x INTRODUCTION 
That the seismogram of a distant earthquake represents 
elastic waves that have travelled through the earth from focus 
to station was early recognised, but the well-known result that 
a solid body transmits longitudinal and transversal waves with 
different speeds did not at first seem to throw much light on 
the meaning of the seismogram, which by its apparent com- 
plexity suggested a highly heterogeneous earth. 
It was, however, an important thing for seismological 
theory when Lord Rayleigh (see “ Collected Papers”) showed in 
1885 that there could be propagated along the surface of an 
elastic solid a set of waves travelling with speed rather less 
than that for transversal waves. Such waves play an important 
part in the long wave phase of a seismogram which develops 
some time after the first indication of a disturbance. Milne 
applied this in 1895 to show that the interval of time between 
the apparent “start” and the occurrence of the long waves 
on the record provided a means of estimating the distance of 
the epicentre. Although the estimate is not very accurate, it 
was really the first step in the interpretation of seismograms 
and in the location of the epicentre from observations made at 
distant points. 
About this time we owe to Rudski (‘‘ Physik der Erde’’) 
and to von Kévesligethy (‘‘Seismonomia”) theoretical investi- 
gations of the path of seismic rays within the earth. The first 
application of the well-known theory of longitudinal and trans- 
versal waves to Milne seismograms appears to have been made 
by Oldham (“ Phil. Trans. R.S.,” 1900). Milne seismograms, 
however, partly because of the presence of instrumental vibra- 
tion and partly because of the comparatively slow speed of 
registration, do not readily lend themselves to an accurate a 
priovi estimate of the occurrence of the second or transverse 
phase. Thus no great progress to accuracy seems to have 
been made until the interpretation of seismograms was taken 
up by Wiechert and his pupils at Gottingen. 
Wiechert’s investigations began about 1900, when, at the 
