PRESIDENTIAL AiJDKESS SECTION A 4I 



faster the further they go, and the theory of elastic vibrations 

 shows this to be impossible. Hence we are forced to conclude 

 that they take a nearly direct route, and plunge right through the 

 great unknown depths of the earth. The way in which they 

 emerge entirely confirms this. 



The behaviour and speeds of these waves throw much light 

 on the elastic nature oi the earth's interior. In the first place it 

 is well-known that a liquid or gas cannot propagate a transverse 

 disturbance at all. Hence we are once more definitely shut up to 

 the conclusion that for such rapid vibrations as constitute these 

 waves, with periods of a few seconds, the earth behaves like a 

 stifif, very incompressible body. 



But we can go much further. For it is well-known that 

 from the speeds of longitudinal and transverse vibrations travers- 

 ing a large solid of known density we can calculate its elastic re- 

 sistance to compression and to change of shape. The equations 

 are: — 



Vi (longitudinal vibrations) = \J k-\- ~ ^z=modulus of re- 



d ' sistance to com- 

 pression. 



v., (transverse vibrations )= v/-^ n=:modulus of re- 



" ▼ a, . 



Sistance to 



change of shape, 

 (/zrrdensity. 



Whence k and n are found when V,, \\. and d are known. 



At the surface, Galitzin gives as mean observed values for 

 V\.. 7.17 km./sec. and for Vo 4.0J km. /sec, whence, if we assume 

 a surface density of 2.75 gm/per com. we get for the rigidity 

 of the surface crust a value equal to a little more than half of 

 that of good steel. 



As we go down into the earth we .should naturally expect 

 the increasing temperature and density to cause a progressive 

 diminution in the speeds of these waves. To our surprise, the 

 precise contrary is the case. The speed grows decidedly greater 

 the deeper we go, in spite of the certain great increase of density. 

 DilTferent seismologists have given somewhat different estimates 

 of the increase, the differences arising from the dilBculty of com- 

 paring the records of instruments of different types. But we shall 

 not ibe far wrong in taking 12.7 km./sec. as the maximum speed 

 otf the longitudinal vibrations, and 6.8 km./sec. as the maximum 

 speed of the transverse vibrations when passing through nearly 

 centra] regions. These speeds give evidence of a rigidity ex- 

 ceeding that of steel two or three times. 



The continued investigation of earthquake records is full 

 of promise. For instance, there seems to be evidence of a differ- 

 ent surface speed under oceans from that which obtains under 

 continents. Again, both Oldham and Wiechert believe that they 

 have accumulated clear evidence of more than one shell within 



