622 
ME. B. STEWAET ON THE NATHEE OE THE EOECES 
tensity of the disturbing force, and therefore not as phenomena caused by earth-currents, 
since the great body of the disturbance of which they represent the changes cannot be 
due to any such cause. Auroras and earth-currents being thus removed out of the list of 
causes, might then be supposed to follow rather as effects of these magnetic changes, on 
the principle of voltaic induction already mentioned. As, however, there seems to be 
a tendency to regard the motions of the magnetic needle as due to the direct action of 
earth-currents, it may be desirable here to inquire whether this view of the origin of 
these small and rapid disturbances is capable of being accepted as a tenable hypothesis. 
4. It is almost unnecessary to remark that the hypothesis which asserts that earth- 
currents are not the cause of the small and rapid magnetic disturbances, does not assert 
that such currents have absolutely no influence upon the needle, since we know that a 
current must always act upon a needle ; but it maintains that these currents are induced 
and secondary phenomena, and therefore represent only a small fraction of the whole 
force in operation, and react on the magnet only to a very limited extent. 
5. The interesting observations on earth-currents made under the direction of Mr. C. 
V. Walkee, and recorded by him in a paper recently published in the Transactions of the 
Royal Society, appear to furnish grounds for an hypothesis regarding these phenomena. 
By reference to a map appended to his paper, it appears that a line drawn from the 
Ramsgate to the Margate telegraph station proceeds in a direction 12° 50' W. of true 
north; or, assuming 21° 30' W. as the approximate magnetic declination, this line proceeds 
in a direction 8° 40' E. of magnetic north. In like manner, a line drawn from the 
Ashford to the Ramsgate telegraph station proceeds in a direction 83° 50' E. of magnetic 
north ; also a line drawn from the Ashford to the Margate station in a direction 77° 42' E. 
of magnetic north. 
By Mr. Walker’s observations from August 29 to September 2, 1859, and also from 
August 8 to August 10, 1860, it would appear that a current proceeding Rom Margate 
to Ramsgate was always simultaneous with one proceeding from Ramsgate to Ashford, 
and with one proceeding from Margate to Ashford ; that is to say, simultaneous currents 
proceeding along these lines were either all north or all south currents. 
Let us endeavour to ascertain how far this agrees with the behaviour of the magneto- 
graphs at Kew during these two disturbances on the hypothesis of direct action, assuming 
also with Mr. Walker that earth-currents are derived from a stream of electricity which 
is drifting across the country. 
Conceive now a current resolved into two components, the one proceeding in the mag- 
netic meridian, and the other in a direction perpendicular to it ; and let us agree to con- 
sider as positive those currents flowing from magnetic north to south, and from magnetic 
east to west. Of these two components, the flrst only will affect the freely suspended 
declination magnet, a positive current tending to increase the westerly declination; 
while the latter only will affect the horizontal-force magnet which has been twisted 
into a position at right angles to the magnetic meridian, a positive cui-rent increasing 
the horizontal force. 
