THE STERN OF THE PORCUPINE.
The frame of the largest dredge used weighed 225 pounds. The bag was double, the outside of strong twine netting, lined with canvass. Three sinkers, one of 100 pounds, and two of 56 pounds each, were attached to the dredge rope at 500 fathoms from the dredge. A description of the sounding made in the Bay of Biscay on the 22d of July, 1869, will give an idea of the process. When the depth had been ascertained, the dredge was let go about 4:45 p.m., the vessel drifting slowly before a moderate breeze. At 5:50 p.m. the whole 3,000 fathoms of rope were out. While the dredge is going down the vessel drifts gradually to leeward; and when the whole 3,000 fathoms of rope are out, she has moved so as to make the line from the dredge slant. The vessel now steams slowly to windward, and is then allowed to drift again before the wind. The tension of the vessel's motion, thus instead of acting immediately on the dredge, now drags forward the weight, so that the dredging is carried on from the weight and not directly from the vessel The dredge is thus quietly pulled along, with the lip scraping the bottom, in the position it naturally assumes from the center of weight of its iron frame and arms. If, on the contrary, the weights were hung close to the dredge, and the dredge was dragged directly from the vessel, owing to the great weight and spring of the rope the arms would be continually lifted up, and the lip of the dredge be prevented from scraping. In very deep water this operation of steaming up to windward until the dredge rope is nearly perpendicular, after drifting for half an hour or so to leeward, is usually repeated three or four times. At 8:50 p.m. hauling-in is commenced, and the donkey-engine delivers the rope at a little more than a foot a second. A few moments before 1 o'clock in the morning the weights appear, and a little after one, eight hours after it was cast, the dredge appears and is safely landed on deck, having in the meantime made a journey of over eight miles. The dredge, as the result of this haul, contained 1 1/2 hundred weight of characteristic pale grey Atlantic ooze. The total weight brought up by the engine was as follows:
| 2,000 fathoms of rope, | 4,000 | |
| 1,000 " " | 1,500 | |
| 5,500 | ||
| Weight of rope reduced to 1/4 in water | 1,375 | |
| Dredge and bag | 275 | |
| Ooze | 168 | |
| Weight attached | 224 | |
| 2,042 | pounds. |
In many of the dredgings at all depths it was found that while few objects of interest were brought up within the dredge, many echinoderms, corals and sponges came to the surface sticking to the outside of the dredge bag, and even to the first few fathoms of the rope. The experiment was therefore tried of fastening to a rod attached to the bottom of the dredge bag, a half dozen swabs, such bundles of hemp as are used on ship-board for washing the decks. The result was marvelous; the tangled hemp brought up everything rough and movable that came in its way, and swept the bottom of the ocean as it would have swept the deck. So successful was this experiment, that the hempen tangles are now regarded as an essential adjunct to the dredge, and nearly as important as the dredge itself, and when the ground is too rough for using the dredge, the tangles alone are used.
The mollusca have the best chance of being caught in the dredge; their shells are comparatively small bodies mixed with the stones on the bottom, and they enter the dredge with these. Echinoderms, corals and sponges, on the contrary, are bulky objects, and are frequently partially buried in the mud, or more or less firmly attached, so that the dredge generally misses them. With the tangles it is the reverse, the smooth heavy shells are rarely brought up, while the tangles are frequently loaded with specimens; on one occasion not less than 20,000 examples came up on the tangles in a single haul.
In the Porcupine both derricks were furnished with accumulators, which were found of great value. The block through which the sounding line or dredging rope passed was not attached directly to the derrick, but to a rope which passed through an eye at the end of the spar, and was fixed to a bitt on the deck. On a bight of this rope, between the block and the bitt, the accumulator was lashed. This consists of thirty or forty, or more, vulcanized india-rubber springs, fastened together at the two extremities, and kept free from each other by being passed through holes in two wooden ends like barrel heads. The loop of the rope is made long enough to permit the accumulator to stretch to double or treble its length, but it is arrested far within its breaking point. The accumulator is valuable in the first place as indicating roughly the amount of strain upon the line; and in order that it may do so with some degree of accuracy it is so arranged as to play along the derrick, which is graduated, from trial, to the number of hundred weights of strain indicated by the greater or less extension of the accumulator; but its more important function is to take off the suddenness of the strain on the line when the vessel is pitching. The friction of one or two miles of cord in the water is so great as to prevent its yielding to a sudden jerk, such as is given to the attached end when the vessel rises to a sea, and the line is apt to snap.
The results which have been gained by deep sea dredging are so important that the English Government recently fitted out another vessel, the Challenger, for such a cruise, with every appliance. This vessel is now due in New York.
AQUARIUM.