on Earthquakes in Japan. 



365 



Velocity of 

 normal vibra- 

 tions, in centi- 

 1 metres per 

 second. 



Velocity of 

 transverse vi- 

 brations, in 

 centimetres 

 per second. 



Eatio of normal 

 to transverse. 



Granite S95X10 3 



Marble 381 XlO 3 



Tuff 285X10 3 



Clay-rock 348 XlO 3 



Slate 451xl0 3 



219 XlO 3 



208 XlO 3 



209 XlO 3 

 254 XlO 3 

 286 XlO 3 



1-80 



1-83 

 1-36 



1-37 

 1-58 



Deductions of this kind, however, are invalidated by the fact 

 that the specimens used for experiment are, of course, nearly- 

 homogeneous, whilst the earthquake passes through a mass 

 which is heterogeneous and more or less fissured. The results 

 here given are deduced from moduli which were determined 

 by cross-bending and twisting of long round cylinders. Mallet, 

 by experiments " on the compressibility of solid cubes of these 

 rocks, obtained the mean modulus of elasticity," with the result 

 that " nearly seven eighths of the full velocity of wave-transit 

 due to the material if solid and continuous, is lost by reason 

 of the heterogeneity and discontinuity of the rocky masses as 

 they are found piled together in nature." 



(b) Artificial Earthquakes. — So far as we are aware, the 

 presence of the normal and transverse vibrations has never 

 been satisfactorily proved ; although Mallet proved that their 

 presence could be observed in artificially produced vibrations. 



In performing a similar set of experiments to those carried 

 out by Mallet, we succeeded in obtaining records of both 

 sets. These records proved: — 1st, that the two sets of vibra- 

 tions exist ; 2nd, that they travel at different rates, which 

 rates were measured ; 3rd, that they are differently affected 

 by distance, one set dying out before the other; 4th, that there 

 is a difference in their period, the transverse wave having 

 rather the longer ; 5th, the amplitude of normal vibrations is 

 approximately in the inverse ratio of the distance from the 

 source. 



These vibrations were produced by dropping a heavy iron 

 ball, weighing about a ton, from heights varying up to 

 35 feet. The ground through which the vibrations were pro- 

 pagated was a hardened mud. The records here referred to 

 were written by a pair of bracket- seismographs, with their 

 indices resting on a smoked glass plate drawn along by clock- 

 work. 



