OCEANIC MOVEMENTS.

position of the continents. Owing to the obstruc-
tions offered by the continents, and by inequalities
in the bed of the ocean, a very considerable re-
tardation of the tidal wave is effected, so that a
high tide may not occur at a place until long
_after the moon has passed over it.

Solar Tides—The sun also produces a system
of tidal waves, but owing to its greater distance
from the earth, the tides thus produced are much
smaller than those of the moon, upon which, there-
fore, they exert but a modifying influence. The
tide-producing power of the moon is greater than
that of the sun, in about the proportion of 800
to 855. That is, the tide produced by the moon
is about 24 times greater than that produced by
the sun.

‘The tidal wave moves, in general, from east to west, or in
the opposite direction to the rotation of the earth. The motion
of so large a mass of water thus opposed to the earth’s ro-
tation, must gradually diminish the axial velocity, and,
eventually, entirely stop the rotation of the earth; in this
way an increase in the length of day and night should be
produced, but so far, however, no increase has been de-
tected, although astronomical observations extend back-
ward for long periods. The increased axial velocity, pro-
duced by the contraction of the globe, probably balances
the retarding influence of the tides.

In the deep ocean, and near the mouths of rivers, the
duration of the flood and ebb are about equal; but in most
rivers, at some distance from the mouth, the ebb is longer
than the flood. The cause is to be found in the fact that
the outflowing river current meets and temporarily neu-
tralizes the inflowing flood tide, thus diminishing its dura-
tion, and afterward, adding its motion to the ebb, makes
the difference between the two still greater.

The tidal wave often ascends a stream to a much greater
elevation above the level of its mouth than the height of
the tide at the river’s mouth. In large rivers, like the
Amazon, the tidal wave advances up the river as much as
100 feet above the sea-level.

Neap Tid
flood and eb!
moderate.

 

Some of the proofs of the connection between the tides
and the attraction of the moon and sun are as follows:

(1.) The interval between corresponding high tides at
any place is the same as the interval between two succes-
sive passages of the moon over that place: 24 hours, 52
minutes,

(2.) The tides are higher when the moon is nearer the
earth.

(8:) The tides are higher when the sun and moon are
simultaneously acting to cause high tides in the same

places.
“D /

Quarter.

Fig. 78, Cause of the Phases of the Moon,

Phases of the Moon.—An inspection of Fig. 73 will
show, that during new and full moon, the earth, moon,
and sun are all in the same straight line, but, that during
the first and last quarters, they are at right angles. The
portions of the earth and moon turned toward the sun are
illumined, the shaded portions are in the darkness. To
an observer on the earth, the moon, at a, appears new,
since the dark part is turned toward him; at b, however,
it must appear full, since the illumined portions are toward
him. At, and d, the positions of the quarters, only one-
half of the illumined half, or one quarter, is seen.

Spring Tides,
flood and ebb
excessive.

Fig. 74, Position of the Earth, Moon, and Sun during Spring and Neap Tides.

211. Spring and Neap Tides.——When the sun
and moon act simultaneously, on the same hemi-
sphere of the earth, as shown in Fig. 74, the tidal
waye is higher than usual.
then h‘ghest, and the ebb tides lowest.
are called spring tides.

These
They occur twice during

The flood tides are’

every revolution of the moon—once at full, and
once at new moon. The highest spring tides oc-
cur a short time before the March and the Sep-
tember equinoxes, when the sun isover the equa-
tor.

When, however, the sun and moon are 90°