ON FORCE. 285 



energies would enable them to perform an amount of work 

 just equivalent to that which the annual produce of our 

 coal-fields would be able to accomplish. 



Comparing with ordinary gravity the force with which 

 oxygen and carbon unite together, chemical affinity seems 

 almost infinite. But let us give gravity fair play by per- 

 mitting it to act throughout its entire range. Place a body 

 at such a distance from the earth that the attraction of our 

 planet is barely sensible, and let it fall to the earth from 

 this distance. It would reach the earth with a final ve- 

 locity of 36,747 feet a second; and on collision with the 

 earth the body would generate about twice the amount of 

 heat generated by the combustion of an equal weight of 

 coal. We have stated that by falling through a space of 

 16 feet our lead bullet would be heated three-fifths of a 

 degree; but a body falling from an infinite distance has al- 

 ready used up 1,299,999 parts out of 1,300,000 of the earth's 

 pulling power, when it has arrived within 36 feet of the 

 surface; on this space only one one million three hundred 

 thousandths 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 expenditure of 

 the sun as regards heat; and by an easy calculation 

 we ascertain the precise amount of the expendi- 

 ture 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 temperature of the sun's surface. Besides, 

 were the sun a burning 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 outlay? We have already regarded the case 

 of a body falling 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 



