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.