espect,

Dear Sir, your most obedient

and most humble servant,

Th: Jefferson.

LETTER XXXVI.--TO MR. VAUGHAN, December 29, 1786

TO MR. VAUGHAN.

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 hy