Though the above letter was in itself very clear and explicit as to the general traces of the method, y et some doubts occurring to me, a farther explanation became ne cessary. A copy of my letter not being preserved, the purport of it may be inferred front the answer, which was as follows : Dear Friend, York, 13th March, 1748-9.
I think, in your last, you seem to be ap prehensive of some difficulties in drilling the hoop for dividing : First, that the centre of the hole in the hoop might not be precisely in the centre of the hole of the steel chaps it was drilled in ; but if I described fully to you the method I used, I can see no danger of error there; for my chaps were very thick, and the two corresponding holes were a little conical, and ground with a steel pin ; first one pair, and then the other, alternately, till the pin would go the same depth into each. Then, for drilling the hoop, I took any common drill that would pass through and bore the hole. After that 1 took a five-sided broach, which opened the hole in the brass betwixt the steel chaps, but would not touch the steel ; so, consequently, the cen tre of the holes in the brass must be concentric with the holes in the chaps ; and for alterations by air, heat, cold, &c. I was not above two or three hours in drilling a row of holes, as far as I remember.
2dly, For drilling, in a right line, I had a thin brass plate, fastened between the steel chaps, for the edge of the hoop to bear against, whilst I thrust it forward from hole to hole. What you propose, of an iron frame with a lead outside, will be better than my wooden block ; but considdring the little time that past betwixt transferring the divisions of the hoop to the divisions of my dividing plate, I did not suffer much that way It was when I drilled the holes in my dividing plate that I used a frame for drilling, which had one part of it that had a steel hole ; that, in lying upon the plane of the dividing plate, was fix ed last in its place for the point of the drill to pass through; then, at the length of the drill, there was another piece of steel, with a hole in it, to receive the other end of the drill, to keep it at light angles to the plane of the plate. This piece was a spring, which bended at the end, where it was fastened to the frame of the lathe, at about 18 inches from the end of the drill ; so it pushed the drill through with any given force the drill would bear ; and though that end of the chill moved in the arch of a circle, it was a very small part of it, being no more than equal to the thickness of the dividing plate. My good wishes. Conclude me yours, HEN. Ilism.Ev.
Nothing ever surpassed in originality the method de scribed in the above letters, which is in ho respect like any other. There is not a tool employed in it, except the
lathe, but what may either be found in the shop of the most common worker in metals, or made by him. It is, how ever, like Hook's and Roemer's, a system of uncontroulecl stepping ; and, like theirs, if only applied to an arc, would have ended in the same uncertainty ; but being extended to the whole circle, it secures the intended number of divi sions, and closes without a remainderovhich is what theirs never could have clone.
To avoid errors occasioned by expansion, which Hindley scents not to have been aware of, Smeaton recommends that the work should be done when the air is of a mode rate temperature ; and that, to pi event the materials from being heated, it should be carried on at short intervals ; but his chief improvement was directed to correct the effect of long continued stepping. Mr Smeaton would divide his circle into 1440 parts, or quarters of a degree. To effect the correction last mentioned, two pieces of brass must be provided, in every respect like that which is to be the hoop, except that they need not be longer than is required to contain 30° ; these pieces, which are called straps, he would drill in the manner that Hindley pre scribes, and it should be mentioned, that the first hole in each must be made in a short piece of hardened steel, which in the first instance had been soldered to the ends of the straps. For the purpose of obtaining the total length, the straps are drilled from end to end ; but no more than the first, middle, and last holes are used. The zero hole being made in the long, or hoop-piece, the middle holes of the straps are to be pinned to it on opposite sides, and the steel pieces directed forwards ; the three pieces extended in a right line are to be pressed together, and the 60th hole in the hoop-piece bored coincident with those of the straps. The straps are now taken off, and the 59 intermediate holes drilled by means of the chaps only. Again, the extreme holes of the straps are to be pinned to the zero holes of the hoop-piece, and arranged as be fore, when the 120th hole of the hitter is to be bored agreeable to the steel holes in the former ; and recourse again had to the chaps for the next 59 holes. As the pro cess described above fills up as much of the hoop-piece as is equal to 30°, eleven similar double operations will complete the whole length ; and, as Smeaton observes, produce 12 master checks, and 12 subordinate ones. The next thing to be done is, to fasten the ends of the long piece together, so as to form it into a hoop, by pressing it upon the edge of a chock, as directed by Hindley.