This instrument is designed to measure the distance covered by a ship. In this design, a brass rotor trails in the water and spins around as the boat 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 while the log is in the water by a brass swivelling cover. This form of log was patented by Edward Massey (about 1768-1852), whose name is inscribed on the converter, along with the trademark of three Ls in a triangle.