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 still (or other heavenly 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-staff 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 object 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 various lengths were used. The as trolabe, which was construeted 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 obserying. The line of sight was usually a diameter of the circle and a pointer was supplied by which the angle could he read.
In 1504 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 Clay 13, 1731, and a few days later exhibited the instrument. About a year earlier, Thomas Godfrey, of Phila delphia, designed a sextant. lie made an instru ment about November, 1730, and it was in actual use at sea before the end of the year. Hadley's instrument may have been the outcome of Bon guer's improved cross-staiT, but Godfrey's scents to have been quite an independent invention. It may be noted also that Newton designed a double reflecting instrument, similar to the sextant. and a description of it was found in Newton's own handwriting among Hadley's papers in 17-12. llooke 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.