1317. CLIMATE, Humidity gauge.—
I think Mr. Rittenhouse never published an
invention of his in this way, which was a very
good one. It was of an hygrometer which, like
the common ones, was to give the actual moisture
of the air. He has two slips of mahogany
about five inches long, three-fourths of an inch
broad, and one-tenth of an inch thick, the one
having the grain running lengthwise, and the
other crosswise. These are glued together by
their faces, so as to form a piece five inches
long, three-fourths of an inch broad, and one-third
of an inch thick, which is stuck by its
lower end into a little plinth of wood, presenting
their edge to the view. The fibres of the
wood, you know, are dilated, but not lengthened
by moisture. The slip, therefore, whose
grain is lengthwise, becomes a standard, retaining
always the same precise length. That
which has its grain crosswise, dilates with
moisture, and contracts for the want of it. If
the right hand piece be the cross-grained one,
when the air is very moist, it lengthens, and
forces its companion to form a kind of interior
annulus of a circle on the left. When the air
is dry, it contracts, draws its companion to the
right, and becomes itself the interior annulus.
In order to show this dilation and contraction,
an index is fixed on the upper end of two of the
slips; a plate of metal or wood is fixed on the
upper end of two of the slips; a plate of metal
or wood is fastened to the front of the plinth,
so as to cover the two slips from the eye. A
slit, being nearly the portion of a circle, is cut
in this plate, so that the shank of the index
may play freely through its whole range. On
the edge of the slit is a graduation. The objection
to this instrument is, that it is not fit
for comparative observations, because no two
pieces of wood being of the same texture exactly,
no two will yield exactly alike to the
same agent. However, it is less objectionable
on this account than are most of the substances
used. Mr. Rittenhouse had a thought of trying
ivory; but I do not know whether he executed
it. All these substances not only vary
from one, another at the same time, but from
themselves at different times. All of them,
however, have some peculiar advantages, and
I think this, on the whole, appeared preferable
to any other I had ever seen.—
To Mr. Vaughan. Washington ed. ii, 83.
(P.
1786)
