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Longitude Essays

Board of Longitude > Longitude Essays


In the eighteenth and nineteenth centuries, dead reckoning was one of the most common means of calculating the position of a ship at sea. Four pieces of information were required. First, the known position of the ship at a previous time. Second, the time elapsed since that position. Third, the direction in which the ship had been travelling. Fourth, the speed at which the ship had been moving. Given these, a navigator could compute the current position of a ship, or alternatively plot it on a chart. In order for this method to be accurate, navigators needed to regularly record the speed of their ship, along with its direction of travel.

Traditionally, in order to measure the speed of a ship, a navigator would use a wooden board [RMG icon] attached to a coil of rope knotted at set intervals. This was known simply as a log. By towing the wooden board behind the ship, and counting the number of knots of rope taken out over a set time, a navigator could acquire an estimate of the speed.

Despite the widespread use of this method, inaccuracies could easily arise: the direction of the waves, the stretch in the line, and the speed of the current could all affect the accuracy of dead reckoning. As such, a number of alternative devices were proposed [RGO 14/31]. Many of these were sent to the Board of Longitude for evaluation as part of its broader remit to improve the general practice of navigation. Mechanical devices were particularly popular. Edward Massey, the Staffordshire clockmaker, patented a mechanical log [RMG icon] in 1802. In this design, a brass rotor trails in the water and spins around as the ship moves along. This rotary motion is then passed through a register. Three dials marked '1/4-1', '1-10' and '10-100' give a readout of the distance travelled in nautical miles. The dials are also protected whilst the log is in the water by a brass swivelling cover. Massey forwarded this design to the Board of Longitude. In 1807 they concluded [RGO 14/7] that it did not represent a sufficient improvement over the traditional wooden log. Nonetheless, Massey continued to sell his mechanical logs privately throughout the nineteenth century and marketed them alongside his more successful sounding machine.

James Poskett
History and Philosophy of Science
University of Cambridge