is a wall, or walled arch, placed exactly in the plane of the meridian, i.e., upon the meridian line, for the fixing of a large quadrant, sextant, or other instrument, to observe the meridian altitudes, &c., of the heavenly bodies.

Tycho Brahe was the first who used a mural arch in his observations; after him Hevelius, Mr Flamsteed, De la Hire, &c., used the same means. See ASTRONOMY.