Fig. 1379.
Fig. 1380.
In [Fig. 1378] d is the head of this pillar, p the screw which turns the wheel. e1 e1 are the boxes, which are made conical so as to prevent any shake and to hold the screw firmly. Circles of brass, f and v, are placed on the arbor of the screw, and as their circumference is divided into 60 parts, each division consequently amounts to a motion of the wheel of 10 seconds, and 60 of them will equal 1 minute. Revolution is given to the screw by means of the treadle b′ and the cord y, which runs over the guiding screw w, [Fig. 1379], and is finally attached to the box u. A spring enclosed in the box u causes it to revolve, and winds up the slack of the cord whenever the treadle is relieved. In the original drawing the head of the pillar p was carried in a parallel slip in the piece surrounding its head. The construction as shown in [Fig. 1379] is somewhat different. The result attained, however, is identical, and the spindles and attachments are held so as to have no lateral motion. The wheels v and x have stops upon them, so arranged that the screw may be turned definitely to a given point and stopped. These wheels are at the opposite ends of the screw w. A detail of one of them is shown at v in [Fig. 1380], where x is the ratchet-wheel. This figure also illustrates the construction of the bearings for the screw arbor. We have not space to explain the method by which the perfection of the screw was obtained, nor to discuss the means by which was obtained the success of so eliminating the errors as to make the division of the instrument more perfect than anything which had been attempted previously. Success, however, was obtained, and by means of the first or tooth-cutting screw the teeth were brought to such a considerable uniformity that, together with the fact that the screw took hold of a number of teeth at one time, most of the errors which would have been expected from this method of operation were eliminated. The method of ruling lines upon the instrument was most ingenious. The frame l l, is connected to the head d, of the pillar p in front, by the clamps i and k, and to the centre m by the block r. A frame n n stiffens the back. The blocks o, o on the frame q′ are secured to the frame l l, by set-screws c, c.
Fig. 1381.
[Fig. 1381] shows a side view of the frame q′, which it is seen carries a V-shaped piece q, which in turn carries another V-shaped piece s, [Fig. 1378]. The piece q is supported on pointed screws d, d, and the piece s is supported on two similar screws f, f. The point of this piece s carries the cutting tool e, [Fig. 1378]. Of course s can move only in a radial line from the centre m towards the circumference. If the sextant, octant, or other instrument be fastened to the large wheel a, with its centre at m, and the large wheel be rotated by the screw, all lines drawn upon it by e will be radial, and the distances apart will be governed by the number of turns made by the screw. This improvement, we think, was originated by Mr. Ramsden, and was a very great advance over the old method of the straight-edge, and has been used in some of the Government comparators and dividing engines. The following is Mr. Ramsden’s own description of the graduation of the machine, and of his method of operating it. It shows the extreme care which he took in correcting the mechanical errors in the construction:—