log, instrument for measuring the speed of a ship through water. The first practical log, developed about 1600, consisted of a pie-shaped log chip with a lead weight on its curved edge that caused it to float upright and resist towing. When the log was tossed overboard, it remained more or less stationary while an attached line (marked off with equally spaced knots) was let out behind the vessel for a measured interval of time (measured with a sandglass). The line and log were then hauled aboard and the speed of the ship determined by dividing the length of the line by the time interval.

In the 19th century the log chip was replaced by a towed rotor or propeller connected by a line to automatic speed- and distance-measuring equipment. Two logs in use today are the pitometre log and the electronic log. The pitometre uses a pitot tube (see Henri Pitot) projecting through the bottom of the ship. The tube has one forward-facing and two side-facing orifices. When the ship is moving, pressure in the forward-facing tube exceeds the pressure in the side tubes; this differential is transmitted to equipment that translates it into a speed measurement. In the electronic log, which also protrudes through the bottom of the ship, a water-driven rotor turns a small electric generator, the current from which is proportional to the speed of the ship. This current is similarly used to produce a speed measurement.