THE TELEGRAPH INSTRUMENT GALLERIES, GENERAL POST OFFICE.
In the General Post Office West, facing the oldest of the three great buildings in St. Martins-le-Grand, are the Telegraph Instrument Galleries, measuring three hundred feet by ninety. On presentation of a reference the public are permitted to visit this very interesting branch of the Postal Service. There are five hundred instruments here, worked night and day by the telegraphists, male and female, though men only are required to do night duty. In the north court are four steam engines, of fifty-horse power each, used for the purpose of sending messages to the other offices in the City and in the Strand district. In one year the Post Office has to deal with about 71½ million of telegrams, and the London local messages alone number over 6¼ millions.
source: The Queen's London : a Pictorial and Descriptive Record of the Streets, Buildings, Parks and Scenery of the Great Metropolis, 1896