CHAPTER VI. 
THEORY OF A SOLID ISOTROPIC EARTH. 
A COMPARISON of seismograms obtained at different stations 
suggests at once that we are concerned with mechanical effects 
propagated from the region in which the earthquake oc- 
curred. 
We are thus led to inquire what is the nature of the effects 
propagated and to form a working theory as to the physical 
properties of the Earth, which will enable us to co-ordinate the 
observations. 
At the present time the evidence in favour of a solid Earth 
is very great, but the alternative view that the interior of the 
Earth is fluid retarded for a considerable time the progress of 
seismological theory, which requires the Earth to possess the 
properties of an elastic solid. 
As astronomical theory agrees with seismological in de- 
manding a solid earth we accept this as a primary condition. 
The simplest assumption we can make is that the physical 
properties of the Earth are uniform throughout, and although 
we shall find that seismology requires a modification of this 
assumption, yet many important features of a seismogram 
become intelligible on the basis of this simple hypothesis, and 
quantitatively the differences are not so great but that we may 
regard a uniform isotropic Earth as giving a good first ap- 
proximation to the co-ordination of results. Accordingly it is 
instructive to begin by a consideration of the effects to be 
expected on this view, as it prepares us to make a first inter- 
pretation of a seismogram and to see on what lines the modi- 
fication has to proceed. 
The fundamental equations of motion of a uniform isotropic 
solid are so fully dealt with in treatises on elasticity (e.g. Love’s 
37 
