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. 507 Whence the formula T'=T [1 X 0-000068 (60°— t)], in which T is the time of vibration at any station, t, the temperature of the cylinder in degrees of Fahrenheit, and T' is the equivalent time at a standard temperature of 60°. The thermometer was noted at the beginning and ending of every set of vibrations, and was always placed in the box with the cylinder. At all Captain Fitz-Roy's stations the apparatus was placed for observation on a stand, which raised it from two to three feet above the ground, thereby rendering the cylinder somewhat less liable to be disturbed by local influences: it was not furnished with a means of examining the strict horizontality of the cylinder, that improvement having been introduced into M. Hansteen's appa- ratus at a later period. On this point Captain Fitz-Roy remarks : ''A small leaden tripod was used as a stand, whose upper surface " was adjusted by a small spirit-level — or roughly by the trough " of an artificial horizon, filled with mercury. Upon the leaden " stand the box containing the needle was adjusted by its foot- " screws, so that the suspending fibre of silk hung centrally in " the wooden tube, the needle's centre being over that of the " graduated circle, and the needle itself near, but not touching, " the bottom of the box. The needle was not always strictly " parallel to the bottom of the box, nor strictly horizontal, be- " cause I would not move the brass stirrup in which it was sus- " pended, but its deviation from strict horizontality never exceeded " two degrees, and was seldom neai-ly so much." The time of completing every tenth vibration was recorded. The time of performing 300 vibrations is deduced from a mean generally of seven partial results : i. e. from the 0th to the 300th i 10th to the 310th ; and so on, to the 60th and 360th vibrations; the commencing vibration being always at an arc of 20°. In a very few instances the number of vibrations observed, after the com- mencing arc of 20°, was less than 360 ; in such cases the first vibra- tion after the arc had become 20° has still been taken as the com- mencing one, though previous ones may have been recorded; it being kept strictly in view, to obtain the relative time of vibration in arcs as nearly the same as possible, and not exceeding 20° as the initial. The arc was reduced to 10° generally about the 100th vibration ; and one thousandth of the time of vibration has been taken throughout the series as the correction to infinitely small arcs. &