i6.6 GENERAL SCIENCE 



on bicycles or in automobiles if it were not for the 

 condensed air in the tires, making them act as springs. 

 The Westinghouse air-brake, which is used on trains and 

 street cars, depends upon compressed air for its opera- 

 tion. Compressed air is also used in drilling and riveting 

 machines, without which modern skyscrapers reinforced 

 with steel could not be built. Many kinds of hammering 

 and stone-cutting machines are operated by compressed air. 



The gas formed by the explosion of gasoline vapor in 

 a gasoline engine requires more space than the vapor, 

 and so it sets the piston of the cylinder in motion. By 

 a series of such explosions enough power is developed 

 to run the machine. The compressed gas that drives 

 the locomotive is steam. The force that compresses the 

 steam comes from the burning fuel which causes the 

 water to evaporate. Water is made to evaporate by 

 causing the molecules to move so fast that they will not 

 stay in the liquid. So many of the molecules of steam 

 hit the sides of the boiler that they sometimes have 

 a pressure of 250 pounds per square inch. When the 

 engineer opens the valves the molecules rush through at 

 an enormous speed and exert enough pressure on the 

 piston in the cylinder to turn the wheels of the locomotive 

 and thus move the whole train of cars. 



Heat causes the molecules of a gas to move faster and 

 to strike the sides of the containing vessel with greater 

 force. To apply heat to a gas, then, will increase its 

 pressure without increasing its weight or density. If 

 the pressure on a gas is not increased when heat is ap- 

 plied, "the gas will expand and become less dense. Bal- 

 loons are sometimes filled with hot air, but they will 

 descend as soon as the air in them cools to about the 

 temperature of the air around them. Why? The air 



