PNEUMATICS. 133 



This effect cannot be accounted for upon any 

 other principle than the pressure of the air ; as the 

 common idea of suction can have nothing to do in 

 the case of the small receiver, which is fixed down 

 merely by letting in the air round it. We ought, 

 therefore, to attribute all those effects which are 

 vulgarly ascribed to suction, such as the raising of 

 water by pumps, &c. to the weight and pressure 

 of the atmosphere. 



A square column of quicksilver, 29i inches 

 high, and an inch thick, weighs just 15 pounds, 

 consequently, the air presses with a weight equal 

 to 15 pounds upon every square inch of the earth's 

 surface ; and 144< times as much, or 2160 pounds 

 upon every square foot. 



The earth's surface contains, in round numbers, 

 200,000,000 square miles; and as every square 

 mile contains 27,876,400 square feet, there must 

 be 5,575,080,000,000,000 square feet on the 

 earth's surface; which number multiplied by 2160 

 pounds (the pressure on each square foot) gives 

 12,043,468,800,000,000,000 pounds for the pres- 

 sure, or whole weight of the atmosphere. 



Reckoning the surface of a middle-sized man to 

 be about 14 square feet, he sustains a pressure 

 from the air equal to 30,210 lbs. troy, or 11 tons 

 2 cwt. and 18^ lbs. It may be asked, how it 

 happens that we are not sensible of so great a 

 pressure? The reason is, that such pressures only 

 are perceived by us as move our fibres, and put 

 them out of their natural situations. Now the 

 pressure of the air is equal on all parts of the body, 

 and it is balanced by the spring of the air con- 

 tained in the body; therefore it cannot possibly 

 displace any of the fibres, but on the contrary, 

 braces, and keeps them all in their relative situations. 



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