SWABING THE BOTTOM OP THE SEA. 



735 



an essential adjunct to the dredge, and nearly as important as 

 the drcd'jfc itself, and when the ground is too rough for using 

 the dredge, the tangles alone are used. 



The moll usca 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 tne 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. AVith the tangles it is the reverse, the smooth heavy 

 shells are rarel y 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 accu- 

 mulators, 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, fas- 

 tened 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 doable or treble its length, 

 but it is arrested far within its breaking point. The accumula- 

 tor 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 num- 

 ber 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 

 wheL the vessel is pitching. The friction of one or two miles 



