In the case
of longitude there is no fixed spot to which reference can be
made. The rotation of the earth makes the existence of such a
spot impossible. The question of longitude is purely a question
of TIME. The circuit of the globe, east and west, is simply
represented by twenty-four hours. Each place has its own time.
It is very easy to determine the local time at any spot by
observations made at that spot. But, as time is always changing,
the knowledge of the local time gives no idea of the actual
position; and still less of a moving object--say, of a ship at
sea. But if, in any locality, we know the local time, and also
the local time of some other locality at that moment--say, of the
Observatory at Greenwich we can, by comparing the two local
times, determine the difference of local times, or, what is the
same thing, the difference of longitude between the two places.
It was necessary therefore for the navigator to be in possession
of a first-rate watch or chronometer, to enable him to determine
accurately the position of his ship at sea, as respected the
longitude.
Before the middle of the eighteenth century good watches were
comparatively unknown. The navigator mainly relied, for his
approximate longitude, upon his Dead Reckoning, without any
observation of the heavenly bodies. He depended upon the
accuracy of the course which he had steered by the compass, and
the mensuration of the ship's velocity by an instrument called
the Log, as well as by combining and rectifying all the
allowances for drift, lee-way, and so on, according to the trim
of the ship; but all of these were liable to much uncertainty,
especially when the sea was in a boisterous condition.
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