THE EARTH. 7 



ilants upon some of the greater, are subject to the same laws ; 

 they circulate with the same exactness ; and are, in the same 

 manner, influenced by their respective centres of motion. 



Besides those bodies which make a part of our peculiar 

 system, and which may be said to reside within its great circum- 

 ference, there are others that frequently come among us, from 

 the most distant tracts of space, and that seem lijce dangerous 

 intruders upon the beautiful simplicity of nature. These are 

 Comets, whose appearance was once so terrible to mankind ; 

 and the theory of which is so little understood at present : all 

 we know is, that their number is much greater than that of the 

 planets ; and that, like these, they roll in orbits, in some measure 

 obedient to solar influenc*?.* Astronomers have endeavoured to 



atmosphere of Ceres is 675 English miles high, while that of Pallas rises to 

 the height of 468 miles. Now the height of any of these atmospheres is 

 greater than the united heights of the atmospheres of all the other planet?, 

 and is above a thousand times higher than it ought to have been, according 

 to the ratio which exists between the globes and the atmospheres of all the 

 other bodies of the system. Astronomers were so forcibly struck with the 

 magnitude of these atmospheres, that a dispute arose whether Ceres and 

 Pallas should be called planets or comets, and the discussion terminated, by 

 giving them tlie name of asteroids, a class of bodies which were supposed to 

 partake of the nature both of planets and comets. But to draw this argument 

 still closer upou the subject, let us inquire from what other source these atmo- 

 spheres could be derived, if they were not imparted by the comet of 1770. If 

 the four new planets are the fragments of a larger body, endowed with an 

 extensive atmosphere, each fragment would obviously carry off a portion of 

 atmosphere proportioned to its magnitude ; but two of the fragments, Juno 

 and Vesta, have no atmosphere at all, consequently the atmospheres of Ceres 

 and Pallas could not have been derived from the original planet, but must 

 have been communicated to them at a period posterior to the divergency of 

 the fragments. It would have been a satisfactory addition to the preceding 

 arguments, if we had been able to show, by direct calculation, that Ceres 

 and Pallas were at the same instant with the comet in that part of their or- 

 bits which was crossed by its path, and that the position of the planes of the 

 orbits was such, ls to permit a near approximation. But as we have no data 

 sufficiently correct for such a calculation, we must leave this part of the sub- 

 ject to some future opportunity. There is one fact, however, which in some 

 measure supplies its place, and which is therefore worthy of particular no- 

 tice. The nodes of the comet of 1770, lie exactly between the nodes of Ceres 

 and Pallas, an arrangement which is absolutely indispensable to the truth oi. 

 the preceding theory." 



* "When examined through a good telescope, a comet resembles a mass of 

 •queous vapours encircling an opaque nucleus of different degrees of dark, 

 ness in different comets, though sometimes, as in the case of several dis. 

 covered by Dr Herschel, no nucleus can be Been. As the comet edvaucea 



