RAILWAY CONSTRUCTION. 339 



block-telegraph instruments are required, one for the section of 

 the line to the left hand of the cabin, and the other for the 

 section to the right. At the terminal stations only one instru- 

 ment is required. 



In the instrument shown in Fig. 511, the upper arm of the 

 miniature signal-post is coloured RED, and is moved by electricity 

 through the medium of the block telegraph instrument in the 

 signal-cabin in advance ; and until this RED signal be lowered to 

 the line clear position by the signalman in the cabin in advance, 

 no train must be allowed to start from or pass the cabin in the 

 rear. The lower signal-arm coloured white is lowered by the 

 plunger A on its own instrument by the signalman in charge, 

 and at the same moment lowers by electricity the upper or RED 

 arm of the block-telegraph instrument in the signal-cabin at the 

 other end of the section. The lower or WHITE arm is thus re- 

 stricted to the signals sent away from the signal -cabin, while the 

 upper or RED arm is restricted to signals received in the signal- 

 cabin. In the centre there is a round handle B, which rotates a 

 circular disc inside the instrument, and on this disc are painted 

 three distinct train inscriptions, only one of which can be seen 

 at a time through the glazed opening. One inscription has the 

 words ALL CLEAR painted in black letters on a WHITE ground ; 

 another has the words TRAIN ON LINE painted in white letters 

 on a RED ground ; and the third has the words TRAIN OFF, BUT 

 SECTION BLOCKED painted in black letters on a GREEN ground. 

 The instrument is considered to be in its normal position when 

 the GREEN inscription is in view, and both the miniature signal- 

 arms raised to danger. 



Fig. 512 represents a portion of double line divided out into 

 sections, or working blocks, between the stations B, C, and D. 

 Each station is provided with the necessary block- telegraph 

 instruments, and the usual distant, home, and starting semaphore 

 signals. 



Fig. 513 is a diagram sketch showing the pair of instruments 

 as they stand on the instrument-tables in the signal-cabins B and 

 C, where B 2 and C 1 are the instruments which work together 

 for the block section BC. Supposing a DOWN train proceeding 

 from A in the direction of F, and approaching the signal-cabin 

 of the block station at B, the DOWN starting signal standing at 

 danger ; then by the code of signals on the bell or gong the 

 signalman at cabin B would communicate with the signalman 



