CH. xxxv. MECHANICAL EQUIVALENT OF HEAT. 355 



calculation ; but when a horse walks round and round, we 

 cannot measure how much strength he gives out, and in 

 order to prove that energy by itself can produce heat we 

 must measure exactly how much work produces a definite 

 increase of temperature, say of i Fahr. and then see if 

 that amount of heat can be turned back again into. work. 

 This was done by Dr. Joule, of Manchester, a celebrated 

 physicist, who is still living. 



In 1839 a Frenchman named M. Seguin, and in 1842 

 a German physician, Dr. Mayer, of Heilbronn, both sug- 

 gested that by careful experiments it might be found out how 

 much work must be done to produce a certain increase of 

 temperature, and Dr. Mayer made many calculations about it. 

 In 1843, without having heard of Dr. Mayer's suggestion, Dr. 

 Joule, who had previously discovered in 1840 the law cf 

 heat evolved by Voltaic electricity, began those famous ex- 

 periments which have formed the foundation of the dynami- 

 cal theory of heat, or the theory of heat produced by mechanical 

 energy, and he completed them in 1849. A description of 

 one of his experiments will explain the results he obtained. 

 He took a weight, A, Fig. 62, p. 349^ which weighed ilb. and 

 fastened it by strings to the roller,// On to the wheel, B, 

 of this roller he wound another string which passed round 

 the roller r, and this roller was attached to a paddle which 

 was shut into the box of water, c. He next wound up the 

 string on the roller / so as to draw up the weight A, and 

 then set it free. Immediately the force of gravity drew the 

 weight down to the ground, and in doing so pulled round 

 the wheel B, and consequently the roller which turned the 

 paddle in the water. When the weight readied the ground 

 he took out the little pin, p, which fastened the paddle to 

 the roller, so that he could wind up his apparatus without 

 disturbing the water and begin again. 



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