A means, before the days of echo sounders or sonar, of finding the depth of water near coasts. It was probably the earliest device used by coastal navigators to facilitate safe navigation, especially in thick or hazy weather. It comprised a hemp line to which was attached, by means of a leather becket or a rope strop, a lead weight, or plummet, of about 3 kilograms (7 lb). The lower end of the weight was cup shaped, into which a lump of tallow could be pressed, an action known as arming the lead. From the material that adhered to the tallow it was often possible for those aboard to identify what type of sea bottom the ship had beneath it. When the tallow came up clean, it indicated the bottom was rock.
A lead line of about 25 fathoms (150 ft/45.7 m) was used in shallow water and an experienced and skilful leadsman could measure depths of as much as 20 fathoms (120 ft/36.5 m), with a ship making moderate headway of about 10 knots, by heaving the lead ahead of the ship so that when it reached the bottom it would be vertically beneath him. Traditionally, the line was marked with materials which could be easily distinguished by texture when taking a sounding in the dark. There were pieces of leather at 2 (two strips), 3 (three strips), and 10 (a square piece with a hole in it) fathoms; white duck (strong cotton fabric) at 5 and 15 fathoms; red bunting at 7 and 17; and blue serge (durable worsted) at 13 fathoms. The mark at 20 fathoms was a piece of cord with two knots in it, also easily distinguishable in the dark.
The fathoms on a longer lead line were marked with a piece of cord with one knot in it at each 5 fathoms and with three, four, five knots at 30, 40, and 50 fathoms (180, 240, 300 ft/55, 73.2, 91.5 m). When a seaman took a sounding with the lead line he called out the depth of water according to the mark on the line that was on or very near the surface of the sea when the lead reached the bottom beneath him. For example, if he saw that the first piece of red bunting was on the surface after his cast, he called out ‘By the mark, seven’.
If, after heaving the lead, there was no distinguishing mark level with the sea, the leadsman had to estimate the depth of water by the nearest mark he could see above the sea. This was known as a deep, and the leadsman would call the depth of water with the words ‘Deep —’. The navigator would then know that he had only got an approximate depth.For England when, with favouring gale, Our gallant ship up Channel steered,And, scudding under every sail, The high blue western land appearedTo heave the lead the seaman sprungAnd to the pilot cheerly sung, ‘By the deep, nine’.(W. Pearce, c.1793)For measuring depths greater than was possible with a hand lead line, the deep-sea lead line, which could measure depths of up to about 100 fathoms (600 ft/183 m), was used. In this case, the lead was about double the weight of a hand lead and the line was marked at each multiple of 10 fathoms with cord having a number of knots equal to the number of tens of fathoms. Each intermediate fifth fathom was marked with a piece of cord having a single knot. The lead was always cast from the weather side of the ship, in the case of the hand lead from a platform called the chains, with the depths being reported to the navigator or officer of the watch by singing them out loudly in a traditional manner.