""D. Joao de Castro carried out a series of experiments that succeeded in detecting phenomena, in particular related to magnetism and the magnetic needle on board. It should be assumed that such knowledge to Pedro Nunes, of course the direct inspiration of all the observations he has done in his travels. When on August 5, 1538, D. João de Castro decided to determine the latitude of Mozambique, found the cause that dictated the astonishing uneasiness of needles; noted the deviation of the needle, discovering it 128 years before Dennis Guillaume (1666) of Nieppe, which is recorded in History of Sailing as if he were the first to know about this phenomenon. His point near Baçaim, on December 22, 1538, a magnetic phenomenon, for which there were variations of the needle because of the proximity of certain rocks, confirmed four centuries later, was called local attraction. D. João de Castro refuted the theory that the variation of magnetic declination is not formed by geographic meridians. His comments are the most important record of values of magnetic declination in the Atlantic and Indian oceans, in the sixteenth century, and useful for the study of terrestrial magnetism. It was one of the personalities of this century European experimental science, linking the importance of this study with the sailing.""