been utilized to assist in regulating these train movements, but it was felt that a mere telegraph message was not sufficient to ensure positive safety, and that something more tangible was required in the shape of a staff, or token, without which no train should be allowed to travel on a single line of railway. To meet this requirement, the electric train-tablet, and the electric train-staff instruments have been invented, each of them being so arranged that upon any one section, or pair of instruments, a tablet or train-staff may be taken out from the instrument at either end of the section, but when once taken out, no other tablet or train-staff can be withdrawn from either instrument until the first has been delivered and placed again in one or other of the two instruments. Figs. 514, 515, and 516 are sketches of an electric train-staff instrument which has been very largely adopted on single lines, both at home and abroad. In a similar manner to the block-telegraph instruments for double line, the electric train-staff instruments have each a bell or gong by which the adjacent signalmen can communicate their calls and answers in accordance with a regulation code. In the signal-cabins of the intermediate stations two instruments are required, one for the staffs belonging to the section to the left of the cabin, and the other for the staffs of the section to the right. At a terminal station only one instrument is required. [Illustration: Fig. 514, 516, 515, 517] The head of the instrument contains the electrical and mechanical locking apparatus which controls the withdrawal of a train-staff, or is acted upon by its insertion. The circular name-plates and pointers, together with the galvanometer in the centre, serve as indicators to guide the signalmen in carrying out the various operations. The staffs usually consist of thin steel tubes, solid at the ends, with metal rings fixed upon them, as shown in the sketch, the number and position of the rings varying according to the section or pair of staff stations to which they belong; this difference in the rings effectually preventing the possibility of one set of staffs being used or inserted in either of the instruments of the adjoining sections. The staffs rest normally in the long vertical slot A, with the rings fitting in vertical grooves, which prevent the removal of any staff except by passing it along the curved slot BC, and out by the opening 346 D, of large diameter. The electrical and mechanical locking apparatus is placed at the curved slot, and until the locking-bolt, which stands across the passage of the curved slot, be lifted by the joint operations of the signalmen and their instruments at both ends of the section, no staff can be withdrawn. When the instruments are standing in their _normal_ position of “staffs in,” the signalmen can arrange between them to withdraw a staff--say either from the NORTH cabin