ON FORCE 395 



of the surface; on this space only i^oooths of the whole 

 force is exerted. 



Let us now turn our thoughts for a moment from the 

 earth to the sun. The researches of Sir John Herschel 

 and M. Pouillet have informed us of the annual expendi- 

 ture of the sun as regards heat; and by an easy calcula- 

 tion we ascertain the precise amount of the expenditure 

 which falls to the share of our planet. Out of 2,300 million 

 parts of light and heat the earth receives one. The whole 

 heat emitted by the sun in a minute would be competent 

 to boil 12,000 millions of cubic miles of ice-cold water. 

 How is this enormous loss made good whence is the 

 sun's heat derived, and by what means is it maintained? 

 No combustion no chemical affinity with which we are 

 acquainted would be competent to produce the tempera- 

 ture of the sun's surface. Besides, were the sun a burn- 

 ing body merely, its light and heat would speedily come 

 to an end. Supposing it to be a solid globe of coal, its 

 combustion would only cover 4,600 years of expenditure. 

 In this short time it would burn itself out. What agency 

 then can produce the temperature and maintain the out- 

 lay? We have already regarded the case of a body fall- 

 ing from a great distance toward the earth, and found 

 that the heat generated by its collision would be twice 

 that produced by the combustion of an equal weight of 

 coal. How much greater must be the heat developed by 

 a body falling against the sun! The maximum velocity 

 with which a body can strike the earth is about 7 miles 

 in a second; the maximum velocity with which it can 

 strike the sun is 390 miles in a second. And as the heat 

 developed by the collision is proportional to the square 

 of the velocity destroyed, an asteroid falling into the sun 



