lines which it crosses. And this is the meaning of the numbers marked on the different levers. No. 10, let us suppose, has 5, 7, and 23 marked on Its side. He may pull at No. 10 as long as he pleases, but he cannot move it till Nos. 5, 7, and 23 have first been moved; and so throughout the whole system. No signal lever can be moved to safety unless the point levers corresponding with it have first been moved, and no point levers can be moved while there stands at safety any signal lever that ought to stand at danger. Every lever is under lock and key, each being a part of the key which unlocks some of the others, and each forming a part of the lock which secures some of the others against possible movement, while each is at the same time subject to the control of all those which are related to it.
"This result, complex and difficult as it seems, is achieved by mechanism of great simplicity and beauty. Immediately under the floor of the platform, and just in front of the levers, are arranged several series of vibrating and sliding bars, somewhat like the tumblers of a lock, placed horizontally. These bars have projections here which stand in front of certain levers as obstacles to their motion, and notches there which permit certain levers to travel. Some of these have sloping faces, so that when a lever moves along them it edges them to one side, and this transverse motion being communicated to others of the series, brings the proper projections or notches in front of those other levers to which the moving lever is related. Thus by the movement of one lever some others are stopped, and some are left free, and this simple principle, carefully applied
