An Improved Instrument for Setting out Large Circles such as Railway Curves.
Year: 1889

page_text: Date of Application, 1st Jan., 1889
Specification Accepted, 2nd Mar., 1889
An Improved Instrument for Setting out Large Circles such as
Railway Curves.
I, ALEXANDER PELHAM TROTTER, of 2 Victoria Mansions in the City of Westminster'
Civil Engineer, do hereby declare the nature of this invention and in what manner
the same is to be performed, to be particularly described and ascertained in and
by the following statement:-
--
My invention relates to that class of instruments which is used for setting out
circles of considerable size, or parts of circles, such as railway curves, or the like;
especially where the centre of the circle is inaccessible, or where the curve has to be
set out through two or more given points.
The principle of the instrument is that of the centrolinead, viz. the angles in the
same segment of a circle are equal to one another. In place of the arms of the
centrolinead, I use two lines of sight, proceeding from fixed points at the end of
the chord of the segment. One of these is observed by the operator, direct, the other
is seen by reflection in a mirror.
In carrying out my invention, I provide at one extremity of a bar, a sight, which
may be provided with a lens, or a telescope, if required. At the other end of the
bar is a half-silvered mirror, as used for sextants. The mirror is mounted on an axis,
so that its inclination to the sight may be adjusted, and graduations are provided
for determining the adjustment.
To use the instrument, a pole, or other mark is set up at each end of the chord,
and the mirror is suitably adjusted. When the poles are seen, the one direct, the other.
reflected in the mirror, in apparent coincidence, the mirror is at a point on the curve
required.
I am aware that sextants have been used for setting out curves in this manner,
but a table of chords is required for the purpose of setting the mirror. In my
invention no table is needed. The graduations are numbered with reference to the
ratio of the length of the chord to the radius of the curve.
For example, if the length of the chord is 200 feet, and the radius of the required.
curve is 500 feet, the mirror is adjusted to the graduation 0-4 this being the ratio
of the chord to the radius. The mirror will then make an angle of 5° 46' 6.5" with
the axis of the sight, being one quarter of the angle at the centre subtended by
the chord.
When it is desired to set out a curve through three given points, the operator
holding the instrument at the third or intermediate point, adjusts the mirror so that
the poles are seen in apparent coincidence.
The mirror being adjusted, and clamped in position, the operator walks in the
direction of the curve, and at suitable intervals, places himself so that the poles at

page_text: the extremities of the chord are seen in apparent coincidence. The instrument is
then at a point on the curve. Where accuracy is required for small curves, the
exact spot may be indicated by a plumb-bob suspended from the instrument.
The graduations on the instrument may be marked on an arc, and the mirror set
by means of an arm bearing a fiducial line. It would be difficult to estimate
fractional parts of such graduations, since they would be of varying magnitude.
Again, the mirror may be adjusted by means of an arm and suitable link work,
connected to a slide bearing a fiducial mark, reading against a uniformly graduated
scale. Such linkwork would be liable to produce error on account of play in the
joints and slides. I therefore prefer to effect the adjustment of the mirror by the
means shown in the accompanying drawings.
Fig. 1 is a plan, and Fig. 2 a side elevation of the instrument. The bar A is
provided with a sight B at one end, and a half-silvered mirror C at the other end.
A clamping screw D is provided for holding the mirror securely at any desired angle.
An eye E is provided for the attachment of a plumb-line. The instrument may be
held in the hand, or a tripod may be used if necessary.
On the bar A is a scale uniformly divided; the graduations being numbered by
decimals up to the digit 2. The numbers express the ratio of the chord to the
radius; that ratio being 2 when the curve is a semicircle.
To the mirror is attached the plate F having a bevelled edge. This edge is the
fiducial curve by which the scale is read. When the, plate F is in a median or
symmetrical position, the edge cuts the centre line of the scale at the extreme
graduation marked 2. The mirror is then inclined at an angle of 45° to the axis of
the sight. The reflected ray then makes a right angle with the direct ray. The
angle in a semicircle being a right angle, poles placed at the ends of the diameter
will appear to coincide when observed from any point on a semicircle.
In Fig. 3, A B is a chord, and A Ca radius, the ratio being 15. The mirror is
set so that it makes an angle a little less than 24° 15' with the axis of the sight.
The observer sees the pole at the point B direct, and the pole at the point A by
reflection.
If the instrument be placed at A, and the pole at B is observed direct; a point
at F will be seen in apparent coincidence with B when the line A F is tangent to
the curve A E B.
The figure of the plate F Fig. 1. is a polar curve whose equation is ;-
where a is the distance from the zero graduation to the axis of the mirror, and bis
the length of the scale from zero to 2.
A slide, micrometer screw, and vernier may be provided, the slide having a stop
against which the edge of the plate is to be set, the edge of the plate being formed
so as to allow for the size of the stop..
Having now particularly described and ascertained the nature of my said
Invention, and in what manner the same is to be performed, I declare that what
I claim is ;-
The construction of an instrument for ascertaining the position of points on.
a curve, substantially as herein described and illustrated by the accompanying
drawings.
Dated this First day of La 1000
ALEXANDER PELHAM TROTTER,
Applicant.