Chap. 9.] Preffure of Air en tie Human Body. 427 



teen pounds upon every fquare inch. Computing, 

 therefore, the furface of a man's body at 15 iquare 

 feet, the whole preflure, which each pcrfon fuftains, 

 will be nearly equal to 3 3,480 pounds. By this enor- 

 mous preflure we fliould undoubtedly be crufhed in a 

 moment, if every part of our body was not filled with 

 air, or fome other elaftic fluid, the fpring of which is 

 fufficient to counteract the preflure. <c We are fear- 

 " fully and wonderfully made!" 



The whole quantity of preflure upon the earth muft 

 thus be immenfe, and has been computed equal to 

 that of a globe of lead of fixty miles in diameter. 



It is the gravity of the air which caufes that ftrong 

 preflure upon the hand, when it is placed upon the 

 mouth of a receiver open at the top, in which a va- 

 cuum has been made by an air-pump ; for as foon as 

 the air in the receiver has been* rarefied by the action 

 of the machine, it is no longer capable of fuftaining 

 the exterior preflure of the, air, as it would have been 

 if its denfity.had not been altered. It is, therefore, 

 the weight of the exterior air, which prefles the hand 

 with fuch force to the edges of the receiver; and this 

 preffure is according to the fize of the aperture of the 

 receiver, becaufe the column of air is enlarged in pro* 

 portion to the diameter of the aperture. 



Our furprife is excited by obferving, that notwith- 

 ftanding this great prefiure upon a glafs receiver, when 

 a vacuum is obtained, the glafs is not darned to pieces 

 as might be expected. Its prefervation is in a great 

 meafure owing to the rotundity of its figure, and to 

 the excels of the exterior furface over the interior; for 

 the fubflance which compofes the body of the veflfel 

 refembles, in this cafe, the fubftance which compofes 

 an arch in a bridge. We may be convinced of the 

 trutlj of this obfervation by taking a receiver of ano- 

 ther 



