A brass cock was fastened to the iron frame, which pass ing over the endless screw, applied itself to the slip on the brass plate A, a small silver wire was stretched across a hole of half an inch diameter in the end of the cock. The coincidence of the bisections with this wire was examined by a small magnifier in a brass tube, fixed exactly over it.
The plate A being on the iron frame, the bisection mark ed I, towards the right Nand, was set to coincide with the wire, and the division numbered 50 on the wheel, was set to the first division on the nonius.
The plate A was then pressed against the endless screw, by turning the finger screw F ; then, by means of the winch, the endless screw was turned towards the left six teen revolutions, till the bisection marked o, was brought to the wire ; this done, the plate was detached from the endless screw, by unturning the finger screw F, and the division numbered 2 was set to coincide with the wire, the division 50 on the wheel being previously set to its index, and the edge of the plate was pressed against the screw by turning the finger screw F ; then, by means of the winch, the endless screw was again turned round its axis 16 revo lutions towards the left hand, till the bisection I was brought to coincide with the wire. The plate was again detached as bef,re, and the bisection marked 3 was set to coincide with the wire, and in this manner the edge of the plate was ratched from end to end three or four times, till the threads had made a good impression, which afterwards was ratch ed from end to end, without ever disengaging the plate from the screw, till the teeth were entirely finished.
The notched endless screw, with the rod and winch, were then removed, and the plain screw was put in its place ; having the divided wheel on one end of the screw arbor, and two sets of ratchet wheels on the other end. These sets are each composed of three wheels, having teeth round their circumference ; one in each set bath 32 teeth, another 48, and the third 50. These two sets being one for turning the screw, and the other for stopping it, they have, for this purpose, their ratchet teeth cut in op posite directions.
Plate CCLXXX. Figs. 1, 2. I, represents a cylinder of brass, having on one end two steel rings a and b, with their edges that are towards each other cut into ratchet teeth ; these teeth are cut in contrary directions, so as to fit into each other ; on one of these rings is an index, and the other bath its teeth numbered with 10, 20, up to 50. The
other end of the cylinder is made hollow, and contains one of the sets of ratchet wheels There are two slits opposite each other, pierced through the hollowed pat t of the cylin der W. In each of these slits is a click on an axis, and is pressed into the teeth of the ratchet wheel by a small spring. The clicks may be moved along their axis, so as to catch in any one of the three ratchet wheels, and may be fastened at that place, by tightening the small screw S.
The cylinder I, (Figs. 1, 2. 4.) with the clicks, Ste. turns on a steel axis X, firmly attached to the piece K, and in a line with the axis of the endless screw. lotion is given to this cylinder round its axis by a piece of cat-gut, which bath one end fastened to the ratchet ring b ; and the other end, after passing four or five times round the cylinder, is fastened to a treadle ; and on pressing the treadle down wards, the clicks S catch in the teeth of one of the ratchet wheels, by which means the cylinder I, together with the endless screw, are turned round their axis, which moves the plate along the iron frame, and at the same time winds up the spiral spring rt. On releasing the treadle, the spring 11 unbends itself, the clicks quit the ratchet wheel, and leave the endless screw at rest, while the cylinder I turns in an opposite direction, and raises the treadle to where it was before.
Figs. 2, 4. V is a small square bar of steel, having both its extremities cylindric. These cylinders move in holes lined with hardened steel, one in the piece D, and the other in the piece K. This bar carries three different pieces, which are of tempered steel ; the middle one t, is made to lie in the interval between the threads of the screw cut on the cylinder, and passes nearly half round its circumference ; it is kept in the threads by a spring c, which presses on a piece q, screwed to the iron frame. This piece being attached to the bar V, by the screw p, turning the cylin, er I on its axis, will give a longitudinal motion to the bar V.