MOVEMENTS AND DEFORMATIONS OF THE EARTH'S BODY. 529 



that the focus is not so deep as implied by the simple backward projec- 

 tion of the lines of emergence. 



A second correction must be made for the differences of continuity 

 of the upper rock in the vertical and horizontal directions. In the 

 outer part of the earth, the continuity in horizontal directions is inter- 

 rupted by vertical fissures. Were these not usually filled with water, 

 they would soon kill the horizontal component of the seismic wave, 

 and the residual portion would be directed almost vertically to the sur- 

 face, for the width of the fissures is almost always greater than the ampli- 

 tude of the seismic vibrations. The water restores the continuity, in a 

 measure, but not perfectly, for the elasticity of water is much less than 

 that of rock. It is clear that in horizontal movement there must be a 

 constant transfer from rock to water and from water to rock, and this must 

 retard, as well as partially destroy, the vibrations. In a vertical direction, 

 however, the rocks rest firmly upon one another, and this gives measur- 

 able continuity, the only change being from one layer or kind of rock to 

 another. It seems certain, therefore, that the vertical component of 

 the seismic wave will be less damped and less retarded in transmission 

 than the horizontal. It will, therefore, reach the surface sooner and 

 will have the greater effect on bodies at the surface, not only for the 

 reasons given, but also because it emerges more nearly in the line of 

 least resistance and of freest projection. On this account, a second 

 correction must be added to the correction for elasticity, and this must 

 further reduce appreciably the first estimate of the depth of the focus. 



Observation shows that in some way a seismic wave becomes sepa- 

 rated in transmission into portions of different natures and speeds, but 

 their interpretation is yet uncertain. These separated portions probably 

 consist of the compressional, the distortional, and the undulatory waves, 

 and perhaps of refractions and reflections of these (see Fig. 448). 



A most important recent achievement is the detection and investi- 

 gation of seismic tremors that appear to have come through the earth. 

 The transmission of such waves promises to reveal much relative to the 

 nature of the deep interior, when enough data are gathered to warrant 

 conclusions. The rate of propagation in the central parts is found to 

 be greater than in the outer parts, implying high elasticity within. 



The amplitude of the vibrations. — From the very disastrous effects of 

 severe earthquakes, it is natural to infer that the distinctive oscillations 

 must have large amplitude, but in fact it is the suddenness of the vibra- 



