of tides is now
known to be obtained at the expense of the earth's rotation; and
accordingly our day must be slowly, very slowly, lengthening. The tides
of past ages have destroyed the moon's rotation, and so it always turns
the same face to us. There is every reason to believe that in geologic
ages the moon was nearer to us than it is now, and that accordingly our
tides were then far more violent, rising some hundreds of feet instead
of twenty or thirty, and sweeping every six hours right over the face of
a country, ploughing down hills, denuding rocks, and producing a copious
sedimentary deposit.

[Illustration: FIG. 119.--Tide-predicter, for combining the ascertained
constituents into a tidal curve for the future.]

In thus discovering the probable violent tides of past ages, astronomy
has, within the last few years, presented geology with the most powerful
denuding agent known; and the study of the earth's past history cannot
fail to be greatly affected by the modern study of the intricate and
refined conditions attending prolonged tidal action on incompletely
rigid bodies. [Read on this point the last chapter of Sir R. Ball's
_Story of the Heavens_.]

[Illustration: Fig. 120.--Weekly sheet of curves. Tides for successive
days are predicted on the same sheet of paper, to economise space.]

I might also point out that the magnitude of our terrestrial tides
enables us to answer the question as to the internal fluidity of the
earth. It used to be thought that the earth's crust was comparatively
thin, and that it contained a molten interior. We now know that this is
not the case. The interior of the earth is hot indeed, but it is not
fluid. Or at least, if it be fluid, the amount of fluid is but very
small compared with the thickness of the unyielding crust. All these,
and a number of other most interesting questions, fringe the subject of
the tides; the theoretical study of which, started by Newton, has
developed, and is destined in the future to further develop, into one of
the most gigantic and absorbing investigations--having to do with the
stability or instability of solar systems, and with the construction and
decay of universes.

These theories are the work of pioneers now living, whose biographies it
is therefore unsuitable for us to discuss, nor shall I constantly
mention their names. But Helmholtz, and Thomson, are household words,
and you well know that in them and their disciples the race of Pioneers
m