CH. xxxiv. CONVERSION OF MOTION INTO HEAT. 337 



motion. Looking at the diagram, try to picture to your- 

 self what would be taking place if the weight was able to 

 fall the whole 772 feet without stopping. First, a man must 

 wind up the weight, and in doing this he uses force to over- 

 come the force of gravitation which is pulling the weight 

 down to the earth ; so that the machine starts with a certain 

 stock of force stored up in the weight, and its amount is 

 called 772 foot-pounds because it has raised a weight of 



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



A, Weight, u, Wheel of the roller, f f. c, Vessel containing water and the paddle. 

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

 the vessel, C. ff. Roller turned by the falling weight, r. Roller turning the 

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

 in the vessel, c. 



i lb. to a height of 772 feet. This stock of force philoso- 

 phers 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. If the water were free, 



16 



