5564. MOUNTAINS, Barometrical measurement.—
The method of estimating
heights [of mountains] by the barometer, is
convenient and useful, as being ready, and furnishing
an approximation to truth. Of what
degree of accuracy it is susceptible we know
not as yet; no certain theory being established
for ascertaining the density and weight of that
portion of the column of atmosphere contiguous
to the mountain; from the weight of which,
nevertheless, we are to infer the height of the
mountain. The most plausible seems to be that
which supposes the mercury of barometer divided
into horizontal lamina of equal thickness;
and a similar column of the atmosphere into
lamina of equal weights. The former divisions
give a set of arithmetical, the latter of geometrical
progressionals, which being the character,
of logarithms and their numbers, the tables of
these furnish ready computations, needing, however,
the corrections which the state of the
thermometer calls for. It is probable that in
taking heights in the vicinity of each other in
this way, there may be no considerable error,
because the passage between them may be quick
and repeated. The height of a mountain from
its base, thus taken, merits, therefore, a very
different degree of credit from that of its height
above the level of the sea, where that is distant.
According, for example, to the theory above
mentioned, the height of Monticello from its
base is 580 feet, and its base 610 feet 8 inches,
above the level of the ocean; the former, from
other facts, I believe to be near the truth; but a
knowledge of the different falls of water from
hence to the tide-water at Richmond, a distance
of seventy-five miles, enables us to say that the
whole descent to that place is but 170 or 180
feet. From thence to the ocean may be a distance
of one hundred miles; it is all tide-water,
and through a level country. I know not what
to conjecture as the amount of descent, but certainly
not 435 feet, as that theory would suppose,
nor the quarter part of it. I do not know
by what rule General Williams made his computations.
He reckons the foot of the Blue
Ridge, twenty miles from here, but 100 feet
above the tide-water at Richmond. We know
the descent, as before observed, to be at least
170 feet from hence, to which is to be added
that from the Blue Ridge to this place, a very
hilly country, with constant and great waterfalls.
His estimate, therefore, must be much
below truth. Results so different prove that for
distant comparisons of height, the barometer
is not to be relied on according to any theory
yet known. While, therefore, we give a good
degree of credit to the results of operations between
the summit of a mountain and its base,
we must give less to those between its summit
and the level of the ocean.—
To Capt. A. Partridge. Washington ed. vi, 495.
(M.
1815)
