46 PHILOSOPHICAL TRANSACTIONS. [aNNO I786. 



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 2 or 3 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, while I thrust it for- 

 ward from hole to hole. What you propose of an iron frame with a lead out- 

 side, will be better than my wooden block ; but considering the little time that 

 past, between 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 on the plane of the dividing 

 plate was fixed fast 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 right 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 drill 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. 



Henry Hindley. 



Whoever attentively considers the communication contained in the above 

 letters will see, that more happy expedients could not have been devised to pro- 

 cure a set of divisions, where there should be the most exact equality among 

 neighbours ; and which, for the purposes of clock-making, is the principal thing 

 to be wished for. But herein, as in M. Roemer's method, there were no means of 

 checking the distant divisions, which run on to 360: now such a check, when 

 the expansion of metals is considered, and particularly the difference of expan- 

 sion between brass and steel, seems absolutely necessary for the purpose of divi- 

 sions on instruments, where the accurate mensuration of large angles is required, 

 as much as the equality of neighbouring divisions.* With this view the inven- 

 tion of this ingenious person suggested to him the thought of making his curved 

 screw to lay hold of 1 5 teeth or degrees together : this, in effect, becomes a 

 pair of compasses, 24 removes of which complete the whole circle, and produce 

 24 checks in the circumference: and whoever considers the very exquisite de- 

 gree of truth that results from the grinding of surfaces in contact, as already 



* The ingenious Mr. StanclifFe, some years a workmen of Hindley's, has suggested, that the 

 difference of expansion between the steel chaps and the brass hoop may be avoided by makino^ the 

 diaps of brass also, with hard steel holes set separately in them, somewhat similar to the jewelled 

 holes of watches. — Orig. 



