grometer, 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 the two 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 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. Not knowing whether you had heard of this
instrument, and supposing it would amuse you, I have taken the liberty
of detailing it to you.

I beg you to be assured of the sentiments of perfect esteem and respect
with which I have the honor to be, Sir, your most obedient

and most humble servant,

Th: Jefferson.

LETTER XXXVII.--TO JOHN JAY, December 31, 1786