70 EXPERIMENTAL GENERAL SCIENCE 



brakes on street cars and railway trains are set by compressed 

 air stored in a tank under each car and controlled by the motor- 

 man or engineer. Pneumatic tubes for carrying mail and other 

 light articles are also operated by compressed air. The steam 

 engine derives its power from water vapor under pressure. 

 This, usually called steam, is fed into a cylinder at just the 

 right times to give the piston a back and forth motion which is 

 turned into a rotary motion at the driving wheels. The gas 

 engine so familiar from its use in automobiles, pumps and the 

 like, operates in the same way through energy derived from 

 gasoline. Since the pressure in this type of engine is derived 

 from explosions of gasoline in the cylinders themselves, it is 

 called an internal-combustion engine. Steam under pressure is 

 also used to run certain engines called turbines, which are essen- 

 tially large wheels containing an immense number of curved 

 blades enclosed in a case. The steam is admitted at many 

 points in such a way as to strike these blades and cause the 

 wheel to revolve rapidly. A similar contrivance makes use of 

 the energy in falling water. 



Practical Exercises 



1. Fill a florence flask half full of water and heat over the bunsen 

 burner until it boils. Remove from the fire, cork, and invert on some con- 

 venient support such as a ring stand. Then pour some cold water on the 

 bottom of the flask. This causes the air within to cool. How does this 

 affect its pressure? 



2. How does it affect the boiling point? Why? 



3. Why can you not raise the temperature of boiling water in an 

 open vessel to 100C. in your locality (104)? 



4. Why does it take longer to cook potatoes in Denver than it does 

 in New Orleans? 



