204 THE ELECTRIC TELEGRAPH. 



the moment the discharge takes place the electricity with 

 which the line is charged is suddenly set free and seeks the 

 earth. It is not seldom that it takes its way over the posts 

 and through the station, where, unless sufficiently protected, 

 it does serious damage. 



There are, of course, many ways in which the positions of 

 the clouds and their motions, with respect to the direction of 

 the line, modify the conditions of the current. The passage 

 of the electric discharge between two clouds over the line is 

 alone able -to induce a powerful current, under certain 

 circumstances, independent of the liberation of the static 

 charge. 



The remaining phenomena of atmospheric electricity con- 

 fine themselves to the production of currents of more or less 

 intensity in the line. These are also dangerous to the 

 apparatus, but not to the same extent as the stroke of 

 lightning. 



These currents are produced in different ways. The 

 atmosphere is everywhere electrical, either positive or nega- 

 tive ; its electrical state depending partly upon the height 

 above the surface of the earth, partly upon the hour of the 

 day, and upon other causes of which our knowledge is 

 limited. When a telegraph line runs through a region 

 which, by reason of great difference of level or any other 

 cause, at one end is positively electrical and at the other end 

 negatively, the line taking the electrical tension of the 

 atmosphere at all points, a constant current must pass 

 through it so long as there are opposite electricities to 

 combine in the circuit. Baumgartner, Henry, and others 

 have made an especial study of this subject, and observed 

 the phenomena under different conditions of height, weather, 

 hour of the day, &c. 



The passage of a single charged cloud over the line occa- 

 sions sometimes also a considerable and long- continued cur- 

 rent through the apparatus. As the cloud approaches the line 

 it induces in it an opposite charge. To do this the natural 

 electricity of the line must be decomposed ; the electricity of 

 the same kind as the cloud is repelled to earth through the 



