4621 HYDROGRAPHIC MANUAL PaGE 384 



4621. Marking Leadlines 



A leadline should be from 15 to 30 fathoms in length, depending on the depths in 

 which it will likely be used and whether it is intended for use on a small boat or survey- 

 ship. 



Each leadline should be identified by a consecutive number stamped on a metal 

 disk attached at the inboard end of the line at the time of graduation. This number 

 should be retained throughout the life of the leadline or until it is necessary to re-mark it. 



The braided covering of a leadline tends to shrink with use when wet. When this 

 occurs, the wire core eventually buckles, and strands break and are likely to protrude 

 through the covering and injure the leadsman's hands. This can be prevented by the 

 following preseasoning before it is marked: 



A leadline is prepared for use by soaking it in salt water for 24 hours. Then while 

 wet it should be laid out where the cotton covering can be worked along the wire by 

 hand until about a foot or so of the wire for each 10 fathoms protrudes from the covering. 

 This is a tedious proceeding, several men have to cooperate, the covering can be 

 pushed back only a few inches at a time, and this length of slack has to be pushed nearly 

 the full length of the line before another few inches can be started. The excess wire is 

 cut off. Experience has proved that a leadline so prepared, if the covering is worked 

 back the correct amount, wiU maintain an almost constant length in future use. The 

 covering must not be worked back too far or it will form bulges along the wire. 



After the above preparation the line should be dried under considerable tension 

 and then soaked again for another 24 hours. A leadline should never be boiled as this 

 removes the waterproofing with which the covering is impregnated. 



After the lead has been attached, the line while still wet should be placed under a 

 tension equal to the weight of the lead, while it is being graduated. If temporarily 

 marked at this time the permanent marks can be seized on afterwards. 



The marks on a new leadline may be laid ofi" with a tape, but the most convenient 

 arrangement, and one which will be needed later for leadline verification, is to mark the 

 correct distances permanently with copper tacks on the deck of the ship, or on a wharf 

 if the survey party is based at a shore station. 



Two units of measure, the fathom and the foot, are used by the Coast and Geodetic 

 Survey for the measurement of depths, but the two units are not commingled on the 

 same leadline. One system of marking utilizes feet only and the other sj^stem fathoms 

 and tenths of fathoms. Every survey party that has occasion to use both depth units in 

 accordance with the instructions in this Manual shall be equipped with at least one 

 leadline marked in each unit. 



Leadlines in fathoms shall be marked as follows: 



Fathoms Marks 



1, 11, 21 One strip of leather. 



2,12,22 Two strips of leather. 



3, 13, 23 Blue bunting. 



4, 14, 24 Two strips of leather secured in the middle (so that two ends 



point upward and two downward). 



5, 15, 25 White bunting. 



6, 16 White cord with one knot. 



7, 17 Red bunting. 



8, 18 Three strips of leather. 



