36 



COMETS. 



lion of volume according to the distance from 

 the sun is particularly striking. The physical 

 explanation of the phenomenon cannot well be 

 sought for in any increased density of the lay- 

 ers of the world-ether at distances progressive- 

 ly nearer the sun ; for it is difficult to conceive 

 the vaporous envelope of the comet's nucleus 

 as vesicular, and impenetrable to the ether that 

 fills the universe("). 



The very dissimilar excentricities in the el- 

 liptical orbits of comets has led in recent times 

 (1819) to brilliant additions to our knowledge 

 of the solar system. Encke made the discov- 

 ery of a comet of so short a period that it always 

 remains within the limits of our planetary or- 

 bits ; he found that the place of its aphelion or 

 greatest distance from the sun lay between the 

 orbits of the telescopic planets and that of Ju- 

 piter. The excentricity of this comet's orbit is 

 0-845, that of Juno (the greatest excentricity 

 among all the planetary orbits) being 255. 

 Encke's comet has repeatedly been seen with 

 the naked eye, although it is not easily discov- 

 ered ; it was seen, however, in Europe in 1819, 

 and, according to Riimker, in New Holland in 

 1823. The period of this comet is nearly 3| 

 years ; but from careful comparisons of the 

 times of its return to the perihelion, the remark- 

 able fact has been discovered that its periods 

 from 1786 to 1838 have been going on regularly 

 contracting from revolution to revolution, viz., 

 in the course of 52 years, by one and f ^ths of 

 a day. So remarkable a circumstance has led 

 to the admission of the very probable existence 

 of a vaporiform matter diffused in planetary 

 space, and capable of opposing a certain resist- 

 ance to bodies in motion through it. Some- 

 thing of the kind, indeed, seems necessary in 

 order to bring the most careful consideration of 

 every source of planetary perturbation into har- 

 mony with the results of observation and calcu- 

 lation. The tangential force is diminished, and 

 vifith it the greater axis of the cometary orbit. 

 The value of the constant of resistance appears, 

 moreover, to be somewhat different before and 

 after the passage of the perihelion, which is 

 perhaps to be ascribed to the altered form of 

 the small nucleus, and to the effect of inequality 

 in density of the layers of ether in the sun's 

 vicinity(*^). These facts, and their explanation, 

 must be reckoned among the number of the 

 most interesting results of modern astronomy. 

 And then, if Encke's comet led us at an earlier 

 period to subject the mass of Jupiter — always 

 so important in every reckoning of perturbation 

 — to a closer scrutiny, its course has subse- 

 quently obtained for us the first, although mere- 

 ly approximative, determination of an inferior 

 mass for Mercury. 



Th« first comet of short period, namely, 

 Encke's, of 3^ years, was followed, in 1826, by 

 another planetary one, the aphelion of which 

 lies beyond the orbit of Jupiter, but much within 

 that of Saturn. This, or Biela's comet, com- 

 pletes its revolution in 6| years. Its light is 

 still more feeble than that of Encke's comet. 

 The motion of both these comets is direct, 

 whilst that of Halley's is retrograde — contrary 

 to the motion of the planets properly so called. 



Biela's comet presents the first certain in- 

 stance of the orbit of a comet intersecting that 

 of the Earth ; its path is, therefore, one of pos- 



sible danger — if we can regard as dangero»ir a 

 phenomenon which has not been observed within 

 the historical period, and of which the conse- 

 quences are doubtful. Small masses, possess- 

 ed of enormous velocities, may certainly exei- 

 cise a notable force ; but, though Laplace de- 

 monstrated the mass of the comet 'of 1770 tf> 

 be less than the l-5000th of that of the Earth, 

 he supposes, with a certain degree of probabil- 

 ity, that the average masses of the comets are 

 much below the one hundred thousandth part 

 of the Earth's (about the l-1200th of the moon's; 

 mass("). We must not confound the passage 

 of Biela's comet through our earth's orbit, with 

 its proximity to, or absolute encounter with the 

 Earth itself When this passage took place on 

 October 29th, 1832, the Earth was still a full 

 month off from the point of intersection of the 

 two orbits. 



The orbits of these two comets of short pe- 

 riod mutually intersect each other ; and it ha.s 

 been correctly observedC'*), that owing to the nu- 

 merous perturbations which such small celestial 

 bodies suffer from the planets, it is not impos- 

 sible for them to encounter, and that, should 

 this occur about the middle of the month oi 

 October, the inhabitants of the Earth might be- 

 hold the extraordinary spectacle of a cosmicat 

 combat ; in other words, of the mutual pene- 

 tration of two comets, of their agglutination, or 

 of their destruction, in consequence of exhaust- 

 ive emanations. The immense ethereal ex- 

 panse may have witnessed during millions of 

 years several events of this kind, consequences 

 of deviations produced by perturbing masses, 

 or of originally intersecting orbits ; still the) 

 are insulated phenomena, having as little gen- 

 eral influence in modifying the form or state ol 

 the universe, as the appearance* or extinction 

 of a volcano it. the limited sphere of the Earth. 



A third planetary comet of short period was 

 discovered by Fayt on November 22d, 1843, at 

 the Paris Observatorj Its elliptical orbit ap- 

 proximates more nearly Va a circle than that of 

 any other known comet, &nd is included be- 

 tween the paths of Mars and Saturn. Tiiis 

 comet (which Goldschmidt say& stretches be- 

 yond the orbit of Jupiter), is therefore one of 

 the few known which has its perihelion neyond 

 the orbit of Mars. It accomplishes its revolu- 

 tion in 7-29 years, and probably owes the pres- 

 ent form of its orbit to its great proximity to 

 Jupiter at the close of 1839. 



When we consider comets in their closed 

 elliptical orbits as members of our solar system, 

 with reference to their major axes, their ex- 

 centricities, and their periods of revolution, it 

 seems probable that in the last particular the 

 three planetary comets (Encke's, Biela's, and 

 Faye's) are immediately succeeded by Messier's 

 of 1766 (supposed by Clausen to be identical 

 with the third comet of 1819), and by the fourth 

 of 1819, discovered by Blanpain, which Clausen 

 considers identical with that of 1743, but which, 

 as well as Lexell's, has suffered great orbital 

 changes from the proximity and attraction of 

 Jupiter. These two comets appear to have a 

 period of from five to six years, and their aphe- 

 iia fall in the neighbourhood of the orbit of Ju- 

 piter. From 70 to 76 years are occupied in 

 their revolutions, by Halley's comet (so impor- 

 tant in a theoretical point of view, of which 



