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VII. Surface Reflexion of Earthquake Waves. 
By George W. Walker, A.R.C.Sc ., M.A., F.R.S., formerly Fellow of 
Trinity College , Cambridge. 
Beceived June 28,—Bead November 1, 1917. 
[Plates 3 and 4.] 
A seismogram, or instrumental time record of the earth movement produced by an 
earthquake at some distance, may be divided into three main parts, known as the 
first phase, the second phase, and the long-wave phase respectively. As a rule, the 
experienced observer has little difficulty in distinguishing these phases, and although 
difficult cases do arise, there is general agreement that the three divisions are 
characteristic and porrespond to definite properties of the earth. 
The beginning of the first phase is denoted by P and the beginning of the second 
phase by S, and these are interpreted as representing the arrival of longitudinal and 
transversal waves which start simultaneously at the focus of the earthquake and 
reach the observing station at different times. By general convention P and S are 
also used to represent the time interval between the occurrence of the earthquake 
and the arrival of the corresponding waves at the observing station. 
Time curves giving the values of P and S as functions of the epicentral distance 
were obtained by Zoppritz in 1907 (‘ Gottinger Nachrichten’). The tables of values 
are in general use at seismological observatories. They have proved remarkably 
successful in determining epicentres of earthquakes, and the theoretical analysis by 
Wiechert has led to inferences about the interior of the earth which are of the 
greatest interest. Doubtless as observational data improve these time curves may 
have to be modified, but only by way of correction and not as regards the main broad 
features. 
In addition to the primary features called P and S, a seismogram may show 
well-marked sharp impulses, and in Wiechert’s opinion these represent the arrival 
of waves that have undergone one or more reflexions at the earth’s surface. On this 
view PR n would denote a longitudinal wave that has been reflected n times at 
the surface, thus dividing the epicentral distance into (^+1) equal parts, and a 
similar notation SB„ would apply to transversal waves. We may also contemplate 
waves P n S TO that have undergone change from longitudinal to transversal (or vice 
versa) on reflexion. 
YOL. CCXVIII.—A 567. 3 D 
[Published. June 12, 1919. 
