QUADRANT, denotes a mathematical instrument, of great service in astronomy, and, consequently, in navigation, for tak ing the altitudes of the sun and stars ; as also for taking angles in surveying. Those chiefly in use, are Adams's, Cole's, Gun ter's, Hadley's, Sutton's, or Collins's, the horodictical, the sinical, the astronomical, and the common surveying quadrant.— Many of these are made of wood, gene rally ebony, mounted with ivory ; but such are subject to warp, which occasions those made of brass to be preferred, for very hot or very cold climates ; though their expansion and contraction, under various temperaments, is some drawback on their merits : however, that being the lesser evil, and scarcely ever amounting to more than two or three seconds in the whole arch of the quadrant, cannot be considered as any great defect.
Although these instruments are gene rally termed quadrants, they are, in truth, but octants, since they occupy but one. eighth of a circumference ; but as each of the 45° they contain on the arch ac tually measures two, while taking the ob servation, they do not receive their desig nation improperly. We have another de scription of this instrument, called the sextant, which has 60° marked onits arch, and includes in real measurement. This is peculiarly calculated for the obser vation of various celestial bodies, so as to ascertain their distances at any particular moment : this often could not be effect ed by an instrument which embraced only 90° ; whereas, we rarely find any two planets suitable to the purposes of navigation, at so great a distance as 120°.
The manner in which the quadrant is held, relieves it from the effect of the ves sel's motion ; although, in the first in stance, some difficulty may occur in suit ing the body to the rolling, or pitching, of a vessel ; yet, in a very short time, the operator will become so habituated, as to overcome that trifling impediment.
Hadley's quadrant (or his sextant) is the only instrument, hitherto known, on which the mariner can depend for a correct ob servation. It may be called the " por table observatory?' The first idea of this machine originated with the celebrated Dr. Hooke ; it was completed by Sir Isaac
Newton, and first offered to the public by Mr. Hadley ; however, it has undergone many changes since that time. The great perfection it exhibits, with respect to the accuracy of the angles it defines, is con siderably enhanced by the facility with which it may be rectified ; so that errors may be avoided: a matter of supreme importance, when we consider the rough usage to which the instrument is subject ; and, that an error of one degree in the in dex makes two in the observation.
Description of s Quadrant. (Fig.
1. Plate XIII. Miscel.) shows the quadrant, as usually constructed. The following parts compose the instrument. B C, the arc of 45° : A D, the index, moving on a pivot, under the centre of the index-glass, E; which glass is in the exact direction of the index, and stands at right angles upon it. 1', the fore-horizon-glass, which receives the reflection from the index glass. G, the back-horizon-glass. The former stands parallel with the leg, A C ; the latter at right angles thereto. K, is a pivot, on which three dark glasses, or screens, move, so that any one, or more, may be placed between the index-glass and the horizon-glass, to diminish the lustre of the reflected planet. H and 1, the vanes, or sights. The arc, B C, is call ed the limb, or quadrantal arc ; what is beyond 0, is the arc of excess : the residue of the arc usually is graduated up as far as 100°.
A large portion of the lower part of the index is open, so as to show the grada. tions on the arc : the lower edge is chain fered, that it may come close down to them, and is there divided into smaller portions : this scale is called the nonius, and shows the smaller divisions in a more correct and obvious manner than could be done by the quadrantal arc, on which each degree is subdivided into no more than three equal parts, of 20' each. Now the nonius, being divided into 21 equal parts, shows at what portions of the arc the index cuts the division of 20 minutes ; therefore it shows every mi nute.