6 3 o SCIENCE PROGRESS 



sound-areas with regard to the source of sound. It is only in 

 the third of the accompanying maps that the normal relations 

 are clearly shown. For the Spithead minute-guns (Fig. i), the 

 inner sound-area cannot be mapped. In Fig. 2, the form of 

 this area is exceptional. As a general rule, the source of sound 

 lies close to one side or one end of the inner area (as in Fig. 3), 

 and the outer sound-area lies, not in the direction of greatest 

 extension of the inner area, but in exactly the opposite direction. 



That direction, as will be seen from the maps, is by no means 

 uniform. The outer sound-area lies to the north of the origin 

 for the Spithead minute-guns and the East London explosion, 

 towards the east in the Forde and Hayle explosions and the 

 Antwerp bombardment, and towards the west in the Wiener- 

 Neustadt explosion. In the Japanese volcanic explosions, it 

 lies on the west side of the Asamayama, but on the north side 

 of the Sakura-jima in the south of the empire. 



How closely the directions of the Asamayama sound-areas 

 are connected with the direction of the wind in the upper atmo- 

 sphere has been clearly shown by Prof. Omori. The column of 

 ashes from the volcano is shot up to a height of two or three 

 miles, to a region of the air in which the wind is generally from 

 the west. The ashes are thus carried in an easterly direction, 

 and are deposited along a narrow band, sometimes as much as 

 one hundred miles in length. The boundary of the inner sound- 

 area may be deformed slightly by local winds, but, as a whole, 

 the area is arranged more or less symmetrically about the band 

 of ashes. 



There is no hard-and-fast rule binding the position of the 

 outer sound-area with regard to the direction of the ashes-zone. 

 But it is noticeable that, when the ashes are carried due east, 

 the centre of the outer area lies to the west of the Asamayama. 

 When the ashes drift to the east-south-east, the centre of the 

 outer area is shifted towards the north ; and, when they are 

 borne to the east-north-east, the centre is deflected to the south. 

 Thus, as a rule, the direction of the outer area is nearly opposite 

 to that in which the band of ashes is drawn out by the wind. 



(4) The dimensions of the silent zone, and especially those 

 of its outer boundary, have an important bearing on the origin 

 of the zone. The mean radius of the boundary varies between 

 rather wide limits. In European explosions it may be as low 

 as 50 miles (Spithead minute-guns), or as high as 112 miles 

 (Jungfrau railway explosion). In the volcanic explosions of 

 Japan, the distance lies between 54 and 105 miles, or about the 

 same limits as in Europe. The average of all known measure- 

 ments of the outer radius is 87 miles. 



(5) One more property, not so much of the silent zone as of 

 the outer sound-area, may be noticed. Considering its distance 



