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, extends 319 out beyond the rails, and is formed into a flat bar or plate, J, sliding through the guide-holes K, K in the casting L. Short upright levers, M and N, work on trunnions fixed in the casting, and to M and N are attached the wires leading from the signal-cabin and continuing on to the signal-posts, as shown in elevation in Fig. 483. Two slots are cut in the plate J to receive the curved arms of the levers M and N when they are drawn downwards to pull off the corresponding signals. Neither of the levers, M or N, can be drawn over unless there is a slot immediately under the curved arm into which it can enter. When there is solid plate under a curved arm, the short lever cannot be pulled over, and the signal therefore remains at danger. The slots in the plate J are spaced so that one will be brought into position for one of the curved arms, when the points are close home for the main line, and the other slot for the other curved arm, when the points are set for the branch line or siding. The two slots cannot be under the two curved arms at one and the same time, as one of the signals corresponds to the main line and the other to the branch line or siding. In some forms of signal-detector the transverse rod I is joined on to a vertical bar which slides through guide-holes in a casting something similar to the arrangement shown in the casting L. Longitudinal guide-holes, parallel to the line of rails, are made in the casting a little above the transverse rod-bar, and through the longitudinal guide-holes slide two vertical bars which are attached to, and form part of, the wire connections to the two signals. The wire bars have each a small tongue or rectangular fin forged on to the under side of the bar, and there is one corresponding channel cut in the transverse rod-bar. When the switches are properly closed in one position, the channel cut of transverse bar will be opposite one of the wire bar fins, and will allow one of the signals to be pulled over, but the other wire bar cannot be moved. The closing of the switches in the reverse position moves the channel cut so as to allow the other wire bar to be pulled through, but as there is only one channel cut in the transverse bar, only one signal can be pulled over for each position of the switches. [Illustration: Fig. 484, 485, 486, 490, 491, 487, 492, 488, 493, 496, 489, 494, 495, 497]