'
.
,
-
223
and in general there is adecrease in temperatureWith depth.
Usually the effects of the sun’s warmth are not felt to a
_
greater depth than three hundred fathoms.
Actually
‘
the heat rays are rapidly absorbed by the upper few inches
of water and any -warmth that is tranSported to deeper
layers is mostly brought about by the mixing of the warmer
with thecolder water.
.
.
‘
:
Owing to this very slow warming up of the sea. there is
_a lag in the seasonal change of temperature, and while
our latitudes the h0ttest time of the year is June, the
water does
not reach its maximum temperature until
August.
Now. we have seen that the deeper layers only
receive their heat by mixing with the warm upper water
and
while there is a lag between the raising
the
_
'
temperature of the air and
the water— there is a
'
further lag in the warming of the deeper water, until at
_
.
about fty ,iathoms we get a complete reversal of Seasons,
There the hottest part of the year "is in December; that is
mid—winter, and the” coldest about May or June. Below a
hundred fathoms there is no seasonàl change whatever in
temperature, and, year in year out, the conditions are
_
uniform.
From this depth downwards the temperature
'
gradually falls until in_the ocean abysses it remains always
at somewhere near the freèzing point.
:
-
_
The actual weight or specic gravity of the water depends
on its temperature andupori its saliriitÿ, the warm water
being lighter than the cool, and the fresh water lighter
than salt
In the tropics therefore the high temperature
-
‘
makes _;the water light, but at the.sàme time _owing to
'
,-evaporation the" salinity is high, which teds*to make the
water heavy.
-
'
'
‘
_
-
'
The pressure in the Sea varies with the depth.
At every
'
ten— metres depth the pressureis increased by one atmos—
_
phere, that is by 14 lbs. ‘to the square inch.
In the great