RAM 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. Inclination of the whole instrument introduces no
appreciable error. If the eye piece be pointed up or down hill, the
instrument is thrown a little to one side or other of the tip of the
staff, but in a plane tangent to the circle. Errors made in setting
out a curve with the Trotter curve ranger are not cumulative, as in
the method of tangential angles with a theodolite. No corrections for
inaccurate hitting of the final rod can occur, for the curve must
necessarily end at that point. It should be observed that the
instrument is not intended to supersede a theodolite, but it has the
great advantage over the older instrument that no assistant or chains
or trigonometrical tables or any knowledge of mathematics are
required. The data being given, by a theodolite or otherwise, an
intelligent platelayer can easily set out the curve, while the trained
engineer proceeds in advance with the theodolite. No time is lost; as
in chaining, since the marks may be made wherever and as often as
convenient. In work where high accuracy is required this instrument is
well adapted for filling in, and where a rough idea of the nature of a
given curve is required, the mirror being adjusted for any three
points upon it, the various elements may be read off on the scales. A
telescope is provided, but the errors not being cumulative, it is
rarely required. The curve ranger weighs 1 lb. 10 oz., and is
manufactured by Messrs. Elliott Bros., St. Martin's Lane, London. It
is the invention of Mr. Alex. P. Trotter, Westminster.--_The
Engineer._

* * * *