196 EAETHQUAKES. 



a simple backward and forward motion being quite suffi- 

 cient. If one block of stone rests upon another, and the 

 two are shaken backwards and forwards in a straight line, 

 and if the vertical through the centre of gravity of the 

 upper block does not coincide with the point where there 

 is the greatest friction between the blocks, rotation must 

 take place. If the vertical through the centre of gravity 

 falls on one side of the centre of friction, the rotation 

 would be in one direction, whilst, if on the other side, the 

 rotation would be in the opposite direction. 



Although the above explanation is simple, and also in 

 many cases probably true, it hardly appears sufficient to 

 account for all the phenomena which have been observed. 



Thus, for instance, if the stones in the Yokohama 

 cemetery, at the time of the earthquake of 1880, had been 

 twisted in consequence of the cause suggested by Mallet, 

 we should most certainly have found that some stones 

 had turned in one direction whilst others had been twisted 

 in another. By a careful examination of the rotated 

 stones, I found that every stone — the stones being in 

 parallel lines — had revolved in the same direction, namely 

 in a direction opposite to that of the hands of a watch. 



As it would seem highly improbable that the centre of 

 greatest friction in all these stones of different sizes and 

 shapes should have been at the same side of their centres 

 of gravity, an effect like this could only be explained by 

 the conjoint action of two successive shocks, the direction 

 of one being transverse to the other. 



Although fully recognising the sufficiency of two trans- 

 verse shocks to produce the effects which have been 

 observed in Yokohama, I will offer what appears to me to 

 be the true explanation of this phenomenon : it was first 

 suggested by my colleague, Mr. Gray, and appears to be 

 simpler than any with which I am acquainted. 



