1S4- PNEUMATICS. 



But if the pressure be removed from any particular 

 part, the pressure on the neighbouring parts im- 

 mediately becomes sensible. Thus, if we take a 

 receiver open at the top, and cover it with the 

 hand, upon exhausting the receiver, and thereby 

 taking off the pressure from the palm of the hand, 

 we shall feel it pressed down by an immense 

 weight, so as to give pain that would soon be in- 

 supportable, and endanger the breaking of the 

 hand. 



If the top of the receiver be covered by a piece 

 of flat glass, upon exhausting it, the glass will be 

 broken to pieces by the incumbent weight ; and 

 this would happen to the receiver itself, but for 

 the arched top, that resists the weight much more 

 than a flat surface. 



This experiment may be varied, by tying a piece 

 of very thin wet bladder over the open mouth of 

 the receiver, and leaving it to dry till it becomes 

 as tight as a drum. Upon exhausting the receiver, 

 you will perceive the bladder rendered concave, 

 and it will yield more and more, until it break 

 with a loud report, which is occasioned by the air 

 striking forcibly against the inside of the receiver, 

 upon being re-admitted. 



Air is one of the most elastic bodies in nature ; 

 that is, it is easily compressed into less compass, 

 and when the pressure is removed, it immediately 

 regains its former bulk. 



Let mercury be poured into a bent tube A B C D 

 (Plate 6. fig. 4.) open at both ends, to a small 

 height, asBC; then stopping the end D with a 

 cork, or otherwise, air-tight, measure the length 

 of confined air D C, and pour mercury into the 

 other leg A B, till the height above the surface of 

 that in C D be equal to the height at which it 



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