The upper end of the piece f (Fig. 2.) is formed into a hook, and may be set to catch in the teeth of any of the ratchet wheels, and then fastened to the bar -v by the screw i. Towards the other end of the bar is a piece j, which serves to stop the cylinder in turning back, so as to limit the number of revolutions or parts. It is fastened to any required place on the bar v, by the finger screwf.
When the engine is used, the treadle is pressed down wards, which, by means of the cat-gut string, turns the cylinder I round its axis ; (Plate CCLXXX. Figs. 2, 4.) and the piece t moves along the thread, till a stud r on the cylinder, striking on the top of the curved piece t, bends the spring e, till that piece rests on the piece q. By bend ing this spring, the square bar is turned a little on its axis, and pulls the hookfinto the teeth on the ratchet wheel R. Then releasing the treadle, the spiral spring turns back the cylinder, till the piece j is brought under the stop on the ratchet ring b.
The parts of a revolution are regulated, by setting the number required on the ratchet ring b, (Figs. 1, 2.) to the index on the fixed ring a. Each of the teeth answers to a motion of of an inch of the plate A ; and the number of revolutions, each of which moves the plate A inch, is regulated by setting the piece j on the bar.
L (Figs. 1, 2, 3 ) represents the steel frame, in which the tracer is fixed. This frame turns between the conical points of two screws n, n of tempered steel, which are screwed in the frame Q. There are also two similar screws in the same frame at m, in. The points of these screws, which are also of tempered steel, turn in conical holes in the piece P. By means of this parallel motion, the tracing point, by which the divisions are cut, will always describe the same line, without any lateral bend ing. The tracer is put in the hole in the axis b, and is fixed there by tightening the four screws!, which presses the piece c against the flat part of the axis.
This axis, which hath its pivots formed in double cones, turns between the half holes at d, and may be fixed when the tracer is set to any required inclination, by tightening the screw S.
S (Fig. 6.) is a brass ruler, having its edges very straight and parallel. It hath two thin pieces of steel g attached to it, which turn on joints at h, exactly equidistant from the edges of the ruler. The interval between the pieces g, g, is exactly the same with the width of the steel frame L. There are angular notches on the lower edge of the pieces g, similar and equidistant from their centres ; so that when any two corresponding notches are put on the screws n, n, between the frames Q and L, the screws be ing on that part made cylindric, and both of the same diameter, then the edge of the ruler will always be at right angles to a line drawn by the tracer. The ruler S,
in this manner attached to the cutting frame, may be set parallel, or to any required inclination with the edge of the plate A, by turning the handle T, which moves the piece P with the cutting frame and ruler on the centre x, and may be fixed there by tightening the nut 1'.
From a centre J, on the plate A, are drawn two cir cular arcs. The outer one is divided into degrees, and numbered from 1 to 9. Each degree is again subdivided into six parts, or every ten minutes. The inner circle is divided in the proportion, that the cosines of the angles of inclination with the edge of the plate A bears to the radius, supposing the radius 10,000, and the divisions are num bered every fOth with 10, 20, to 140. But the use of this apparatus may be perhaps better understood by an ex ample.
Let it be required to divide a line of the length of 9 into the same number of divisions, and in the same manner as if it were 10 inches long. Put the ruler S to the cutting frame L, and turn the handle T till the same edge of the ruler cuts the centre J, and the first division from the 0 of the inner arc. Then screw the instrument to be divided firmly on the plate A, so that the line to be divided may be parallel to the edge of the ruler, which may be now removed. When the plate has moved 10 inches in its own direction, the whole length of the divi sions on the line divided will only be 9 inches." The screw of a dividing engine, whether for right lines or circles, must be considered as a most essential part, and should be of the most perfect workmanship. Proba bly from knowing, that there are several good methods of producing a screw now well known to artists, Troughton omitted to send an account of that by which his was made, at the time he transmitted to us the description of his engine. We will, however, annex the method of making one ; and as that employed by Ramsden for his straight line engine, may, for aught we know, be as good as any, it has been selected. The following is Mr Ramsden's own directions for making the screw, and the Plate is a copy from his original.