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 nearly so mucK." 

 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 : z. 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. 



