Richard MacKenzie - Collections, Maritime Museum of British Columbia, Explaining how to use a lead line

This is a lead line for finding the depth of water that you're in and also the nature of the bottom. You can see there is a hole here that you could stuff full of tallow and when you brought your lead line up from the bottom it would have sand and grit and things in it and from that the navigator could tell what the bottom was like. This tube would have tags on it measured off in fathoms or six feet. It would be lowered over the side of the ship down, down, down, down and then when it hit the bottom and was vertical you could read off how deep the water was. Then, they would bring it up and they would examine what had been caught in the bottom. If it was sand and shale, then it was good anchorage, if it was muddy, anchors wouldn't hold so they would move on. For navigation as you got close to land you wanted to know whether there was any shoals and so the captain of the ship would have someone taking depths continually as they came into a harbour so they didn't run aground.