ATMOSPHERIC ELECTRICITY. 392 



Wh B this ■^^^ experiment, as thus performed, was one of great risk 

 experiment one and danger, since the whole amount of electricity contained in 

 eerf^^' ^^^ ^^'^ thimder-cloud was liable to pass from it, by means of 

 the string, to the earth, notwithstanding the use of the silk 

 insulator.* 



Wh t ■ fh ^rom whatever cause electricity is present in the air, the 



cause of light- clouds appear to collect and retain it ; and when a cloud over- 

 '""S ^ charged with electric fluid approaches another which is under- 



charged, the fluid rushes from the former into the latter. In a like manner, 

 the fluid may pass from the cloud to the earth, and in such cases elevated 

 objects upon the earth's surface, as trees, steeples, etc , appear to govern its 

 direction. 



. "When a cloud highly charged with electricity is near to the 



circumstances earth, the surface of the earth, for a great extent, may also 

 does lightning become highly charged by induction ; and when the tension 

 earth to the of the electricity becomes sufficiently great, or the two elec- 

 clouds . ^j.jg gmfaces come sufiBciently near, a flash of lightning not 



unfrequently passes from the earth to the clouds. In this way an equilibrium 

 of the two elements is restored. 



Lightning clouds are sometimes greatly elevated above the surface of the 

 earth, and sometimes actually touch the earth with one of their edges ; they 

 are, however, rarely discharged in a thunder-storm when they are more than 

 700 yards above the surface of the earth. 



„ 765. Li";htninor has teen divided into three 



How many , . . 



kinds of light- kinds, viz., zio-zao;, or chain-licrhtninff, sheet- 



mngare there? . , 



lightning, and ball-lightning. 

 Explain the The zigzag, or forked appearance of hghtning, is believed to 

 diverse ."ppear- ^e occasioned by the resistance of the air, which diverts tho 

 ance of light- electric current from a direct course. Tho globular form of 



lightning sometimes observed, is not satisfactorily accounted 

 for. "What is called "sheet," or "heat" lightning, is sometimes the reflection 

 in the atmosphere of lightning very remote, or not distinctly visible ; but gen- 

 erally this phenomenon is occasioned by the play of silent flashes of electricity 

 between the clouds, the amount of electricity developed not being suflScient to 

 produce any other effects than the mere flash of light. 



76G. The usual explanation of thunder is, 



VfTiat is the 



cause of thun- that it is duc to a sudden displacement of the 



particles of air by the electrical current. Others 



have supposed that the passage of the electricity creates 



* When the experiment was subsequently repeated in France, streams of electric fire, 

 nine and ten feet in length, and an inch in thickness, darted spontaneously with loud re- 

 ports from the end of the string confining the kite. During the succeeding year. Prof. 

 Eichman of St. Petersburg, in making experiments somewhat Eimilar, and havias hi« 

 apparatus entirely insulated, was immediately killed. 



17* 



