1885.] Fluctuations of Declination at Kew and Stonyhurst. 373 



that secondary currents must have an influence, perhaps a powerful one, in causing 

 disturbances. 



In order to fix the mind, let us here imagine that this secondary current influence 

 (exhibited probably in the shape of an earth current) is opposed in direction to the 

 true magnetic change. We should, therefore, expect something of the following 

 nature. 



ED or E'D' = magnetic change, first movement. 

 DF or DT' = magnetic change, second movement. 



DC oi-D'C' = — . 



In the first of these diagrams AB denotes a true magnetic descending change, 

 while ACB is the observed disturbance couplet. In the second A'B' denotes a 

 true magnetic ascending change, while A'C'B' is the observed disturbance couplet. 



In our various measurements, therefore, it is assumed that we pass from a point 

 of no disturbance, A or A', to another, C or C', in which there is a magnetic change 

 and a superposed secondary current change, or from a point in which these two 

 forces act to a final point. B or B', of no disturbance. Now the maximum earth 

 current force will depend upon the maximum rate of magnetic change. This maxi- 

 mum rate we cannot tell, but we may imagine it to be proportional to the mean 

 rate of magnetic change, being possibly represented in an approximate manner by 

 the expression — 



Max. current force = a constant x ma 8 ne ^ lc <-'h«nge I n other words, our general 



duration 



functions of the text would be replaced by the expressions (taking both branches of 

 the curve) — 



K=*(lTf) 



s = *H) 



It would appear from this as well as from the diagrams that the first turn of a 

 couplet should be less than the second. 



The results in our paper cannot, therefore, be regarded as a final analysis, but 

 merely as being of sufficient interest to demand a fuller inquiry. — November 4th. 

 1885. 



