THE EAHl'H, 201 



them ; so here we do not perceive the weiglit of the ambient 

 fluid til] a part of it is taken away. If, by any means, we con- 

 trive 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 moutli of a vessel 

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

 on one side and none on the other j upon which we shall in- 

 stantly 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 experiments we learn the exact weight with which it presses. 

 First, if the air be exhausted out of any vessel, a drinking ves- 

 sel, for instance,' and this vessel be set with the mouth down- 

 wards in water, the water will rise up into the empty space, and 

 lill 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 be- 

 ing 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 be- 

 ing 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 fee: 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 cover- 

 ing 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, 



1 This may be done by burniiia' a bit of p^ipiT in the same, and then 

 quirkly turuing it down upon the watir. 



