to the jar V. which stands upon the shelf of a
small pneumato-chemical apparatus GHIK, the inside of which is shown Pl.
IX. Fig. 1. The second tube is applied against the outside of the vessel
LMNO from 6 to 7, is continued at 8, 9, 10, and at 11 is engaged below
the jar V. The former of these tubes is intended for conveying gas into
the machine, and the latter for conducting small quantities for trials
under jars. The gas is made either to flow into or out of the machine,
according to the degree of pressure it receives; and this pressure is
varied at pleasure, by loading the scale P less or more, by means of
weights. When gas is to be introduced into the machine, the pressure is
taken off, or even rendered negative; but, when gas is to be expelled, a
pressure is made with such degree of force as is found necessary.

The third tube 12, 13, 14, 15, is intended for conveying air or gas to
any necessary place or apparatus for combustions, combinations, or any
other experiment in which it is required.

To explain the use of the fourth tube, I must enter into some
discussions. Suppose the vessel LMNO, Pl. VIII. Fig. 1. full of water,
and the jar A partly filled with gas, and partly with water; it is
evident that the weights in the bason P may be so adjusted, as to
occasion an exact equilibrium between the weight of the bason and of the
jar, so that the external air shall not tend to enter into the jar, nor
the gas to escape from it; and in this case the water will stand exactly
at the same level both within and without the jar. On the contrary, if
the weight in the bason P be diminished, the jar will then press
downwards from its own gravity, and the water will stand lower within
the jar than it does without; in this case, the included air or gas will
suffer a degree of compression above that experienced by the external
air, exactly proportioned to the weight of a column of water, equal to
the difference of the external and internal surfaces of the water. From
these reflections, Mr Meusnier contrived a method of determining the
exact degree of pressure to which the gas contained in the jar is at any
time exposed. For this purpose, he employs a double glass syphon 19, 20,
21, 22, 23, firmly cemented at 19 and 23. The extremity 19 of this
syphon communicates freely with the water in the external vessel of the
machine, and the extremity 23 communicates with the fourth tube at the
bottom of the cylindrical vessel, and consequen