Walker patent log
Walker's Harpoon Log
A patent log is a self-recording device consisting of a rotator towed behind a ship. The number of revolutions of the rotator in a certain time period could be used to calculate the speed of a ship. Thomas Walker was widely associated with the development of the modern patent log. His "Harpoon" log with dials incorporated in the outer casing of the rotator was patented in 1861, and his famous "Cherub" log was introduced in 1884. By this time engineering development had allowed revolutions of the rotator astern to be transmitted accurately to an inboard register. These were also called taffrail logs.