78 THE VOYAGE OF H.M.S. CHALLENGER. 



thimble being let go they travelled down the curve of the rope until they were brought up 

 at the toggle G. The dredge and rope by means of this additional weight now succes- 

 sively assumed the positions A C, A D, and A E, until finally the dredge reached the 

 bottom at F, the weights being in the positions G', G", G'", G"". 



It must be evident that, provided there were no surface or under current, the dredge 

 would reach the bottom with the swabs trailing fairly behind it, if sufficient time were 

 allowed for it to sink. The surface current could always be ascertained and allowed for ; 

 when the dredge, therefore, came up foul, as it occasionally did, this could only be ascribed 

 to the influence of some under-current, which need not necessarily have been at the 

 bottom, or to the rope when new twisting the dredge round and round with the weights 

 on the toggle. It was found by experience that about three hours were required to sink 

 the dredge in this manner when the depth was about 2500 fathoms. When it was once 

 down the ship was allowed to drift broadside to the wind for a certain time, and the 

 accumulators pointed out, by their expansion and contraction, that the dredge was 

 being dragged slowly over the ground. When it fouled anything the strain of the ship 

 immediately stretched the accumulators to their utmost, and the line was at once let go 

 to prevent its carrying away, the ship being brought head to wind and kept stationary, 

 while the rope was hove in easily. Did the dredge still continue foul, the ship was 

 steamed ahead of and all round the supposed position of the dredge to endeavour to clear 

 it (in the same manner as a boat's anchor is cleared when jammed on a coral reef or amongst 

 rocks), until the dredge was freed by the stop (see fig. 19 D) breaking, or the line carrying 

 away. Supposing no accident occurred, when the dredge had been on the bottom a 

 sufficient time — from half an hour to an hour — the rope was brought to the donkey-engine, 

 and the dredge hove up. It was found that the strain on the line was so great that the 

 men could not hold on to it while it was being hove in, when turns were passed round 

 only one drum of the engine. Fortunately, the engine was fitted with drums of the same 

 diameter on each side of the deck, so that by taking a number of turns with the rope 

 round the drum on one side, and then leading it abaft through two blocks across the 

 deck, it was possible to take a number of additional turns round the drum of the 

 engine at the other side, so that the men were enabled to hold on to it easily, and a 

 great support was given to the bearings of the engine. 



On one or two occasions, when, owing to the depth (over 3000 fathoms), sufficient 

 time could not be spared to allow the dredge or trawl to sink in this manner, a 

 sounding rod was fastened to the bottom of the dredge or trawl and 4 cwt. put on the 

 rod. The dredge was then let go perpendicularly, the ship being kept stationary, until 

 sufficient line had been paid out to allow the rod to reach the bottom and disengage the 

 weights, when the ship was allowed to drift a little way, and then the weights were 

 attached to the thimble and allowed to slide down the rope to the toggle. This is a 

 very successful way of dredging or trawling quickly in deep water. 



