[Illustration: The Figure of the Quadrant and Manner of Observation. (Davis's Quadrant.)] [Illustration: The Figure of the Cross-Staff and the Manner of Observation.] "Davis's Quadrant," invented by the celebrated navigator of that name in Queen Elizabeth's time, was the best of these. This instrument was known also as "the back-staff" from the position of the observer with his back to the sun when using it. The cross-staff or fore-staff was, however, still used, as it was in the time of Columbus; this was simply a four-sided straight staff of hard wood, about three feet long, having four cross-pieces of different lengths made to slide upon it as the cross-piece does upon a shoemaker's rule. These cross-pieces were called respectively the ten, thirty, sixty, and ninety cross, and were placed singly upon the staff according to the altitude of the sun or star at time of observation; the angle measured being shown by a scale of degrees and minutes intersected by the cross-piece on that side the staff to which it (the cross) belonged. Besides the cross-staff, a form of small quadrant, called an "Almacantas staff," was used just after sunrise, and before sunset, for finding the sun's azimuth, and the variation of the compass, while in latitudes north of the line, the "Nocturnal" gave the hour of the night, by observing with it the hands of the great star-clocks, Ursa Major and Minor, as they turned about the Pole Star. [Illustration: The Figure of the Nocturnal.] [Sidenote: _Letter to the Governor of St. Antonio._]