CH. xxxv. DYNAMICAL THEORY OF HEAT. 



357 



the weight down to the earth ; so that the machine starts 

 with a certain stock of energy stored up in the weight, and its 

 amount is called 772 foot-pounds because it has raised a 

 weight of i Ib. to a height of 772 feet. This stock of 

 working-power philosophers call potential energy, or possible 

 energy which may be called into use at any time. When 

 the man sets the weight free, it begins to fall, drawn down 

 by the force of gravity, and the stock of energy is set free. 

 What becomes of it ? It passes by the wheel B into the 

 roller r, and, turning the paddle in the box enters the water. 



FIG. 62. 

 Joule's Experiment on the Conversion of Motion into Heat (Phil. Trans.) 



A, Weight. B, Wheel of the roller, ff, c, Vessel containing water and the paddle. 

 D, Scale to measure the distance that the weight falls. E, Paddle contained in 

 the vessel, c. _/"_/", Roller turned by the falling weight, r. Roller turning the 

 paddle. /, Pin which joins the roller and the paddle. /, Thermometer plunged 

 iu the vessel, c. 



If the water were free, the motion would pass on into the air 

 and we should lose sight of it ; but the water is shut in and 

 the energy cannot escape, so now it employs itself in dashing 

 to and fro all the little particles which make up the water, 

 and producing the effect we will call heat. Thus the 



