356 DE. CHAELES DAVISON ON THE [A.Ug. 1907, 



Instrumental Records. 



The seismographic records of the earthquake do not add to our 

 knowledge of its nature. It was recorded at 9 h 45* m a.m. by Milne 

 horizontal pendulums at Shide and Bidston, which are 135 and 

 130 miles respectively from the principal epicentre. At Shide, 

 the maximum amplitude was 0*2 millimetre and the duration of 

 the disturbance 3 minutes ; at Bidston, the maximum amplitude 

 was 0-5 mm. and the duration 6-8 minutes. The register of the 

 Omori horizontal pendulum at Birmingham, which is 110 miles 

 from the principal epicentre, shows with the aid of the lens a 

 series of minute waves or rather serrations, lasting for about 

 a quarter of a minute. At Newport (Mon.), 45 miles east of the 

 epicentre, the pressure-gauges at the gas-works recorded the earth- 

 quake-vibrations at about 9.50 a.m. (the time not being exact 

 within a few minutes), similar marks being also made on the 

 barograph and thermograph charts. Lastly, in London (in 

 Finsbury Square, E.G.), 170 miles east of the epicentre, a piece 

 of magnetite suspended by a fine wire in a glass case oscillated 

 distinctly about 9.45 a.m. 



Sound-Area. 



Towards the south and east, the boundary of the ■ sound-area 

 cannot be distinguished from the isoseismal 4, and the two curves 

 probably coincide approximately towards the west. The boundary 

 of the sound-area, however, extends about 15 miles north of the 

 isoseismal 4 (as shown by the dotted line in PI. XXV), and 

 thus the length of the sound-area is about 237 miles from east to 

 west, its width 218 miles, and it covers about 40,600 square miles. 



For a considerable distance from the epicentre, the sound was 

 heard by nearly all observers, the percentage of audibility being 

 97 within the isoseismal 8, 90 between the isoseismals 8 and 7, 

 and 92 between the isoseismals 7 and 6. Beyond the isoseismal 6, 

 the intensity of the sound diminished somewhat rapidly, being 75 

 between the isoseismals 6 and 5, 60 between the isoseismals 5 and 

 4, and 25 between the latter isoseismal and the boundary of the 

 sound-area. Throughout the whole of this area, 85 per cent, of" 

 the observers heard the earthquake-sound. 



The uniformly-high percentage of audibility in the region sur- 

 rounding the epicentres, from which alone numerous observations 

 are forthcoming, renders it impossible to construct the isacoustic 

 lines of the earthquake. Prom the distribution of the perceptible 

 variations in audibility-percentage, it is clear, however, that the 

 isacoustic lines do not present those marked distortions outwards 

 which are noticeable when the two parts of the shock coalesce 

 along a synkinetic band. 1 



1 As, for example, in the Derby earthquakes of 1903 & 1904 ; see Quart. 

 Journ. Geol. Soc. vol. lx (1904) pi. xix, and vol. lxi (1905) pi. ii. 



