e connected by Laplace with
an analytical theory in which the physical conditions of the question
figure for the first time. Accordingly calculators, to the immense
advantage of the navigation of our maritime coasts, venture in the
present day to predict several years in advance the details of the time
and height of the full tides without more anxiety respecting the result
than if the question related to the phases of an eclipse.

There exists between the different phenomena of the ebb and flow of the
tides and the attractive forces which the sun and moon exercise upon the
fluid sheet which covers three fourths of the globe, an intimate and
necessary connection from which Laplace, by the aid of a series of
twenty years of observations executed at Brest, deduced the value of the
mass of our satellite. Science knows in the present day that
seventy-five moons would be necessary to form a weight equivalent to
that of the terrestrial globe, and it is indebted for this result to an
attentive and minute study of the oscillations of the ocean. We know
only one means of enhancing the admiration which every thoughtful mind
will entertain for theories capable of leading to such conclusions. An
historical statement will supply it. In the year 1631, the illustrious
Galileo, as appears from his _Dialogues_, was so far from perceiving the
mathematical relations from which Laplace deduced results so beautiful,
so unequivocal, and so useful, that he taxed with frivolousness the
vague idea which Kepler entertained of attributing to the moon's
attraction a certain share in the production of the diurnal and
periodical movements of the waters of the ocean.

Laplace did not confine himself to extending so considerably, and
improving so essentially, the mathematical theory of the tides; he
considered the phenomenon from an entirely new point of view; it was he
who first treated of the stability of the ocean. Systems of bodies,
whether solid or fluid, are subject to two kinds of equilibrium, which
we must carefully distinguish from each other. In the case of stable
equilibrium the system, when slightly disturbed, tends always to return
to its original condition. On the other hand, when the system is in
unstable equilibrium, a very insignificant derangement might occasion an
enormous dislocation in the relative positions of its constituent parts.

If the equilibrium of waves is of the latter kind, the waves engendered
by the action of winds,