The great risk in dredging in very deep water is that of the dredge running down nearly vertically and sinking at once into the soft mud, and remaining imbedded until hauling in commences. During the earlier part of the voyage of the Challever this accident frequently defeated, at least partially, the object of the operation; and, after various suggestions for modifying the dredge, it was proposed to try some form of the trawl in order to insure, so far as possible, the capture of any of the larger marine ani mals which might be present, and thus to gain a better general idea of the nature of the fauna. A 15-ft. beam-trawl was sent down off cape St. Vincent to a depth of 600 fath la ins; the experiment looked hazardous, but the trawl duly came up, and contained, with many of the larger invertebrata, several fishes. The trawl seemed to answer so well that it was tried again a little further s. in 1090 fathoms, and again it was perfectly suc and during the remainder of the voyage it was employed almost as frequently, and in nearly as deep water (3,125 fathoms in the Pacific), as Ball's dredge was in the Atlantic, where the deepest haul was at 3,150 fathoms. During the voyage of the Chal lenger, a course of about 70,000 nautical miles was traversed in three years and a half, and 362 observing stations were established at intervals as nearly uniform as circum stances would permit; and at the greater number of these, dredging or some modifica tion of the process was successfully performed-52 times at a depth greater than 2,000 fathoms, and thrice at depths beyond 3,000 fathoms. So fully convinced were the Chal lenger officers that they could dredge at any depth, that it was only want of time and daylight which prevented their doing so at their deepest sounding, 4,575 fathoms. The Atlantic was crossed five times, and an erratic route through the Pacific gave a good idea of the conditions of the abysses of that ocean, while in the s. Indian ocean dredg ing and trawling were carried down close to the Antarctic ice-barrier.
The results of this expedition were of the most interesting nature. Animal life was found to exist at all depths, although probably in diminishing abundance as the depth becomes extreme; and in various parts of the world at depths beyond 400 or 500 fath oms the fauna had much the same general character. The species usually differed in widely separated areas, but the great majority of forms, if not identical, were so nearly allied that they might be regarded as representative and genetically related. Although all marine invertebrate classes were represented, echinoderms in their different orders, sponges and ervstacea preponderated, while corals and mollusea were comparatively scarce. In the two groups first named, many forms occurred allied to families which had been previously regarded as extinct or nearly so; thus among the echinoderms, stalked crinoids were by no means rare, and many species of regular eehinidea related to the chalk genus echiiiothuria, and many irregular species allied to ananehytes and dysaster, occurred. The sponges were mainly represented by the hexactinellidie, the beautiful order to which the glass-rope sponge of Japan and the marvelous " vrenus's flower-bas ket" of the Philippines belong, the order'to which the ventriculites of the chalk must also be referred.
Dredging at its greatest depth is a difficult and critical operation, and although by its means some idea of the nature-and distribution of the abyssal fauna of the ocean has already been attained, it will be long before the blanks are filled up; for of the area of 140,000,000 sq.m. forming the "abyssal province," the actual amount hitherto traversed by the naturalist's dredge may still be readily reckoned by the square yard. [The sub stance of this article is from Encyclopaedia Britannica, ninth edition.] a machine used for clearing out or deepening the channels of rivers, harbors, etc. Dredging-machines are variously constructed, the simplest being like the oyster-dredge described above, only having a perforated cowhide bag instead of the chain-net, and a stronger " spoon " or iron mouth to the bag. This is attached to the end of a pole, and with tackle by men from a barge in such a manner that the loose matter of the bottom is scooped up into the barge. The bucket dredging machine is much more efficient. It consists of a long stage or framework overhanging the side of the barge. This frame has a wheel at each end, upon which works a power ful endless chain, to which is attached a series of perforated iron buckets, each with a shovel-shaped steel mouth projecting considerably on one side. The overhanging framework forms an inclined plane, along which the buckets run, descending on one side, and ascending on the other. They are so arranged that they descend empty, and on reaching the bottom, the projecting shovel or scoop-mouth digs into the bottom, and partially fills the bucket with the silt; it then turns round on the wheel at the lower end of the incline, and runs up it till near the top, when it turns over the upper end, and in doing so its contents are emptied into a second attendant barge. This action is continued by every succeeding bucket of the endless chain. The perforations are for the passage of the water. By varying the inclination of the framework, the working depth may be increased or diminished. Some dredgers are fitted with two complete sets of buckets, one on each side of the vessel. A steam-engine and boiler, suitably placed in the dredger, are provided for giving motion to the machinery, and sometimes also to a screw-propeller placed at the stern. Perhaps nowhere has river-dredging been carried such a length as in the case of the Clyde, which, by this process of scooping, has at and below Glasgow been converted from a river navigable only for small vessels into an estuary capable of bearing the largest ships. The dredgers employed for this purpose are moved by steam, the materials scooped out being carried out to sea by lighters. These have a large open tank amidships, while the two ends are decked over, and afford such accommodation for crew or machinery as may he necessary. The sides of the hold are hinged from the top, and open outwards, and thus its contents can easily be emptied into the sea.