[Illustration: From Sir Charles Wyville Thomson's _Voyage of the "Challenger."_ By permission of Macmillan & Co., Ltd
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.