a principal fixed instrument in all the great public observatories, is generally of large dimensions, and is attached to a stone wall, or pier of solid masonry, placed in the meridian. It is employed to measure the zenith distances of stars. Tycho Brahe was the first who used the mural arc in his observations; and Flamsteed erected the first one at Greenwich in 1689. Mural arcs and quadrants were superseded at that observatory by the mural circle of Troughton in 1812, an instrument of much greater accuracy, and much less liable to derangement. (See Astronomy, part iv., chap. iv., § 5.)