CHAP. XX,] METHOD OF SWEEPING 449 



face, occasioned by the rise and fall of tide, so were the weights 

 lowered or raised and the attending lines adjusted. 



Two boats were employed to tow the spar broadside on, tow- 

 Unes being attached to where the lead-lines were made fast to 

 the spar, thereby making it 50 feet between the boats — one 

 boat acting as the directing one, working on transits, allowing 

 each successive sweep to sUghtly overlay the previous one, fixes 

 being frequently obtained to plot progress of work as on a hne 

 of soundings. Weights of 200 pounds being found too large for 

 convenient handhng in boats, those of 100 pounds were used; 

 in the first instance, 100 pounds was lowered with the bottom 

 line between, the second 100 pounds being lowered on the sus- 

 pending Unes, the whole weight then allowed to come upon the 

 same. 



It was calculated that the tension on the bottom line, between 

 the weights, at a depth of 30 feet and using 200 pounds at each 

 end, was 52 pounds; if the spar has good buoyancy, greater 

 weights could be used to increase the tension between them on 

 the bottom line, thus assuming the character of an iron bar. 



As a test of this method of sweeping, the MerUn Rock, with 

 29 feet over it at low water, was swept for, and was found 

 at once, the sweep having been adjusted to about one foot 

 below the level of the rock. 



REPORT ON THE SWEEPING OF SHEERNESS BAR. 



By LIEUTENANT-COMMANDER L. D. PENFOLD, 

 H.M. Surveying Ship "Triton." 



A lighter was specially fitted with wooden spars, rigged up 

 over the stern, as shown in Fig. 106. From the spars mentioned 

 a railway iron 30 feet long was slung athwartships by two wire 

 pennants about 6 fathoms long, shackled on near each end of 

 the iron, enabling the bar to be raised cr lowered as required 

 by means of winches. These pennants were marked in feet 

 from 25 to 32 feet with spun yarn for graduating the depths 

 of the bar. 



The lighter was moored over the area swept by four light 

 anchors on 3-|-inch hemp hawsers, which were spliced so as to 

 be continuous each side of the deck (Fig. 107). 



29 



