“Here,” says one of his early biographers, “he entertained himself with a new and strange kind of agriculture, for he cut down fruit-bearing trees and planted wild woods, perhaps to show that he was so disinterested that not even from the earth would he expect reward. Yet it is no wonder if one who disdained the rubies and diamonds of the East should think little of the products of Cintra’s rocks.” It was to the _matos_ of the Serra de Cintra that he longed to return in 1546. But he certainly did not despise the fruits of the soil, and probably occupied himself with grafting experiments. In the spring of 1538, as perhaps previously in the spring of 1537, he sailed to India as captain of a ship. The fleet arrived at Goa in September 1538 and went on to the relief of Diu. In March of the following year he returned to Goa, and two years later accompanied the new Governor, Dom Estevão da Gama, to the Red Sea. On all these occasions Castro kept a log or _roteiro_, from Lisbon to Goa, from Goa to Diu, and from Goa to the Red Sea. They display a strong scientific interest, a spirit thoroughly modern--nothing, however small it might be, was to him necessarily unimportant or negligible--or perhaps ancient, since he complains that in his day the scientific investigations of the ancients were no longer in vogue. The logs are written with that vivid directness which mark his letters, “written,” he said, “not for the ladies and gallants of the Court and royal palaces, but for the mariners of Leça and Mattosinhos.” His descriptions are precise and accurate, which does not prevent them from being often picturesque. He notices many birds, including one white and grey which, he says, the sailors call _frades_ (monks). “I pay great attention to eclipses of the moon,” he writes, as also to longitudes and latitudes, fishes, seaweeds, currents, winds, the colour of the Red Sea, and every detail that might concern the art of navigation, to the delight of his friends Dr. Pedro Nunez and Prince Luis, who had furnished him with special instruments and other assistance for his voyage. In the summer of 1542 he was back at Cintra, but in December of that year he was appointed to the command of the coast fleet, the main duties of which were to keep clear the coast of Portugal from pirates, such as Mondragon, who perpetually hovered in wait for the priceless spoils and cargoes of Portuguese ships homeward bound from India. He seems to have gone to sea before the end of the year and held this post for two years, with a brief interval in 1543 when he commanded the Portuguese fleet sent to co-operate with the Spanish against Barbarossa. They did not come to an engagement, and Dom João, after visiting Ceuta, returned to Portugal.