Walker's Harpoon Ship Log
This mechanical log tells the distance the ship sails through the water by recording the number of times the five-finned section of the mechanism turns. The indicator shows nautical miles sailed, in hundreds, tens, and ones. The log was towed astern of the ship and hauled in for readings. Though not perfect, the harpoon log was significantly more accurate than the traditional chip log and sand glass. This instrument was patented September 18, 1866.
Thomas Walker & Son was a British firm that patented their first log in 1861. They continued to develop and patent logs into the 1930s.