Gen,. Roy’s Account of 
Art. XXL To fet the vertical Axis perpendicular. 
This may be done by either level, but beft with the axis 
level, which being fufpended on its pivots, muft be brought 
parallel with two of the feet of the inftrument ; and by the 
ferews of thefe two feet, the bubble is to be brought between 
its indexes. The circle being then turned round 180% if the 
bubble changes its place, half the difference is to be corrected 
by one of the feet ferews, and the other half by two capftan- 
headed ferews, that ad againft each other, under and belong- 
ing to one of the Y’s* or fupports, in which the pivots reft. 
When the bubble is found to be juft in thefe two pohtions, turn 
the circle 90°, which will neceflarily bring the axis over the 
third foot of the inftrument. Then corred any error there 
may be by that foot ferew. In this manner the circle will be 
made to revolve again and again, without any alteration what- 
ever of the bubble, which fhews that the vertical axis is then 
truly perpendicular to the horizon. 
Art. XXII. To make the line of Golhmation in the telefcope at 
right angles with the tranfverfe Axis . 
The pivots refting in their Y’s, dired the telefcope to fome 
diftant well-defined objed, and let the circle be clamped. 
Then reverfe the axis, that is, turn the telefcope upfide down. 
If the interfedion of the wires does not co-incide with the 
objed in both portions, half the difference muft be correded 
by the motion of the circle with the HookVjoint, and the 
other half by the motion of the brafs Hide in the eye end of 
the telefcope, by applying the milled-head key in the fmall focket 
feen on the right hand fide in Plate VI. and fo repeatedly until 
the difference wholly difappears. 
7 
Art. 
