332 RAILWAY CONSTRUCTION. 



Figs. 504 and 505 show plan views of four levers in a signal 

 cabin taken just above the level of the tappets. In Fig. 504, all 

 the levers are in their normal or forward position, with the 

 home and distant signals at danger, and the facing points 

 leading into loop or siding lying for main line. Previous to the 

 approach of a train on the main line, the home and distant 

 signals have to be lowered, and will require the pulling over of 

 levers 1 and 2 ; but these levers cannot of themselves be moved, 

 as the wedges P and Q are locked by the straight side of lever 

 3. The operation would therefore be as follows : points lever 

 4 being set in its normal position for the main line would remain 

 forward, lever 3 working the facing point bolt-lock would be 

 pulled over, and in doing so would move the wedge R to the 

 right into the recess of tappet of lever 4, locking that lever, and 

 presenting the recess of its own tappet ready to receive the 

 wedge Q. Lever 2 can then be pulled over, and will move the 

 wedge Q to the right into the recess of tappet of lever 3, and 

 present its own recess for wedge P. The pulling over of lever 1 

 completes the series, by moving the wedge P over to the right 

 into the recess of tappet of lever 2. Fig. 505 shows the positions 

 of the tappets and wedges with the levers 1, 2, and 3, pulled 

 over to make the combination described. Upon examination, it 

 will be seen that levers 2, 3, and 4, are all securely locked, the 

 points cannot be moved, nor the facing point bolt-lock with- 

 drawn, nor the home signal changed until the lever 1 is pushed 

 over again into its normal or danger position. To restore the 

 levers to their forward position, they must be set back in the 

 reverse order to which they were pulled over. To simplify 

 the explanation, only four levers are shown in the above 

 sketches, but the principle is constantly extended out to a very 

 large number of levers, and in many cases necessitates the intro- 

 duction of several rows of wedges as indicated by the dotted 

 lines. In some instances a combination is effected by pulling a 

 certain lever only half over. In some systems the preliminary 

 action or spring handle locking is adopted, in which the locking 

 is actuated by the small side lever, similar to the one marked M 

 on Fig. 503. The advocates of this arrangement claim increased 

 security and precision in the interlocking, while on the other 

 hand it is alleged that the mechanism is rendered more com- 

 plicated without any corresponding advantage. 



Detached Lock. Sometimes there is in the vicinity of a 



