IN FORT WILLIAM, &C. £g 



their flat furfaces, and the middle one was the biggeft; 

 that below was the next in fize ; and the upper one 

 was the leaft, and only equal to the brafs plate on 

 which the fpeculum was fcrewed. 



A plate of brafs, nearly one tenth thick, broad 

 enough to admit of a hole as big as the under circular- 

 part of the turned wood, and to afford a rim of half an 

 inch broad, was then fixed into the lath, and had a hole 

 turned in it of that fize : on one fide it had an arm as 

 broad as the wooden index was. 



A fecond plate of the fame kind was alfo prepared ; 

 but the hole was larger, though lefs than the middle 

 part of the turned wood. 



The turned piece was then fixed to the o£tant by its 

 pin, and the plate with the fmaller hole beneath it. 

 As they fitted very nicely, the brafs plate turned upon 

 the wood round the center of the o£tant, if that were 

 held fait : and both turned on the center pin if they 

 were prefied together. 



The plate with the large hole was then laid above 

 the turned wood, its center coinciding with the com- 

 mon center : the wooden arm of the index had the 

 end neareft the center cut away, above and below, 

 equal to the thicknefs of the plates of brafs : it was 

 there fixed to the octant in the fame manner as before 

 it was cut off from the center, and the brafs plates 

 were drilled, and rivetted to it. 



When thefe plates were prefled together, they held 

 the turned piece as it were in a vice; when thev were 

 forced afunder, the turned piece might be moved in- 

 dependently ; and there were in the direction of the 



radius 



