122 A HISTORY OF 



from this experiment, therefore, we learn, that the earth, .nd all 

 things upon its surface, are every where covered with a ponderous 

 fluid, which rising very high over our heads, must be proportionally 

 heavy. For instance, as in the sea, a man at the depth of twenty feet 

 sustains a greater weight of water, than a man at the depth of but ten 

 feet ; so will a man at trie bottom of a valley have a greater weight of 

 air over him, than a man on the top of a mountain. 



From hence we may conclude, that we sustain a very great weight of 

 air ; and although, like men walking at the bottom of the sea, we can- 

 not feel the weight which presses equally round us, yet the pressure 

 is not the less real. As in morals, we seldom know the blessings that 

 surround us till we are deprived of them ; so here we do not perceive 

 the weight of the ambient fluid till a part of it is taken away. If, by 

 any means, we contrive to take away the pressure of the air from any 

 one part of our bodies, we are soon made sensible of the weight upon 

 the other parts. Thus, if we clap our hand upon the mouth of a ves- 

 sel from whence the air has been taken away, there will thus be air 

 on one side, and none on the other ; upon which, we shall instantly 

 find the hand violently sucked inwards ; which is nothing more than 

 the weight of the air upon the back of the hand that forces it into the 

 space which is empty below. 



As, by this experiment, we perceive that the air presses with great 

 weight upon every thing on the surface of the earth, so by other ex- 

 periments we learn the exact weight with which it presses. First, it 

 the air be exhausted out of any vessel, a drinking vessel for instance,* 

 and this vessel be set with the mouth downwards in water, the water 

 will rise up into the empty space, and fill 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 experi- 

 ment, of above thirty-two feet high ; in which, upon being exhausted, 

 the water has always 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 a 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 sur- 

 face 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 quick- 

 silver ; and it is easily found, by computation, that to raise water thirty- 

 iwo feet, will require a weight of fifteen pounds upon every square 

 inch. Now, if we are fond of computations, we have only to calcu- 



* This may be done by burning a bit of paper m tlie same, an A fnen quickly turning i 

 town upon the water. 



