4o6> An Account of the Measurement 
round go°, and the same operation gone through. This being 
properly done, the interior frame is made perfectly upright. 
The next step to be taken, is that of placing the long level 
on its axis above, and rectifying that axis by means of the Y 
plate screws. If this be done carefully, the bubble will remain 
between the pointers of the level, whatever position the sector 
may be placed in. Having thus rectified the instrument, by 
making the internal frame upright, and the axis horizontal, the 
only remaining point to engage attention is, placing the plumb- 
line at a proper distance from the arch : this is done by means 
of the screw acting on the spring just under its point of suspen- 
sion. If great care be used in going through these several 
adjustments, the instrument may, at any future time, be accu- 
rately adjusted for observation, by turning the proper screw 
belonging to the azimuth circle, and bringing the arch to its 
usual distance from the wire. 
Laying off the Points, or dividing the Limb of the Sector. 
The first step preparatory to finding the length of the radius, 
was to mount the sector in its frame, and adjust the counter- 
poising weights. By attention to the proper points on the levers, 
the axis was kept from bending, the pivots not having more 
than a weight of two pounds to support. This done, a tool with 
a well defined point was made to press lightly against the face 
of the arch, and firmly sustained in that situation, while the 
sectorial tube was slowly moved to the right and left : a fine 
line was by this means described on the limb, passing through 
the centres of the golden pins. The arch having been thus 
struck, the telescope was taken out of its frame, and laid on the 
