Page:Popular Science Monthly Volume 22.djvu/343

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together, and each group has a separate relay, in order that the shorter may not unduly deprive the longer lines of their share of the current. The four relays are all worked by the hourly signal from Greenwich, and therefore act simultaneously. The lines of the Metropolitan group are used only for time purposes, and are therefore always connected with their relay, and distribute the signals hourly. But the lines of the other groups are in use generally for ordinary telegraphic purposes, and distribute time-signals only at ten and one o'clock. At these hours, therefore, the wires must be switched off from their ordinary duty, and placed in communication with their respective relays to be