The officers and captain navigated the ship by the sun and stars, using various instruments to work out the position of the ship or the time, and a lead to check the depth of water to avoid running aground. The other essentials were a compass and a chronometer. The sounding lead was lowered over the side of the ship and the line let out until the lead hit the bottom. By about 1600, the line was marked in fathoms (6ft or 1.8m) with pieces of material. This sounding lead was used aboard the trow (a small cargo boat used mainly for river trading) Nora, trading locally from Bristol, but something very similar would have been in use on slaving ships. A sounding lead was especially important in unknown shallow waters, as the risk of grounding was high.