122 GENERAL SCIENCE 



where the air pressure is being maintained, it is necessary 

 that the change in air density affecting their bodies be made 

 gradually. This is done in order to permit an equalization 

 of density of air within the body and without. Serious 

 results follow if sufficient time is not taken to provide for 

 this. 



SUMMARY 



The great pressure exerted by compressed air makes it an effective 

 agent for doing work. Its escape from pipes and hose, by means of 

 which it can be taken anywhere, is so easily controlled that its uses 

 are manifold and of great economic importance. 



Air-brakes have made possible a large increase in the speed of railway 

 trains, and a decrease in danger from accidents, by reason of the fact 

 that the engineer is given complete control of a train through their use. 



Air drills have revolutionized mining, tunnel building, and all engi- 

 neering enterprises involving the removal of rock by blasting. By aid 

 of compressed air excavations can be made beneath water when not 

 too deep, and solid masonry put in place under water from the bed- 

 rock upward. 



Where drilling is done by use of compressed air in mines and in 

 tunnel headings, the escaping air serves to provide the necessary 

 ventilation for the workmen. 



Compressed air in the tires of bicycles and automobiles makes riding 

 a comfort. Without spraying outfits employing compressed air the 

 fruit grower could not wage profitable warfare upon insect enemies 

 and fungus growths. Submarine navigation involves the use of com- 

 pressed air in controlling submergence of the boats, and for air supply 

 to the persons aboard the boats. 



Water pumps commonly employ the pressure of the atmosphere to 

 raise water above a valve placed some thirty feet more or less above 

 the water surface. This height varies with the density of the atmos- 

 phere, and its consequent pressure. The flow of water through siphons 

 where it runs "up-hill" part of the way is due to atmospheric pressure. 



Exercises 



Get information from all sources at hand upon some or all of the topics 

 named below. Describe the construction and operation of these appliances. 

 Use drawings wherever helpful to clearness. Be concise in the descriptions. 



