AIR 118 



sure, however, that there are some at a height 

 of one hundred miles, because about that dis- 

 tance away grains of dust falling toward the 

 world Are set on fire by friction with them, and 

 become shooting .stars. See METEOR. 



Buoyancy.* As the tendency of air is to ex- 

 pand, any portion of it presses upward against 

 the weight of the air above. Normally this 

 upward pressure exactly equals the downward 

 pressure at the same point. If it were less, the 

 air would be further compressed; if it were 

 greater it would expand. For the same reason 

 the pressure is equal in all directions. Without 

 this elastic upward pressure you could not in 

 the open air turn the leaf of this book, for the 

 weight of the air upon it would be nearly a 

 half a ton. 



If any object is lighter than the air which 

 it displaces, its downward pressure, due to 

 gravity, is less than the upward pressure of the 

 air underneath it, and it rises. Thus a balloon 

 filled with light gases ascends till it reaches a 

 point where it equals in weight the rarer at- 

 mosphere it displaces. 



An opened newspaper floats gently to earth, 

 while the same paper, folded, falls rapidly. In 

 the first instance the downward force of its 

 weight is distributed over a larger surface, thus 

 reducing the pressure which drives the air from 

 under the paper in order that it may fall. The 

 flight of a heavier-than-air machine depends 

 upon this buoyancy; its planes do not keep it 

 from falling, but make it fall more slowly, so 

 that its engines are able to support it (see 

 FLYING MACHINE). In a vacuum all things fall 

 with equal rapidity, and only in a vacuum can 

 objects be accurately weighed. 



What the Air Is Made of. The proportion 

 of gases in the air varies with place and height. 

 About seventy-nine per cent is nitrogen, nearly 

 twenty-one per cent, oxygen. Water vapor, 

 carbonic acid gas, traces of ammonia, ozone, 

 argon, helium, neon, krypton, xenon and 

 minute particles of animal, vegetable and min- 

 eral matter form the very small balance. Be- 

 cause plants absorb carbonic acid gas and re- 

 turn oxygen to the air, while men reverse this 

 process, city air has less oxygen than country 

 air. Coal, oil, natural gas and other substances 

 in the earth which have been formed by plants 

 or animals formerly living on it, contain gases 

 extracted from the air, so there is probably 

 slightly less atmosphere around us than there 

 was a few thousand years ago. But it is be- 

 lieved that the proportions of its parts remain 

 the same. 



AIR BRAKE 



What the Air Does for Us. Besides furnish- 

 ing oxygen for us to breathe, the air in other 

 ways enables us to live. It absorbs heat from 

 the sun, giving it out to us during the hours of 

 darkness; without an atmosphere the earth 

 would be burning hot by day and intensely 

 cold by night. It is also a medium for sound 

 waves, which, unlike light, travel only through 

 material bodies. It turns our windmills ; makes 

 air brakes possible; we ride on air in automo- 

 biles; we have many machines which operate 

 by compressed air; the housewife uses air in 

 vacuum cleaners to make her work less tire- 

 some. See WIND; Am BRAKE; COMPRESSED 

 AIR; LIQUID AIR; GEOLOGY. C.R.M. 



Outline for the Study of Air 



1. What air Is 



2. Weight 



(a) How discovered 



(b) Weight of air at sea level 



(c) Why air far above the earth weighs 



less 



3. Buoyancy 



(a) Equality of pressure 



(b) Why balloons rise 



(c) Why airships do not sink 



4. Composition 



(a) Nitrogen 



(b) Oxygen 



(c) Water vapor 



(d) Other gases 



5. What air does 



(a) Furnishes oxygen 



(b) Absorbs sun's heat 



(c) Permits passage of sound 



(d) Operates mechanical contrivances 



AIR BRAKE , a. device which uses the power 

 of compressed air to stop railway trains or 

 regulate their speed. Before the invention of 

 the air brake the only way of bringing a train 

 to a stop was by the hand brake attached to 

 each car. The brakeman had to run from one 

 end of the train to the other, setting each 

 brake as he went. If the train was long, sev- 

 eral minutes elapsed before it finally could be 

 brought to a stop. Many accidents occurred 

 because of defective brakes and also because a 

 train could not be stopped quickly. 



One of these railroad accidents, which he wit- 

 nessed, set a certain twenty-year-old youth to 

 thinking. Two years later, in 1868, this youth, 

 George Westinghouse, had perfected a device 

 with which the engineer in his cab, by turning 

 a valve, could stop his train. Westinghouse 

 took his plans to Cornelius Vanderbilt, then 

 the foremost railroad man in the United 

 States; but the old "Commodore" somewhat 

 angrily told him that he had no time to waste 



