A subsidiary cause of tides is found in the revolution of the earth and moon about their common centre of gravity.  Revolution about an axis through the centre of a sphere enlarges the equator by centrifugal force.  Revolution about an axis touching the surface 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 three inches.

This constant ebb and flow of the great sea is a grand provision for its purification.  Even the wind is sent to the sea to be cleansed.  The sea washes every shore, purifies every cove, bay, and river twice every twenty-four hours.  All putrescible matter liable to breed a pestilence is carried far from shore and sunk under fathoms of the never-stagnant sea.  The distant moon lends its mighty power to carry the burdens of commerce.  She takes all the loads that can be floated on her flowing tides, and cheerfully carries them in opposite directions in successive journeys.