Description
This circle was used out of doors by surveyors and map-makers to measure the horizontal angle between a reference point and a prominent feature such as a tower, hilltop, or other signal.

The plate carries a graduated scale cut on the inlaid circle, and a rotating index arm with a fixed mirror, verniers at each end and a single clamp. In front of the telescope is a prism, half its height. When the mirror is parallel to the long side of the prism, two images of the same object coincide, one seen directly through the upper part of the telescope, the other reflected by the mirror and prism. The index arm is clamped, and the bearing taken as the mean of the reading on the two verniers.

The index arm is then turned until an object on the right can be reflected into coincidence with the direct view. Again clamping the index, the angular distance between the first and second objects is given by the bearing.

This process can be continued round the horizon, except where the observer’s head gets in the way. When the index is on 0°, the reflected rays are at their most feeble; when the index is at 130° they are at their brightest.
04/08/2008
Created by: Dr Anita McConnell on 04/08/2008