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 16 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 
