The practice of gazometry would still have laboured under great
difficulties, without farther precautions than those above described.
When the jar A sinks in the water of the cistern LMNO, it must lose a
weight equal to that of the water which it displaces; and consequently
the compression which it makes upon the contained air or gas must be
proportionally diminished. Hence the gas furnished, during experiments
from the machine, will not have the same density towards the end that it
had at the beginning, as its specific gravity is continually
diminishing. This difference may, it is true, be determined by
calculation; but this would have occasioned such mathematical
investigations as must have rendered the use of this apparatus both
troublesome and difficult. Mr Meusnier has remedied this inconvenience
by the following contrivance. A square rod of iron, 26, 27, Pl. VIII.
Fig. 1. is raised perpendicular to the middle of the beam DE. This rod
passes through a hollow box of brass 28, which opens, and may be filled
with lead; and this box is made to slide alongst the rod, by means of a
toothed pinion playing in a rack, so as to raise or lower the box, and
to fix it at such places as is judged proper.
When the lever or beam DE stands horizontal, this box gravitates to
neither side; but, when the jar A sinks into the cistern LMNO, so as to
make the beam incline to that side, it is evident the loaded box 28,
which then passes beyond the center of suspension, must gravitate to the
side of the jar, and augment its pressure upon the included air. This is
increased in proportion as the box is raised towards 27, because the
same weight exerts a greater power in proportion to the length of the
lever by which it acts. Hence, by moving the box 28 alongst the rod 26,
27, we can augment or diminish the correction it is intended to make
upon the pressure of the jar; and both experience and calculation show
that this may be made to compensate very exactly for the loss of weight
in the jar at all degrees of pressure.
I have not hitherto explained the most important part of the use of this
machine, which is the manner of employing it for ascertaining the
quantities of the air or gas furnished during experiments. To determine
this with the most rigorous precision, and likewise the quantity
supplied to the machine from experiments, we fixed to the arc which
terminates the arm of the beam E, Pl. VIII. Fig. 1. the brass sector l
m, divided into degrees and half degrees, which consequently moves in
common with the beam; and the lowering of this end of the beam is
216