256 ELEMENTARY LESSONS ON [CHAP. iv. 



ing at the surface of the cloud, and the other parts of 

 the cloud will now react upon the discharged portion, 

 producing internal attractions and internal discharges. 

 The internal actions thus set up will account for the 

 usual appearance of a thundercloud, that it is a well- 

 defined flat-bottomed mass of cloud which appears at the 

 top to be boiling or heaving up with continual move- 

 ments. 



3O4. Lightning and Thunder. Three kinds of 

 lightning have been distinguished by Arago : (i.) The 

 Zig-zag flash or " Forked lightning" of ordinary occur- 

 rence. The zig-zag form is probably due either to the 

 presence of solid particles in the air or to local electrifi- 

 cation at certain points, making the crooked path the 

 one of least resistance, (ii.) Sheet lightning, in which 

 whole surfaces are lit up at once, is probably only the 

 reflection on the clouds of a flash taking place at some 

 other part of the sky. It is often seen on the horizon at 

 night, reflected from a storm too far away to produce 

 audible thunder, and is then known as " summer light- 

 ning." (iii.) Globular lightning, in the form of balls of 

 fire, which move slowly along and then burst with a 

 sudden explosion. This form is very rare, but must be 

 admitted as a real phenomenon, though some of the 

 accounts of it are greatly exaggerated. Similar phe- 

 nomena on a small scale have been produced (though 

 usually accidentally) w r ith electrical apparatus. Cavallo 

 gives an account of a fireball slowly creeping up the 

 brass wire of a large highly charged Leyden jar, and 

 then exploding as it descended ; and Plante* has recently 

 observed similar but smaller globular discharges from 

 his "rheostatic machine" charged by powerful second- 

 ary batteries. 



The sound of the thunder may vary with the con- 

 ditions of the lightning spark. The spark heats the air 

 in its path, causing sudden expansion and compression 

 all round, followed by as sudden a rush of air into the 



