Connexion with the Phenomena of Terrestrial Magnetism. 439 



other means) in a position perpendicular to the magnetic meri- 

 dian, and is measured, in terms of X, by the relative 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. 



Now e=-a—yjr } 



in which a is the azimuth of the line connecting the two stations 

 measured from the true meridian eastward, and -ty the magnetic 

 declination measured in the same direction. The observations 

 of Sir James Ross at Derby, give -ty = — 22° 25' ; and we have, for 

 the line connecting Derby with Rugby, 



a=-13°7', «-^=+9°18'; 

 and for the line joining Derby and Birmingham, 



«=+33°27', a-^=-f 55° 52'. 

 The first column of the following Table contains the mean 

 variations 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 hori- 

 zontal 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 respectively, 

 and expressed in terms of the same units. These latter numbers 

 are by hypothesis proportional to the intensities of the currents 

 directed along the connecting wires. 



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

 traversing the Wires uniting Derby and Rugby, and Derby 

 and Birmingham, respectively. 





A^. 



AX 



Derby and 



Derby and 





X' 



Rugby. 



Birmingham- 



1 A.M. 



1-8 



0-4 



5-1 



2-6 



3 



25 



- 16 



76 



5-5 



5 



3-9 



- 37 



11-9 



9-5 



7 



5 2 



- 8-4 



16-2 



15-4 



9 



21 



-16-9 



8-9 



175 



11 



-41 



-15-9 



- 9-3 



6-4 



1 P.M. 



-71 



- 31 



— 19 8 



- 90 



3 



-51 



61 



-157 



-13-4 



5 



-1-8 



14-2 



- 76 



-14-8 



7 



0-3 



14 6 



- 1-5 



— 11 6 



9 



10 



90 



13 



- 5-9 



11 



1-3 



5-2 



29 



- 22 



The galvanometric observations instituted by Mr. Barlow on 



