tides and neap tides.The third, the annual
period, is what will bring about the expected
high tide of the 16th or 17th of September—
because the tide which will happen about
midnight (according to the locality) between the
16th and the 17th may perhaps attain the
maximum of elevation. The annual period is
manifested at the equinoxes, in March and
September, by the spring tides being higher and
the neap tides feebler than at other epochs of
the year. At the equinoxes, there is a greater
inequality in the tides generally; at the solstices,
there is a greater general equality.

The cause of these equinoctial high tides, is,
that the Sun is then crossing the equator, at
which point he is able to give the hardest pull
at the bulging waters; he is either actually on
the line, or only a little above or below it at the
time when the Moon also crosses the equator,
and is in syzygy, and sometimes also at her
shortest distance from the Earth. It appears,
therefore, that the tides are the effect of a
combination of varying forces, and that their
magnitude is exactly proportional to the strength
of those forces. By elaborate and complex
calculations, modern astronomers, led on by
Laplace, are able to predict the hour and the
height of every tide, with a precision which is
the admiration of thinking persons. If we
suppose the Moon's orbit so changed, that at
certain times she would approach the Earth
much nearer than she does, the consequence
would be tides of such force and elevation as to
devastate whole continents. Yet their height,
and the date of their occurrence, would be
calculable, if men were left to calculate them.
However curious we may be to have a nearer
view of our splendid satellite, it is better for
us, on the whole, that she should continue to
keep her present respectful distance.

The exact state of a tide, at any moment,
as well as the points of high and low water,
may be known in a seaport town by the
contrivance of a well having a subterranean
communication with the sea, so that the water shall
rise in it during the flow, and sink in it during
the ebb. By causing the water to enter a tube of
this kind by a small orifice, the agitation of the
waves without is rendered insensible.
Tide-wells of this kind, constructed by M. Chazallon,
the French naval engineer, exist at Cherbourg
and at Brest. The height of the tide is self-
registered by an instrument called a marégraphe.

Laplace was not satisfied with perfecting the
mathematical theory of the tides; he looked at
it in quite a novel point of view, and was the
first to treat of the stability of the equilibrium
of the seas. All systems, or collections and
combinations of bodies, whether solid or liquid,
are susceptible of two sorts of equilibrium, which
must be carefully distinguished. In stable
equilibrium, the system, if slightly disturbed
from its usual position, has a ceaseless tendency
to return to it. A well-ballasted ship, sailing
with a side wind, leans a little out of the
perpendicular, but rights again as soon as
the wind falls. The weight of the hull and
position of the centre of gravity of that weight
are sufficient to retain the whole vessel, with,
its masts and rigging, in constant stable
equilibrium. In the case of unstable equilibrium
the reverse takes place; a very slight disturbing
force suffices to upset the whole system so
constituted. An acrobat balances a ladder on
his shoulder; on the ladder, perhaps will mount
a child carrying flags, chairs, and sundry articles.
The whole are in unstable equilibrium.
Although the skill of the acrobat may put the
whole system, for an instant, in exact balance,
still the slightest tremor or puff of air, causing
his burden to lean ever so little on one side,
would precipitate the whole to the ground,
were he not, by slightly changing his position,
to obviate that tendency by restoring the
balance.

If the equilibrium of the Ocean be of this
latter kind, unstable, the waves caused by the
action of the winds, by earthquakes, by sudden
displacements of the bottom of the sea, have
been able in former times, and will be able at
future times, to rise to the summits of the
highest mountains. The geologist will have
the satisfaction of drawing from these prodigious
oscillations, a rational explanation of a
great number of phenomena; but then the
world must be regarded as liable to new and
terrible catastrophes.

We may take comfort, however, from Laplace's
assurance that the equilibrium of the Ocean is
stable: on the express condition, however (which.
is established by indubitable facts), that the
mean density of the liquid expanse be inferior
to the mean density of the Earth. He also
assumes that no change will ever occur in the
position of the Earth's centre of gravity, like
that which forms the basis of Adhemar's theory
of periodical deluges. But if, for the actual
Ocean (everything else remaining in the same
state), we substitute an ocean of mercury, the
stable equilibrium will have vanished, the
liquid will frequently burst its limits to rush
sweeping over terra lirma, and will mount even to
the snowy peaks above the clouds.

Although the phenomena of the tides be
owing to the action of the Moon and the Sun,
nevertheless many peculiarities attending them
still remain imperfectly explained. For instance,
between the tropics, with a few exceptions, the
tides are very feeble, although the action of the
two great luminaries is there perpendicular to the
surface of the water. In some of the South Sea
Islands, there is only one tide per day
. Calculation demonstrates that the rising of the waters
is slight in proportion as a sea is small; and we
find that the tides are scarcely perceptible in
the Caspian, Mediterranean, White, and Baltic
Seas, which are almost lakes: having either no
real or no considerable point of communication
with the Ocean. In the Black Sea the tides are
almost insensible; they ought to be still feebler
in the Baltic and the White Seas, in
consequence of their distance from the equator. In
the Gulf of Venice, the tide is more perceptible
than in the rest of the Mediterranean: which