3i8 RAILWAY CONSTRUCTION. 



of the rocking-bar is sufficiently below the top of the rail to be 

 well clear of the flange of any passing wheel ; but while chang- 

 ing from the one to the other position, and when the pivoted 

 arras are vertical, or at half-stroke, the upper surface of the 

 rocking-bar is about level with the top of the rail, and right in 

 the pathway of the wheel-flange. It is evident, therefore, that 

 when the pivoted arms are set in the forward or backward 

 position, and one of the wheels of a train or vehicle has passed on 

 to the rail over the rocking-bar, the latter cannot be changed or 

 raised and pulled over to the opposite extremity so long as any 

 one of the wheels of the train or vehicles remain over the 

 rocking-bar. 



As the same ground-crank which pulls over the pivoted 

 arms from backward to forward also withdraws the locking-bolt 



O 



B, the latter is thus held securely in the hole or slot of the trans- 

 verse rod, E, until all the wheels of the train have passed off the 

 rocking-bar. The operation of changing the points from one 

 road to another is very simple. By means of the rodding G, 

 worked by a lever in the signal-cabin, the locking-bolt B is first 

 withdrawn from the slot ; the points are then pulled over into the 

 reverse position by the rodding H, and the locking-bolt B is 

 again set back into one of the slots by the rodding G. Some- 

 times, for economy, the points, bolt-lock, and rocking-bar, are all 

 three worked by one lever in the signal-cabin, and one set of 

 rodding on the ground, as shown in Fig. 482 ; but the arrange- 

 ment is neither so perfect nor so secure as that shown in Fig. 

 480. Where there are two sets of rodding and gearing, the 

 failure or breaking of either of them prevents the complete com- 

 bination being made, and indicates at once to the signalman that 

 something is wrong ; but when there is only one set of rodding 

 a breakage may occur without giving any tangible evidence to 

 the signalman of the defect, and he may proceed to pull over his 

 signal lever in ignorance that the points have not been properly 

 made and bolted. To avoid an accident taking place from the 

 failure of either rodding or gearing, the signal-detector has been 

 devised, so as to prevent the possibility of pulling over the signal 

 wire until the points and locking-bar are both in their proper 

 positions. 



The signal-detector is applied in several forms ; the one shown 

 in Figs. 480 and 483 will explain the principle on which its 

 efficacy depends. A transverse rod, I, attached to the sliding-rail, 



