2 66 THE POPULAR SCIENCE MONTHLY. 



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 

 7^ 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 inter- 

 est 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 ob- 

 tained, 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 to- 

 gether, 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, 2-J inches in 

 circumference, with a breaking strain of 2- tons, and was 18,000 feet, 

 or nearly 3^- miles long. A row of about 20 large iron pins, about 2^ 

 feet in length, projected over one side of the quarter-deck, rising 

 obliquely from the top of the bulwark. Upon these the rope was con- 

 tinuously coiled, as shown in the figure, which also represents the 

 dredge in position for descent. 



