'Plimsoll lines' on the side of a cargo ship. The original Plimsoll lines (the circle with a line through it) were introduced after the 1876 Merchant Shipping Act, largely due to the work of Samuel Plimsoll (1824-98). This shows the level of water at which the ship is fully laden. Later, the load line system (upper left) was introduced which takes account of different types of water. The abbreviations used are: TF (tropical fresh); F (fresh water); T (tropical salt water); S (salt water, summer) and W (salt water, winter). At right is a scale showing the distance to the keel of the ship in metres (here about 8.6m).