PROPERTIES OF GASES 



99 



pressure because of the elasticity of the fluids of the body, 

 which exert a similar outward pressure. When travelers 

 rapidly ascend mountains of great height, or airmen reach 

 high levels in the air, blood sometimes bursts through the 

 thin membranes of the nose and eyes. This is because 

 the pressure outside the body has diminished while the 



FIG. 46. AN AIR PUMP 



A receiver (r) fits closely upon a metal plate. A tube () connects 

 the space in the receiver with the cylinder of the pump. When the piston 

 (p) is lifted, the space c is enlarged and air moves into it from r, equalizing 

 the pressure. When the piston goes down, V closes and V opens and 

 allows air to pass through. A part of the air originally in the receiver is 

 removed. A gauge (g) connected with t shows' that the pressure in the 

 receiver is now less than normal. 1. If one half of the air of the receiver 

 is removed by one double stroke of the piston, how much of the original 

 amount will be left after two such strokes? 2. How much after four strokes? 



internal pressure has not had time to change during the 

 rapid ascent. 



108. The Air Pump. The air pump is a machine for 

 taking air out of a closed space, in somewhat the same 

 way that water is pumped from a well. An inverted jar, 

 called the receiver, is placed upon a brass plate, which has 

 an opening into a tube connected with the cylinder of the 

 pump. The receiver fits the plate air-tight, and at each 



