A lecture delivered to the Institute of Navigation on 22 October 1959 at the Royal Geographical Society in London, with H. R. H. The Duke of Edinburgh in the Chair. The lecture was the first of the Duke of Edinburgh"s Lectures, a series sponsored by the Institute to extend the horizons of navigational interest. The paper was originally published in The Journal of the Institute of Navigation, Vol. XIII, Nº. 1. This version of it is presented to the International Congress of the History of the Discoveries being held in Lisbon in September 1960 to commemorate the fifth centenary of the death of Prince Henry of Portugal. At the Royal Geographical Society. The Institute of Navigation. London.

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