doubt not 
 can furnish much better illustrations of the fact. 
 
 Another material effect which seems attri- 
 butable to the ascending of the solar atmosphere, 
 is the rotation of the planets. It has been remark- 
 ed that progressive motion through a medium is 
 requisite for the production of rotary motion. 
 An inert and quiescent body floating in a me- 
 dium, cannot be supposed thereby to acquire 
 rotary motion. But the planets are not qui- 
 escent in the solar stream, inasmuch as their 
 greater gravity than the fluid makes them resist 
 
 being
 
 being carried to a greater distance from the Sun, 
 whilst the ascending of the fluid resists their 
 nearer approach to the Sun. The constant 
 ascension of the solar stream is to the planets 
 equivalent to, in fact is, so much progress through 
 a resisting medium. In seaman's phrase, the 
 planets by their gravity maintaining their ground 
 against the ascending of the solar fluid, is similar 
 to a vessel stemming the tide. 
 
 To attribute the rotation of the planets, and 
 their being sustained against the power of gravity, 
 to the ascending of the solar fluid, seems allow- 
 ing great effect to extremely light means. It 
 has been remarked that the Sun's attraction at 
 those distances is extremely light ; it is also to 
 be remarked that the rotations of the planets are 
 extremely gentle if compared with the rapidity 
 of their progress in orbit. The Earth, for 
 example, travels in orbit more than sixty times 
 the measure of her own circumference to each 
 time that she revolves round her own axis. 
 Whence must be concluded that the rotary 
 motion is produced by the most mild means, 
 consonant to the old and established maxim, 
 that Nature performs all her functions in the 
 least difficult manner. 
 
 c 4 It
 
 40 
 
 It may be conceived that the planets are a 
 greater weight upon the rotation of the Sun, than 
 equal magnitudes of the solar fluid at the same 
 distances, and that therefore they maybe a longer 
 time in performing a revolution round the Sun 
 than the fluid ; which, by the gliding of the fluid 
 past the planet, might contribute to rotation of 
 the planet. But against this supposition it occurs, 
 that if the solar fluid circulated faster than the 
 planet it would have a tendency towards causing 
 rotation from East to West, the fluid being most 
 dense on the inferior side of the planet. Another 
 small circumstance to notice is, that the rays of 
 the Sun are believed to be material ; which, if 
 true, and they are not merely corruscations fol- 
 lowing in rapid succession, but have more than 
 an instant of existence, they may by the rotation 
 of the Sun, communicate a degree of lateral 
 pressure on bodies in their way and by which they 
 are broken ; and thereby give a small progressive, 
 impulse in the direction of a tangent to the arc 
 which the striking part of the ray was describing. 
 
 But these are minute considerations. From 
 a comparison of observations made in times 
 past, with the observations of the present time, 
 it is believed that the Sun and the whole of 
 
 our
 
 41 
 
 our solar system are gravitating towards the 
 constellation of Hercules. * To ascertain this 
 point with certainty, will probably require a long 
 series of observations ; but that the Sun has pro- 
 gressive motion may be regarded as proved by 
 its having rotation round an axis of its own. 
 Arguing on the belief that matter is always the 
 same in quantity, that what is at any time dis- 
 persed from masses is diffiised in other parts of 
 space, and that the Sun is in progress always 
 through space new to it, much of the medium 
 nearest to the line of progress must continually 
 be drawn into the Sun's atmosphere, and that 
 which gravitates most, becomes part of the Sun. 
 It has been conjectured that small masses, and 
 sometimes even comets or wandering stars, fall 
 into the Sun and help to its recruit. According 
 as the expenditure from the Sun, or other orb, 
 exceeds, or is less than, the supply, the orb must 
 diminish or increase ; and it is to be imagined 
 that every orb in the firmament is liable to in- 
 crease or decrease. 
 
 The belief of the Sun having progressive motion 
 does not appear to have been entertained till long 
 after its rotary motion was discovered. It has of 
 
 late 
 * La Place. Lav. IV. ch. 16.
 
 42 
 
 late so much gained ground, that astronomers 
 have conjectured an orbit of the Sun round a 
 more powerful centre of motion; and some have 
 not scrupled to say, * round the centre of gravity 
 of the whole universe,' which seems travelling 
 far ; but is the contrary, for every point of in- 
 finite space is as exactly its centre as any other 
 point ; which Pascal has happily expressed, in 
 describing the illimitableness of space, * whose 
 centre,' he says, * is every where, and whose cir- 
 cumference is no where.' 
 
 The Sun is one of innumerable stars visible to 
 us, among which, it is reasonable to suppose, are 
 many similar, in being each the centre of a system 
 of minor stars or planets j but we have no ground 
 for conjecturing of any particular star, that it is 
 of greater or less magnitude than the Sun. 
 
 In either case the course of the Sun's progres- 
 sion will be determined by the predominance of 
 the attractions. Revolution in orbit of one body 
 round another proceeds from a superiority which 
 commands it. If the Sun was under the influence 
 of such a controlling power, it seems probable 
 that some symptom or indication of it would be 
 apparent to us, but no such indication has been 
 discovered ; nor is there reason to suppose revo- 
 lution
 
 43 
 
 lution round a central attraction to be universally 
 of necessity, whilst progressive motion appears to 
 be universally inevitable. 
 
 It may be argued, that if the Sun does not 
 move in orbit round a central attraction, but is 
 moved by attractions continually varying, its 
 motion cannot be uniform ; and every variation 
 in its motion must cause variation in the whole 
 solar system. Without supposing eternal exist- 
 ence to the Sun, this is to be supposed the case. 
 In such (to our apprehensions) immense distances 
 it may require centuries to produce small per- 
 ceptible variations. If orbicular motion round 
 a central attraction can be supposed universally 
 of necessity, it may also be supposed that the 
 central attraction of the Sun performs revolution 
 round a still greater, and that round another, ad. 
 wfinitum.. 
 
 The planets and comets, it is remarked, whilst 
 in the same parts of the heavens, move with dif- 
 ferent velocities and in different directions. 
 Those called the regular planets have permanent 
 situation in the equatorial regions of the system, 
 where their motions are governed by the regular 
 stream of the solar atmosphere. Comets are 
 mostly bodies of the higher latitudes. As in the 
 
 extra
 
 44 
 
 extra tropical latitudes of the Earth, the Earth's 
 atmosphere appears not to be under any certain 
 regular direction, so in the higher latitudes of 
 the solar space the rotation of the Sun does not 
 act with the same strength as in the equatorial 
 parts, and may be supposed not to give there the 
 constancy of direction necessary to preserve one 
 regular course of the medium. A comet or 
 planet of the polar regions of the system, if 
 carried by a temporary stream towards the Sun, 
 would be aided in descent by an increasing force 
 of gravity, and by degrees contract that velocity 
 of progress with which we see those bodies ap- 
 proach the Sun, and cross the equatorial stream. 
 Some comets have exhibited much the appear- 
 ance of being planets in a state of dissolution and 
 dispersion. It has been conjectured, that the 
 moon has been an extra tropical planet, which, 
 coming within the equatorial vortex of the atmos- 
 phere of the Earth, has been detained in its 
 service. This does not oppose M. de la Place's 
 opinion, that none of the regular planets have 
 ever been comets. The moon I apprehend to 
 be a much less perfect orb than the earth ; and 
 the great eccentricity observable in the orbits of 
 comets, appears to me a symptom of decadence. 
 
 Where
 
 45 
 
 Where the plane of rotary motion coincides, or 
 nearly coincides with the progressive direction, 
 it seems favourable for collecting back to the 
 revolving body the weightier parts of the atmos- 
 phere which rises from it. A body not well 
 balanced, may, by the preponderance of density 
 at some one part, have the axis of its rotary 
 motion to coincide with the lirie of its progress. 
 Such possibly is the case with some of the comets. 
 
 It will assist investigation to compare the two 
 systems, that of revolution in orbit being pro- 
 duced by rotation of a superior, and that of its 
 resulting from the application of a projectile 
 force restrained by gravity. Some of the most 
 important phenomena may be found consistent 
 with either ; and principally that of the planet 
 being retained by the power of gravity, whether 
 from being carried off by a projectile velocity, 
 or by the ascending of the solar atmosphere. It 
 is however most to the purpose to examine cir- 
 cumstances, for such there are, which will not 
 accord indifferently with either system. 
 
 The laws of elliptic motion are better un- 
 derstood than the causes why the planets and 
 satellites move in elliptical orbits ; and astro- 
 nomers are obliged to acknowledge that ' elliptic 
 
 motion
 
 46 
 
 motion will not exactly coincide with modern 
 observation.' 
 
 I will suppose for a moment (what seems to 
 have been once a general opinion, but which it 
 would now be deemed absurd to believe), that 
 the Sun was stationary and at rest, except in its 
 rotative motion, and that its rotation proceeded 
 from some property inherent in itself, or from 
 the application of a force appointed and limited 
 to that purpose. Let it at the same time be sup- 
 posed, that there was but one planet revolving 
 round the Sun. Whether the planet was moved 
 by a projectile impulse uniformly and exactly 
 balanced by the force of gravity, or was carried 
 round in the vortex of the Sun's atmosphere at 
 a distance determined by its density, it is to be 
 supposed that the planet would describe an orbit 
 exactly circular, the causes of motion being uni- 
 form. As the case really is, there are many 
 well understood circumstances in the varying 
 positions of the planets from each other, and in 
 the motions of their satellites, to cause what 
 astronomers call perturbations in the courses 
 of the planets, which perturbations are small 
 irregularities of motion. But I apprehend the 
 great and regular cause of the elliptic orbits of 
 
 the
 
 4? 
 
 the planets to be the progressive motion of the 
 Sun. The Sun, drawn itself by some powerful 
 attraction, draws after it a train of attendant 
 orbs, linked in a chain of dependencies, which 
 revolve in orbits in subordinate order. In all 
 these revolutions, the secondary (that is to say, 
 the planet with respect to the Sun, or the satellite 
 with respect to the planet), is at one time in 
 advance, and at another time in the rear, of its 
 primary : in the most rapid part of its progress 
 through space it is most a weight, possibly some 
 drawback, upon the progress of its primary, 
 and in its retrogradation it wants not assistance. 
 When it is in advance of its superior, its superior 
 presses towards it ; and the contrary when in 
 the opposite part of its orbit. The secondary is 
 thus made to partake of the progressive move- 
 ment of its primary, and carried round in orbit, 
 by an alternate increase and relaxation of the 
 force applied, whence may arise the periodical 
 accelerations and retardations, as likewise the 
 alternate approximations and recessions, observed 
 to be regular in the motions of the planets, and 
 less regularly periodical in the motions of the 
 satellites. 
 
 The radius vector is an imaginary strait line 
 
 from
 
 from the centre of the Sun to the centre of a 
 planet. As the planet revolves in its orbit, the 
 radius vector passes over areas of the space 
 within the orbit, forming sectors comprehended 
 between the radii and the arc of orbit described 
 by the planet. The orbits of the planets hav- 
 ing eccentricity, the radius vector is not always 
 of the same length ; but it has been discovered 
 that the areas described by it are equal in equal 
 times, or proportional to the times in which 
 they are described. Accordingly in an uniform 
 equal velocity of the planet, the receding of 
 the planet from the Sun must cause an apparent 
 retardation of progress in orbit, that is to say, 
 a diminution of angular advance ; but it is 
 found that the receding of the Earth from the 
 Sun is attended with a real diminution of velo- 
 city, and of advance in orbit ; and that on re- 
 approaching the Sun, the Earth recovers its 
 velocity to what it had been before. 
 
 The half revolution of the Earth in orbit from 
 the vernal to the autumnal equinox, occupies 
 seven days and sixteen hours more time than 
 the half revolution from the autumnal to the 
 vernal ; which is a greater retardation and ac- 
 celeration than the difference of distance from 
 
 the
 
 the Sun will account for. As this is a point of 
 importance, it is worth entering into more 
 particularly. 
 
 The greatest distance of the Earth from the 
 Sun in the course of her revolution is about 
 96,500,000 miles, ancf the daily apparent or 
 angular progress of the Earth in orbit when at 
 that distance, is 57' 11", which reckoned as an 
 arc of a circle of 96,500,000 miles radius, will 
 give the absolute diurnal progress then, 
 1,606,000 miles. The least distance of the Earth 
 from the Sun in the course of a revolution, is 93J 
 millions of miles, and the angular diurnal progress 
 of the Earth is then 1 I' 12", which reckoned 
 as an arc of a circle of 93 millions of miles 
 radius, gives absolute daily progress 1,665,000 
 miles. Here is a difference of nearly 60,000 
 miles that the daily progress of the Earth in 
 orbit is greater at one part of its revolution 
 round the Sun than at another. A real increase 
 and decrease of velocity * does not, on mecha- 
 nical principles, correspond with motion com- 
 municated by a projectile impulse. The in- 
 crease or diminution of the force of gravity, by 
 
 approach 
 
 * Ily a un ralentissement reel dans le mouvement de cet astrc 
 (le soldi) lorsqu'il s'eloigne de la terre. De la Place. 1.15. 
 
 D
 
 50 
 
 approach towards, or by recess from, the centre 
 of gravity, affects only in the vertical direction, 
 and has no connection with the increase or de- 
 crease of the projectile velocity. A propelling 
 impulse which is not continued, cannot cause 
 alternation of velocity j and once abated, it can- 
 not of itself recover. On the other hand, re- 
 tardation of progress in orbit as a consequence 
 of receding from the Sun, and acceleration as a 
 consequence of approach, are effects which cor- 
 respond with the hypothesis of the planets 
 being carried in the vortex or circulating stream 
 of the Sun's atmosphere. 
 
 For the Sun, or the Earth, turning on its axis, 
 and atmosphere continually rising from the sur- 
 face, is wrapped in a constant succession of 
 fresh atmosphere, drawing after it the old, 
 which is constantly in ascension and elongation. 
 Hence it is that the planets are longer in per- 
 forming a revolution round the Sun in a propor- 
 tion as they are more distant ; and hence any 
 planet receding from the Sun experiences a 
 diminution of progress, and the contrary on 
 re-approaching. That the circulation of the 
 ethereal solar fluid is more slow as the distance 
 from the Sun is greater, may be exemplified by 
 
 the
 
 51 
 
 the gyration of any fluid, and the reader may 
 observe it accords with the preceding table of 
 distances and times of revolution, the circum- 
 ferences of the orbits being in like proportion to 
 each other as the distances from the Sun. 
 
 Much of effect similar to what is caused by 
 the Sun on the planets, is caused by the planets 
 on their satellites. If the satellites had moved 
 in as regular order as the planets, it being known 
 that the moon at her last quarter is in advance 
 of the Earth, by some similar adaptation, ground 
 for estimating when the Earth was in advance of 
 the Sun might have been obtained ; that is to 
 say, towards what part of the heavens the Sun 
 is advancing. 
 
 A projectile and instantaneous impulse, which 
 after being once imparted leaves the projected 
 body to itself, it has been shown, is treated by 
 astronomers as an adventitious cause of motion. 
 The similarity of movement in all the planets of 
 the solar system is however very differently re- 
 garded by M. le Comte de la Place, who remarks; 
 " On est etonne de voir toutes les planetes se 
 " mouvoir autour du Soleil, d'occident en orient, 
 " et presque dans le meme plan j les satellites 
 " en mouvement autour de leur planetes, dans le 
 D 2 " meme
 
 52 
 
 " meme sens, &c. Un phenomene aussi extra^ 
 " ordinaire n'est point PefFet du hazard ; il 
 " indique tme cause generale qui a determine 
 " tous ces mouvemens." And afterwards, 
 " quelle que soit sa nature, puisqu'elk aproduitou 
 " dirigi lesmouvemens des planetes, ilfaut qu'elle 
 " ait embrasse tous ces corps - 9 et vu la distance 
 " prodigieuse qui les separe, elle ne pent avoir 
 " etc qu'un Jluide d'une immense etendue. Pour 
 " leur avoir donne dans le meme sens, un mouve- 
 " ment presque circulaire autour du Soleil, iljaut 
 " que ce Jluide ait environni cet astre comme une 
 " atmosphere. La consideration des mouvemens pla- 
 " netaires nous conduit done a penser qu'en vertu 
 " d'une chaleur excessive, I' atmosphere du Soleil 
 " s'est primitivement- ttendue au dela des orbes 
 " de toutes les planetes, et quelle s'est resserree 
 " successivement, jusqu'a ses limites dctuelles." 
 i. e. " We are astonished to see all the planets 
 " move round the Sun, from West to East, and 
 " almost in the same plane ; the satellites in 
 '* movement round their planets, in the same 
 " order, &c. A phenomenon so extraordinary 
 '* is not the effect of chance ; it indicates a 
 " general cause, which has determined all these 
 ** motions." And afterwards, " whatsoever is 
 
 its
 
 53 
 
 " its nature, since it has produced or directed the 
 " movements of the planets, it must have em- 
 " braced all those bodies ; and seeing the prodi- 
 " gious distance which separates them, it cannot 
 " be other than a fluid of an immense extent. 
 " To have given them in the same order, a 
 " movement nearly circular round the Sun, this 
 " fluid must have environed this star (the Sun) 
 " as an atmosphere. The consideration of the 
 " planetary movements conducts us then to 
 " think, that in consequence of an excessive heat, 
 " the atmosphere of the Sun primitively extended 
 " itself beyond the orbs of all the planets, and 
 " that it was afterwards gradually contracted to 
 " within its actual limits." * 
 
 It is remarkable in the above quotation, that 
 M. de la Place has entered as largely into the 
 hypothesis of the extension of the Sun's atmos- 
 phere as Kepler and Descartes j and also that he 
 has supposed it to have been afterwards con- 
 tracted within its original limits. 
 
 I will briefly sum up the whole argument. 
 
 Admitting duration to matter and that cir- 
 cumstance^ occasions no absolute exhaustion; 
 
 admitting 
 
 * Exposition du Systeme du Monde. Liv. V. ch. 6. Vol. II. 
 p. 420. 425.
 
 admitting also the power of gravity to be uni- 
 versal; all else appears to follow consecutively. 
 Gravity causes progression. Progression causes 
 rotation. Whilst progression and rotation dis- 
 perse and circulate, gravity or attraction collects. 
 Herein is the vivifying power which supports 
 and continues the action of the Sun. The Sun 
 communicates motion in consequence of having 
 received it ; and these principles seem calcu- 
 lated to preserve matter in perpetual action of 
 interchange. 
 
 M. de la Place has noticed a conjecture made 
 by M. Buffon respecting the formation of our 
 planetary system. M. Buffon supposed that a 
 comet fell upon the Sun and drove thence a tor- 
 rent of matter, which when arrived at a distance, 
 formed new junctions in a number of separate 
 orbs. Not very dissimilar to this is a conjecture 
 which has recently been made respecting the 
 four small planets discovered between the orbits 
 of Jupiter and Mars ; i. e. that they had been 
 parts of one and the same planet, and came to 
 their present form by convulsive separation. A 
 mass of matter so broken would probably be 
 dispersed in almost every direction, and in parts 
 of various sizes. The impulses given by the ex- 
 plosion
 
 plosion would gradually diminish, and the parts 
 that were too heavy to be sustained in that alti- 
 tude of the solar fluid, would gravitate towards 
 the Sun. If any of the parts were large enough 
 to acquire, by rotation in the descent, atmosphere 
 adequate to rendering them buoyant, their farther 
 descent would be impeded or stopped, and they 
 would be carried in the solar stream, and so be 
 made to perform revolutions in orbit. Or if any of 
 the smaller parts in gravitating towards the Sun, 
 came within the influence of some other mass, 
 whose nearer attraction acted on it more power- 
 fully, they would join that mass, or revolve round 
 it as a satellite. 
 
 It seems quite open to conjecture, of the orbs 
 in the firmament, that some have received their 
 present form by the disruption of larger masses, 
 and that some have acquired it by gradual accu- 
 mulation. It may be remarked as within our 
 own observation, that in a perturbed state of our 
 atmosphere, the earthy and metallic particles of 
 matter which are floating in the air, being moved 
 by variable streams, have then greater opportu- 
 nities of attracting and joining each other, than 
 in times when the streams of air flow more regu- 
 larly and quiet. Such collections, being of in- 
 flammable
 
 56 
 
 flammable nature, if struck by some electric 
 power, may be made to undergo an instantaneous 
 and more powerful fusion than can be given by 
 human art j and by the shock and their more 
 collected weight, be made to descend. In the 
 superior air or medium, matter may be supposed 
 to collect more gradually, to be less subject to 
 disturbance, and thus be allowed to attain to the 
 state of an infant planet. 
 
 The planets discovered since the commence- 
 ment of the present century evince how small 
 a mass of collected matter is capable of re- 
 volving in a regular orbit round the Sun ; one 
 of those planets being reckoned not to exceed 
 100 miles in diameter. There does not appear 
 any demonstrable objection why the stones we 
 denominate meteoric, may not either be formed 
 in our own atmosphere, or be part of the wreck 
 of some other mass. 
 
 There is infinite room for fancy in imagining 
 comparisons between our Earth and the other 
 planets, as habitable bodies. The size and weight 
 of a man of the Sun has been calculated, the cal- 
 culator meaning the Sun to have a race of beings 
 in form like his own fellow-creatures. Some, 
 however, conjecture the planets to be variously 
 
 peopled,
 
 57 
 
 peopled, each with beings whose form, organ- 
 ization, powers and attributes, are peculiarly 
 adapted to itself. And seeing that man, which 
 is the most perfect being of this Earth, is yet 
 too imperfect to be with much show of reason 
 supposed of the first order of beings in the whole 
 solar system, the conjecture appears natural. 
 The argument goes on to show why the Earth is 
 not to be esteemed either the most desirable or 
 the least desirable planet of the solar system ; 
 for as there are planets larger, and some smaller, 
 some nearer the Sun and some more distant, 
 some constituted of matter more dense and 
 solid, and some of matter more rare ; there 
 seems reason for considering our Earth among 
 the planets, as one of average value. 
 
 The orb we inhabit, from geological appear- 
 ances, has been conjectured to have undergone 
 great changes before the present race of man- 
 kind had arrived at the knowledge of making 
 records which should be intelligible to their pos- 
 terity. That mankind have been some thousand 
 years in a progress, though slow and uncertain, 
 of improvement, can be shown or argued, in 
 numberless instances ; but this would seem wide 
 of the present subject. The Earth itself, I 
 esteem to be in a progress of physical improve- 
 E ment,
 
 5$ 
 
 ment, of which I regard as an indication, that 
 the plane of the Earth's rotation or diurnal 
 motion is gradually becoming less oblique to 
 the plane of her motion in orbit ; the effect of 
 which will be to lessen the inequality of the 
 seasons. It may be regarded as another indi- 
 cation, that the excentricity of the Earth's orbit 
 is diminishing. If this had been discovered in 
 the lifetime of Fontenelle, he would probably 
 have made it an argument in favour of mankind 
 becoming more rational. 
 
 Many are glad to join in the opinion, that the 
 Sun is not a globe of fire, from unwillingness to 
 believe that a mass containing so large a portion 
 of the matter of the whole system, shall not be 
 habitable and inhabited. That belief is strength- 
 ened by heat and fire being produced many 
 ways besides being received from the Sun. The 
 strong light we receive from the Sun may pro- 
 ceed from the density and richness of the Sun's 
 atmosphere ; and possibly, attrition of the Sun's 
 rays with the surface of the earth in its rotation, 
 may be as great a cause of the heat we experi- 
 ence from the Sun, as the quality of the rays. 
 
 By inadvertence we sometimes call an hypo- 
 thesis of the motions of the heavenly bodies, a 
 system of the Universe. But as the whole of our 
 
 view
 
 59 
 
 view of space, (vast as it seems to the generality 
 of our conceptions,) we must be sensible is not 
 equal to the smallest fractional part capable of 
 being expressed by figures, of the Universe, it 
 is not amiss to conceive, that there may exist in 
 space matter and properties, of which we are 
 not capable of entertaining any idea. Our rea- 
 soning by analogy, therefore, should not be 
 supposed to comprehend the Universe. 
 
 Finally, it should not escape our notice, that 
 the orderly arrangement observable in the system 
 of our Sun has been contrived for the benefit of 
 conscious existence j and from all that we can 
 understand of the government of the heavenly 
 bodies, we find cause to believe, that a perfect 
 justice prevails through all nature. It is no 
 departure from the strict line of justice, that 
 benefit is conferred without having been merited, 
 where Almighty power has so much to bestow ; 
 and when, in the course of human events, we 
 see unmerited misery, or triumphant iniquity, 
 it is to be regarded as a small part of a large 
 running account. 
 
 I will only add, that I entered on the fore- 
 going enquiry without bias or prepossession in 
 favour of any particular system of the heavenly 
 motions j but was moved thereto by objections 
 
 which
 
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