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204 Lattitude & [and] Longitude
and that every change in her
course is taken account of, as
well as every change in her rate
of advance. This is, strictly speaking
what is called "Navigation" in
which the captain of a ship in
reality relies on terrestrial measurements
to decide his position. If all this
could be accurately managed, the
position of the ship at any hour
could be known; but unfortunately
such accuracy is impossable, from
the fact that a ships compass is
not an instrument affording
perfectly Exact indications. It is
plainly impossable Even for the
most skillful steersman to keep
a ship rightly to one course for
any length of time on such a
shifting pathway, and to this
may be added the difficulty
arising from what is Known as
the "variation of the compass"
in differant parts of the Earth
as well as its liability to be
affected by the presence of iron
in the ships cargo. Again the
log line gives only a rough idea
of the ships rate of progress, as
of course a ships rate does not
remain constant, Even when she
is under steam alone. Currents
too carry the ship along sometimes
with considerable rapidity, and
the logline affords no indication
of their action. Thus the distance
made on any course may differ
considerably from the Estimated
distance, and when several
days sailing are dealt with,
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Latitude & [and] Longitude 205
an error of large amount may readily
accumulate.
For these and other
reasons, a ships captain places little
reliance on what is called the
days work, that is, the change
in the ships position from noon
to noon, as Estimated from the
compass courses Entered in the
log book and the distances supposed
to be run on these courses. It is
absolutely Essential that such
Estimates should be carefully
made, because under unfavorable
conditions of weather there may
be no other means of guessing
at the ships position, but the only
really reliable way of determining
a ships place is by astronomical
observation. It is on this account
that the almanacs published by the
Admiralty in England, and by the
observatory authorities at Washington,
in which the position and apparent
motions of the celestial bodies are
indicated four or five years in
advance, are called "par Excellance"
the Nautical Almanacs. By thier
aid, the seaman, in the processes
now to be considered, trusts to
celestial observation, independently
of all terrestrial measurements.
The points to be
determined by the voyager are his
latitude and longitude. The
latitude is the distance north or
south of the Equator, and is
measured always from the Equator
in degrees, the distance from
Equator to pole, being divided
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