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HYDROGRAPHY 



3421 



When there are waves or swells the leadsman must be careful to allow for their 

 height so that the reading of the leadline will give the depth from the mean water 

 surface. The leadline should always be read at the surface of the water. To read 

 the handlead at night from a ship, a floodlight should be installed where it can be 

 directed on the water below the sounding chair. The depth should never be obtained 

 by subtracting the estimated height above the water of a leadline mark in the leadsman's 



Figure 60. — Handlead sounding from launch. 



hand, or by subtracting the height of the deck of the sounding chair from the leadline 

 reading at this point. 



The rate at which the lead on a leadline sinks when thrown from a moving boat 

 varies according to its weight, its shape, and the skill with which it is thrown. An idea 

 of this rate may be gained from the fact that an ordinary 10-pound lead used in sounding 

 will reach bottom in 6 fathoms about 5 seconds after it strikes the surface. The same 

 10-pound lead, dropped vertically from a stationary boat, will sink to the bottom in 6 

 fathoms in about 3 seconds. 



Errors occur in leadline soundings when the leadline is not straight or the lead is 

 not vertically below the leadsman when the depth is read. It is almost impossible to 

 avoid a curve or underwater bight in the leadline when sounding in strong currents and 

 there may be a similar error in soundings taken by inexperienced leadsmen in the absence 

 of current (see 3464). 



