October. 1912. 



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cloud half a inilr fuitlu'r (i\cilu ad. In this case the 

 sound of the flash will be spread over rather more 

 than two and a quarter seconds. If both clouds 

 were higher up, but the same distance apart, the 

 '■ period of total arrival," as we may call it, w ould 

 be the same, all the sound taking longer to tra\el 

 the increased distances, all arri\ing later b\- the 

 same interval of time. The Mash would make a crash 

 equalh' shar[) but less loud because further awaw 



Turning now to Figures 421 and 422, if a Hash 

 starting from a point E, a mile 

 awa\' from the observer, strikes 

 from cloud to cloud as in Figure 

 421, the noise from the far end ' 



reaches the observer about two ; 



and a quarter seconds after th 

 noise from the nearer end. II 

 however, a flash which is a mil 



away at its nearest point strikes i 



the cliff as in Figure 422, the j 



whole sound of the crack reaches I 



us in about one-seventh part of a 

 second. We should, therefore, 

 hear a crack about sixteen times as sharp as in 

 the former case. Being also nearer it would for 

 that reason be somewhat louder, but the effect of 

 the new direction is, clearl}-, far more important. 

 Apart from such differences as are produced by the 

 positions of the clouds which are between us and 

 parts of some of the flashes, we now see how it is 

 that a very brilliant flash such as that in Figure 421 

 might be, may produce a noise much less sharp than 

 that of a less vivid flash taking such a path as that 

 in Figure 422. 



It must not be supposed that the noise will be 

 greater because the flash strikes the ground. \Mien 



the water more quickly than through the air, for 

 sound travels more than four times as fast through 

 water as it does through air; but on land the differ- 

 ence is not so great. Through solid granite sound 



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electricity passes along, or is distributed in solids or 

 liquids, nothing happens at all corresponding to what 

 produces the crack or clap in the air : the expansion 

 due to the heat of the flash and the sudden contrac- 

 tion following it. When lightning strikes into or near 

 water the sound mav reach the observer bv wav of 



FtGURK 42.5. 



travels half as fast again as through air : but since its 

 speed through granite as it usually occurs in the hills 

 is not much greater than the speed through air ; 

 and since, too, the rate at which it travels through 

 wet sand is a good deal less than its speed through 

 air, it is clear that as a rule the sound coming 

 through the ground will lag behind that which 

 comes to us directly from the flash. 



.\ Flash Striking the Ground. 



When a flash strikes the ground the sharpness of 

 the crash mav depend upon where it strikes, and the 

 nearer stroke is not necessarily the louder — strength 

 and length being equal. Suppose 

 that a flash starting from H in 

 Figure 423, strikes the ground 

 at Q, the difference between the 

 nearest distance and the furthest 

 distance is six hundred and 

 twenty feet giving a " period of 

 total arrival" of rather more 

 than half a second. If a similar 

 flash strikes at P, the nearest 

 point is three thousand five 

 hundred and forty feet awa}', 

 the furthest five thousand two 

 hundred and eighty feet, and the 

 difference one thousand seven 

 hundred and forty feet, giving 

 a period of total arrival of more 

 than one and a half seconds. 

 In the former case the handicap 

 is from Q to X, in the latter, 

 This figure, in which an arc 

 centre at A is drawn through 

 suggests that some of the loudest 

 crashes and those that are very loud considering 

 their distance from the observer, are due to the 

 htning following a path which approximates to 



from H to 

 of a circle 

 H and X, 



W. 

 with 



