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 146 tude was a much more difficult task, which in the earlier stages could only be performed by guesswork and dead reckoning.

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 £10,000 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