The Lead Line, a device for measuring the depth of the water as well as obtaining a sample of the ocean floor, is one of the oldest of all navigating tools. The word "lead" is pronounced the same way as in "lead pencil".
It began with the oldest known boat trading peoples, the Egyptians. We have images of their river trading craft going back to about 3400 BCE. Then, as now, it was inconvenient to run aground….it could ruin your entire day just as quickly as a collision. The earliest device to measure depth was a stick. At first it was unmarked with any depth scales. In time, it would have been. As trading expanded beyond the rivers of Egypt and onto the coast and into the Mediterranean Sea, a stick was no longer adequate. A rock could be tied to the end of a line and dropped over the side. The depth of the water could be measured as you retrieved the line and stretched the line between your arms. By the Fifth Century, BCE, the Greeks were using a lead line which is mentioned by the Greek historian Herodotus.
By 1600 in England, the lead line was being marked at certain depths to make the reading easier: 2, 3, 5, 7, 10, 15, 20, and 25 fathoms. A fathom, from the Old Norse word, fathmr, for "outstretched arms" was standardized at six-feet: an average distance between a man's outstretched arms as he held the lead line. The standard lead line was 20 fathoms long--120 feet--and the lead weight 7 pounds. That number may seem curious now, but in England of the 1600's, weights were routinely measured in a 14 pound increment called a "stone". A half stone, or a "clove" was seven pounds. The lead itself was cylinder-like, but slightly fatter at the bottom than the top and a loop was cast into the metal so that a line could be attached.
Besides the depth, the lead could also tell the mariner about the type of ocean bottom he was sailing over. The bottom of the lead weight was hollowed out so that a glob of tallow, or animal fat, could be inserted. A glob is a non-scientific, but highly descriptive term for tallow about the size of a golf ball. Since Golf hadn't been invented yet, the "glob" would do. When the fat glob hit the bottom, some of the material stuck into the fat. Just as on land the surface varies from rock, to sand, to dirt, to pebbles, so too does the ocean bottom. Knowing the material on the bottom along with the depth was an additional means to determine where you were on the featureless ocean as the bottom changes drastically as you travel.
As the leadsman retrieved the line, he'd call out the depth. If it were exactly as measured on the line at 2, 3, 5, 7, etc. fathoms, he called out a "mark": "By the mark seven". If he estimated it to be one-quarter less, he'd say "A quarter less seven". If it were more," and a quarter seven", or "and a half seven". If he estimated a reading in whole fathoms, but not marked, he called it a "deep"; "by the deep four". Estimates were only made in quarter, half fathoms and whole fathoms.