VOL. XLVII.] PHILOSOPHICAL TRANSACTIONS. 8,47 



LXIX. Improvements made in the Air Pump. By Mr. S. Smeaton, p. 415. 



The chief causes of imperfection in the common pumps arise, first, from the 

 difliculty in opening the valves at the bottom of the barrels ; lA\y, from the pis- 

 ton's not fitting exactly, when put close down to the bottom ; which leaves a 

 lodgment for air, that is not got out of the barrel, and proves of bad effect. 



In regard to the first of these causes; the valves of air-pumps are commonly 

 made of a bit of thin bladder stretched over a hole generally much less than one 

 tenth of an inch diameter ; and to prevent the air from repassing between the 

 bladder and the plate, on which it is spread, the valve must always be kept moist 

 with oil or water. It is well known that at each stroke of the pump, the air is 

 more and more rarefied, in a certain progression, which would be such, that an 

 equal proportion of the remainder would be taken away, were it not aftected by 

 the impediments just mentioned: so that when the spring of the air in the re- 

 ceiver becomes so weak, as not to be able to overcome the cohesion of the blad- 

 der to the plate, occasioned by the fluid between them, the weight of the bladder, 

 and the resistance that it makes by being stretched, the rarefaction cannot be 

 carried further, though the pump should still continue to be worked. 



It is evident, that the larger the * hole is, over which the bladder is laid, a 

 proportionably greater force is exerted on it by the included air, in order to lift 

 it up ; but the aperture of the hole cannot be made very large, because the pres- 

 sure of the incumbent air would either burst the valve, or so far force it down 

 into the cavity, as to prevent its lying flat and close on the plate, which is abso- 

 lutely necessary. To avoid these inconveniences as much as possible, instead of 

 one hole, Mr. S. made use of 7? all of equal si^ie and shape ; one being in the 

 centre, and the other six round it : so that the valve is supported at proper dis- 

 tances, by a kind of grating, made by the solid parts between these holes : and 

 to render the points of contact, between the bladder and grating, as few as 

 possible, the holes are made hexagonal,' and the partitions filed almost to an 

 edge. As the whole pressure of the atmosphere can never be exerted on this 

 valve, in the construction made use of in this pump ; and as the bladder is 

 fastened in four places instead of 2, the breadth of the hexagons are made -^ of 

 an inch ; so that the surface of each of them is more than Q times greater than 

 usual. But as the circumference of each hole is more than 3 times greater than 

 common, and as the force that holds down the valve, arising from cohesion, is, 

 in the first moment of the air's exerting its force, proportionable to the circum- 

 ference of the hole ; the valve over any of these holes will be raised with 3 times 

 more ease than common. But as the raising of the valve over the centre hole is 



* If we examine the force, that air rarefied 140 times can exert in a common valve through a hole 

 of one tenth of an inch diameter, we shall find it not to exceed 0' grains at a medium. — Orig. 



