THOUGHTS ON NATURAL PHILOSOPHY. 21 



admitted in face of the notorious fact that there is no 

 historical evidence that the sun is growing colder. 

 We have not the slightest reason to think that the 

 radiation from the sun is measurably weaker now than 

 it wa,s a couple of thousand years ago, yet it can be 

 shown that, if the sun were merely radiating heat as 

 simply a hot body, then it would cool some degrees 

 every year, and must have cooled many thousands of 

 degrees within the time covered by historical records. 

 We therefore conclude that the sun has some other 

 source of heat than that due simply to incandescence. 

 We can also conceive that the heat of the sun might 

 be supplied by something analogous to combustion. 

 It would take 20 tons of coal a day burned on each 

 square foot of the sun's surface to supply the daily 

 radiation. Even if the sun were made of one mass of 

 fuel as efficient as coal, that mass must be entirely 

 expanded in a few thousand years. We cannot there- 

 fore admit that the source of the heat in the sun is 

 to be found in any chemical combination taking place 

 in its mass. Where then can we find an adequate 

 supply of heat ? Only one external source can be 

 named : the falling of meteors into the sun must yield 

 some heat just as the flash of a shooting star yields 

 some heat to our atmosphere, but the question is 

 whether the quantity of heat obtainable from the 

 shooting stars is at all adequate for the purpose. It 

 can be shown that unless a quantity of meteors in 

 collective mass equal to our moon were to plunge into 

 the sun every year the supply of heat could not be 

 sustained from this source. Now there is no reason 

 to believe that meteors in anything like this quantity 



