404 



Messrs. Carpenter, Jeffreys, and Thomson [Nov. 18, 



and in the 'Lightning' Expedition. — As it was considered advisable by 

 Capt. Calver that provision should be made for carrying on Sounding and 

 Dredging at either end of the ship, a " derrick " (A C) with an " accumu- 

 lator" * (E F) was rigged out both at the bow and the stern, on the plan 

 shown in the accompanying figure. 



Fig. 1. 



c 



* The Accumulator is composed of a number of strong vulcanized India-rubber 

 springs combined at their extremities E F ; and its use is twofold,— first, to indicate by 

 its elongation any excessive strain upon the sounding or dredging line I II, which passes 

 through the block G ; and second to ease off the suddenness of such strain, and give time 

 for the action by which it may be relieved. This is especially valuable in Deep-sea 

 Sounding and Dredging when the vessel is pitching; for the friction of two or three 

 miles of immersed line is so great as to prevent its yielding to any sudden jerk, such as 

 that given to its attached extremity by a vertical motion of a few feet when the vessel 

 rises to a sea. And it is absolutely needful when Dredging is carried on from a vessel as 

 large as the 4 Porcupine since, whenever the dredge 'fouls,' the momentum of such 



