But for the same purpose of taking the height of the sun there was employed an instrument called the cross-staff; of which the Spanish word (adapted from the Greek) was the “balla stella.” The drawback to the astrolabe was that it was difficult to use it with accuracy owing to the rolling and pitching of the ship. Therefore the cross-staff, being more steadily held in the hand, began to supersede the astrolabe. Bourne, the famous Elizabethan navigational expert, insisted that because the sea “causeth the shippe to heave” the best way to take the sun’s height was with the cross-staff: furthermore, the degrees on this instrument were marked larger than on the astrolabe. Also in a larger instrument an error was seen sooner. The method of use in taking the height of the sun, he explained, was as follows: Note with your compass the sun when the latter approaches the meridian. When it has arrived at S. by E. then begin to take the sun’s height thus: Put the “transitorie” (or cross-piece) on the long staff, set the end of the long staff close to the eye, “winking with your other eye,” and then move the transitory forwards or backwards until you see the lower end of it (“being just with the horizon”) and the upper end of it (“being just with the middle of the sun”), “both to agree with the sunne and the horizon at one time.” Observe the same until you see the sun at the highest and beginning to descend. You have then finished. [Illustration: A SIXTEENTH-CENTURY NAVIGATOR USING THE CROSS-STAFF.] [Illustration: A SIXTEENTH-CENTURY COMPASS CARD.] It is not my intention to digress from the path of historical continuity, but let the reader bear in mind how very little the navigator of this period had to help him. He had the compass for indicating the direction of the ship’s head, and he had the astrolabe and cross-staff for showing him his altitude. But two intensely important data he could not yet obtain accurately: (1) his longitude, and (2) the distance run by the ship in any given time. Very great errors were made in both of these. It was not until the introduction of the log-line in the seventeenth century that a ship could tell with even approximate accuracy her daily run. For many a long year all the cunning Jews and Arabs, all the philosophers, the astronomers and physicians, all the cleverest men out of Portugal, Spain, Genoa, Venice, and the Balearic Isles had tried but failed to solve this proposition. And the coming of the perfect chronometer for finding the longitude was delayed even longer still.