Intelligence and Miscellaneous Articles. 561 



do not show a daily course of the earth current either in respect of 

 magnitude or number of oscillations. But taking the mean of 24 

 term-days, a sufficiently distinct, though small, daily course is seen. 

 The current in the S.-N. line indicates a maximum between 4 and 

 5 o'clock in the morning, and a minimum at 8 o'clock in the evening ; 

 and the current in the W.-E. line a maximum at 8 a.m., and a 

 minimum at 1 p.m. The amplitude of this oscillation is very small, 

 only corresponding for the two components to about 0-0008 volt. 

 This variation does not coincide with the normal variation of the 

 magnetic elements ; and it follows that the daily variation of the 

 earth currents cannot be regarded as the origin of the daily varia- 

 tion of the magnetic elements. 



4. As soon as the earth current shows itself in the two lines 

 with greater force, the magnetic instruments begin to diverge from 

 their regular course ; and these disturbances increase in general 

 with the strength of the earth currents without there being any 

 strict proportionality between the two classes of phenomena. On 

 the 15th of November, 1882, for instance, the earth currents 

 showed themselves in greater strength in the two lines than on the 

 1st of March, 1883 ; and nevertheless the course of the magnetic 

 instrument on the first of these days was far more irregular than on 



his latter day. 



5. If, as Sir Gr. Airy has already done for similar observations 

 at the Greenwich Observatory, we compare the current in the 

 S.-N. line with the changes in the declination, and the current in 

 the W.-E. line with the changes in horizontal intensity, we often 

 find, especially for somewhat slow changes, perfect equality between 

 these two kinds of disturbances ; but the variation of the current 

 always precedes that of the earth's magnetism by at least five 

 minutes, so that the current appears as the primary cause of this 

 latter. Thus the motion of the magnetic apparatus is always that 

 w r hich would be required by the fundamental law of electromag- 

 netism, if the earth current had a direct influence on the magnets. 

 If, for instance, a positive current from north to south shows 

 itself in the N.-S. line, the easterly declination of the needle in- 

 creases at the same time ; and if in the E.-W. line a positive cur- 

 rent goes from east to west, the horizontal intensity of terrestrial 

 magnetism becomes greater — that is to say, the north pole of the 

 bifilar moves towards the north. 



The retardation which the indications of the magnetic instru- 

 ments experience relatively to those of the galvanometer could be 

 explained by the hypothesis that the earth current, which directly 

 influences the magnets of the galvanometers, only acts on the varia- 

 tion instruments by the magnetization of the more or less thick 

 layers of earth, and this requires a certain time. 



6. This retardation in the effects of the earth current explains, 

 partially at any rate, the want of strict proportionality between 

 the variations of the current and the magnetic elements. It is 

 observed that the proportionality fails particularly when the changes 

 in the current are very frequent and alternating ; and it is more 



