In The Reign Of Queen Anne The Celebrated
Act Was Passed, Appropriating Certain Sums For Encouraging Attempts To
Ascertain The Longitude.
Stimulated by this, Mr. Harrison invented his
time-keeper, which on trial was found to answer the purpose with such
tolerable accuracy, that he was deemed worthy to receive the sum awarded by
parliament:
It went within the limit of an error of thirty miles of
longitude, or two minutes of time, in a voyage to the West Indies. Since
this period, chronometers have been much improved, and excellent ones are
very generally used: perhaps the most trying circumstances in which any
were ever placed, existed during the voyage for the discovery of a
northwest passage by Captain Parry; and then most of those he had with him
were found to be extremely accurate.
It is evident, however, that chronometers are liable to a variety of
accidents, and that in very long voyages the means of verifying their rate
of going seldom occur. Hence the lunar method, or the method of
ascertaining the longitude by means of the motions of the moon, is more
useful and valuable. Here again, the profoundest researches of Clairaut,
Euler, D'Alembert, and La Place, were brought practically to bear on
navigation. Guided and aided by these, Tobias Mayer, of Gottingen, compiled
a set of solar and lunar tables, which were sent to the lords of the
admiralty, in the year 1755; they gave the longitude of the moon within
thirty seconds. They were afterwards improved by Dr. Maskelyne and Mr.
Mason, and still more lately by Burg and Burckhardt; the error of these
last tables will seldom exceed fifteen seconds, or seven miles and a half.
The computations, however, necessary in making use of these tables, were
found to be very laborious and inconvenient; to obviate this difficulty,
the nautical almanack, suggested by Dr. Maskelyne, was published, which is
now annually continued.
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