MEASURE OF THE SUN^S HEAT. 271 



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 prob- 

 abilities, approaches the infinite. 



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

 agther 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 OBther. 

 Littrow, amongst others, expresses himself on this point as 

 follows : " The assumption that the planets and the 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 aBther), this alone is sufficient to alter the mo- 

 tion of the planets in the course of time and the arrangement 

 of the whole system 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 results 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, which revolves round the 

 sun in the short space of 1207 days, shows a regular acceler- 

 ation 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 

 shortening of the diameters of their orbits proceeds, as might 

 be expected, very slowly, and is up to the present time inap- 

 preciable. The smaller the cosmical masses are, on the con- 

 trary, other circumstances remaining 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 dimmish one metre, a small asteroid 



