CAUSE OF THE ROLLING OF THUNDER. 



supposing the intensity of the lightning to be uniform from A to 

 D, there will be three degrees of loudness in the sound produced, 

 the least between A and c and between b and D, the greatest 

 between c and b along the zigzag, and the intermediate at the 

 points c c and B b. 



It is evident, that from the infinite variety of form and position 

 with relation to the observer, of which the course of the lightning 

 is susceptible, the variations of intensity of the rolling of thunder 

 which may be explained in this way have no limit. 



15. Since the loudness of a sound diminishes as the square of 

 the distance of the observer is increased, it is clear that this affords 

 another means of explaining the varying loudness of the rolling of 

 thunder. 



16. As the rolling of thunder is much more varied and of 

 longer continuance in mountainous regions than in open plain 

 countries, it is no doubt also affected by reverberation from every 

 surface capable of reflecting sound, which it encounters. A 

 part therefore of the rolling must be in such cases the effect 

 of echo. 



It has been also conjectured that the acoustic effects are 

 modified by the effects of interference. 



17. A cloud charged with electricity, whatever be the quality of 

 the fluid or the state of the atmosphere around it, exercises by 

 induction an action on all bodies upon the earth's surface imme- 

 diately under it. It has a tendency to decompose their natural 

 electricity, repelling the fluid of the same name, and attracting to 

 the highest points the fluid of a contrary name. The effects thus 

 actually produced upon objects exposed to such induction, will 

 depend on the intensity and quality of the electricity with which 

 the cloud is charged, its distance, the conductibility of the 

 materials of which the bodies affected consist, their magnitude, 

 position, and, above all, their form. 



Water being a much better conductor than earth in any state 

 of aggregation, thunder clouds act with great energy on the sea, 

 lakes, and other large collections of water. The flash has a 

 tendency to pass between the cloud and the water, just as the 

 spark passes between the conductor of an electric machine and the 

 hand presented to it. 



18. This explains the fact that lightning sometimes penetrates 

 strata of the solid ground, under which subterranean reservoirs of 

 water are found. The water of such reservoirs is affected by the 

 inductive action of an electrified cloud, and in its turn reacts 

 upon the cloud, as one coating of a Ley den jar reacts upon the 

 other. When this mutual action is sufficiently strong to over- 

 come the resistance of the subjacent atmosphere, and the strata of 



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