- Sounding line
A sounding line or lead line is a length of thin
rope with aplummet , generally oflead , at its end. No matter what metal the plummet is made of, it's still referred to as "the lead."In the recent past, depths were measured in
fathom s, although sincemetrication this has mostly changed tometre s. MostU.S. charts use fathoms or feet; the U.S.A. also uses a differentchart datum . Sounding lines were widely used innavigation until the development of ultrasonic depth-measuring devices. The sounding line has been superseded byecho sounding . Ultrasonic depth sounders can be cheap, accurate, and provide a real-time graphical profile of the depth of the seabed. To work continuously and reliably, they do needelectric al power and shelter from rain and spray, something that can be difficult to maintain on smaller and openboat s.ounding technique
It is easy to measure a length of line or rope as a rough number of fathoms by repeatedly stretching the rope between the two outstretched arms. Water depths have traditionally been measured this way using a weighted sounding line. The word "fathom" can be used as a verb to describe this process.
At sea, in order to avoid repeatedly hauling in and measuring the wet line by stretching it out with one's arms, it became traditional to tie marks at intervals along the line. These marks were made of
leather , calico,serge and other materials, and so shaped and attached that it was possible to "read" them on sight by day or at night by the feel of each one. The marks were at every second or third fathom, in a traditional order: at 2, 3, 5, 7, 10, 13, 15, 17, and 20 fathoms. The "leadsman" called out the depth as he read it off the line. If the depth was at a mark he would call "by the mark" followed by the number, if the depth was between two marks, he would call "by the deep" followed by the estimated number.On the
Mississippi river in the1850 s, the leadsmen also used old-fashioned words for some of the numbers; for example instead of "two" they would say "twain". Thus when there were only two fathoms left under the boat they would call "by the mark twain!". The American writerMark Twain , a former river pilot, likely took hispen name from this cry.Sometimes
tallow was placed at the recess in the bottom of the plummet to pick up traces of any loose material (such asmud ,sand , orshingle ) from theseabed , providing information useful forpilotage and anchoring. If the plummet came up clean, it meant the bottom was rock.Nautical chart s now provide information of the seabed materials at particular locations.See also
*
Fish finder
*Echo sounding References
P. Kemp, ed., "The Oxford Companion to Ships and the Sea" (London: Oxford University Press, 1976).
External links
* [http://www.navyandmarine.org/planspatterns/soundingline.htm An explanation of the fathom marks used at sea] (retrieved Sept 2006).
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