But, whoever was the real inventor of this admirable instrument, it is certain that, in little rnore than half a century after it was applied to nautical purposes in Eu rope, navigation assumed a bolder and more adventurous character. Human curiosity, which had been long re pressed by scholastic dogmas, and superstitious prohi bitions, began, about the same period, to shake off the fetters which had been imposed by false philosophy and mistaken views of religion, and to search, in the free ex ercise of the mental powers, for objects of more rational pursuit. The spirit of enterprise and discovely kept pace with the means which the progress of human knowledge gradually unfolded, to aid and direct its ex ertions; and the facility of mutual communication which navigation began to open up between remote countries was equally conducive to the benefit of individuals, and the general advantage of nations. From this era we may date the conuneneement of that kind of intercourse between the inhabitants of different countries, which pro perly deserves the appellation of commerce.
The Portuguese were among the first to signalize themselves in the career of geographical discovery which was now opened up by means of the marinet's compass. Under John I. a prince devoted to philosophical put.
suits, and well skilled in the mathematical sciences, a small Ileet was fitted out in 1412, to explore the west ern coast of Africa, and bring back whatever informa tion could be obtained respecting the inhabitants and productions of these unknown regions. 'Die expedi tion was sufficiently successful to keep alive the spirit of adventure by which it had been planned and conduct ed, and even to encourar.:,e the hope, that discoveries of still greater importance might be the result of a more extended system of maritime enterprise. Henry, Duke of Vise°, the fourth son of King John, who, like his fa ther, hacl been instructed in all the sciences of that age, and was particularly versant in geography, was accord ingly entrusted with the sole management of these new and cl.tring undertakings ; and certainly no individual could have been selecttd, who was better qualified to superintend their execution. Guided by his directions, the Portuguese ranged along the coast of Africa till they approached within a few degrees of the equator, and discovered, at a little distance from the shores of that vast continent, the island of 'Modena, the Cope Veld Isles, the Azores, and several other islands of inferior note.
It is unnecessary, in this sketch of the history of ma thematical navigatioo, to give a detailed view of the va rious discoverie...which Wele made under the direction of this enlightened prince ; it is more itnmediately con nected with our subject to state, that he seems to have been the first who sugges•ed to the mariner the use of plane charts to delineate his course and distance from one place to another. His endeavours to improve the state of nautical science were ably seconded by Joseph and Roderic, two mathematicians of some Tminence at that period, who were employed as physicians at the court of Lisbon. With their assistance, and that of Mar tin de Bohemia, a Portuguese by birth, who had studied under the celebrated Regiomantanus, lie caused tables to be constructed of the sun's declination, and furnished navigators with instruments for deducing their geogra phical position by celestial observation.
The mariner's compass had not been long employed as a guide at sea, before it WaS discovered that the mag netic needle did not coincide exactly with the direction of the true meridian, and that its deviation from that line was even different at different places. This deviation, which is commonly called the variation of the compass, seems to have been first observed by the celebrated Co lumbus, on the 14th of September 1492, during his first voyage to America, though the discovery is also ascrib to Sebastian Cabot, a distinguished coternporary naviga tor. Subsequent observations, made with better instru ments, and under more favourable circutnstances, have not only established this important fact, but demonstrat ed that the quantity of thc variation is subject to regu lar periodical changes, the north indicated by the mag net being sometimes to the eastward, and sometimes to the westward of thc true north. It hos even been ascer tained that a local variation, of sufficient magnitude to lead to considerable errors in a ship's reckoning, may frequently be occasioned by the attractive influence of the ferruginous particles diffused through the mass of materials of which the hull is composed ; though the subject has not yet been investigated with sufficient at tention to explain the anomalous appearances to which this source of variation is supposed to give rise. See Bain On the Variation.
The art of navigation, notwithstanding the great ad vantages which it had derived from the mariner's com pass, was still in a very imperfect state. The use of the plane chart, first introduced by Prince Henry of Portugal, contributed, without doubt, to abridge the la bour of the mariner in his attempts to delineate a repre sentation of the ship's tract and position, but the errors which it introduced into his reckoning, especially in high latitudes, more than counterbalanced the simplicity of the principles on which it proceeded. The methods too, which are recommended, and attempted to be re duced to practice, for determining the ship's place by celestial observation, though founded on just views, were rendered in a great measure useless, by the rude and inadequate nature of the instruments which were employed to obtain the necessary data ; while the astro nomical tables, which were requisite in the calculation of the ultiniate result, had not yet been drawn up with sufficient accuracy for that purpose. Thus, though it was suggested by \Verner, so early as in the middle of the 16th century, that the longitude might be ascertain ed by observations of the moon, the lunar tables were much too limited and imperfect to enable the mariner to derive any benefit from the proposal ; and watches, which had been recommended by Gemma Frisius about the same time, and for the same purpose, having been but recently invented, could not be used with any ad vantage for the solution of a problem, in which a high degree of accuracy is essential, for obtaining even an approximation to the truth.