44 PHILOSOPHICAL TRANSACTIONS. [anNO 1786. 



In the year 1748 I came to settle in London; and the first employment I met 

 with was that of making philosophical instruments and apparatus. In this situa- 

 tion, my friend Hindley, from a principle the reverse of jealousy, fully com- 

 municated to me, by letter, his method of division ; and though I was enjoined 

 secrecy respecting others, for the reasons already mentioned, yet the communi- 

 cation was expressly made with an intention that I might apply it to my own 

 purposes. The following are extracts from 2 letters, which contain the whole 

 of what related to this subject ; and since I have many things to observe on 

 them, so that the paraphrase would be much greater than the text, I think it 

 best not to interrupt the description with any commentary, as perhaps his own 

 mode of expression will more briefly and happily convey the general idea of the 

 work, than any I can use instead of it. 



MY dear FRIEND, York, 14 Nov. 1748. 



As to what you was mentioning about my brother's knowing how I divided my 

 engine plate, I will describe it as well as I can myself ; but you will want a good 

 many things to go through with it. The manner is this : first chuse the largest 

 number you want, and then chuse a long plate of thin brass ; mine was about 1 

 inch in breadth, and 8 feet in length, which I bent like a hoop for a hogshead, 

 and soldered the ends together ; and turned it of equal thickness, on a block of 

 smooth-grained wood, on my great lathe in the air, (that is, on the end of the 

 mandrel :) one side of the hoop must be rather wider than the other, that it may 

 fit the better to the block, which will be a short piece of a cone of a large dia- 

 meter : when the hoop was turned, I took it off, cut, and opened it straight 

 again. 



The next step was to have a piece of steel bended into the form 

 as per margin ; * which had 2 small holes bored in it, of equal size, 

 one to receive a small pin, and the other a drill of equal size. I 

 ground the holes, after they were hardened, to make them round 

 and smooth. The chaps formed by this steel plate were as near to- 

 gether as just to let the long plate through. Being open at one end, 

 the chaps so formed would spring a little, and would press the long 

 plate close, by setting in the vise. Then I put the long plate to a 

 right angle to the length of the steel chaps, and bored one hole 

 through the long plate, into which I put the small pin ; then 

 bored through the other hole ; and by moving the steel chaps a hole 

 J forward, and putting in the pin in the last hole, I proceeded till I 

 had divided the whole length of the plate. 

 The next thing was to make this into a circle again. After the plate was cut 



* The figure is considerably less than the real tool should be. — Orig. 



