SEXTANT (from Lat. sextons, sixth part, from sextus: sixth, from sex, six). An instru• meat used for measuring angles between distant objects. The sextant finds its greatest field of usefulness in navigation, hut it is also employed in marine surveying. It consists of a fame in the form of a stetor embracing somewhat more than one-sixth (usually about one-fourth) of the whole circle; two mirrors (one wholly silvered and one silvered over one-half its surface) ; movable arm pivoted at the centre of the sector and carrying the fully silvered mirror and a vernier; an are along the circumference of the sector graduated into degrees, minutes, and see onds; and an eye-piece. The common form of the instrument is shown in Fig. 1.
The frame is of brass. AA is the limb in which is inlaid a strip of silver on which are the gradu ations of circular measure: the smallest divi sions are usually 10' to 30', and the vernier en ables angles to be read to at least 1' and usually to 10". The handle H by which the instrument is held in the hand is of wood. The mirrors Bf and in are of plate glass. The former has all its surface, while the latter has but the lower half silvered. Both are fitted with small screws for adjusting them in perpendicularity to the plane of the front face of the frame and in paral lelism to each other when the index arm is set at 0°. E is the eye-pieee of the telescope, which is Besides the ordinary telescope the instrument is usually provided with an inverting telescope, and a tube without glasses, F; also colored eye pieces to use in place of the colored shade glasses, 1' and Q, and an adjusting wrench or serew driver. The theory of the instrument is shown in Fig. 2. AUG is the frame of the instrument in the form of a circular sector. VO is the in dex arm carrying the index glass, I, and the ver nier, V, and is pivoted at 0 on the frame. 11 is the horizon glass. which is set in a clasp seeurely attached to the frame in a position parallel to 00 (the position of the index arm when set at 0° of the are), but is susceptible of adjustment if thrown out of position. 1.0 is parallel to Ain't'. To determine the angle at the eye (STAI) between two distant objects, S and Al. the pro cedure is as follows: Turn the instrument until one object (D1) can be seen through the telescope and the unsilvered half (which is the half farthest away from the plane of the instrument) of the horizon glass (H). Then turn the instru ment until its plane coincides with that passing through both NI and S. Now move the index arm until the reflection of S appears in the sil vered half of H. By slightly turning the instru ment both objects will be brought together—one just on and one just clear of the edge of the silvered surface of H. Perfect the coineidenee
of the two objects and the reading of the vernier at V will give the angle. For purposes of navi gation the angle commonly measured is that be held in position by the adjustable clasp K. The mirror Al is secured to the index arm S, which is pivoted beneath the centre of 1?1 and earries a vernier on its other end. R is a small magnifying glass for reading the vernier. C is the clamp for holding the index arm to the limb. B is the tangent screw for moving the arm slight ly to perfect the angle: it only acts when the clamp screw C is set up. P and Q are colored shade glasses for use when observing the sun.
tween the sea horizon and the sun, moon, star, or planet. The angle is called the altitude of the heavenly body: in the ease of a star it. can only be taken at twilight or when the moon is up, beeause the stars are not plainly seen by daylight and the horizon is not clearly visible at night. From an inspection of the sketch ( Fig. 3) it is readily seen that the angle through which the index arm moves is one-half that of the angle measured.
For n = angle of incidence and n' = the angle of reflection at the surface of the mirror I and x and x' the same at the mirror H: let LI he drawn parallel to HT. Then the angle measured is SIL = a -I- a ; — a = n —a = = 2,r; y 2n = n — a + 2y; it + = 2y; y= il` it + a = 2 it The are, or limb, of the sextant has a gradu ated scale cut in an inlaid silver strip. The fine ness of the graduation varies: in high-grade instruments the smallest division of the settle is 10 minutes; in some cheaper instruments the smallest division is one degree. To read the angle with great closeness sextants, like other similar instruments, are fitted with verniers. In the ac eompanying figures the smallest division of the limb of the sextant is 10 minutes, and the least count of the vernier is 1 minute. To effect this 10 divisions of the vernier are made equal to 9 of the limb. If the 0 of the vernier rests on the zero of the limb then tho first division of the vernier will fall short of the first division of the limb by 1 min ute, the second division of the vernier will fall short of the second division of the limb by 2 minutes, and so on. If Fig. 4 represents the position of the vernier after measuring a certain angle we proceed to read it as follows: First we note that the zero of the vernier is be tween 17° 50' and IS° 0'. Next we find that the third division of the vernier is in coincidence with a division of the limb. Therefore, we add 3' to 17" 50' and find the angle to be 17° 53'.