70 



GENERAL SCIENCE 



The Steam Engine. When water changes to steam, 

 its volume increases about 1600 times. That is, one 

 liter of water will make 1600 liters of steam at standard 

 pressure. Steam is a gas, and the molecules of all gases 

 move about rapidly. The higher the temperature the 



more rapidly they 

 move and the 

 harder they strike 

 the walls of any 

 containing ves- 

 sel. Since this is 

 true, the mole- 

 cules of com- 

 pressed steam, as 

 of any other com- 

 pressed gas, will 

 rush rapidly 

 through any pas- 

 sage that is of- 

 fered for them 

 from a chamber 

 which incloses 

 them. This 

 stream of gas has 

 the power of doing 

 work. Figure 37 

 shows the essen- 

 tial parts of a double-acting steam engine. The steam 

 is produced under high pressure in the boiler and is 

 allowed to expand in the cylinder, first on one side 

 of the piston and then on the other, thus forcing the 

 piston rapidly to and fro. This motion is converted into 

 the type of* motion desired, by a shaft and cogwheels 



FIG. 67. A Windmill in Holland. 



