The unique use of the sandglass, after the sixteenth century, was measuring a ship’s speed. Knots were tied at seven-fathom intervals on a line tied to a log chip that would float astern. A sailor dropped the log chip off the end of the speeding ship and counted off the number of knots paid out while a small sandglass measured a half-minute. If five knots passed in the interval, the ship was making five nautical miles an hour. Throughout the nineteenth century, sailing vessels still “heaved the log” every hour to keep track of the speed.
