1^2 Gen . Roy’s Account of 
centers of whofe pivots an imaginary line paffes that fhould at 
the fame time have pafled through the center of the femicircle. 
Mr. Ramsden took the beft method that could be devifed to 
render the excen tricity as little as pofiible. Having framed the 
femicircle, and fcrewed it in its place on the axis, he made a 
Reel point firmly fixed to the horizontal bar defcribe the con- 
centric arcs whereon the divifions were afterwards to come, and 
then marked the point for zero, when the telefcope by adjuft- 
ment had been brought as nearly horizontal as pofiible. Thefe 
previous fteps being taken, the femicircle w r as removed, divided 
on the engine, and replaced in its orginal fituation. Never- 
thelefs, when the inftrument was carried into the field, and 
fcrupuloufly adjufted, the error was found, as has above been 
faid, 12", which of courfe became the constant quantity to be 
applied with its proper fign, when angles of elevation or de- 
prefiion were taken. 
Art. XVII. Eye-glafjes of the tekfcopes , and inechanifm of the 
wires in their foci . 
It has been already mentioned, that the telefcopes of the in- 
ftrument are furniftied w T ith eye-glafles of different magnifying 
powers for ere£t and inverted vifion, fix for each telefcope, as 
follows, viz . 
Ere<ft vifion. 
Inverted vifion. 
r 
1 
/ — 
N° 
Power. 
N 3 
Power?, 
rN° 1. 
5 s. 
N° 1. 
4 . 3 - 
For the lower telefcope, < 
2. 
88. 
2% 
59 - 
3 * 
1 1 7 ' 
3 - 
87. 
N° 1. 
54 * 
N° 1 » 
4 - 0 . 
For the upper telefcope, < 
2. 
8 1, 
55 - 
n 
108. 
3 * 
80. 
Wirh 
