198 EAETHQUAKES. 



be any revolution. If we divide our section A B c D up into 

 eight divisions by lines through these directions, we shall 

 see that any shock the direction of which passes through 

 any of the octants which are shaded will cause a positive 

 revolution in the body — ^that is to say, a revolution corre- 

 sponding in its direction to that of the movements of the 

 hands of a watch ; whilst if its direction passes through 

 any of the remaining octants the revolution will be nega- 

 tive, or opposite to that of the hands of a watch. From 

 the direction in which any given stone has turned, we can 

 therefore give two sets of limits between one of which the 

 shock must have come. 



Further, it will be observed that the tendency of the 

 turning is to bring a stone, like the one we are discussing, 

 broadside on to the shock ; therefore, if a stone with a 

 rectangular cross section has turned sufficiently, the direc- 

 tion of a shock will be parallel to one of its faces, but if 

 it has not turned sufficiently it will be more nearly parallel 

 to its faces in their new position than it was to its faces 

 when in their original position. 



If a stone receives a shock nearly parallel with its 

 diagonal, on account of its instability it may turn either 

 positively or negatively according as the friction on its 

 base or some irregularity of surface bearing most influence. 

 Similarly, if a stone receives a shock parallel to one of 

 its faces, the twisting may be either positive or negative, 

 but the probability is that it would only turn slightly ; 

 whereas in the former case, where the shock was nearly 

 parallel to a diagonal, the turning would probably be great. 



Determination of direction froTYi instruments, — 

 When speaking about earthquakes it was shown, as the 

 result of many observations, that the same earthquake in 

 the space of a few seconds, although it may sometimes 

 have only one direction of motion, very often has many 



