l6 THE FLOOR OF THE OCEAN 



The Push waves race through every part of the earth and 

 emerge even at the antipodes. On the other hand, the Shake 

 waves are greatly or entirely damped out when they enter the 

 earth's so-called Iron Core, which begins at the depth of about 

 2900 kilometers or 1800 miles and continues to the center, 4000 

 miles below sealevel. Incidentally, this apparent failure of the 

 Shake waves to pass entirely through the Iron Core suggests 

 that the Core is fluid, at least in part — fluid in spite of the in- 

 conceivable pressure on the material, ranging from 1,300,000 

 atmospheres at its top to more than 3,000,000 atmospheres, or 

 45,000,000 pounds to the square inch, at the center of the globe. 

 See Figure 22. 



The late Lord Rayleigh showed that, if the earth were 

 everywhere uniform in density and elastic properties, there 

 would be only one kind of Surface wave. Its passage would be 

 accompanied by motion of the rock particles in paths that are 

 partly in the vertical direction and partly horizontal in the 

 direction of propagation. Seismologists have named such a 

 theoretically deduced wave a Rayleigh wave. The earth is not 

 homogeneous, but it has been found that some of the oscilla- 

 tions registered in seismograms are much like those forecast by 

 Rayleigh's mathematical analysis, and are accordingly called 

 pseudo-Rayleigh waves. 



And there is a second kind of Surface wave, expected be- 

 cause of proof that the earth is layered and vertically heteroge- 

 neous, each layer being in general more incompressible and 

 more rigid than the layer above it. Professor A. E. H. Love of 

 Oxford University asked himself the question: what kind of 

 Surface wave should be set up in such a planet when vibrating 

 under Nature's hammer-blow ? His mathematical treatment of 

 the problem showed that, in addition to the pseudo-Rayleigh 

 wave, there must be another type, characterized by motion of 

 the rock particles in the horizontal direction and at right 



