424 THE SCIENTIFIC PAPERS OF 



but considering that the earth's apparent diameter as seen from 

 the sun is only seventeen seconds, the earth can intercept only the 

 2,250-millionth part. Assuming that the other planetary bodies 

 swell the intercepted heat by ten times this amount, there remains 

 the important fact that f-f o (HHr f the solar energy is radiated 

 into space, and apparently lost to the solar system, and only 

 utilised. 



235000000 



Notwithstanding this enormous loss of heat, solar temperature 

 has not diminished sensibly for centuries, if we neglect the periodic 

 changes apparently connected with the appearance of sun-spots 

 that have been observed by Lockyer and others ; and the question 

 forces itself upon us how this great loss can be sustained without 

 producing an observable diminution of solar temperature even 

 within a human lifetime. 



Amongst the ingenious hypotheses intended to account for a 

 continuance of solar heat is that of shrinkage, or gradual reduction 

 of the sun's volume suggested by Helmholtz. It may, however, 

 be urged against this theory that the heat so produced would be 

 liberated throughout his mass, and would have to be brought to 

 the surface by conduction, aided perhaps by convection ; but we 

 know of no material of sufficient conductivity to transmit any- 

 thing approaching the amount of heat lost by radiation. 



Chemical action between the constituent parts of the sun has 

 also been suggested ; but here again we are met by the difficulty 

 that the products of such combination would ere this have ac- 

 cumulated on the surface, and would have formed a barrier against 

 further action. 



These difficulties led Sir William Thomson to the suggestion 

 that the cause of maintenance of solar temperature might be found 

 in the circumstance of meteorolites falling upon the sun, not 

 from great distances in space, as had been suggested by Mayer 

 and Waterston, but from narrow orbits which slowly contracted 

 by resistance until at last the meteorolites became entangled in the 

 sun's atmosphere and fell in ; and he shows that each pound of 

 matter so imparted would represent a large number of heat units 

 without disturbing the planetary equilibrium. But in considering 

 more fully the enormous amount of planetary matter that would 

 be required for the maintenance of the solar temperature, Sir 

 William Thomson soon abandoned this hypothesis for that of 



