278 ON EARTH-CURRENTS, AND THE DIURNAL 



and for the line connecting Derby with Bugby, 

 = - 13 7', = + 9 18'. 

 Introducing these values in the expression for /, 



(85T\ 

 000287^-0-16-^-). 

 A / 



The following Table gives the values of the quantity within 

 the brackets in this formula, for the days on which the force of 

 the currents was observed, and for the even hours of Gottingeu 



$X 

 mean time, the unit being 10 ^ 00 . The values of <ty, and -=- are 



obtained from the Greenwich observations.* 



* In my first calculation of the Intensity of the Currents, communicated to the- 

 Academy on the llth November, I employed the mean values of Sif and for the 



month of May, as deduced from observations made at the Dublin Magnetical Obser- 

 vatory in the years 1840-1843. My reason for this was, that in the year 1848, which 

 was stated in Mr. Barlow's Paper to have been the year of his galvanometric observa- 

 tions, the daily observations at the Dublin Observatory were not sufficiently numerous 

 for the required comparison ; while the Greenwich published observations were then 

 limited to term-days, and days of unusual disturbance. But I soon after learned from 

 Mr. Barlow himself, that his observations were made in 1847, and that the date in the 

 heading of his Tables had been misprinted by oversight. And as the Greenwich 

 observations were taken twelve times in the day in 1847, 1 have recalculated the forces 

 from the simultaneous elements thus furnished. The comparison is, of course, in 

 every way more satisfactory than the former. 



