Professor TyndaPs Lecture on Force. 245- 



possess an enormous store of mechanical power. This pound of 

 coal, which I hold in my hand, produces by its combination with 

 oxygen an amount of heat, which, if mechanically applied, would 

 suflBce to raise a weight of one hundred lbs. to a height of twenty 

 miles above the earth's surface. Conversely, one hundred pounds 

 falling from a height of twenty miles, and striking against the 

 earth, would generate an amount of heat equal to that developed 

 by the combustion of a pound of coal. Wherever work is done 

 by heat, heat disappears. A gun which fires a ball is less heated 

 than one which fires blank cartridge. The quantity of heat com- 

 municated to th^ boiler of a working steam-engine is greater than 

 which could be obtained from the re-condensation of the steam 

 after it had done its work ; and the amount of work performed 

 is the exact equivalent of the amount of heat lost. Mr. Smyth 

 informed us in his interesting discourse, that we dig annually 84 

 millions of tons of coal from our pits. The amount of mechanical 

 force represented by this quantity of coal seems perfectly fabu- 

 lous. The combustion of a single pound of coal, supposing it to 

 take place in a minute, would be equivalent to the work of 300 

 horses; and if we suppose 108 millions of horses working day" 

 and night, with unimpaired strength, for a year, their united 

 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 energy of the force 

 with which oxygen and carbon unite together, the chemical aflBnity^ 

 seems almost infinite. But let us give gravity fair play : let us 

 permit it to act throughout its entire range. Place a body at 

 such a distance from the earth that the attraction of the earth is 

 barely sensible, and let it fall to the earth from this distance. It 

 would reach the earth with a final velocity of 36,747 feet in a 

 second ; and on collision with the earth the body would gener- 

 ate 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 sixteen feet, our lead bullet would be heated 

 three-fifths of a degree ; but a body falling from an infinite dis- 

 tance has already used up 1,299,999 parts out of 1,300,000 of 

 the earth's pulling power, when it has arrived within IG feet of 

 the surface; in this space only Taooooo^^'s of the whole force 

 is exerted. 



Let us now turn our thoughts for a moment from the earth to- 



