VOL. LXXIII.] PHILOSOPHICAL TRANSACTIONS. 455 



of oil-silk, and passing by the sides of it, and also through the large perforation 

 of the piece 2,4, will go upwards, &c; but can by no means return backwards, 

 since any pressure that the air makes on the upper part of the oil-silk will only 

 stop the passage more effectually. 



A valve much like this is in the piston, the parts of which are shown sepa- 

 rately in fig. 2 : u is a perforated brass piece screwed to the cylindrical handle or 

 axis, which is also perforated with a short and bent hole. The piece x is screwed 

 into fhe part u, and contains a valve, viz. a small piece 6 with a slip of oil-silk 

 tied round its groove yy, which slip of oil-silk bears against the hole 5. The 

 piece x screwing into the other piece u, fastens the round leathers, about 30 in 

 number, which form the stopping part of the piston, and rub with their edges 

 against the cavity of the barrel. This is a very useful improvement, since the 

 common way of using two leathers turned over corks is both troublesome to 

 make, and seldom fits exactly. 



Hence, it appears, that the air can pass through the valve from without to 

 within the barrel, but not vice versa. It will be also easily conceived, that the 

 air can pass from the cavity of the tube of communication only when the said 

 air has elasticity, or force enough to push up the oil-silk. Now the principal 

 improvement in this machine is, to lift up the oil-silk by a power applied exter- 

 nally, when the weakened elasticity of the air within the cavity of the tube is 

 not capable of doing it by itself, and the description of this mechanism is as 

 follows. The double ring 8, 8, fig. 3, which holds the oil-silk, is fastened to 2 

 steel wires 9, 9. Those wires pass through collars of leathers held in proper 

 brass boxes ho., screwed to the piece k, and furnished with caps, 11, 11. The 

 lower extremities of the wires are fastened to the cross bar, 7, 7, of a brass frame. 

 If this frame is moved upwards, the wires 9, 9, and the double ring 8, 8, with 

 the oiled-silk, being all connected together, will be pushed also upwards ; con- 

 sequently, the oiled silk being removed from the hole of the piece k, a free 

 communication is opened between the cavity of the tube and the cavity of the 

 barrel, through which the air, however rarefied or weakened in elasticity, can 

 pass without the least impediment. 



In order to move the brass frame upwards, the end of a lever bears against it. 

 When the valve is to be opened, the foot of the operator must press on the ex- 

 tremity of the lever, by which means the other extremity, with the frame, the 

 wires 9, 9, and the double ring 8, 8, with the oiled silk, are all lifted up. But 

 in order to bring down again all those parts, and to shut the valve when the 

 pressure of the foot is removed, there is an open brass tube fastened to the piece 

 k, which contains a spiral spring, that, bearing against the extremity of the 

 brass frame, pushes it downwards. 

 There are several other minuteand particular appendages to thismachine,which seem 



