336 NINETEENTH CENTURY. PT. III. 



pulled round the wheel B, and consequently the roller which 

 turned the paddle in the water. When the weight reached 

 the ground he took out the little pin, /, which fastened the 

 paddle to the roller, so that he could wind up his apparatus 

 without disturbing the water and begin again. 



Now observe how this measured the motion and the 

 heat. Every time the weight fell, it turned the paddle, and 

 so, by agitating the water, added to its heat. The scale, D, 

 told him exactly how far the weight fell, while the thermo- 

 meter, t, in the box told him how much hotter the water 

 grew. At the end of an hour, therefore, he had only to see 

 how many feet his pound-weight had fallen, and how 

 many degrees of Fahr. the heat of his water had risen ; 

 and after allowing for the friction of his machineiy and 

 for the heat lost in the cooling of his vessel, both of which 

 he ascertained by careful experiments, he could tell how 

 much motion had been used up in producing the heat. In 

 this way he found that a weight of i Ib. would have to fall 

 772 feet in order to make i Ib. of water warmer by i Fahr. 



He next tried the same experiment with oil and with 

 mercury instead of water, and also measured the heat pro- 

 duced by rubbing together two plates of iron ; and in every 

 case he found that a certain amount of work gave a certain 

 amount of heat and no more. For example, if the weight 

 in Fig. 53 fell double the distance, the heat of the water was 

 raised two degrees instead of one, while if it fell only half the 

 distance, or 386 feet, the water was only raised half a degree. 

 In this way Dr. Joule established what is called the 

 mechanical equivalent of heat, namely that the fall of a pound 

 weight through 772 feet equals the heating of a pound of 

 water i Fahr. And now you must try to foim a clear idea 

 what this means, and how it proves that heat is altered 



