SEA WATER
237
interval between one high tide and the next is not exactly
twelve.hours, but on the average twelve hours and twenty
minutes.
Thus the time of high Water is not the same
.
every day, but there is a progressive change, the high tide
of to—morrow aftemoon being about forty minutes later
than that of this aftemoon.
So throughout the years
there is practically no time of day at which high or low
water may not Coeur at any place.
Together with this change in the time of the tides there
.
is also to be n0ticed a change in the actual range of the
tides, the distance between high and low—water marks
becoming progressively greater throughout a certain period
and
then
correspondingly smaller.
These
periods are
known as the periods of ‘-‘ spring
and of “
neap
”
tides.
During the “ spring
”
tides
(Saxon :
sprùngen,
“
to
bulge ”) the low—water mark is lowest and the high—water
mark highest, that is, the range between the tide marks
’
is greatest; during the nèaps the reverse is the case, the
«tide only coming in “and gôing out a short distance. Besides'
the attraction of the moon there is also a smaller attraction
exerted by the sun.
It is because of the sun’s_ attraction
_
that the spring and neap tides are caused… When the sun
_
and moon are both exerting their pulls in the same direction
.
_
‘
the force will be at its greatest andthe big tide's or springs -
will occur.- This happens shortly after both new and-full
moon (Fig. 50).
But the range
of the tides_ will be greatest
at
becauseÏthen the sun-and moon are both;
on the same side of the earth ; and the spring tides are
less
_
at full moOn, the sun and
then opposite one
.
another.
When the forces of the sun and 'moon are at
,
right angles to one another,
_
at the periods of
half
_
moon, the range of the tides will be least and neap tides
,
Will result. -
,
'
‘
'
‘
"
-_
‘
.
-
The tidês must not be confused with “tidal streams -’.’.