ON THE AGE OF THE SUN'S HEAT. 359 



contracting mass, by mutual gravitation of its 

 parts. Hence, in contracting by one-tenth per 

 cent, in his diameter, or three-tenths per cent, in 

 his bulk, the sun must give out something either 

 more, or not greatly less, than 20,000 years' 

 heat ; and thus, even without historical evidence as 

 to the constancy of his diameter, it seems safe to 

 conclude that no such contraction as that calcu- 

 lated above (one per cent, in 860 years), can have 

 taken place in reality. It seems, on the contrary, 

 probable that, at the present rate of radiation, 

 a contraction of one-tenth per cent, in the sun's 

 diameter could not take place in much less than 

 20,000 years, and scarcely possible that it could 

 take place in less than 8,600 years. If then, 

 the mean specific heat of the sun's mass, in its 

 actual condition, is not more than ten times that of 

 water, the expansibility in volume must be less 

 than 1/4000 per 100 Cent., (that is to say, less 

 than i/io of that of solid glass,) which seems 

 improbable. But although from this consideration 

 we are led to regard it as probable that the sun's 

 specific heat is considerably more than ten times 



