RAILWAY CONSTRUCTION. 323 



rod from the switches, and serve merely as indicators to show 

 whether the switches are lying for or against an engine passing 

 out of the siding. In other cases they are worked independently 

 from the signal-cabin by a separate lever and wire connection, 

 the interlocking being so arranged that the lever working the 

 switches must be pulled over before the lever working the 

 disc signal can be moved. In one type the disc signal is fixed 

 to a short vertical axis, as shown in Fig. 494, and by means 

 of a cranked arm is made to rotate a quarter of a circle, so 

 as to exhibit either a stop or advance signal according to the 

 position in which the switches are lying. In another type, the 

 lamp is fixed, and the red disc, with a red glass in the centre, is 

 made to assume a horizontal or vertical position by a rod and 

 crank, as shown in Fig. 495. 



A simple arrangement of rodding and rollers for switch con- 

 nections is shown in Fig. 496, the number of sets of rodding 

 being determined by the number of connections to be made. 

 Fig. 497 is a rodding compensator, to compensate or adjust for 

 the difference in length of the rodding arising from variations 

 in the temperature. The compensator may be used either 

 vertically or horizontally, according to space or circumstances. 



Strong wrought-iron or steel cranks of different angles will 

 be required when changing the direction of the rodding, or con- 

 necting to switches and facing point-locks. They must be firmly 

 secured to strong timber framework well bedded in the ballast. 

 For cranks working switches and bolt-locks, it is better to use 

 extra long timbers under the rails instead of the ordinary sleepers. 

 Cross-pieces can be bolted to the ends of the long timbers, and 

 the cranks placed practically on the same timbers carrying the 

 permanent way. By this means the rails and cranks can always 

 be maintained in their proper relative positions as to distance, 

 line, and level. 



Without a large series of diagrams it would be impossible to 

 adequately describe the extent of signalling and interlocking 

 required at large terminal stations and important roadside 

 stations, but one or two simple examples may serve to illustrate 

 the general principles. 



Fig. 498 represents the modern grouping of signals considered 

 necessary at an ordinary double-line junction, showing all the 

 signals at their normal or danger position. The numbers 

 marked on each indicate the numbers of the levers in the inter- 



