516 FIRST REPORT UPON MAGNETIC WORK

II.——TO DETERMINE THE TRUE MERIDIAN BY OBSERVATION ON POLARIS
AT ELONGATION WITH A PLUMB-LINE AND PEEP SIGHT.

1. Attach the plumb-line to a support situated as far above the
ground as practicable, such as the limb of a tree, a piece of board
nailed or otherwise fastened to a telegraph pole, a house, barn, or
other building affording a clear view in a north and south direction.

The plumb-bob may consist of some weighty material, such as a
brick, a piece of iron or stone, weighing four to five pounds, which
will hold the plumb-line straight and vertical fully as well as one of
turned and finished metal.

Strongly illuminate the plumb-line just below its support by a lamp
or candle, care being taken to obscure the source of light from the
view of the observer by an opaque screen.

For a peep sight, cut a slot about one-sixteenth of an inch wide in
a thin piece of board, or nail two strips of tin, with straight edges, to
a square block of wood, so arranged that they will stand vertical when
the block is placed fiat on its base upon a smooth horizontal rest, which
will be placed at a convenient height south of the plumb-line and
firmly secured in an east and west direction, in such a position that
when viewed through the peep sight, Polaris will appear about a foot
below the support of the plumb-line.

The position may be practically determined by trial the night pre-
ceding that set for the observation.

About thirty minutes before the time of elongation, as given in
the tables of elongation, bring the peep sight into the same line of
sight with the plumb-line and Polaris.

To reach elongation the star will move off the plumb-line to the
east for eastern elongation, or to the west for western elongation,
therefore by moving the peep sight in the proper direction, east or
west, as the case may be, keep the star on the plumb-line until it
appears to remain stationary, thus indicating that it has reached its
point of elongation.

The peep sight will now be secured in place by a clamp or weight,
and all further operations will be deferred until the next morning.

4. By daylight place a slender rod at a distance of two or three