ially to the aid of navigation: we have already
seen how, even in the rudest state of the latter, it derived its chief
assistance from this sublime science, confined as it then was to a
knowledge of the position of a few stars. Astronomy enables the navigator
to ascertain his latitude and longitude, and to find the variation of the
compass. The principal difficulty in ascertaining the latitude at sea,
arose from the unsteady motion of the ship: to remedy this, several
instruments were invented. We have already alluded to the astrolobe; but
this, as well as the others, were imperfect and objectionable, till such
time as Hadley's quadrant was invented, the principle and uses of which
were first suggested by Newton.

To ascertain the longitude was a much more difficult task: there are
evidently two methods of doing this,--by time-keepers or chronometers, and
by making the motions of the celestial bodies serve instead of
time-keepers. About the middle of the seventeenth century, Huygens proposed
the pendulum clock for finding the longitude at sea; but it was unfit for
the purpose, for many and obvious reasons. Watches, even made with the
utmost care, were found to be too irregular in their rate of going, to be
depended upon for this purpose. 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 practical