But all the tables of declination are of no use without an instrument with which to measure accurately the angle between the sun and the horizon, and such an instrument was slow in coming. The first apparatus used was called a “cross staff.” It was made of two rods, one about thirty-six inches and the other about twenty-six inches long. The shorter staff was arranged so that its centre slid along the other while it remained rigidly at right angles to the longer staff. To work out one’s latitude with this instrument the observer waited until noon was almost upon him. He then took his cross staff and, placing one end of the longer crossbar to his eye and holding the instrument so that the shorter bar stood in a vertical plane, moved the shorter bar back and forth until he could sight over the upper end at the sun and, at the same time, beneath the lower end at the horizon. As the sun continued to mount to its highest point he pulled the cross staff slowly toward him, measuring a greater and a greater angle. When the sun had reached its highest point and the angle between it and the horizon began to lessen, his “sight” was completed, and carefully holding the crossbar where it marked the greatest angle he laid it on a table on which a circle was inscribed. The end that had been at his eye he placed at the centre of the circle, and the segment marked by the lines from the centre past the two ends of the crossbar showed the number of degrees in the angle he had measured between the horizon and the sun. But any one who has attempted to sight a gun accurately while standing on an irregularly moving platform will have some idea of the difficulty these old sailors had in sighting accurately at the horizon and the sun at identically the same time from the deck of a bobbing ship. The glare of the sun, the motion of the ship, and the inaccuracy of the instrument itself could not be expected to give more than approximate results, especially as the several corrections on the angle now known to be necessary (the refraction of the sun’s rays as they enter our atmosphere is one) were either not recognized or were inaccurately known. [Illustration: USING AN ASTROLABE _This instrument was meant to improve on the cross staff. One man held it, when it was supposed to hang with the horizon line horizontal. Another man sighted at the sun or the stars, and a third read and recorded the angle. Needless to say the instrument was very inaccurate._ ]