586 THE POPULAR SCIENCE MONTHLY. 



to actuate, with any degree of certainty, these exposed signals, has not 

 yet been attained. It has become necessary, therefore, to rely upon 

 small electrical instruments, miniatures of the out-door semaphores, 

 which direct the signal-man in the box how to exhibit his out-door sis:- 

 nals by displaying the signals which they themselves ought to give. 

 The same principle which produces a blow upon the bell-signal lowers 

 the semaphore arm on the miniature to " all clear." A current of 

 electricity, flowing through the wire of an electro-magnet, converts 

 the iron core into a magnet, and exerts precisely the same action upon 

 a rocking lever that a pull or strain of a signal-wire does upon the 

 large rocking lever of a signal-post. A counter-weight, when the cur- 

 rent ceases, restores the arm to " danger," as it does in ordinary rail- 

 road-signals. So that the miniature semaphore will remain at " all 

 clear " so long as a current flows, but, the moment the current ceases, 

 the arm by the action of gravity flies up to " danger." It is impossi- 

 ble to lower the signal at one station except by the action of an elec- 

 tric current, and to maintain that signal at " all clear " except by the 

 persistent effect of the battery at the other station. The signal, there- 

 fore, is under the sole control of the signal-man toward whom the train 

 is approaching. The instrument employed to raise and lower this 

 miniature signal is called a " switch," from the similarity of its ap- 

 pearance and construction to the switch-handles or levers employed to 

 raise and lower the larger signals on the line. Its electrical construc- 

 tion is precisely similar to that of the " plunger." By removing the 

 handle over from one side to the other, it places the battery in connec- 

 tion with the line wire, and thereby causes a current of electricity to 

 flow which lowers the signal. 



There were four of these miniature " switches " in our signal-box, 

 and this was the way they appeared to us to work : When the switch- 

 handle was placed so as to be nearest us, or On, no current was trans- 

 mitted, and the little signal stood at " danger ; " when, however, it 

 was pushed over farthest from us to Off, a current flowed, and the lit- 

 tle arm was lowered to " all clear." As the arm could only be low- 

 ered when a current was flowing, it was only when the switch-handle 

 was pushed over to Off that the "all-clear" signal could be given. 

 Similarly, when the switch-handle was at On, the flow of electricity at 

 once ceased, and the signal flew to " danger." The signal " all clear " 

 could therefore only be given when the little switch was intentionally 

 placed over to Off, and there was no other means of accomplishing 

 this object by willfulness or accident. ISTo accident, mechanical or 

 electrical, could alter the miniature danger-signal. The man at our 

 signal-box had the sole and complete control over the signal at the 

 next box, and it was simply impossible for him to interfere with or 

 alter the signal in his own box. This in effect is the " block " sys- 

 tem, which answers so admirably on the principal lines of English 

 railroad. The instructions given to signal-men who work the signals 



