234 JWr. Lax ow <3 Method of examining 
To the face of the rim which surrounds the azimuth circle, 
and with its left end close to the stand which supports the 
micrometer on the east side, an arc of brass, concentric with 
the circle itself, and a little more than 90° in length, an inch 
in breadth, and one eighth of an inch in thickness, is firmly 
fixed by screws, so as to have the plane parallel to the plane 
of the circle, and a small portion of its lower surface resting 
upon the extreme part of the rim. The screws pass through 
a brass arc, which is fastened to this at right angles, and lies 
with its broad side against the face of the rim. Upon the first 
mentioned arc, a strong upright piece of brass, about six inches 
in length, is made to slide, the lower part of it embracing the 
arc as a groove and having a clamping screw underneath, for 
the purpose of fixing it firmly to the arc at any point re- 
quired. To the top of the upright piece of brass is attached a 
microscope, with a moveable wire in its focus, pointing down 
to the division upon the circle, not directly, however, but with 
an inclination to the left of about 30°. This inclination is given 
to it, in order to make it point to the same division upon the 
circle, which is immediately under the micrometer itself, when 
it has been moved up as near to the micrometer, as it is capable 
of approaching. The microscope has attached to it a small 
graduated circle of brass, and an index, by which the seconds, 
and parts of a second, moved over by the wire are deter- 
mined. 
To the vertical circle there is likewise an arc applied, of 
the same length and breadth as the former, but somewhat 
thicker, and of a radius exceeding that of the circle by about 
two inches. This greater thickness is given to it, on account 
of its being supported in a manner which renders additional 
