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..