The observation made at Cayenne, that a pendulum which beat seconds there, must be shorter than one which beat seconds at Paris, was explained by Huygens, to arise from the diminution of gravity at the equator, and from this fact he inferred the spheroidal form of the earth.  The application of the pendulum to clocks, one of the most beautiful and useful acquisitions which astronomy, and consequently navigation and geography have made, was owing to the ingenuity of Huygens.  These are the principal discoveries and inventions, relating to astronomy, which were made prior to the eighteenth century, so far as they are connected with the advancement of the art of navigation and the science of geography.

The discoveries of Columbus and Gama necessarily overturned the systems of Ptolemy, Strabo, and the other geographers of antiquity.  The opinion that the earth was a globe, which had been conjectured or inferred prior to the voyage of Magellan, was placed beyond a doubt by that voyage.  The heavenly bodies were subjected to the calculations of man by the labours of Copernicus, Tycho Brahe, and Galileo.  Under these circumstances it was necessary, and it was easy, to make great improvements in the construction of maps, in laying down the real form of the earth, and the relative situations of the countries of which it is formed, together with their latitudes and longitudes.  The first maps which displayed the new world were those of the brothers Appian, and of Ribeiro:  soon afterwards a more complete and accurate one was published by Gemma Frisius.  Among the geographers of the sixteenth century, who are most distinguished for their science, may be reckoned Sebastian Munster; for though, as we have already mentioned, he joins Greenland to the north of Lapland in his map, yet his research, labour, and accuracy were such, that he is compared by his contemporaries to Strabo.  Ortelius directed his studies and his learning to the elucidation of ancient geography; and according to Malte Bran, no incompetent judge, he may yet be consulted on this subject with advantage.

But modern geography may most probably be dated from the time of Mercator:  he published an edition of Ptolemy, in which he pointed out the imperfection of the system of the ancients.  The great object at this time, was to contrive such a chart in plano, with short lines, that all places might be truly laid down according to their respective longitudes and latitudes.  A method of this kind had been obscurely pointed at by Ptolemy; but the first map on this plan was made by Mercator, about the year 1550.  The principles, however, on which it was constructed, were not demonstrated till the year 1559, when Wright, an Englishman, pointed them out, as well as a ready and easy way of making such a map.  This was a great help to navigators; since by enlarging, the meridian line, as Wright suggested and explained, so that all the degrees of longitude might be proportional to those of latitude, a chart on Mercator’s projection shews the course and distance from place to place, in all cases of sailing; and is therefore in several respects more convenient to navigators than the globe itself.  Mercator, in his maps and charts, chose Corvo, one of the Azores, for his first meridian, because at that time it was the line of no variation of the compass.