enience
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
measured by the fixed index 29, 30, which has a Nonius giving hundredth
parts of a degree at its extremity 30.

The whole particulars of the different parts of the above described
machine are represented in Plate VIII. as follow.

Fig. 2. Is the flat chain invented by Mr Vaucanson, and employed for
suspending the scale or bason P, Fig. 1; but, as this lengthens or
shortens according as it is more or less loaded, it would not have
answered for suspending the jar A, Fig. 1.

Fig. 5. Is the chain i k m, which in Fig. 1. sustains the jar A. This
is entirely formed of plates of polished iron interlaced into each
other, and held together by iron pins. This chain does not leng