38S Dr. J. R. Mayer on Celestial Dynamics. 



the regions governed by the other fixed stars is a vacant soli- 

 tude destitute of matter. We shall leave, however, all supposi- 

 tions concerning subjects so distant from us both in time and 

 space, and confine our attention exclusively to what may be 

 learnt from the observation of the existing state of things. 



Besides the fourteen known planets with their eighteen satel- 

 lites, a great many other cosmical masses move within the space 

 of the planetary system, of which the comets deserve to be men- 

 tioned first. 



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

 fore the not inconsiderable number of actually observed comets 

 obliges us, according to the rules of the calculus of probabi- 

 lities, 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 considered 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 neighbourhood of the earth, they pro- 

 duce the phenomena of shooting-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 numbers; 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 thousands of millions. This, without 

 doubt, is only a small fraction of the number of asteroids that 

 move round the sun, which number, according to the rules of 

 the calculus of probabilities, approaches the infinite. 



As has been already stated, on the existence of a resisting 

 aether it depends whether the celestial bodies, the planets, the 

 comets, and the asteroids move at constant mean distances round 

 the sun, or whether they are constantly approaching that central 

 body. 



Scientific men do not doubt the existence of such an aether. 

 Littrow, amongst others, expresses himself on this point as fol- 

 lows : — "The assumption that the planets and the comets move 

 in an absolute vacuum can in no way be admitted. Even if the 



