222 EAKTHQUAKES. 



be that one mass of rock has been sliding across another 

 mass, giving rise to shearing strains, and producing waves 

 of distortion. 



The first seismologist who attacked the problem of 

 finding out the dimensions and position of such a fissure 

 was Mallet, when working on the Neapolitan earthquake 

 of 1857. The reasons that the origin should, in the first 

 place, have been a fissure, rather than any other form of 

 cavity, was that such a supposition seemed to be a priori 

 the most probable, and, further, that it afforded a better 

 explanation of the various phenomena which were observed, 

 than that obtained from any other assumption. 



The method on which Mallet worked to determine 

 the form and position of the assumed fissure, which 

 method was subsequently more or less closely followed 

 by other investigators, was as follows i — 



From an observation of the various phenomena pro- 

 duced upon the surface of the disturbed area, a map of 

 isoseismals was constructed. These were seen, as has 

 been the case with many earthquakes, not to distribute 

 themselves in circles round the epicentrum, hut as distorted 

 oval or elliptical figures, the major axes of which roughly 

 coincided with each other. Further, the epicentrum, did 

 not lie in the centre of these ovals, but was near to the 

 narrow end where they converged. 



This at once showed, if the reasoning respecting the 

 manner in which waves are propagated from an inclined 

 fissure be correct, that the fissure was at right angles to 

 the major axis of the curves, dipping from their narrow 

 end downwards, in the direction of their larger wide- 

 spread ends. 



The next weapon which Mallet employed to attack 

 this problem was the sound which was heard at different 

 points round about the focus. These sounds appear to have 



