Pocks and the Velocity of Seismic Waves. 65 



goes on decreasing. The former supposition makes the 

 velocity of elastic waves increase from the surface towards 

 the centre of the earth, while the latter implies the existence 

 of the stratum of maximum velocity of propagation. Such a 

 stratum, if it exists, will lie pretty deep in the earth's crust, 

 and will be inaccessible to us ; but the question will be settled 

 by the seismologists. 



Velocity of Propagation of Seismic Waves. — A glance at the 

 table of elastic constants will show the complex elastic nature 

 of rocks composing the earth's crust. The path pursued by 

 waves of disturbance must necessarily assume very compli- 

 cated forms, as they are subject to manifold reflexion, re- 

 fraction, and dispersion. We can perhaps borrow analogy 

 from a kindred optical phenomenon of curved rays in a 

 medium of heterogeneous density, studied experimentally by 

 Mace de Lepinay and Perot, and theoretically discussed by 

 A. Schmidt and Wiener. The phenomena presented by the 

 seismic wave will be of still more complex character as the 

 medium is of quasi-crystalline nature, and the wave may suffer 

 refraction something akin to that of light in Iceland-spar and 

 arragonite. The elastic constants of rocks through which 

 the disturbance propagates will rarely satisfy the condition 

 of giving rise to purely longitudinal or distortional waves, so 

 that the seismic wave will be of a mixed character. What 

 Mr. Milne designates earthquake echoes or reverberations 

 will partly find explanation in the intricate behaviour of 

 diverse rocks in relation to the elastic wave travelling through 

 them. The waves propagated from the centre of disturbance 

 will appear on the seismograph as undulations of irregular 

 periods, especially near the origin. At a distance waves of 

 short period will gradually die out owing to the greater 

 damping effect, while those of long period will still leave 

 their mark, although not felt by us as a shock. 



The investigation of tine seismic waves affords the best 

 means of feeling the jpulse o' the interior of the earth ; the 

 elastic nature and the densify distribution of the constituent 

 rocks, or even the condition *f the inaccessible depth, will in 

 some future day be 'brought to light by the patient study 

 of the disturbances, which traverse the strata of hetero- 

 geneous structure and appear as tremors or earthquakes on 

 the earth's surface. 1 think the introduction of the hori- 

 zontal pendulum is a great progress in that branch of study 

 which relates to the earth's interior, not that it records the 

 apparent surface-movement of the soil, but that it does not 

 fail to record earthquakes of distant origin, which, though 

 insensible to us, sometimes appear as slow waves of gigantic 



Phil. Mag. S. 5. Yol. 50. No. 302. July 1900. F 



