On land a vehicle's speed is measured by a speedometer that reacts to the revolutions of the wheels. At sea, ships use a device called a log for the same purpose. In its earliest form, the log literally consisted of a log or other wooden float attached to a line. The log was let out at the stern of the ship and allowed to drift away from the ship for an interval of time. Then it was reeled in. The length of the line told the mariners how far they had traveled during that time interval. The original log was not an accurate method of measuring speed, however, for the log itself moved about in the water as the ship moved forward.

An accurate reading would have required that the log remain stationary. In the sixteenth century, a triangular chip weighted with lead was let out in the same manner as the log. Lowering the chip to a moderate depth made it less susceptible to surface drift. In 1801 Edward Massey, an Englishman, created a mechanical log that automatically measured the line as it was let out. This was followed by the taffrail log, which consisted of a rotator at the end of a tow line connected to a recording device. Because it measured distance traveled, a mariner had to take two readings to establish the ship's speed.

The modern pitometer log uses a device called a Pitot tube that also projects from the ship's bottom. The tube has one forward-facing orifice and two side-facing orifices. The difference in pressure caused by the ship's forward movement is calculated to establish the ship's speed. When the ship is at rest, the difference is zero. The best modern method of measuring a ship's speed is by counting the revolutions of the propeller. Since no two ships are exactly alike, the standard speed of a ship relative to its propeller revolutions must be calculated in a series of trial runs shortly after the ship is launched. Still, such factors such as weather and aging of the ship's hull can affectthe accuracy of this method.

The book in which readings are recorded is called the logbook, and in mariner's jargon it, too, is often shortened to "log." All of the events of the ship's journeys, both routine and unusual, are noted in the logbook.