METHOD OF EXAMINING ASTRONOMICAL INSTRUMENTS. 363 



plane of the azimuth circle, and standing in aline 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 micrometers 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 divi- 

 sions 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 while 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 movable 

 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 Q 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 micrometer at the 

 same time. 



I will now proceed to show you, in what manner the Method of ex- 

 examination of the divisions upon either circle may be P er - viTionTby iL 

 formed. 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 Q , which 

 must be done in the usual way, by bringing the points of 

 and 180° to the movable wires of the opposite micrometers ; 

 then turning the circle half-way round, and bisecting the 

 points again with the movable wires ; and lastly, taking half 

 the difference betwixt the distances of the wires in the two 

 positions of the circle for the errour at the point of 180°. 

 Having now bisected the point of zero with the movable 

 wire of the micrometer, which is intended to be used in the 

 rest of the process (for we shall have no farther occasion for 

 both), we must slide the microscope along the arc, till by 

 moving the wire a little we can bisect the point of 90°, and 

 then the micrometer must be firmly clamped to the arc. The 

 circle must then be turned till the point of 180° is brought 

 to the microscope, and that of 90? to the microme- 

 ter, so that we may be able to bisect each by a slight 



motion 



