262 C. Davison — Vorticose Earthquake Slwclcs. 



Corresponding, then, to the portion of each sphere included within 

 the focus, there will be an equivalent centre of disturbance, and 

 these equivalent centres will not coincide with one another, but their 

 positions will be dependent upon the form" of the seismic focus 

 and upon the intensity of the disturbance at each point of it. If, 

 however, the focus be such that any two points of it can be con- 

 nected by a line lying wholly within it, and if the intensity vary 

 continuously from point to point, the equivalent centres will lie 

 upon a continuous curved line. It will be seen that for diiferent 

 positions of P, the distribution Qf equivalent centres may, and pro- 

 babl}'' will, be entirely different. 



We may therefore suppose the seismic focus to be replaced by a 

 number of equivalent points, and a corre- 

 sponding disturbance at each point, each 

 aifecting P at a different instant of time. 

 Suppose A, B, C, D, E, etc., to be the series 

 of equivalent points for any point P on the 

 surface of the earth (in the order in which 

 the disturbances from them affect P) ; a, b, 

 c, d, e, etc., the corresponding apparent 

 seismic vertices. Then the disturbances at 

 P will appear to come along the surface 

 from a, b, c, d, e, etc., in the direction and 

 order indicated by the numbered arrow- 

 heads. 



Fixing our attention upon any one of the 

 spherical portions, let us suppose, for a 

 moment, the rest of the focus annihilated, 

 and consider the motion of the particle at 

 P. It will begin to move in some definite 

 direction, and will continue to move until 

 the molecular forces brought into action by 

 its displacement compel it to stop ; it would 

 then i-eturn nearly, or quite, to its original 

 position, but before the movement is completed, disturbances from 

 succeeding spheres will reach the point P, and will affect its 

 subsequent motion, though not the vibratory character of that motion. 



Hence, remembering that the earth's crust is seldom or never quite 

 homogeneous and isotropic, that the seismic focus may be of any 

 form and magnitude and may even consist of detached portions, that 

 the disturbances at different points of the seismic focus are not 

 necessarily of equal intensity, and that the disturbances do not 

 necessarily take place simultaneously throughout the whole extent of 

 the seismic focus, it is evident that we have a cause sufficient to pro- 

 duce any vorticose shock however complicated, and any variety in 

 the changes of direction. 



It will be seen also that the same reasoning may be applied to 

 earth-waves consisting of transverse vibrations, whether these 

 vibrations are executed in straight lines or curves. 



As an illustration, let us take the case of a seismic focus (drawn 



