THE ISOSEISTS. 301 



through zones of greater and greater destruction to the pleistoseismic 

 area which must lie above the place of origin. 



No one doubts that the surface during an earthquake is in a com* 

 plicated state of molecular strain and movement, the earth particle 

 (as revealed by seismographs) swinging in ever-changing paths that 

 yield, as factors, the period, velocity, amplitude and acceleration of 

 that particle. These vary in amount at different places, generally 

 increasing as regards the energy involved and the damage caused to 

 buildings, etc., up to some point, line, plane or other epicentral surface 

 feature, from which also the time records frequently show that the 

 movement sprung and spread. 



But, beyond this, if we try to analyse directions of shock as felt 

 by observers, as evinced by swaying and fallen objects, or as deduced 

 from strains resulting in fissures that have been set up in all solid 

 objects resting on the earth, and even the surface of the ground itself; 

 and if we try to square that analysis with theoretical speculations as 

 to what the nature of the wave or waves started by the seismic im- 

 pulse ought to be — then do we find ourselves entangled in a hopeless 

 mass of irreconcilable data. At least such seems to be the prevailing 

 experience among many students of earthquakes, and it is certainly 

 borne out by the study of the present earthquake. 



Whether, then, the original impulse is condensational and in the 

 direction of propagation of the shock, or distortional and transverse 

 to it, or both ; whether such wave or waves reach the surface by this 

 or by that path, and with varying speeds or not — current explanations 

 regarding which the writer thinks do not yet thoroughly accord with 

 facts — need not be considered for the purposes of this descriptive 

 memoir. Assuming a below-the-surface origin for the shock, the first 

 news we really have of it is as expressed at the surface of the ground 

 in damage which varies in character and intensity. To the general 

 description and summarising of this, accompanied by a few remarks on 

 the character of the wave motion as so expressed or recorded in the 

 various isoseismal compartments I now propose to turn. 



