Page:The New International Encyclopædia 1st ed. v. 18.djvu/43

SEXTANT. 29 SEXUAL SELECTION. smallest division is one degree. To read the angle with great closeness se.tauts, like other similar instruments, are fitted with verniers. In the ac- 1 -20 . 1 1 1 1 1 i , IS ( •1 1 1 1 1 111 1 1 1 1 1 1 M 1 1 1 1 ITTl ITTl 11 nil /Ml 1 III 1 1 IV llllllll Flu. 5. I'm. i. companying 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 of the vernier rests on the zero of the limb then the 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 18° 0'. Next we find that the third divsion 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'. When used on shore and the sea horizon can- not be seen an 'artificial horizon' is used. This consists of a shallow tray filled with mercury and protected by a gable-roofed cover of thin plate glass framed in brass. The angle measured is that between the sun (or other heavenlj' body) and its reflection from the level surface of the mercury. As is readily seen, this angle is double the altitude of the body. In place of the tray of mercury, silvered glass, laid horizontal by means of a set of levels and screws, is sometimes used. As stated in the article on Navigation, the sextant is a development of the cross-stall' and astrolabe. The former consisted of a staff on which a cross was fitted so as to slide along, its axis being perpendicular to that of the staff. The observer would sight from one end of the staff at the distant oljject and then move the cross until its end was in line with it and the eye. The angle was first measured by laying the instrument on paper and constructing the angle. Later the angles were marked on the staff and crosses of ^•arious lengths were used. The as- trolabe, which was constructed in several forms, consisted of a ring or disk with graduated scale and was provided with sights through which the navigator could view the sun or other heavenly bodies he was observing. The line of sight was usually a diameter of the circle and a pointer was supplied by which the angle could be read. In 1594 the celebrated navigator .John Davies published in his pamphlet, The Seaman's Secrets, a description of his improved cross-staff. - In using this instrument the observer stood with his back to the sun and looked at the horizon through a sight at the end of the staff while the shadow of a movable projection fell on the sight box. In 1729 Pierre Bouguer devised an im- proved form of the Davies instrument, and this was immediately followed by the appearance of the sextant. .John Hadley described a double- reflecting octant in a paper dated Hay 13, 1731, and a few days later exhibited the instrument. About a year earlier, Thomas Godfrey, of Phila- delphia, designed a sextant. He made an instru- ment about November, 1730, and it was in actual use at sea before the end of the year, lladley's instrument may have been the outcome of liou- guer's improved cross-stall', but Godfrey's seem^s to have been quite an independent invention. It may l)e noted also that Newton designed a double- rcllccting instrument, similar to the sextant, and a dcscri])tion of it was foimd in Newton's own handwriting among Hadley's papers in 1742. Hooke also devised a similar one as early as 1674. It does not apear that any actual instru- ments were ever made on Hooke's or Newton's plans. SEXTET (from Lat. S(?a:< lis, sixth). In music, a composition for six voices or instruments, or for six obligato voices with instrumental accom- paniment. Instrumental sextets are generally cyclical compositions in sonata form. SEXTUPLET (from Lat. sextus, sixth -f- -plus, -fold). In music, a group of six equal notes to be performed in the time of four. The true sextupfet is composed of three groups, of two notes each, being, in ... ^ fact, a triplet (q.v.) , with sub- 1^ — ] each of its notes divided into two: But a group composed of two successive triplets is sometimes also called a sextuplet and written as such, though it is more correct to divide it into its component trip- lets thus : SEX'TUS EMPIR'ICUS. A Greek physician of the first half of the third century a.d. He was a pupil of Herodotus of Tarsus, who was probably contemporary with Galen. Nothing is known concerning his life, excejit that he was a physician, and of the school of the Empirics, whence his cognomen: but in his writings his philosophical opinions are sufficiently clear. His first work, the celebrated Pi/rrhonic Sketches, is a repository of the doctrines of the Skeptics : his second, in eleven books, attempts to refute every item of positive knowledge that man has ever acquired. Both works combined furnish the best account extant of ancient skepticism and its methods of assailing all manner of opinions. His works are edited by Fabricius (Leipzig. 1718, with a Latin translation), and by Bekker (Ber- lin. 1842). Consult: Brochard. Les sceptiqucs (jrecs (Paris, 1887); Jourdain. Excursions his- toriques et philosophigues (Paris, 1888) ; Pat- rick, Sextus Empiricus and the Greek Sceptics (Cambridge. 1899. with translation of book i. of the Pyrrhonic Sketches). SEXUAL SELECTION (Lat. sexualis, re- lating to sex. from sr.Tiis. secus, sex). This prin- ciple depends, as Darwin states, not on a strug- gle for existence, but on a struggle between the males for possession of the females. The result is not death to the unsuccessful competitor, but few or no offspring. In many cases, however, victory depends not on general vigor, but on the possession of special weapons confined to the male sex, as the spurs of the cock or the horns of the stag. The war is perhaps severest between the males of polygamotis animals, and these seem oftenest provided with special weapons of offense. .Among birds the contest is often less gross and fierce, the males rivaling each other in attracting the