CHAP, v.] TELEGRAPHIC LINES. 625 



years a marvel of quickness ; we must add the constancy, the almost 

 absolute continuity of the working of the apparatus, on the sole con- 

 dition of taking care to keep the batteries, the line, and the transmit- 

 ting and receiving arrangements, in good working order. The optical 

 telegraph of Ohappe, was only of use by day, and even then but in 

 clear weather, so that sometimes an important message arrived only in 

 part at its destination, with this statement Interrupted by the fog, 

 or by the night. 



Nothing like this is to be feared with the electric telegraph, which 

 can work all the year round, night and day. But we must make one 

 reservation, however the transmission of electrical currents is 

 sometimes interfered with. During storms the wires of the line 

 are partially electrified, whence disturbances arise 'in messages which 

 come from points far removed from this accidental phenomenon. 

 The aurora borealis produces similar effects and irregularities, for 

 which there is not yet any certain cure. These disturbances may be 

 strong enough to cause damage either to the line or to the stations 

 and their apparatus. In storms of considerable violence, the lightning 

 may break- the poles or the porcelain insulators ; the magnets and the 

 compass needles may become demagnetized which will not astonish 

 the reader if he is acquainted with the electro-magnetic phenomena 

 we have described in the Forces of Nature. The armatures and the 

 bars of soft iron forming the electro-magnets may, on the contrary, 

 receive, under these circumstances, a permanent magnetization which 

 will render them useless. 



There is no remedy for this except a careful surveillance of the 

 line and the apparatus at the stations, and the repeated testing 

 of their proper working, especially in times of storm, or when 

 auroras make their appearance. In cases of damage the broken parts 

 must be replaced ; but as these things are now foreseen, well 

 organised lines keep the most indispensable parts for renewal at 

 all the most important stations, and in the result the interruption 

 is not for long. 



There is, however, a danger which may be foreseen and effectually 

 guarded against, and it is one which threatens the security and the 

 life of the clerks at the stations. In the first days of electric 

 telegraphy, strong sparks sometimes passed between the metallic 

 parts of the apparatus ; the discharge broke them, scattered them to 



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