THE BOUNDARIES OF ASTRONOMY. 247 



of it is intercepted by the earth, hut this portion is less than one two- 

 thousand-millionth part of the whole ; the planets also, no doubt, 

 each intercept a small portion of the solar radiation ; but the great 

 mass of radiated heat from the sun entirely escapes. This heat is 

 supposed not to be restored to the sun. The sun certainly must re- 

 ceive some heat by the radiation from the stars ; but this is quite in- 

 finitesimal in comparison with the stupendous radiation from the sun. 

 "We therefore conclude that the sun's heat is being squandered with 

 prodigal liberality.* We also know that the store of heat which the 

 sun can possess, though no doubt enormously great, is still limited in 

 amount. It is, indeed, a question of very great interest to decide 

 what are the probable sources by which the sun is able to maintain 

 its present rate of expenditure. The sun must have some source of 

 heat in addition to that which it would possess in virtue of its tem- 

 perature as an incandescent body. If we suppose the sun to be a vast 

 incandescent body, formed of materials which possess the same spe- 

 cific heat as the materials of which our earth is composed, the sun 

 would then cool at the rate of from 5 to 10 per annum. At this 

 rate the sun could not have lasted for more than a few thousand years 

 before it cooled down. "VVe are therefore compelled to inquire 

 whether the sun may not have some other source of heat to supply 

 its radiation beyond that which arises merely from the temperature. 



Of the various sources which have been suggested, it will here only 

 be necessary to mention two. It has been supposed that the heat of 

 the sun may be recruited by the incessant falling of meteoric matter 

 upon the sun's surface. If that matter had been drawn only by the 

 sun's attraction from the remote depths of space, it would fall upon 

 the sun with an enormously great velocity, amounting to about 300 

 miles a second. It follows from the principle of the equivalence be- 

 tween heat and mechanical energy that a body entering the sun with 

 this velocity would contribute to the sun a considerable quantity of 

 heat. It is known that small meteoroids abound in the solar system ; 

 they are constantly seen in the form of shooting-stars when they dash 

 into our atmosphere, and it can hardly be doubted that myriads of 

 such bodies must fall into the sun. It does not, however, seem likely 

 that enough matter of this kind can enter the sun to account for its 

 mighty radiation of heat. It can be shown that the quantity of mat- 

 ter necessary for this purpose is so large that a mass equal in the 

 aggregate to the mass of the earth would have to fall into the sun 

 every century if the radiation of the sun were to be defrayed from 

 this source. That so large a stream of matter should be perennially 

 drawn into the sun is, to say the least, highly improbable. But it is 



* A remarkable theory has recently been put forward by Dr. Siemens, according to 

 which the sun's radiant energy is ultimately restored to the sun. Even the possibility of 

 some such theory being true most seriously affects the abore arguments in favor of the 

 nebular hypothesis. 



