KNOWLKDGE. 



OCTOHEK, 1912. 



strcnj^tli of tlic Hash docs, of course, mako a differ- 

 ence : Init there is something else whicli will 

 seemingly make the noise of one flash many times 

 louder than that of another which is quite as strong 

 and is no further away. This is the 

 direction taken by the flash relatively 

 to tin- (losition of the observer. 



Why TiiiiNDEKCi-Ais \'.\ky. 



Probably a good many of us have 

 noticed that a very brilliant flash, 

 quickly followed by thunder and there- 

 fore quite near, often causes onl}- a 

 moderately loud tearing noise, though 

 a long and loud peal follows it. Soon 

 after, perhaps, there comes a less vivid 

 flash followed by an explosive crash or 

 crack of great violence, though the 

 peal which follows it is less loud than 

 that which followed the gentler rending 

 noise of the preceding flash. The 

 explanation is very simple ; the length 

 and strength of the peal depend mainlv 

 upon the strength and size of the 

 flash and only partly upon its position, 

 while the loudness and sharpness of 

 the crack, which comes before the peal, 

 depends chiefly upon the direction 

 taken by the flash. The crack, or 

 rending noise, comes from the flash 

 itself ; the peal consists of echoes of this noise, 

 coming from the clouds. 



Now, the noise of the actual flash comes to us. 

 not from one spot, but from all along the path of the 

 flash, and it is because of the length of this path 

 that the time taken for the sound to come from the 

 farther end of it is much greater than the time taken 

 for the sound to 

 come from the 

 nearer end. It 

 is only a matter 

 of seconds, but 

 it makes all the 

 difference in the 

 sharpness and 

 loudness of the 

 crack or rending 

 noise. If the 

 whole of this 

 noise, from all 

 along the path 

 of the flash, 

 reaches us in a 

 quarter of a 

 second, say, it 

 sounds like a 

 terrific thump 



or sudden crash ; but if it takes two or three 

 seconds for all of it to reach us, we hear it as a long 

 rending noise. By noting with accuracy the dura- 

 tion of the rending noise of a big flash which could 

 be identified by the exact time at which it occurred, 



FlGlRK 420. 



three or four pairs of observers could measure the 

 length of a flash of lightning and fix its position in 

 the sk\'. The observers should be in pairs, and one 

 of each jiair would note the interval between the 

 flash and the beginning of the crash, 

 so as to get the distance, while the 

 -^ _^ other, with a stop-watch, noted the 



duration of the crash. The possibility 

 of doing this depends upon the two 

 facts, that the time taken by the flash 

 to traverse its path is immeasurably 

 short, and that the time taken for the 

 light of it to travel to the observer 

 may also be counted as nothing. 



Some Typic.\l Ex.\mples. 



The differences in the positions of 

 ~ flashes relative to the observer, and the 

 effects on the duration of the crash or 

 rending noise as distinct from the 

 pealing echoes, are illustrated by the 

 accompanying diagrams. If, for in- 

 stance, the observer is at the point A 

 in I-'igure 418, and a flash of lightning 

 half a mile long and half a mile awav 

 from him passes from B in the cloud 

 to C" in another cloud or on the hill, 

 the sound from the two ends of the 

 flash will reach him in twenty-nine 

 hundredths — less than one-third — of a 

 fter that from the middle of the flash, 

 onlv has to travel three hundred and 

 Sound travels through air at 



second 

 since it 



twenty feet further 

 a speed of one thousand one hundred and forty- 

 two feet per second. Suppose, now, that a flash 

 half a mile long occurs as in Figure 419, passing 

 from 1) to E. then the difference in distance between 



the longest path 

 ^ and the shortest 



path traversed 

 by the sound is, 

 in round figures, 

 one thousand 

 and ninety feet, 

 and the noise 

 from E will 

 reach the ob- 

 server nearly a 

 second later 

 than that from 

 D. He will hear 

 the noise of the 

 crash spread out 

 as a rending 

 sound over the 

 space of one 

 second, and the 

 noise will only be about a third as sharp as that of 

 the former flash of the same strength. 



Let us now look at Figure 4_'0, which represents 

 a flash half a mile long, passing between a cloud 

 w hich is half a mile above the observer and another 



