THE PRESIDENTIAL ADDRESS. 13 
the material of the central core is unable to withstand shearing stress, 
just as if it were fluid, but this must at present remain, I think, uncertain. 
It should be remarked that the wave-velocities by themselves do not 
furnish any information as to the elasticities or the density of the material, 
since they involve only the ratios of these quantities. The relation between 
the two velocities is however significant, and it is satisfactory to note that 
it has much the same value as in ordinary metals or glass. 
It is to be regretted that at present so little is being done in the way of 
interpretation of seismic records. Material support in the way of more and 
better equipped stations is certainly needed, but what is wanted above all 
is the co-ordination of such evidence as exists, the construction of more 
accurate tables, and the comparative study of graphical records. These 
latter present many features which are at present hard to interpret, and a 
systematic comparison of records of the same earthquake obtained at 
different stations, especially if these are equipped with standardised 
instruments, should lead to results of great theoretical interest. The task 
will be a difficult one, but until it is accomplished we are in the position 
of a scholar who can guess a few words in an ancient text, possibly the most 
significant, but to whom the rest is obscure. 
Even on this rapid review of the subject it should be clear that there is 
an apparent inconsistency between the results of two lines of argument. 
On the one hand, the thermal evidence points to the existence of a high 
temperature at a depth which is no great fraction of the earth’s radius, 
so high indeed as to suggest a plastic condition which would readily yield 
to shearing stress. On the other hand, the tidal arguments, as well as the 
free propagation of waves of transversal vibration at great depths, indicate 
with certainty something like perfect elasticity in the mathematical 
sense. The material with which we are concerned is under conditions far 
removed from any of which we have experience; the pressures, for instance, 
are enormous; and it is possibly in this direction that the solution of the 
difficulty is to be sought. We have some experience of substances which 
are plastic under long-continued stress, but which behave as rigid bodies as 
regards vibrations of short period, although this combination of properties 
is, I think, only met with at moderate temperatures. It is conceivable 
that we have here a true analogy, and that the material in question, 
under its special conditions, though plastic under steady application of 
force, as for instance centrifugal force, may be practically rigid as regards 
oscillatory forces, even when their period is so long as a day ora fortnight. 
But beyond that we can hardly, with confidence, go at present. 
