214 PNEUMATICS. 



surface of the earth the air is about eight hundred times 

 hghter than the water, and the higher we ascend, the 

 rarer or lighter it becomes, from the diminished press- 

 ure of its weight above. At seven miles high, it is 

 four times lighter than at the surface ; at twenty-one 

 miles, it is sixty-four times lighter ; and at fifty miles, 

 about twenty thousand times lighter. At this height 

 it ceases to refract the rays of the sun so as to render 

 it visible at the earth's surface ; but if it decreases at 

 the same rate upward, at a hundred miles high it must 

 be nearly a thousand miUion times rarer than at the 

 earth. 



If the atmosphere were uniformly of the same dens- 

 ity, with its present weight, it would reach only five 

 miles high. Although so much lighter than water, 

 yet, from its great height, it presses upon the surface 

 of the earth as heavily as a depth of thirty-three feet 

 of water. This is nearly equal to fifteen pounds on 

 every square inch, or more than two thousand pounds 

 to the square foot. This enormous weight would in- 

 stantly crush us, did not air, like liquids, press in every 

 direction, so that the upward exactly counterbalances 

 the downward pressure, and the air within the body 

 counteracts that without. 



The weight of the atmosphere is strikingly shown 

 by means of an air-pump, which pumps the air from a 

 glass vessel, placed mouth downward upon the brass 

 plate of the machine {Fig. 178). When the air is 

 pumped out, and the upward or counterbalancing air 

 removed, so heavy is the load upon the glass vessel, 

 that a strong man could scarcely remove it from the 

 plate, although it be no larger than a small tumbler. 



