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. [Illustration: SECT. III. The Description and Use of the Plough. _The Description of the Plough._ This Instrument was antiently in use amonght Mariners, although at this day it is not so commonly used as formerly; it consists of a Staff having a small Arch, and three Vanes. _The Figure of the Plough._ The Staff is about two foot and a half long, or three foot at the most; at the Center-end of which is erected a small Arch, that is divided into 85 degrees; on the side of the Staff are set off the Graduations proper to the Plough, beginning at five or six degrees, and encreasing to ten degrees towards the Arch, every degree being divided into single minutes. The Vanes are a _Horizon-Vane_, as A, and _Shadow-Vane_, as B, (to be used as in the Quadrant) and a _Sight-Vane_ moving upon the Staff, as at C. PAGE FROM "PRACTICAL NAVIGATION," BY JOHN SELLERS, LONDON, 1676] [Illustration: 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.]