Descripiion of an improved Air-Pump. 39 
tion and procures a close fitting of the parts; and if after 
use it should at any time be found not air-tight, the defect 
is instantly remedied by means of four screws ; which is a 
considerable advantage over stop-cocks. The manner of its 
application is shown in fig. 1. (Plate II.); the perspective is 
considerably strained for the purpose of rendering the con- 
struction more intelligible. AB is the barrel, having a strong 
plate of brass, CDEF, firmly fixed to the bottom with two 
small holes, G and H, that enter the inside; but near the 
edge of the barrel IKLM is another plate of brass similar 
to the former, having two holes, N and O, that correspond 
with G and H. In the hole N is closely screwed the duct P, 
which leads to the receiver of the pump; and in the hole O 
is screwed the duct Q, through which the air is discharged. 
The plates, being ground smooth, are faced with leather, 
pierced opposite the holes, and smeared with grease; then 
the polished brass slide RS, fig. 2, is placed between them. 
In the slide is a-hole T, which is put over the hole N. 
The plates are then pressed together by the four screws 
V,U, W,X, which screw into nuts that are fixed into the 
bottom part of the machine; they also serve to direct the 
motion of the slide, which is drawn backwards and forwards. 
When the hole T is between N and G, the piston Y is 
drawn up, which causes the air to rush out of the receiver 
through the duct P into the barrel; the slide is then drawn 
by the ring Z, for the hole T to be between O and H. ~The 
communication between the barrel and discharging duct Q 
is opened ; the piston is then forced down, and the air is 
discharged. The slide is again shifted; which closes up the 
discharging aperture and opens the former, and the process 
is continued until the receiver is exhausted. At each end 
of the’ slide is a small ridge of brass that prevents its being 
pushed or pulled too far, so that the hole in the slide will 
exactly correspond with the’ holes in the plates. 
_If the pump has two barrels, by making the slide a little 
longer, and with two holes, it will serve both by only once 
moving ; and, from what has been said, it is evident that, 
merely by changing the motion of the slide, the machine 
will condense as well as exhaust. When the receiver is ex- 
C4 hausted, 
