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.
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