PNEUMATICS. 
^liinh terminate the atmospheric cohiran 
at bottom are actually occupied by the 
mercury. In general, if we take the ratio 
between the first pressure from the column 
of the atmosphere, and any other pressure 
whatever exerted by that same column, 
and by the mercury superadded, the cor- 
responding'Spaces, occupied by the com- 
pressed air, will be respectively in the in- 
verse ratio of the pressures; whence it is ob- 
vious, that the air contracts itself, as we have 
stated, in proportion to the weights com- 
pressing it. If we afterwards take out the 
mercury at several distinct times, the air 
will expand by reason of its elasticity, and 
the spaces which it will successively occupy 
in a contrary order will still conform to the 
inverse ratio of the pressures. 
Having given this brief account of the 
general properties of air, we shall refer to a 
few experiments, and the instruments which 
are commonly used in performing these ex- 
periments ; beginning with the air-pump, 
which has been already described in a 
general way. Fig. 2, is an air pump, much 
in use. A A are two brass barrels, each 
containing a piston, with a valve opening 
upwards. They are worked by means of 
the winch, B, which has a pinion that fits 
into the teeth of the racks, C C, which are 
made upon the ends of the pistons, and by 
this means moves them up and down alter- 
nately. On the square wooden frame, D E, 
there is placed a brass plate, G, ground 
perfectly flat, and also a brass tube, let 
into the wood communicating with the two 
cylinders and the cock, I, and epening into 
the centre of the brass plate at «. The 
glass vessel, K, to be emptied or exhausted 
of air, has its rim ground quite flat, and 
rubbed with a little pomatum, or hog’s-lard, 
to make it fit more closely upon the brass 
plate of the pump. Sometimes thin slips 
of moistened leatiier are used for this pur- 
pose. These vessels are called receivers. 
Having shut the cock, I, the pistons are 
worked by the winch, and the air being 
suffered to escape when the piston is forced 
down, because the valve opens upwards, 
but prevented from returning into the 
vessel, for the same reason the receiver is 
gradually exhausted, and will then be fixed 
fast upon the pump-plate. By opening the 
cock, I, the air rushes again into the re- 
ceiver. 
To the air pump is attached the gauge, z, 
or instrument for measuring the degree of 
rarefaction, or exhaustion, produced in the 
Receiver, and which is a necessary appen- 
dage to the air-pump. If a barometer be 
included beneath the receiver, the mercury 
will stand at the same height as in the open 
air, but when the receiver begins to be ex- 
hausted, the mercury will descend, and rest 
at a height, which is, in proportion to its form- 
er height, as the spring of tlie air remain- 
ing in the receiver is to its spring before 
exhaustion. Thus, if the height of the mer- 
cury, after exhaustion, is the thousandth 
part of what it was before, we say that the air 
in the receiver rarefied is a thousand times. 
On account of the inconvenience of includ- 
ing a barometer in a receiver, a tube, of 
six or eight inches in length, is filled with 
mercury, and inverted in the same manner 
as the barometer. This being included, an- 
swers the same purpose, with no other dif- 
ference, than that the mercury does not 
begin to descend till after about three- 
fourths of the air is exhausted : it is called 
the short barometer gauge. This is gene- 
rally placed detached, but communicating 
with the receiver by a tube concealed in 
the frame, as is represented in the figure ; 
another and better guage was invented by 
Mr. Smeaton, and called from its form, the 
pear-guage. It consists of a glass vessel, 
in the form of a pear (fig. 3), and sufficient 
to hold about half a pound of mercury ; it 
is open at one end, and at the other end is 
a tube hermetically closed at top. The 
tube is graduated, so as to represent pro- 
portionate parts of the whole capacity. 
This gauge, during the exhaustion of the 
receiver, is suspended in it by a slip wire, 
over a cistern of mercury, placed also in 
the receiver. When the pump is worked 
as much as is thought necessary, the gauge 
is let down into the mercury, and the air 
re-admitted. The mercury will immediately 
rise in tlie gauge ; but if any air remained 
in the receiver, a certain portion of it would 
be in the guage ; and as it would occupy 
the top of the,tube above the mercury, it 
would shew by its size the degree of ex- 
haustion ; for the bubble of air would be 
to the whole contents of the gauge, as the 
quantity of air in the exhausted receiver 
would to an equal volume of the common 
atmospheric air. If the receiver contained 
any elastic vapour generated duritig the 
rarefaction, it would be condensed upon 
the re-admissipn of the atmospheric air, as 
it cannot subsist in the usual pressure. The 
pear-gauge, therefore, shows the true quan- 
tity of atmospheric air left in the receiver. 
Hence it will sometimes indicate that all 
the permanent air is exhausted from the 
