70 



HYDROGRAPHIC MANUAL 



by a small notch cut in the pole. Paint the 

 entire pole white, and the spaces between the 

 2- and 3-, the 7- and 8- and the 12- and 

 13-foot marks black. Other foot marks are 

 indicated by 14-inch colored bands, red at 

 the 5- and 10-foot marks, and black at the 

 1-, 4-, 6-, 9-, 11-, and 14-foot marks. Half- 

 foot marks are l^-inch bands, white where 

 the pole is black, and vice versa. 



3-65 Leadline. — Portable echo sounders 

 have replaced the leadline as the principal 

 instrument for obtaining soundings in rela- 

 tively shoal water. Nevertheless, every unit 

 engaged in hydrographic surveys where the 

 general depths are less than 20 fathoms 

 should have one or more leadlines properly 

 marked and calibrated. Leadlines are re- 

 quired to search for, or confirm, the least 

 depths on shoals and sunken rocks ; to con- 

 firm echo soundings in kelp or grass; to ob- 

 tain bottom samples; to obtain vertical cast 

 comparisons with echo soundings; and are 

 occasionally used when making temperature 

 and salinity observations from a launch. 



The Coast and Geodetic Survey has 

 adopted, as standard leadline material, ma- 

 hogany-colored tiller rope with a phosphor- 

 bronze wire center, of six strands of seven 

 wires each. No. 33-B. and S. gage. The wire 

 core should not break with prolonged flexing. 

 The rope is size No. 8, about one-quarter inch 

 in diameter made of solid-braided long-staple 

 cotton, and water-proofed. The braid should 

 be tight so that broken wire strands are less 

 likely to protrude through the covering to 

 injure the leadman's hands. The Washing- 

 ton Office will furnish this material upon 

 requisition. 



3-66 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 lead- 

 line 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 lead- 

 man'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 cover- 

 ing. 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 an- 

 other 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, will main- 

 tain 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 ten- 

 sion 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 

 off with a tape, but the most convenient 

 arrangement, and one which will be needed 

 later for leadline vertification, 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 co-mingled on the same 

 leadline. One system of marking utilizes feet 



