In the fifteenth century there was an easterly declination. This gradually diminished, so that in about the year 1657 the needle pointed due north. After this the needle pointed towards the west, and continually more and more, so that scientific men, having had experience only of a continual shifting of the needle in one direction, began to form the opinion that this change would continue, so that the needle would pass, through north-west and west, to the south. In fact, it was imagined that the motion of the needle would resemble that of the hands of a watch, only in a reversed direction. But before long observant men detected a gradual diminution in the needle’s westerly motion. Arago, the distinguished French astronomer and physicist, was the first (I believe) to point out that ‘the progressive movement of the magnetic needle towards the west appeared to have become continually slower of late years’ (he wrote in 1814), ‘which seemed to indicate that after some little time longer it might become retrograde.’ Three years later, namely, on the 10th of February, 1817, Arago asserted definitively that the retrograde movement of the magnetic needle had commenced to be perceptible. Colonel Beaufoy at first oppugned Arago’s conclusion, for he found from observations made in London, during the years 1817-1819, that the westerly motion still continued. But he had omitted to take notice of the circumstance, that London and Paris are two different places. A few years later the retrograde motion became perceptible at London also, and it has now been established by the observations of forty years. It appears, from a careful comparison of Beaufoy’s observations, that the needle reached the limit of its western digression (at Greenwich) in March 1819, at which time the declination was very nearly 25°. In Paris, on the contrary, the needle had reached its greatest western digression (about 22½°) in 1814. It is rather singular that although at Paris the retrograde motion thus presented itself five years earlier than in London, the needle pointed due north at Paris six years later than in London, viz., in 1663. Perhaps the greater amplitude of the needle’s London digression may explain this peculiarity.