368 POPULAR LECTURES AND ADDRESSES. 



initial heat, 10,000,000 times a year's supply at 

 present rate, but 50,000,000 or 100,000,000 as pos- 

 sible, in consequence of the sun's greater density 

 in his central parts. 



The considerations adduced above, in this paper, 

 regarding the sun's possible specific heat, rate of 

 cooling, and superficial temperature, render it pro- 

 bable that he must have been very sensibly warmer 

 one million years ago than now; and, consequently, 

 if he has existed as a luminary for ten or twenty 

 million years, he must have radiated away consider- 

 ably more than the corresponding number of times 

 the present yearly amount of loss. 



It seems, therefore, on the whole most probable 

 that the sun has not illuminated the earth for 

 100,000,000 years, and almost certain that he has 

 not done so for 500,000,000 years. As for the 

 future, we may say, with equal certainty, that 

 inhabitants of the earth cannot continue to enjoy 

 the light and heat essential to their life, for many 

 million years longer, unless sources now unknown 

 to us are prepared in the great storehouse of 

 creation. 



