Theory of Magnetic Disturbance by Earth- Currents. 441 



column the calculated, and the third the observed values of 

 the force, the unit being as usual 10~ 5 Gauss. 



Table III. 



y- 



F calculated. 



F observed. 



•38 

 •64 

 •79 

 •92 



301 

 7-48 



12-8 



7-3 



5-8 



49 



•99 



•21 



23-17 



10-6 



5-6 



43 



•95 



•24 



1-54 



1-5 



1-04 



It is clear that at the two shorter distances the formula 

 fails to represent the facts. 



In the case of the next four stations, that is for a range 

 from # 79 to 7*48 kilometres, or from half a mile up to five miles, 

 the agreement is practically exact. 



In the case of the experiment recorded in the last line 

 the river Havel, which from a small map printed with the 

 paper is apparently nearly half a kilometre in width, and 

 another smaller stream lie between the observing-station and 

 the line. This may possibly account for the discrepancy 

 between theory and experiment. 



It is easy from Professor Riicker's theory to calculate the 

 horizontal resultant disturbance. Taking, as in fig. 1, A 

 and B to be the source and sink and the observatory, 

 we have to find the resultant of two forces Le/OB and 

 I/c/OA acting at right angles to OB and OA respectively, in 

 such a way that if the one tends to increase the angle MBO, 

 the other tends to decrease the angle MAO. 



This resultant is easily shown to be equal to 



I . k .a 



To apply this I have assumed the whole leak to take place 

 at the ends of the line, which is clearly quite an extreme case y 

 and have thus obtained the following table. 



Phil May. S. 6. Yol. 1. No. 4. April 1901 



2G 



