When I had the honour 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. Mr. 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 moister. I did not then know of any experiments which might authorize a denial of it. Speaking afterwards on this 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 2d. vol. 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 it’s mean degree of moisture, and enable us thus to make with more certainly a comparison between the humidities of different climates. May I presume to trouble you with an enquiry 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 peice, and under like circumstances, that sending one to America, I may rely on it’s 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. Colo. 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 5 I. long, ¾I. broad and 1/10 I. thick, the one having the grain running lengthwise, and the other crosswise. These are glued together by their faces, so as to form a peice 5 I. long, ¾I. broad and ⅕ I. thick, which is stuck, by it’s lower end, into a little plinth of wood thus [Fig. 1] presenting their edge to the view. The fibres of 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 it’s grain crosswise, dilates with moisture and contracts with the want of it. If the right hand peice above represented be the cross grained one, when the air is very moist, it lengthens and forces it’s companion to form a kind of interior annulus of a circle on the left thus [Fig. 2] When the air is dry, it contracts, draws it’s companion to the right, and becomes itself the interior annulus, thus [Fig. 3] In order to shew this dilatation 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 it’s whole range. On the edge of the slit is a graduation, so that the instrument shews somewhat thus [Fig. 4] The objection to this instrument is that it is not fit for comparative observations, because no two peices of wood being of the same texture exactly, no two will yeild 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 & most humble servant,