436 HYDROGRAPHICAL SURVEYING [chap. xx. 



is secured to the shackle or the band of the buffer and brought 

 inboard. By means of this Hne the buffer may be controlled 

 and secured at any desired position on the jackstay. 



On releasing the hauling aft line attached to the traveller, 

 the lead-line being clear for running, the traveller slides down 

 the jackstay, and on impact with the releasing l)iiffer the tripper 

 will be raised, releasing the tongue ; the weight of the lead will 

 revolve the tumbler and the ball will be dropped through the 

 carrier horns, thus releasing the lead. As soon as the lead 

 becomes disengaged the traveller will automatically reset itself 

 by means of the counterpoise weight. The sounding is ob- 

 tained by the leadsman in the usual way by hauUng the inboard 

 part of the sounding hne taut when the outboard part is ujj and 

 down. The lead is then hove up and the traveller hauled 

 aft again ready for engaging. 



SWEEPING. 



The ever-increasing draught and value of modern shipping 

 renders necessary, in certain areas, more minute and thorough 

 search for hidden dangers than can be afforded by the use of the 

 lead alone. 



Various methods of sweeping have been devised and carried 

 out in recent years by H.M. surveying vessels, of ■s^hich the 

 following pages give some account. 



It is necessarily a laborious process, and the object yet to be 

 attained is to cover a sufficiently wide front on a single Hne of 

 sweep. 

 Sweeping Captain F. C. C. Pasco, R.N., when commanding H.M.S. 

 of the*°^ Research, used the ship drifting broadside to the current, with 

 Ship. a sweep consisting of 1-inch wii'e rope weighted at each end 

 with 150 pounds, and supported at each end with 3-inch hemp 

 rove through convenient blocks, the distance between the sup- 

 porting blocks being 40 feet greater than the length of sweep. 

 This gives tension sufficient to keep the sweep taut. Up and 

 down Unes, marked in feet, support the sweep at equidistant 

 intervals. 



The arrangement is shown in Fig. 95. 



Under favom-able conditions, in calm weather with from 

 1 to 1 2 knots tide, the action of the sweep touching the bottom 



