On the Errors of Dividing Engines. 347 



Akt XIII. ASTRONOMICAL AND NAUTICAL 

 COLLECTIONS, No. II. 



i. Report of the Committee of the Board of Longitude, 

 for examining Instruments and Proposals, upon the' Mode em- 

 ployed for determining the Errors of dividing Engines. 



[Published by permis^ioo of the Board.] 



1. Two double circles of brass were prepared, each consisting 

 of two concentric circles moving on the same axis, and sup- 

 ported by radii, so as to have their upper faces in the same 

 plane, the one turning in perfect contact with the other ; they 

 were eighteen inches in diameter at the common circle of 

 contact. 



2. One of these double circles was fixed on each of the engines 

 to be compared, and divided into degrees, by lines drawn across 

 the circle of contact ; care being taken to prevent the cutting 

 tools entering too deep, where the division of the circles inter- 

 rupted the lines. 



3. The divisions being completed, and the degrees numbered, 

 the circles were fixed on an axis, attached to a small board or 

 table, in order for examination by means of a micrometer in 

 the possession of Captain Kater. 



4. The zero of the inner circle was moved so as to correspond 

 with a division of the outer circle at the distance of some 

 aliquot part of the circumference from 0, and there fixed by a 

 clamp, while the imperfections of the coincidences of all the 

 other points, at the same distances from each other, were 

 separately examined by means of the micrometer. 



