332 Professor John Milne [Feb. 12, 



surface over a distance of 40 miles, many opportunitieis presented 

 themselves for the observation of sound waves. Often a subterranean 

 boom was heard, unaccompanied by any sensible shaking, but more 

 frequently it was a warning that within a very few seconds there would 

 be a more or less violent shaking. 



If we assume that the sounds originated at the same foci as the 

 after-shocks, the velocity with which the former were transmitted was 

 therefore higher than that at which the latter were transmitted. But 

 inasmuch as observation showed that the earth waves had a velocity 

 seven times as great as an air wave, the conclusion is that whatever 

 may be the mechanical action producing the earthquake sound, it is a 

 vibratory motion transmitted tl^roifgh the rocks ; and because it is never 

 audible at many miles distant from its source, the vibrations producing 

 it either raj)idly die out or change in character. 



Another interesting investigation, which is by no means completed, 

 has been to note the effects produced by earthquakes upon the lower 

 animals, several of which are apparently more alive to the existence 

 of minute tremors than human beings. The effect produced by earth- 

 quakes on human beings, which partakes largely of an emotional and 

 moral character, is a subject about which many interesting facts have 

 been collected. 



Perhaps the greatest triumph in seismological investigations is 

 the fact that \ye are now assured that if a large earthquake occurs in 

 any one portion of our globe, it can with suitable instruments be 

 recorded in any other portion of the same. Because the rate at which 

 these movements are propagated is so very high, in some instances 

 approaching 12 km. per second, or double the rate at which a wave of 

 pompression could pass through steel or glass ; because at a given 

 station we have never recorded two disturbances which we should 

 expect had the movement like a barometrical wave been transmitted 

 in all directions round the earth ; and finally, because it appears that 

 the velocity to points at a great distance from an origin is higher than 

 that to points relatively near to the same, the conclusion for the 

 present is that the motion, rather than being propagated round our 

 world, is propagated through the same. 



Inasmuch as thpse velocities throw light upon the effective rigidity 

 of the materials constituting the paths along which they were de- 

 termined, the importance of establishing, say at twenty existing 

 observatories willing to co-oj)erate, instruments to record these earth 

 movements is at once apparent. The cost of such a set of instruments, 

 required to carry out a seismic survey of the world, would be about 

 1000^. 



At the observatories where these instruments were established, in 

 addition to the speedy announcements of great catastrophes in distant 

 places, the records of these, and of disturbances of a more local origin, 

 would throw light upon some of the otherwise unaccountable de- 

 flections sometimes fc>hQ\vn in phqtograms from magnetographs, baro- 

 graphs and other instruments sensible to slight displacements ; 



