Based on this instrument, by laying out the circle on a table, John Davis, the explorer, devised his quadrant in 1586. At first the observer used it by facing the sun, as the cross-staff had been used, but a better form was made later where the observer had the sun at his back. This instrument has been called by sailors “jackass quadrant” and, supposedly from its shape, “hog-yoke.” In early books on navigation it is called “sea-quadrant.” The earlier form used by the observer standing back to the sun had a solid “shade vane” which slid along the smaller arc of the instrument. By adjusting this a little short of the supposed altitude of the sun and sighting the horizon through the minute hole in the “sight vane” until it was seen through the “horizon vane” at the apex of the instrument, and then gradually moving the “sight vane” along the larger arc until the shadow of the “shade vane” met the horizon line, the sum of the degrees on the two scales indicated the sun’s altitude. This was really the second form of the Davis quadrant. In the third, the solid “shade vane” was replaced by one with a low-power lens inserted in it arranged to focus on the “horizon vane,” thus approaching the idea of the reflected sun in the Hadley quadrant and the sextant. A most interesting instrument, half-way between a cross-staff and the Davis quadrant, is illustrated in Seller’s book on navigation published in 1676. He calls it a “Plough.” Above, it has the small arc of the Davis quadrant with the sliding rod of the cross-staff below. These were, of course, imperfect instruments, but still a great advance over previous devices to obtain time and latitude.
PAGE FROM “PRACTICAL NAVIGATION,”
BY JOHN SELLERS, LONDON, 1676
DAVIS QUADRANT
“Made by William Williams in King St. Boston.” An ivory plate has “Malachi Allen 1769.” Mahogany, 24 inches long, convex glass in the shade vane; fine example of cabinet work. In Peabody Museum, Salem.
The Davis quadrants are usually made of ebony, rosewood, or other dark woods, with boxwood scale arcs and could be made by expert wood-workers. The numerous examples preserved attest the skill of the old cabinet-makers, for they are never warped or twisted while their jointing is a Chinese puzzle. Probably the Mayflower carried a Davis quadrant and quite likely an astrolabe, and of course, a compass, for the compass had been in use for two centuries.
Whether the compass was independently invented in Europe or was borrowed from the Chinese is uncertain. The old marine compasses were set in gimbals. The magnet was a thin bar attached, usually with sealing wax, to the under side of the compass card, the whole mounted in a thin bowl of turned wood. These were the compasses of the eighteenth century. There is one in the Salem collection inscribed,—“Benjamin King Salem in New England”, with the date “1770” cut in the box; another has the mark of Benjamin King, 1790. A surveyor’s compass, wooden throughout, including wooden sights, is inscribed,—“Made by James Halsey near ye draw bridge Boston.” The liquid compass first suggested by Francis Crow in 1813 and improved by E. S. Ritchie of Boston, has largely displaced the older devices.
The “nocturnal”, used at night, as its name signifies, appeared at an early date, exactly when it does not seem possible to say. One in the Salem collection is marked,—“Nathaniel Viall 1724”. By adjusting the movable discs to the date on the scale for the day of the month, sighting the north star through the hole in the center and then bringing the arm against the “guard stars”, the hour was indicated with reasonable accuracy. Good pictures and descriptions of the nocturnal may be found in old books on navigation.
In 1730, John Hadley in England and Thomas Godfrey in Philadelphia, independently invented the octant, known for nearly two hundred years as Hadley’s quadrant. Both Hadley and Godfrey received awards for their devices. Although called quadrant in this country it is generally known elsewhere as octant, which is the better name, for the instrument represents but one eighth of the circle. By the principle of reflection, however, it covers ninety degrees and the scale is so marked. The Davis quadrant with its two arcs does represent one fourth of the circle and for that instrument the name is correct.