ances of about an inch, and through these strong iron rings about
an inch in diameter are passed, and two or three similar rings run on
the short rods which form the ends of the dredge-frame. A light iron
rod, bent to the form of the dredge opening, usually runs through
these rings, and to this rod and to the rings the mouth of the
dredge-bag is securely attached by stout cord or strong copper wire.
Various materials have been used for the bag, the chief of which are
hide, canvas and netting. The hide was recommended by its strength,
but it is now abandoned. Canvas bags fill quickly with mud or sand and
then cease to operate: on the other hand wide mesh net fails to retain
small specimens. Probably the most suitable material is hand-made
netting of very strong twine, the meshes half an inch to the side, the
inter-spaces contracting to a third of an inch across when the twine
is thoroughly soaked, with an open canvas or "bread-bag" lining to the
last 6 in. of the net. A return to canvas covering has latterly
occurred in the small dredge called the mud-bag, trailed behind the
trawl of the "Albatross" for obtaining a sample of the bottom, and in
the conical dredge.

The dimensions of the first dredges were as follows: Frame about 12
in. by about 4 in.; scraping lips about 2 in. wide; all other iron
parts of round iron bar 5/8 in. diameter; bag rather more than 1 ft.
long. These small dredges were used from rowing boats. Larger dredges
were subsequently made for use from yawls or cutters. The mouth of
these was 18 by 5 in., the scraping lips about 2 in. wide and bag 2
ft. deep; such a dredge weighs about 20 lb. The dredge of the
"Challenger" had a frame 4 ft. 6 in. by 1 ft. 3 in. and the bag had a
length of 4 ft. 6 in.; the "Porcupine" used a dredge of the same size
weighing 225 lb. Doubtless the size of Ball's dredge would have
grown still more had it not been proved by the "Challenger" expedition
that for many purposes trawls could be used advantageously instead of
dredges.

_Operation of the Dredge from Small Vessels._ For work round the coasts
of Europe, at depths attainable from a row-boat or yawl, probably the
best kind of line is bolt-rope of the best Russian hemp, not less than
1-1/2 in. in circumference, containing 18 to 20 yarns in 3 strands. Each
yarn should be nearly a hundredweight, so that the breaking strain of
such a rope ought to be about a ton. O