This high-quality working example of a Taffrail Log was submitted to the U.S. Patent Office when John Edward Massey, its British inventor, applied for a patent in 1877. This two-part mechanical log measures a vessel's speed though the water. The four-bladed rotator is towed astern and registers the distance traveled and speed on an integral dial. Closer to the ship, or even mounted to the ship's rail, a second mechanism measures rotations of the towing rope to record the same information. The rotator must be hauled in to be read, but the second log can be monitored while the first is still in operation.
Since the sixteenth century, sailors had determined vessel speed using a log. This device was basically a rope with knots tied at intervals along its length. With a board attached to one end to create drag, the log-line would be heaved overboard and allowed to run out for a short period of time. The number of knots counted off during that time indicated the speed. (The unit of speed at sea is therefore the knot, one knot being equal to one nautical mile per hour). Logs provided vital information for navigation but were susceptible to a variety of errors, and inventors in the seventeenth and eighteenth centuries sought ways to improve their accuracy. Edward Massey, a leading London nautical instrument maker, received the first patent for a successful mechanical log (British patent 2,601, March 24, 1802), and many ships used his firm's "patent logs" throughout the nineteenth century. John Edward Massey inherited the business and continued to patent improvements to his father's work.
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