440 Professor Cargill G. Knott [Feb. 14, 



WEEKLY EVENING MEETING, 



Friday, February 14, 1919. 



The Right Hon. Lord Wrenbury, P.O. M.A., Vice-President, 

 in the Chair. 



Professor Cargill G. Knott, D.Sc. LL.D., Gen. Sec.Pt.S.E. 



The Propagation of Earthquake Waves through the 

 Substance of the Earth. 



[Abstract,] 



It has long been recognised that every earthquake is the source of a 

 succession of elastic waves propagated through the earth and over 

 its surface. Three distinct types of waves have been discriminated 

 the one from the other. Two of these consist of elastic vibrations 

 transmitted along definite paths or rays and emerging at correspond- 

 ing distances from the epicentre of the earthquake origin. It is now 

 universally admitted that these two types of waves are the com- 

 pressional and distortional waves of elastic theory, frequently known 

 as the longitudinal and transverse waves. Of the many workers 

 whose investigations helped to elucidate the character of these waves 

 one calls for special mention— namely, the late Professor John Milne — 

 who began his seismological studies while Professor of Mining in 

 Japan in the last quarter of the nineteenth century, and continued 

 them at Shide, Isle of Wight, to the day of his death in 1913. 

 Supported by the Seismological Committee of the British iVssociation, 

 Milne was able to establish at a large number of stations all over the 

 earth's surface a certain simple type of instrument for recording the 

 tremors which, originating at the earthquake source, were sufficiently 

 strong to affect the delicate seismometers installed at these stations. 

 From a comparison of the records obtained at these stations and at 

 other stations provided with different types of instruments, the 

 following points were soon estabUshed : — 



(1) A good earthquake record has an unmistakable character, the 

 well-marked first movement being followed after a certain interval 

 of time by the advent of a larger and well-marked second movement. 



(2) The interval of time between the advents of the first and 

 second characteristic movements increases with the distance of the 

 station from the epicentre. 



(8) From the measurement of this time interval on any seismo- 

 gram the distance of the epicentre from the station at which the 



