274 THE SEAS 



but a difference lies in the means by which the mouth of 

 the net is kept open as it sweeps over the bottom. 



In the front of the net in the beam-trawl is a frame, 

 consisting of two D-shaped iron runners, known as " shoes " 

 or " trawl heads," joined together above by a long and very 

 stout wooden beam. To this beam the upper edge of the 

 mouth of the net is attached, the short side of the mouth 

 being fixed to the runners while the heavy foot-rope, 

 curving backwards at the centre (Fig. 57), drags along the 

 ground. The opening of the net is therefore determined 

 by the size of this frame, its width being the length of the 

 beam and its height the same as that of the shoes. The 

 length of the beam may vary from forty-five to fifty feet 



35^ 



Fig. 57. — Beam-trawl, showing frame and front portion of net. 



down to much smaller dimensions to suit the size of the 

 boat in which it is used. The height of the shoes for a 

 full sized trawl is three to three and a half feet, so that the 

 largest opening a net could have would be a rectangle 

 fifty feet by three and a half feet. The net itself may 

 extend backwards up to 100 feet in length. 



The principle upon which the mouth of the net is kept 

 open in the otter-trawl is entirely different. In this case 

 there is no frame to which the front of the net can be 

 attached. Advantage is here taken of the fact that if a 

 kite or any flat object set at an oblique angle be drawn 



