f 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.

In dredging in sand or mud, the dredge-rope may simply be passed through
the double eye formed by the ends of the two arms of the dredge-frame;
but in rocky or unknown ground it is better to fasten the rope to the
eye of one of the arms only, and to tie the two eyes together with three
or four turns of rope-yarn. This stop breaks much more readily than the
dredge-rope, so that if the dredge get caught it is the first thing to
give way under the strain, and in doing so it often alters the position
of the dredge so as to allow of its extrication.

The dredge is slipped gently over the side, either from the bow or from
the stern--in a small boat more usually the latter--while there is a
little way on, and the direction which the rope takes indicates roughly
whether the dredge is going down properly. When it reaches the ground
and begins to scrape, an experienced hand upon the rope can usually
detect at once a tremor given to the dredge by the scraper passing over
the irregularities of the bottom. The due amount of rope is then paid
out, and the rope hitched to a bench or rowlock-pin. The boat should
move very slowly, probably not faster than a mile an hour. In st