The curve can then be set out, and fully marked,
and the elements of the curve can be read on the scales and recorded
for reference.
[Illustration: FIG. 4.--DIAGRAM ENGRAVED ON THE INSTRUMENT.]
Various other cases which may occur in practice can be rapidly met by
one or other of the various scales. Suppose the angle A G B between
the tangents be given, together with the middle point F on the curve,
Fig. 3. Subtract this angle from 180 deg., the difference gives the
angle at the center A O B. Take half this, and set the instrument to
the angle thus found. Walk along the tangent until a rod set up at
some point in the tangent, say E, is seen in coincidence with a rod
set up at B. The position of the instrument then marks the point of
departure A. A rod being placed at A, the first half of the curve may
be set out; or, if B is invisible, the instrument may be reset for the
angle E A B, and the whole curve set out up to B. No cutting of hedges
is necessary, as with theodolite work, for a curve can easily be taken
piece by piece.
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