From the mere fact that Chaucer said as much as he did, it is clear enough that English merchant ships at this time were trading up and down Europe between Sweden and the Spanish coast, and that as a regular thing. And some of these seamen were now also navigators. Chaucer, who died in the year 1400, has left behind a treatise on the astrolabe by which a mariner could “knowe the altitude of the sonne, or of other celestial bodies.” An astrolabe was the ancestor of the sextant, an instrument which had been used by the Arabs for long years before it was used by the Christian seamen of the Mediterranean or the North. As far back as the eighth century a learned Rabbi named Messahala had written a treatise on the astrolabe, and it was chiefly from that author that Chaucer derived his knowledge on the subject. During the early Tudor period the cross-staff began to supersede the astrolabe. The growth of the English Mercantile Marine in the fourteenth century, and the friction which occurred between these ships and the vessels belonging to Norway, Prussia, Flanders, Scotland, Spain and Genoa, led to the assertion by Edward III that his royal progenitors had been lords of the English sea “on every side,” and in 1344 he had coined a gold noble whereon he was represented standing in a ship crowned, with sword and shield, emblematic of his sovereignty of the seas. It was a statesmanlike attempt on behalf of the commercial ships to make the sea a peaceful highway, when for so long it had been such a dangerous thoroughfare. It had its counterpart in the Mediterranean; for the raids by the Saracens had caused the Pisans to exercise the rights of commercial and naval supremacy on the west coast of Italy; and in 1138 the Genoese had exercised a similar authority in the Gulf of Lyons. Similarly Venice had claimed sovereignty over the whole of the Adriatic in the thirteenth century, though prior to that it had exacted a toll from vessels passing through that sea.