B is a strong iron frame, forty-eight inches long, hav ing two edges a, b rising half an inch above its surface ; these two edges are made very straight, and in the same plane ; the inside of the edge a is also made as straight as possible.
(Fig. 5.) The plate A slides on the two edges of the iron frame ; beneath it are two springs c, c, each fastened at the extreme ends to the plate A by the screws s ; at the other end of each spring is a roller e of tempered steel, turning on an axis in these springs ; there is also a third roller d of tempered steel, (Fig. 3.) let into the iron frame near where the threads of the endless screw act ; this roller bath a long axis, one end turning in the iron frame at g, (Figs. 2. 3 ) and the other in the lever h ; this lever torus on a centre at i, and with it the roller d may be raised or depressed, by turning the capstan head screw o, which presses on a strong spring.
The use of the rollers is to diminish the friction of the plate A. when moving on the iron frame II : for this pur pos:, tile strength of the springs is regulated by turning the two screws n, n, and of the roller d, by the capstan-head screw o, till the weight of the plate A be very nearly car ried on these rollers.
(Fig. 2.) C is the endless screw, which is of tempered steel, and has its pivots formed in the shape of two frus tums of cones joining each other at their smaller end by a cylinder, as shewn in the description of the circular engine, p. 5, Plate Ill. of that work. These pivots turn in half holes of the same form in the pieces of brass DD, which are fit mly screwed to the iron frame ; the hall holes are kept together by the screws in, 7/1, which may be tightened at any time, to prevent the endless screw from shaking.
On one end of the screw arbor is a wheel h, (Fig. I.) having its circumference divided into 50 parts, and num bered at every tenth division with I, 2, to 5 ; and these divi sions are again subdivided into five parts by the vernier t.
G, G represent two frames of steel ; each of these frames turn on centres K, fastened to the under-side of the plate A, and equidistant from the edge of it. In each frame is a roller y of tempered steel, turned very concentric to their pivots, and exactly of the same diameter.
The frames G, G (Plate CCLXXX. Fig. 5.) are connect ed together by the brass plate E, which turns on a stud in each frame ; the studs must be at equal distances from the centres K on which the frames turn, and the distance be tween the holes in the plate E in which the studs act, must be the same with the distance between the centres k, so that the plate E may always move parallel to itself, and that the circumference of the rollers may he always equidistant from that edge of the plate A, which is intended to be ratch ed. This apparatus serves to press the edge of the plate A, with a motion parallel to itself, against the threads of the endless screw.
On the end of the plate A (Fig. 1, 2.) is a spring of tem pered steel, which acts as a bent lever. The spring end of this lever has a ketch, which passes under the head of the stud I, that is on the end of the connecting piece E. While the other end of the lever is pressed gradually down to wards the Plate A, by turning the finger screw F, the con necting piece E is drawn forward, so that the steel rollers pressing against the edge a of the iron Frame, may force the side of the plate against the endless screw.
Then having two screws of tempered steel exactly of the same diameter and number of threads, viz. 20 in an inch, one of these screws was notched across the threads, so as to cut in the manner of a saw ; this screw was put in the half holes in the pieces D, D ; (Fig. 2.) On the opposite end of the screw arbor to that whereon the wheel is, there is at tached a long rod, of such length, that the winch on the end of it, by which the rod and endless screw are turned round, may be clear oldie iron frame.
Z (Fig. I.) is a narrow slip of brass, having its edges exactly parallel, which is screwed on the plate A, and stea dy pinned.
The edge of this slip is parallel to the edge of the plate ; a distance of 25.6 inches was set off on a line on the slip parallel to its edge ; this distance was bisected continually, till the distance between each bisection was of an inch.