In Order To
Find The Variation Of The Needle With The Least Error Possible, The
Seamen Take This Method:
They observe the point the sun is in by
the compass, any time after its rising, and then take
The altitude
of the sun; and in the afternoon they observe when the sun comes to
the same altitude, and observe the point the sun is then in by the
compass; for the middle, between these two, is the true north or
south point of the compass; and the difference between that and the
north or south upon the card, which is pointed out by the needle, is
the variation of the compass, and shows how much the north and
south, given by the compass, deviates from the true north and south
points of the horizon. It appears clearly, from what has been said,
that in order to arrive at the certain knowledge of the variation,
and of the variation of that variation of the compass, it is
absolutely requisite to have from time to time distinct accounts of
the variation as it is observed in different places: whence the
importance of Captain Tasman's remarks, in this respect,
sufficiently appears. It is true that the learned and ingenious Dr.
Halley has given a very probable account of this matter; but as the
probability of that account arises only from its agreement with
observations, it follows those are as necessary and as important as
ever, in order to strengthen and confirm it.
CHAPTER III: REMARKS ON THE VARIATION OF THE NEEDLE.
On the 6th of November, I was in 49 degrees 4 minutes south
latitude, and in the longitude of 114 degrees 56 minutes; the
variation was at this time 26 degrees westward; and, as the weather
was foggy, with hard gales, and a rolling sea from the south-west
and from the south, I concluded from thence that it was not at all
probable there should be any land between those two points. On
November 15th I was in the latitude of 44 degrees 33 minutes south,
and in the longitude of 140 degrees 32 minutes. The variation was
then 18 degrees 30 minutes west, which variation decreased every
day, in such a manner, that, on the 21st of the same month, being in
the longitude of 158 degrees, I observed the variation to be no more
than 4 degrees. On the 22nd of that month, the needle was in
continual agitation, without resting in any of the eight points;
which led me to conjecture that we were near some mine of loadstone.
This may, at first sight, seem to contradict what has been before
laid down, as to the variation, and the causes of it: but, when
strictly considered, they will be found to agree very well; for when
it is asserted that veins of loadstone have nothing to do with the
variation of the compass, it is to be understood of the constant
variation of a few degrees to the east, or to the west:
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