Narrative of the surveying voyages of his majesty's ships Adventure and Beagle (vol.1): between the years 1826 and 1836 : describing their examination of the southern shores of South America, and the Beagles's circumnavigation of the globe

504' MAGNETIC INTENSITY. laid, are important when extreme accuracy is sought; as for example, in comparing the force at stations which have served as the unities of different observers : their effect isj however, of little moment in observations which include great differences of the terrestrial intensity. But when the magnetic condition of the needle has varied, and interpolation becomes necessary, the in- stances are rare in which it can be done with entire satisfaction. The time of vibration of this cylinder at Plymouth, with correc- tions applied for the chronometer's rate, the temperature, and the arc, was, in December 1831, 770,8 seconds, with a dip of 69° 27',6, and in October 1836, 777,3 seconds with a dip of 69° 17', 5. The difference in its magnetic state, at the two periods, is shown by the squares of the times of vibration multiplied by the cosine of the dip observed at the respective periods. This makes known what the time of vibration of the same cylinder would have been, had it been free to move in the direction of the dipping-needle, instead of being suspended horizontally ; and, consequently, if it had been acted upon by the total magnetic intensity, instead of by the horizontal component only. It is here assumed that the total terrestrial intensity is constant at the same place. This is doubtless not strictly true ; but the amount of the change must be too small to require consideration in the period occupied by Captain Fitz- Roy's observations. The horizontal component must, however, necessarily vary with the changes in the dip : and it is, therefore, from the time of vibration in the direction of the dipping-needle, and not from the time of horizontal vibration, that the change, or otherwise, in the magnetism of the needle is to be inferred. We find, then, the equivalent time of vibration of this cylinder in the direction of the dipping-needle to have been 456,4 seconds in December 1831, and 462,2 seconds in October 1836; showing a difference of 5,8 seconds in fifty-eight months. If we compare this amount with the far greater loss of intensity sustained by this cylinder in the preceding voyage, it seems a probable supposition that, at the commencement of Captain Fitz-Roy's voyage, the cylin- der had nearly attained its permanent magnetic state ; and that its further loss of magnetism, occasioning an increase of 5,8 seconds in the time of vibration^ took place in the early part of the voyage supposing the loss to have been progressive, and not sudden, as from accidents, of which the observations give no indications. Con-