The log served to measure the ship's speed. It consisted of a wooden board tied to a rope carrying a series of knots at intervals of approximately 50 feet (15.43 meters), or 1/120th of a nautical mile (1,852 meters). The board would be cast into the sea from the stern and a sailor would count the knots passing through his hands, while another sailor would measure the time with a 30-second water clock or sandglass. As 30 seconds are 1/120th of an hour, the knots counted by the first sailor gave the ship's speed in miles per hour. To this day, a knot denotes a speed of one nautical mile per hour. The officer of the watch would perform the measurement every half hour and record the direction and distance traveled on a traverse board. Every four hours, the measurements would be added up, and at the end of the day the route and distance would be recorded on the sea charts.