les the surface of the sea
into ripples, and then, acting with ever-increasing power on the little
surfaces thus raised, blows _them_ up into waves, and finally into great
billows. Sometimes, however, winds burst upon the calm ocean with such
sudden violence that for a time the waves cannot lift their heads. The
instant they do so, they are cast down and scattered in foam, and the
ocean in a few minutes presents the appearance of a cauldron of boiling
milk! Such squalls are extremely dangerous to mariners, and vessels
exposed to them are often thrown on their beam-ends, even though all
sail has been previously taken in. Generally speaking, however, the
immediate effect of wind passing either lightly or furiously over the
sea is to raise its surface into waves. But these waves, however large
they may be, do not affect the waters of the ocean more than a few yards
below its surface. The water below their influence is comparatively
calm, being affected only by ocean currents.

The tides of the sea--as the two great flowings and ebbings of the water
every twenty-four hours are called--are caused principally by the
attractive influence of the moon, which, to a small extent, lifts the
waters of the ocean towards it, as it passes over them, and thus causes
a high wave. This wave, or current, when it swells up on the land,
forms high tide. When the moon's influence has completely passed away,
it is low tide. The moon raises this wave wherever it passes; not only
in the ocean directly under it, but, strange to say, it causes a similar
wave on the opposite side of the globe. Thus there are two waves always
following the moon, and hence the two high tides in the twenty-four
hours. This second wave has been accounted for in the following way:
The cohesion of particles of water is easily overcome. The moon, in
passing over the sea, separates the particles by her attractive power,
and draws the surface of the sea away from the solid globe. But the
moon also attracts the earth itself, and draws it away from the water on
its opposite side thus causing the high wave there, as represented in
the diagram, _figure 1_.

The sun has also a slight influence on the tides, but not to such an
extent as the moon. When the two luminaries exert their combined
influence in the same direction, they produce the phenomenon of a very
high or spring-tide, as in _figure 2_, where the tide at _a_ and _b_ has
risen extremely high, while at _c