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
plain the air in the branch BCDE will now be pressed upon by a weight
equal to twice 28 inches of mercury, or twice the weight of the
atmosphere; and experience shows, that, in this case, the included air,
instead of filling the tube from B to E, only occupies from C to E, or
exactly one half of the space it filled before. If to this first column
of mercury we add two other portions of 28 inches each, in the branch
AB, the air in the branch BCDE will be pressed upon by four times the
weight of the atmosphere, or four times the weight of 28 inches of
mercury, and it will then only fill the space from D to E, or exactly
one quarter of the space it occupied at the commencement of the
experiment. From these experiments, which may be infinitely varied, has
been deduced as a general law of nature, which seems applicable to all
permanently elastic fluids, that they diminish in volume in proportion
to the weights with which they are pressed upon; or, in other words,
"_the volume of all elastic fluids is in the inverse ratio of the weight
by which they are compressed_."

The experiments which have been made for measuring the heights of
mountains by means of the barometer, confirm the truth of these
deductions; and, even supposing them in some degree inaccurate, these
differences are so extremely small, that they may be reckoned as
nullities in chemical experiments. When this law of the compression of
elastic fluids is once well understood, it becomes easily applicable to
the corrections necessary in pneumato chemical experiments upon the
volume of gas, in relation to its pressure. These corrections are of two
kinds, the one relative to the variations of the barometer, and the
other fo