tly, by means of the
perpendicular tube st, Pl. IX. Fig. 4. with the air contained in the
jar. He likewise cements, at 16, Pl. VIII. Fig. 1. another glass tube
16, 17, 18, which communicates at 16 with the water in the exterior
vessel LMNO, and, at its upper end 18, is open to the external air.

By these several contrivances, it is evident that the water must stand
in the tube 16, 17, 18, at the same level with that in the cistern LMNO;
and, on the contrary, that, in the branch 19, 20, 21, it must stand
higher or lower, according as the air in the jar is subjected to a
greater or lesser pressure than the external air. To ascertain these
differences, a brass scale divided into inches and lines is fixed
between these two tubes. It is readily conceived that, as air, and all
other elastic fluids, must increase in weight by compression, it is
necessary to know their degree of condensation to be enabled to
calculate their quantities, and to convert the measure of their volumes
into correspondent weights; and this object is intended to be fulfilled
by the contrivance now described.

But, to determine the specific gravity of air or of gasses, and to
ascertain their weight in a known volume, it is necessary to know their
temperature, as well as the degree of pressure under which they subsist;
and this is accomplished by means of a small thermometer, strongly
cemented into a brass collet, which screws into the lid of the jar A.
This thermometer is represented separately, Pl. VIII. Fig. 10. and in
its place 24, 25, Fig. 1. and Pl. IX. Fig. 4. The bulb is in the inside
of the jar A, and its graduated stalk rises on the outside of the lid.

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 inconv