is
very critical epoch of its history be compared with a grindstone
which is being driven so rapidly that it is on the very brink of
rupture. It is remarkable to note, that a cause tending to precipitate
a rupture of the earth was at hand. The sun then raised tides in the
earth as it does at present. When the earth revolved in a period of
some four hours or thereabouts, the high tides caused by the sun
succeeded each other at intervals of about two hours. When I speak of
tides in this respect, of course I am not alluding to oceanic tides;
these were the days long before ocean existed, at least in the liquid
form. The tides I am speaking about were raised in the fluids and
materials which then constituted the whole of the glowing earth; those
tides rose and fell under the throb produced by the sun, just as truly
as tides produced in an ordinary ocean. But now note the significant
coincidence between the period of the throb produced by the sun-raised
tides, and that natural period of vibration which belonged to our
earth as a mass of molten material. It therefore follows, that the
impulse given to the earth by the sun harmonized in time with that
period in which the earth itself was disposed to oscillate. A
well-known dynamical principle here comes into play. You see a heavy
weight hanging by a string, and in my hand I hold a little slip of
wood no heavier than a common pencil; ordinarily speaking, I might
strike that heavy weight with this slip of wood, and no effect is
produced; but if I take care to time the little blows that I give so
that they shall harmonize with the vibrations which the weight is
naturally disposed to make, then the effect of many small blows will
be cumulative, so much so, that after a short time the weight begins
to respond to my efforts, and now you see it has acquired a swing of
very considerable amplitude. In Professor Fitzgerald's address to the
British Association at Bath last autumn, he gives an account of those
astounding experiments of Hertz, in which well-timed electrical
impulses broke down an air resistance, and revealed to us ethereal
vibrations which could never have been made manifest except by the
principle we are here discussing. The ingenious conjecture has been
made, that when the earth was thrown into tidal vibrations in those
primeval days, these slight vibrations, harmonizing as they did with
the natural period of the earth, gradually acquired amplitude; the
result being that