THUNDER AND LIGHTNING. 



But since a separate sound is produced at every point through 

 which the flash passes, and as these points are at distances from 

 the observer which vary according to the position, length, 

 direction, and form of the flash, it will follow necessarily that the 

 sounds produced by the same flash, though practically simul- 

 taneous, because of the great velocity with which the electricity 

 moves, arrive at the ear in comparatively slow succession. 



The varying loudness of the successive sounds heard in the 

 rolling of thunder, proceeds in part from the same causes as the 

 varying intensity of the light of the flash. But it may, perhaps,, 

 be more satisfactorily explained by the combination of the suc- 

 cessive discharges of the same cloud, rapidly succeeding each 

 other, and combining their effects with those arising from the 

 varying distances of different parts of the same flash. 



14. It appears to us that the varying intensity of the rolling of 

 thunder may also be very clearly and satisfactorily explained by 

 the zigzag form of the flash, combined with the effect of the 

 varying distance ; and it seems extraordinary that an explanation 

 so obvious has not been suggested. Let A, B, c, D (fig. 1), be a 



part of a zigzag flash seen by an observer at o. Taking o as a 

 centre, suppose arcs c c and B & of circles to be drawn, with o c 

 and o B as radii. It is clear that the points c and c, and B and &, 

 being respectively equally distant from the observer, the sounds- 

 produced there will be heard simultaneously, and, supposing 

 them equal, will produce the perception of a sound twice as loud 

 as either heard alone would do. All the points on the zigzag- 

 c B c b are so placed that three of them are equi- distant from o. 

 Thus, if with o as centre, and m as radius, a circular arc be 

 described, it will intersect the path of the lightning at three 

 points m, m', and m", and these three points being, therefore, at 

 the same distance from o, the sounds produced at them will 

 reach the observer at the same moment, and if they be equally 

 intense will produce on the ear the same effect as a single 

 sound three times as loud. The same will be true for all the 

 points of the zigzag between c and b. Thus, in this case, 

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