Page:Philosophical Transactions - Volume 054.pdf/323



this machine ( XVL Fig. 1. and 2.) AAAA is a frame of wainscot or mahogany, grooved in the innermost edges of the two longest sides, for holding a pannel BBBB of white deal-board, without pinching it. The pannel is about the thickness of a crown piece, and fifteen inches in length, cross-wise to the grain of the wood. The middle part projects outward from the upper and lower edges, at C and C, where it is fastened into the frame by two screws, to keep the middle part always in the same place, whilst the rest of the pannel expands by moist air toward both ends of the frame, and contrasts toward the middle when the air is dry. F is a pin fixt into the pannel near one of its ends, and a round pin is fixt near the other end of the pannel, on which the large pulley H turns, and also the small pully [sic] G which is fixed to H. One end of a small flexible cord is fastened to the pin F, and the other end goes round the pully [sic] G, and is fixt into the bottom of its groove, as at h. One end of another small cord is fixed into the bottom of the groove of the large pulley H, as at a, from which it goes round the part  and in its way thence to M it goes round a small pulley L, in which an axis is fixt, and turns in the piece O, which lies above the pulley, and is screwed to the upper side of the frame at C. This cord goes over the pulley M (which turns on a round pin fixt into the pannel) and has a flattish weight N hung to it.—The pullies G and L are of equal diameters in their grooves, which is only equal to a tenth part of the diameter of the large pulley H in its groove. The pulley M may be of any convenient size.

Now it is plain, that as much as the pannel expands between F and G, so much will the pully G be removed farther from the pin F; and just so much will the cord turn the pulley G backward; and any point in the