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

MAGNETIC INTENSITY. 503 change which the cylinder had undergone in the preceding voyage, they persevered in diligently observing, and carefully recording, its time of vibration, at most of the principal ports which they visited in their voyage of five years' duration. Nor was it until their retui'n to the Cape Verd Islands, in September 1836, that they could infer, from observations repeated at the same spot as in their outward passage in 1832, that the cylinder had not varied in any thing like the degree that it had done in the prece- ding voyage, and that the care and pains they had bestowed were therefore likely to be recompensed by success. This appears a fitting opportunity to remark, how much the establishment in England of a depository for magnetic needles is needed ; whence officers, and persons desirous of making such observations, might be supplied with instruments, which had been kept a sufficient time to have attained their permanent magnetic state, and had been examined from time to time to prove that they had done so. The correction for temperature should be ascertained for each needle, and given with it; as well as the time of vibration (or whatever else constituted the measure of intensity, — as, for example, the angle of deflection in Mr. Lloyd's statical needles,) observed at the spot which should be selected as most suitable for a point of general comparison ; and the observations should be repeated at the same spot on the return of the needle. The want of such an establishment has long been greatly felt ; and opportunities, where nothing was wanting but proper instruments, have been lost in consequence, where determinations of great value might have been obtained, in parts of the world of the highest magnetic interest, and where such opportunities are of rare occurrence. The corrections necessary to render the times of vibration at the different stations strictly comparable with each other, are as follows. 1st, For the rate of the chronometer. 2d, For the temperature of the needle. 3d, For the arc of vibration. 4th, For any change in the magnetic condition of the cylinder. In extensive voyages, the last-named correction, or that for the change in the cylinder itself, is the one which requires principal consideration. The corrections for temperature, and for the arc, on the first of which particularly much stress has sometimes been