148 EIGHTH REPORT — 1838. 



§ 2. By the Method of Vibrations. 



The observations by this method include twenty- seven sta* 

 tions ; i. e. 18 by Captain Ross ; 7 by Major Sabine ; and 2 by 

 Mr. Lloyd. 



1st. Captain Ross's determinations were made with a cylin- 

 der (X) vibrated in an apparatus on the well-known plan of -M. 

 Hansteen. The loss of magnetism sustained by the cylinder 

 during the time of its employment, from July 1837 to June 1838, 

 was very considerable, and was occasionally so irregular as 

 to prevent any satisfactory conclusion whatsoever being drawn 

 from the observations. On a careful examination, there ap- 

 peared two intervals, viz. from the middle of September to the 

 middle of November 1837, — and from April 24 to June 5, 1838, 

 — during which there was reason to infer that the loss of mag- 

 netism, though considerable, had been tolerably uniform and 

 regular. During the second interval, viz. from April 24 to 

 June 5, 1838, on both which days the cylinder was vibrated in 

 London, the increase in the time of vibration at the same sta- 

 tion affords a direct measure of the diminution in its magnetic 

 intensity ; and being divided by the number of days comprised 

 in the interval, furnishes the amount of the daily correction. 

 But in the first interval we have the additional disadvantages 

 of having no direct observation showing the amount of the loss 

 of magnetism, and no direct comparison with the force in 

 London : and it is necessary, consequently, to have recourse to 

 indirect means for the purpose of determining these particulars. 

 On the 19th of September, 1837, Captain Ross vibrated cylin- 

 der X at Birkenhead ; and on the 21st of September, at Dou- 

 glas, in the Isle of Man. In Table XLVII. we have the 

 value of the intensity at both these stations relatively to the 

 London unity, determined by Mr. Phillips ; and in Table 

 XLIV. we have Mr. Lloyd's determination of the force at 

 Birkenhead. We may employ these determinations to supply 

 the time of vibration in London corresponding to the observa- 

 tions with the cylinder at Douglas and Birkenhead. In like 

 manner we may accomplish a second indirect comparison with 

 London by means of Captain Ross's observations at Falmouth 

 on the 18th of November, 1837, combined with the values of 

 the intensity at that station determined by Mr. Fox, (Table 

 XLVIIL), and Major Sabine, (Table XLV.). The several 

 observations and processes by which the times of vibration 

 of the cylinder in London have been derived at different epochs, 

 are comprised in Table XLIX. ; and in its final column is 



