584 THE POPULAR SCIENCE MONTHLY. 



dow ; 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 espe- 

 cially interesting and not unconnected with the gaining a full and ac- 

 curate 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 com- 

 panies 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 : 



1 Depression of the " key " Acknowledgment. 



2 Depressions Passenger- train. 



3 " Goods. 



4 " Special. 



5 " Obstruction-signal, 



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 Eng- 

 lish 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 an- 

 other. 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, 



