226 
THE ASTRONOMER ROYAL ON TERRESTRIAL CURRENTS. 
It must be observed that the investigations of any one month are totally unconnected 
with the investigations of every other month. Bearing this in mind, and remarking the 
strong similarity in the laws of the numbers (under each division of the Table) in pro- 
ceeding from month to month, with change in the magnitude of the numbers and small 
change in the epochs of the argument evidently depending on the season, it is impos- 
sible to doubt that these numbers are real, the true representation of a galvanic and con- 
sequent magnetic action, with remarkable diurnal variation, in the surface-materials of 
the earth. 
In every month there is a constant term of considerable magnitude (in reference to 
the scale of forces before us) towards the North. Of the origin of this term we can 
give no certain account ; but it may not improbably arise from the different oxidabili- 
ties of the terminal plates. The variations of magnitude probably depend on the 
changes which were made from time to time in the earth-connexions. In any case, 
there is no reason to doubt that the same term exists in the exhibition of forces on days 
of great disturbance. And, referring to the tabular values of these constant terms, and 
to the apparent increase of northern force in the disturbed days as measured by the 
scales at the sides of their diagrams, it will be seen that the magnitude of these con- 
stant terms fully explains the apparent increase in northerly force which was remarked 
in the discussion of the magnetic effects of earth-currents on the days of great disturb- 
ance. The last of the apparent anomalies, which exhibited itself in that discussion, is 
therefore entirely removed. 
In every month there is a constant term of still greater magnitude towards the west. 
And, on referring to the diagrams applying to the days of great disturbance, it will be 
seen that there is in them a greater increase of force to the west, well corresponding in 
magnitude to that larger constant term. 
The peculiarities of the law of diurnal inequality will be well seen in the diagrams 
attached to this paper. The general type of the curve is a double lobe, somewhat mo- 
dified in one or two months, but always preserving the duplicity. It must be remem- 
bered that these curves, which are formed by use of the smoothed numbers, are slightly 
inaccurate in regard to the more rapid inequalities. 
Neither in magnitude nor in law are these inequalities, consequent on the galvanic 
currents, competent to explain the ordinary diurnal inequalities of magnetism. 
The discussion of the galvanic currents on the Croydon and Dartford Lines may now, 
perhaps, be considered as exhausted. 
