286 CELESTIAL DYNAMICS. 



The continual bombardment of the sun by these cosmical 

 masses ought to increase its volume as well as its mass, if 

 centripetal action only existed. The increase of volume, 

 could scarcely be appreciated by man ; for if the specific grav- 

 ity of these cosmical masses be assumed to be the same as 

 that of the sun, the enlargement of his apparent diameter to 

 the extent of one second, the smallest appreciable magnitude, 

 would require from 33,000 to 66,000 years. 



Not quite so inappreciable would be the increase of the 

 mass of the sun. If this mass, or the weight of the sun, 

 were augmented, an acceleration of the motion of the planets 

 in their orbits would be the consequence, whereby their times 

 of revolution round the central body would be shortened. 

 The mass of the sun is 2*1 quintillions of kilogrammes ; and 

 the mass of the cosmical matter annually arriving at the sun 

 stands to the above as 1 to from 21 42 millions. Such an 

 augmentation of the weight of the sun ought to shorten the 

 sidereal year from ii^^th to &jnSj&fh f ^ ts length, or from 

 f ths to f ths of a second. 



The observations of astronomers do not agree with this 

 conclusion ; we must therefore fall back on the theory men- 

 tioned at the beginning of this chapter, which assumes that 

 the sun, like the ocean, is constantly losing and receiving 

 equal quantities of matter. This harmonizes with the suppo- 

 sition that the vis viva of the universe is a constant quantity. 



VI. THE SPOTS ON THE SUN'S DISC. 



THE solar disc presents, according to Sir John Herschel, 

 the following appearance. " When the sun is observed 

 through a powerful telescope provided with coloured glasses 

 in order to lessen the heat and brightness which would be 



