sounding line

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Sounding aboard a frigate of the 19th century.

A sounding line or lead line is a length of thin rope with a plummet, generally of lead, at its end. Regardless of the actual composition of the plummet, it is still called a "lead."

Sounding lines were widely used in navigation until the development of echo sounding. Ultrasonic depth sounders provide an accurate graphical profile of the depth of the seabed.

The word derives from the Old English sund, meaning swimming, water, sea.^[1]

Sounding 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 the 1850s, 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 writer Mark Twain, a former river pilot, likely took his pen 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 as mud, sand, or shingle) from the seabed, providing information useful for pilotage and anchoring. If the plummet came up clean, it meant the bottom was rock. Nautical charts now provide information of the seabed materials at particular locations.

See also

• Fishfinder
• Echo sounding

References

• P. Kemp, ed., The Oxford Companion to Ships and the Sea (London: Oxford University Press, 1976).

External links

• An explanation of the fathom marks used at sea (retrieved Sept 2006).