﻿22 DR. CHARLES DAVISON ON [Feb. I905, 



of 1896, and in the Derby earthquakes of 1903 and 1904. The 

 band is only a few miles in width, and crosses the central curves at 

 right angles to their longer axes. In the Derby earthquake of 

 1903 the synkinetic band was straight, and in the other two earth- 

 quakes curved, the concavity in the case of the Hereford earthquake 

 facing south-eastward, and in that of the Derby earthquake of 1901 

 south-westward. In every case, it is worthy of notice that two 

 maxima of intensity, connected by weaker tremulous motion, were 

 distinguished by careful observers close to the boundaries of the 

 synkinetic band. 



Sound-Phenomena. 



As a rule, though not invariably, the sound is heard with each 

 part of the shock ; sometimes the shock is not felt, and two 

 sounds are alone observed. The stronger part of the shock is 

 accompanied by the louder sound, but the two parts of the sound 

 differ less in intensity than the two parts of the shock ; also, the 

 sound is often heard before and after the shock begins and ceases 

 to be sensible. Thus, when the two parts of the earthquake coalesce 

 within and near the synkinetic band, the sound receives a notable 

 increase of strength over a comparatively-wide region; while the 

 strongest vibrations of the two parts of the shock are of such 

 brief duration that they coalesce only near the synkinetic line. 

 Observations of the reinforced sound are therefore common, and those 

 of the reinforced shock are rare : the result being that the isoseismals 

 show little, if any, deflection near the synkinetic band ; while the 

 isacoustic lines (or lines of equal sound-intensity) are so much 

 distorted that they cling to the synkinetic band, and share in its 

 straightness or curvature. 



IV. Twin-Earthquakes connected with a Twin-Focus. 



In several earthquakes, the isolation of the two series of vibra- 

 tions has been so marked that seismologists have felt it necessary 

 to offer some explanation of their origin. The brevity of the 

 interval has been tacitly assumed to preclude a repetition of the 

 impulse within the same focus ; and the two series have been 

 generally referred to a single impulse, the separation of the earth- 

 waves being effected by subterranean reflection or refraction, or 

 ascribed to the existence of direct and transverse waves. Thus, in 

 the Neapolitan earthquake of 1857, Mallet noticed the difference in 

 direction between the two parts of the earthquake, and regarded 

 the second part as a reflected shock, although he offered a somewhat 

 different explanation for the outlying region of Naples. In the 

 Colchester earthquake of 1884, Prof. Meldola & Mr. White also 

 attribute the second part to reflection ; they consider the existence 

 of two initial disturbances as improbable, although they notice that 

 the distribution of damage to buildings gives the impression that 

 there were two separate foci. Profs. Taramelli & Mercalli suggest 



