r the column of water or mercury contained in the jars. I shall
endeavour to explain these by examples, beginning with the most simple
case.

Suppose that 100 cubical inches of oxygen gas are obtained at 10 deg.
(54.5 deg.) of the thermometer, and at 28 inches 6 lines of the barometer,
it is required to know what volume the 100 cubical inches of gas would
occupy, under the pressure of 28 inches[58], and what is the exact
weight of the 100 inches of oxygen gas? Let the unknown volume, or the
number of inches this gas would occupy at 28 inches of the barometer, be
expressed by x; and, since the volumes are in the inverse ratio of
their superincumbent weights, we have the following statement: 100
cubical inches is to x inversely as 28.5 inches of pressure is to 28.0
inches; or directly 28 : 28.5 :: 100 : x = 101.786--cubical inches, at
28 inches barometrical pressure; that is to say, the same gas or air
which at 28.5 inches of the barometer occupies 100 cubical inches of
volume, will occupy 101.786 cubical inches when the barometer is at 28
inches. It is equally easy to calculate the weight of this gas,
occupying 100 cubical inches, under 28.5 inches of barometrical
pressure; for, as it corresponds to 101.786 cubical inches at the
pressure of 28, and as, at this pressure, and at 10 deg. (54.5 deg.) of
temperature, each cubical inch of oxygen gas weighs half a grain, it
follows, that 100 cubical inches, under 28.5 barometrical pressure, must
weigh 50.893 grains. This conclusion might have been formed more
directly, as, since the volume of elastic fluids is in the inverse ratio
of their compression, their weights must be in the direct ratio of the
same compression: Hence, since 100 cubical inches weigh 50 grains, under
the pressure of 28 inches, we have the following statement to determine
the weight of 100 cubical inches of the same gas as 28.5 barometrical
pressure, 28 : 50 :: 28.5 : x, the unknown quantity, = 50.893.

The following case is more complicated: Suppose the jar A, Pl. XII. Fig.
18. to contain a quantity of gas in its upper part ACD, the rest of the
jar below CD being full of mercury, and the whole standing in the
mercurial bason or reservoir GHIK, filled with mercury up to EF, and
that the difference between the surface CD of the mercury in the jar,
and EF, that in the cistern, is six inches, while the barometer stands
at 27.5 inches. It is evident from these data, that the air contained in
ACD is pressed upo