PNEUMATICS. 
46 ! 
and the receiver is exhausted, the stop- 
screw B, at the bottom of the pump, must 
be unscrewed, to admit the air into the re- 
ceiver ; but when the gages are not all used, 
the stop-screw at«, or either of the others in 
the place of gages, may be unscrewed for 
this purpose. The mechanism and object of 
the barrels D, D, the racks C, C, the plate 
G, and handle II, will be easily understood 
from the figure. CD fig. 2. represents a sec- 
tion of one of the barrels of the pump, F the 
collar of leathers which renders it air-tight, 
G a hollow cylindrical vessel to contain oil ; 
K. is- also an oil-vessel, which receives the 
ail that is driven with air through the hole 
an, when the piston is drawn upwards; and 
when this falls, the oil is carried over with the 
air along the tube T, into the oil-vessel G : 
c c is a wire which is driven upwards from 
the hole a a by the passage of the air ; and 
as soon as this is escaped, falls down again 
by its own weight, shuts up the hole, and 
prevents any air from returning by that way 
into the barrel ; at d d are fixed two pieces 
of brass, to keep the wire c c in such a direc- 
tion as may preserve the hole air-tight. H 
is a cylindrical wire, which carries the piston 
I, and is made hollow to receive a long wire 
q q, that opens and closes the hole L, which 
forms the communication with the receiver 
standing on the plate : m is part of a pipe, 
one end of which is screwed into the wire 
q q, that opens and shuts the hole L ; and 
upon the Other end O, is screwed a nut, 
which, stopping in the smaller part of the 
hole, prevents the wire from being lifted too 
high. This wire and screw are more dearly 
seen in fig. 3. ; they slide through a collar 
of leathers r r (see figs. 3 and 5,) in the 
middle of the piston. Figures 5 and 6, are 
the two main parts which compose the pis- 
ton ; and when the pieces in tigs. 7 and 4 
are added to it, the whole is represented in 
fig. 3. Figure 5 is a piece of brass, turned 
in a conical form, with a shoulder or ledge at 
the bottom ; a long female screw is cut into 
it, about two-thirds of its length ; and the 
remaining part of the hole, in which there 
is no screw, is about the same-sized diame- 
ter as the screw part ; except a thin plate at 
the end, which is of a breadth exactly 
equal to the thickness of q q fig. 2. That 
part of the inside of the conical piece of 
brass, in which no thread is cut, is filled with 
oiled leathers with holes in them, through 
which q q can slide air-tight ; there is also a 
male screw with a hole in it which is fitted to 
q q, and serves to press down the leathers 
r r. In fig. 6, a a a a is the outside of the 
piston, the inside of which is turned exactly 
to fit the outside of fig. 5 ; b b are round 
leathers, cc is a circular plate of brass of 
the size of the leathers, and dd is a screw 
which serves to press them down as tight as 
it is necessary. The male screw at the end 
of fig. 7, is made to fit the screw in fig. 5. 
If fig. 4. is put into fig. 5, and that again 
into fig. 6, and fig. 7 screwed into the end 
of fig. b, these will compose the whole pis- 
ton as represented by fig. 3. II in fig. 2, is 
that part to which the rack is fixed. If this, 
therefore, is drawn upwards, it will make 
fig. 5 shut dose to fig. 6, and drive out the 
a'r above it; and when it is pushed down- 
wards, it will open as far as the shoulder's a a 
will allow, and suffer the air to pass through. 
AA fig. 8. is the receiver-plate ; B is a long 
square piece of glass screwed to the under- 
most side of the plate, through which a hole 
is drilled, corresponding with that in the 
centre of the receiver-pia;e, and with the 
three female screws b b c. 
To conceive how the rarefaction of the air 
is effected, suppose the piston to be at the 
bottom of the barrel, and a receiver to stand 
upon the plate ; the inside of the barrel from 
the top of the piston to a is full of air, and 
the piston shut: when drawn upwards, by 
the cylindrical wire FI, it will drive the air 
before it through the hole an into the oil- 
vessel 11, and out into the atmosphere by 
the tube T. The piston will then be at the 
top of the barrel at a, and the wire q q will 
stand nearly as it is represented in the 
figure, just raised from the tube L, and pre- 
vented from rising higher by means of the nut 
n. While the piston is moved upwards, the 
air will expand in the receiver, and be driven 
along the bent tube m into the inside of the 
barrel. Thus the barrel will be filled with 
air, which, as the piston rises, will be rare- 
fied in proportion as the capacity of the re- 
ceiver, pipes, and barrel, is to the capacity 
of the barrel alone. When the piston is 
moved downwards again by H, it will force 
the conical part tig. 5, out of the hollow 
part fig. 6, as far as the shoulders a a . ; fig. 
3. will rest upon a a, fig. 6, which will then 
be so far open as to permit the air to pass 
freely through it, while at the same time the 
end q q is forced against the top of the hole, 
and closes it in order to prevent any air 
from returning into the receiver. Thus the 
piston, while moved downwards, suffers the 
air to pass out between the figs. 6 and 5, 
and when it is at the bottom of the barrel, 
will have the column of the air above it ; 
and, consequently, when drawn upwards, it 
will shut and drive out this air, and by open- 
ing the hole L, give a free passage to more 
air from the receiver. This process being 
continued, the air will be exhausted out of the 
receiver as far as the expansive power will 
permit : for in this instrument there are no 
valves, as in common air-pumps, to be forced 
open by the air in the receiver, which, when 
its elasticity is diminished, it becomes unable 
to effect ; nor is there any thing to prevent 
the air from expanding to the greatest de- 
gree. 
The oil-vessel G, fig. 2, must be always 
kept about half-full of oil ; and when it has 
stood long without using, it will be right to 
draw a table-spoonful or more through it, by 
pouring it into the hole a, in the middle of 
the receiver-plate, fig. 1. when the piston is 
at the bottom of the barrel ; then by moving 
the winch FI backwards and forwards, the 
oil will be drawn through all the parts of 
the machine, and the superfluous part will 
be forced out through the tube T, into the 
oil-vessel G. Near the top of the cylindri- 
cal wire FI, fig. 2, is a square hole, which is 
intended to let in some of the oil from the 
vessel G, that the oiled leathers, through 
which the wire qq slides, may always be 
duly supplied with it. Fig. 9. is a repre- 
sentation of a condensing - apparatus used 
with this pump. 
Mr. Cuthbertson has by many experiments 
shewn the great powers of exhaustion of 
which this pump is capable. With the 
double syphon-gage, and also with the long 
gage, compared with an attached barometer 
in which the mercury has been well boded, 
the difference between the heights of the 
mercurial column proved no more than 
of an inch: the barometer standing at 30 
inches, which is an exhaustion of 1200 times ; 
and on some occasions, when the air was 
very dry, he observed the difference to be 
as low as .j-i_ of an inch, which gives more- 
than double tliat degree oi rarefaction. 
We must not omit the American air-pump, 
invented by Mr. Prince, who first took away 
the valves, which w'ere long known to pre- 
vent the air from entering the barrel above 
the piston. Fiis next attempt was to expel, 
the air more perfectly oi.t of the barrel than 
Mr. Smeaton had done, by making a better 
vacuum between the piston and the top 
plate, so that more of the air might be al- 
lowed to expand itself into the barrel from 
the receiver. Mr. Prince also contrived to 
connect the valves on the top plate with the 
receiver occasionally by means of a pipe and 
cock, by the turning of which the machine 
might be made to exhaust or condense at 
pleasure. In order to remove the pressure 
of the atmosphere from the valve on the top 
plate, so that this valve might open as easily 
as the piston-valve, he connected with the. 
duct on the bottom piece, which conveys the 
air from the valves to the cock, a small 
pump of the same construction as the large 
one, having the barrel opening into the cis- 
terfT ; the piston-rod, which is solid, moving 
through a collar of leathers,, and a valve near 
the top, through w hich the air is forced into 
the atmosphere. This pump with one barrel 
is called the valve-pump ; its chief use being, 
to rarefy the air above the valves, or to re- 
move the weight of the atmosphere from 
them. When this valve-pump is used, the 
passage through the cock is shut up ; and, 
therefore, instead of placing three ducts 
at equal distances round the cock in the 
manner of Mr. Smeaton’s, Mr. Prince divid- 
ed the whole into five equal parts, leaving 
the distance of one-fifth part between the 
ducts leading" from the cistern and valves to 
the cock, and two-fifths between each of 
these and the one leading from the cock to 
the receiver. By this adjustment, when the 
communication is open between the receiver 
and the valves for condensation, the other 
hole through the cock opens the cistern to 
the atmosphere; but when the communica- 
tion is made between the cisterns and the 
receiver for exhaustion, a solid part of the 
key comes against the duct leading to the 
valve and shuts it up, and the air which is 
forced out of the barrel, passes through the 
atmosphere into the valve-pump ; for the 
valve of the small pump may be kept open 
while the great one is worked. 
Upon this construction, the pump with two 
barrels may be made like the common pump, 
which cannot be conveniently done where the 
lower valve is retained. In this pump, the 
pistons do not move the whole length of the 
barrels ; an horizontal section being made in 
them a little more than half-way from the 
bottom, where the top plates are inserted. . 
The pump is thus made more convenient and 
simple ; as the head of it is brought down 
upon the top of the barrels, in the same man- 
ner as in the common air-pump. r I he barrels 
