It was thanks to the aid of Martin Behaim, a distinguished cosmographer who came to Lisbon to co-operate with the learned men there assembled, that an improved _sea_ astrolabe was adapted for the purpose of determining the distance from the Equator, by means of the altitude of the sun or stars at sea. There had, indeed, been in use for some time a _land_ astrolabe for finding the latitude of a place, and it was but a natural advance that this instrument should be adapted for use on board ship, so that the mariner might be able to ascertain his position on the vast expanse of trackless ocean. We are all most ready to admire and extol the men and the ships which made such daring voyages and discoveries in the past; but I submit that nothing like adequate recognition has been paid to the essential value of the astrolabe and cross-staff, or their successor, the modern sextant. Even if in those days which marked the close of the Middle Ages there had suddenly been invented and built a whole fleet of turbine steamships with capable crews, yet still without the instrument of finding latitude they could have had only vague ideas as to their position and would only have been able to produce unsatisfactory charts. Indeed, as a modern writer has remarked, it was this improved sea astrolabe which “removed the last doubt in Columbus’s mind as to the possibility of carrying out his plans of discovery.” Thus it came about that the man who could work an astrolabe was a person of some importance. He was held in high honour by the crew, since he alone was able to state the ship’s position and her course thence to her nearest port. Naturally, therefore, those Arabian pilots and Oriental astronomers who had been brought to the Iberian peninsula would go swaggering along the streets of Lisbon wearing these sea-rings conspicuously both as their badge of office and as indicative of their dignity. It was Behaim’s astrolabe which was used by Columbus, by Vasco da Gama, by Diaz, and others in their stupendous voyages: and still more valuable was it with the addition of the tables of the sun’s declination, first reduced by Behaim also. Nevertheless, we must not omit to bear in mind that as far back as the eighth century Messahala, a learned Rabbi, had already written a treatise on the astrolabe, and that even earlier still--in the sixth century B.C.--the astrolabe for use on shore had been invented by Hipparchus. But had the achievements of the ancients much influence, do you ask, on the cosmographers and astronomers of the fifteenth and sixteenth centuries? The answer is most certainly in the affirmative; and the greatest experts of this period had a very complete knowledge of the work of their predecessors.