CH. XXXIV. DYNAMICAL THEORY OF HEAT. 335 



of heat, but of all the extra force which was holding the 

 particles asunder. 



Dr. Joule's Experiments on the Conversion of Motion 

 into Heat, 1849. — It had now been shown that motion 

 could be turned into heat, and Rumford had even calculated 

 roughly how much heat was caused by a certain amount of 

 motion. But when a horse walks round and round you cannot 

 measure how much strength he gives out, and in order to 

 prove that motion by itself can produce heat we must 

 measure exactly how much motion produces a definite 

 quantity — say 1° Fahr. of heat, and then see if that amount 

 of heat can be turned back again into motion. This was 

 done by Dr. Joule, of Manchester, a celebrated physicist, 

 who is still living. 



In 1839 a Frenchman named M. Se'guin, 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 quantity 

 of heat, and Dr. Mayer made many calculations about it. In 

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

 Joule began those famous experiments which have formed 

 the foundation of the dynamical theory of heat, or the theory 

 of heat produced by motion, 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. 53, which 

 weighed i lb. 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 



