386 Mr. cavendish’s Account of 
brafs frame on which it turns; and the method of ob- 
ferving is to move the box, till a line drawn on it points 
exactly to the end of the needle ; which being done, the 
angle that the needle makes with the fide of the frame 
is fhewn by the index. Fig. 3. is the plan of the inftru- 
ment; ABba is the brafs frame, the fides ab and ab being 
parallel ; Ee is a circular plate fattened thereto, on which 
.CD dc, the box which holds the needle, turns as on a cen- 
ter; n n is the needle, the pin on which it vibrates, being 
fixed in the center of the plate Ee; b b is the divifion on 
the brafs frame; and g the index fattened to the box 
cd dc, furnithed with a vernier divifion; the divifion and 
vernier being conftrudted fo as to fhew the angle which 
the line f/ makes with ab or ab. The inftrument is 
placed in the meridian by the teiefcope m m, the line of 
vcollimation of which is parallel to ab, and is pointed to a 
mark fixed due North of it. 
Fig. 4. is a vertical ledtion of the inftrument pafling 
along the line f f; ab is the brafs frame; cecIc the box 
which holds the needle; e e the circular plate on which 
it turns : n n is the needle; p and p are fmall plates of brafs 
fixed to the ends of it, on each of which is drawn a line 
ferving by way of index. Thefe pieces of brafs are raifed 
to fuch a height that their tops are on a level with the 
point of the pin on which the needle turns. The ufe of 
them is, that it is much eafier obferving this way than 
when the lines, ferving by way of index, are drawn on 
the needle itfelf, as by this means the inconvenience pro- 
ceeding from one kind of vibration in the needle is 
avoided. 
