Vol. 60.] DERBY EARTHQUAKES OF 1903. 223 



The time of the first disturbance is 13.30.44 [that is, l h 30 U1 44^ p.m.], as nearly 

 as it can be read off the diagram. The record gives evidence of but one impulse 

 which has moved the pendulum towards the west : the subsequent oscillations 

 of the pendulum, which are carried on for about 55 seconds, being due to the 

 original disturbance. The greatest amplitude of oscillation is about - 7 milli- 

 metre, and, as the movement dies away, there is no trace of the ordinary period 

 of the pendulum, which is about 16 seconds. The vibrations appear to have 

 accomplished themselves in a shorter time, so that the successive vibrations have 

 run into each other.' 



One of the most interesting features of this record is the fact that 

 only one impulse was detected. Bidston being so close to the 

 rectilinear band, the interval between the two prominent vibrations 

 was too short to allow of their separate registration. 



At the time of the earthquake, as Dr. Wiechert kindly informs me, 

 rather strong pulsations were being registered by his pendulums at 

 Gottingeu ; and, on this account, all measurements are to some 

 extent uncertain. The determinations of the epochs, for instance, 

 mav err bv as much as 5 seconds on either side of the times given. 

 The preliminary tremors, though very small, were distinctly recog- 

 nized with the aid of a lens, beginning at l h 33 m 32 s p. 31. (G.M.T.); 

 their period was about 1 second, and their amplitude about -0001 

 millimetre. They were succeeded by a series of larger waves, 

 beginning at l h 34 m 20 s , and attaining their maximum at l h 34 m 40 s , 

 with a period of between 2 and 3 seconds and an amplitude of about 

 •0007 millimetre. The total duration of the movement was about 

 1 j minutes. 



Velocity of the Earth-Waves. 



The most accurate determinations of the time are probablv those 

 given by the pendulums at Birmingham and Gottingeu. the distances 

 of which places from the epicentre are 66 and SOS kilometres 

 respectively, or 41 and 502 miles. The interval between the 

 arrival of the first vibrations at these places being 193 seconds, and 

 of the maximum of the principal waves 257 seconds, it follows 

 that the preliminary tremors travelled with a velocity of 3*8 kilo- 

 metres (or 2-4 miles) per second, and the larger waves at the rate 

 of 2*9 kilometres (or 1*8 miles) per second. The former of these 

 values many be inaccurate, for we cannot be certain that the first 

 tremors recorded in Birmingham corresponded with those registered 

 in Gottingen ; the latter value agrees closely with the estimates 

 made for many other earthquakes. 



Sotjxd-Phenomexa . 

 Isacoustic Lines and Sound-Area. 



As persons differ considerably in their powers of hearing very 

 deep sounds, the short-period vibrations, in spreading outwards 

 from the origin, tend to become inaudible to a continuallv in- 

 creasing number of observers ; and the rate of decline in audibility 

 may be represented by a series of isacoustic lines, or curves drawn 



