270 CELESTIAL DYNAMICS. 



stars is a vacant solitude destitute of matter. We shall 

 leave, however, all suppositions concerning subjects so distant 

 from us both in time and space, and confine our attention ex- 

 clusively to what may be learnt from the observation of the 

 existing state of things. 



Besides the fourteen known planets with their eighteen 

 satellites, a great many other cosmical masses move within 

 the space of the planetary system, of which the comets de- 

 serve to be mentioned first. 



Kepler's celebrated statement that " there are more com- 

 ets in the heavens than fish in the ocean," is founded on the 

 fact that, of all the comets belonging to our solar system, 

 comparatively few can be seen by the inhabitants of the earth, 

 and therefore the not inconsiderable number of actually ob- 

 served comets obliges us, according to the rules of the calcu- 

 lus of probabilities, to assume the existence of a great many 

 more beyond the sphere of our vision. 



Besides planets, satellites, and comets, another class of 

 celestial bodies exists within our solar system. These are 

 masses which, on account of their smallness, may be consid- 

 ered as cosmical atoms, and which Arago has appropriately 

 called asteroids. They, like the planets and the comets, are 

 governed by gravity, and move in elliptical orbits round the 

 sun. When accident brings them into the immediate neigh- 

 bourhood of the earth, they produce the phenomena of shoot- 

 ing-stars and fireballs. 



It has been shown by repeated observation, that on a 

 bright night twenty minutes seldom elapse without a shooting- 

 star being visible to an observer in any situation. At certain 

 times these meteors are observed in astonishingly great num- 

 bers ; during the meteoric shower at Boston, which lasted 

 nine hours, when they were said to fall " crowded together 

 like snow-flakes," they were estimated as at least 240,000. 

 On the whole, the number of asteroids which come near the 

 earth in the space of a year must be computed to be many 



