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 at cabin C, to obtain _line clear_, so as to allow the 341 approaching train to proceed on to C. If the previous train in the same direction had already passed C, and there was not any obstruction on the line, the signalman at C would give _line clear_ for the DOWN train, and to do so he would turn his circular disc to show the WHITE inscription ALL CLEAR, and then push in the plunger of his C^1 instrument, lowering the DOWN or white arm, K, of his own instrument to the position shown by the dotted lines, which operation would at the same moment lower by electricity the DOWN or red arm, G, of the instrument B^2 in cabin B to the position of the dotted lines. The signalman at B would then lower his starting signal, to allow the DOWN train to proceed on towards C, and immediately the train had passed the starting signal he would, by means of his bell or gong advise the signalman at C that the train had entered the section, or block BC, and the signalman at C would at once turn his circular disc to show the RED inscription TRAIN ON LINE, and use his plunger to raise to _danger_ the DOWN or white arm, K, of his own instrument, and at the same time raise by electricity the DOWN or red arm, G, to danger in the instrument B^2 in cabin B. The section BC would then remain blocked until the DOWN train had arrived, or passed the station C, when the signalman there would, by means of his bell or gong advise the signalman at B that the DOWN train had passed out of the section, and would turn his circular disc to show the GREEN inscription TRAIN OFF, BUT SECTION BLOCKED. Both instruments would then be in their _normal_ positions, with the arms raised to danger, and ready for further train operations. In a similar manner for the UP-line trains on the section or block between C and B, the signalman in B cabin would turn his circular disc, and use his plunger to lower the UP or white arm, H, in his own instrument, B^2, and at the same moment lower by electricity the UP or red arm, I, of the instrument C^1 in cabin C, the other operation for train on line and train off being carried out for the UP train in the same routine as for the DOWN train. The outdoor fixed signals, or distant home and starting semaphore signals, have all to be worked to correspond to the block telegraph signals, and as the latter are always received well in advance of an approaching train, it follows that when the line is clear, the outdoor