202 THE ELECTRIC TELEGRAPH. 



extreme end is fastened a cast-iron screw, or bore, which is 

 screwed into the earth ; the upper end is surmounted by an 

 ornament and lightning discharger, and is furnished with 

 binding screws for receiving the wires leading to the 

 apparatus. 



VIII. ATMOSPHERIC ELECTRICITY. 



The experiments of Franklin, and various physicists since 

 his time, have proved that the atmosphere is always more or 

 less charged with electricity ; that in some parts the charge 

 is positive, in others negative. Accumulations of atmospheric 

 electricity occur particularly in the clouds, which become 

 charged in their formation, their passage, or otherwise, with 

 high tension. 



It is this charge of electricity which probably tends in a 

 great measure to prevent the clouds falling readily in the 

 form of rain. It is well-known that bodies charged with the 

 same kind of electricity repel each other ; and it must be the 

 same with the water-particles of which a cloud is composed ; 

 when they are charged with electricity they repel each other, 

 and this repulsion prevents them combining and falling to 

 the earth. But when the electricity is discharged and the 

 repulsion over, the water*particles are free to unite, and to 

 descend to the earth in drops. This phenomenon we call 

 a thunderstorm. It is the discharge and passage of such 

 clouds which often prove destructive to telegraph lines and 

 stations, and still more often disturb the regular service. 



The most terrible effects are to be attributed to the electrical 

 discharge into the line, or direct stroke of the lightning. 

 This occurs when a charged cloud passes over and attracts, 

 in the earth's surface immediately underneath it, the opposite 

 electricity ; and where points occur over the surface, or any 

 object stands up high, these points and objects are charged 

 with greater tension during the passage of the cloud. The 

 induction between the cloud and the earth then resembles 

 that of a Ley den jar, the dielectric being represented by the 



