276 Mr. C. Davison. [June 10, 



connected with the first of the 1ST.W. and S.'B. faults of the Pentlands 

 on the north-west side of the axis ; a fault which passes close by the 

 centre of the sound-area, and has a downthrow to the north-west. 

 The inclination of this fault is unknown, but is probably about 75 

 to the horizon ; the depth of the seismic force may therefore be about 

 8| miles. It is shown that the earthquake was probably caused by 

 the impulsive friction produced by a slip of the fault referred to ; 

 that this slip took place near the middle of the length of the fault ; 

 that the slip increased the throw of the fault ; that the slip-area was 

 very short, possibly less than a mile in horizontal length, but that it 

 extended from a depth of several miles to within a short distance of 

 the surface. 



2. Lancashire Earthquake, February 10. 22 h. 36 m. Intensity VI. 

 The disturbed area is nearly circular in form, about 55 miles in 

 diameter, and 2480 square miles in area. The nature of the shock 

 varied with the position of the place of observation. In, or nearly 

 in, a line with the Irwell fault the number of vibrations was gener- 

 ally greater than in places more remote. As in the Edinburgh 

 earthquake, the usual sounds were heard in many places which are 

 confined to a nearly circular area, which is neither coextensive nor 

 concentric with the disturbed area. The duration of the sound was 

 generally greater at places in, or nearly in, a line with the Irwell 

 fault than at places more remote. The epicentrum, which is prob- 

 ably coincident with the common centre of the disturbed area and 

 of the isoseismal line of intensity V, is at a point 2 miles N.N.E. of 

 Bolton, and the centre of the sound-area is about 3^ miles S.S.E. of 

 the epicentrum. The earthquake was probably caused by a slip 

 of the great Irwell fault, which passes close by the centre of the 

 sound-area, having a downthrow to the north-east. If so, the slip 

 must have increased the throw of the fault. The horizontal length 

 of the slip-area was possibly not much more than a mile. The 

 seismic focus is perhaps at a depth of about 3| miles, but the slip 

 seems to have extended to within a short distance of the surface. 



The excentricity of the sound-area in these two cases throws light 

 on the origin of the sound- vibrations. Seismographic records show 

 that near the beginning of an earthquake the period increases with 

 the amplitude, and it is suggested that the sound- vibrations are the 

 very minute vibrations of rhort period which proceed from the upper 

 and lateral margins of tfl n '^lip-area. It is pointed out that this 

 theory explains all the knotfK characteristics of earthquake-sounds. 



3. Ben Nevis Earthquake, May 22. 13 h. 58 m. Intensity about 

 IV. This bhock may have been connected with the great fault which 

 crosses Scotland from Inverness in a south-west direction. 



4. Kintyre Earthquake, July 15. About 18 h. Intensity V. The 

 disturbed area is roughly elliptical in form, the longer axis being in 



