The beam trawl consists of a triangular purse-shaped net, about seventy feet long, forty wide at the mouth, gradually diminishing to four or five at the commencement of the cod, as the smaller end is called.  This part of the trawl, about ten feet long, is of a uniform breadth to the extremity, which is closed by a draw-rope, like the string of a purse. The upper part of the mouth is made fast to a beam about forty feet in length, which keeps the net open.  This beam is supported by two upright iron frames, three feet in height, known as the trawl heads, or irons; the lower being flattened, to rest on the ground.  The under side of the net is made with a curved margin.  The outside is guarded from chafing, when the trawl is being worked over the bottom, by pieces of old net. The meshes vary in size according to the part of the trawl.  Near the mouth, they are four inches square, and in the cod, an inch and a quarter.  The trawl is hauled along by a bridle, that is to say, by two ropes of about fifteen fathoms each, which are fastened to the ends of the trawl heads, and unite at a warp, one hundred and fifty fathoms long, which serves to haul the net along.  Trawling, as a rule, is carried on in the direction of the tide, although sometimes across it, but never against a stream.  It is usually kept down for one tide, and its rate of progress is generally from half a mile to two miles an hour faster than that of the stream.  The fish caught are turbot, skate, soles, though others are occasionally taken in the net.  The trawl can only be used with advantage on smooth ground; and, of course, a sandy bottom is preferred, not only from that being the usual resort of several valuable kinds of ground fish, but from the less danger there is on such a surface of tearing the net to pieces.