316 
MR. A. W. RtrCKBR AND DR. T. E. THORPE ON A MAGNETIC 
of the two stations. It changes very rapidly, falling to 26' W. within a mile and a 
quarter, and to nearly 30' E. at Loughborough, which is thirteen miles distant. If 
currents produce it they must therefore be very local. On the other hand we know 
that the disturbances produced during magnetic storms are simultaneous over areas 
such as that of the United Kingdom, and if the currents produce the deflections, they 
also must be widespread, so that if we assume that the same earth-current is required 
at Melton to produce a permanent deflection as is observed at Greenwich when an 
equal temporary deflection takes place, we are not overstating the case. 
Now, during the whole of the year 1886, there w^ere only a few occasions on which 
the earth-currents w^ere stronger than on that which we have selected as an example, 
and we may therefore say that on the earth-current theory there should be permanently 
at Melton a potential difierence of 0'3 volt per mile (for the smaller deflection of 26' is 
1’6 times that observed at Greenwich on the selected occasion) an amount which is only 
registered at Greenwdch during violent storms, and is nearly thirty times greater than 
that observed in a circuit only two miles in length in the neighbourhood of Melton itself. 
So far we have dealt only with the magnitudes of the currents. The case becomes 
much stronger when we consider their directions. On all the circuits out of Melton 
Mowbray referred to in the table on p. 315, the earth-currents flowed from that station. 
Hence, on all the currents except that to Oakham, the direction of the current was 
such as to produce a deflection of the North Pole to the east, i.e., in the opposite 
direction to that which was actually observed, wdiile the P.D. between Oakham and 
Melton was the smallest of those measured, being only 0'002 volt per mile. We do 
not lay stress on this, as the distance was perhaps rather too great, the main fact being 
that the earth-currents between Asfordby and Melton, on the same side of the latter 
town as that on which our second station was placed, were not only much too small (if 
Ave may judge from what is observed at Greenwich) to produce the observed deflection, 
but that they were actually in the wrong direction. 
To make certain that notliing in this argument depends upon the particular deflec¬ 
tion obtained at Greenwich on the selected occasion, we have taken tArenty other 
examples in Avhich Declination disturbances of from 3' to 22' were accompanied by 
changes in the earth-currents. To avoid the necessity for correcting for diurnal 
variation they Avere chosen from the nocturnal hours Avhen the magnet is normally 
steady, and occasions Avere selected on Avhich one circuit only Avas appreciably affected. 
Except in these particulars they were chosen haphazard. 
If we divide the E.M.F. per mile in volts (V) by the Declination disturbance (A) aa^ 
get a number which expresses a relation betAveen the two, and Avhich, if they Avere 
cause and effect, ought to be constant. The following table shoAvs that the selected 
example Avas not particularly taAmurable to our vieAvs. As the first station at Melton 
was on the south side of the toAvn Ave assume that the potential difference was 
the same as that betAveen Melton and Oakham. At the second station we take it to 
