Reference has been made to Blaeu’s ability as a maker of mathematical and astronomical instruments, which work claimed more or less of his attention to the end of his days. The fact, however, is a very remarkable one that from the many years of activity as an instrument maker with Tycho Brahe, and independently at Amsterdam, but one example of his should have come down to us. Vossius says that “no other scholar ever deserved so much praise as Blaeu on account of an extraordinary and beautiful quadrant, which can be seen at Leyden in the tower, where astronomical observations are being made.”[11] His reference is to a quadrant now kept in the Leyden Observatory. It is described by Kayser in his Annals of the Observatory as a wooden quadrant, with a brass rim, having a radius of seven feet.[12] In the year 1632 it was purchased from the estate of Willebrod Snellius, and may be said to have induced the founding of this observatory in the year 1633. With the conscientious map makers the problem was ever present, how shall accuracy in space relations, accuracy in location of places on the map be attained. Ptolemy had been a guide in the earlier years of great geographical discoveries and explorations, but geographical knowledge soon extended to regions beyond those known to Ptolemy, and the inaccuracies of his records, even for those regions more or less familiar to the ancients, became very apparent. In the maps of the seventeenth century we have interestingly exhibited the tortuous advance of geographical knowledge. They do not show a steady and continuous progress toward accuracy. Information which may have been accepted as truth respecting certain regions or geographical areas, and have been recorded as such by one or by many of the map makers, would often be held as doubtful by those of succeeding years, to be accepted and rejected again in turn. Blaeu’s intimate acquaintance with mathematics and astronomy as related to geography, particularly as related to cartography, admirably fitted him for a certain leadership in this field. We have from Vossius the information that Blaeu undertook the measurement of a degree on the surface of the earth to the end that he might aid the map maker in improving his work and serve especially the cause of navigation. In his attempts to find a new and better method for terrestrial measurements, it seems probable that he anticipated the work of the great mathematician Snellius, whose results were first published in the year 1617, and to whom credit is generally given for having employed a new method of procedure based upon triangulation.[13] Blaeu’s measurements, made along the coast of the Netherlands from the mouth of the Meuse to the Texel, were never published, probably for the reason, as suggested by Vossius, that he may have distrusted their accuracy. The mathematician Picard, in his Voyage d’Uranibourg,[14] writes in the month of July, 1671, a