7 Experimental Philosophy. [Lecture 6. 



vious experiment. If we take a vessel of a long 

 or cylindrical shape, (fig, 30.) which is open at 

 the top, and place it on the pump, where the 

 receiver stands in fig. 28, then press it on the 

 top with the hand so as to exclude the external 

 air, we shall find, as the vessel begins to be 

 exhausted of air, a considerable pressure on the 

 back of the hand ; and if the operation is conti- 

 nued, that pressure will even become painful, and 

 we shall perceive it impossible to remove the 

 hand. This evinces that the weight of that co- 

 lumn of air which is above must be considerable, 

 and that the calculation above stated, of the 

 weight which a man's body usually bears, is not 

 overrated. If, instead of the hand, a piece of 

 bladder is tied over the open top of the vessel, 

 we shall see the bladder gradually sunk in like a 

 jelly-bag, and at length burst with considerable 

 force by the pressure of the external air ; a flat 

 piece of thin glass, placed in the same situation, 

 will be broken in pieces. Why then is the glass 

 receiver, which, we see, is placed on the pump in 

 fig. 1, not broken ? The reason of this is, first, the 

 shape of the glass, which is globular or arched at 

 top, and this is found, by long experience, to be 

 the best form for supporting a weight ; secondly, 

 these receivers are generally made of thick glass, 

 and with particular care, so as to sustain a 

 greater pressure than that of fifteen pounds on 

 a square inch without any danger of breaking. 

 A beautiful experiment to evince the pressure 



