SECT. ii. MECHANICAL EQUIVALENT OF HEAT. 29 



Although the specific heat of a substance remains 

 the same, its sensible and absorbed heat may vary reci- 

 procally to a great extent. 



As there can be no direct measurement of heat inde- 

 dendent of matter, its mutations and action on matter 

 are the sole means we have of forming our judgment 

 concerning its agency in the material world. 



Mr. Joule has proved that the quantity of heat 

 requisite to raise the temperature of a pound of water 

 one degree of the centigrade thermometer is equivalent 

 to the mechanical work or force that would raise the 

 same mass of water to the height of 1,389 feet. This 

 is the unit, or mechanical equivalent of heat. 



In fact, for every unit of force expended in percussion, 

 friction, or raising a weight, a definite quantity of heat 

 is generated ; and conversely, when work is performed by 

 the consumption of heat, for each unit of force gained, 

 a unit of heat disappears. For since heat is a dynami- 

 cal force of mechanical effect, there must be an equi- 

 valence between mechanical work and heat as between 

 cause and effect. That equivalence is a law of nature. 

 The mechanical force exerted by the steam engine is 

 exactly in proportion to the consumption of heat, neither 

 more nor less ; for if we could produce a greater quantity 

 than its equivalent we should have perpetual motion, 

 which is impossible. When steam is employed to per- - 

 form any work, the temperature of the steam is lowered ; 

 the heat that disappears is transformed into the force 

 that performs the work, and is exactly proportional to 

 the work done, and vice versa. 



The heat which is the motive force in the steam 

 engine is due to the chemical combination of the 

 carbon of the fuel with the oxygen of the atmosphere. 

 A pound weight of coal when consumed in one of our 

 best steam engines produces an effect equal to raising a 

 weight of a million of pounds a foot high, yet marvellous 



