CH. l] 



THE EXPLORATION OF THE SEA 



17 



two to five feet in length and about nine inches to two feet in 

 breadth. To this frame is attached a bag of twine netting, the 

 mesh of which is always fine, but varies of course with the purpose 

 for which the dredge is intended. From either end of the fi:ame 

 an arm is fastened which passes forward and to which the dredge 

 rope is attached. Only one of the eyes at the ends of the arms is 

 attached to the dredge rope ; the other is attached to the eye 

 which is shackled to the rope, by a lashing of rope yarn, the 

 object of this device being to free the dredge in case it may 

 become fast on some obstacle on the sea bottom. If this occurs 

 the direction of strain on the dredge is altered and the latter 

 usually becomes free. Sometimes a bar is attached to the end 

 of the bag and to this are fastened a number of swabs or 

 tangles of yarn ; to these many delicate objects become fast 

 which otherwise might be damaged by the accumulation of debris 

 in the bag. 



Even a small trawl is a more efficient instrument than the 

 dredge, although it may be a less suitable instrument for some 

 purposes than the latter. In Fig. 7 I give the form and 

 dimensions of a trawl used for scientific purposes. The trawl 



Fig. 7. Diagram of a small beam trawl used for scientific investigation. 



consists of two " irons," stirrup-shaped contrivances which are 

 joined together by a beam of wood. The length of this beam 

 may be about 50 feet in the largest commercial trawls carried by 

 fishing smacks. The flat side of the iron runs on the sea bottom 



J. F. 2 



