3 



ti^e changes of the horizontal intensity, taken negatively. Hence the 

 force of the current in any given direction may be determined in terms 

 of the same units. 

 ISTow 



6 = a - 



in which a is the azimuth of the line connecting the two stations, mea- 

 sured from the true meridian eastward, and the magnetic declination 

 measured in the same direction. The observations of Sir James Ross, 

 at Derby, give = - 22° 25'; and we have for the line connecting 

 Derby with Rugby, 



a = - 13° 7', a - -f = + 9°18'; 

 and for the line joining Derby and Birmingham, 



a = + 33° 27', a - = + 55° 52'. 



The first column of the following Table contains the mean variation 

 of the magnetic declination at the alternate hours, for the month of 

 May, as deduced from four years' observation of that element at the 

 Dublin Magnetic Observatory. The second contains the corresponding 

 values of the changes of the horizontal intensity, in ten-thousandths of 

 the whole intensity ; and the third and fourth the calculated values of 

 the deflecting forces, in the line perpendicular to that connecting the 

 earth contacts at Derby and Rugby, and at Derby and Birmingham, re- 

 spectively, and expressed in terms of the same units. These latter 

 numbers are, by hypothesis, proportional to the intensities of the cur- 

 rents directed along the connecting wires. 



Table I. — Calculated Values of the Intensity of the Currents, traversing 

 the Wires uniting Derby and Rugly, and Derby and Birmingham^ 

 respectively. 



Hour. 



All/' 



AX 



Derby and 



Derby and 



X 



Rugby. 



Birmingham. 



1 A.M. 



l'-8 



0 4 



5-1 



2-6 



3 



2-5 



- 1-6 



7-6 



5-5 



5 



3-9 



- 3-7 



11 -9 



9-5 



7 



5-2 



- 8-4 



16-2 



15-4 



9 



2-1 



- 16-9 



8-9 



17-5 



11 



- 4-1 



- 15 9 



- 9-3 



6-4 



1 P.M. 



- 7-1 



- 3-1 



- 19-8 



- 9-0 



3 



- 6-1 



6-1 



- 15-7 



- 13-4 



5 



- 1-8 



14-2 



- 7-6 



- 14-8 



7 



0-3 



14-6 



- 1-5 



- 11 -6 



9 



1-0 



9-0 



1-3 



- 5-9 



11 



1-3 



5-2 



2-9 



- 2-2 



The galvanometric observations instituted by Mr. Barlow on these 

 two lines were continued for fourteen consecutive days, commencing 



