16 IN STARRY REALMS. 



This is a question which cannot be decided by mere off- 

 hand reasoning ; we must submit it to the test of actual 

 calculation. As a foundation for this calculation, we 

 know the quantity of heat that would be contained in a 

 globe of white-hot iron of the same dimensions as the sun. 

 We also know the quantity of heat which represents the 

 sun's daily expenditure. It is therefore a simple matter 

 of arithmetic to find the number of days' supply which a 

 white-hot sun could contain. The result is not a little 

 startling ; it demonstrates that to supply the current 

 expenditure of solar radiation at its present rate the 

 temperature of the sun would have to decline through 

 some degrees every year if the constitution of the body 

 were what we have supposed. It seems that if the sun's 

 temperature were suffering an abatement at a rate any- 

 thing like so fast as this supposition would involve, then 

 the effects of cooling would be perceptible in a continuous 

 fall of the sun's efficiency as a radiator. It is therefore 

 obvious that our first notion, which suggests that the 

 sun is merely a white-hot globe cooling down, must be 

 abandoned. 



Another conceivable origin of the sun's heat must also 

 be considered for a moment before receiving its dismissal 

 as utterly incapable of affording a solution of the problem. 

 As heat is generated by the consumption of fuel, it might 

 seem not unreasonable to suppose that there may be some 

 process analogous to combustion at present in progress in 

 the sun on a scale of sufficient magnitude. But there are 

 insuperable difficulties about such a view, which will be- 

 come apparent when we call to mind the actual nature 

 of combustion. The coal which is glowing so cheerfully 



