Blaeu’s charts are drawn on a plane or cylindrical projection, although the projection of Mercator had been known for more than half a century. This fact seems to suggest the idea that in publishing his book he had not so much in view the question of the advance of the art of navigation as to meet a desire for a work in agreement scientifically with the knowledge of seamen of his day. In point of accuracy, Blaeu’s charts in this work are wanting much, but we must not forget the imperfect methods employed in his day for obtaining geographical location. If accuracy was wanting in the land maps, much less could we expect to find accuracy in the marine charts. Seamen in part were accustomed to estimate longitude and distance from change in latitude, and the direction in which they sailed. The speed of a ship was often estimated from the size and the number of sails used. It was not until about the middle of the seventeenth century that the log was brought into common use, consequently Blaeu makes no reference to it. It is interesting to note that a somewhat similar instrument was employed about the middle of the sixteenth century for measuring the angle which the axis of the ship made with its track as observed on the surface of the water. In his “Spieghel der Zeevaert” of 1584, Waghenaer states that for measuring this angle, “it is necessary to let the plumb line log behind with a piece of wood attached or otherwise.” A comparison of the maps of Blaeu with those of his distinguished predecessors, Lucas Jansz. Waghenaer and Willem Barentszoen, warrants our assigning to him first place, both in point of execution and in the nearer approach to correctness in his drawing. John Blaeu, in his Atlas of 1664, referring to the maps of Waghenaer and Barentszoen, says that “my late father not only greatly improved both of these, but also enlarged them for the benefit of navigation, adding to them so much that was lacking that his may justly be called a new work.”