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.
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.
HADLEY QUADRANTS (OCTANTS) IN PEABODY MUSEUM, SALEM
| 1. “Made by John Dupee 1755 for PatrickMontgomerie.” All wood, ebony, arm 22 incheslong. | 2. “Made by Ino. Gilbert on Tower HillLondon 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. | |
| 4. “Spencer Browning and Rust London.”Ebony frame, brass arm 17 inches, ivory scale,pencil inserted in cross piece, ca. 1800, used byCapt. Henry King. | 5. “J: Urings London.” All brass, arm 20inches, back sight broken off, ca. 1780, rare. |
To obtain the ship’s latitude with comparatively good results was an easy matter with the quadrant and its fore-runners, but the great problem for centuries was how to find the longitude, now universally and quickly obtained by the chronometer and simple observations in the morning or at noon. Spring clocks and watches appeared about 1530 but they were unreliable and of no use on long voyages. Sand glasses like those of the old Colonial churches were used on ships and so conservative is the British mind that some were in use on British naval vessels as late as 1828 and one authority states as late as 1839. Greenwich Observatory was established in 1675 and a Royal Commission was soon appointed with authority to award prizes for important inventions in aid of navigation. A prize of £20,000 was finally offered for a time-keeper that should meet certain requirements which practically meant absolute accuracy. In 1767, John Harrison produced the chronometer, based on the principle of an invention of 1735, and eventually he received the reward. Chronometers were so expensive and so hard to obtain that few New England ships had them until more than a half a century later. Other devices were tried to obtain longitude by lunar observations and by Jupiter’s satellites, but these observations were too difficult to be of practical use. Today, fine watches serve for short trips and chronometers are carried by nearly all vessels making long voyages.