198 FERGUSON'S LECTURES. 



LECT. atmosphere upon that in the bason, is evident, ; for, 

 ^J 1 - if the bason and tube be put under a glass, and the air 

 be then taken out of the glass, all the quicksilver in the 

 tube will fall down into the bason ; and if the air be let 

 in again, the quicksilver will rise to the same height as 

 before. Therefore, the air's pressure on the surface of 

 the earth, is equal to the weight of 29i inches depth of 

 quicksilver all over the earth's surface, at a mean rate. 



A square column of quicksilver, 29J inches high, and 

 one inch thick, weighs just 15 pounds, which is equal 

 to the pressure of air upon every square inch of the 

 earth's surface ; and 144 times as much, or 2160 pounds 

 upon every square foot; because a square foot contains 

 144 square inches. At this rate, a middle-sized man, 

 whose surface may be about 14 square feet, sustains a 

 pressure of 30,240 pounds, when the air is of a mean 

 gravity : a pressure which would be insupportable, and 

 even fatal to us, were it not equal on every part, and 

 counterbalanced by the spring of the air within us, 

 which is diffused through the whole body ; naid re- 

 acts with an equal force against the outward pres- 

 sure. 



Now, since the earth's surface contains (in round huai- 

 bers) 200,000,000 square miles, and every square mile, 

 27,878,400 square feet, there must be 5,575,680,000, 

 millions of square feet on the earth's surface ; which 

 multiplied by 2160 pounds (the pressure on each square 

 foot, gives 12,043,468,800,000,000,000 pounds for the 

 pressure or weight of the whole atmosphere. 



When the end of a pipe is immersed in water, and 

 the air is taken out of the pipe, the water will rise in it 

 to the height of 33 feet above the surface of the water 

 in which it is immersed ; but will go no higher : for it 

 is found, that a common pump will draw water no 

 higher than 33 feet above the surface of the well : and 

 unless the bucket goes within that distance from the 

 well the water will never get above it. Now, as it is 



