THE AIR 151 



lower than 30 inches the height of the water will be less 

 than 34 feet. In the year 1640 it was accidentally learned 

 that air would not lift water in a pump more than 32 feet 

 above the surface of the water in the well. But at that 

 time no one knew why. They did not know that the 

 downward pressure of the 32 feet of water was equal to 

 the pressure of the air at that particular place. Galileo, 

 the great Italian scientist, was living at that time and he 

 proceeded to investigate this strange action of the water. 

 He died before finishing his task, and so it was left to his 

 pupil, Torricelli, to continue the investigation and learn 

 the truth in the matter. This he did in 1643. 



Air is an elastic substance and so it can be compressed. 

 The air at sea level bears the weight of all the air above 

 it, and therefore it is very much compressed, and the 

 molecules are much closer together than they are in the 

 upper regions of the atmosphere. Twelve cubic feet of 

 air at sea level weigh a pound, but twelve cubic feet of 

 air at an altitude of 16,000 feet weigh only half a pound, 

 and at an altitude of 15 miles the same volume weighs 

 only about half an ounce. This shows that air decreases 

 rapidly in density as one goes to higher altitudes. 



This rapid decrease in the density and pressure of the 

 air as we go upward explains why man and other animals 

 cannot ascend to the tops of the highest mountains; 

 also why balloons and flying machines cannot go many 

 miles above the earth's surface. All birds and flying 

 devices made by man depend upon the pressure of the 

 air to sustain them and keep them from falling to the 

 earth. The monoplanes and biplanes-are, of course, heavier 

 than the air which they displace, but they are kept from 

 falling by being moved forward at a great speed with 

 the planes set at such an angle that the air, which is being 



