238 Mr. Lax on a Method of examining 
again with the moveable wires ; and lastly, taking half the 
difference betwixt the distances of the wires in the two posi- 
tions of the circle for the error at the point of 180°. Having 
now bisected the point of zero with the moveable wire of the 
micrometer, which is intended to be used in the rest of the 
process (for we shall have no further 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 mi- 
crometer must be firmly clamped to the arc. The circle must 
then be turned till the point of 180° is brought to the micro- 
scope, and that of 90° to the micrometer, so that we may be 
able to bisect each by a slight motion of their respective wires. 
This being done, we must observe, from the positions of the 
wires, how much the interval betwixt them has increased or 
decreased in the measurement of the new arc ; and this in- 
crease or decrease must be noted down with a -f* or — ac- 
cordingly. In the same manner we must proceed through 
the remaining two arcs of 90°, observing and noting down 
the difference betwixt each and the original arc. 
The point of zero must now be brought again to the mi- 
crometer, and bisected by the moveable wire, and the micro- 
scope be made to slide back along the arc, till by moving the 
wire a little we can bisect the point of 6 o°, and when this is 
done, the microscope must be clamped. We must then mea- 
sure the arc of 6 o° against every succeeding arc of 6 o° in the 
circle, precisely in the same way that we measured the first 
arc of 90° against the other three. The arc of 45 0 is next to 
be measured against every succeeding arc of 45 0 , and this will 
complete all that js necessary to be done in the early part of 
the morning before the heat of the sun can have affected the 
