Interlocking.--There are several systems of interlocking, each of them varying considerably in the form and mode of application, but all of them having the same general object of securing or releasing the necessary levers for each combination of signalling movements. A brief description of one of the systems will explain the order in which the movements have to be made, and the security which can be obtained by the locking. Figs. 503, 504, and 505, are sketches illustrating one of the types of wedge and tappet interlocking. Each lever works on a fulcrum or pinion as at A, and has a lower arm B for lifting the rods leading off to points or signals, and an arm C to carry a counterweight when necessary. Cast-iron braces D are placed at convenient distances between the series of levers to carry the top frame E on which the lever floor casing F is bolted. This casing is continuous from end to end of the locking frame, with the exception of the narrow openings through which the levers travel when moving backwards or forwards. The sleeve-block G, resting in the depressed portions of the arc, retains the lever in position. When taking hold of the main lever L, the signalman’s hand draws the small side lever M, close to the main lever, and raises the sleeve-block G sufficiently high to pass over the top of arc F, the lever L can then be pulled or pushed over, and the block G will fall into the depression at the end of the stroke when the hand is removed. N is a tappet or thin flat bar attached to the main lever, and which works backwards or forwards between the wedges in the wedge frame O. The wedges move horizontally between guide pieces, and work either singly or are connected by the lower slide bars to other wedges some distance away on the frame according to the position of the levers which have to stand or move in unison for the releasing or locking. A strong cover is placed over the wedge frame to keep out the dirt. [Illustration: Fig. 503, 504, 505]