NATURAL PHILOSOPHY. HI 



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. \Vhen accident brings them into the immediate 

 neighborhood of the earth, they produce the phenomena of shooting 

 star's and iireballs. 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 great American meteoric shower, which lasted nine hours, when 

 they were said to fall, "crowded together like snowflakes," 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, ap- 

 proaches infinity. 



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

 depends whether the celestial bodies, the planets, the comets, and the 

 asteroids move at constant mean distances around the sun, or whether 

 they are constantly approaching that central body. Scientific men do 

 not doubt the existence of such an ether. Littrow, amongst others, 

 expresses himself on this point as follows: "The assumption that 

 the planets and comets move in an absolute vacuum can in no way be 

 admitted. Even if the space between celestial bodies contained no 

 other matter than that necessary for the existence of light (whether 

 light be considered as emission of matter or the undulations of a 

 universal ether), this alone is sufficient to alter the motion of the 

 planets in the course of time and the arrangement of the whole sys- 

 tem itself ; the fall of all the planets and the comets into the sun and 

 the destruction of the present state of the solar system must be the 

 final result of this action." 



A direct proof of the existence of such a resisting medium has been 

 furnished by the academician Encke. He found that the comet named 

 after him, Avhich revolves round the sun in the short space of 1,207 

 days, shows a regular acceleration of its motion, in consequence of 

 which the time of each revolution is shortened by about six hours. 



From the great density and magnitude of the planets, the shorten- 

 ing of the diameters of their orbits proceeds, as might be expected, 

 very slowly, and is up to the present time inappreciable. The smaller 

 the cosmical masses are, on the contrary, other circumstances remain- 

 ing the same, the faster they move towards the sun : it may therefore 

 happen that in a space of time wherein the mean distance of the earth 

 from the sun would diminish one meter, a small asteroid would travel 

 more than 1,000 miles towards the central body. As cosmical 

 masses stream from all sides in immense numbers towards the sun, 

 it follows that they must become more and more crowded together 

 as they approach thereto. The conjecture at once suggests itself that 

 the zodiacal light, the nebulous light of vast dimensions which sur- 

 rounds the sun, owes its origin to such closely packed asteroids. 



* Compare Prof. Newton's computation of the approximate number of meteors 

 in the August ring alone, which makes it more than 300,000,000,000,000 : Silliman's 

 Journal, xxxii. 451. 



