National Library of Canada
Sir Alexander Mackenzie, Voyages From Montreal to the Frozen and Pacific Oceans, in the Years 1789 and 1793. London: R. Noble, 1801
Jefferson needed no prompting from anyone's journal on the need for his explorers to take good measurements of latitude and longitude across the continent, but he must been impressed by Mackenzie's diligence in the task. The Scot recorded 34 latitudes on his round trip from Fort Chipewyan to the Pacific, obtained by measuring the sun's noon altitude with his sextant. Special perseverance was required when Indians flocked around to watch the white wizard perform this mysterious rite; on one occasion some coastal residents anxiously told Mackenzie to put his sextant away, lest it scare away the salmon on which their lives depended.
The latitude readings helped Mackenzie realize how far south he drifted while aiming west. The graffiti rock on the Dean Channel stands more than 6 degrees of latitude southward of his Fort Chipew-yan starting point, about equal to the north-south distance between Chicago and Nashville.
With similar diligence on their longer trip Lewis and Clark computed just over 80 latitudes in the field. The Americans stumbled, however, by shunning Mackenzie's method of getting relatively accurate longitudes. On his London visit in early 1792 Mackenzie bought a telescope big enough to pick out the four brightest moons of the planet Jupiter. He also got a British Nautical Almanac predicting the Greenwich times these moons would disappear behind the planet. By comparing his local time of seeing these eclipses with the Almanac times, the explorer could figure his longitude west of Greenwich at the rate of 15 degrees for each hour of time difference.
Mackenzie had to lug the telescope on his own back during the final march over the mountains from the Fraser River to the saltwater inlet at the modern village of Bella Coola, British Columbia. When he set up the instrument in the fjord near his graffiti rock, Mackenzie got a rare cloudless shot at Jupiter and a fairly good longitude of 128°02' W. That was by far the most important of the six longitudes obtained from Jupiter during his round trip.