In practice to find the true meridian, two observations must be made at
intervals of six hours, or they may be made upon different nights. The
first is for latitude, the second for azimuth at elongation.
To make either, the surveyor should provide himself with a good transit
with vertical arc, a bull's eye, or hand lantern, plumb bobs, stakes,
etc.[1] Having "set up" over the point through which it is proposed to
establish the meridian, at a time when the line joining Polaris and
Alioth is nearly vertical, level the telescope by means of the attached
level, which should be in adjustment, set the vernier of the vertical
arc at zero, and take the reading. If the pole star is about making its
_upper_ transit, it will rise gradually until reaching the meridian as
it moves westward, and then as gradually descend. When near the highest
part of its orbit point the telescope at the star, having an assistant
to hold the "bull's eye" so as to reflect enough light down the tube
from the object end to illumine the cross wires but not to obscure the
star, or better, use a perforated silvered reflector, clamp the tube in
this position, and as the star continues to rise keep the _horizontal_
wire upon it by means of the tangent screw until it "rides" along this
wire and finally begins to fall below it. Take the reading of the
vertical arc and the result will be the observed altitude.
[Footnote 1: A sextant and artificial horizon may be used to find the
_altitude_ of a star. In this case the observed angle must be divided by
2.]