Let the magnetic variation be
N. 15 W., and the observed bearings A 290, B 320, then the
position stands as in Fig. 38, or, correcting for magnetic
variation, as in Fig. 39, from which it will be seen that the
true bearing of C from A will be 275-180=95 East of North, or
5 below the horizontal, and the true bearing of C from B will
be 305-180=120 East of North, or 35 below the horizontal.
These directions being plotted will give the position of C
by their intersection. Fig. 40 shows the prismatic compass in
plan and section. It consists practically of an ordinary compass
box with a prism and sight-hole at one side, and a corresponding
sight-vane on the opposite side. When being used it is held
horizontally in the left hand with the prism turned up in the
position shown, and the sight-vane raised. When looking through
the sight-hole the face of the compass-card can be seen by
reflection from the back of the prism, and at the same time the
direction of any required point may be sighted with the wire in the
opposite sight vane, so that the bearing of the line between the
boat and the required point may be read. If necessary, the
compass-card may be steadied by pressing the stop at the base of
the sight vane. In recording the bearings allowance must in all
cases be made for the magnetic pole.
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