METHODS O F SAMPLING THE BENTHOS
197
There is therefore a tendency for the grab to be jerked off the bottom
while it is closing, particularly if the ship is rolling (cp. Davis, 1926 ;
Thorsoii, 1057a). This can be mitigated to some extent by manual
closing of the grab, the grab being operated on a rope hauled in by a
FIG:. IS. The Okeaii grab, ready for lowering. The 0.25 m* grab ehown here is weighted
(Photo: Institute of Oceanology of the U.S.S.R. Academy of
to 130 kg or niore.
Sciences, Noscow.)
capstan or siinilar arrangtkment which allows the rope to be surged
(paid-out) ir-hcn a sudden strain occurs.
Variotis means of increasing the closing forces, by levers or pulleys,
have resulted in improved penetration of grabs. The simplest is the
van Veen grab (Thamdrup, 1938) in which the scoops are closed through
the pincer-like action of two long arms to which the bridles are attached
(Figs. 13, IS), and levers are also responsible for the final part of the