4°9 
of an Arc of the Meridian. 
plane passing through the axis and zero cuts the arches. This 
being done, the telescope and Ys were moved laterally, till the 
vertical wire bisected the speck. The system of wires was then 
turned, till the meridional one was made exactly perpendicular 
to the axis, as seen from the mark being bisected in every part 
of the wire, when the end of the telescope was moved up and 
down by the adjusting-screw. The axis was then carefully 
taken out of the Ys, and inverted : it was afterwards placed as 
before, and the distance between the spot and vertical wire esti- 
mated by the eye. The telescope was then moved in azimuth, 
half that quantity, and the meridional wire brought to a bisec- 
tion on the speck. Repeating this operation twice or thrice, the 
vertical wire became accurately perpendicular to the line passing 
through the centre of the conical axis, and also in the plane 
passing through the centre of the tube. 
The next step was, to move the whole system of wires in the 
direction of the perpendicular, in order that the horizontal one 
(at right angles to the vertical wire by construction) should be 
also brought into its proper position. For this purpose, the tele- 
scope was moved a little in azimuth, and the proper wire made 
to bisect it accurately, at which time, the wire of the micrometer 
before mentioned was brought over the dot on the edge of the 
limb. 
In this position of things, the instrument was taken off the 
Ys, and turned over; it was then again carefully placed in its 
former position, and the end of the telescope brought up by the 
adjusting-screw, till the distant speck was bisected by the hori- 
zontal wire. Now, if this horizontal wire had been, by accident, 
placed so that the point of intersection of the two wires was 
