440 



HYDROGRAPHICAL SURVEYING [CHAr. xx. 



"Sea- 

 lark " 

 Sweep, 



it might be advantageous to hang the sweep under the ship 

 athwartships, its extremes being shnig from the outer ends of 

 the lower booms, on which chains are rigged for the sounders. 

 The ship might then be steamed slowly ahead against the 

 current without any appreciable slant from the vertical being 

 given to the sweep. This has been tried successfully. The 

 objection to this method is that, should the sweep foul the 

 bottom, the lower booms arc liable to be carried away unless 

 one end of the sweep is shpped at once, and that a shallow patch 

 is not so easily detected if it occurs in the central part of the 

 sweep. 



SWEEPING BY BOATS. 



This sweep, designed by Lieutenant J. M. Jackson, R.N., 

 consists of 50 yards of three-stranded wire, which is kept taut 

 and at any convenient depth by two submarine kites towed 

 by two steamboats (see Fig. 97). 



The kites are towed from sweeping machines (see Figs. 98 



Distance Line (2y^' grass) 50 yds. 



Sweeping Machine 

 ^ (see detailed drawing) 



I 



a 



■5 



Sweeping Machine 

 (see detailed drawing) 





late 



Wire Sweep 50 yds. '3 Stranded wire) 



Fig. 97. 



and 99) mounted on a sounding machine platform in each boat ; 

 the kite wire, consisting of three parts of the same wire as that 

 used for the sweep, laid up together. The platforms are 

 strengthened to take the strain of the kites. 



The boats are connected by a distance line of 2|-inch grass 



