DABLORM OFA SOLID DSODROPIE\ LA RTL, 39 
evidence that the primary disturbance at the focus consists of 
a concentrated shock or limited series of shocks occurring 
within a very short time, a matter of some seconds. In any 
case we are certainly not concerned with unlimited trains of 
waves proceeding from the focus, so that our discussion must 
now proceed in the light of Stokes’ ‘‘ Dynamical Theory of 
Diffraction ” (Collected Papers) in which he considers the effect 
of an arbitrary initial disturbance produced in the vicinity of 
a point. 
In an unlimited medium the disturbance spreads in spheri- 
cal shells from the origin. If the primary disturbance is of 
short duration, the effects observed at a point distant 7 will be 
first a short disturbance at the time taken for the longitudinal 
waves of velocity V, to reach the point, then a period of 
quiescence followed by a second short disturbance when the 
transversal waves of velocity V.(<V,) reach the point, after 
which the motion at 7 ceases. The relative magnitudes of 
these effects depends not only on the distance 7, but also on 
how the primary disturbance can be analysed into the two 
types, and in particular one or other may vanish. We have 
also to note that the effects are not the same at all points at 
distance 7, but depend on the axis or axes of the constituents 
of the initial disturbance. 
When we pass to the case of the Earth, we shall suppose 
in accordance with observation that the origin of disturbance 
is situated at a point comparatively near the surface of the 
earth. We may still expect that the seismogram obtained at 
a point on the earth’s surface will, in general, be characterized 
by a pronounced movement corresponding to the arrival of the 
longitudinal disturbance, and by a pronounced movement when 
the transversal disturbance arrives, both of which have travelled 
by the brachistochronic path (in this case a straight line) from 
the focus to the station. These are accordingly indentified 
with the beginning of the first phase P and the second phase 
S of a seismogram. 
From observations made at comparatively small distances 
(<1000 km.) from the focus, Zoppritz and Geiger find that 
V,=7'17 km. per second, and V,=4:'0o1 km. per second, and 
