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XXVIII. On the Nature of the Forces concerned in producing the greater Magnetic 
Bisturhances. By Balfour Stewart, M.A.^ F.B.S. 
Eeceived June 14, — Eead June 19, 1862. 
1. In a previous communication submitted to the Eoyal Society on June 28th, 1861, 
and since pubhshed in their Transactions, I ventured to make a suggestion regarding 
the nature of that connexion which subsists between magnetic disturbances, earth- 
currents, and auroras. 
In this hypothesis the earth was viewed as similar to the soft iron core of a Euhm- 
KORFP’s machine, in which a primary disturbing current was supposed to induce mag- 
netism. Earth-currents and auroras, on the other hand, were viewed as induced or 
secondary currents, caused by the small but abrupt changes which are constantly taking 
place in the strength of the primary disturbing current, these changes being very much 
heightened in effect by the action of the iron core, that is to say, of the earth. 
2. These small and rapid changes are very observable in the photographic traces given 
by the Kew magnetographs during a time of disturbance. At such a time the curves 
for all the three elements invariably present a serrated appearance, which, when the 
disturbing cause is very powerful, is magnified into a succession of sharp peaks and 
hollows, and these exhibit a frequency of change which makes them comparable in this 
respect with earth-currents and auroras. 
These peaks and hollows are therefore regarded by this hypothesis as denoting the 
changes which take place in the action of the primary disturbing current upon the needle 
through the intervention of the earthfs magnetism, or, since the existence of a primary 
current is really unnecessary as an explanation, we may dispense with it altogether *, and 
regard these photographic peaks and hollows as simply representing the changes which 
take place in the magnetism of the earth, without speculating upon the cause of such 
changes. 
3. In the paper already alluded to, in discussing the great magnetic storm extending 
from August 28 to September 7, 1859, I endeavoured to show that the first effect of the 
superimposed disturbing force was to diminish both elements of the earth’s magnetism 
during a period of about six hours. This grand wave, constituting the great body of 
force, could not, I remarked, be supposed to be due to any combination of earth-currents 
of which the period is only a few minutes. Under these circumstances it would seem to 
be the most natural supposition to regard the peaks and hollows, which occur as it were 
on the surface of the great disturbance wave, as representing rapid changes in the in- 
* Tliis was suggested to me by Professor Txndall. 
