ar smaller
vessels to undertake deep sea work. The "Challenger," however, may be
said to have established the practicability of dredging at any known
depth.

_Operating Dredges and Trawls in deep Seas._--Dredging operations from
large vessels in deep seas present numerous difficulties. The great
weight of the ship makes her motion, whether of progress or rolling,
irresistible to the dredge. The latter tends to jump, therefore, which
both lowers its efficiency and causes it to exert a sudden strain on the
dredge rope.

The efficiency or evenness of dredging was secured, therefore, by the
special device of fastening a heavy weight some 200 or 300 fathoms from
the dredge end of the dredge rope. This was either lowered with the
dredge or sent down after by means of a "messenger," a ring of rope
fixed round, but running freely on, the dredge rope. The latter plan was
used on the "Challenger"; the weights were six 28 lb. leads in canvas
covers: their descent was arrested by a toggle or wooden cross-bar
previously attached to the rope at the desired point. When, however, the
rope used is of wire this front weight is unnecessary.

The possibility of sudden strain necessitates a constant watching of
the dredge rope, as the ship's engines may at any moment be needed to
ease the tension by stopping the vessel's way, and the hauling engines
by paying out more rope. The use of accumulators both renders the
strain more gradual and gives warning of an increase or decrease;
indeed they can be calibrated and used as dynamometers to measure the
strain. One of the best forms of accumulator consists of a pile of
perforated rubber disks, which receive the strain and become
compressed in doing so. The arrangement is in essence as follows. The
disks form a column resting on a cross-bar or base, from which two
rods pass up one on each side of the column. Another cross-bar rests
on the top disk, and from it a rod passes freely down the centre
perforation of disks and base. Eyes are attached to the lower end of
this rod and to a yoke connecting the side rods at the top: a pull
exerted on these eyes is thus modified by the elasticity of the
dredge. In the "Porcupine" and other early expeditions the accumulator
was hung from the main yard arm, and the block through which the
dredge rope ran suspended from it. In more recent ships a special
derrick boom is rigged for this block, and a second accumulator is