A mechanical instrument for measuring time, depending on the uniform flow of sand or other fine-grained material through a narrow aperture between two glass bulbs. In the simplest and commonest form the two bulbs are of equal volume, and the unit of time measured is that taken for the whole of the sand content to flow from one bulb to the other. Occasionally, the assembly consists of one large bulb and four smaller bulbs, the time unit being thus divided into four. The earliest-known records of sand glasses date from the 14th century, and one is shown in a fresco of 1338 by Ambrogio Lorenzetti in the Palazzo Pubblico in Siena. There are records of sand glasses in ships' inventories from about this date, and it seems that their principal early use was for dead reckoning at sea, for measuring the progress of a ship requires a timekeeper. Slow-running glasses, emptying in two or four hours, were also used at sea for timing the duty watches. Sand glasses were sometimes assembled and mounted in groups of four timed for 1/4-hour, 1/2-hour, and 3/4-hour and one-hour periods. All early sand glasses were made with two glass bulbs separated by a thin metal plate with a central hole, the assembly being waxed together and covered with thread. From the mid 18th century, however, they were blown in one piece, the central constriction being of glass and the opening sealed by a cork. From c. 1800 the opening was sealed after filling by the glass blower, making the whole airtight.