SURVEY OF THE BRITISH ISLES FOR THE EPOCH JYHUARY 1, 1886. 315 
earth-currents by dislocations of the strata rather than in rock magnetism. We will 
consider these arguments in the inverse order to that in which we have stated them. 
Mr. Preece, F.R.S., Chief Electrician to the General Post Office, has been good 
enough to have measurements of the earth-currents made in several of the districts 
in which we have found large disturbances of the Declination. The currents flowing 
between various post-offices have been observed, and the dmections and intensities of 
the currents noted. 
The following Table gives the data obtained on telegraph lines between Melton 
Mowbray and stations in its neighbourhood. The letters P.D. signify Potential 
Difference. 
Station. 
Bearing from 
Melton. 
Distance 
from 
Melton in 
miles. 
Earth cur¬ 
rents in 
milliamperes. 
Direc¬ 
tion of 
current. 
Resistance 
of circuit 
in ohms. 
P.D. 
per mile in 
volts. 
Long Clawson .... 
K 17° W. 
.0 
0-062 
From 
912 
0-009 
Oakham. 
S. 50° E. 
9-0 
0-045 
Melton 
477 
0-002 
Asfordby. 
W. 
2-5 
0-070 
in all 
317 
0-009 
„ (Private Line) 
W. 15° N. 
1-9 
0-067 
cases 
317 
0-011 
These potential differences per mile are much smaller than those which occur 
during magnetic storms, and which on the earth-current theory must, we suppose, be 
regarded as the causes of the deflections of the Declination needle wdiich then take 
place. During a very violent storm in 1881 Mr. Preece found a P.D. of 1’9 volt 
per mile. We will take for comparison a less extreme case. Yearly records of the 
movements of the Declination needle, and of the reg-isters of the earth-currents 
apparatus, are published by the Greenwich Observatory, and the Astronomer-Poyal 
informs us that half an inch on the earth-current registers corresponds approximately 
to a P.D. of one volt in circuit. 
On September 10, 1886, an increase of 16' took place in the Declination between 
22'’ and 22'’‘5, and between 2l‘’'8 and 21'’'2 a change in the intensity of the current in 
one of the circuits occurred which corresponded to a change of P.D. of about 0'9 volt. 
The direction of the current was such as to produce the observed movement of the 
needle, and if wo regard the current as its cause, since the distance between the earth 
plates is 3 miles, we find that a deflection of 16' was produced by a potential difference 
of 0'3 volt per mile. 
There are two earth-current circuits mutually at right angles, both of which are 
inclined at about 45° to the magnetic meridian. The second was but slightly affected, 
and, therefore, the effective difference of the potential was about 0’3 cos 45°= 0'2 
volt per mile nearly. 
At Melton Mowbray we find the disturbance of the Declination to be 33' W. at one 
2 s 2 
