In CERTAIN ERRORS, Wright says: ". . . the sun being under 50 degrees in height, you may easily take the height because you may easily see or view the upper and nether end of the cross-staff [transversary] at one time..." but, he cautions, ". . . if it doth exceed 50 degrees, then by means of casting your eye upwards or downwards so much, you may soon commit error." He adds, "you must [then] leave the cross-staff and use the Mariners Ring, called by them the Astrolaby."