is a wall, or walled arch, placed exactly in the plane of the meridian, that is, 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 Brabe was the first who used a mural arch in his observations; after him Hevelius, Mr Flamsteed, De la Hire, and others, employed the same means.