នាឡិកាខ្សាច់ ឬនាឡិកាកែវ គឺជាឈ្មោះឧបករណ៍ម្យ៉ាង សម្រាប់វាស់ពេលវេលារយៈពេលខ្លី ទៅតាមតម្រូវការរបស់អ្នកប្រើប្រាស់។ នាឡិកាខ្សាច់នេះ មានវាស់ពេលចំនួន៣នាទី ក៏មាន ១ម៉ោងក៏មាន ទៅតាមខ្នាតដែលគេឆ្នៃធ្វើមក។ តាមភាសាបរទេស គេអាចហៅថាអេកថែមមើ (eggtimer) ព្រោះជាឧបករណ៍ ដែលគេប្រើសម្រាប់កំណត់ពេលស្ងោរស៊ុត។ នាឡិកាប្រភេទនេះ មានអំពូលពីរធ្វើអំពីកែវ តភ្ជាប់គ្នា ជាទំរង់បញ្ឈរ ដើម្បីឲ្យខ្សាច់ហូរពីអំពូលកែវផ្នែកខាងលើ ចុះមកផ្នែកខាងក្រោម តាមរន្ធដ៏តូចនៅត្រង់ផ្នែកកណ្តាល។ នៅពេលដែលខ្សាច់ហូរអស់ពីអំពូលកែវផ្នែកខាងលើ គេអាចត្រឡប់ឧបករណ៍នេះបញ្ច្រាស់មកវិញ គឺអំពូលដែលនៅផ្នែកខាងក្រោម ដាក់នៅផ្នែកខាងលើវិញ ហើយចាប់គិតរយៈពេល ដែលខ្សាច់ហូរចុះមកអំពូលខាងក្រោម ឡើងវិញ។ កត្តាដែលធ្វើឲ្យឧបករណ៍នេះ វាស់ពេលបានត្រឹមត្រូវ មិនលំអៀង គឺ គេត្រូវគិតអំពីចំនួនខ្សាច់ ដែលត្រូវដាក់បញ្ចូល ក្នុងអំពូលនោះ ទំហំរបស់អំពូល ទំហំផ្នែកកណ្តាល (ផ្នែក ក) ដែលខ្សាច់ត្រូវហូរចុះ និងគុណភាពរបស់ខ្សាច់។ ក្រៅពីខ្សាច់ប្រភេទនេះ គេអាចប្រើម្សៅសំបកស៊ុត ឬម្សៅម៉ាបក៏បាន។  (sources disagree on the best material). បច្ចុប្បន្ននេះ នាឡិកាខ្សាច់ ត្រូវបានគេប្រើសម្រាប់វាស់រយៈពេលស្ងោរស៊ុត ឬវាស់រយៈពេលគិត របស់ដៃគូ ក្នុងការលេងល្បែងមួយចំនួន ដូចជាល្បែងអុក ត-អក្សរ ឬ រៀបអក្សរ ជាដើម។
ដើមកំណើតនៃនាឡិកាខ្សាច់មិនត្រូវបានគេដឹងច្បាស់លាស់ទេ តែវាត្រូវបានណែនាំទៅក្នុងទ្វីបអឺរ៉ុបនៅសតវត្សទី៨ ដោយសង្ឃមួយព្រះអង្គ ព្រះនាម Luitprand ដែលមកពីព្រះវិហារគ្រឹស្គមួយនៅឆាត្រេស ប្រទេសបារាំង។ Fresco "Allegory of good Government" ត្រូវបានបង្កើតដំបូងនៅឆ្នាំ១៣៣៨ ដោយលោកAmbrogio Lorenzetti។ នាឡិកាខ្សាច់ ត្រូវបានគេពេញនិយមប្រើប្រាស់ នៅអំឡុងដើមសតវត្ស៍ទី១៤ 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. 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 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.
The shape behind the hourglass has hardly any written evidence of why its external form is the shape that it is. The glass bulbs used, however, have changed in style and design over time. While the main designs have always been ampoule in shape, the bulbs were not always connected. The first hourglasses were two separate bulbs with a cord wrapped at their union that was then coated in wax to hold the piece together and let sand flow in between. It was not until 1760 that both bulbs were blown together to keep moisture out of the bulbs and regulate the pressure within the bulb that varied the flow.
ប្រភេទនាឡិកានេះ ខ្លះគេដាក់គ្រាប់ខ្សាច់ម្យ៉ាងដ៏ល្អិតនៅខាងក្នុង ហៅថា នាឡិកាខ្សាច់ ខ្លះទៀតគេប្រើល្បាយដែលលាយដោយម្សៅម៉ាប សំណប៉ាហាំង និងកំទេចសំបងស៊ុតដែលដុតរួច។ ក្រោយមក គេក៏ធ្វើការប្រៀបធៀបលម្អិតទាំងនោះ ដើម្បីរកឲ្យឃើញថា តើលម្អិតណាមួយហូរយ៉ាងទៀងទាត់ពីអំពូលលើចុះមកអំពូលក្រោម។ ទីបំផុតគេរកឃើញថា ដើម្បីសម្រេចបានលំហូរឥតខ្ចោះមួយ សមាមាត្រនៃលម្អិតនោះមួយៗត្រូវមានទំហំ1/12 ឬច្រើនជាងនេះ ប្រៀបនឹងក-អំពូល តែមិនអាចធំជាង1/2នៃទំហំក-អំពូលឡើយ។
Hourglasses were an early dependable, reusable and accurate measure of time. The rate of flow of the sand is independent of the depth in the upper reservoir, and the instrument will not freeze in cold weather.
From the 15th century onwards, they were being used in a range of applications at sea, in the church, in industry and in cookery.
During the voyage of Ferdinand Magellan around the globe, his vessels kept 18 hourglasses per ship. It was the job of a ship's page to turn the hourglasses and thus provide the times for the ship's log. Noon was the reference time for navigation, which did not depend on the glass, as the sun would be at its zenith. More than one hourglass was sometimes fixed in a frame, each with a different running time, for example 1 hour, 45 minutes, 30 minutes, and 15 minutes.
Modern practical uses[កែប្រែ]
While they are no longer widely used for keeping time, some institutions do maintain them. Both houses of the Australian Parliament use three hourglasses to time certain procedures, such as divisions.
The sandglass is still widely used as the kitchen egg timer; for cooking eggs, a three-minute timer is typical, hence the name "egg timer" for three-minute hourglasses. Egg timers are sold widely as souvenirs.[ត្រូវការអំណះអំណាង]
The hourglass, sometimes with the addition of metaphorical wings, is often depicted as a symbol that human existence is fleeting, and that the "sands of time" will run out for every human life. It was used thus on pirate flags, to strike fear into the hearts of the pirates' victims. In England, hourglasses were sometimes placed in coffins, and they have graced gravestones for centuries. The hourglass was also used in alchemy as a symbol for hour.
The former Metropolitan Borough of Greenwich in London used an hourglass on its coat of arms, symbolising Greenwich's role as the origin of GMT. The district's successor, the Royal Borough of Greenwich, uses two hourglasses on its coat of arms.
Modern symbolic uses[កែប្រែ]
Recognition of the hourglass as a symbol of time has survived its obsolescence as a timekeeper. For example, the American television soap opera Days of our Lives, since its first broadcast in 1965, has displayed an hourglass in its opening credits, with the narration, "Like sands through the hourglass, so are the days of our lives," spoken by Macdonald Carey.
កំព្យូទ័រភាគច្រើន graphical user interfaces នៅពេលដែលកំពុងដំណើរការកម្មវិធីណាមួយ ទស្សន៍ទ្រនិច (cursor) ដែលស្ថិតនៅក្នុងកម្មវិធីនោះ ដូរទៅជារូបនាឡិកាកែវ បញ្ជាក់ថា កម្មវិធីនេះកំពុងធ្វើប្រតិបត្តិការ ហើយមិនមានប្រតិកម្ម (មិនអនុញ្ញាត) ឲ្យអ្នកប្រើប្រាស់បញ្ចូល ឬបញ្ជាលើកម្មវិធីនេះឡើយ។ សញ្ញានាឡិការកែវនៅក្នុងកម្មវិធីមួយ មិនអាចមានឥទ្ធិពលលើកម្មវិធីដទៃឡើយ។ អ្នកប្រើប្រាស់អាចធ្វើការងារលើកម្មវិធីដទៃទៀត កំឡុងពេលដែលកម្មវិធីនេះ កំពុងធ្វើប្រតិបត្តិការ។ នៅពេលដែលរូបសញ្ញានាឡិកាកែវនេះរលត់បាត់ បញ្ជាក់ថា កម្មវិធីនេះនៅក្នុងលក្ខណៈរង្វិលជុំ រង់ចាំការបញ្ចប់ពីអ្នកប្រើប្រាស់ ឬធ្វើការងារបន្តទៀត ដូចជា បញ្ចូលCDជាដើម។ នៅក្នុងប្រព័ន្ធយូនីកូដ រូបនាឡិកាកែវនៅ
Because of its symmetry, graphic signs resembling an hourglass are seen in the art of cultures which never encountered such objects. Vertical pairs of triangles joined at the apex are common in Native American art; both in North America, where it can represent, for example, the body of the Thunderbird or (in more elongated form) an enemy scalp, and in South America, where it is believed to represent a Chuncho jungle dweller. In Zulu textiles they symbolise a married man, as opposed to a pair of triangles joined at the base, which symbolise a married woman. Neolithic examples can be seen among Spanish cave paintings. Observers have even given the name "hourglass motif" to shapes which have more complex symmetry, such as a repeating circle and cross pattern from the Solomon Islands. Both the members of Project Tic Toc,from television series the Time Tunnel and the Challengers of the Unknown use symbols of the hourglasse representing either time travel or time running out.
- Lua error in Module:Cite_web at line 38: attempt to concatenate field 'month' (a nil value).
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- Balmer, R. T. "The Operation of Sand Clocks and Their Medieval Development." Technology and Culture, Vol. 19, No. 4 (Oct., 1978), pp. 615-632 Balmer, R. T. "The Operation of Sand Clocks and Their Medieval Development." Technology and Culture, Vol. 19, No. 4 (Oct., 1978), pp. 615-632.
- Peterson, Ivars. "Trickling sand: how an hourglass ticks". Science News, Vol. 144, No. 11 (September 11, 1993). p. 167
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- Room, Adrian (1999). Brewer's Dictionary of Phrase and Fable. New York: HarperCollinsPublishers. http://www.phrases.org.uk/bulletin_board/17/messages/288.html. "Time is getting short; there will be little opportunity to do what you have to do unless you take the chance now. The phrase is often used with reference to one who has not much longer to live. The allusion is to the hourglass."
- Ewbank, Thomas (1857). A Descriptive and Historical Account of Hydraulic and Other Machines for Raising Water, Ancient and Modern With Observations on Various Subjects Connected with the Mechanic Arts, Including the Progressive Development of the Steam Engine. Vol. 1. New York: Derby & Jackson. p. 547. http://books.google.com/?id=PxEKAAAAIAAJ. "Hour-glasses were formerly placed in coffins and buried with the corpse, probably as symbols of mortality—the sands of life having run out. See Gent. Mag. vol xvi, 646, and xvii, 264."
- Splendid Heritage: treasures of native american art (search on "hourglass")
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- An African Valentine: The Bead Code of the Zulus, edunetconnect.com
- Greenman, E.F. The Upper Palaeolithic and the New World in Current Anthropology Vol. 4, No. 1 (Feb., 1963), pp. 41-91 (NB: includes reviews disputing the central thesis and methodology)- via JSTOR (subscription)
- Image, "Croquis 1872" (click to enlarge) at colonias.iespana.es
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- Morris, Scot (September 1992). "The floating hourglass". Omni: p. 86
- Peterson, Ivars (September 11, 1993). "Trickling sand: how an hourglass ticks". Science News