An hourglass (sandglass, sand timer, sand watch, sand clock, egg timer) measures the passage of a few minutes or an hour of time. It has two connected vertical glass bulbs allowing a regulated trickle of material from the top to the bottom. Once the top bulb is empty, it can be inverted to begin timing again. The name hourglass comes from historically common hour timing. Factors affecting the time measured include the amount of sand, the bulb size, the neck width, and the sand quality. Alternatives to sand are powdered eggshell and powdered marble (sources disagree on the best material). In modern times, hourglasses are ornamental, or used when an approximate measure suffices, as in egg timers for cooking or for board games. The origin of the hourglass is unclear, although it may have been introduced to Europe by an 8th-century monk named Luitprand, who served at the cathedral in Chartres, France. It was not until the 14th century that the hourglass was seen commonly, the earliest firm evidence being a depiction in the 1338 fresco Allegory of Good Government by Ambrogio Lorenzetti. Unlike its predecessor the clepsydra, or water clock, which may have been invented in ancient Egypt, the hourglass is believed to have originated in medieval Europe. This theory is based on the fact that the first written records of it were mostly from logbooks of European ships. Written records from the same period mention the hourglass, and it appears in lists of ships stores. An early record is a sales receipt of Thomas de Stetesham, clerk of the English ship La George, in 1345: "The same Thomas accounts to have paid at Lescluse, in Flanders, for twelve glass horologes (" pro xii. orlogiis vitreis "), price of each 4½ gross', in sterling 9s. Item, For four horologes of the same sort (" de eadem secta "), bought there, price of each five gross', making in sterling 3s. 4d." Hourglasses were very popular on board ships, as they were the most dependable measurement of time while at sea. Unlike the clepsydra, the motion of the ship while sailing did not affect the hourglass. The fact that the hourglass also used granular materials instead of liquids gave it more accurate measurements, as the clepsydra was prone to get condensation inside it during temperature changes. Seamen found that the hourglass was able to help them determine longitude, distance east or west from a certain point, with reasonable accuracy. The hourglass also found popularity on land as well. As the use of mechanical clocks to indicate the times of events like church services became more common, creating a 'need to keep track of time', the demand for time-measuring devices increased. Hourglasses were essentially inexpensive, as they required no rare technology to make and their contents were not hard to come by, and as the manufacturing of these instruments became more common, their uses became more practical. Hourglasses were commonly seen in use in churches, homes, and work places to measure sermons, cooking time, and time spent on breaks from labor.[4] Because they were being used for more everyday tasks, the model of the hourglass began to shrink. The smaller models were more practical and very popular as they made timing more discreet. After 1500 A.D., the hourglass was not as widespread as it had been. This was due to the development of the mechanical clock, which became more accurate, smaller and cheaper, and made keeping time easier. The hourglass, however, did not disappear entirely. Although they became relatively less useful as clock technology advanced, hourglasses remained desirable in their design. Some of the most famous hourglasses are the twelve-hour hourglass of Charlemagne of France and the hourglasses of Henry the VIII of England, made by the artist Holbein in the 16th century. The oldest known surviving hourglass resides in the British Museum in London. Not until the 18th century did the Harrison brothers, John and James, come up with a marine chronometer that significantly improved on the stability of the hourglass at sea. Taking elements from the design logic behind the hourglass, they were able to invent a marine chronometer that was able to accurately measure the journey from England to Jamaica, with only a miscalculation of five seconds, in 1761.

• published: 28 Oct 2013
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