DETERMINATION OF EARTHQUAKE ORIGINS. 197 



If any columnar-like object, for example a prism 

 of which the basal section is represented by A B c D (see 

 fig. 30), receives a shock at right angles 

 to B c, there will be a tendency for 

 the inertia of the body to cause it 

 to overturn on the edge B c. If the 

 shock were at right angles to DC, the 

 tendency would be to overturn on the 

 edge D c. If the shock were in the 

 direction of the diagonal CA, the 

 tendency would be to overturn on the 

 point C. Let us, however, now suppose Fig. 30. 



the impulse to be in some direction like E G, where G is 

 the centre of gravity of the body. For simplicity we may 

 imagine the overturniiig effect to be an impulse given 

 through G in an opposite direction — that is, in the direction 

 G E. This force will tend to tip or make the body bear 

 heavily on c, and at the same time to whirl round C as 

 an axis, the direction of turn being in the direction of 

 the hands of a watch. If, however, the direction of im- 

 pulse had been e' G, then, although the turning would 

 still have been round c, the direction would have been 

 opposite to that of the hands of a watch. 



To put these statements in another form, imagine 

 G e' to be resolved into two components, one of them 

 along G C and the other at right angles, G F. Here the 

 component of the direction G c tends to make the body 

 tip on c, whilst the other component along GF causes 

 revolution. Similarly G E may be resolved into its two 

 components G c and G f', the latter being the one tending 

 to cause revolution. 



From this we see that if a body has a rectangular 

 section, so long as it is acted upon by a shock which is 

 parallel to its sides or to its diagonals, there ought not to 



