Thomas Walkers Patent Harpoon Ship's Log

This curious object is a harpoon ship log. It was used for measuring a ship’s speed and the distance it had traveled. Harpoon logs were produced in Birmingham and London by Thomas Walker and Son, a firm of scientific instrument makers. The idea was patented in 1861 and this example was made in the same year. The harpoon log is essentially a finned copper tube with a set of interlinked gears inside. The gears operate a set of three counters which record mileage. The log was attached to a length of rope and towed behind the ship. The finned section at the end of the log rotated as the device was pulled through the water. This action turned a series of levers which recorded the distance the vessel had run. To prevent corrosion the log had to be regularly hauled on board and sluiced with fresh water. The history of this particular nautical log is obscure but it may have been used by either Captain John McBride or Captain William McBride of Port Logan.