strict reference to the _changes of elevation_ in the water, while
they distinguish the recurrent streams as _tidal currents_. Hence,
also, _rise_ and _fall_ appertain to the tide, while _flood_ and _ebb_
refer to the tidal current.

The _cause of the tides_ is the combined action of the sun and moon.
The relative effects of these two bodies on the oceanic waters are
directly as their mass and inversely as the square of their distance;
but the moon, though small in comparison with the sun, is so much
nearer to the earth that she exerts the greater influence in the
production of the great _tide wave_. Thus the mean force of the moon,
as compared with that of the sun, is as 2¼ to 1.

The attractive force of the moon is most strongly felt by those parts
of the ocean over which she is vertical, and they are, consequently,
drawn toward her. In the same manner, the influence of the luminary
being less powerfully exerted on the waters furthest from her than on
the earth itself, they must remain behind. By these means, at the two
opposite sides of the earth, in the direction of the straight line
between the centers of the earth and moon, the waters are
simultaneously raised above their mean level; and the moon, in her
progressive westerly motion, as she comes to each meridian in
succession, causes two uprisings of the water--two high tides--the one
when she passes the meridian above, the other when she crosses it
below; and this is done, not by drawing after her the water first
raised, but by raising continually that under her at the time; this is
the _tide wave_. In a similar manner (from causes already referred to)
the sun produces two tides of much smaller dimensions, and the joint
effect of the action of the two luminaries is this, that instead of
four separate tides resulting from their separate influence, the _sun