THE EARTH. 57 



of but ten feet ; so will a man at the 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 cannot 

 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 

 vessel 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 no- 

 thing 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 experiments 

 we learn the exact weight with which it presses. 

 First, if 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 



* This may be done by burning a bit of paper in the same, and then 

 quickly turning it down upon the water. 

 VO L. I. R 



