36 Proceedings of the Royal Society of Edinburgh. [Sess. 
As regards application to the problems suggested by seismic phenomena, 
we are practically concerned only with cases in which the angle of incidence 
is small. In the immediate vicinity of the epicentre, where the motions are 
large and very complex, any application of an elastic solid theory is out of 
the question. With this limitation to small incidences the numbers in the 
first and second tables show that an incident condensational wave is 
accompanied by a comparatively small tangential displacement, but that an 
incident distortional wave is accompanied by a large tangential displace- 
ment. An instrument therefore which records horizontal movements of the 
earth’s surface will be more sensitive to the outcrop of a distortional wave 
than to the outcrop of a condensational wave. If the first preliminary 
tremors are the result of condensational waves propagated through the 
earth, horizontal pendulum records at stations far distant from the earth- 
quake source will tend to be retarded in their appearance. 
The above investigation deals with the very simplest case of plane 
waves. But plane waves in earthquake tremors will be the exception. 
The nature of the original disturbance, and the influence of the heterogeneity 
of the crust through which the disturbance must pass before it reaches the 
surface, will combine to produce a great complication of movements. Let 
us suppose that the horizontal and vertical components of these movements 
could be accurately measured. If these components happened to be cophasal, 
we might be able to evaluate the real angle of emergence. If, as is highly 
probable, they are not cophasal, then the resultant motion will be almost 
certainly much more complicated than the elliptic motion established above. 
It is hopeless to expect any results of consequence to be drawn from such 
indications. 
Note added October 1909. 
While this paper was being printed I came across a memoir on Earth- 
quake Waves by E. Wiechert and K. Zoeppritz, published in the Nachrichten 
von dev koniglichen Gesellschaft dev Wissenschaften zu Gottingen (1907). 
The memoir is a long one of 138 pages, and touches upon many points of 
interest. Fully one-third of it is a discussion of the elastic problems 
treated in my early papers of 1888 and 1899, with which apparently the 
authors were unacquainted. Of the other questions investigated one is 
very similar to the problem discussed in “Seismic Radiation,” but their 
mode of approach is quite different. Broadly speaking, the conclusions are 
very similar. In a very ingenious manner Wiechert and Zoeppritz work 
from the measurable surface phenomena of earthquake transmission to the 
mode of propagation through the heart of the earth, using for this purpose 
