124 CHAPTER Or VARIETIES. 



cloud. And that such is the case, has been ascertained by direct 

 observation on the sound of cannon. Messrs. Arago, Mathieu, and 

 Prony, in their experiments on the velocity of sound, observed that, 

 under a perfectly clear sky, the explosions of their guns were always 

 heard single and sharp, whereas when the sky was overcast, or 

 even when a cloud came in sight over any considerable part of the 

 horizon, they were frequently accompanied with a long continued 

 roll like thunder, and occasionally a double sound would arrive from 

 a single shot. 



But there is, doubtless, also another cause for the rolling of 

 thunder, as well as for all its sudden and capricious bursts and vari- 

 ations of intensity, of which our knowledge of the velocity of sound 

 furnishes a perfect explanation. To understand this, we must pre- 

 mise, ceeteris paribus, the estimated intensity of a sound will be pro- 

 portional to the quantity of it, (if we may so express ourselves,) which 

 reaches the ear in a given time. Two blows equally loud, at 

 precisely the same distance from the ear, will sound as one of double 

 the intensity ; a hundred, struck in an instant of time, will sound as 

 one blow a hundred times more intense than if they followed in such 

 slow succession that the ear could appreciate them singly. Now let 

 us conceive two equal flashes of lightning, each four miles long, both 

 beginning at points equi-distant from the auditor, but the one running 

 out in a straight line directly away from him ; the other describing 

 an arc of a circle having him in its centre. Since the velocity of 

 electricity is incomparably greater than that of sound, the thunder 

 may be regarded as originating at one and the same instant in every 

 point of the course of either flash, but it will reach the ear under 

 very different circumstances in the two cases. 



In that of the circular flash, the sound from every point will arrive 

 at the same instant, and affect the ear as a single explosion of stun- 

 ning loudness. In that of the rectilinear flash, on the other hand, 

 the sound from the nearest point will arrive sooner than from those 

 at a greater distance 3 and those from different points will arrive in 

 succession, occupying altogether a time equal to that required by 

 sound to run over four miles, or about twenty seconds. Thus the 

 same amount of sound is in the latter case distributed uniformly 

 over twenty seconds of time, which in the former arrives at a single 

 burst 5 of course, it will have the effect of a long roar, diminishing 

 hi intensity as it comes from a greater and greater distance. If the 

 flash be inclined in direction, the sound will reach the ear more 

 compactly, (i. e. in shorter time from its commencement,} and be 



