394 FRAGMENTS OF SCIENCE 



than one which fires a blank cartridge. The quantity of 

 heat communicated to the boiler of a working steam- 

 engine is greater than that which could be obtained from 

 the recondensation 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 in- 

 formed us, in his interesting discourse, that we dig an- 

 nually 84 millions of tons of coal from our pits. The 

 amount of mechanical force represented by this quantity 

 of coal seems perfectly fabulous. 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 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 

 velocity 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 

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

 earth's pulling power, when it has arrived within 16 feet 



