HISTORY AND PROGRESS. 203 



thickness of air intervening between the cloud and the nearest 

 points above the surface of the earth. As soon as this thick- 

 ness is so diminished that the tension of the electricity is 

 able to overcome it, a neutralisation ensues in the form of 

 lightning. The telegraph poles and line wires are especially 

 favourable for the discharge of atmospheric electricity, being 

 extended over an immense surface and offering numerous 

 points. 



As a cloud approaches, it induces an opposite charge in the 

 earth's surface and in the line wire ; this charge becoming 

 greater and greater as the cloud nears the line. While this 

 goes on, electricity of the same kind as the charge of the 

 clouds is driven along the line in the form of a current to the 

 earth-plates of the stations at each end. When the cloud is 

 near enough, it discharges itself into the line, and, more than 

 neutralising the electricity of the latter, leaves the line some- 

 times heavily charged with the fluid, which seeks the nearest 

 road to earth. When the charge is very intense the lightning 

 not unfrequently melts the line wire in its passage along it, 

 and splinters several of the posts. Should it succeed in 

 entering a station and reaching the apparatus, the usual con- 

 sequence is that it melts the coils of the receiving apparatus, 

 and perhaps shatters the station. 



Schellen, who has written a beautiful chapter on this sub- 

 ject in his book,* says that the destruction of the posts 

 is probably caused by the water, which during rain creeps 

 into the numerous pores and cracks of the wood, being 

 decomposed by the lightning, the sudden expansion following 

 its conversion into gas bursting them asunder. 



At times the earth plays no part in the discharge of the 

 clouds. This is the case when two clouds charged with 

 opposite electricities meet in the air and neutralise each 

 other. A line of telegraph being underneath one of them 

 is charged by induction, as we have already seen, in com- 

 mon with the surrounding earth's surface, although to a 

 greater extent, with accumulated electricity. This charge 

 does not change so long as the clouds remain tranquil ; but 



* Der Elektromagnetische Telegraph, p. 341. 



