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,