small currents due to thermal effects have no value, the electrochemical effect being evaluated, the daily marked features of earth currents may be observed, but their smaller fluctuations escape observation. To ascertain the thermal effect at each moment would be very difficult indeed; and it would seem as if the magnetical observations of our observatories will form the best guides at some future time in arriving at a satisfactory explanation of the phenomena of earth currents, unless,

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... the effect is purely a thermal one in the earth's strata, when it may be estimated in telegraphic circuits.