13:2 PNEUMATICS. 



water immediately under it; consequently the 

 water is obliged to yield in that part to the pres- 

 sure on the surface. 



It is upon this principle that all those pumps 

 called suckmg-pumps act: the piston, fitting tightly 

 the inside of the barrel, by being raised up, re- 

 moves the pressure of the atmosphere from that 

 part, and consequently the water is drawn up by 

 the pressure upon the surface. 



These effects, arising from the weight and pres- 

 sure of the asmosphere, were formerly attributed 

 to suction; a word which ought to be exploded, 

 as it conveys a false notion of the cause of these 

 and similar phenomena. To prove that an ex- 

 hausted receiver is held down by the pressure of 

 the atmosphere, take one open at top, and ground 

 quite fiat, as A, (Plate 6. fig. 3.) and covered with 

 a brass plate B, which has a brass rod passing 

 through it, working in a collar of leather, so as to 

 be air-tight: to this rod suspend a small receiver 

 within the large one, a little way from the bottom - y 

 place the receiver A upon the pump-plate, and ex- 

 haust it: it will now be fixed fast down ; but the 

 small receiver may be pulled up or down with per- 

 fect ease, as it is itself exhausted, and all the air 

 which surrounded it removed, consequently it can- 

 not be exposed to any pressure ; let then the small 

 one down upon the plate, but not over the hole by 

 which the air is extracted, and re-admit the air 

 into the large receiver, which may then be re- 

 moved j it will be found that the small one being 

 itself exhausted, is held down fast by the air, 

 which is now admitted round the outside. If the 

 large receiver be again put over it and exhausted, 

 the small one will be at liberty, and so on, as often 

 35' the experiment is repeated. 



