urface
of a flexible globe converts it into an egg-shaped body, with the
longer axis perpendicular to the axis of revolution. In Fig. 56 the
point of revolution is seen at the centre of gravity at G; hence,
in the revolution of earth and moon as one, a strong centrifugal
force is caused at D, and a less one at C. This gives greater height
to the tides than the attraction of the moon alone could produce.

[Page 147]
[Illustration: Fig. 56.]

If the earth had no axial revolution, the attractive point where
the tide rises would be carried around the earth once in twenty-seven
days by the moon's revolution about the earth. But since the earth
revolves on its axis, it presents a new section to the moon's attraction
every hour. If the moon were stationary, that would bring two high
tides in exactly twenty-four hours; but as the moon goes forward,
we need nearly twenty-five hours for two tides.

The attractive influence of the sun also gives us a tide four-tenths
as great as that of the moon. When these two influences of the sun
and moon combine, as they do, in conjunction--when both bodies
are on one side of the earth; or in opposition, sun and moon being
on opposite sides of the earth--we have spring or increased tides.
When the moon is in its first or third quarter, _i. e._, when a
line from the moon to the earth makes a right angle with one from
the sun to the earth, these influences antagonize one another,
and we have the neap or low tides.

It is easy to see that if, when the moon was drawing its usual
tide, the sun drew four-tenths of the water in a tide at right
angles with it, the moon's tide must be by so much lower. Because
of the inertia of the water [Page 148] it does not yield instantly
to the moon's influence, and the crest of the tide is some hours
behind the advancing moon.

The amount of tide in various places is affected by almost innumerable
influences, as distance of moon at its apogee or perigee; its position
north, south, or at the equator; distance of earth from sun at
perihelion and aphelion; the position of islands; the trend of
continents, etc. All eastern shores have far greater tides than
western. As the earth rolls to the east it leaves the tide-crest
under the moon to impinge on eastern shores, hence the tides of
from seventy-five to one hundred feet in the Bay of Fundy. Lakes and
most seas are too small to have perceptible tides. The spring-tides
in the Mediterranean Sea are only about t