d correctly, to
the loss of _vis viva_, and consequently of speed, by the _discontinuity
of the materials_.

And some indication of the general truth of the fact was derivable from
comparing the rude previous approximations to the transit rate of some
great Earthquakes. In the case of that of Lisbon, estimated by Mitchell
at 1,760 feet per second. It was still desirable to extend similar
experiments to the harder classes of stratified and of contorted rocks.
This I was enabled to carry into effect, at the great Quarries at
Holyhead (whence the slate and quartz rocks have been obtained for the
construction of the Asylum Harbour there), taking advantage of the
impulses generated at that period by the great mines of powder exploded
in these rocks.

The results have been published in the "Philosophical Transactions for
1861 and 1862 (Appendix)." They show that the mean lowest rate of wave
transit in those rocks, through measured ranges of from 5,038 to 6,582
feet, was 1,089 feet per second; and the mean highest, 1,352 feet per
second; and the general mean 1,320 feet per second.

By a separate train of experiments on the compressibility of solid cubes
of these rocks, I obtained the mean modulus of elasticity of the
material when perfectly continuous and unshattered, with this
remarkable result--that in these rocks, as they exist at Holyhead,
_nearly seven-eighths of the full velocity of wave transmission due to
the material, if solid and continuous, is lost by reason of the
heterogeneity and discontinuity_ of the rocky masses as they are found
piled together in Nature.

I also proved that the wave-transit period of the unshattered material
of these rocks was greatest in a direction _transverse_ to the bedding,
and least in line parallel with that; but the effect of this in the
rocky mass itself may be _more_ than counterbalanced by the
discontinuity and imperfect contact of the adjacent beds.

These results indicate, therefore, that the superficial rate of
translation of the solitary sea-wave of earthquakes may, when over very
deep water, equal or even exceed the transit rate (in some cases) of the
elastic wave of shock itself.

These results have since received general confirmation by the careful
determinations of the transit rates of actual earthquake waves, in the
rocks of the Rhine Country and in Hungary, by Noeggerath and Schmidt
respectively, and by those made since by myself in those of Southern
Italy, to whic