into two parts: first, the consideration of the tides and
currents; and, secondly, the design of the works.

The tidal attraction is primarily due to the natural effect of
gravity, whereby the attraction between two bodies is in direct
proportion to the product of their respective masses and in
inverse proportion to the square of their distance apart; but
as the tide-producing effect of the sun and moon is a
differential attraction, and not a direct one, their relative
effect is inversely as the cube of their distances. The mass of
the sun is about 324,000 times as great as that of the earth,
and it is about 93 millions of miles away, while the mass of
the moon is about 1-80th of that of the earth, but it averages
only 240,000 miles away, varying between 220,000 miles when it
is said to be in perigee, and 260,000 when in apogee. The
resultant effect of each of these bodies is a strong "pull" of
the earth towards them, that of the moon being in excess of
that of the sun as 1 is to 0.445, because, although its mass is
much less than that of the sun, it is considerably nearer to
the earth.

About one-third of the surface of the globe is occupied by
land, and the remaining two-thirds by water. The latter, being
a mobile substance, is affected by this pull, which results in
a banking up of the water in the form of the crest of a tidal
wave. It has been asserted in recent years that this tidal
action also takes place in a similar manner in the crust of the
earth, though in a lesser degree, resulting in a heaving up and
down amounting to one foot; but we are only concerned with the
action of the sea at present. Now, although this pull is felt