HOW THE HEAT IS KEPT UP. 33 



test that we have applied to the other explanations which 

 have been offered of the source of sun heat. When we do 

 so the figures supply abundant confirmation of its truth. 

 Suppose that the sun were to pass by loss of heat and 

 consequent contraction into a globe which was less than 

 its present size by one ten-thousandth part of its diameter. 

 Such a change would, no doubt, be considerable if we 

 merely regarded its absolute dimension in miles. It 

 would amount to a shrinkage in the sun's diameter of 

 87 miles. But on so mighty a globe this alteration is 

 relatively insignificant ; indeed no measurements that 

 could be made at our observatories would be sufficiently 

 delicate to detect a change of this magnitude. Helmholtz 

 has, however, shown that if the sun were to undergo even 

 this small diminution of volume the quantity of heat that 

 would be thereby liberated for the purposes of radiation 

 would supply the sun's current rate of expenditure for 

 nearly two thousand years. 



This point is so important, that for the sake of further 

 illustration, I shall present it from a somewhat different 

 point of view. Imagine two suns of similar materials and 

 of equal temperature, but so that the diameter of one was 

 a ten- thousandth part greater than that of the other, 

 which latter was as big as ours. Then, although these 

 two suns would be practically the same as far as sensible 

 warmth was concerned, and although the difference 

 between their dimensions would be too insignificant to be 

 appreciable to measurements, conducted from such a dis- 

 tance as that at which we are placed, yet, nevertheless, 

 the larger of these two suns would possess an excess of 

 heat, or its equivalent, over that contained in the smaller.. 



