The well-ventilated and well-lighted telegraph instrument room is on the upper floor, and extends from end to end of the building. In it there are 102 telegraph instruments of various kinds in use, viz.: 5 A.B.C.'s, 19 double-plate sounders, 30 sounders, 28 duplexes, 5 quadruplexes, 5 Wheatstone sets, 7 repeaters or relays, 2 concentrators and 1 hexode. Divested of technicalities, it may be said that telegraphing on the A.B.C. instruments is effected by alphabetic manipulative keys, which are depressed by the fingers of the left hand of the sender at the same time that a handle is turned with the right hand, and a corresponding effect is produced on the dial plate of the receiver. The double-plate sounder is read by sound from two small metal hands striking right and left against two pieces of metal. In sending, the working is by means of keys manipulated by the hand. The sending upon the sounder instrument, which is that chiefly used, is done by a small key with handle being depressed and released according to the dots and dashes of the Morse alphabet. The signals by which messages are received and read by the ear are produced by a bar of soft iron striking upon a steel point placed between two coils of wire. With the A.B.C., double-plate sounder, and sounder, only one message can be sent or received on the wire at one time; but the duplex sounder instruments are so constructed that two messages can be sent on the wire--one in each direction--at the same time. Double-current duplex instruments are in use for telegraphing to busy towns such as Plymouth, Exeter, Cardiff, Swansea, &c., &c. The quadruplex consists of two duplex sets upon one wire. Upon these circuits two distinct messages may be sent simultaneously from each end. The hexode has six instruments at each end of a single wire, enabling twelve clerks to operate at the same time--six at each end,--and thus admits of a single wire doing so much work as six wires worked with the ordinary sounder instrument.