Now, if we look at one of these sheets of photographic paper after it has been taken from the drum, we shall see that the north pole of the magnet has moved a little, a very little, towards the west in the early part of the day, say from sunrise to 2 p.m., and has swung backwards from that hour till about 10 p.m., remaining fairly quiet during the night. The extent of this daily swing is but small, but it is greater in summer than in winter, and it varies also from year to year.
MAGNETIC PAVILION—INTERIOR.
(From a photograph by Mr. Lacey.)
Besides this daily swing, there occasionally happen what are called 'magnetic storms;' great convulsive twitchings of the needle, as if some unseen operator were endeavouring, whilst in a state of intense excitement, to telegraph a message of vast importance, so rapid and so sharp are the movements of the needle to and fro. These great storms are felt, so far as we know, simultaneously over the whole earth, and the more characteristic begin with a single sharp twitch of the needle towards the east.
Besides the movements of the magnetic needle, the intensity of the currents of electricity which are always passing through the crust of the earth are also determined at Greenwich; but this work has been rendered practically useless for the last few years by the construction of the electric railway from Stockwell to the City. Since it was opened, the photographic register of earth currents has shown a broad blurring from the moment of the starting of the first train in the morning to the stopping of the last train at night. As an indication of the delicacy of modern instruments, it may be mentioned that distinct indications of the current from this railway have been detected as far off as North Walsham, in Norfolk, a distance of more than a hundred miles. A further illustration of the delicacy of the magnetic needles was afforded shortly after the opening of the railway referred to. On one occasion the then Superintendent of the Magnetic Department visited the Generating Station at Stockwell, and on his return it was noticed day after day that the traces from the magnets showed a curious deflection from 9 a.m. to 3 p.m., the hours of his attendance. This gave rise to some speculation, as it did not seem possible that the gentleman could himself have become magnetized. Eventually, the happy accident of a fine day solved the mystery. That morning the Superintendent left his umbrella at home, and the magnets were undisturbed. The secret was out. The umbrella had become a permanent magnet, and its presence in the lobby of the magnetic house had been sufficient to influence the needles.