The Hadley was a great improvement over the Davis quadrant and other older devices for finding latitude. By moving the arm the sun is reflected by the mirror at the apex and "brought down" to the horizon line and the eye is protected by colored glasses of various degrees of density through which the sun's rays shine. Catching the sun the instant it is on the meridian (noon), the scale indicates the altitude by which the latitude was figured with the Bowditch Navigator, used for more than one hundred years by American seamen, or Moore's before that and numerous others back to the early eighteenth century. The Hadley quadrant is still used in its modern form with telescopic eye-pieces although the sextant--one-sixth of the circle and by reflection one-third--is a more accurate instrument and also may be used to make lunar observations to obtain longitude, a complicated and difficult matter, so difficult that the authors of the older works did not even take trouble to explain the process, for only the most expert could make this observation, nor were the results satisfactory. The sextant was devised about 1757 and as now made is framed wholly of metal. To prevent corrosion, the scale, which is minutely divided, and has a "vernier" with a magnifying glass to show divisions of minutes, is made of gold or platinum in the best instruments. A half-circle has been devised and is exceedingly rare. An example in the Salem collection was made before 1818. A curious double-jointed dividers accompanied it and the entry in the museum catalog reads,--"used to correct a lunar observation for longitude." A full "circle of reflection" is also sometimes used, more often on land than at sea. This is a beautiful instrument and is not often met with in collections or in use. All of these instruments are similar in character and may be traced, as previously stated, to the ancestral astrolabe. [Illustration: NOCTURNAL "Nath'll Viall 1724." Boxwood, arm seven inches from centre to tip. In Peabody Museum, Salem.] The early Hadley quadrants were huge affairs made of wood with an arm twenty-four inches in length. Today they are more generally of metal with arms from ten to twelve inches. Using the sextent or Hadley quadrant the observer stands facing the sun, but old Hadley quadrants were made with a "back sight" so that they could be used like the Davis quadrant, thus making two independent observations the average of which would ensure greater accuracy. [Illustration: HADLEY QUADRANTS (OCTANTS) IN PEABODY MUSEUM, SALEM 1. "Made by John Dupee 1755 for Patrick Montgomerie." All wood, ebony, arm 22 inches long. 2. "Made by Ino. Gilbert on Tower Hill London for Hector Orr Augt. 6, 1768." Ebony, arm 20 inches long. 3. "Norie & Co. London." Ebony and brass, _ca._ 1840. Arm 11-3/4 inches, telescopic eyepieces, used by Capt. John Hodges.