1898-1902. No. 6.] TERRESTRIAL MAGNETISM. 31 



The means of the values found for (x in each of the three years 

 do not, it will be observed, agree very well with one another; but if we 

 take the average of the results from 1899 and 1900, and compare it 

 with the result from 1901, and the certain determinations in the table 

 on p. 29, made before and after the Expedition, indications appear, which 

 make it possible to fix with more or less certainty, the time when the 

 above-mentioned weakening of the magnetic moment of the deflectors took 

 place. We have as follows: 



Ve Ve VIm 



1897 + 1898 wilhelmshaven 0.11785 0.07656 0.09250 



2 



1899 + 1900 Expgjjiio„ 0.11637 0.07664 0.09200 



1901 — 0.07566 0.08854 



1902 Potsdam 0.10604 06943 0.08866 



According to this it may safely be assumed that deflector VI has 

 undergone a change at the end of 1900 or the beginning of 1901, while 

 deflector V" seems to have kept its magnetic moment more or less un- 

 changed until the termination of the observations in the summer of 1901. 

 On the other hand it is a question, as regards the latter magnet, whether 

 the change demonstrated at Potsdam took place before or after the ob- 

 servations made at Station IV during the summer of 1902. 



In order to solve this question, I have proceeded in the following 

 manner. According to equation (2), the following connection is found be- 

 tween the time of vibration at 0° for the two magnets, assuming that 

 H is constant: 



Cy Cvi 



/.IV ToT l-lVI ToVI 



and accordingly 



CyTl 



VI 





Both for 1901 and 1902, I have calculated /ty from the vibration 

 observations made with magnets Fand V7 on the same day, and from 

 the means for Cy and Cn, taken from the table on p. 29, as also from 

 the value of i.in found at Potsdam in 1902, belonging to the long dis- 

 tance, E, and obtain as mean 



;« 



1901. Expedition 0.07599 



1902. - 0.06843, 



