58 HISTORY OF 



the inverted glass ; for the external air will, in 

 this case, press up the water where there is no 

 weight to resist ; as one part of a bed being 

 pressed, makes the other parts, that have no 

 weight upon them, rise. In this case, as was said, 

 the water being pressed without, will rise in the 

 glass, and would continue to rise (if the empty 

 glass- were tall enough) thirty-two feet high. In 

 fact, there have been pipes made purposely for 

 this experiment of above thirty-two feet high ; in 

 which, upon being exhausted, the water has al- 

 ways risen to the height of thirty-two feet : there 

 it has always rested, and never ascended higher. 

 From this, therefore, we learn, that the weight of 

 the air which presses up the water, is equal to at 

 pillar or column of water which is thirty-two feet 

 high ; as it is just able to raise such a column, 

 and no more. In other words^ the surface of the 

 earth is every-where covered with a weight of air 

 which is equivalent to a covering of thirty-two 

 feet deep of water ; or to a weight of twenty-nine 

 inches and a half of quicksilver, which is known 

 to be just as heavy as the former. 



Thus we see that the air, at the surface of the 

 earth, is just as heavy as thirty-two feet of water, 

 or twenty-nine inches and a half of quicksilver ; 

 and it is easily found,; by computation, that to 

 raise water thirty-two feet will require a weight 

 of fifteen pounds upon every square inch. Now, 

 if we are fond of computations, we have only to 

 calculate how many square inches are in the sur- 

 face of an ordinary human body, and allowing 

 every inch to sustain fifteen pounds, we may 



