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 longitude was a much more difficult task, which in the earlier stages could only be formed by guesswork and dead reckonings.

But when clocks had been brought to such a pitch of accuracy that they would not lose but a few seconds or minutes during the whole voyage, they could be used to determine the difference of local time between any spot on the earth’s surface and that of the port from which the ship sailed, or from some fixed place where the clock could be timed.  The English government, seeing the importance of this, proposed the very large reward of L10,000 for the invention of a chronometer which would not lose more than a stated number of minutes during a year.  This prize was won by John Harrison, and from this time onward a sea-captain with a minimum of astronomical knowledge was enabled to know his longitude within a few minutes.  Hadley’s sextant and Harrison’s chronometer were the necessary implements to enable James Cook to do his work, which was thus, both in aim and method, in every way English.

James Cook was a practical sailor, who had shown considerable intelligence in sounding the St. Lawrence on Wolfe’s expedition, and had afterwards been appointed marine surveyor of Newfoundland.  When the Royal Society determined to send out an expedition to observe the transit of Venus, according to Halley’s prediction, they were deterred from entrusting the expedition to a scientific man by the example of Halley himself, who had failed to obtain obedience from sailors on being entrusted with the command.  Dalrymple, the chief hydrographer of the Admiralty, who had chief claims to the command, was also somewhat of a faddist, and Cook was selected almost as a dernier ressort.  The choice proved an excellent one.  He selected a coasting coaler named the Endeavour, of 360 tons, because her breadth of beam would enable her to carry more stores and to run near coasts.  Just before they started Captain Wallis returned from a voyage round the world upon which he had discovered or re-discovered Tahiti, and he recommended this as a suitable place for observing the transit.