METHODS OF OCEANOGRAPHICAL RESEARCH
2 57
Of equal importance in navigation comes the necessity
of knowing the speed and direction of the ocean currents.
The bearing of ”currents on the distribution of the life
drifting in the water layers is a problem that has received
considerable attention in modern shery research. InformaÂ
tion can be gained about currents both directly by noting
the movements of oating and drifting objects or actually
,
measuring the speed and direction by special instruments,
and indirectly by a study of the physical and chemical '
-
condition of the water itself from place to place.
In olden days much of the knowledge gained about ocean
currents was acquired
the mariners themselves noting
»
‘
the position of drifting wreckage or derelict ships and as
reports came in from_ different vessels it was possible to
"
mark off‘on a chart the route taken by a drifting wreck
and therefore to gain some idea of the general trend of
-
the current.
There was however a danger in basing too
denite a conclusion on such observations, as the wreckage
,
always had a portion above the water levelexposed to the
winds so that the path taken by such a wreck was therefore
caused by: a combination of wind and current.
To-day
information
is
sought _by using specially constructed
bottles which are thrown overboard -at‘specied positions. '_
They are known as " drift bottles ”
and they are used to
study the water movements atthe surface or near the sea
bottom.
The surface bottles_are so weighted' that they
.
_
-
oat almost completely submerged, and thus the least
possible area is exposed to wind“ action. The. bottom
bottlesare made just the slightest bit heavier than water
S
and have xed to them abouteighteen inches of sti Wire;
_
this wire trails along the bottom and holds«the bottle
clear of stones and other obstructions that
impede
_
its Vmov‘ement as it'
"carried by_' the current (Plate 90).
In each bottle is placed au addressed postcard with
__
_