234
THE SEAS
’
it is so small as to have no effect in drawing it vertically
_
away from the earth’s surface.
"But the further we go
from this point on the hemisphere directed towards the
moon the more horizontal does the moon’s pull become.
It
does not take nearly so great a.force to make Water slide
horizontally over the earth as to draw it vertically upward,
and the moon’s attraction is sufcient to do the former. The
result is that water is drawn over the surface of the earth
from all around and towards the point beneath the moon
so
that the water becomes
up there.
Where this
bulge of water occurs the tide is high.
.
On the hemisphere of the earth pointing away from the
moon exactly similar forces are acting to cause high tide
at the point furthest from the moon. ’
How these forces
come about can be proved mathematically, but it is somewhat beyond the scope of this book to enter into an explana_
tion.
'
.
In this way there are tWo high tides simultaneously on
the earth, one at a point beneath the moon, and the other
"
on the other face of the earth opposite to the rst.
These
two high tides cause the water lying between them to be
drawn away, so that at tWo points lying
either side
of the earth midway between the two high tide regions
there will be 10w tides.
'
_
-
_
_But the earth is revolving, so that approximately once
eaCh day the moon
exerts its inuence on every meridian
"
the
surface in turn} Under these conditions
—
the points at
high ’tide occurs change with the
changing
of the-moon, and a “ tidal wave” sweeps
'
round the surface of the globe.
(This must not be confused
with the popular. and wrongly named ‘.‘ tid3,1 wave " of
great dimensions and destructive force, Which is usually
a
wave
caused by a submarine earthquàlæ and quite
uneonnected .With the moon’s
.
attraction.) …If the earth
\