ON THE SUN'S HEAT. 



387 



year, the solar radius must have been four 

 times as great as at present. 



(2) If the sun's effective thermal capacity can 

 be maintained by shrinkage till twenty million 

 times the present year's amount of heat is 

 radiated away, the sun's radius must be half 

 what it is now. But it is to be remarked 

 that the density which this would imply, 

 being 1 1*2 times the density of water, or just 

 about the density of lead, is probably too 

 great to allow the free shrinkage as of a 

 cooling gas to be still continued without 

 obstruction through overcrowding of the mole- 

 cules. It seems, therefore, most probable that 

 we cannot for the future reckon on more of 

 solar radiation than, if so much as, twenty 

 million times the amount at present radiated 

 out in a year. It is also to be remarked that 

 the greatly diminished radiating surface, at a 

 much lower temperature, would give out 

 annually much less heat than the sun in his 

 present condition gives. The same considerations 

 led Newcomb to the conclusion "That it is 



C C 2 



