232 Proceedings of Philosophical Societies. [MAkcM, 



sphere over which the attractive energy of the sun extends ; and 

 that this motion combined whh the solar attraction might convert 

 these centres into as many new comets, circulating round the sun, 

 and following the same laws as the planets ; that this may have been 

 the origin of all the planets, and doubtless likewise of the sun and 

 stars ; for if we are obliged to admit the anterior existence of these 

 great bodies, we may as well assign the same date to the much less 

 important bodies that circulate round ihem. 



The new orbits will be circular, or elliptic, or parabolic, or hyper- 

 bolic. 



In the first case the comets will be always invisible, unless we 

 suppose, against all probability, that their mass and inherent light 

 are sufficient to render them visible at so great a distance; for these 

 nuclei or centres perceived by Dr. Herschel are invisible to a com- 

 mon telescope. 



If they are eliptical or parabolic, the comets may come so near 

 the sun tiiat. they will become visible in a portion of tiieir orbit 

 comprehended between the perihelion and the parameter, and even 

 a little beyond it. This supposition will explain sufficiently well 

 the phenomena which the comets hitherto observed have presented. 

 Their greater axis ought to go beyond the sphere of the sun's acti- 

 vity, which must extend much fartber than the orbit of Uranus. 

 Such elongated clipsos must be sensibly confounded with parabolas 

 having the same summit. Tlie revolutions of such comets vvill be 

 so long that we can scarcely expect to see them again, or to know 

 them again, after all tlie alterations which they may have expe- 

 rienced in that part of their orbit where we cannot follow them, and 

 in which so many causes may modify their elements. 



This would likewise give a good explanation of the length and 

 tenuity of their tails. The nebulous matter, condensed by attrac- 

 tion to form the comet, being dilated by the solar heat, would 

 resume nearly its primitive tenuity, and may even evaporate and be 

 lost in space. Having lost its tail and its nebulosity, the comet 

 will be exactly similar to the four little planets. It may be even 

 entirely dissipated, and cease to exist. Astronomers would have to 

 regret the time lost in calculating its elements. It would be suffi- 

 cient for them to be in possession of this coarse approximation, 

 which puts it in their power to satisfy the public at an easy rate 

 during the short time that the comet is visible from the earth. 



But some comets have presented particularities inconsistent with 

 this hypothesis. That of Halley, for example, has the great axis 

 of its orbit smaller than that of Uranus. The comet of 1 770 had 

 a great axis less than that of Jupiter. According to M. Laplace 

 these singularities may be produced by the planetary attractions, or 

 the resistance of the ethereal medium ; but the planetary attractions 

 ought to be very weak at the entrance of the comet into the sphere 

 of activity of the sqn ; and Laplace himself has rendered the re- 

 sistance of the ether very problematic, and inconsistent with thp 

 constancy of the great axes of the planetary orbits, 



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