The value of the Chronometer for finding the Longitude at Sea, being, by the experience of many years trial, fully established, I am induced to offer to the Royal Society some remarks on the change of rate observed in this instrument, when on ship-board. This change of rate, that had usually been supposed to arise from the motion of the ship, has recently heen attributed, by Mr Fisher, who accompanied Captain Buchan in his Voyage towards the North Pole in the year 1818, ‘to the magnetic action exerted by the iron in the ship upon ‘the inner rim of the Chronometer's balance, which is composed of steel.’ I apprehend, however, that it will be very easy to show, that although the alteration of rate may be, and most probably is, owing to magnetism, yet the magnetic action of the iron in the ship, excepting in cases where chronometers are placed in immediate contact with large masses of iron, can contribute but in a very small degree to the error in question.