Here At London, For Instance, In The Year 1580, The Variation Was
Observed To Be 11 Degrees 17 Minutes To The East; In The Year 1666,
The Variation Was Here 34 Minutes To The West; And In The Year 1734,
The Variation Was Somewhat More Than 1 Degree West.
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.
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