HISTORY AND PROGRESS. 205 



apparatus, and a supply of the opposite kind fetched by the 

 same road out of the earth, and held by induction. The one 

 going to and the other coming from the earth form a single 

 current. This continues as long as the cloud nears the line. 

 If it stood still for any length of time the current would 

 cease, but the line would retain its charge. As soon as it 

 moves off, on the other side, the static charge of the line 

 becomes gradually liberated, and endeavours to establish 

 equilibrium with the earth through the coils of the appa- 

 ratus and earth-plates. Then arises a current in the opposite 

 direction to the previous one, which lasts until the electricity 

 of the line is neutralised. 



The last-mentioned phenomena are causes of annoyance 

 rather than danger, but have been known frequently to 

 interrupt the operations of an overland line for many hours 

 in succession. 



Atmospheric electricity is the great enemy of overland 

 lines ; and were it not for the protection which the present 

 system of lightning-'dischargers in some measure affords, it 

 is probable that repeated sacrifices of apparatus, stations, and 

 even the lives of the employes, would long since have com- 

 pelled the rejection of the overland system and the adoption 

 of the subterranean and submarine only. The latter are 

 always free from danger, and can receive no injury from 

 atmospheric electricity, so long as they are not in electrical 

 connection with overland lines. When this is the case, and 

 the latter are not supplied with lightning-dischargers, or they 

 fail to do their duty, the lightning enters the insulated wire, 

 and, bursting through the dielectric in its struggle to reach 

 the earth, ruins the insulation in one or more places. This 

 is always to be guarded against, particularly in dealing with 

 long submarine wires, which may be irreparably injured by 

 want of sufficient foresight in enabling the high-tension 

 electricity to go to earth soon enough. 



The way this protection is provided is by opening a way 

 for the atmospheric electricity from the line to earth, which 

 offers it less resistance, and which it therefore sooner strikes 

 into than the legitimate circuit of the galvanic current. 



