of an Arc of the Meridian. 405 
therefore, close this article with a few observations on the manner 
of adjusting the instrument for observation. 
Manner of adjusting the Instrument for Observation. 
The feet of the external stand should be first carefully brought 
into a horizontal plane; and, when they are so, the azimuth 
circle will be, necessarily, parallel to it, having its centre under 
the middle of the opening in the mahogany frame screwed on 
the top of the stand. This being done, and the instrument set 
up, the plane of the arch should be brought parallel to one of 
the sides of the stand, in which situation, the internal frame is 
to be clamped to the azimuth circle, and the wire brought to its 
proper distance from the limb, by means of the adjusting-screw 
attached to one of the sliders, which carries the concave recep- 
tacle and conical point. The dot at zero should then be brought 
exactly under the plumb-line, as seen through the magnifier, 
and the point on the micrometer-head, at which its index stands, 
noted. The instrument is then to be turned half round ; and, if 
the same dot on the arch still continues bisected, it will afford a 
proof of the internal stand being upright in one direction. But, 
if the dot should not continue bisected by the plumb-line, it 
must be made to do so, and the revolutions, or parts of a revo- 
lution, counted ; half of which is to be turned back on the micro- 
meter-head. The same dot, zero, is then to be brought under 
the wire, (plumb-line,) by means of the other adjusting-screw 
beneath the azimuth circle. If the stand is pretty accurately set 
up, one operation is sufficient for bringing the interior frame 
upright in one direction, viz. either in that of the meridian, or 
the one at right angles to it. The arch is then to be turned 
MDCCCIII, 3 G 
