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 1½ 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. Of course it is never voluntarily exposed to such a strain, but in shallow water the dredge is often caught among rocks or coral, and the rope should be strong enough in such a case to bring up the boat, even if there were some little way on. It is always well, when dredging, to ascertain the approximate depth with the lead before casting the dredge; and the lead ought always to be accompanied by a registering thermometer, for the subsequent haul of the dredge will gain greatly in value as an observation in geographical distribution, if it be accompanied by an accurate note of the bottom temperature. For depths under 100 fathoms the amount of rope paid out should be at least double the depth; under 30 fathoms, where one usually works more rapidly, it should be more nearly three times; this gives a good deal of slack before the dredge if the boat be moving very slowly, and keeps the lip of the dredge well down. When there is anything of a current, from whatever cause, it is usually convenient to attach a weight, varying from 14 lb. to half a hundredweight, to the rope 3 or 4 fathoms in front of the dredge. This prevents in some degree the lifting of the mouth of the dredge; if the weight be attached nearer the dredge it is apt to injure delicate objects passing in.