wireless this could best be done by an observation for local time at some
place the longitude of which was known. Failing such a place of
known longitude the error of the chronometer on Greenwich time had
to be obtained by means of one of the “‘absolute’’ methods of obtaining
longitude. These methods, generally speaking, may be divided into
two classes, the simpler of which consists of simply noting the time of
certain celestial phenomena, the Greenwich time of which has already
been calculated, such as, for example, the eclipse of one of Jupiter’s
satellites. Methods of the other class include the various kinds of
observation on the moon from which the Greenwich time can be
computed by a variety of methods, all more or less laborious. Of the
latter class the English navigators almost invariably used the method
of “lunar distances,” while the Spanish occasionally observed the
occultation of a star by the moon or even a lunar eclipse.

Embarrassed as he was by prospects of attack by hostile Indians,
Mackenzie was fortunate in being able to use the former method and
succeeded in observing eclipses of two of Jupiter’s satellites. This
observation, however, while the simplest of the absolute methods and
one in common use even for scientific purposes a few years before
Mackenzie’s time, cannot be depended upon to give very accurate
results, as the eclipses are not instantaneous and the exact time of
the moments of immersion and emersion, or disappearance and reappear-
ance, of the satellites is difficult to determine. Raper’s “Navigation”
states that the observation is only to be considered complete when both
immersion and emersion of the same satellite are observed on the same
evening under the same circumstances. Mackenzie had a telescope
which was very probably not up to the standard deemed necessary by
Raper, who specified a magnifying power of at least 40 diameters for
satisfactory work.”5

In comparing the results of Mackenzie’s necessarily crude method
with those obtained by skilled observers carrying the best equipment
that England could produce, we find that Vancouver's determination
of the longitude of Nootka differed from that of Cook by 20 minutes of
are.

If another case be examined in detail it is found that Vancouver,
in fixing the longitude of Monterey, used 199 sets of lunar distances,
of six observations each. The 199 sets are divided into 21 groups of
sets and the average error of each group, compared with the mean
result, is 12!/)’. Of these 21 groups of sets, one is 28’ 40” in error and
another 27’ 11’, so that certain individual sets are probably well over
30 minutes, or 21 miles, in error.

As the lunar distance was considered to be a more accurate method
than the eclipse of a satellite, even when the latter was observed under
the most favourable conditions; and as Mackenzie’s determination,
under difficult circumstances, and by the less accurate method, was
incomplete, and possibly obtained by means of a telescope of insufficient
power, it may be considered that the error of approximately 40 minutes
of arc was not excessive.

25 In a note in “‘Les Bourgeois de la Compagnie du Nord-Ouest,” by L. R. Masson,
Quebec, 1889, vol. I, p. 40, it is stated that “Sir A. Mackenzie’s sextant and chrono-
meter are still preserved in the family at Terrebonne.” And in a letter on the same
page written by Mackenzie from the Forks Peace River 10 January 1793, he asks his
cousin Roderick to ‘send me the sextant with all the quicksilver you have, as I have
lost all mine.” It is presumed that the quicksilver was required for the artificial
horizon.

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