Page:The story of geographical discovery.djvu/149

 Rh Ptolemy existed, and, if so, what were its dimensions. It happened that all these problems of coast-line geography, if we may so call it, were destined to be solved by one man, an Englishman named James Cook, who, with Prince Henry, Magellan, and Tasman, may be said to have determined the limits of the habitable land.

His voyages were made in the interests, not of trade or conquest, but of scientific curiosity; and they were, appropriately enough, begun in the interests of quite a different science than that of geography. The English astronomer Halley had left as a sort of legacy the task of examining the transit of Venus, which he predicted for the year 1769, pointing out its paramount importance for determining the distance of the sun from the earth. This transit could only be observed in the southern hemisphere, and it was in order to observe it that Cook made his first voyage of exploration.

There was a double suitability in the motive of Cook's first voyage. The work of his life could only have been carried out owing to the improvement in nautical instruments which had been made during the early part of the eighteenth century. Hadley had invented the sextant, by which the sun's elevation could be taken with much more ease and accuracy than with the old cross-staff, the very rough gnomon which the earlier navigators had to use. Still more important for scientific geography was the improvement that had taken place in accurate chronometry. To find the latitude of a place is not so difficult—the length of the day at different times of the year will by itself be almost enough to determine this, as we have seen in the very earliest history of Greek geography—but to determine the