444 HYDROGRAPHICAL SURVEYING [chap. xx. 



It is safe to sweep at the rate of 5 knots. One man should be 

 stationed at each machine, with one hand on the thumb-screw 

 controlhng the brake, and the other on the wu'e abaft the 

 machine. 



The instant the sweep wire fouls a rock the kites converge ; the 

 brake must be at once eased, letting out wire till the boats have 

 gathered sternway. 



If the sweep wire is badly foul of a rock, there is no danger 



of losing either of the kites, as by bringing a heavy strain on the 



kite wire, the sweep wire will carry away first, its strength 



Ijeing only one-thu'd of that of the former. The kite being 



released, a spare sweep wire can be quickly fitted. 



Sweeping This sweep was also designed by Lieutenant J. M. Jackson, 



Boat"^^ R.N. It is carried out by one boat, the width of the sweep 



The being 42 yards, and " otter boards " the means employed to 



Sweep^ hold the sweep wire taut (see Figs. 102 and 103). The same 



machines and kite wires as previously described are used with 



two slight additions : first, a fairlead of mild steel fitted for 



leading the wire to drum (see g in Fig. 101); second, at least 



40 fathoms of wire must be on each drum. 



The sweep can be used for sweeping at any required depth, 

 alterations in depth being obtained by altering position of 

 " otter boards " on kite wires. 

 To sweejD at a depth of 6 fathoms with 42 yards wire sweep — 



1. Attach one end of sweep wire to one kite (say, starboard), 

 put starboard kite over going slow ahead and paying out sweep 

 wu'e at same time till 8 fathoms of kite wire are out. 



2. From depth scale: Sfafhojns kite wire =5 fathoms 4 feet 

 depth of sweep ; so attach otter board to kite wire at S^ fathoms; 

 put over otter board and pay out kite wire and sweep wu"e 

 together till about 20 fathoms of kite wire are out. (Note. — 

 In jjutting otter board over, one hand should tend its lower 

 tail wu'e to start it on right tack, or it may foul screw.) 



3. Then secure other end of sweep wire to port kite, let out 

 kite and 8 fathoms of kite wire and secure port otter board at 

 same distance above kite as starboard, and pay out to about 

 20 fathoms kite wire. 



4. Increase to half speed, pay out both kite wires together to 

 a length of 29 fathoms, and put band brakes hard on. 



5. Increase to full speed (6-knot boat) when otter boards 



