472 THE ASTRONOMER ROYAL ON TERRESTRIAL GALVANIC CURRENTS, 
One point, of considerable importance, is the relation of the new curves to their zeros. 
I have stated that the readings by means of Scale E, Scale F, &c. have been taken in 
the same way for the zeros of galvanic ordinates as for the points of the curves ; and, by 
means of these, zero-lines have been laid down on the diagrams. From general inspec- 
tion of these, it would seem that, on the days of magnetic storms, the North Force, as 
shown by the galvanic currents, is on the whole increased. But I believe that in all 
examinations of magnetometer results for North Force during magnetic storms, and 
certainly for those in the Royal Observatory (see the Analysis of 177 Magnetic Storms, 
Philosophical Transactions, 1863) it is found that the North Force is diminished. I 
cannot at present explain this discordance. 
The other discordances, both of time and measure, may not improbably depend on 
the following circumstance. The south-western (Croydon) wire is entirely south-west 
of the Royal Observatory ; and the south-eastern (Dartford) wire is entirely south-east 
of the Royal Observatory. It is probable therefore that the times and magnitudes of 
the terrestrial currents which pass through these wires, correspond rather to places near 
the middle of the straight lines connecting the Royal Observatory with Croydon and 
Dartford than to the locality of the Royal Observatory itself. 
It might be a question whether, in continuing these observations, the length of wires 
might not be considerably reduced, and whether an attempt might not be made to give 
such terminations to each that the recording apparatus should be nearly in the middle 
of the length of each. In the actual topographical circumstances, it may not be easy 
to secure the latter condition. 
At present we are unable to say whether the records of the galvanic currents throw 
any light on the origin of the diurnal variations of the magnetic elements. 
On one point, however, we seem to be able to speak with confidence ; they do not 
explain the existence of the principal part of terrestrial magnetism. The hypothesis of 
Professor Barlow in the Philosophical Transactions, 1831, plausible as it undoubtedly 
was, is not supported by the discovery of galvanic currents competent to explain the 
earth’s general magnetism. 
