THE PROPAGATION OF THE DISTURBANCE. 123 
TABLE 13.— Velocities of Earthquake Waves at Various Depths below the Earth’s Surface. 
First PRELIMINARY | SECOND PRELIMINARY 
TREMORS, TREMORS. 
DisTANCE Derpru 
seine Enns or |F80M Earta’s| BELOW 
hh SURFACE. 
PATH. thre 





sossssooor 
WIM MD OMOO 



results we have reached can only be looked upon as fair approximations to the truth; 
and we need more numerous and more accurate determinations of the times of trans- 
mission of earthquake waves, especially to great distances, before we can reach a satis- 
factory knowledge of the velocity of propagation at various depths. 
INTERNAL REFLECTIONS. 
When the waves of the first two phases come to the surface of the earth they are 
reflected, and as the density of the air is insignificant in comparison with that of the rock, 
practically none of the energy escapes into the air. But the reflected energy will be 
divided between two waves, a longitudinal and a 
transverse, each of which, therefore, will be weaker 
than the original waves.t Waves will reach a 
given station, S (fig. 29), after a single reflection, 
from three points on the are between the focus 
and the station. The first is the half-way point B; 
from this point an incident longitudinal wave will 
send a reflected longitudinal wave and an in- 
cident transverse wave will send a reflected 
transverse wave to the station. The second is 
the point C, where the reflected transverse 
waves, due to incident longitudinal waves, pass 
off in the proper direction at an angle of reflec- 
tion smaller than the angle of incidence, because 
their velocity is less than that of the longitudinal. 
The third point is D, where the transverse waves 
are transformed into longitudinal waves, which pass on to S. There would also be three 
analogous points B’, C’, D’, on the major arc. When we consider waves which reach S 
after two reflections, we see that they may follow many different paths, as they transform 
by reflection from one type of wave to the other; there are of course two points, situated 
at one-third and two-thirds the distance to the station, where reflections can take place 






1 The reflection and refraction of waves in elastic media was first thoroly elucidated by Prof. C. G. 
Knott, “ Earthquakes and Earthquake Sounds,” Trans. Seismol. Soc. Japan, 1888, vol. x11, pp. 115- 
136; “Reflection and Refraction of Elastic Waves, with Seismological Applications,” Phil. Mag., 
1899, vol. xLvu1, pp. 64-97, 567-569. He has also given a very interesting account of the subject in 
his recently published work, “The Physics of Earthquake Phenomena.” Prof. E. Wiechert has also 
discust this subject (‘Ueber Erdbebenwellen,” Nach. d. K. Gesells. d. Wissen. zu Gottingen, Math.- 
phys. K1., 1907). 
