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THE TROTTER CURVE RANGER.

This little instrument was exhibited in a somewhat crude state at the meeting of the British Association at Newcastle in 1889.  It has since been modified in several respects, and improvements suggested by practical use have been introduced, bringing it into a practical form, and enabling a much greater accuracy to be attained.  The principle is one which is occasionally employed for setting out circles with a pocket sextant, viz., the property of a circle that the angle in a segment is constant.  The leading feature of the invention is the arrangement of scales, which enables the operation of setting put large curves for railway or other work to be carried out without requiring any calculations, thereby enabling any intelligent man to execute work which would otherwise call for a knowledge of the use of a theodolite and the tables of tangential angles.

[Illustration:  FIG. 1—­PERSPECTIVE VIEW OF INSTRUMENT MOUNTED ON A STAFF.]

The instrument is intended to be thoroughly portable; so much so, indeed, that it is not necessary or even desirable to use a tripod.  It may be held in the hand like a sextant, or may be carried on a light staff.  The general appearance is shown in Fig. 1.  It will be seen that a metal plate, on which two scales are engraved, carries a mirror at one end and an eye piece at the other.  The mirror is mounted on a metal plate, which is shaped to a peculiar curve.  A clamp and slow motion provide for rapid and for fine adjustment.  The eye piece is set at an angle, and contains a half silvered mirror, the upper portion being transparent.  This allows direct vision along the axis of the eye piece, and at the same time vision in another direction, after two reflections, one in the eye piece and the other at the adjustable mirror.  Fig. 2 is an outline plan of the instrument when closed.  In the first form of the instrument only one mirror was provided, but by the double reflection in the improved pattern, any accidental twisting of the rod or handle produces no displacement of the images, since the inclination of one mirror neutralizes the equal and opposite inclination of the other.  No cross line is required with the new arrangement, since it is only necessary that the two images should coincide.

[Illustration:  FIG. 2.—­OUTLINE OF INSTRUMENT SHOWING THE PATH OF THE DIRECT AND OF THE REFLECTED RAY.]

The dotted line A B represents the direct ray, and the line A C D the reflected one.  Fig. 3 shows the different geometrical and trigonometrical elements of the curve, which can be read upon the various scales, or to which the instrument may be set.  An observer standing at C sights the point B directly and the point A by reflection.  A staff being set up at each point, he will see them simultaneously, and in coincidence if the instrument be properly set for the curve.