shortening, by being
less or more charged with weight; to this chain, an iron trivet, with
three branches, a i, c i, and h i, is strongly fixed at i, and
these branches support a large inverted jar A, of hammered copper, of
about 18 inches diameter, and 20 inches deep. The whole of this machine
is represented in perspective, Pl. VIII. Fig. 1. and Pl. IX. Fig. 2. and
4. give perpendicular sections, which show its interior structure.

Round the bottom of the jar, on its outside, is fixed (Pl. IX. Fig. 2.)
a border divided into compartments 1, 2, 3, 4, &c. intended to receive
leaden weights separately represented 1, 2, 3, Fig. 3. These are
intended for increasing the weight of the jar when a considerable
pressure is requisite, as will be afterwards explained, though such
necessity seldom occurs. The cylindrical jar A is entirely open below,
de, Pl. IX. Fig. 4.; but is closed above with a copper lid, a b c,
open at b f, and capable of being shut by the cock g. This lid, as may
be seen by inspecting the figures, is placed a few inches within the top
of the jar to prevent the jar from being ever entirely immersed in the
water, and covered over. Were I to have this instrument made over again,
I should cause the lid to be considerably more flattened, so as to be
almost level. This jar or reservoir of air is contained in the
cylindrical copper vessel, LMNO, Pl. VIII. Fig. 1. filled with water.

In the middle of the cylindrical vessel LMNO, Pl. IX. Fig. 4. are placed
two tubes st, xy, which are made to approach each other at their upper
extremities t y; these are made of such a length as to rise a little
above the upper edge LM of the vessel LMNO, and when the jar abcde
touches the bottom NO, their upper ends enter about half an inch into
the conical hollow b, leading to the stop-cock g.

The bottom of the vessel LMNO is represented Pl. IX. Fig. 3. in the
middle of which a small hollow semispherical cap is soldered, which may
be considered as the broad end of a funnel reversed; the two tubes st,
xy, Fig. 4. are adapted to this cap at s and x, and by this means
communicate with the tubes mm, nn, oo, pp, Fig. 3. which are fixed
horizontally upon the bottom of the vessel, and all of which terminate
in, and are united by, the spherical cap sx. Three of these tubes are
continued out of the vessel, as in Pl. VIII. Fig. 1. The first marked in
that figure 1, 2, 3, is inserted at its extremity 3, by means of an
intermediate stop-cock 4,