96 



KNOWLEDGE. 



March, 1913. 



them were extremely slight, only six being sensible 

 without instrumental aid. According to Professor 

 Omori, they belong to two types of movement. In 

 one, the shock consists only of minute quick 

 vibrations ; in the other, it begins with slow move- 

 ments, mingled after a few seconds with quick 

 vibrations. The shocks of the first tvpe originate at 

 a depth of two or three miles below the base of the 

 volcano ; the vibrations resemble those of ordinary 

 small local earthquakes, and they are probably due 

 to the formation of fissures caused by the under- 

 ground expansive force. The earthquakes of the 

 second type were invariably the results of volcanic 

 explosions. They began with a preliminary tremor 

 lasting about two and a half seconds, which Professor 

 Omori regards as due to the disturbances occurring 

 just before the actual explosion, such as the formation 

 or extension of an underground crack. The slow 

 vibrations, which follow the tremor, are probably of 

 " the nature of a bodily oscillation and due to the 

 first bulging up and the consequent outward forcing 

 of the mountain mass at the moment of the 

 explosion." 



The detailed study of the eruptions of the last 

 few years has led Professor Omori to some interesting 

 conclusions. Among the most valuable are those 

 which relate to the propagation of the detonations 

 resulting from the more important explosions. These 

 have been heard at places on the east coast at a 

 distance as great as one hundred and eighty miles ' 

 from the volcano, and no doubt would have been 



In most cases, the area is of the form indicated by 

 the lightly shaded portion in Figure 92, which repre- 

 sents the region throughout which the sound of the 



Figure 92. 



The sound of the eruption of Asama-yama on December 7th, 1909, 

 was heard on the lightly-shaded area, ashes fell on the darkly- 

 shaded region. 



heard farther if there had been land in this direction. 

 There are some curious anomalies in the forms of 

 the areas over which the detonations were audible. 



Figure 93. 



The shaded portions show the two areas in which the sound of the 

 eruption of Asama-yama was heard on December 25th, 1910. 



eruption of December 7th, 1909, was heard. In 

 Tokyo (eighty-five miles from the volcano), the 

 detonation was loud and like that caused by a powder 

 explosion, shaking the houses strongly, although 

 there was no movement of the ground. The sound 

 was heard at a distance of one hundred and ten 

 miles to the south, and one hundred and seventy 

 miles towards the north-east. Towards the west, 

 however, the sound-area extended but a short dis- 

 tance, for the detonation was inaudible at places 

 only fifteen miles to the west, and twenty-five miles 

 to the north-west of the mountain. The darkly- 

 shaded area represents that within which ashes were 

 precipitated. It is a lens-like zone, about one 

 hundred and twenty miles in length, extending from 

 the Asama-yama in a direction a little south of east. 

 The arrows show the direction of the surface wind 

 at the time of the eruption. In some cases, this 

 agrees roughly with the principal direction in which 

 sound and ashes were carried ; in others, it is almost 

 at right angles to this direction. The ashes, more- 

 over, travelled at the rate of about fifty miles an 

 hour, while the velocity of the surface wind was 

 generally less than twelve miles an hour. It will be 

 noticed, also, that the area of ash-precipation is 

 roughly symmetrical with respect to the sound-area. 

 It is, therefore, clear that the sound and ashes were 

 carried by the upper winds, which appear to have a 

 fairly uniform direction for the greater part of the 

 year between east-north-east and east-south-east, 

 tending as a rule in the latter direction. The height 



