A HISTORY OF 



determine the peculiar form of its parts which fl 

 gives it the spring or elasticity with which it 

 is endued : he might distinguish which of its ; 

 parts were pure incorruptible air, and which 

 only made for a little time to assume the ap- 

 pearance, so as to be quickly returned back 

 to the element from whence it came. But as 

 for us, who are immersed at the bottom of this 

 gulf, we must be contented with a more con- 

 fined knowledge; and. wanting a proper point 

 of prospect, remain satisfied with a combina- 

 tion of the effects. 



One of the first things that our senses in- 

 form us of is, that although the air is too fine ' 

 for our sight, it is very obvious to our touch. 

 Although we cannot see the wind contained 

 in a bladder, we can very readily feel its re- 

 sistance ; and though the hurricane may want j 

 colour, we often fatally experience that it 

 does not want force. We have equal expe- 

 rience of the air's spring or elasticity : the [ 

 bladder, when pressed, returns again, upon 

 the pressure being taken away; a bottle, 

 when filled, often bursts, from the spring of 

 air which is included. 



So far the slightest experience reaches; 

 but, by carrying experiment a little farther, 

 we learn that air also is heavy : a round glass 

 vessel being emptied of its air, and accurate- 

 ly weighed, has been found lighter than when 

 it was weighed with the air in it. Upon com- 

 puting the superior weight of the full vessel, 

 a cubic foot of air is found to weigh some- 

 thing more than an ounce. 



From this experiment, therefore, we learn, 

 that the earth, and all things upon its surface, 

 are every where covered with a ponderous 

 fluid, which rising very high over our heads, 

 must be proportionably 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 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 al- 

 though, 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 5 so here we do not per- 

 ceive the weight of the ambient fluid till a 

 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 

 lias been taken away, there will thus be air 

 on one side, and none on the other; upon 

 which we shall instantly find the hand vio- 

 lently 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 exhaust- 

 ed out of any vessel, a drinking vessel for 

 instance," arid tliis vessel be set with the 

 mouth downwards in water, the water will 

 rise up into the empty space, and fill the in- 

 verted 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 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 

 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. 



This may be done by burning a bit of paper in the 

 same, and then quickly turning it down upon the water. 



