FORM AND LENGTH OF FLASH. 



come within striking distance, the contrary fluids rush to each 

 other, and an electrical discharge takes place. 



The clouds, however, unlike the metallic coatings of the jar, 

 are very imperfect conductors, and consequently, when discharged 

 at one part of their vast extent, they preserve elsewhere their 

 electricity in its original intensity. Thus, the first discharge, 

 instead of establishing equilibrium, rather disturbs it, for the 

 part of the cloud which is still charged is alone attracted by the 

 part of the other cloud in which the fluid has not yet been 

 neutralised. Hence arise various and complicated motions and 

 variations of form of the clouds, and a succession of discharges 

 between the same clouds must take placa before the electrical 

 equilibrium is established. This is necessarily attended by a corre- 

 sponding succession of flashes of lightning and claps of thunder. 



12. The form of the flash in the case of lightning, like that of 

 the spark taken from an electrified conductor, is zigzag. The 

 doublings or acute angles formed at the successive points when 

 the flash changes its direction vary in number and proximity. 

 The cause of this zigzag course, whether of the electric spark or 

 of lightning, has not been explained in any clear or satisfactory 

 manner. 



The length of the flashes of lightning also varies ; in some 

 cases they have been ascertained to extend to from two-and-a- 

 half to three miles. It is probable, if not certain, that the line 

 of light exhibited by flashes of forked lightning are not in reality 

 one continued line simultaneously luminous, but that on the con- 

 trary the light is developed successively as the electricity proceeds 

 in its course, the appearance of a continuous line of light being an 

 optical effect, analogous to the continuous line of light exhibited 

 when a lighted stick is moved rapidly in a circle, the same 

 explanation being applicable to the case of lightning. 



13. As the sound of thunder is produced by the passage of the 

 electric fluid through the air which it suddenly compresses, it is 

 evolved progressively along the entire space along which the 

 lightning moves. But since sound moves only at the rate of 1100 

 feet per second, while the transmission of light is so rapid that 

 in this case it may be considered as practically instantaneous, the 

 sound will not reach the ear for an interval greater or less after 

 the perception of the light, just as the flash of a gun is seen before 

 the report is heard. 



By noting the interval, therefore, which elapses between the 

 perception of the flash and that of the sound, the distance of 

 the point where the discharge takes place can be computed 

 approximately, by allowing 1100 feet for every second in the 

 interval. 



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