Jacob's ladder

Also Jacobs ladder.
1. A flexible hanging ladder consisting of vertical ropes or chains supporting horizontal rungs, used to allow access over the side of a ship, either to transfer between the ship and another vessel alongside it or to perform maintenance tasks along the side of the ship. Sometimes mistakenly referred to as a pilot ladder, which differs from a Jacob′s ladder in its use of spreaders and in terms of specific regulations governing step size and step spacing.
2. A vertical ladder from the ratlines found on square-rigged ships, used to get around the top while climbing between the lower mast and the topmast.