136 Mr. John Milm [March 20, 



require a central bureau to discuss applied seismology than we do to 

 discuss the construction of torpedoes or flying machines. 



A discovery which during the last few years has done much to 

 popularise Seismology is the fact that a very large earthquake origin- 

 ating in any one part of the world may be recorded in any other 

 portion of the same. This means that the opportunity for carrying 

 on Seismological research is not a mono})oly enjoyed only by those 

 who reside in earthquake countries. Although only a few persons in 

 Great Britain have been privileged to feel one of its home-Luade 

 tremors, everyone of its inhabitiints are very many times per year 

 moved by earthquakes. Back and forth motion of the ground is 

 performed too slowly for us to feel, while, if there is a movement like 

 the swell upon an ocean, the undulations are too long and flat for us 

 to see. 



Waves start out from their epicentral area, which is a district 

 that has been fissured and shattered by the formation or exten- 

 sion of large faults in all directions. Observation, however, shows 

 that these waves are propagated farthest in one particular direction. 

 For example, the chief movement following the San Francisco earth- 

 quake which originated from fault lines running parallel to the coast 

 of California, was much more marked in countries lying to the east 

 or west of California than in countries lying towards the south. 

 England and Japan obtained large records of the disturbance, while 

 in Argentina the records were extremely small. In the case of the 

 Jamaica earthquake where the lines of origin ran east and west, the 

 phenomenon was reversed. Toronto received a large quantity of 

 motion, and England a very little. Another peculiarity of this phase 

 of earthquake motion is that it may be propagated in one direction 

 round the world to a greater distance than in an opposite direction. 

 The suggestion is that the initial impulse was delivered in the direc- 

 tion towards which motion was propagated farthest. If for illustra- 

 tion we assume that the slip on a fault line has been downwards towards 

 the east, then the motion would travel towards the east farther than 

 it would towards the west. That which happens, corresponds to 

 what we see if we dip the blade of a spade in water and suddenly 

 push the blade in some particular direction. The water waves thus 

 created travel farthest in the direction of the impulse. 



Another curious phenomenon connected with the large waves of 

 certain earthquakes is that they can pass their equatorial or quadrantal 

 region unobserved. They may be very marked for 1000 miles round 

 their origin, and recordable, but much reduced in size, about their 

 antipodes, but not recordable in between. For example, an earthquake 

 originating near New Zealand may be recorded in that country, but 

 not in India, Egypt, West Asia, or east of Europe, but in Britain it 

 may make itself evident by the thickening of a photographic trace. 

 The phenomenon may be compared to a water wave running down 

 an expandmg estuary. At the mouth of such an estuary it may have 



