ON THE AGE OF THE SUN'S HEAT. 363 



approximate convective equilibrium of heat through- 

 out the whole, if the whole is fluid. That is to 

 say, the temperatures, at different distances from 

 the centre, must be approximately those which 

 any portion of the substance, if carried from the 

 centre to the surface, would acquire by expansion 

 without loss or gain of heat. 



PART III. 



ON THE ORIGIN AND TOTAL AMOUNT OF THE 

 SUN'S HEAT. 



The sun being, for reasons referred to above, 

 assumed to be an incandescent liquid now losing 

 heat, the question naturally occurs, How did this 

 heat originate ? It is certain that it cannot have 

 existed in the sun through an infinity of past 

 time, since, as long as it has so existed, it must 

 have been suffering dissipation, and the finiteness 

 of the sun precludes the supposition of an infinite 

 primitive store of heat in his body. 



The sun must, therefore, either have been 



