448 HYDROGRAPHICAL SURVEYING [chap. xx. 



and passed through parts of kite wire to prevent slijiping. 

 If wire stoppers are used, a wire " preventer " should be fitted, 

 as otter boards sink if free. 



The head and tail wires of otter boards should not be at first 

 secured permanently to the thimble, as, however carefully 

 otter boards are made and wires adjusted (so that to all appear- 

 ances otters are identical except that wires are secured in each 

 case to inner side of board), some slight difference will probably 

 be found either in their surface keeping or station keeping on 

 the quarter, which can only be remedied by experimental 

 adjustment of wu-es. These adjustments should be carried 

 out with kite attached to otter. 



A hghter otter board than the one shown in drawing (Fig. 105) 

 is not satisfactory, not being steady. 



METHOD OF SWEEPING IN THE ENTRANCE TO 



ST. JOHN'S HARBOUR, NEWFOUNDLAND, 



USING BOATS ONLY. 



By captain J. W. F. COMBE, R.N., Admiralty Surveying 

 Vessel " Ellinor." 



In adopting a method for sweeping an area in the entrance 

 to the harbour of St. John's, through the Narrows (1,250 yards 

 in length on the leading marks, and 110 yards wide), for rocks 

 or any other obstructions, a spar 66 feet long was obtained, 

 though a longer one would have been used if procurable (see 

 sketch). 



From each end a weight of 200 pounds was suspended by a 

 H-inch manilla rope, the weights connected below by 50 feet 

 of |-inch wire, called the " bottom line," the horizontal depth 

 of same being regulated by a measured Une vertically over 

 each weight and attached to the spar. 



At distances of 16 and 17 feet along on the bottom Une two 

 light lines were attached, made fast above to small floats by 

 length of line corresponding to depth of bottom line; these 

 floats helped to indicate, by their change of position on the 

 surface relatively with the spar, when the bottom line touched 

 any obstruction. 



According to the depth required to sweep below the sea sur- 



