POPULAR SCIENCE 629 



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of this explosion, there were other air-waves, inaudible to man, 

 but manifested by the shaking of windows and the quivering 

 of trees on which pheasants were roosting. These air-waves 

 evidently travelled along a somewhat less lofty course than the 

 sound-waves. They spread, not only over the two sound-areas, 

 but across the intervening silent zone, at the western end of 

 which their effects were frequently noticed. There can be 

 little doubt, I think, that, if the sound-waves had taken a lower 

 path, the report would have been heard right across this end 

 of the zone. The two sound-areas would then have coalesced, 

 and the silent zone would have been merely a deep gulf in a 

 continuous sound-area. An explosion at Kobe, in southern 

 Japan, on April 3, 19 10, seems to have provided a sound-area 

 of this type, but the observations are insufficient to place the 

 matter beyond doubt. 



(2) The number of explosions in which the form of the sound- 

 area has been traced is comparatively small. In Europe, the 

 silent zone has been mapped seven times during the present 

 century — namely, for the Spithead minute-guns on February 1 , 

 1 90 1, an explosion of dynamite at Forde (Westphalia) on Decem- 

 ber 14, 1903, of nitroglycerine at Hayle (Cornwall) on January 

 5, 1904, of dynamite on the Jungfrau railway on November 15, 

 1908, and of gunpowder at Wiener-Neustadt on June 7, 191 2, 

 the bombardment of Antwerp on October 8, 19 14, and the muni- 

 tions explosion in East London on January 19, 191 7. On the 

 other hand, no trace of a silent zone has been discerned in the 

 sound-areas of the Spithead naval reviews of July 17, 1867, and 

 June 26, 1897, the Avigliana (Northern Italy) explosion of Janu- 

 ary 16, 1900, or the Cherbourg naval review of July 17, 1900. 



In Japan, Prof. Omori has studied the sound-areas of 

 twenty-two important explosions of the Asamayama, from 

 1 9 10 to 191 3, and he finds that single sound-areas and double 

 sound-areas occur with equal frequency. Of the single sound- 

 areas, nine were those of explosions which occurred in the six 

 winter months, and two of explosions in the six summer months ; 

 of the double sound-areas, ten were those of explosions in the 

 summer half of the year, and one of an explosion in the winter 

 half. 



The materials are perhaps insufficient to lead to any general 

 law. It is clear, however, that the silent zone is not present in 

 all explosion sound-areas, that roughly in great explosions it is 

 absent as often as it is present ; and it would seem probable that 

 in European explosions it is chiefly manifested in the winter half 

 of the year, and in the Japanese volcanic explosions in the 

 summer half. 



(3) A third property of the silent zone, of no less significance 

 than its intermittent existence, is the positions of the two 



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