Vol. 60.] THE DERBY EARTHQUAKES OF 1903. 221 



and the next point to be determined is whether the)' occurred 

 within the same focus at different times, within two foci at the 

 same time, or within two foci at different times. The theory of 

 two successive impulses within the same focus is negatived by the 

 existence of the rectilinear band within which the two parts of the 

 shock were superposed, and by the fact that the first part of the 

 shock was not everywhere the stronger. For the same reasons, 

 the double shock cannot be referred to the duplication of a single 

 initial impulse by reflection or refraction at the bounding surfaces 

 of different rocks, or by the separation of its direct and transverse 

 waves. There must, therefore, have been two distinct foci arranged 

 along a line parallel, or nearly so, to the longer axes of the isoseismal 

 curves ; and that the foci were practically detached is evident from 

 the cessation of all sound and movement during the interval 

 between the two parts of the shock. 



One of the most interesting features of twin-earthquakes is the 

 occurrence of the second impulse before the vibrations from the 

 focus first in action have time to reach the other. In other words, 

 the second impulse is not a consequence of the first. In the 

 Hereford earthquake of 1896, the two impulses were separated by 

 a brief interval of time, and the two corresponding parts of the 

 shock coalesced within a hyperbolic band, the convexity of which 

 faced the focus first in action. In the Derby earthquake, however, 

 this band is rectilinear, showing that the two impulses must have 

 occurred at the same instant. They were therefore due to a single 

 generative effort, and it is on this account that I have given the 

 name of •' twins ' to this class of earthquakes. 



Position of the Two Foci. 



In the absence of isoseismal lines for each part of the shock, the 

 exact positions of the two epicentres cannot be determined. From 

 the form of the curves in PI. XIX, however, it is probable that one 

 epicentre was situated near Ashbourne, and the other about 3 miles 

 west of AVirksworth : their centres being, therefore, about 8 or 9 

 miles apart. 



Seismographic Records. 



Records of the Derby earthquake were given by an Omori 

 horizontal pendulum at Birmingham, by a Milne seismograph at 

 Bidston (near Birkenhead), and by an astatic pendulum designed by 

 Dr. E. Wiechert at Gottingen. The first of these, which is the most 

 interesting ever obtained of a British earthquake, is reproduced in 

 fig. 1 (p. 220) from a photographic enlargement of the original record, 

 for which I am indebted to the kindness of Mr. Arnold-Bemrose. 



The Omori pendulum belongs to the type first devised by 

 Mr. Gerard, of Aberdeen, in 1853, and afterwards re-discovered and 

 employed by Prof. Milne in his well-known seismograph. It differs 

 from the latter instrument in its mechanical form of registration, the 

 record being made by a tine point on a rotating surface of smoked 



