A log is used, together with a sand glass or other timing device, to measure the speed of a ship relative to an object floating in the water. The simplest version, known at least since the fifteenth century, was simply a piece of wood that a sailor could throw overboard. Mechanical logs were introduced in the eighteenth century, and self-recording logs in the early nineteenth century. These could also indicate the distance sailed from one time to another.
Ref: J. Broelmann, "Logs," in Robert Bud and Deborah Warner, eds., Instruments of Science. An Historical Encyclopedia (New York & London, 1998), pp. 361-363.