358 



EE. CHARLES DAVISON ON THE 



[Aug. 1907 



per cent, in which the duration of the sound was greater than, 

 equal to, or less than, that of the shock. 



Table II. 



Beginning. 



End. 



Relative 

 Duration. 



V 



c 



33 



31 

 24 

 22 

 15 



28 



28 



/ 



4 



5 



8 



29 



46 



10 



6 



P 



26 



16 

 11 

 13 

 20 



17 



15 



c 



59 



65 

 60 

 42 

 30 



59 



47 



/ 



15 



19 

 29 

 45 

 50 



24 



38 



9 



53 



60 



72 

 56 



61 

 65 



e 



42 



37 

 28 

 39 



35 

 30 



I 



5 

 3 

 5 



4 

 5 



Within isoseismal 8 



63 



64 



68 

 49 

 39 



62 



66 





7 „ 6 



6 „ 5 



, 4 









The number of observations on the epoch of maximum intensity 

 of the sound and shock is 37. The instant when the sound 

 was loudest preceded that when the shock was strongest in 

 30 per cent, of the records, coincided with it in 62, and followed 

 it in 8 per cent. 



Observations in Mines. 



That the intensity of an earthquake is much less in mines than 

 at the surface has long been known ; but, until lately, our know- 

 ledge has been based on isolated observations. In the Hereford 

 earthquake of 1896, observations were made in seven mines, the 

 earthquake being perceived as far as Chasetown, near Walsall, 

 54 miles from the epicentre. In the more distant mines, however, 

 the shock was imperceptible, and the sound alone was noticed. 1 

 The Derby earthquake of 1903 was observed in thirty-two mines, 

 several of them about 20 miles from the epicentre. In the more 

 distant mines, the shock as a rule was not felt ; in several, the sound 

 appeared to be more overhead than below ; and in one mine at 

 Swadlincote (20 miles from the centre), the intensity of the sound 

 was greater in the lower than in the higher workings. 2 



As the district bounded by the isoseismal 7 is one of the most 

 important mining districts in Great Britain, I endeavoured to 

 obtain further information on these points, and, through the 

 courtesy of many managers, engineers, and others, have received 

 53 records from 39 pits. These are distributed over an area 



1 'The Hereford Earthquake of December 17th, 1896' 8vo. Birmingham, 

 1899, pp. 278-80. 



2 Quart. Journ. Geol. Soe. vol. lx (1904) pp. 227-28. 



