'T Derde deel van 't Licht der zee-vaert, inhoudende de beschryvinghe der zeecusten van de Middelandtsche...

A navigational diagram shows a ship’s course across a surface. It features three compass circles labeled with letters. Two lines represent potential paths: a straight line labeled as "straight east and west" and a curved path representing magnetic deviation. A decorative floral ornament sits below the first compass.

Let there be a ship in this second figure at the island of Corvo at A, which from there wishes to sail with a compass that points exactly north at that location, to a place on the coast of Spain situated at the same latitude as B. A C D B is then the line of straight east and west. Such a ship, sailing according to the east of its compass, will at the beginning run straight east along the line A C without noticeable difference, but the Lily of the compass (following the needle) will soon begin to deviate towards the east, as from G to H, and the east point southward, as from I to K. The compass deviating more and more, the ship (following it) is led not from A through C to D, but from A through E to F, and finally, being destined to arrive at B, arrives at land at L, far to the south of it, at a lower latitude. What more is to be said about this can easily be understood from what has been said above.

From this aforementioned, one easily understands that one cannot place the lands on the map of the Mediterranean Sea according to their true bearings and at the same time according to their latitudes, but that one of these two must be set aside. This being noted by the navigators, and those who have made the sea charts of those regions, they have left the latitudes aside and kept to the bearings, as the most necessary thing. And since the sky there is clear most of the time, and the lands are very high, and in many places not situated very far from each other, by which one, losing sight of one land, soon gets the other back in view, the use of latitudes is held in low regard by many there.

How one may use the latitude.

Nevertheless, one could often use the latitude there with significant advantage, but first, the true latitudes of the principal headlands, islands, and