TIDES are regular motions or successive rising and falling of the
waters of oceans and seas. They are caused chiefly by the attraction
of the moon, which draws up the waters as seen in _cut_ No. 1. Thus,
as the moon revolves round the earth, its attracting or drawing power
passes with it over the surface of the ocean, and the elevation of
the waters, following that attraction, is drawn along until the shore
or coast of a country stops its progress; this causes, all along the
coast, a rising of the water, and it is then called high tide. The
tides occur twice in twenty-four hours.
The attraction of the moon in producing tides is affected in some
degree by the sun. When the sun and moon attract in unison, the tides
are very high; and when the sun tends to counteract the action of the
moon the tides are not so high. _Cut_ No. 2.
The _highest_ tide is called _spring tide_, and the _lowest_, _neap
tide_. These tides follow each other in regular succession, the daily
tides gradually decreasing during fifteen days, from the _highest_ to
the _lowest_; and during the next fifteen days, increasing from the
_lowest_ to the _highest_.
[Illustration: MOON’S ORBIT.