FOKCE. 373 



gard the clashing of a falling weight against the 

 earth. The heat produced in both cases is referable to 

 a common cause. A diamond, which burns in oxygen 

 as a star of white light, glows and burns in consequence 

 of the falling of the atoms of oxygen against it. And 

 could we measure the velocity of the atoms when they 

 clash, and could we find their number and weights, mul- 

 tiplying the weight of each atom by the square of its 

 velocity, and adding all together, we should get a 

 number representing the exact amount of heat de- 

 veloped by the union of the oxygen and carbon. 



Thus far we have regarded the heat developed by 

 the clashing of sensible masses and of atoms. Work is 

 expended in giving motion to these atoms or masses, 

 and heat, is developed. But we reverse this process 

 daily, and by the expenditure of heat execute work. 

 We can raise a weight by heat ; and in this agent we 

 possess an enormous store of mechanical power. A 

 pound of coal produces by its combination with oxygen 

 an amount of heat which, if mechanically applied, 

 would suffice to raise a weight of 100 Ibs. to a height of 

 20 miles above the earth's surface. Conversely, 100 Ibs. 

 falling from a height of 20 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 communicated 

 to the boiler of a working steam-engine is greater than 

 that 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 



