We do not intend, by any means, to enter at any length, or
systematically, on these subjects; but a brief and popular notice of them
seems proper and necessary in such a work as this.
Astronomy here comes essentially 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.
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