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

especially those places that one would like to locate by latitude, which are not yet known, would have to be gathered over time from various bearings and compiled into tables. One would also have to observe daily (or at least on the day one would want to use the latitude) and carefully measure how much the Lily the north marker of the compass deviated from the north at the place where one was. Knowing that, one would know at once how much the east point was north or south of the true east, and following which point of the compass the true east was indicated. From this, it would be evident that a place situated on the same latitude as the helmsman found himself would lie in such a direction from him as the true east or west point indicated. For example: A helmsman, being near Cyprus, wished to be at a certain place on the coast of Syria, which he knew to lie at the latitude of 35 degrees. He found himself at that same latitude, and that the Lily of his compass deviated one and a half points towards the west there; the half point between east-by-south and east-south-east would then indicate the true east to him. From this, it is clear that by sailing such a course, he would reach the desired place. Another: A ship being between Sapienza and Sicily. The helmsman found himself at the latitude of Cape Passero in Sicily, and that the Lily of his compass deviated exactly one point towards the west; the point of west-by-north would then indicate the true west to him. If he then sailed west-by-north according to such a compass (which is in fact true west), he would without doubt encounter Cape Passero. As for anyone who might think that while one sails to such a place, the course also changes due to the constant change of the needle's deviation, which one experiences mostly when sailing east or west, no significant error is to be expected on short voyages, because such change is not to be considered in the practice of navigation on short trips. On long voyages, one has new bearings daily, to which a sensible helmsman knows how to adjust himself.

How to find the deviation of the needle, or the change of the compass.

To find the deviation of the compass, that is, how much the Lily points to the east or west of the north, one first seeks the true north (in order to understand the deviation from it) as follows: Make a small box, very straight according to the square, four-sided, of reasonable depth, as follows here A B C D E F G, in which, in the middle under a glass, a compass floats on a pin, such that it comes equally close to all sides of the box, as S T. Divide the outside of the