AIR PU5IP. 57 



weight of the water is sustained by the upward pressure of 

 the air upon the paper. 



The most essential point in which air differs from other 

 fluidSj is by its spring or elasticity, that is to say, its power 

 of increasing or diminisliing in bulk, according as it is more 

 Or less compressed. The elasticity of air differs from that of 

 bodies in general ; for when solid bodies are compressed 

 they have an elastic power, which causes them to resume 

 the same figure they possessed before compression : but on 

 removing the pressure on air, it will not only resume its first 

 bulk, but expand to an indefinite extent. With regard to 

 animal and vegetable bodies, the gravity of the air is de- 

 stroyed by its elasticity. It is true, that the atmosphere presses 

 with a weight of fifteen pounds upon every square inch of 

 the earth's surface, when the air is heaviest, and tliat conse- 

 quently a man's body, which contains nearly fifteen square 

 feet, will sustain a weight equal to about fourteen tons and a 

 half; but this pressure is so great that it would be absolutely 

 insupportable, and even fatal to us, were it not equal in 

 every part, and counterbalanced by the spring of that air 

 which fills all the vesicles of the body, and reacts with an 

 outward force equal to that with which the atmosphere 

 presses inward. 



By means of an air-pump, the air may be drawn out of a 

 large glass vessel, or receiver, and a vacuum produced, in 

 which a great number of curious experiments may be per- 

 formed, showing at once the properties and usefulness of the 

 air. We shall give a brief description of the air-pump, 

 though a view of the machine itself will convey a much bet- 

 ter idea of the important purposes to which it is applied, than 

 any description can afford- Two brass cylinders are closely 

 and firmly fastened down to the table or base of the machine, 

 by means of what are called the head and the columns. The 

 receiver is made to fit very accurately on a brass circular 

 plate, which has a hole in the middle, through which the 

 air passes from the receiver into a tube made of brass, that 

 communicates with the cylinders. Near the bottom of each 

 cylinder is a valve opening upwards, and above these valves 

 are two others in pistons which are moved up and down by 

 toothed rods that fall into a toothed wheel, to the axis of 

 which a handle is fixed. On turning the handle one of the 

 pistons is raised and the other depressed, consequently a ra- 



