level, when the jar is at the bottom of the cistern, is increased by all
the water which it displaces, and is diminished in proportion as the jar
rises to its highest elevation. We next endeavour, by repeated trials,
to discover at what elevation the box 28 must be fixed, to render the
pressure equal in all situations of the beam. I should have said nearly,
because this correction is not absolutely rigorous; and differences of a
quarter, or even of half a line, are not of any consequence. This height
of the box 28 is not the same for every degree of pressure, but varies
according as this is of one, two, three, or more inches. All these
should be registered with great order and precision.
We next take a bottle which holds eight or ten pints, the capacity of
which is very accurately determined by weighing the water it is capable
of containing. This bottle is turned bottom upwards, full of water, in
the cistern of the pneumato chemical apparatus GHIK, Fig. 1. and is set
on its mouth upon the shelf of the apparatus, instead of the glass jar
V, having the extremity 11 of the tube 7, 8, 9, 10, 11, inserted into
its mouth. The machine is fixed at zero of pressure, and the degree
marked by the index 30 upon the sector m l is accurately observed;
then, by opening the stop-cock 8, and pressing a little upon the jar A,
as much air is forced into the bottle as fills it entirely. The degree
marked by the index upon the sector is now observed, and we calculate
what number of cubical inches correspond to each degree. We then fill a
second and third bottle, and so on, in the same manner, with the same
precautions, and even repeat the operation several times with bottles of
different sizes, till at last, by accurate attention, we ascertain the
exact gage or capacity of the jar A, in all its parts; but it is better
to have it formed at first accurately cylindrical, by which we avoid
these calculations and estimates.
The instrument I have been describing was constructed with great
accuracy and uncommon skill by Mr Meignie junior, engineer and physical
instrument-maker. It is a most valuable instrument, from the great
number of purposes to which it is applicable; and, indeed, there are
many experiments which are almost impossible to be performed without it.
It becomes expensive, because, in many experiments, such as the
formation of water and of nitric acid, it is absolutely necessary to
employ two of the same machines. In the present advanced state of
chemistry, very expensive and complicated instruments are become
indispensibly necessary for ascertaining the analysis and synthesis of
218