HEAT: DISTRIBUTION AND MEASUREMENT 79 



and so does not have a freezing temperature when the 

 ground does. This delays the season of frost in the fall at 

 places near the water. Grapes and other fruits which ripen 

 late are more successfully grown near the lakes in New York 

 State than at a distance from them. 



85. Heat and Energy. Heat is a form of energy; it 

 can do work. As the temperature of a body rises, its mole- 

 cules vibrate more rapidly and, in so doing, separate farther 

 apart; that is, the volume of the body is increased. Thus 

 pressure is exerted upon surfaces in contact with the body. 



FIG. 33. THE STEAM CHEST OF AN ENGINE 



In the left-hand figure, steam is shown entering from the boiler at B, 

 passing through D to the cylinder. It exerts pressure on P, pushing it to 

 the end of the cylinder. Steam passes out through C and O to the con- 

 denser. The valve rod VR is moved by other machinery so as to close the 

 entrance to D when the cylinder is filled with steam. C is then open to 

 S. Trace the path of the steam from B to O in the right-hand figure. 



When water is changed to steam, the vapor, if unconfined, 

 occupies about 1700 times as much space as the liquid did. 

 The force of this expansion is used to exert a push on the 

 piston in the cylinder of an engine. When the steam is 

 admitted first into one end of the cylinder and then into the 

 other, the piston is pushed back and forth. By means of 

 a connecting rod, other parts of the machinery are kept in 

 motion. 



