sextant (sekstant)

[F. sextant = Sp. sex tante = Pg. sextante, seistante = It. sestante, IMG:content-0756.png:[merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small] IMG:content-0757.png:[graphic] ing, for measuring the angular distance of two stars or other objects, or the altitude of a star above the horizon, the two images being brought into coincidence by reflection from the transmitting horizon-gla , lettered b in the figure. The frame of a sextant is generally made of bra , the arc h being graduated upon a slip of silver. The handle a is of wood. The mirrors band e are of plategla , silvered. The horizon-gla b is, however, only half silvered, so that rays from the horizon or other direct object may enter the telescope e. This telescope is carried in the ring d, and is capable of being adjusted, once for all, by a linear motion perpendicular to the plane of the sextant, so as to receive proper proportions of light from the silvered and unsilvered parts of the horizon-gla . The figure does not show the colored gla shades which may be interposed behind the horizon-gla and between this and the index-gla c, upon which the light from one of the objects is first received, in order to make the contact of the images more distinct. This index-gla is attached to the movable arm f. The movable arm is clamped by the screw i, and is furnished with a tangent screw j. The arc is read by means of a vernier carried by the arm, sextant with the reading-lensg. In the hands of a competent observer, the accuracy of work with a sextant is surprising. The first inventor of the sextant (or quadrant) was Newton, among whose papers a description of such an instrument was found after his death-not, however, until after its reinvention by Thomas Godfrey, of Philadelphia, in 1730, and, perhaps, by Hadley, in 1731. Chauvenet, Astronomy, II. § 78. 3. [ cap.] Same as Sextans, 2. - Box-sextant, a surveyors' instrument for measuring angles, and for filling in the details of a survey, when the theodolite is used for long lines and for laying out the larger triangles. - Prismatic sextant, a sextant in which a rectangular prism takes the place of the common horizon-gla , and with which any up to 180°