And since it was obvious that discovery had to be made by traversing long expanses of ocean, and that this could only be done by a sound knowledge of navigation, those in authority were not slow to realise that lectures and instruction on this subject at home meant presently an increase of territory and wealth across the seas. Prince Henry on his promontory had been the first to grasp this. Now also Charles V not only established a Pilot Major for the examination of those who sought to take ships to the West Indies, but also founded a lecture on the art of navigation which was given in the Contractation House at Seville. Those anxious to qualify as pilots had to learn thoroughly the use of the astrolabe and quadrant, and obtain a thorough grasp of the theory and practice of sailing a ship from one port to another out of sight of land. For this instruction they had to pay fees, but it more than repaid them many times over when they were able to bring back such valuable commodities. Furthermore, as experience gains knowledge, so every voyage taught them something of their art which hitherto they had not known--the direction of a current, the state of the moon when high tide occurred at such and such an hour, the depth of those new harbours they had entered, the position of the outlying shoals, the landmarks on shore, the temper of the natives, the kind of commodities which could be obtained in the districts, and so on. The pilots brought all these details home at the end of every voyage, made the necessary corrections in the charts (and this not by choice, but by compulsion), so that always there was being compiled a set of sailing directions and an ever improving bundle of charts which were simply invaluable to State and seamen alike.