▼OL. LXXVI.] PHILOSOPHICAL TRANSACTIONS. 41 



diameter, very stout and strong, and cut into 36o teeth ; to which was applied 

 an endless screw, adapted to it. The threads of this screw were not formed on 

 a cylindric surface, but on a solid whose sides were terminated by arches of 

 circles. The whole length contained 1 5 threads ; and as every thread, on the 

 side next the wheel, pointed towards the centre, the whole 15 were in contact 

 together ; and had been so ground with the wheel, that, to my great astonish- 

 ment, I found the screw would turn round with the utmost freedom, inter- 

 locked with the teeth of the wheel, and would draw the wheel round without 

 any shake or sticking, or the least sensation of inequality. How long this en- 

 gine might have been made before this first interview, I cannot now exactly as- 

 certain : I believe not more than about a couple of years ; but this I well re- 

 member, that he then showed me an instrument intended for astronomical pur- 

 poses, which must have been produced from the engine, and which of itself must 

 have taken some time in making.* 



I in reality thought myself much indebted to Mr. Hindley for this communi- 

 cation ; but though he showed me his engine, and told me that the screw was 

 cut by the rotation of the point of a tool, carried round on a strong arm, at the 

 distance of the radius of the wheel from the centre of motion, which arm was 

 carried forward by the wheel itself, and the wheel was put forward by an endless 

 screw, formed on a cylinder to a proper size of thread, cut by his chock lathe ; 

 though he showed me also this chock lathe, and the method employed to make 

 the threads of the screw equiangular with the axis^ that is, to free the screw 



* This instrament was of the equatorial kind ; the wheel parallel to the equator, the quadrant of 

 latitude, and semi-circle of declination, being all furnished with screws containing 15 threads each 

 framed and moved in the same manner as that of the engine j the whole of which instrument was 

 already framed, and the telescope tube in its place, which was intended to be of the inverting re- 

 fracting kind, and to be furnished with a micrometer. This however was not completed tiU some 

 years after ; but in the year 1 748 I received it in London for sale. It was with me 2 years, in which 

 time I showed it to all my mechanical and philosophical friends, among whom was Mr. Short, who 

 afterwards published in the Philos. Trans, vol. 46, an account of a portable observatory, but without 

 claiming any particular merit from the contrivance. However, the model of it differs from Hind- 

 ley's equatorial only in the following articles. He added an azimuth circle and compass at the 

 bottom. He omitted the endless screws, placing verniers in their stead ; and at the top, a reflecting 

 telescope instead of a refractor. This insfrument of Hindley's being afterwards returned to him un- 

 sold, I pointed out the principal deficiencies that I found in it ; viz. that, in putting the instrument into 

 different positions, the springing of the materials was such as in some positions to amount to con- 

 siderable errors. This remained with him in the same state till the year of the first transit of Venus, 



viz. 1761 } when it was sold to Constable, Esq. of Burton Constable, in Holdemess. Mr. 



Hindley, to remedy the evil above-mentioned, applied balances to the different movements. He 

 soon afterwards completed one, de novo, on this improved plan, for his Grace the late Duke of 

 Norfolk. A method of balancing in much the same way, without the knowledge that it had been 

 done before, has been fiilly explained, and laid before the Society, by our ingenious and worthy 

 brother Mr. Nairne. Phil. Trans, vol. 59, p. 108, — Orig. 



VOL. XVI. G 



