I 
SURVEY OF THE BRITISH ISLES FOR THE EPOCH JANUARY I, 1886. 317 
be that between Melton and Asfordbj. Tn both cases the whole is supposed to be 
effective, i.e., it is not resolved along the magnetic meridian. A negative sign means 
that the current flowed in the direction opposite to that which would be required to 
produce the observed deflection. 
Station. 
Greenwich . . . <|^ 
Melton Mowbray. | 
I.argegt value of V/A . 0'028 
Smallest „ ,, . 0'008 
Mean ,, ,, . 0'016 
Value on selected occasion O'OIS 
Station 1.O'OOOOG 
„ 2 . 0-00040 
We are quite aware that an argument of this sort is open to criticism. The exact 
relation between earth-currents and magnetic storms is uncertain; in short circuits 
the eai'th-currents may be masked by those due to the earth-plate, and we have been 
compelled to assume that the conditions at Greenwich and Melton are the same. 
We should not, therefore, have put the argument forward had the conclusion depended 
on any irice balance of figures. As it is, we think it supports the view that the per¬ 
manent earth-currents at Melton Mowbray are very much less, and less extended, 
than the temporary currents observed at Greenwich during ordinary but considerable 
storms, though the permanent Declination disturbance which they are supposed to 
produce is of the same order as the temporary deflections which are observed simul¬ 
taneously with the currents at Greenwich. 
Similar results were obtained near to the Beading and to the Wash disturbances. 
If these are produced by earth-currents they must circulate round the peaks, and thus 
the potentials at points on opposite sides of them should be different. Near the 
Wash the direction of the telegraph wires is not very favourable for a test, but we 
owe to the kindness of Mr. Preece, to whom we must again express our obligation, a 
series of measurements made at Pleading and Windsor. Of these it is only necessary 
to say that, though Reading and Windsor are on opposite sides of the focus of 
disturbance, and though the needle is deflected 11' to the west at Windsor and 
6 to the east at Reading, so that the assumed current circulating round the peak 
ought to run in opposite directions through them, the observed earth-currents were so 
small that no measure of their magnitude could be taken. 
It might be urged, in answer to these arguments, that the earth-cnrrents by which 
: local disturbances are produced are not mere surface currents, but that they flow 
r through the mass of the earth, j^ossibly where the strata are (as recently suggested by 
i Professors Lamb and Schuster) of higher conductivity. 
I To this it may be replied that the greater the depth at which the currents are 
i 
j 
1 
