﻿Vol. 6 I.] DEEBY EARTHQUAKES OF 1904. 13 



director of the Liverpool Observatory at Bidston, near Birkenhead, 

 informs me that, although the shock was felt by many persons 

 there, the trace of the Milne seismograph shows but the slightest 

 disturbance at 3 h 18*8 m p.m. In Birmingham, there was apparently 

 no movement of the Omori horizontal pendulum, and only a minute 

 displacement of the pointer of the Ewing duplex pendulum. The 

 absence of a clearer record is probably due to the comparative 

 weakness of the original disturbance, to the irresponsiveness of 

 long-period horizontal pendulums to rapid vibrations, and, in the 

 case of Birmingham, to the fact that the direction of the epicentre 

 lies very nearly in the plane of the pendulum. 



Isacoustic Lines and Sound-Area. 



In drawing the isacoustic lines for the earthquake of 1903, the 

 whole area was divided into squares by north-to-south and east- 

 to-west lines 10 miles apart. I have made use of the same squares 

 for the recent shock, but have calculated the percentage of audibility 

 corresponding to the corners of these squares, making use of the 

 records within the four contiguous squares. In this way a larger 

 number of observations is at our disposal, the only effect of such 

 a treatment being to smooth away local inequalities in the resulting 

 curves. Notwithstanding this advantage, only one isacoustic line, 

 corresponding to a percentage of 80, can be satisfactorily drawn. 

 This is represented by the irregular dotted line on the map (PI. II). 

 Its peculiar form, especially the branches to the west and south- 

 east, is evident at once. In the former direction, the number of 

 records is too small to complete the curve, and its course may not 

 be quite accurately laid down, but the branch evidently extends 

 some distance farther towards the west. For the south-eastern 

 branch, the observations are more numerous, and it will bo noticed 

 that it lies almost exactly along the course of the median line 

 of the hyperbolic band. The explanation of this peculiarity, of 

 course, is that the sound-waves from the two foci coalesced along 

 the hyperbolic band, and were therefore audible to a larger number 

 of observers. Prom the study of the sound-phenomena, we are 

 therefore again led to the conclusion that the north-eastern focus 

 was in action slightly before the other. With regard to the less- 

 pronounced expansion of the curve towards the north, I can offer 

 no satisfactory explanation. 



With a few isolated exceptions, the sound was heard within the 

 area bounded by the outer dotted line in PI. II, a line which in no 

 X>lace deviates more than 4| miles from the isoseismal 4, over- 

 lapping that curve towards the east, south, and west, but not 

 extending quite so far towards the north. Its dimensions are 

 121 miles from north-west to south-east, and 113 miles from north- 

 east to south-west, and its area covers about 10,700 square miles. 

 The exceptional records come from Branston Green (13 miles to 

 the north of the boundary), Lytham (21 miles to the north-west) 



