374 FRAGMENTS OF SCIENCE. 



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 permitting 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 fr >m 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 of the 

 surface ; on this space only i^J^ths 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 expenditure which falls to the share of our planet. 

 Out of 2300 million parts of light and heat the earth 



