Note:  Do not rely on this information. It is very old.

Mural Circle

Mural Circle is a large metallic circle having a conical axis supported by a stone pier so that the circle is parallel to the face of the pier which is very nearly in the meridian. The instrument is adjusted so that the axis is accurately horizontal and the circle exactly in the plane of the meridian. The use of the instrument is to find the zenith distance of any star as it comes in the meridian. A telescope is firmly attached to the rim of the circle, its optical axis being parallel to the plane of the circle. The rim of the circle is accurately graduated from 0° to 360°, and six microscopes attached to the stone pier are directed straight on to the divisions. The use of six microscopes instead of one is to avoid error. A preliminary experiment determines the reading for the zenith, the circle is then turned so that the star, as it crosses the meridian, is bisected by the horizontal wire of the telescope. The reading for this is got from the microscopes, and the difference between this and that of the zenith gives the zenith distance of the star.