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Maps, plates, charts, etc., may be filmed at different reduction ratios. Those too large to be entirely included in one exposure are filmed beginning In the upper left hand corner, left to right and top to bottom, as many frames as required. The following diagrams Illustrate the method: Les cartes, planches, tableaux, etc., peuvent Atre fllmfo d des taux de reduction diff4rents. Lorsque le document est trop grand pour Atre reproduit en un seul cllchA, II est fllmA A partir de I'angle supArleur gauche, de gauche ii droite, et de haut en bas, en prenant le nombre d'images nAcessaire. Les diagrammes sulvants illustrent la mAthode. 1 2 3 1 2 3 4 5 6 fM "5 W-' '■ ■ Wif. -it: ■ Kr;^, ■■•■tXi.v.- iU4.:::. ■■•v^S *■•< ;> «) a S I ^ I a I I I ^ t*.^ %: \ ' .,!«S»^W- a.) CEJfTRIFUGAL FORCE * GRAVITATIOJir. COMETS AMD The Theory of Cometary Orbits. BT JOHN HARRIS. Pfltttrcal: PRINTED BY THE LOVELL PRINTING & PUBLISHING COMPANY, St. Nicholas Stiikbt. AuQUBT, 1875. 0- l."S> r xo- Mr '■■ \ iiS i \ ': i T ! - ! t » I N 1) E X I'ago rrefuco 5 Chap. 1 — TiiEoiiY OP CoMETARV Orbits. Introductory Ob8orvations 11 (1) Tho Uompound Sidorul Orbit 12 (2) Tho Compound Solar und rianotnry Orbit IG (3) Torrostrial and Solar Uomots 20 (4) Tho Expanding Compound Orbit "JO Chap. II. — The Cometary Phenomena. (1) Theoretical Consideration of tho Phenomena. . . 23 (a) Physical condition of a Comet (2) Explanation of the Phenomena 27 (3) Natural Division of Comets into two classes.:. . . 28 (4) The Theory of Eccentric Orbits, and the Facts ) ^q of Astronomy j (5) Biela's Comet 32 (6) Notices from tho Record, of various Comets 36 (7) The Eccentric Orbit Theory 41 (a) Extravagant hypotheses of the theory 49 (8) The Comet's Luminous Train 51 (9) Concluding Remarks 53 Appendix. , (1) Notice of llalley's Comet ,. 55 (2) The Comet o^ 1744 59 lx\DEX OF PLATES. fttge Plate 1 ) »* Fias ] & 2 I '^'"^ Compound Sideral Orbit. 12 \ Sideral Comet controlled by throe i centres of gravitation 14 Plate 2. Fig. 3.. Plate 3. I The Compound Plaretary and Solar Fiffs.4&5.\ Orbit 16 Plate 4. ) The Expanding Compound Planetary Figs.6&8.j and Solar Orbit 22 Plate 4. ) The Expanding Compound Sideral Fig. 7 ) Orbit « Plate 5. "I The Comet of 1832, (Biela' 8), according Fig. 7. I to the Eccentric Orbit Theory 32 Plate 6 & 7. 1 The same considered as a Planetary Fig.S J and Solar Comet 34 Plate 7. . Fig. 817. . . Plate 8. Fig. 882.. ■;Th( J a The thirteen Elliptic Comets, according to the Eccentric Theory, whose orbits are within that of the planet Saturn. . . 42 he six Elliptic Comets, according to the Eccentric Theory, whose mean distances are nearly equal to that of Uranus «< Plates 9 & 10. " 11 & 12. " 13 u Hallej^'s Comet 58 The Comet of 1819...! 60 The Comet of 1744 ■ <« 14 The Comet of 1680 <« 15 The Comet of 1828 (Encke's Comet).. « (Frontispiece) ) ^^^ ^^^^^^ ^^ ^^^^ ^^^^ ^^ jgg^ Plate 14. (n) ) itiiififtili - ' '-'• ' "■■'""-•' t'""'- PREFACE. WRIPPMH In the historical recoixi of the younger days of the human race, the appearance of a comet was the occasion of wide- spread fear and anxiety; the portentous signal of the wrath of the Deity ; the probable fore-runner of dire calamity, none the less alarming because its nature and the precise character of the impending catastrophe were alike mysteri- ous and unknown. « In reading the narrative of the feeling with which such a natural phenomenon was regarded and the conduct by which that feeling was expressed, the young student of the present day, possessed of only a very elementary knowledge of physical science, is apt to think within himself . . * the mea of that age must surely have been foolish and superstitious in a surprising degree to be astonished and frightened at such a natural occurrence.' Doubtless we, older and somewhat more experienced students of physical science, are not likely to be so hasty in writing down and judging our ancestors as altogether foolish; nevertheless, we may err greatly and in the same direction, although not quite so superficially. Let us bi'iefly consider the case in the way of comparison : — For the reasonableness of a conclusion to be absolute, an abso- lutely perfect knowledge of the subject to which the conclu- sion pertains is requisite. Now such absolute perfection does not belong to human kno .fledge on any subject (unless, perhaps, in regard to some subjects or facts of a quite simple and elementary description). Therefore, in a human sense, reasonableness is dependent or relative. For exam- ple . . a man may have a very limited knowledge of a sub- 6 PREFACE. ■^. ject, and may come to a conclusion quite reasonable relatively to that limited knowledge ; i. e., hia conclusion may be supported by his direct and indirect knowledge of the facts and be limited by the limited extent of that know- ledge. On the other hand, a man may have a much more extensive knowledge of the subject and may come to an unreasonable conclusion ; unreasonable because . . he allows undemonstrated theories and inventions of his own to occupy the place of fact, and bases his conclusion not upon his sound limited knowledge of the subject, but upon the unsound combination of that which is true with that which is false. In the appearance of a comet, our forefathers contem- plated a strange, and to them an alarmingly mysterious phenomenon. An ai)parition in the heavens, almost, if not quite unprecedented,* presents itself suddenly to their astonished vision. There must be a cause. ..Can any known natural cause bo assigned to reasonably account for the phenomenon ? No ; the circumstance is apparently quite unique and irreconcilable with the observed facts belong- ing to astronomy. The character of the celestial visitant evidently differs greatly from that of the stars and planets. It appears suddenl}*, spreads its luminous signal over a con- siderable space in the heavens and, after remaining very conspicuous for a certain time, the menacing apparition suddenl}' disappears. There must be a cause. . .Can any known cause be reasonably assigned ? Yes ; one, and one only. It ! known that the Creator is omnipotent over the laws of nature; it is known, by the ever present facts of creation, by •Supposing tradition, or even the memory of the older men, to have informed them that the visitation was not unprecedented, the accounts of the former appearance would be mixed up with the- fears and prognostica- tions of calamity which it inspired, and, probably, -rame event of a dis- astrous eharacter, which had afterwards taken place, would be connected with it as its consequent. ■p PREFAOB. T tradition and record of the most reliable description, and by the teaching of the inner intellectual consciousness (reason) that God the Creator has power, if He will, to cause such extraoi-dinary phenomenon. But an extraordinary exercise of the divine power suggests some particular and impor- tant purpose. What purpose is it most probable that such a witness is intended to notify? It is a signal evidently, but of what? Its a.>^pect is menacing... its form and appear- ance suggest danger. . It is most probably a signal of the Avrath of the Deity ; but again, for what ? Many individu- als are aware that they have individually been doing what they knew to be wrong ; have been disobeying or, perhaps .setting at defiance His moral laws, and feel that they are criminal in the sight of God ; but it does not seem reasona- bly probable that tha crimes or sins of a few individuals would be considered of sufficient importance to occasion such an extraordinary manifestation of the divine power. The nation 1 Ah . . the nation feels conscious that it has been 'doing wrong ; has been violating the laws and command- ments of God; or, at least, has been neglecting to fulfil some part of that which it knew to be its duty to Him. Ah ; that is the explanation : the nation has angered and provoked the wrath of God and this apparition is the signal that punishment is about to be inflicted. We know now that our ancestors wex'e mistaken in fact with respect to the nature and purpose of the comet's appearance. It was not a miraculous portent having ex- press relation to the sinfulness or misconduct of a nation. It was a natural phenomenon, quite of the same character sls the appearence of a planet or a star. But does it follow, therefore, that the conclusion of our .ancestors was superstitious and unreasonable ? 'Not at all. We opine that the more cai'efuUy the case is considered, the more certainly it will appear that their reasoning, .assuming it to have been as stated, was essentially sound :l ;■ i I; 8 PREFACE. on both the important questions involved. (1) Whether it be within the power of the Creator to cause an exceptional and supernatural phenomenon (such as this was supposed to be) to present itself ; or, to state the question more gener- ally in other words, . . whether the Creator be the living^ Omnipotent God, possessing the power to control and impose such laws upon the Natural world as He pleases ? (2) Admitting the existence and omnipotence of the Crea- tor, the second question is . . Whether there be supervision and direct supernatural intervention, general or special, oy Him in the aflfairs and arrangements of the Natural world ?* The first question belongs pre-eminently to general and physical science. It is a primary or fundamental question which all students of Science, especially those who break ground for themselves and become patient and persistent cultivators of the soil, must sooner or later meet face ta face and consider with careful and earnest attention. Whc*t then, is t,he general conclusion or judgment arrived at v/ith respect to it ? Wo have no hesitation in replying that a very large and overwhelming majority of the men of science- belonging to the present, to the immediate past, and to that earlier age of science preceding the immediate past, ugrec in affirming the proposition here stated in the form of a question. Of the great names inscribed on the muster-roil of Science a very large majority are those of men who have decidedly expressed their conviction that the Creator is; possessed of such omnipotent supernatural power, and who- have tber jfore indirectly expressed their belief that a mater- ial or immaterial cometary apparition might or may he- caused by the Will of God with a special and express pur- pose. The second question, as to continual or occasional supervision and supernatural intervention by the Creator in the natural world, is also one which is answered in the affirmative by a vast majority of the educated men of PREFAOK. t our own time. All those who profess any form or 4system of Christianity agree in such affirmative answer; for a profession of Christianity is manifestly inconsistent with a disbelief as to such spiritual supervision and intervention ; Christianity being, both directly and indirectly, Jbased upon such belief. We find therefore that the reasoning of our ancestors, and .also their conclusion, taking into consideration their very limited knowledge of facts belonging to physical science, is justified by the increased experience and knowledge since acquired by the human race. How stands the case on the ^ther side of the comparison, viz., with regard to ourselves, the present more highly civilized and educated represen- tatives of the human race ? What is it that we are in efibct now taught with respect to a comet and to the nature of a comet ? That it is not a mass of aggregated matter obedient to the law of gi-avitation ; nor is it an animated, intelligent being ; but a mysterious phantom endowed with instinct and capability of the most extraordinary and surpris- ing description, by virtue of which it is enabled to leave the -dominion of the sun and, setting at nought the laws of matter, to retire to the most remote regions of space ; and yet, after the expiration of a certain time, notwithstanding its then enormous distance therefrom, it becomes again cognizant of the existence and influence of the sun, uud, guided apparently by the strange instinct of which we have spoken,* and quite inattentive to the attractions of other stars and stellar systems, it usually, although not always, finds its way with dii*ect and unerring precision, back again • It will be understood thtt the application of the term instinct to the gaiding principle under which the comet of the present doctrine performs its eccentric migrations is ours, and :3 not so used in the astronomical works to which we allude, but we so apply it because instinct is, in fact, the only guiding principle known to science in connection with the mate- Jial world which can be assigned to satisfy the requirements of the doctrine. ^.r. 10 PREFACE. to the immediate neighbourhood of the central body of our particular stellar system. Now this is substantially and essentially the present doc- trine as laid down in the best and most highly reputed treatises on astronomy, and we opine that the ancient repre- sentative of human education, if brought directly into argu- ment with us, would be entitled to say : * Our explanation is, you are obliged to admit, not in itself unreasonable : it is true we have not explained the nature of the comet itself because we have not, and do not pretend to haVe, knowledge of it, but your presumptive explanation is quite unreasonable and untenable. If your comet be wholly spiritual and im- material your explanation should show that the action or behaviour of other spiritual existences is regulated by a similar strange instinct, but if your comet be in any degiee material why is it not subject to the general laws of the material world ? You have had niore time and better oppor- tunities to observe these apparitions than we have had, and we may believe that you have, as you say, found out that they are recurring phenomena, reappearing v 5th regularity after definite periods of absence. This is well, and may be eventually useful as an addition to the knowledge of the foots belonging to the phenomenon, but, in a reasonable sense,, you have confused your statement of a fact by mingling it with certain extravagant, unsupported, and incredible sup- positions, and you have really given no philosophical expla- nation of the case. ... , ., . 1 ■.; .( ? -■: .;i'«<' 'i' CHAPTER I. THE THEORY OF COMETARY ORBITS. ujf "1- ,tr INTRODUCTORY OBSERVATIONS. As already stated, it is not permissible to entertain the supposition that a planet or other mass of aggregated matter, revolving around the sun (or other centre) under the influence of gravitation, can suddenly divest itself of that influence f* consequently the hypothesis which sup- poses the orbital path of a comet to describe a parabola or a hyperbola must certainly be erroneous. But it was also explained in the earlier part of this work, that the deviation from a circle in the orbital motion of a planet revolving around a centre of gravitation is of the nature of an oscillation or vibration, which is kept under con- trol and restricted in amount, by the gravitating influence in the one direction and by the centrifugal force in the other. If this teaching is correctly understood, it will become apparent, on attentive consideration, that, al- though the elliptical orbit of a mass of matter (planet) may vary as to the eccentricity of the ellipse described by its path, such variation can be only within certain narrow limits determined by the particular circumstances of the case ; the favourable conditions for the development or permanence of a larger amount of eccentricity being a great angular velocity and a short distance from the centre of gravitation ; whereas, under the reverse conditions, • Part. Fint, page 55 ; Also Part Third, page 24t,etitq. 12 COMPOUND SIDtRIAL ORBIT. i '.■ viz., an orbit of much greater diameter and proportionally lesser angular velocity, the deviation from a circular path will be so much less. It is true, a perturbing influence may interfere and cause a considerable increase in the deviation ; but this increased deviation c;\ n only become perninnent, ns a constant (periodical) oscillation, if the conditions are favourable ; otherwise, the effect of the perturbation (if permanently any) would be to modify the average distance from the centre of gravitation throughout the entire orbit. It will therefore also follow that it is not allowable to attribute to a planet or comet, revolving around the sun as its primary centre, an ellip- tical orbit having a very great degree of eccentricity, and of which, therefore, the aphelion distance is very much greater than the perihelion distance. (1) TJw compound sidereal orbit. — We say that such an hypothetical orbit is inadmissable because it is irrecon- cilable with the law of gravitation. It is, nevertheless, quite possible for a planetary or cometary mass of mat- ter to enter the solar system, and being within the sun's gravitating influence, to approach the sun, and even to make a partial revolution about the sun, and then to de- part or return to another system. To explain this more particularly we refer to Fig 1, (PI. 1,) where A. represents the sun, and B. represents the central star of a neighbouring system ; C is a comet or cometary mass of matter ; m. y. n.p. q. v. is the comet's supposed orbit. From the place w., tiiO comet moves in the direction of the arrows through the circular arc m. c. n., having B., the star, for the centre of gravitation ; having arrived at the point n., the direction of motion is the tangent to the arc, viz., n. o. Now if C, the comet, were ^^ -'-t3-— - ■^' / \ s .-V \^. \~ ■<5i? H 'y / (7) / ^ V o--^Y. -% fS-'K /:> •i^y'f /. -i- ^ ^^^ ^■pup COMPOUND IIDEBEAL ORBIT. n tBtrictly a member of the system belonging to B., and con- fined to that syst'^'^, that is to say, beyond the influence of any other gravitating centre, then, the influence of B., being counteracted only by the centrifugal force of the moving comet, would restrain it from deviating out of the circular path ; but the distance of the comet C. from B., is so great that, when it has arrived at n., the influence of the sun A., has already begun to act upon it, and by coun- teracting the influence o{ B., lessens the effect of the lat- ter ; consequently the motion of C. deviates outside the ciicle and towards the tangent *, so that the orbital path n. t. is interme'^'iate between the arc of the circle and the tangent. At this point, being at about the half distance between A. and B., tlieir opposing influences are about -equal and C, therefore, moves in the direction of the tan- gent. The comet is now ropidly receding from B., and Jipproaching A. ; when it has arrived at the place p., the comparatively feeble influence of B. will be effective only in retarding the motion and diminishing the velocity, which will have just previously increased in consequence of ^.'s influence during the approach of the comet to- wards i4., whilst moving from n. to p. After passing the place J)., the influence of ^. will be alone eflfective in re- straining and governing the motion of the comet, which will therefore move in a circular orbit round A., until having passed q., it arrives at v., the point corresponding to that of n. in the neighbouring system. The conditions will be now similar to those preceding, when the comet was at «., and moving towards A ., only that the relation of the two centres of gravitating influence to each other in respect to the comet will now be reversed ; and the comet will now leave A.j and approach A, moving through i.itii i-jMr -•^'■•''^^''*^"1 '¥ 14 COMPOUND SIDEREAL ORBIT. the compound curve v. t. m., which is similar to the curve n. t. p., through whicli A. was approached. From m. the comet will again traverse the same compound orbital path ; and so on continuously, moving in the direction of the arrows.* . If now we assume that such compound orbital path of u siderial comet may be in a plane vertical to that of the solar Hystem, or in the same plane, or in aplane oblique at some angle to the plane of the solar system ; it will be at once apparent that to a spectator observing the comet from the earth the difficulty of correctly determining the orbital j»ath by observation must be very great. Fig. 2 (PI. 1) may serve to convey a clearer idea of the difficulty. E. re- presents tlie earth, and the orbit of the comet is supposed to be vertical to the ecliptic (or to the plane of the sun's equator). If the comet, on entering its solar orbit from t. in the direction t. p.f became visible from the earth, the orbital motion of the earth would be apparently trans- ferred to the comet, which apparent motion, in the re- verse direction to the actual motion of the earth, would combine itself with the real motion of the comet ] and thus give the appearance to an observer on the earth of an approach to the sun in an oblique direction. The law of gravitation permits us to suppose that a mass of aggregated matter may thus have its motion con- * This explanation in respect to tbe uniformity of the comet's distance from the centre of gravitation is provisional only : it will be seen hereafter that our theory supposes an expanding compound orbit, viz., that the comet on leaving the one orbit and entering the other approaches nearer than its average distance to the centre of gravitation, and then commences and continues to recede (spirally) from that centre, until it (tbo eomet) again eaters the former orbit where, in like manner, it approaches that centre and then commences to recede, and so on. See itineration and remarkt, page 22. ■■'"' i ' '. • [>• :r( -,:-A^-' '•m mm ipfppff mmmm. SOLAR AND PLANETARY ORBIT. OT trolled and regulated by two distinct centres of gravitat- . ing influence ; nor are we prevented from supposing that the .orbit may be yet more complex, and that three or even several systems may be traversed in a similar man- ner and in obedience, as already explained, to the re- cognized law of gravitation. Fig. 3 (PI. 2) shows the orbital path of a comet which is supposed to be con- trolled by three distinct centres of gravitating influence ; the arrows and the explanation already given will suffi- ciently indicate the manner in which the orbit is com- pounded, 'i i^'V ;'.-::/'-'•', i '/i;^_.-'-/ • -'-:• /.. ■" In either of these cases it is evident that the comet would be periodic ; and if, in any part of its orbit; it approached the earth to within visual distance, the time of its return, after several such visitations had been observed and noted, might be safely predicted. (2) The compound solar and planetary orbit. ' It may be objected to the foregoing that there are certain comets which are known as belonging altogether to the solar system, of which the periods are too short to admit the supposition of their travelling beyond the influence of the sun, and of which the orbits and elements have been calculated on the eccentric hypothesis, and the results of the calculations confirmed and verified by actual observation. But a planet, which is secondary to the sun as the general centre of the system, may be, if of sufficiently large size, primary to bodies of much less mass, as for instance the earth to the moon, or either one of the large planets to the satellites which revolve about them as their centre of gravitating influence. Evidently therefore, the law of gravitation allows us to suppose that a planet of large size, which, as a planet, is secondary «iu> ■P 16 SOLAR AND PLANETARY ORBIT. to the sun, may also serve together with the sun as one of two primaries controlling the motion and determining the orbital path of a comet : the requisite conditions of the case being that the relative distance of the cometary body from the sun and from the planet is proportional to the relative masses of the sun and the planet. For example, in Fig. 4, (PI. 3),/. represents the planet Jupiter, S. the Sun, and C. a cometary body: the comet's orbital path is indicated by the arrows. The conditions of this case will be essentially similar to those explained in the example of Fig. 1. In that example the two centres of gravitat- ing influence were supposed equal ; and in this, the piass of the sun is much greater than that of Jupiter, but the orbital distance of the body C. from the sun is assumed to be also gre&ter than its orbital distance from Jupiter, in the Siime proportion ; and therefore when the body in Fig. 4 arrives at (about) the point m. it will be essen- tially in the same case as at the point »., in Fig. J , viz. : the attraction of the planet, adding its influence to the centrifugal force, will in the first place cause a deviation towards the tangential direction outside the circular orbit ; a little further on, the attractions of the planet and of the sun will be equal, and the body will move in the tangential direction, thereby receding from the sun and approaching the planet j thus when the point n. has been reached the more distant and feeble influence of the sun will operate only in diminishing the (increased) velocity, and the cometary body becomes a satellite of the planet throughout n. a, p., about three-fourths of a revolution, until on arriving at (about) the place p. the former conditions are reversed, and the comet receding from the planet returns to its solar orbit q. b. m. It is :'t " ph,t>- 3. / C (1) \. >^"' P ^^, / / / J m \ .^ 'VI Ii\^. i . c >»;.; ^ j.r' ^». A f iD ^z- ^^'- ^ ^ H 9 /.,i ,/ / I r '/ \ \ \^ \ \ /. 'i^ "^ ^ ^ 1 a %: 1 !J: ■v §^ J? 5! ■>* 1 i s^ i 1 ii :*; c ^ rl ,5S V. ih CHAPTER II. THE COMETARY PHENOMENA. (1) Theoretical consideration of the pheftoniend.- The peculiar appearance of the coma, and the luminous characteristic ol the train or tail of many of the comets, are appearances of which no satisfactory explanation has been given. With respect to the first, we think tliat a are- ful consideration of the evidence which geology furnishes, as to what was certainly the condition of the earth at a time antecedent to the existence of animal and vegetable life thereon, will enable us to understand the nebulous appearance of the coina and the comparatively small size and solid appearance of the nucleus. Geolo^jical theories explaining the primary condition of the earth, appear to be at present in a somewhat incomplete and crude state, contravening more or less the known physical laws of matter. The explanation now perhaps most generally accepted is to the efiect that the entire mass, including all the varieties of matter compounding the earth as it now exists, was originally in a state of vapor. This entirely vaporous condition of the earth is supposed to have been succeeded by a liquid nucleus occupying the central part of the vaporous sphere and consisting of the denser vari- eties of matter in a molten state ; after a time, loss of heat having been caused by radiation, a crust is supposed to have been formed on the surface of the liquid (fluid) nucleus,which,being subsequently acted upon by volcanic agency and earthquakes, acquired stability as the cooling ^4 PHTSIOAL CONDITION OF A COMKT. I : \ i i process went on, and eventually became fitted for water to remain on its surface, and for the support of vegetable and animal existence. Now this hypothetical explanation in the first place takes for granted that all those varieties of matter, whether compour.d or elementary substances, which are now known to us in the solid state may be volatilized by the influence of heat. The evidence of chemistry, in the present state of the science, does not certainly do more than allow of such a supposition as a possibility ; it would be at least as reasonable, on chem- ical grounds, to suppose that many of these varieties of matter now recognized by us as elementary are not in yac^ eleniertary, and would be decomposed and separated into their elements if exposed to tlie exceedingly high temperature contemplated ; and it might be assumed with a greater measure of probability, tliat even tlie intense heat supposed would be unable to vaporise (volatilize) or even to liquefy some of those substances now known to us as solids, but that some of them would resist lique- faction even at the highest temperature. But allowing, for a moment, the •possibility that intense heat, under favourable conditions, might liquefy and volatilize all the solid forms of matter, yet we find the hypothesis tacitly assuming that the entire mass or quantity of matter com- pounding the earth has not undergone augmentation ; but that, whether in its present partially liquid and par- tially solid and gaseous condition ; or, as formerly, in a partially or wholly vaporous state, the aggregate quantity of matter has remained the same. It therefore follows that the (vaporous) centre — that is, the matter (in a va- porous condition) occupying the centre, must have been under the same pressure from the gravitation of the PHT8ICAL CONDITION OF A COMET. 25 superincumbent matter as that to which in the same situ* ation it is now subjected. This consideration at once much increases the difficulty of imagining many of those substances, at present only known to us as solids, in a fluid or vaporous state; because we are called on to suppose them able to assume and retain that condition under enormous pressure. It seems much more reason- able to suppose that at the very elevated temperature of the hypothesis the conditions would be . . . the centre of the earth composea of matter in the liquid (fluid) state ; exterior to or upon this, a crust of solid matter : then a stratum of dense vapor, becoming more gaseous and attenuated as the distance from the centre increased. On this supposition, as the cooling process gradually advanced^ chemical combination and reaction of the materials upon each other would take place within, upon, and above the crust, and, also, the potent agency of volcanic action would be at work from the first in supplying and modifying the constituents, and in fashioning the form of the crust for the ulterior purpose it vas intended to serve. We think that a careful consideration of the evidence now afforded by geology, together with the teaching of chemical and physical (meteorological) science, will be found to sub- stantiate this supposition as to the primary condition of the earth. If then we assume that the earth at some former period was in a physical condition substan- tiaUy such as we have just described, there can be no difficulty in supposing that some masses of aggregated matter, i. e., planetary or cometary bodies, may be at the present time in a similar condition ; indeed, it at once suggests itself as a probability that some of those very numerous bodies, of which astronomical observation has 26 TBI OOMIT's luminous TRAIN. made known to us the existence, are now in such a pri- mary or igneous condition.* Keeping this probability in mind, let us now examine the appearances presented to a terrestrial observer by a comet. HcrscJieVs Outlines of Astronomy. (550) " Comets consist for the most part of a large, and more or less splendid, but ill-defined, nebulous mass of light called the head, which is usually much brighter towards its centre, and offers the appearance of a vivid nucleus, like a star or planet. From the head and in a direction opposite to that in which the sun is situated from the comet appear to diverge two streams of light, which grow broader and more diffused at a distance from the head, and which most commonly close in and unite at a little distance behind it, but sometimes continue distinct for a great part of their course ; producing an effect like that of the trains left by some bright meteors, or like the diverging fire of a sky-rocket (only without sparkle or perceptible motion). This is the tail." (557) " The tail is, however, by no means an invari- able appendage of comets, many of the brightest have been observed to have short and feeble tails, and a few great comets have been entirely without them. Those of 1585, and 1763, offered no vestige of a tail ; and Cassini describes the comets of 1665, and 168*2, as being as round and as well defined as Jupiter. On the other hand instances are not wanting of comets furnished with many * If the condition of all ihe planetary bodies known to ua was found to be, bo far as we coald observe, precisely similar and aniform, the probability would be against the above supposition ; but since, on the contrary, observation has made certainly known to us that the present conditions of the various planets are dissimilar and differ very considerably, the probability is strongly in favor of the supposition. TBI OOUKT'S luminous TRAIN. 27 tails or streams of diverging light. That of 1744 had no less than six, spread out like an immense fan, extend- ing to a distance of nearly 30° in length. The small -comet of 1823 had two, making an angle of about 160°, the brighter turned as usual from the sun, the fainter towards it, or nearly so. The tails of comets, too, are often somewhat curved, bending, in general, towards the region which the comet has left, as if moving somewhat .more slowly, or as if resisted in their course." Lardner's Astronomy. (3092) "The comet (Halley's comet 1835) first became visible as a small round nebula, without a tail, and having & bright point more intensely luminous than the rest eccentrically placed within it." Also, see Illustrations, Plates 9, 10, 11, 12, 13. (2) Explanation of the Phenomena. The description given by others of the general appear- ance of comets, is in agreement with the foregoing, viz., as consisting of a nebulous mass, more or less lumiuous, at or near the centre of wliicli is the nucleus having the appearance of concentration orsolidi':y, and which is also more vividly luminous ; the tail or train of luminous mat- ter which forms part of the usual cometary appearance, varying greatly in furm and extent. Now if we suppose a planetary mass of matter in a condition similar to that of the earth in its primary state, moving at a very considerable distance from the earth, the appearance it might be expected to present, leaving «ut of consideration for the moment the luminous train or tail, would be precisely that described as belonging to •the comet ; viz., the spherical mass of matter in a liquid <^molten or fluid) state occupying the central part of the Ji EXPLANATION OF THB PHENOMENA. body, covered by the solid crust in an intensely heated' condition and surrounded by the vaporous and gaseous envelope would give the appearance of the nucleus and the coma. The supposition that the peculiar general appearance of cometary bodies is correctly accounted for in this manner is strengthened by astronomical observa- tion which teaches us that all comets do not present this peculiar appearance but, are sometimes more similar and sometimes more dissimilar to ordinary planets. Thus *' Cassini describes the comets of 1665 and 1682 as being as round and well defined as Jupiter ; " the comets of 1585 and 1763 offered no vestige of a tail ; " and " the smaller comets, such as are visible only in telescopes or with difficulty by the naked eye, and which are by far the most numerous, offer very frequently no appearance of a tail, and appear only as round or somewhat oval vaporous masses, more dense towards the centre, where,, however, they appear to have no distinct nucleus, or anything which seems entitled to be considered as a solid body. " (Herschel's Outlines.) (3) Natural division of comets into two classes. From the explanation which has been now given as to the orbital paths of comets, it follows that the observed comets would divide themselves into two classes,* viz.,, *A third class would be those comets (if we suppose there are any> which belong entirely to some other sjstem, and become occasionally- visible from the earth ; there is a probability that those comets of long- period which have their orbital plane vertical or nearly vertical to the- ecliptic, will be found to belong to this third class; and still more so where the motion is in the reverse direction to that of the planets belonging to the solar system. Again, the planetary comets might be divided into terres^ trial and planetary comets ; the first group containing all the comets of which the earth is the secondary centre of gravitation, and the second, all those having one of the other planets, to wit: Venus, Mars, Jupiter^ Saturn, as the secondary centre. ? ' .' '" ■< CLASBIFipATION OF COMETS. 2» sidereal (and solar) and planetary (and sOlar) comets ; the former only partially, and the latter wholly belonging t» the solar system. The former would evidently have orbital distances from the sun of great magnitude com- pared to the latter ; and, in cases where the periodical return is observable, the periods of those belonging to the first class would be proportionately greater than those of the second. In comparing this reference with the record of actual observation, we find : " Herealso we may notice a very curious remark of Mr. Hind (Ast. Nach. No. 724) respecting periodic comets, viz., that so far as at present known, they divide themselves for the most part into two families, the one havingperiods of about 75 years, corres- ponding to a mean distance about that of Uranus ; the other corresponding more nearly with those of the aste- roid8,and with a mean distance between those small planet» and Jupiter. The former group consists of four members ; Halley's comet revolving in 76 years, one discovered by Oblers in 74, De Vice's 4th comet in 73, and Brorsen'a 3rd in 76, respectively. Examples of the latter group are to be seen in the tables at the end of this volume." {HerscheVs Outlines.) " We may add, too, a marked ten- dency in the major axis of periodical comets to ground themselves about a certain determinate direction in space, that is to say, a line pointing to the sphere of the fixed stars northward to 70° long, and 30° N. lat. or nearly towards the star ^ Persei (in the Milky Way), and in the southern to a point (also in the Milky Way) diametrically- opposite." (Ast. Nach. No. 853.) 90 XHIORT or OOMITARr 0BBIT8. (4) The prevalent Theory of Cometary Orbits, and the facts of Astronomy. Persons who, it maybe, are only slightly acquainted with astronomy, in a scientific sense, are likely to somewhat misunderstand the nature of the connection between the prevalent astronomical theory as to the cometary revolu- tions, and the astronomically observed facts belonging to the same subject. They are informed, or may so under- stand the matter, that the orbit of a comet having been calculated according to a theory affirming its path h6 in an ellipse of extreme eccentricity, and the p return of the comet having been found to agree or very nearly so with a prediction based on the result of that calculation, that such argument constitutes a strong probability as to the correctness of the theory; and since, in a number of instances, the predicted return of the comets, of which the orbits have been so calculated, has been verified by the actual return in agreement with the prediction, that the theory is demonstrated by the observed facts, and therefore it is safe to conclude that the eccentric theory of the cometary orbit is establ.sh- ed. Such a conclusion is indeed very far from safe. It is true that certain computations based upon the theory are shown to bring out results which are in agreement with certain observed facts, but the nature of the case, which is of a compound character, makes it necessary to examine very carefully whetlier all the elements of the computation are in agreement with all the elements of the case, or, in other words, with all the known circum- stances belonging to the fact, because, computations in which the elements vary greatly, comparing those of the one respectively with those of the other, may bring out TECORY or OOMITART ORBITS. 81 the same general result, ond in this particular case the in- ference is that, as the result of the computation agrees with a certain fact (of observation), therefore all the elements of the computation are necessarily true, or according to fact, also. To point the objection to such an inference, we will observe that any compound arithmetical number may be arrived at, as a result, by combinations, in two or more computations, of elements which respectively (or taken separately) may dift'er considerably in the one computation from those in the other; for example, take ithe number 72, which results from 3x6x4, and also from 3 X 8 X 3, in one of which the 6 and the 4 differ respectively from the 8 and the 3 in the other : and, that the reader may correctly appreciate the merits of the case, we will suppose that the question is not as to whether the result is, or will be 72, because that is known beforehand, but as to the particular elements by which the result is produced. With respect, therefore, to the cometary predictions, they seem to amount to, but little more than this ; a comet having been visible at a cer- tam date and its appearance noted, and a definite number of years thereafter a comet, closely resembling the first, and apparently the 8ttme,having appeared ; and again after the same definite number of years, the comet having reappeared ; a strong probability suggests itself that the reappearances will be periodic at such intervals, and the next appearance or return of the comet is predicted accordingly. It appears that certain computations based upon a particular theory (the eccentric orbit theory) have been made to harmonize with the intervals of absence and re-appearance of the comets, but there is no sufficient evidence at present, so far as we are aware, of ! ; i 82 THEORY OF COMETARY ORBITS. a relation between the computations and the actual periods of the comets of such a kind as to justify the inference that the theory is supported, or in any way strengthened, by the return of certain comets at definite times, predicted in the manner just stated. Figures 7 and 8 will serve to illustrate the practical application of this argument. (3) BielcCs Comet. Fig. 7, PI. 5, is taken from Aragd's scientific notices of comets, and shows the theoretical orbit of Biela's comet, with the supposed relative position of the orbital path of the earth. This comet was seen in 1826, 1832, and 1846 ; and it is also supposed to have been seen in 1772^ and 180f5, etc. Its orbit, according to Biela, is a very eccentric ellipse described about the sun in 2410 days, or about 6f years. The following quotation from Lardtier's Astronomy is noteworthy as indirectly illustrating the preceding argument and the succeeding application thereof: " 3024. Corrected Estimate of the Mass of Mercury,— The masses of comets in general are, as will be explained, incomparably smaller than those of the smallest of the planets ; so much so, indeed, as to> bear no appreciable ratio to them. A consequence of this is, tliat wbile the effects of their attraction upon the planets are altogether insensible, the disturbing effects of the masses of the planets upon them are considerable. These disturbances, being proportional to the disturbing masses, may then be used as measures of the latter, just as the movement of the pith-ball in the balance of torsion sup- plies a measure of the physical forces to which that instrument i» applied. Encke's comet near its perihelion passes near the orbit of Mercury, and when that planet at the epoch of its perihelion happens to be near the same point, a considerable and measurable disturbance is ^ > <' Qrbii of Jupiter. % / oi. "T^l^-ssss^-- 0'^^ tea' ^"'1^^^^«^^4. 8 I 1 CO '^^ftdnj* fo 'M.H) in ■pWP|^?f"WP?»W m'mf ",ipwWf?iit|!!#' W^ BIELA'a COMIT. 33 onanifeBted in the comet's motion, whicli being obserred supplies a measure of tlie planet's mass. This combination of the motions of the planet and comet took place under very favourable circumstances, on the occasion of the peri- Jielion passage of the comet in 1838, the result of which, according to the calculations of Professor Encke, was the discovery of an error of .large amount in the previous estimates of the mass of the planet. After making every allowance for other planetary attractions, and for the effects of the resisting niedium, the existence of which it appears necessary to admit, it was inferred that the mass assigned to Mercury by Laplace was too great in the proportion of 12 to 7 This question is still under examination, ..ud every succeeding perilielion passage of the comet will increude tlie data by which its anore exact solution may be accomplished. 3025. Biela's Comet.— On February 28th, I82p, M. Biela, an Austrian officer, observed in Bohemia a comet, which was seen at -Marseilles at about the same time by M. Gambart. The path which it pursued, was oliservod to be similar to that of comets which had appeared in 1772 nnd ls06. Finally, it was found that this body moved round the sun in an oval orbit, and that the time of its revolu- tion was about 6 years and 8 months. It has since returned at its predicted times, and has been adopted as a member of our system, under the name of Biela's comet. Biela's comet moves in an orbit whose plane is inclined at a small -angle to those of the planets. It is but slightly oval, the length })eing to the breadth in the proportion of about four to three. When nearest to the sun, its distance is a little less than that of the earth ; -and when most remote from the sun, its distance somewhat exceeds ihat of Jupiter.* Thus it ranges through the solar sywtem, between Jlie orbits of Jupiter and the Earth . This comet had been observed in 1 772 and in 1806 ; but in the elliptic form of its orbit, and consequently its periodicity was not . ■discovered. Its return to ijerihelion was predicted and observed in 18.12, in 1846, and in 1852 ; but that which took place in 1838 escaped observation, owing to its unfavourable position and extreme faint- ness." • The distance of Jupiter to that of the earth is, in round numbera, about £ : 1, therefore the above orbit should be . . the length to the breadth in the proportion of about 6 : 3J, instead of 4 : 3. 34 ORBIT OF BIELA's COHBT, M '-: Fig. 8, PI. 6, exhibits a theoretical orbit of the same* comet which we propose to substitute for that of Biela, on the ground that the orbit now proposed affords a reasonable explanation of the observed facts, and which the former (Biela's) does not. The object of contrasting these two- figures is, in the first place,, to show that the situations in which the comet was actually seen at the various times of the observations, as well as the definite periods of i^s absence and of its return, i.e., from the time when it becomes invisible until the time when it again becomes visible, can be explained by attributing to the comet an orbit essentially different from that of Biela. We divide the so-called period of the comet, 2410 days by three,, and we consider the resulting number, 803^ days, to be (about) the actual period of the comet, that is to say, from the time of an observed appearance until the next. The orbit, as shown by the figure, is compound, belonging in pait to the planet Jupiter. It is evident that if we assume these relative periods for the comet and the earth, that the earth will make two complete revolutions and be in advance of the comet by about 73^ days in the 1st period ; in the 2nd period, the earth will make two complete revolutions and gain another 73^ days, making together 146§ days, and at the end of the third period,, the earth will have made six annual revolutions and have gained 220 days. At this time the comet again becomes visible from the earth in a situation nearly the same relatively to the earth as when it was observed 2410 days previously. During this longer term the comet might be twice visible from the earth ; but the frequency of the comet's re-appearance would be, in the first place, dependant upon the relative situation in its orbit of the planet Jupiter, because if the comet was in its planetary mmmmm w^^ M ORBIT OF BIEL^'S COMET. 3» orbit (revolving around Jupiter) at the time that the earth passed by, the comet would not be visible from the earth until overtaken again in the next revolution ; and, in the second place, it should be observed that, when the comet becomes visible in November or February, the earth is situated (vertically) much below the plane of the ecliptic, not very far from its point of maximum depression ; now, if tlie comet was seen at that time of the year when the earth is near its point of maximum elevation and therefore much above the plane of the ecliptic, the difference in the apparent relative situation might alone prevent the recognition of the comet. * According to this explanation the comet's true period considered as its third return to the same sidereal (- - fixed) place in the heavens (i.e., to a place situated at the same point of the so^r compass) will be somewhat (about 145 days) more than 7 years, because when the comet again becomes visible the earth requires 145 days to reach its former situation, which would complete the 7 years, and the comet moves with only-iibout one-half the angular velocity of the earth. And, also, since the period of Jupiter is nearly 12 years, that planet would make rather more than half a revolution during the 7 years, so that a great number of these septennial re- appearances might occur before the planet's situation in the zodiac would cause the comet to leave the solar orbit at that particular time of the year when its return was expected, and so prevent its being seen from the earth at the time of its usual re-appearance. * And moreover the comet must certainly have its periods of vertical elevation and depression which, instead of coinciding with those of the earth, may be in opposition thereto, and hence considerably increase the apparent difference in the relative situation.) »-! wm '- 1 36 BALLET 8 COMET. (4) Notices from tlie Record of various comets. HaUetf^s Comet. Fig. 9* represents the supposed orbit of Halley's comet, And is a fair illustration of the elliptical orbit of extreme eccentricity, which is now attributed to cometary bodies. We observe that the comet, having nearly reached its perihelion, makes about one-third of a revolution around t? the sun in moving from A to li, but having arrived at B, and still being comparatively very near to the sun, it no longer obeys the restraining power of the sun's gravi- tating, influence, but recedes in an almost direct line to a great distance, tlien, describing a slight curve towards the major axis of the ellipse, it gradually approaches its (supposed) aphelion C!, Notwithstanding that the conaet when at B, comparatively close to the sun, was unaffected •From Dick's "Sidereal Hearens." pppppp wmm^m ^mm HALLXT'S OOMIT. 37 % the enormous gravitating force to which it must have been at that place subjected, now, when near C at the very great distance S. (7, it becomes suddenly and sensi- tively attentive to the comparatively very feeble influence of the sun an4 describes the short curve shown at C (the supposed aphelion) ; but, here again, it appears quite evi- •dent that if the velocity of the comet at this place is so small and the sun's influence sufficiently great to cause the comet to make the comparatively sudden curve shown at C, the further result will be the motion of the comet in an almost direct line towards the sun as shown in Fig. 10. The following are Mr. Dick's observations having refer- ence to the figure : " The orbit of Halley's comec is four times longer than it is broad, and the orbits of those comets whose periodical revolution exceeds a hundred or a thousand years must be still more elongated and eccentric. The following figure (Fig. 9) represents the orbit of Hal- ley's comet nearly in its exact proportions — E. C. repre- sents the length of the ellipse in which it performs its revolution; E.D. the orbit of the earth somewhat longer than it ought to be in proportion to the comet's orbit ; S. the sun in one of the foci of the ellipse ; Sat. the propor- , tional distance ofthe planet Saturn from the sun; and U, the proportional distance of Uranus. The orbit of this comet extends to nearly double the distance of Uranus, and considerably beyond the orbit of the lately discovered planet Neptune." The following extract from Dr. Lardner^s Treatise on Astronomy will serve to illustrate more especially the sub- ject of the ' planetary comets ' by which we mean those which have a compound solar-and-planetary orbit such .as we have attributed to the comet known as Biela's. 88 lexell's comet. (3036) " Lexell's comet. — The history of Astronomy has recorded one singular example of a comet which appeared in the system, made two revolutions round the sun in an elliptic orbit, and then disappeared, never hav> ing been seen either beiore or since This comet was discovered by Messier, in June, 1770, in the constellation of Sagittarius between the head and the northern extremity of the bow, and was observed during that month. It disappeared in July, being lost in the sun's rays. After passing through its perihelion, it reappeared about the 4th of August, and continued to be observed until the first days of October, when it finally disappeared. All the attempts of the astronomers of that day failed to deduce the path of this comet from the ob- servations, until six years later, in 1776, Lexell showed that the observations were explained, not as had been assumed previously, by a parabolic path, but by an ellipse, and one, moreover, without any example at that epoch, which indicated the short period of 5i^ years. It was immediately objected to such a solution that its admission would involve the consequence that the comet, with a period so short, and a magnitude and splen- dour such as it exhibited in 1770, must have been fre-* quently seen on former returns to perihelion ; whereaa no record of any such appearance was found. To this Lexell replied, by showing that the elements of its orbit, derived from the observations made in 1770^ were such, that at its previous aphelion, in 1767, th& comet must have passed within a distance of the planet Jupiter fifty-eight times less than its distance from the sun ; and that consequently it must then have sustained an attraction from the great mass of that planet more than ^t lexill's comet. 89 three times more energetic than that of the sun ; that coiisequently it was thrown out of the orbit in vvhich it actually moved in 1770 ; that its orbit previously to 1767 was, according to all probability, a parabola ; and^ in fine, that consequently moving in an elliptic orbit from 1767 to 1770, and having the periodicity consequent on such motion, it nevertheless moved only for tiie first time in its new orbit, and had never oome within the sphere of the Sun's attraction before this epoch. Lexell further stated, that since tlie comet passed through its aphelion which nearly intersected Jupiter's orbit at intervals of o^ years, and it encountered the planet near that point in 1767, the period of the planet being somewhat above 11 years, the planet after a single revolution and the comet afler two revolutions must necessarily again encounter each other in 1779 ; and, that since the orbiti was such that the comet must in 1779 puss at a distance from Jupiter 500 times less than its distance from the sun, it must suffer from that planet an action 250 times greater than the sun's attraction, and that therefore it would in all probability be again thrown into a parabolic or hyperbolic path ; and, if ^o, th^t it wo'ild depart for ever from our system to visit other spb<;rfcd of attraction. Lexell, there- fore, anticipated the final disappearance of the comet, which actually took place. In the interval between 1770 and 1 779, the comet returned once to perihelion ; but its position was such that it was above the horizon only during the day, and could not in the actual state of science be observed." (3037) '^ At this epoch analytical science had not yet supplied a definite solution of the problem of cometary disturbances. At a later period the question was assumed rnm^ K^ mm^^m^m^fmmmmiiilim 40 lbxill's oomkt. ■ U-^ '' by Laplace, who in his celebrated work, the M^canique Cileste, gave the general solution of the following prob- lem : ' The actual orbit of a comet being given, what was its orbit before, and what will be its orbit after being submitted to any given disturbing action of a planet near which it passes ?" (3038.) " Applyinor this to the particular cose of Lexell's comet, and assuming as data the observations recorded in 1770, Luplace showed that before sustaining the dis- turbing action of Jupiter in 1767, the comet must have moved in an ellipse, of which the semi-axis major was 13'293, and consequently that its period, instead of being 5^ yean must have been 48^ years ; and that the eccen- tricity of the orbit was such, that its perihelion distance would be little less than the mean distance of Jupiter, and that consequently it could never have been visible. It followed also, that, after suffering the disturbing action of Jupiter in 1779, the comet passed into an elliptic orbit, whose semi-axis major was 7 "3 ; that its period was con- sequently 29 years, and its eccentricity such that its perihelion distance was more than twice the dis- tance of Mars, and that in such an orbit, it could not become visible." • (3039.) " This investigation has recently been revised by M. Le Verrier (See Mem. Acad, des Sciences, 1847, 1848,) who has shown that the observations of 1770 were not sufficiently definite and accurate to justify conclusions so absolute. He has shown that the orbit of 1770 is sub. ject to an uncertainty, compassed between certain definite limits ; that tracing the consequences of this to the posi- tions of the comet in 1767 and 1779, these positions are subject to still wider limits of uncertainty. Thus he p^^^ IPWP •.,i"W T^ff-; lexcll'8 comet. 41 K shows that compatible with the observations of 1770, the comet might in 1779 pass either considerably outside or considerable inside Jupiter's orbit, or might, as it was supposed to have done, have passed actually within the orbit of his Satellites. He deduces in iine, the following general conclusions: 1. That if the comet had passed within the orbits of the Satellites, it must have fallen down upon the planet and coalesced with it; an incident which he thinks improbable, though not absolutely impossible. 2. The action of Jupiter may have thrown the comet into a parabolic or hyperbolic orbit, in which case it must have departed from our system altogether, never to return except by the consequence ofsome disturbance produced in another sphere of attraction. 3. It may have been thrown into an elliptic orbit, hav- ing a great axis and a long period, and so placed and formed that the comet could never become visible ; a sup- position within which comes the solution of Laplace. 4. It may have had merely its elliptic elements more or less modified by the action of the planet, without losing its character of short periodicity *, a result which M. le Verrier thinks the most probable, and which would render it poasible that this comet may still be identified with some one of the many comets of short period which the -activity and sagacity of observers are every year discover- ing." - r •/!■ ■<■♦ »;•'■' 41 (7)THI lOOINTSIO-OKBlT THIOBT. TABLE III* ^ SYNOPSIS OF THE MOTION OF THE ELLIPTIC COMETS WHICH REVOLVE WITHIN THE ORBIT OF SATURN. DMigiwuun. Enoka. DIcM. r»ys. D« TIco. Brorwn. D. Amit. Cteuoan. Barckhwrdt, liMCll. BUinpliui, Poos. PI«ott. Paten. M"i>n lllntliUCC Karth-l 17M 17M 1770 1819 181» 1788 1848 3. 1148 S.A34« 8.8118 8.1018 8.1488 8.4818 8.0918 9.9887 8.1AM 1.84M 8.1601 4.6496 6.8106 8.196 6.617 7.141 6.469 8.881 6.641 8.488 S.OIA 8.607 4.809 8.618 10,018 18.990 Inollnk- tlon. 13 7 14 11 14 OS 11 nsi 1 M 48 80 87 81 18 86 11 1 88 U 8 1 48 I 14 18 9 1 16 10 41 48 17 tt 00 18 1 14 Tlmt of Parihallon pMHga. 18h. 18 8 11 7 Much 14, 1881. Fabry. 10, 1846. Oct. 17, 1848... Bapt. 1,1844.. ., Fabry. 18, 1846, Jnly8, 1881 16 Jut. 8,1748.... 4 April 16, 1766.. ..18 Auf.l»,1770....11 Nor. 10, 1819.... 6 July 18, 1819.... II Not. 19, 1738.... 18 Jana 1,1846 1 41 88 M 67 48 89 44 88 8 46 80 40 DIreo- tion of motion D D D D D D D D D D D D 3048. " Diagram of the orbits. In Fig. 817, the orbits of these thirteen comets, brought to a common plane, are represented rougu< ^ but in their proper proportions and relative positions, so as to exhibit to the eye their several ellipticities, and the relative directions of their axes. All those bodies, without one exception, revolve in the com- mon direction of the planets." 3049. " It is not alone, however, in the direction of their motions that the orbits of these bodies have an analogy to those of the planets. Their inclinations, witli one exception, are within the limits of those of the plan- ets. Their eccentricities, though incomparably greater than those of the planets, are, as will presently appear, incomparably less than those of all other comets yet dis- covered. Their mean distances and periods (with the exception of the last two in the table), are within the limits of those of the planetoids." * We gire hero only a part of the Ubie ; omitting the elements of the elliptical orbits, calculated on the basis of the cometary theory. The complete tables may be found in Lardtur'* Aitronomy, from which they are quoted. M Fnm Larcbut'* Jttronomjf. P&atbT. Fia.UT. is ;■», f From Lardttn't Ailrono: y. Pun 8. Fio 882. .j4-.i 'if.- >>^ p^^w mum. mm THI lOOBNTRIO-OBBIT THEORT. 43 TABLE v.* SYNOPSIS OF THE MOTION OF '^HE ELLIPTIC COMETS WHOSE MEAN DISTANCES ARE NEARLY EQUAL TO THAT OF URANUS. Mean |S(N Inrllna- Time of Dine- Sealgnatlon. distance tion. perihelion tion of Earth'=l . " passage. II. m. motion. Halley. 17.9876 76.680 17 44 S Nov. IS. 1888... 33 41 R Pom. 1813 17.09M 70.068 78 87 8 Sept. 18,1813... 7 41 D Ulben. ISIS 17.68S8 74. OM 44 39 88 April 38, 1818... 3buch8, 1846... 38 88 D De Ttco. 184« 14.SS86 7.1.280 84 87 18 14 1 D Bronen. 1847 17.779S 74.870 1» 38 Sept. 9,1847.... 18 11 D WMtpbal. 18«2 IS.MOfl 67.770 40 88 S3 Oct. 13, 1883.... IS 6 D Diagram of the orbits. " In Fig. 820, is presented apian of their orbitsbrought upon a common plane, and drawn according to the scale indicated. This figure shows, in a manner sufficiently- exact for the purposes of illustration, the relative magni- tudes and forms of the six orbits, as well as .he directions of their several axes with relationto that of the first point of Aries." • Sm Note to page 42. I ' , .- _^ . ■.i ■'A 'I. I , :.- , ; - ■ ;■. ■ ^ . ">r 1 -.'J . ^^■^' ^TT"**T^'^W»WW^^^^^"^^^^^' 1 iii.jiBi. i| III! imvfB^i^^v^ift^^p^^ppvTffPn^ipM 44 THE ECCENTRIC-ORBIT THEORT. TABLE VI.* SYNOPSIS OF THE MOTIONS OF THE ELLIPTIC COMETS WHOSE MEAN DISTANCES EXCEED THE LIMITS OF THE SOLAR SYSTEM. Detlg nation. 1 1680 2 I6S3 3 1763 4 1760 6 1780 6 ]7»3 7 1807 8 1811 9 1811 10 182-2 11 182« 12 1827 13 1830 14 1840 15 1840 16 184» 17 1844 18 184« 19 18UI 20 1846 21 1848 Mean Distance Earth Neptnne =1. =1 427 as 217 168 1787 06 143 211 91. 809 267 189. 1KI6 S77 49. 06. 213S 89, 1»4. M. 164. 6487 0»I0 .4074 4S75 9200 2000 8i62 022A S088 JSOOO .9440 6187 .0000 1099 1100 0000 0000 7600 8000 4192 2000 t 4.2.547 I.IOIO 7. 2469 5.4488 fi9.a973 1.9000 4.79fi2 7.0340 8.0S03 10.8200 8.9314 6.8206 01.2000 19.2370 1.6370 18.6000 71.2600 1.3200 64 9000 1.8139 0.4733 Period in Years. Inclina- tion. 8813 190 .32ai 2089 7S838 421 1725 soo.; 870 0444 4:^80 2U11 60200 13864 344 376 100000 201 3719 401 8375! 60 40 16 88 47 46 72 34 10 40 40 50 04 23 12 51 31 10 63 10 20 73 2 21 31 17 11 02 39 10 33 82 89 04 4 42 21 16 59 13 20 07 57 23 30 41 9 48 36 1 48 41 08 47 26 6 29 18 47 66 59 2 Time of perihelion paseage. Dec. 17, 1680 Jnl7l2,1683 Not. 1,1763 Oct. 7, 1769 .Sept. 80, 1780.... Nov. 28,1798.... Sept. 18,1807.... Sept. 13, 1811.... Nov. 10,1811 .... Oct. 23, 1822 Dec. 10,1820,.... Sept. 11,1837.... April 9,1830 March 13, 1840. . . Nov. 13, 1840 ... Febi}.37, 1813.. Oct. 17, 1844 June 0,1840 Jon. 33, 1846 June0, 1846 JnneS, 1839 h. m. 33 00 17 80 31 4 10 3 23 33 5 6 17 03 6 30 28 06 18 38 16 31 16 47 7 10 88 06 IS 87 9 47 8 10 16 10 3 10 13 80 4 10 Direction of motion. D R D D R D D H D K R R D R D R R R D R D t There is a want of strict agreement between this column and that of Earth = 1. For instance, in No. 2... the distance of Earth = 1 : distance, of Neptune = 1 (nearly) : : 33 : 1. In the average of the other numbers it is, more nearly, : : 31 : 1. (3072.) " The distance to which the comet of 1680 recedes in its aphelion is 38^ times greater than that of Neptune. The apparent diameter of the sun seen from that distance would be 3", and the intensity of its light and heat would be 730,000 times less than at the Earth,, while their intensity at the perihelion distance would be 26,000 times greater, so that the light and heat received by the comet in its aphelion would be 26,000 x 730,000^ = 18,980 million times less than in perihelion. The greatest aphelion distances in the table are those of Nos. 5, 13, and 17, the comets of 1780, 1830 and 1844, amounting to from 100 to 140 times the distance of Neptune ; the eccentricities differing from nnity by less than Tb'ffts- These orbits, though strictly the results of calculation, must be regarded as subject to considerable- uncertainty." * See Note to page 42. > m^ ^SlM,iif'l\».-*^.-^t.^.<^ ,.,-,, ,^, THE ECCBNTRIC-OBBIT THBOBT. 4^ "3073. Plan of Vie form and relative magnitude of the orbits. — To convey an idea of the fonn of the orbits of the comets of this group, and of the proportion which their magnitude bears to the dimensions of the solar system, we have drawn, in Fig. 821, an Ellipse, which may be considered as representing tie form of the orbits of the comets Nos. 15, 6, 9, 12 and 1 of the Table VI. If the Ellipse represent the orbit of the comet No. 16, the circle a, will represent on the same scale the orbit of Neptune. If the Ellipse represent the orbit of the comet No. 6, the circle b, will represent the orbit of Neptune. If the Ellipse represent the orbit of No. 9, the circle c, will represent the orbit of Neptune. If the Ellipse represent the orbit of No. 12, tlie circle df will represent the orbit of Neptune. If the Ellipse represent the orbit of No. 1, the circle e, will represent the orbit of Neptune." TABLE VII.» SYNOPSIS OF THE MOTIONS OF THE HYPERBOLIC COMETS. Time at periheUon Iluwl8,173S... April 1», 1771,.. 8 ▲ngiut It. 1774. -Dte.«,181«..... Stirt. St.lSM... Jmi.4, 1840 7p»y 8,1848 6 1» i 16 iO fi ee 1 34 10 88 1 80 PeriheUon dUtonce £atth-l 4.0488 O.DOSS 1.4339 •.aiAO I.OfiOO 0.6184 0.6IU8 Inclination. • 77 » 10 II 16 1» 8S 30 311 63 34 M 86 fi» M S 83 a3 44 46 Dirertion of motion. O 1» D R D U D .v^'t •Be« Note to pace 43. ?3 ip,Pf!ii, J.|iii!l|jppilii I „ , I4«iiij,i^i|,i«ii|if,ipi, ■''i^Ji^ff^PiPpiPiPiipi 46 THI lOCINTRIC-ORBIT THKORT. TABLE VIII. or OTHER OBSERVED COMETS. 1 9 I 4 • a T 8 » 10 11 1> u 14 If U IT 11 1* 90 91 99 « 94 31 90 s 21 t in 1877. llctober 3tl 188U, Nnrember 38.. 18A3. M:iy « I88S, Uccoher 8 ..... 1890, Febiaary 8. . . . 169.1. July 18 , 1496, July 3.'> 1618. Augnat 17.. .., 1618, November 8... 16.Vi, November 13 . 166l,J»naary 36..., 1664, December 4... 1668, April 34 1668, February 34 . . 1668. Febrnary 38 .. 1673, March 1 1677,May6 1678, Auguat 36. . . 1684, Juue 8 1686, Heptamber 16. 1689, December I . . . I6».'i, November 9.. 1698, October 18.... 1699, January 18. . . 1701, October 17.... 1703, March 18.... 1706, January 80. . , h, m. O. 0. 0. 19.13 4.48 3.34 38.81 14.81 11.81 6.39 6.48 18.33 33.81 Perihelon dis- tance Barth>l. rery nnall 1. 01 0. 0. 80 88 0. 730 0. 873 0. 841 0. 883 0. 889 0. 9«8 0. 608 0. 680 8.80 0. 883 38.61 0. 668 0. 0. 730 0. 0. 938 91.37 0. 788 7.13 0. 948 38.81 0. 480 7.39 0. 818 38.81 0. 640 19.13 0. 9.17 38.81 1. 000 4.81 0. 4.W 0.00 0. 968 6.14 0. 774 4.34 0. 839 16.48 3. 108 9.80 0. 888 6.18 0. 839 11.17 0. 788 16.83 0. 886 33.18 0.8091 31.11 0.8369 19.81 0.3038 o.:u O.8O40 19.38 0.8778 33.38 0.1776 P. 48 0.8888 16 00 0.3367 9.JI 1 0984 0.89 0.8877 18.39 0.0891 6. 9 0.8673 3. 3 8130 8.2.1 8896 18.41 0.8476 31. 9 4437 11.63 1.0388 8.16 0.1068 18.46 0.3811 19.13 0.0048 8.88 6974 0.88 0.3806 14. 4 1.3880 10.17 0.9603 14.84 0.3980 14.86 0.0169 16.81 0.8488 16.88 0.0018 8.33 7940 9.81 0..'>936 14.88 0.6468 4.38 0.4388 Inclination o < « 30 30 70 10 40 17 44 10 89 4 43 69 10 to 13 79 17 17 38 73 80 31 31 88 0' 8 I) 31 6 83 18 77 14 30 SO 44 19 1 88 81 87 48 1 )>3 86 38 14 88 48 30 13 13 78 39 76 8 43 64 83 61 38 8 « 4 39 39 44 87 88 61 88 10 31 38 87 11 81 79 38 88 88 31 18 30 76 8 37 7 38 80 ^ 88 83 10 79 8 16 8 4 30 68 48 40 81 91 40 69 17 33 11 46 69 90 41 89 4 34 44 86 14 10 Direction of motion. R R D R R R D D R D D R R R R D D D D R R R D R R R R D R R D R D D? RT D R R D R D D D D D R R D? R7 D R D D D R D R R R D D ^PW m^^mm^ bi THl 1031NTRIC-ORBIT TRCORT. TABLE Vlll.—ContinucfJ. 47 Tim* of Perihelion IMHima. 64 M a« •7 «8 «» .70 .71 •.7a 78 74 70 76 77 78 79 «0 81 82 8» 84 «A 8« 87 88 88 90 91 83 8.S «4 8S m 87 98 W 100 101 102 10.1 I0< lOA 104 107 108 108 110 III 113 113 114 lis 116 117 118 119 180 131 133 138 134 13S 136 137 138 139 110 181 lis 17l8,JMinary 14.... 1738, September 37.. 1738,Jnnel3 1787, Janiwrr 30. . . . 1737, June 8 1788,Jnne17 1713, Febrnary 8.... 1743,JuinBry 10 1743, September 30. . l744.March 1 1746, febroary IS.. 1747, March SI 1748, April 38 1748, Jane 18 1757, October 31 l7S8,Jnnell 1758, November 37. . 17S8, December 16 . . 1763, Hay 28 17U4, Febmary 13... 17««, Febmary 17 . , 1770. November 23. , 1778, September S... 1779, Jannary 4.. .. 1780, November 38.. 1781, July 7 1781, November 39. 1784, January 21... 1 785, Jannary 37... l78S,April8 178«,Jniy 7 1787, May in 1788, November 10. 1788, November 30. 1790, Jannary IS... 1780, Jannary 38. . . 178U,May31 1783, Jannarj- 13. . . 1783, December 27 . 1783, November 4.. 1786, April 3 17»7,Jnly» 1788, April 4 1788, Dec. 31 1788, September 7 .. 1789, December 33 . 1801, Aognft8 1803, September 9.. 1804, Febmary 13 .. 1806, December 38.. 1808, May 13 1808,Jnlyl3 1810, October 8 1818, March 4 1818, May 19 1816, March 1 1818, Febmary 38.. 1818, December 4.. 1819,Jnna37 1881, Match 31 1833, Mays 1883, July 16 1838, December 9 ... 1814, Jnly 11 1638, May SO 1I8«, AngnttlS.... 1(36, April 31 1836, April 39 1830, October 8 h. m. Perihvlion div tanre Earth^I. 31.42 Ifl. 4 17. SI M.2I 7.:m 10 IH) 4.au 311.20 21.17 8.17 0. 7.11 18.44 21.18 7.SS 3.18 2. ID 31 4 8. 2 13 42 S.4' ff.:m 1 .S4 2. 4 20.31 4.32 12.33 4 47 7.4» 8.S!I 21. SI iu.4a 7.2S -.1« S. «i 7 :iti 3 47 13.3S «. 8 20.12 19.48 a. 31 11.32 13.17 fi.3» 2i.:ii 13.33 21.23 IS. 31 22.21 33. S2 S.16 19.4S 12.38 10. » 8 18 23 1 32 36 17 11 13 S3 14 33 12 4S 10 39 12 9 13 9 17 4 33 i 86 32 31 1 0234 9988 4 0431 2228 81170 n tl73)l 7637 8382 S2I6 3221 96 3 1988 84m n2.vt (r 3373 21.34 7lt83 n iiwio 1 (HWH 8.162 3033 8282 1 1269 7132 3182 7738 t) imio 7079 1 1434 4273 4101 n 3489 1 00.10 7373 7.381 1 0633 7980 1 2M:iO 9063 4034 1 3782 S2K6 4848 779S 8399 6238 2017 1 0941 1 0723 1 0816 )8»9 6073 9)131 H!)91 1 2101 0488 1 1978 8331 3410 0918 8044 8367 2268 8913 8891 »»U 3 0111 1881 8334 Inclination 3 Direction at Motion. »1 a 6 80 18 77 8 18 18 20 48 39 14 8 88 42 44 M .39 14 2 16 16 43 48 21 47 8 .36 6 79 6 20 83 28 23 67 3 28 12 80 20 68 19 no 78 89 22 4 81 32 83 :18 13 82 S3 31 40 80 20 31 28 88 61 14 17 32 30 87 72 3 30 81 43 26 27 13 8 81 9 12 70 14 12 87 31 .34 60 84 28 48 13 31 12 27 40 64 30 24 81 .34 13 86 88 13 63 82 27 3» 46 38 48 1 48 60 31 00 64 34 23 30 40 34 43 32 16 42 26 4 80 :,6 27 77 1 38 21 20 37 47 86 84 20 .33 2 SO 48 43 7 M> 17 24 62 46 17 21 13 33 81 2 28 43 S 26 89 43 48 63 8 2» 80 44 44 73 83 7 S3 37 24 38 12 39 76 II 87 84 34 1» 86 41 6 89 41 47 40 2 33 5 17 2 23 37 18 R R D D U R R D R D D R R U D D D R D R R R D D R D R R D R D R R D R n R R R R R R D R R U R D D R It U U R R D D R D R R R R R R O D R D * 1 IfP'IfPW' mtm ^T -P^PJH 'I'' '■iJll'l -'Wl^iJiiPl 48 THC XCCENTBIC-OBBIT THIORT. TABLE VIII.— CoN^mugrf. t i-te l»tl la? i:iK nil 14(1 141 14'J I4:i 144 14f> 14)1 Time of Perihelion pauage. Ifllli, 18i7, ISi7, IKiO. IM8'J, IXilK, I8H4, IS:IA. 1840, 1X44. IH4A. 1845, 1846, 147I184II, 1481847, 14!) 1847. 1W|847, lAl 1847, ImIi847, 18,'l 1848, lA4llg4», ISS 184», Ifi6 I8.'>0, u,vm>». Ift8,l8fil, IM'IMl, iBolia^K. Xuvcnibcr 18, . .. Febniary 4 June 7 December 27 September 3S. . . . Septcnilier 10. ,.. April 2 March 'i* April 2 Ueceiiiber ii ... December la ... . JaimarjrH April 21 M.iy37 October 29 March HO Jnne 4 Augstit 9 Augrnet 9 .iJoveuiber 14.... 8epU'iubcr8 Januar}' 19 May 28 July 2a.. October 19 Aagost2« September 30.... April 19 b, m. !) M 22 7 20 9 lA Al 12 .'il 4 28 1« i 1.1 .'.0 13 M 22 "8 It! 211 a 44 tl 45 21 .'.7 17 M « 28 1« :u 8 17 10 :i7 4 14 1 6 8 Vl 11 •■4 12 28 8 10 7 20 10 12 l:i A3 Perihelion dls- tuuoo £arth-l. 0209 &(Ni5 8l»l 12o8 1 I8:;(i 4AH4 il50 2 III 43 7421 AIU4 U 2Alil 1 JHI.VJ 1 2.'. 17 1 27U:l 8ao 77 115 S.i *H an 45 44 45 30 4a U » 7 21 3 S 56 A3 9 7 :i» 75 51 34 7a 34 4 45 36 31 46 SO ;i'i 56 23 311 57 35 50 49 41 17 48 39 49 79 33 4.1 33 88 47 83 27 1 72 lU 51 84 34 50 85 2 54 «7 9 39 68 12 8 40 5 37 37 43 57 73 50 44 48 53 54 R H R R R D D & D R D D D R I) 1) R R R R R D U I) D D D R We have given these last tables us affording a sort of classified record of the observed comets ; and, inasmuch as they embody the results of certain more or less careful observations of those bodies, they have undoubtedly some value. i i" irf'JS Iffw^ippf mn^mm ^^^^^m^m^^^nmm 4 I THI KCCBNTRIO-ORBIT TIIKOBr. 49 In respect to table VI. in particular, we must again q-einark the wildly improbable statements which the stu- dent of astronomical science is culled upon to accept as fact. In the very extreme cases of this table, Dr. Lard- ner cautions the reader (art. 3072, quoted page 25) that the results must be regarded as subject to uncertainty, but this very caution may be taken to mean that in the less extreme cases there is no uncertainty ; consequently the stud "^t is to understand that a comet may recede to a distance from the sun many times greater than the dis- tance of the planet Neptune, and then be brought back again by the direct influence of solar gravitation after the lapse of some hundreds or, perhaps, even, of several thou- sands of years. The immediate corolliiry to these state- ments would be tliat ' gravitation ' as it is known to us must be confined to the sun and the members of the solar system; because it is evident that, in receding to such n distance, the comet must enter other f- ' eal systems where, if the law of gravitation was in ope. m, it (the comet) would be subjected to the influenc* of the pri- mary, as well as some of the secondary, centres of gravi- tation pertaining to those systems. It seems almost impos- sible to avoid the inference that it would become itself a planet, if not the satellite of a planet, belonging to some sidereal system or other ; not to speak of the numerous perturbations to which, in such a prolonged and uncon- trolled journey, it would be certainly subjected. The following statement from Dr. Lardner's Astronomy together with the accompanying diagram illustrating it, will serve to define the present doctrine of cometary or- bits and to show that we have not exaggerated the extra- vagant characters of the hypotheses by means of which iM. * ^:>" 50 THE E(CB JlRIC-iHUIT THKORT, it huabeen^'oug it to reconcile timt iloctriiie with the law* of the material wurld. "Subject to thfse limita- ItionB, however, a body majr move round the sun in anjr orbit, at any distaiicf, in any plane, and in any direction whatever. It may describe an ellipse of any eccentricity^ from a perfect circle to the most elongated oval. This eUipse may be in any plane, from that of the ecliiitic ta one at right angles to it, and the body may move in such ellipse, either in the same di- rection as the earth or in the contrary direction. Or the body thus subject to sulur at- tractions may move in a para- bola with its point of perihe- lion at any distance whatever from the sun, either grazing its very surface or sweeping beyond the orbit of Neptune, or, in fine, it niuy sweep round the sun in an hyperbo'.u entering and leaving the system in two divergent lirections. To render these explanations, which are of the greatest interest and importance in relation to the subject of com- ets, more clearly understood, we have represented in Fig. 816, the forms of a very eccentric ellipse a h a'h\ a para- bola a j>j/ and a hyperbola a h h' havings as their com- mon focus." • .lafe'- . ,,., . \ , THE COMCI 8 LUMINOUS TBAIN. u (8.) The ComeVs Luminaus Train. The luminous character of the cornet, atul the peculiar appearance and characteristics of the lutuinous train or tail, have yet to be considered. Illustrations of these appearances will be found in plates 6, 9, 10, 11 and 13. In thegeneral description of a comet already given, the most usual appearance of the tail is defined in our <|iiuta- tion (page 1 1) from art. 550, of HersshcVs Outlines, which art. continues thus ■ '' This magnificent appendage attains occasionally an immense length. Aristotle relates of the tail of the comet of 371 u. c, that it occupied a third of the hemisphere, or 00° ; that of a. d. 1018 is stated to have been attended by a train no less than 1U4° in length. The comet of 10^0, the most celebrated of modern times, and on many accounts the most remarkable of all, with a head not exceeding in brightness a star of the second magnitude, covered with its tail an extent of more than 70° of the heavens, or, assome accdunts state, 90° ; tinit of the comet of 1709 extended 97°, and that of the lust great comet [1843] was estimated at about 05° when longest. The figure (plate 11) is a representation of the comet of 1819 — by no means one of the most considerable, but which was, however, very conspicuous to the nuked eye.'^ In some instances there are several streams of light diverging from the head as in that of the comet of 1744,* which "had no less than six, spread out like an immense fan, extending to a distance of nearly 30° in length." And in some cases (very frequently) the comet is, as already stated, without any luminous train or tail." t A circum- • See Plate 13. (t) It bag been aUo noticed tbat " the tails of comets are often somewhat curred ; bending, in general, towards the regions which the comet has left, as if moving somewhat more slowly, or as if resisted in their course." V-l .Mti.msx^ «9 THK COMST'S LCMINOCS TRAIN. a {itance which constitutes itself a difficulty in the way of all such hypotheses as have been suggested to explain the luminous cometary characteristics,— or, perhaps it would be more, correct to say, which at once negatives thosti hypotheses, — is thus described (also in the words of Sir John Herschel) : " Since it is an observed fact that even those larger comets which have presented the appear- ance of a nucleus have yet exhibited no j)A<»<;5, though we cannot doubt that they shine by the reflected solar light, it follows that even these can only be regarded as great mosses of thin vapor susceptible of being penetrated through their whole substance by the sunbeams, and reflecting them alii^e from their interior parts and from their surfaces. Nor will any one regard this explanation as forced, or feel disposed to resort to a phosphorescent quality in the comet itself, to account for the phenomena in quu8cion, when we consider the enormous magnitude of the space thus illuminated, and the extremely small mass which there is ground to attribute to these bodies." In order to give a satisfactory explanation of the appearnncus thus presented by the comet itself and by the luminous train, which is attached to or accompanies the comet,- - an explanation, that is, in harmony with PT)d supported by the observed facts, and by those law» knoc/n to govern and regulate the material world, - - it is necessary in the first place, to have a definite un- derstanding as to the distinction between a luminous and non- luminous body, and the essential difference between a body which is luminiferous in the sense of emitting, and a body which is luminous in the sense of reflecting. It will be therefore necessary to vapke a brief but general investigation into the source and nature of that which h I?- ■ FROM DICKS SIDEBIAL HEAVENS. "I'la-^ IL'. TBS comet's luminous TRAIN. 53 directly causes the recogni2i36 ; and which description applies to tiie illustrations in Plates 9 and IC. (9) Concluding Remarks. —In treatises on astronomy the student is instructed that the planets belonging to the solar system revolve in orbits which are not circles but ellipses. This circumstance is also usually inculcated and insisted upon as one of the established facts belong- ing to theoretical astronomy, of the greatest im^jortance. To such instruction in itself we do not object ; but is the instruction carefully defined I and what is the result, as it is usually imparted, on the mind of the student f We are under the impression that very many persons having some knowledge of astronomy, and, perhaps pos- sessing even a not inconsiderable knowledge of astrono- mical science, would be astonished and almost shocked if informed suddenly that the above circumstance is true only in a strictly exact (mathematical) sense, and that, speaking in ordinary language to a person not expressly cautioned, the most correct (true) information will be conveyed by stating that the planets revolve in circles and not in ellipses. The actual deviation from a true circle is perfectly well known and correctly taught in all sound works on as- tronomy, nevertheless, illustrations are constantly put before the student in which the very small actual ellip- ticity is enormously exaggerated without a word of cau- tion that it is so. 54 THE COMET S LUMINOUS TRAIN. ! The following statement, in the words of Dr. Lardner, distinctly defines the actual amount of deviation from a circle. " The orbits of the planets generally are ellipses, but having eccentricities so small that, if describe^^ on a large scale in their proper proportions on paper, they could be distinguisliable from circles only by measuring accurately the dimensions taken in different directions, and thus as- certaing that they are longer in a certain direction than in {inother at right angles to it." In concluding our present examination of this subject. we will leave in the hands of the reader the two follow- ing questions which pertinently suggest themselves ia this connexion. (1) ... Is there any reasonable ground whatever for be- lieving or supposing that a comet differs in its nature from a planet, to wit : that, a planet being a mass of aggregated matter, a comet differs therefrom in kind, and is immaterial f (3) . . . Since a planet, subjected to per- turbing and interfering causes, is unable in any case to deviate from a circle more thi n to a very slight and scarcely appreciable extent, how are we to accept a pro- position that a comet, if it be a body of the same nature and subject to the same laws, may choose its orbit in the most capricious manner out of several figures, and not only deviate immensely from a circle but may, if it plea«e, suddenly discard altogether the influence and con- trol of the sun, and depart from the 'n,\,xr tyi»t'?m in a direct line f i APPENDIX. See Plates 9 and 10. '< Although the appearance of this celebrated comet at its last apparition was not such as might be reasonably considered likely to excite lively sen- sations of terror, even in superstitious ages, yet, having been an object of the most diligent attention, in all parts of the world to astronomers, furnished with telescopes very far surpassing in power those which had been applied to it at its appearance in 1769, and indeed toany of the greater comets on record, the opportunity thus afforded for study- ing its physical structure, and the extraordinary pheno- mena which it presented when so examined have rendered this a memorable epoch in cometic history. Its first ap- pearance, while yet very remote from the sun, was that of a small round or somewhat oval nebula, quite destitute of tail, and having a minute point of more concentrated light within it. It was not ijefore the 2nd of October that the tail began to be developed, and thenceforward increased pretty ropidly, being already 4** or 5" long on the dth. It attained its greatest apparent length (about 20°) on the 15th of October. From that time, though not yet arrived as its perihelion, it decreased with such rapidity, that already on the 29th it was only 3", and on November the 0th, 2i° in length. Tijere is every reason to believe that before the perihelion, the tail bad altogether disappeared, as, though it continued to be observed at Pulkowa up to th29 2.0ct-.3.30ct29. IMAGE EVALUATION TEST TARGET (MT-3) 1.0 UiW22 ■50 *^" ■^ ^ 12.2 2f |i4 ""l™ 1.1 i.-^l^ 1 L25 1 U ||i.6 ^ 6" ' ► Sciences Carporalion 23 WIST MAIN STRUT WIISTIR,N.Y. USSO (71*) 173-4903 4^ ;\ I L HALLEYS comet departing JROM the sun in I8:4(] Tlate 10. 1.Fe"b.7. 2..¥eb.10.3Jek1G 4Pe>» 23. ^s^^enman^M APPENDIX. 59 tion. By degrees this also faded, and the last appearance presented by the comet was that which it offered in its first appearance in August ; viz : that of a small round nebula with a bright point in or near the centre." Plate 14. (Frontispiece) Encyclopedia Britannica. — '' Fig. 106 is a representation of the celebrated comet of 1680, taken from Lemonnier's Hisioire CSleste. It exhibits the nucleus or disk with its surrounding atmosphere. Above is a sort of ring, wider at the summit and nar- rower towards the sides. A coma or beard succeeds the ring ; and lastly, an immense train of luminous matter, somewhat less vivid than the nucleus. This luminous train, or tail as it is called, is by far the most singular and striking figure presented by the comets. That of tha comet of 1744. was one of the most remarkable. It was divided into six branches all diverging, but curved in the same direction; and between the branches the stars were visible." It is represented in Figure 107, Plate 14. , :l VHP .!'!■■ ■■>''■< ':u.r(i/ iJ i:- «■' < Irom DicVt Siderial Mtaeetu, Flats U. s ■ 1 . N From Diek'n Siderial neareni. Platb 18. % I 'i From Diek'i Side rial Htav*ni. PlATI 14. tJNCKKS COMRT, IS28 Tlr?*-. r 1"N"ov.7. ,?lTov.>'^G