Quād'rant, n.

(Synop., § 130), n. [Lat. quadrans, a fourth part, a fourth of a whole, fr. quatuor, four; It. & Pg. quadrante, Sp. cuadrante, Pr. quadrant, quadran, Fr. cadran.]

1. The fourth part; the quarter. [Obs.] Browne.
2. (Geom.) The quarter of a circle, or of the circumference of a circle: an arc of 90°, or one subtending a right angle at the center.
3. An instrument for measuring altitudes, variously constructed and mounted for different specific uses in astronomy, surveying, gunnery, &c., consisting commonly of a graduated arc of 90°, with an index or vernier, and either plain or telescopic sights, together with a plumb-line or spirit-level for fixing the vertical or horizontal direction.
4. (Arch.) The same as QUADRANGLE. [Obs.]

Gunner's quadrant, an instrument, consisting of a graduated limb, with a plumb-line or spirit-level, and an arm by which it is applied to a cannon or mortar for regulating its elevation and pointing. — Hadley's quadrant, an instrument, used chiefly at sea, for measuring altitudes and other angles, consisting of an octant with graduated limb, and an index carrying at one end a vernier, and at its center of motion a mirror, called the index-glass, from which a ray of light from one of two given objects is reflected to another mirror, called the horizon-glass, fixed to one of the arms, and thence to a sight-hole on the other arm, at which, when the index is rightly set, the eye sees the reflected image of the object apparently in contact with the other object seen directly, the angle between the two being shown by the vernier; — called also octant. — Quadrant of altitude, an appendage of the artificial globe, consisting of a slip of brass of the length of a quadrant of one of the great circles of the globe, and graduated. It may be fitted to the meridian, and being movable round to all points of the horizon, serves as a scale in measuring altitudes, azimuths, &c.