MEASUREMENT OF PRESSURE. 305 



fifteen pounds to the square inch is an atmosphere of press- 

 ure. This is a fundamental principle or fact which it is 

 very important to remember. It comes continually into 

 philosophical and mechanical calculations. 



The meaning of the principle thus stated is, that, light 

 and rare as the atmosphere seems to us, in moving through 

 it, it extends to so great a height above the surface of the 

 earth, and the quantity is in the whole so great, that the 

 weight of it is equal to fifteen pounds upon every square 

 inch that it presses upon. That is to say, if you place a 

 small block of wood an inch square upon a table, and a fif- 

 teen pound weight upon the block, the additional pressure 

 would be that of one atmosphere; and this additional press- 

 ure would be just equal to the original pressure of the at- 

 mosphere itself, so that, with the pressure of the weight, the 

 whole pressure would be exactly doubled. 



Now this original pressure, great as it is, is not felt by 

 us, because it acts in every direction ; just as a fish swim- 

 ming in the water does not feel the weight of the water 

 over him, because, the water being so perfect a fluid, the 

 pressure resulting from the weight diffuses itself and bal- 

 ances itself in every direction, so that the fish floats in it, 

 as it were in the pressure, I mean, not the water and is 

 not sensible of it at all ; so w r e ourselves float, as it were, 

 in the pressure of the air, which acts from above and be- 

 low, upon every substance and upon every side of it, equal- 

 ly, and even from the pores and interstices within it out- 

 wardly, so that it produces, in ordinary cases, no percepti- 

 ble effect. The amount of it, however, in every direction 

 and from every side, is fifteen pounds to the square inch. 



Now this pressure is really much greater than one would 

 at first imagine. The surface of one side of a man's hand, 

 for example, contains not less, including the fingers, than 

 twenty square inches; consequently, the weight of the air 



