I left this island on the 8th of October, and continued my course to the south to the latitude of 40 degrees or 41 degrees, having a strong north-west wind; and finding the needle vary 23, 24, and 25 degrees to the 22nd of October, I sailed from that time to the 29th to the east, inclining a little to the south, till I arrived in the latitude of 45 degrees 47 minutes south, and in the longitude of 89 degrees 44 minutes; and then observed the variation of the needle to be 26 degrees 45 minutes towards the west.
As our author was extremely careful in this particular, and observed the variation of the needle with the utmost diligence, it may not be amiss to take this opportunity of explaining this point, so that the importance of his remarks may sufficiently appear. The needle points exactly north only in a few places, and perhaps not constantly in them; but in most it declines a little to the east, or to the west, whence arises eastern and western declination: when this was first observed, it was attributed to certain excavations or hollows in the earth, to veins of lead, stone, and other such-like causes. But when it was found by repeated experiments that this variation varied, it appeared plainly that none of those causes could take place; since if they had, the variation in the same place must always have been the same, whereas the fact is otherwise.
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.