624 
ME. B. STEWAET ON THE NATUEE OE THE EOECES 
But this difficulty will disappear when we reflect that earth-currents may be twisted 
in passing over the surface of our globe. Although, therefore, we may not find that 
oppositeness of character which we naturally associate with induced currents, yet the 
study of particular earth-currents in connexion with simultaneous magnetic disturbances 
may perhaps serve to throw some light upon the nature of the former. 
7. The following is brought forward as an instance of this. On September 1st, 
11.20 A. M. G. M. T., a strong southerly current was noticed on the Ashford and Margate 
line which lasted till 11.26 A.M., while from 11.28 a.m. to 11.35 a.m. a slight noHherly 
current was observed on the same line. 
If we take into account a slight delay in the starting of the Kew curves (equal to 
about 2 minutes) due to the slackness of the toothed wheels which drive the cylinders, 
we find the first recorded appearance of the south current to be, in point of time, 
1-| minute behind that of a very abrupt disturbance which affected aU the magnetic 
elements at Kew, and which occurred simultaneously with the outbreak of a curious 
phenomenon on the sun’s disk. Whatever be the cause of this apparent difierence in 
time, I do not think that it argues any want of simultaneity between the magnetic dis- 
turbance and the earth-current, and we may safely suppose that the first outbreak of 
the south current was really simultaneous with the commencement of the disturbance 
at Kew. 
The magnets at Kew were affected in the following manner. The westerly declination 
was at first rapidly increased ; but soon the disturbing force attained its maximum, and 
then gradually diminished, until ultimately the needle attained nearly the same position 
which it had before the disturbance. The horizontal force was at first rapidly 
diminished, then as the disturbing force died away it also gradually came back to its 
previous value. A direct current which would cause a disturbance of this nature would 
be one at fii’st rapidly increasing, then gradually diminishing, but always preserving the 
same name, whereas the induced effect of such a disturbance would be at first a strong 
current in one direction, and afterwards a weak current in the opposite direction. The 
latter character agrees very well with that of the currents between Ashford and Margate, 
the former character not in the least. Judging therefore from this disturbance, and, 
though only a single example, it appears to be an unexceptionable one, it would seem 
that earth-currents do not cause magnetic disturbances, but are rather the induced 
currents which the latter give rise to. 
8. I shall now endeavour to show that we have grounds for supposing the magneto- 
graph peaks and hollows to be due to changes in the value of the disturbing force. 
The action of any such disturbing force is of a twofold nature. 
1°. It raises a curve above its normal position, or depresses it below the same, visibly 
and for a somewhat lengthened period of time. 
2°. The curve which represents this definite action of the force is at the same time 
studded with sharp peaks and hollows. 
Now if the second of these two effects denotes the changes which occur in the force 
