Dr. C. Davison — British Earthquakes, 1900. 359 



OcHiL Earthquakes of Sept. 17 and 22, 1900. 



The two undoubted earthquakes occurred on Sept. 17 at 10.15 p.m. 

 and Sept. 22 at 4.30 p.m. There were also four other reported 

 shocks, whose seismic character is not established, at the following 

 times : — 



(a) Sept. 17, 3.30 p.m., Menstrie. 



(6) Sept. 17, 10.5 p.m., Alva. 



(c) Sept. 18, 2 a.m., Alva. 



{d) Sept. 18, about 2.55 a.m.. Bridge of Allan. 



The first two were noticed by several persons at each of the places 

 mentioned, but I have no record of them except the statement that 

 slight shocks were felt. 



Earthquake of Sept. 17, at 10.15 p.m. — I have received BQ accounts 

 of this earthquake from 26 places, in addition to negative records 

 from 11 other places.^ The epicentre is situated among the Ochil 

 Hills, and consequently the intensity of the shock in the central 

 region is unknown. At places near the boundary, in the valleys of 

 the Forth and Allan, the intensity was 4, and it can hardly have 

 exceeded this degree in any part of the disturbed area. 



The boundary of the disturbed area, which corresponds to an 

 isoseismal line of intensity slightly less than 4, is roughly elliptical 

 in form, 15 miles long and 9^ miles broad, and includes 117 square 

 miles. Its longer axis is directed E, 13° N. and W. 13° S., and the 

 centre of the area is 3 miles N. 32° W. of Alva. In spite of the 

 absence of observations from the neighbourhood of Glendevon, it is 

 probable that the curve is drawn with a fair approach to accuracy. 



The shock seems to have been nearly uniform in its character all 

 over the disturbed area, a single prominent vibration succeeded by 

 a tremor such as would be caused by a heavy weight falling on the 

 floor and making the building shake, and lasting altogether not more 

 than three seconds. 



Of the 55 observers who provide detailed accounts of the earth- 

 quake, 48 distinctly heard the sound, 4 are doubtful or fail to 

 answer the question, while 3 state that they heard no sound at all. 

 Thus, the percentage of those who heard the sound is not less 

 than 87. A few observers describe the sound as a loud sharp crash 

 or a low rumbling sound, while as many as 41 refer it to one of the 

 ordinary types. Of these, 29 per cent, compared it to the noise of 

 heavy waggons or traction-engines passing, 10 per cent, to thunder, 

 5 per cent, to wind, 12 per cent, to the tipping of a load of coal or 

 bricks, 17 per cent, to the fall of a heavy body or the banging of 

 a door, 12 per cent, to blasting or explosions, and 15 per cent, to 

 miscellaneous sounds, such as the trampling of horses, a distant 

 waterfall, a large flock of pheasants flying over the house, or the 

 rush of heavy rain against the window. On the whole the frequency 

 of comparison to sounds of short duration is unusual, and this no 

 doubt is due mainly to the prominence of the heavy thud that 



' The outer curve ou the map aud all places marked refer to this earthquake. 



