512 FIRST REPORT UPON MAGNETIC WORK

elongation of Polaris, as given by the tables of elongation, No. XVI,
set up the transit firmly, with its vertical axis exactly over the mark,
and carefully level the instrument.

3. Illuminate the cross wires by the light from a bull's-eye lantern
or other source, the rays being directed into the object end of the
telescope by an assistant; while great care will be taken to see that
the line of collimation describes a truly vertical plane.

4. Place the vertical wire upon the star, which, if it has not reached
its elongation, will move to the right for eastern and to the left for
western elongation.

5. While the star moves towards its point of elongation, by means
of the tangent screw of the vernier plate it will be continually cov-
ered by the vertical wire, until a point is reached where it will appear
to remain on the wire for some time, then leave it in a direction con-
trary to its former motion; thus indicating the point of elongation.

6. At the instant the star appears to thread the vertical wire, de-
press the telescope to a horizontal position; about 100 yards north
of the place of observation set a stone or drive a wooden plug, upon
which by a strongly illuminated pencil or other slender object, exactly
coincident with the vertical wire, mark a point in the line of sight
thus determined; then quickly revolve the vernier plate 180°, repeat
the observation, and as before mark a point in the new direction; then
the middle point between the two marks, with the point under the
instrument, will define on the ground the trace of the vertical plane
through Polaris at its eastern or western elongation, as the case may
be.

7. By daylight lay off to the east or west, as the case may require,
the proper azimuth taken from the table No. XVI; the instrument
will then define the true meridian, which may be permanently marked
by monuments for future reference.