526 



THUNDER-STORMS. 



1 *^-^-^l^ 



A B, one on each side of L 1 ; let other two lines, O L 3 , exceeding O L 2 

 by one hundred and ten feet, be also inflected on A B, and in the same manner 

 let a series 'ot lines, such as L 2 , O L 3 , O L 4 , be successively inflected on A 

 B, each line exceeding that which precedes it by one hundred and ten feet. If 

 we suppose sounds to be simultaneously produced at the points L J , L 2 , L 3 , 

 thai v.hich is produced at L 1 will be first heard by the observer. Since sound 

 iiK.'Vfs at the rate of eleven hundred feet per second, it will take the tenth of a 

 second to move through one hundred and ten feet ; therefore the two sounds 

 emitted at L 2 will arrive together at the ear of the observer a tenth of a second 

 alter the sound at L 1 has been heard. In the same manner, the two sounds 

 eimued at L 3 will arrive after another ten-th of a second, and so on. Thus ev- 

 ery ten sounds of the series, though simultaneously produced, would take a 

 second in being heard, and would be recognised by the ear as a distinct, though 

 rapid succession of ten sounds. 



If it be admitted, then, that the electric fluid, in passing through the air with 

 the great velocity it is proved to have by the experiments of Professor Wheat- 

 stone, produces sonorous vibrations of this kind in the air, the rolling of thun- 

 der vvourd be a necessary consequence. 



According to this manner of viewing the phenomena, the thunder would be 

 loudest which proceeds from L 1 , the nearest point to the observer, and would 

 gradually be enfeebled for points more and more distant from L 1 . Therefore 

 the roll would always be loudest at the commencement, and would gradually 

 diminish in force until it becomes inaudible. This is not in accordance with 

 the actual phenomena. 



But the preceding explanation proceeds on the supposition that the lightning; 

 moves continually in the same straight line. Let us see what the effects of a 

 zigzag course would be, such as that represented by the line A, B. Taking 



Fig. 4. 



the place of the observer, O, as a common centre, let a series of circular arcs 

 be drawn with radii increasing in magnitude each successive distance exceed- 

 ing the last by one hundred and ten feet. These arcs will intersect the zigzag 

 course of the lightning in several points more or less in number, according to 

 the position of the directions of the lightning, and the magnitude of the radius 

 of the circle. The first sound which will reach the observer will be that pro- 

 duced at the points where the least of the circles meets the lightning, and the 

 succeeding sounds will correspond to those emitted at the point of intersection 

 of the succeeding circles with the course of the lightning. It is easy to con- 

 ceive, that the mutual position of the zigzag lightning and the observer may be 

 such that the number of points of intersection of the circles with the lightning 

 may alternately augment and diminish in a manner corresponding to any sup- 

 posable variations in the intensity of the rolling of the thunder. 



It is evident that, independently of the infinite varieties of sound capable of 

 being explained by this hypothesis applied to zigzag lightnings, the changes 

 are not le*,s various for lightning which preserves a single course, the same 



