They have a nice collection of navigational instruments at the French Maritime Museum, so I thought I would share some with you. The little gadget shown above is called a ship log, and it is a way of determining the speed of the boat through water. This is the evolved form of throwing a chip of wood overboard and seeing how far it goes in a few seconds. In the above picture you can see a thirty second sandglass. The idea is to throw it overboard, let it float there for 30 seconds, while the ship moves away. The line has knots on it every so often and thus you can measure the speed of the boat.