(Quadrans), in geometry, the arch of a circle containing ninety degrees, or the fourth part of the entire periphery. The name is also applied to a mathematical instrument, of great use in astronomy and navigation, for taking the altitudes of the sun and stars, as also for taking angles in surveying. This instrument is variously contrived, and furnished with different apparatus, according to the various uses for which it is intended; but they all have this in common, that they consist of a quarter of a circle, the limb of which is divided into ninety degrees. Some have a plummet suspended from the centre, and are furnished with sights to look through. The principal and most useful quadrants are the common surveying quadrant, the astronomical quadrant, Adams's quadrant, Cole's quadrant, Gunter's quadrant, Hadley's quadrant, horodictical quadrant, Sutton's or Collins's quadrant, and the sinical quadrant. The quadrant is now entirely superseded for astronomical purposes by the mural circle. (See MURAL CIRCLE, and Sextant.)