532 GEOLOGY. 



emerges at an angle of about 45°, but various influences modify this 

 result. Lines drawn through points of equal effect (isoseismals) are 

 not usually regular circles or ellipses about the epicentrum, as they 

 would be under ideal conditions. The various divergencies represent 

 differences of effective elasticity, of surface, and of other influ- 

 ences. As most earthquakes originate from lines, planes, or masses, 

 rather than points, there are doubtless differences of intensity of vibra- 

 tion at different points on the lines, planes, or areas of origin, and 

 these differences introduce inequalities in propagation and in surface 

 effects. 



April 6-6. 190L, 



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Fig. 448. — Illustrations of the records made by earthquake tremors after distant 

 transmission through the earth. The four diagrams represent the same set of tremors 

 as received at Shide, Kew, Bidston, and Edinburgh in Great Britain. The move- 

 ment was from left to right. (Milne.) 



Rate of propagation. — The progress of a seismic wave varies very 

 greatly. Both experimental tests and natural observations give very 

 discordant results. At present, they justify only the broad statement 

 that the velocity of propagation varies from several hundreds to several 

 thousands of feet per second at the surface. The rate seems to be 

 greater for strong vibrations than for weak ones, and hence it is faster 

 near the origin than farther away. The strength of a vibration dies away, 

 theoretically, according to the inverse square of the distance from the 

 point of origin. Practically there is to be added to this the partial 

 destruction of the vibrations by conversion into other forms of motion. 



