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 mercury in the barometer descend in proportion to
its elevation? or, what is the same thing, according to what law or
ratio do the several strata of the atmosphere decrease in density? This
question, which has exercised the ingenuity of natural philosophers
during last century, is considerably elucidated by the following
experiment.
If we take the glass syphon ABCDE, Pl. XII. Fig. 17. shut at E, and open
at A, and introduce a few drops of mercury, so as to intercept the
communication of air between the leg AB and the leg BE, it is evident
that the air contained in BCDE is pressed upon, in common with the whole
surrounding air, by a weight or column of air equal to 28 inches of
mercury. But, if we pour 28 inches of mercury into the leg AB, it is
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