260_
'
THE SEAS
obtained keeps its original temperature, and this is then
read off from a thermometer xed in the bottle itself.
This is'a reliable method when a knowledge of the tempera—
ture is required only in shallow waters but a further
complication steps in if one wishes to study the deepest
water layers. . »
The water at this depth is under very ’great pressure,
but when the sample has reached the surface the. pressure
”
is very much reduced.
if water is compressed its
‘
temperature is-slightly raised, and conversely if the pressure
is reduced the-water becomes cooled.
In the passage
of a
sample in the insulated water bottlefrom'a great depth
'
to.the surface, the water, cold as it was at the start,
be slightly .colder when the observer reads the temperature.
This error
must therefore be counteracted.
Accordingly
'
,
a “ reversing thermometèr ” is used, that is one in which
théreis an S—shaped bend, so that if it be suddenly turned
_upside dom the thread of mercury is broken and a
‘
permanent record of the temperature is obtained. The
thermometer on reaching the required depth is reversed
by means of a weight Which slide‘s down the wire and
releases a spring catch.
Nowadays by using at the
'
same
time -both_ the reversing thermometer and that
enclosed in- the 'insulated water bottle it has become
possible to compile a table from which the error due to the
release of pressure can be calculated, 'so that the water“
bottle can always be used;
At big depths a thermometer
enclosed in an
Outer case of’thick“ glass is used as a safe—
guard against,the pressure. »_
'
_
'
,
With the insulated- water bottle samples of water are
obtained, which, besides—giVing information on
perature,
be drawn off into clean, stoppered bottles
to await future chemical analysis.
The…water is subj ected
to extremely delicate
which give information