The morning news. [volume] (Savannah, Ga.) 1887-1900, 1896-07-06
RISE AND Fall OF THE TIDES.
Attraction of the Moon Different i'm
Parts OF the Paciac.


From knowledge.
The attraction Of the moon or OF the
sun is equally exercised upon all parts
OF sheet Of water 81 such size as the
Mediterranean. 50 that scarcely any tide
wave Is raised there. but in the Pacific
ocean. which is the chief cradle Of the
tides. the attractions Of. say, the moon, is
at any moment aDpreciably different in
different parts. and the waters heap them
selves up into a long. low hillOck. which
follows the moon from east TO west, ira
versing the ocean at vast speed as
forced long wave. Two such pillows are
formed by the moon in each twenty-four
hours, for when the moon is on the Op-
pOsite side of uhe earth the solid globe
Is pulled away from the waters, leaving
them heaped uD. so that the result is
much the same as when the moon is
overhead. The long. low pillows which
are formed by the moons action in the
equatorial regions where the ocean girdles
the earth, travel on as free waves into
the higher attitudes, penetrating every
open sea and channei- These free TIDE
waves run under the action Of the earJh
gravity. almost exactly in the same way
that the solitary wave travels on IN
a canal when the motion OF the canal
bvdL mas DEen arrested. Like llc SpildAg
wave formed in the canals the tide wave
is a long wave. for the moons attraction
being practically equal al the surface and
at the bottom OF the sea the motion OF
the water PARTICLES is the same at the
bottom and at the surface The motion
Of the water IS mainly backward and
forward swing, the vertical motion being
very small compared with the horizontal
motion.
The rapidity with which such a wave
travels when left to itself-or rather TO
the force of the earths gravity-depends
upon the depth of the channel, because
the deeper the channel the smaller is the
quantity of water which has to be trans
mitted through each unit of cross sec
tion in order 10 maintain the hillock upon
the surface In the deep waters of the
open ocean the hight of the tide billow is
very small-say. from one to three feet.
As the wave approaches the Swallowing
slope of the shore the front of the billOw
is retarted, while the hinder parts press
on, and the hight of the wave is thereby
increased. The hight of the wave is also
increased when il reaches a narrowing
channel in which the billoW is laterally
compressed. Thus the highest tides are
not in open ocean, where the tides are
generated, but in the distant channels,
bays, and inlets, where the tide wave
penetrates after the moon has left 1t TO
run its course. Once every twelve hours,
as we have seen, the moon raises a billow
and drives or drags IL forward as a pulse
Of water in the southern oceans, much
as the heart in its rhythmic beat,
drives the blood pulse into the arteries.
But, whereas the blood pulse runs the
whole length of the artery, and is lost
in the capillaries before the next stroke
of the heart sends out a second pulse,
the tide wave, on the other hand, has
not reached the most distant shore when
the next succeeding pulse of water is set
going by the moon. The tide wave of
which the crest may round cape of Good
Hope at noon, travels up the west coast
of Africa and reaches the Azores about
midnight BY this time it is followed by
a second pulse, which is now rounding the
cape.
It is O'Clock in the morning before
the crest of the first wave has reached
the entrance of the English channel,
where the shallower water makes 1T trav-
ell slower. lit takes another six hours to
run the length of the channel, reaching
the straights of Dover at 10 a. m, where
the hight of the wave is considerable,
owing, partly to the narrowing of the
channel. Thus, at Dover the water rises
Fl feet and this occurs at Dover about
six hours after the wave crest passed
the Lands End. Now, the next incoming
tide billow on its way from the South
ern ocean has not yet got as far as the
Azores, and the water at the entrance
to the channel is at a low level-tllis be-
between the two succeeding tide pillows.
Therefore, we have the water heaped up
(and pent up, too, for the straits are mar
row near Dover, the water being low at
the other end of the channel, the action
of the earths gravity sets long wave
travelling back from Dover towards the
Lands End and this is the ebb tide in
the English channel. The time which
free, long wave takes to travel the length
of the channel being about six hours, the
the channel of nearly as much water as
was sent in by the tidal billOW