Sounding

By the mid-eighteenth century, there were two kinds of plummets in common use. One was called the hand-lead. This was used in shallower waters. It weighed around 4 kilograms, and was attached to a line 20 fathoms in length, marked at every two or three fathoms. It was important that the marks on the line were very different and made distinctive so that they could be seen by day or at night.

The other plummet was the deep-sea lead. It was much larger, commonly weighing between 10 and 13 kilograms; and as its name implies it was used when vessels were well off shore.

Sounding with the hand-lead was called 'heaving the lead'. It was done standing in the main-chains to windward with the line ready to run out. A seaman would first swing the hand-lead back on a fathom of line back and forth three or four times. He would then use the momentum to swing it around his head, and then let it fly out forward of the vessel.

This ensured that as the lead sunk and the ship moved forward, the line would be perpendicular when it hit the bottom. The lead swinger then cried out what he calculated the depth to be.

The deep sea lead require the ship to be slowed, and the plummet to be thrown as far as possible ahead of its course. As with the hand-lead, the depth of water was discovered by reading marks on the attached line.

The bottoms of deep-sea leads were often coated with tallow or some similar substance. This allowed some idea of the nature of the seabed to be gauged either from marks made by coral or rocks, or from gravel, shells or mud sticking to the lead.

Soundings were carefully recorded in the ship's log book.