Page:Popular Science Monthly Volume 4.djvu/602

584 ; the work does not appear to be very laborious, and what there is of it (which, truth to tell, is a great deal) would seem to be especially interesting and not unconnected with the gaining a full and accurate knowledge of the working of the electric telegraph. On the left-hand wall of the box as you enter, and on a level with the eye, are a number of little ebony handles, technically known as "keys," but variously termed in English railroad parlance "piston-keys" or "plungers." They are on the principle of the little "pea" bell fitted to the bedrooms of most large American hotels, and communicate telegraphically between the stations—"up" and "down." Over each of these "plungers" is an electric bell, which rings to give notice of the approach and departure of a train, its nature—that is, whether it is a passenger or goods, express or special—and to which company it belongs, when two or more companies have running powers over the same line. The custom universally adopted by English railroad companies to distinguish trains, and I believe it is the case with our own, too, in addition to the particular disks and lights carried upon the buffer-plank of the locomotive, is to blow the whistle a certain number of times when approaching a junction or station. The same system is adopted with the electric signals in the signal-box, only, in place of blowing the whistle, a bell is sounded. Every depression of the "plunger" transmits a current of electricity to the other station "up" or "down," as the case may be, which there sounds a bell or gong, and by varying the number of currents sent a code of signals is formed. For instance:

and so on. This code may, of course, be varied at pleasure; and it is possible to give fifteen distinct and unmistakable signals upon a bell by varying the number of beats and repetitions. By means of these bells, then, a perfect means of communication is kept up between two stations, signal-boxes, or gate-houses, on a railroad.

The fundamental principle of the "block" system which we are now endeavoring to explain is, that no train traveling in the same direction shall ever approach nearer to another than the distances which the signal-men's boxes are apart. These distances vary on English roads according to circumstances, but, so long as the signals are properly made by the signal-man, and attended to by the driver of the locomotive, it becomes simply impossible for one train to run into another. For sake of illustration, let us take three signal-boxes, which we shall call B, and C, and D, on a line of railroad between B and E. We will suppose that the express from a station A has arrived at B,