Page:Popular Science Monthly Volume 3.djvu/278

266 dredging from a small boat, at a less depth thaii 600 feet, is a frame 18 inches long and five inches in width. The scrapers are three inches wide, and are so set that the distance across between their edges is 7½ inches.

The dredge used for deep-sea work was larger, the frame being four feet six inches in length, and six inches wide at the throat or narrowest part. The weight of the frame was 225 lbs., but Dr. Thomson thinks it was too large and heavy. The dredge-bag was double, the outer being of strong twine netting lined within with "bread-bag," a light, open kind of canvas.

It was found by experience that very often, when nothing of interest was brought up within the dredge, many echinoderms, corals, and sponges, came to the surface, sticking to the outside of the bag, and even to the first few fathoms of the dredge-rope. This suggested the attachment of swabs, used for washing the decks, to the dredge. The tangled hemp turned out to be very efficient, picking up great numbers of objects that would not be otherwise secured. The bag took the mollusks, which, from their shelly forms, could not be otherwise obtained, while the echinoderms, corals, and sponges—bulky objects that could not readily enter the bag—were more easily caught by the swabs, although, unfortunately, it mutilated them, and brought them up in fragments. So important was this expedient, that a long iron bar was attached to the bottom of the dredge-bag, to which the hempen bundles were suspended, as shown in Fig. 6.

The arrangements for sounding and dredging from the Porcupine are fully described and illustrated in Prof. Thomson's work. The vessel was a 382-ton gunboat, with a steam-engine of 12 horse-power, stationed amidships, with drums of different sizes, from which lines were led fore and aft for working either at the bow or stern. Two powerful derricks were rigged for sounding and dredging, one over the stern and one over the port-bow. 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 "bit," a piece of timber going through the deck. On a bight of this rope between the block and the "bit" was a piece of apparatus shown in Fig. 7, and called the "accumulator." This consisted of 30 or 40 strong India-rubber springs, working together, and its use was to yield by stretching, when, from any cause, as the pitching of the ship, there was an unusual strain upon the line. The dredge-rope of the Porcupine was of Russian hemp, 2½ inches in circumference, with a breaking strain of 2½ tons, and was 18,000 feet, or nearly 3½ miles long. A row of about 20 large iron pins, about 2½ feet in length, projected over one side of the quarter-deck, rising obliquely from the top of the bulwark. Upon these the rope was continuously coiled, as shown in the figure, which also represents the dredge in position for descent.