B is the barrel provided with a solid piston P whose rod slides air 

 tight in the collar of leathers M : C and D are valves opening into 

 the atmosphere. The barrel communicates at E with a smaller bar- 

 rel G, containing the receiver-valve, or rather the apparatus that per- 

 forms its office. This is soldered to the body of the pump, and its 

 orifice is in the upper flanch of it, so that the top plate when screwed 

 down covers both ; it has a piston //, rather less than half its length, 

 accurately fitted, which is perforated and provided with a valve at I 

 opening upwards. The rod of this slides also in a collar of leathers, 

 and is supplied with a spiral spring of sufficient strength to keep it 

 always at the top of its barrel. K is the receiver pipe communicat- 

 ing with the barrel G by a small aperture O. In the operation of 

 this instrument, two distinct modifications are presented ; at first it 

 is equivalent to the action of a double barrelled pump of the com- 

 mon construction, exhausting both in the up and down strokes : but 

 when the exhaustion becomes the quantity which I denote by the 

 symbol v, its action changes, and it becomes equivalent, or indeed 

 superior to, a single barrel of Cuthbertson's construction. Suppose 

 the piston at the lop of JB, and let it be depressed, the air below it 

 is driven through C, while that contained in the receiver comes 

 through K O, and raising the valve I, passes at jE into the space 

 above P. On the return of the piston the valve I closes, and nearly 

 all the air is discharged into the atmosphere through D ; till at the 

 end of the stroke, when P has risen above the aperture F (which 

 communicates with the passage E by a perforation drilled in the 

 solid brass of the barrel.) I opens again, and the air of the re- 

 ceiver expands through F into the space below P. This is dis- 

 charged at C in the next stroke, and so on, as long as the elasticity 

 of the air is able to open I. But in this we have supposed the pis- 

 ton H to remain fixed, which is not the case ; for when P is at 



VOL. XV. JO 



