Lead and line 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", often referred to as
the lead. Leads were swung, or cast, by a leadsman, usually standing in the
chains of a ship, up against the
shrouds. Measuring the depth of water by lead and line dates back to ancient civilization. It continues in widespread use today in recreational boating and as an alternative to electronic
echo sounding devices.
Greek and
Roman navigators are known to have used sounding leads, some of which have been uncovered by archaeologists. Sounding by lead and line continued throughout the
medieval and
early modern periods and is still commonly used today. The
Bible describes lead and line sounding in
Acts 27:28, whilst the
Bayeux Tapestry documents the use of a sounding lead during
William the Conqueror's 1066 landing in
England. Lead and line sounding operates alongside sounding poles, and/or
echo sounding devices particularly when navigating in shallower waters and on rivers. At sea, in order to avoid repeatedly hauling in and measuring the wet line by stretching it out with one's arms, it is common practice to tie marks at intervals along the line. These marks are made of
leather,
calico,
serge and other materials, and so shaped and attached that it is possible to "read" them by eye during the day or by feel at night. Traditionally the marks were at every second or third fathom: 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, while if it was between two marks, he would call "by the deep" followed by the estimated number; thus "by the mark five", since there is a five-fathom mark, but "by the deep six", since there is no six-fathom mark. Fractions would be called out by preceding the number with the phrases "and a half", "and a quarter", or "a quarter less"; thus 4 3/4 fathoms would be called as "a quarter less five", 3 1/2 as "and a half three", and so on. Soundings may also be taken to establish the ship's position as an aid in
navigation, not merely for safety. Soundings of this type were usually taken using leads that had a wad of
tallow in a concavity at the bottom of the plummet. The tallow would bring up part of the bottom sediment (sand, pebbles, clay, shells) and allow the ship's officers to better estimate their position by providing information useful for
pilotage and
anchoring. If the plummet came up clean, it meant the bottom was rock. Nautical charts provide information about the seabed materials at particular locations. One of the most widely adopted sounding machines was developed in 1802 by Edward Massey, a clockmaker from
Staffordshire. The machine was designed to be fixed to a sounding lead and line. It featured a rotor which turned a dial as the lead sank to the sea floor. On striking the sea floor, the rotor would lock. Massey's sounding machine could then be hauled in and the depth could be read off the dials in fathoms. By 1811, the Royal Navy had purchased 1,750 of these devices: one for every ship in commission during the
Napoleonic Wars. The
Board of Longitude was instrumental in convincing the Royal Navy to adopt Massey's machine.
Echo sounding Both lead-and-line technology and sounding machines were used during the twentieth century, but by the twenty-first,
echo sounding has increasingly displaced both of those methods. A sounding line can still be found on many vessels as a backup to electronic depth sounding in the event of malfunction.
GPS has largely replaced the sextant and chronometer to establish one's position at sea, but many mariners still carry a sextant and chronometer as a backup. Many small craft still rely solely on a sounding line. The first practical
fathometer (literally "fathom measurer"), which determined water depth by measuring the time required for an echo to return from a high-pitched sound sent through the water and reflected from the sea floor, was invented by
Herbert Grove Dorsey and patented in 1928. ==See also==