the Divisions of astronomical Instruments. 235 
strength necessary. It is fixed with its broad convex side 
downwards upon two brass pillars, screwed fast to the plane of 
the azimuth circle, and standing in a line parallel to the plane 
of the vertical circle at the distance of about four inches from it, 
and on the right side of the pillars which support the microme- 
ters belonging to this circle. The pillar, to which the left end 
of the arc is fastened, is placed close to the lower micrometer 
of the vertical circle, and the other contiguous to the elevated 
rim, in which the divisions of the azimuth circle are cut. The 
right end of the arc reaches beyond this pillar about ten 
inches. The pillars are of such a height, and so proportioned 
to each other, that whilst the left end of the arc, which lies 
horizontally, is raised to within about two inches of the height 
at which the lowest point of the vertical circle is placed, the 
whole arc runs parallel to the circle through an extent of 
something more than 90°. Upon the arc a microscope, with a 
moveable wire in the focus, is made to slide as in the former 
case, and to point to the divisions upon the vertical circle, not 
directly, but with an inclination of about 30° to the left, in 
order that the same division ( which is the lowest upon the 
circle ) may be seen through it, and through the lower micro- 
meter at the same time. 
I will now proceed to shew you, in what manner the exa- 
mination of the divisions upon either circle may be performed. 
The process is precisely the same in both cases, and will of 
course be described in the same words. 
The first point to be examined is that of 180°, which must 
be done in the usual way, by bringing the points of o and 180° 
to the moveable wires of the opposite micrometers, and then 
turning the circle half-way round, and bisecting the points 
