SCIENCE. 



[Vol. II., No. 25. 



of color from I'ed'to white, without any 

 change of form, conveys no informatioii what- 

 ever In certain states of the weather and with 

 certain baclvgrounds. Other signals are alike, 

 back and front. Facing the train, they signify 

 danger ; standing edgewise, they mean safety : 

 but unfortunately it is difficult to know 

 whether they refer to an east-bound train or a 

 west-bound ' train ; and, though they may be 

 placed on the right hand of the engineer to 

 whom they refer, this' arrangement is not 

 always free from ambiguitj-. 



Tlie semaphore signals, as shown at the ex- 

 position, consist of vertical posts which have 

 one or more arms pivoted at their upper ends ; 

 and these arras are capable of moving through 

 a right angle in a vertical plane. An arm 

 raised to a horizontal position signifies danger ; 

 inclined at an angle of about 45°, it signifies 

 safety. A powerful lamp is fixed near the top 

 of the post ; and, when the arm stands hori- 

 zontally, a disk of red glass stands in front of 

 the lens of the lantern, which then, of course, 

 shows a red light, indicating danger. When 

 the arm drops to an angle of 45°, the red disk 

 moves, and leaves the lantern unobscured, 

 showing a white light, and indicating safety. 



The semaphore arms are weighted, so that 

 their normal position is horizontal, indicating 

 danger ; and the signalman has to overcome 

 this weight in pulling them to safety. The 

 object of this arrangement is, that the break- 

 age of the connection between the lever in the 

 signalman's cabin and the semaphore will 

 release the signal, and let it fly to danger. 



It is usual to place one signal at or as near 

 as possible to both the signalman's cabin and 

 the spot where the engine of an advancing train 

 should stop if the signal is against the train. 

 This signal is called the ', home ' or ' main ' 

 signal. Another signal is placed some dis- 

 tance oflT in the direction from which the train 

 comes : this is termed the ' distant ' signal. 

 The object of this arrangement, is, that, on 

 catching sight of the distant signal, the engi- 

 neer is warned, and has some time and distance 

 in which to stop his train before he reaches 

 the home signal, beyond which the danger lies. 



As the levers work switches and signals at 

 a considerable distance, the connections be- 

 tween them have to be carefullj' made and 

 protected from accidental injury and the 

 effects of the weather, while the diflference in 

 length due to difference in temperature has 

 to be compensated for ; so that the signal is 

 moved with certainty, though the wire or 

 pipe connecting it to the lever vary in length 

 several inches in the twenty-four hours, owing 



to the difference in temperature 'between 

 the day and night. The Pennsylvania steel 

 companj- exhibits an especially neat device 

 for keeping the wire or connection to a dis- 



M- 



SEMAPHORE. 



tant signal always tight. The wire is kept 

 stretched by an ingenious application of the 

 pull of a weight, which acts only when the 

 signal is in its normal position of danger to 

 which it is weighted. When the signal is 



