14 



THE OCEAN OF AIR 



valve worked much as the trigger on a gun. 



Compressed air is used in spraying many sub- 

 stances. The most modern way to apply paint and 

 varnish is by means of a compressed air spray. Nearly 

 the same type of spray is used for spraying fruit trees 

 to protect them from pests. Pressure is first built up in 

 a tank by means of a pump and then is released in 

 such a way that it carries with it an atomized spray of 

 the liquid being applied. Such sprays operate in much 

 the same manner as the common household spray used 

 for killing flies and mosquitoes. 



Compressed air is used in diving suits and deep- 

 sea work. Divers who work far below the surface of 

 the water must be supplied with air for breathing and 

 to help them withstand the tremendous pressure 

 caused by the water. Diving suits are equipped with 

 a hose through which compressed air is forced. 



Compressed air is used to transport parcels. In 



head 



FIG. \29< AIR BRAKE 



Note the changed positions of the slide valve and the piston 

 head in the lower diagram. 



many cities mail is sent from one station to another 

 by means of compressed air. The device is similiar to 

 the cash transporting systems used in many large 

 department stores. The material to be sent is placed 

 in a container, usually made of leather, which fits 

 tightly into a tube carrying a blast of air. The con- 

 tainer acts much as a piston ; the pressure increases 

 behind it and pushes it along. 



Compressed air is used in the air brake. Before 

 1868, all trains were stopped by means of hand brakes. 

 George Westinghouse, then a young man, saw one 

 of the many accidents of the period and determined to 

 improve on the old hand brakes. In 1868 the first train 

 to be equipped with air brakes was run between 

 Pittburgh, Pennsylvania, and Steubenville, Ohio. 

 Since that time not only steam trains but electric 

 trains, street cars, and buses have adopted it for use. 

 In the operation of the airbrake, air is compressed 

 in a compressor either by steam, electric motor, or gas 

 engine, to a pressure of seventy or eighty pounds per 

 square inch. From a central tank in the engine com- 

 pressed air is distributed the entire length of the 

 train by means of pipes and hose connections between 

 the cars. Beneath each car is a brake cylinder and 

 reservoir. The cylinder contains a piston which may 

 be moved by the pressure in the reservoir. When the 

 brakes are to be applied, the engineer pushes a lever 

 to a certain position, thus reducing the pressure in 

 the pipes which come from the engine. Through a 

 valve arrangement the higher pressure in the reser- 

 voir under the car is released and the piston is moved. 

 This motion is transmitted through levers which 

 cause the brake shoes to press against the wheels. 

 When the brakes are to be released, the engineer 

 moves the lever in the engine to another position 

 thus causing the pressure in the pipes to increase. 

 When this pressure is equal to that in the car reser- 

 voir, a spring pushes the piston back, thus releasing 

 the brake shoes from the wheels. 



All these devices are able to aid man because air 

 which has been compressed has a great deal of energy 

 and can do work when the pressure is released and 

 the air expands to its original volume. 

 ry How does compressed air enable men to work un- 

 rder water? The building of bridge foundations and 

 tunnels which go under water frequently makes work 

 at the bottom of the river necessary. The caisson is a 

 large hollow steel case open at the bottom end. Air 

 can be forced into such a chamber under enough pres- 

 sure to keep the mud and water out and allow men to 

 work with pick and shovel digging down to bed rock 

 where the foundation may rest firmly. Figure 30 will 

 teach you many interesting things about work in 

 a caisson. Notice how the gravel and dirt are raised to 

 the top without letting the air out. The pressure in 



