The "Log" was first described as being a piece of shaped wood with a weight attached at one end in order that it should float upright in the water. The log was attached by three short pieces of rope to the so-called "log line" and the log was cast astern of the vessel. The log line was knotted at regular intervals and the number of knots played out during a period of half a minute, again timed by a log glass, was counted. This operation provides the origin of the expression for describing the speed of a ship in "knots".
A number of mechanical logs were patented by during the eighteenth century although examples date as far back as the sixteenth century. However, the first reliable log was not developed until 1802, when Edward Massey, invented a spinning brass rotor which recorded the distance travelled by a ship on a series of dials, and those could be read when the log was retrieved from the sea. A modification of this was the taff-rail log, introduced in the late 1840's in which the recording device was attached to the "taff", or stern rail, of the ship and connected to the rotor by means of a length of rope. Such logs were made in considerable quantities are are still quite easy to find today.