350 POPULAR LECTURES AND ADDRESSES. 



universe ; and therefore science points rather to an 

 endless progress, through an endless space, of 

 action involving the transformation of potential 

 energy into palpable motion and thence into heat, 

 than to a single finite mechanism, running down 

 like a clock, and stopping for ever. It is also 

 impossible to conceive either the beginning or 

 the continuance of life, without an overruling 

 creative power ; and, therefore, no conclusions of 

 dynamical science regarding the future condition 

 of, the earth can be held to give dispiriting views 

 as to the destiny of the race of intelligent beings 

 by which it is at present inhabited. 



The object proposed in the present article is an 

 application of these general principles to the dis- 

 covery of probable limits to the periods of time, 

 past and future, during which the sun can be 

 reckoned on as a source of heat and light. The 

 subject will be discussed under three heads : 



I. The secular cooling of the sun. 



II. The present temperature of the sun. 



III. The origin and total amount of the sun's 

 heat. 



