248 PHILOSOPHICAL TRANSACTIONS. [aNNO 1751. 



assisted on all sides by those placed round it ; and as they all together contribute 

 as much to raise the bladder over the centre hole, as the air immedietely acting 

 under it ; on this account the valve will be raised with double the ease, that we 

 have before supposed, or with a 6th part of the force commonly necessary. It is 

 not material to consider the force of the cohesion, after the first instant : for 

 after the bladder begins to rise, it exposes a greater surface to the air underneath, 

 which makes it move more easily. He has not brought into this account the 

 force that keeps down the valve, that arises from the weight of the bladder, and 

 the resistance from its being stretched ; for he conceives these as small in compa- 

 rison of the other. 



But supposing all those difficulties to be absolutely overcome, the other defect 

 mentioned in the common construction, would hinder the rarefaction from 

 being carried on beyond a certain degree. For as the piston cannot be made to 

 fit so close to the bottom of the barrel, as totally to exclude all the air ; as the 

 piston rises, this air will expand itself; but still pressing on the valve, according 

 to its density, hinders the air within the receiver fi-om coming out : hence, were 

 this vacancy to equal the 150th part of the capacity of the whole barrel, no air 

 could ever pass out of the receiver, when expanded 1 50 times, though the piston 

 was constantly drawn to the top ; because the air in the receiver would be in 

 aequilibrio with that in the barrel, when in its most expanded state. This I 

 have endeavoured to overcome, by shutting up the top of the barrel with a plate, 

 having in the middle a collar of leathers, through which the cylindrical rod 

 works, that carries the piston. By this means, the external is prevented from 

 pressing on the piston ; but that the air, that passes through the valve of the 

 piston from below, may be discharged out of the barrel, there is also a valve ap- 

 plied to the plate at the top, that opens upwards. The consequence of this con- 

 struction is, that when the piston is put down to the bottom of the cylinder, the 

 air in the lodgment under the piston will evacuate itself so much the more, as 

 the valve of the piston opens more easily, when pressed by the rarefied air above 

 it, than when pressed by the whole weight of the atmosphere. Hence, as the 

 piston may be made to fit as nearly to the top of the cylinder, as it can to the 

 bottom, the air may be rarefied as much above the piston, as it could before have 

 been in the receiver. It follows therefore, that the air may now be rarefied in 

 the receiver, in duplicate proportion of what it could be on the common prin- 

 ciple ; every thing else being supposed perfect. 



Another advantage of this construction is, that though the pump is composed 

 of a single barrel,* yet the pressure of the outward air being taken oft' by the 



• It is obvious that these improvements will equally obtain, whether the pump is conslnicled 

 with a single or double barrel. — Orig. 



