50 MODERN SEISMOLOGY 
The second phase lasts for a time depending on the dis- 
tance, and then the whole appearance of the seismogram changes 
and assumes a strongly periodic and sinusoidal character. The 
point at which the change takes place is only rarely sharply 
marked and is not characterized by a large movement such as 
we have with PandS. This phase (undae longae) is initiated 
by L. For distances not less than 2000 km. the general 
appearance of this phase is marked by first a few waves of 
period about 20 seconds, gradually increasing in amplitude and 
looking as if they had been drawn with a shaking hand, then 
a rapid development of extremely smooth waves of rather 
shorter period which reach a maximum amplitude, subside, 
pass through a succession of maxima before merging into the 
tail of the earthquake or Coda. 
For short distances, however, this description does not hold 
good. L succeeds S very quickly, shorter periods of about 12 
seconds prevail, and the duration of the whole phase becomes 
very short. 
These remarks apply as a whole to (XY) and Z; but, as a 
rule, the development of this phase in Z comes rather later than 
in (XY). 
Following the maximal or long wave phase we have the 
Coda. The amplitudes are now small and the movements are 
somewhat irregular and lacking in smoothness. Still the 
motion here is on the whole periodic and sinusoidal (about 12 
seconds). 
If the earthquake is a very large one, we may after about 
24 hours observe the arrival of long waves that have 
travelled by the opposite side of the earth. In this way 
Galitzin has found from the records of the great Messina 
earthquake of December, 1908, that the long waves travel 
round the earth with a surface velocity of 3°53 km. per second, 
which agrees well with the theoretical value for Rayleigh 
waves, Viz. O°919 x 4:01 = 369 km. per second. 
The view that P and S represent the arrival of the longi- 
tudinal and transversal waves that have travelled by brachisto- 
chronic paths from the focus to the station may be accepted 
without much question. The difficulty that attaches to the 
