MUSEUM OF COMPARATIVE ZOOLOGY. 453 



this way three thousand fathoms were used for a depth of two thou- 

 sand four hundred and thirty-five fathoms. 



Dredging may be carried on from a boat, down to ten or twelve fath- 

 oms, with wind enough to propel it, or a good crew to pull it if calm. 

 From a sailing yacht two hundred and fifty fathoms have been attained, 

 though probably with some difficulty. For greater depths a steamer is 

 almost indispensable, provided also with a donkey-engine to haul up the 

 line. Without the latter the labor of the men is very heavy, and the 

 time consumed a great drawback. To ascertain the strain on the rope, 

 some kind of a dynamometer ought to be attached to it. A convenient 

 form is the " accumulator "• used in the, Porcupine. It cousists of a 

 number of solid india-rubber springs, about two feet long, connected at 

 each end with a disk of wood. One end of the accumulator is fastened 

 below to the derrick and above to a rope, which, passing through i 

 block at the top of the derrick, supports the snatch-block through 

 which the dredge-line goes overboard. The motions of the accumu- 

 lator show the variable strain on the line. 



Many valuable specimens were obtained on board the Porcupine by 

 means of hempen tangles or swabs, attached to the ends of a trans- 

 verse bar made fast to the dredge. According to Mr. Jeffreys, it is, 

 however, more advisable to use them separately, as they interfere with 

 good work of the dredge itself. 



In dredging it is preferable to have the dredge go over the bow, 

 especially on rough bottom, as it is easier, in case of fouling, to steam 

 ahead to release the dredge than to back up. Otherwise the stern is 

 more convenient, as the working-table, alcohol jars, &c, are usually 

 stowed on the quarter-deck. 



After the dredge is brought up the contents may be emptied into a 

 tub, the more delicate specimens picked out and transferred to clean 

 sea-water if they are to be observed alive, and the residue washed in 

 graduated sieves of copper wire. A rough list of the contents is noted 

 in a book, with specification of the depth, temperature of the water, 

 fee. Until they can be sorted, the contents of every dredgeful can be 

 cied up in a bag of some loose stuff (old bunting from worn signal-flags 

 was used in the Coast Survey), a label written with ink and folded up 

 being enclosed, and the whole put in alcohol. Thus a large metallic 

 vessel, filled with alcohol, can be used to contain the results of many 

 dredgings ; bottles being reserved for the more delicate objects. Econ- 

 omy of space can thus be gained to a considerable extent. 



