12 



shown in the diagram. It consists of a cylindrical bo* 

 of metal, through the cover of which passes a shaft, 

 carrying upon its lower end a set of paddles, immersed 

 in water within the box, and upon its upper portion a 

 drum, on which arc wound two cords, which, passing in 

 opposite directions, run over pulleys, and are attached 

 to known weights. The temperature of the water with- 

 in the box being carefully noted, the weights are then 

 allowed to fall a certain number of times, of course in 

 their fall turning the paddles against the friction of the 

 liquid. At the close of the experiment the water is 

 found to be warmer than before. And by measuring 

 the amount of this rise in temperature, knowing the dis- 

 tance through which the weights have fallen, it is easy 

 to calculate the quantity of heat which corresponds to a 

 given amount of motion. In this way, and as a mean 

 of a large number of experiments, Mr. Joule found that 

 the amount of mass motion in a body weighing one 

 pound, which had fallen from a hight of 772 feet, was 

 exactly equal to the molecular motion which must be 

 added to a pound of water, in order to heat it one de- 

 gree Fahrenheit. If we call the actual energy of a 

 body weighing one pound which has fallen one foot, a 

 foot-pound, then we may speak of the mechanical equiv- 

 alent of heat as being 772 foot-pounds. 



The significance and value of this numerical constant 

 will appear more clearly if we apply it to the solution of 

 one or two simple problems. During the recent war two 

 immense iron guns were cast in Pittsburgh, whose weight 

 was nearly 112,000 pounds each, and which had a caliber 

 of 20 inches." Upon this diagram is a calculation of the 

 effective blow which the solid shot of such a gun, assum- 



