298 DISCUSSIONS IN COSMOLOGY. 



being an inquiry with which the physicist has nothing 

 whatever to do— but simply its origin as a sun, i.e., as 

 a source of light and heat. Our first question must 

 therefore be, What is the origin of the sun's heat ? 

 From what source did he derive that enormous amount 

 of energy which, in the form of heat, he has been dissi- 

 pating into space during past ages ? Difficult as the 

 question at first sight appears to be, it is yet simplified 

 and brought within very narrow limits, as was shown 

 in the last chapter, when we remember that there are 

 only two conceivable sources. The sun must have 

 derived his energy either from Gravitation, or from 

 that other source which has just been considered, Motion 

 in Space* All other sources of energy put together 

 could not have supplied our luminary with one 

 thousandth part of that which he has possessed. We 

 are therefore compelled to attribute the sun's heat to 

 one or other of these two, or to give up the whole 

 inquiry as utterly hopeless. The important difference 

 between the two, as was shown, is that the store of 

 energy derivable from Gravitation could not possibly 

 have exceeded 20 to 30 million years' supply of heat 

 at the present rate of radiation ; whereas the store 

 derivable from Motion in Space, depending on the rate 

 of that motion, may conceivably have amounted to any 

 assignable quantity. Thus a mass equal to that of the 

 sun, moving with a velocity of 476 miles per second, 

 possesses in virtue of that motion energy sufficient, if 

 converted into heat, to cover the present rate of the 



* A new theory of the sun was advanced a few years ago by Dr. 

 C. W. Siemens, Proc. Koy. Soc., vol. 33, p. 389. According to this 

 theory the greater part of the energy in the form of heat radiated 

 from the sun is arrested and returned over and over again to that 

 luminary. By this means it is concluded the radiation of the sun can 

 be maintained to the remotest future. The theory, I fear, is not 

 likely to gain acceptance among physicists. 



