PARIS, December 29, 1786.
SIR,--When I had the honor of seeing you in London, you were so kind as
to permit me to trouble you sometimes with my letters, and particularly
on the subject of mathematical or philosophical instruments. Such a
correspondence will be too agreeable to me, and at the same time, too
useful, not to avail myself of your permission. It has been an opinion
pretty generally received among philosophers, that the atmosphere of
America is more humid than that of Europe. Monsieur de Buffon makes
this hypothesis one of the two pillars whereon he builds his system of
the degeneracy of animals in America. Having had occasion to controvert
this opinion of his, as to the degeneracy of animals there, I expressed
a doubt of the fact assumed, that our climates are more moist. I did
not know of any experiments which might authorize a denial of it.
Speaking afterwards on the subject with Dr. Franklin, he mentioned to
me the observations he had made on a case of magnets, made for him by
Mr. Nairne in London. Of these you will see a detail, in the second
volume of the American Philosophical Transactions, in a letter from Dr.
Franklin to Mr. Nairne, wherein he recommends to him to take up the
principle therein explained, and endeavor to make an hygrometer, which,
taking slowly the temperature of the atmosphere, shall give its mean
degree of moisture, and enable us thus to make with more certainty, a
comparison between the humidities of different climates. May I presume
to trouble you with an inquiry of Mr. Nairne, whether he has executed
the Doctor's idea, and if he has, to get him to make for me a couple of
the instruments he may have contrived? They should be made of the same
piece, and under like circumstances, that sending one to America, I may
rely on its indications there, compared with those of the one I shall
retain here. Being in want of a set of magnets also, I would be glad if
he would at the same time send me a set, the case of which should be
made as Dr. Franklin describes his to have been, so that I may repeat
his experiment. Colonel Smith will do me the favor to receive these
things from Mr. Nairne, and to pay him for them.
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 dilatation and contraction, an
index 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 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.