DETERMINATION OF EARTHQUAKE ORIGINS. 195 



which a body having a regular form like a prisraatic 

 gravestone or a cylindrical column is overturned some- 

 times gives the means by which we can determine the 

 direction from which a movement came. 



The rotation of bodies, — It has often been observed 

 that almost all large earthquakes have caused objects like 

 tombstones, obelisks, chimneys, &c., to rotate. 



One of the most natural and at the same time most 

 simple explanations is to suppose that during the shock 

 there had been a twisting, or backward and forward screw- 

 like motion in the ground. Amongst the Italians and the 

 Mexicans earthquakes producing an effect like this are 

 spoken of as 'vorticosi.' In the Calabrian earthquake, 

 not only were bodies like obelisks twisted on their bases, 

 but straight rows of trees seem to have been left in 

 interrupted zigzags. These latter phenomena have been 

 explained upon the assumption of the interference of 

 direct waves and reflected waves, the consequence of 

 which being that points in close proximity might be 

 caused to move in opposite directions. Eeflections such 

 as these would be most likely to occur near to the junction 

 of strata of different elasticity, and it may be remarked 

 that it is often near such places that much twisting has 

 been observed. 



Another way in which it is possible for twisting to 

 have taken place would be by the interference of the 

 normal and transverse waves which probably always exist 

 in an earthquake shock, or by the meeting of the parts 

 of the normal wave itself, one having travelled in a direct 

 line from the origin, whilst the other, travelling through 

 variable material, has had its direction changed. 



Mallet, however, has shown that the rotation may 

 have been in many cases brought about without the 

 supposition of any actual twisting motion of the earth — 



