FORCE. 373 



gard the clashing of a falling weight against the earth. 

 The heat produced in both cases is referable to a com- 

 mon 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, 

 multiplying 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 disap- 

 pears. A gun which fires a ball is less heated than one 

 which fires blank cartridge. The quantity of heat com- 

 municated 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 in- 

 formed us in his interesting discourse, that we dig 

 annually 84 millions of tons of coal from our pits. 



