The large central hall facing the main entrance to the building is set apart for the circulation of telegrams received from the various branch offices connected with the Central Office by pneumatic tube, and in the reverse directions for delivery from these offices. A large staff of telegraphists is engaged upon this work. The pneumatic tubes used for forwarding and receiving telegrams to and from certain branch offices in the city, western central, and western district offices, and so obviating telegraphic transmissions, are led into this hall. These tubes are laid at the depth of about two feet underground. They extend as far distant as Billingsgate on the eastern side, House of Commons and West Strand on the south-western side, and the western district office on the western side, and allow of the rapid collection and distribution of telegrams over a very busy area. The tubes make the various offices arms practically of the Central Station so far as telegrams are concerned. The message forms are enclosed in gutta-percha carriers covered with felt, and having attached to their forward ends a number of felt discs which exactly fit the internal circumference of the tubes and prevent any escape of air around them. An elastic band at the mouth of the carrier prevents the messages from escaping. The outgoing carriers containing the messages are propelled through the tubes from the Central Office by forcing compressed air into the tubes behind them at a pressure of about 10 lbs. to the square inch, the incoming carriers being drawn through by vacuum, so that the normal atmosphere exerts behind them a pressure of about 7 lbs. to the square inch. All the tubes are worked on the block system, and by an electrical contrivance the traffic is regulated. In long-distance tubes delay would arise if it were only possible for one carrier to be in the tube at one time, and to meet this intermediate automatic signallers are inserted at various distances in the tube, so that as soon as a carrier passes one section, it automatically notifies the sending section, enabling a second carrier to be inserted. Thus several carriers equidistant from each other may be passing through the one tube at the same time. The power by which these tubes are worked is derived from large compound pneumatic pumping engines fitted in the basement, but eventually the power station which has recently been established at Blackfriars will supply the power required. There are at present thirty-seven pneumatic tubes connecting the various branch offices with the Central Office, in addition to seven which connect the offices of the various cable companies with the cable room.