454 PHILOSOPHICAL TRANSACTIONS. [ANNO 1783. 



of the best air-pumps made before Mr. Smeaton's invention, the rarefaction of 

 the air within the receiver could never have been brought to more than 40 or 50 

 times, if the heat of the place was about 57°; that even with Mr. Smeaton's 

 pump the receiver could not be exhausted beyond 70° or 80° of rarefaction, when 

 moist leathers were used, or moisture was in any way introduced within the re- 

 ceiver ; but that when this pump is quite free from moisture, and is newly 

 cleaned, oiled, and put together, then the air may by it be rarefied about 600 

 times, and not farther.* 



The chief cause which prevents this pump from exhausting farther than that 

 limit is the weakened elasticity of the air remaining within the receiver ; which, 

 decreasing in proportion as the quantity of the air within the receiver is dimi- 

 nished, becomes at last incapable of lifting up the valve, which opens the com- 

 munication between the receiver and the barrel ; consequently no more air can 

 in that case pass from the former to the latter. To remove this imperfection of 

 the best air-pumps had been attempted by several ingenious persons ; but Mr. C. 

 thinks was never obtained before the contrivance of the air-pump here described. 

 This, he says, is the contrivance of a Mr. Haas, an ingenious maker of philo- 

 sophical instruments ; and that besides this capital improvement, his air-pump is 

 rendered altogether more convenient for philosophical experiments, by answering 

 several purposes. The description of the pump is then given, and illustrated by 

 3 large engraven plates. As the chief improvement in the machine is said to 

 consist in the contrivance of the valve which opens the communication between 

 the receiver and the barrel, we shall more particularly attend to that part in our des- 

 cription. Now this valve which lets the air pass upwards, but prevents its return, 

 is so contrived as that, when the piston is drawn quite to the top of the barrel, 

 the least possible quantity of air should be left in the barrel. The parts which 

 form this valve are shown separately in fig. 1, pi. 7 ; where 1,3 is a brass piece 

 that screws into a proper cavity made for its reception, and which is hollow, ex- 

 cept its lower part, where it consists of a thin lamina perforated with a small 

 hole 3. Into the hollow of the last-mentioned part is screwed the other per- 

 forated piece 2,4, having a slip of oil-silk stretched over its lower part 4, and 

 tied round a small indenture or groove made on its lower part. This slip of oil- 

 silk answers better than a piece of bladder or leather: it just covers the hole 3, 

 and is about 4 times broader than the diameter of the hole. It will be easily 

 conceived, that when the air is forced through the hole 3, it will lift up the slip 



* The degree of rarefaction shown by what is called the pear-gage, when any vapour of water is 

 within the receiver, is not to be considered as the degree of rarefaction of the elastic fluid in the re- 

 ceiver, but only of the air ; for though the air may be exhausted, yet the vapour of water vt ill 

 .supply its place ; we shall therefore only take notice of the exhaustion when no vapour or moisture 

 is within the receiver. See Nairne's Experiments, Phil. Trans, vol. 47. — Orig. 



