necessary corrections upon the volume of the Gasses, according
to the pressure of the Atmosphere._

All elastic fluids are compressible or condensible in proportion to the
weight with which they are loaded. Perhaps this law, which is
ascertained by general experience, may suffer some irregularity when
these fluids are under a degree of condensation almost sufficient to
reduce them to the liquid state, or when either in a state of extreme
rarefaction or condensation; but we seldom approach either of these
limits with most of the gasses which we submit to our experiments. I
understand this proposition of gasses being compressible, in proportion
to their superincumbent weights, as follows:

A barometer, which is an instrument generally known, is, properly
speaking, a species of syphon, ABCD, Pl. XII. Fig. 16. whose leg AB is
filled with mercury, whilst the leg CD is full of air. If we suppose the
branch CD indefinitely continued till it equals the height of our
atmosphere, we can readily conceive that the barometer is, in reality, a
sort of balance, in which a column of mercury stands in equilibrium
with a column of air of the same weight. But it is unnecessary to
prolongate the branch CD to such a height, as it is evident that the
barometer being immersed in air, the column of mercury AB will be
equally in equilibrium with a column of air of the same diameter, though
the leg CD be cut off at C, and the part CD be taken away altogether.

The medium height of mercury in equilibrium with the weight of a column
of air, from the highest part of the atmosphere to the surface of the
earth is about twenty-eight French inches in the lower parts of the city
of Paris; or, in other words, the air at the surface of the earth at
Paris is usually pressed upon by a weight equal to that of a column of
mercury twenty-eight inches in height. I must be understood in this way
in the several parts of this publication when talking of the different
gasses, as, for instance, when the cubical foot of oxygen gas is said to
weigh 1 oz. 4 gros, under 28 inches pressure. The height of this
column of mercury, supported by the pressure of the air, diminishes in
proportion as we are elevated above the surface of the earth, or rather
above the level of the sea, because the mercury can only form an
equilibrium with the column of air which is above it, and is not in the
smallest degree affected by the air which is below its level.

In what ratio does the