VARIATIONS OF THE MAGNETIC NEEDLE. 



275 



5. For the foregoing reasons, the magnetometric observations 

 are here compared, not with the single corresponding deflections of 

 the galvanometer needles, but with the means of several. The 

 observations instituted by Mr. Barlow on the two lines of telegraph 

 were taken at intervals of five minutes, and were continued for 

 fourteen consecutive days, commencing May 17, 1847. In the 

 Tables given in his Paper, the individual results are grouped into 

 means corresponding to the beginning of each hour ; so that each 

 number of the Table is the result of twelve distinct readings. 

 The means so obtained are, notwithstanding, more affected by 

 irregularities than the single readings of the magnetometers cor- 

 responding to the same epochs ; and I have found it advisable to 

 group the results still further, by taking the mean of each pair of 

 succeeding hourly means. The following Tables, accordingly, 

 contain the means of the results belonging to the hours, 1 A.M. 

 and 2 A.M., 3 A.M. and 4 A.M., &c. ; they therefore correspond to 

 the epochs, P 30 m , 3 h 30 m , &c., each number being the mean 

 of twenty-four single readings. The positive numbers indicate 

 currents proceeding towards Derby ; and the negative, currents in 

 the contrary direction. It is only necessary to observe further, 

 that of the fourteen days of observation, four, viz., May 17, 23, 

 24, 30, are very incomplete, and are therefore omitted altogether 

 in this place : 



TABLE I. 



Intensity of the Currents traversing the Wire connecting Derby and Rugby. 



