COMETS. 



observed and unobserved, which constantly traverse the solar 

 system in all conceivable directions ; notwithstanding the per- 

 manent revolution of the periodic comets, whose presence and 

 orbits have been ascertained ; notwithstanding the frequent visits 

 of comets, which so thoroughly penetrate the system as almost to 

 touch the surface of the un at their perihelion, the motions of 

 the various bodies of the system, great and small, planets major 

 and minor, planetoids and satellites, go on precisely as if no 

 such bodies as the comets approached their neighbourhood. 

 Not the smallest effects of the attraction of such visitors are 

 discoverable. 



Now since, on the other hand, the disturbing effects of the 

 planets upon the comets are strikingly manifest, and since the 

 comets move in elliptic, parabolic, or hyperbolic orbits, of which 

 the sun is the common focus, it is demonstrated that these bodies 

 are composed of ponderable matter, which is subject to all the 

 consequences of the law of gravitation. It cannot, therefore, be 

 doubted that the comets do produce a disturbing action on the 

 planets, although its effects are inappreciable even by the most 

 exact observation. Since, then, the disturbances mutually pro- 

 duced are in the proportion of the disturbing masses, it follows 

 that the masses of the comets must be smaller beyond all calcula- 

 tion than the masses even of the smallest bodies among the planets 

 primary or secondary. 



The volumes of comets in general exceed those of the planets 

 in a proportion nearly as great as that by which the masses of the 

 planets exceed those of the comets. The consequence obviously 

 resulting from this, is that the density of comets is incalculably 

 small. 



Their densities in general are probably thousands of times less 

 than that of the atmosphere in the stratum next the surface of 

 the Earth. 



192 



