The frame of the "Challenger" trawl consisted of a 15 ft. wooden beam
which in use was drawn over the sea-bed on two runners resembling
those of a sledge, by means of two ropes or bridles attached to eyes
in the front of the runners or "trawl heads." A net 30 ft. long was
suspended by one side to the beam by half-a-dozen stops. The remainder
of the net's mouth was of much greater length than the beam, and was
weighted with close-set rolls of sheet lead; it thus dragged along the
bottom in a curve approximately to a semicircle, behind the beam. The
net tapers towards the hinder end, and contains a second net with open
bottom, which, reaching about three-quarters of the way down the main
net, acts as a valve or pocket. Both heels (or hinder ends) of the
trawl heads and the tail of the net were weighted to assist the net in
digging sufficiently and to maintain its balance--an important point,
since if the trawl lands on its beam the net's mouth remains closed,
and nothing is caught.
The main differences of this trawl from the dredge are the replacement
of scraping lip by ground rope, the position of this ground rope and
the greater size of the mouth. The absence of a lip makes it less
effective for burrowing and sessile creatures, but the weighted ground
rope nevertheless secures them to a very surprising extent. The
position of the ground rope is an important feature, as any free
swimming creature not disturbed until the arrival of the ground rope
cannot escape by simply rising or "striking" up. This and the greater
spread make the trawl especially suitable for the collection of fishes
and other swiftly moving animals. The first haul of the "Challenger"
trawl brought up fishes, and most of our knowledge of fish of the
greatest depths is due to it.
A tendency to return to the use of the small beam trawl for deep-sea
work has lately shown itself. That used by Tanner on the "Albatross"
has runners more heart-shaped than the "Challenger's" instrument; the
net is fastened to the downward and backward sloping edge of the
runner as well as to the beam, being thus fixed on three sides instead
of one; and a Norwegian glass float is fastened in a network cover to
that part of the net which is above and in front of the ground rope in
use, to assist in keeping the opening clear. These floats can stand
the pressure at great depths, and do not become waterlogged as do cork
floats. The largest "Albatross" trawl has a beam 11 ft. long, runners
2 ft. 5 in. high, and its frame weighs 275 lb.
_Agassiz or Blake Trawl._--This is generally considered to possess
advantages over the preceding, and is decidedly better for those not
experts in trawling. Its frame (fig. 18) consists of two iron runners
each the shape of a capital letter D, joined by iron rods or pipes
which connect the middle of each stroke with the corresponding point
on the other letter. The net is a tapering one, its mouth being a
strong rope bound with finer rope for protection till the whole
reaches a thickness of some 2 in. It is fastened to the frame at four
points only, the ends of the curved rods, and thus has a rectangular
opening.
[Illustration: From Alexander E. Agassiz's _Three Cruises of the
"Blake."_ By permission of Houghton, Mifflin & Co.