The Hourglass (Edited from Wikipedia) SUMMARY An hourglass (or sandglass, sand timer, sand watch, or sand clock) is a mechanical device used to measure the passage of time. It comprises two glass bulbs connected vertically by a narrow neck that allows a regulated trickle of material (historically sand) from the upper bulb to the lower one. Factors affecting the time interval measured include the sand quantity, the sand coarseness, the bulb size, and the neck width. Hourglasses may be reused indefinitely by inverting the bulbs once the upper bulb is empty. HISTORY Antiquity The origin of the hourglass is unclear. Its predecessor the clepsydra, or water clock, may have been invented in ancient Egypt. According to the American Institute of New York, the clepsammia or sand-glass was invented at Alexandria about 150 BC. According to the Journal of the British Archaeological Association the so-called clepsammia were in use before the time of St. Jerome (335 AD), and the first representation of an hourglass is in a sarcophagus dated c. 350 AD, representing the wedding of Peleus and Thetis, discovered in Rome in the 18th century, and studied by Winckelmann in the 19th century, who remarked the hourglass held by Morpheus in his hands. Reappearance in the Early Middle Ages There are no records of the hourglass existing in Europe prior to the Early Middle Ages. In the 8th century it is mentioned by a monk named Luitprand, who served at the cathedral in Chartres, France. But 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. Use of the marine sandglass has been recorded since the 14th century. The written records about it were mostly from logbooks of European ships. In the same period it appears in other records and lists of ships stores. The earliest recorded reference that can be said with certainty to refer to a marine sandglass dates from c. 1345, in a receipt 1 of Thomas de Stetesham, clerk of the King's ship La George, in the reign of Edward III of England. Marine sandglasses 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 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, 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. The oldest known surviving hourglass resides in the British Museum in London. DESIGN, MATERIAL, AND USES Little written evidence exists to explain 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. 2 While some hourglasses actually did use sand as the granular mixture to measure time, many did not use sand at all. The material used in most bulbs was a combination of "powdered marble, tin/lead oxides, and pulverized, burnt eggshell". Over time, different textures of granule matter were tested to see which gave the most constant flow within the bulbs. It was later discovered that for the perfect flow to be achieved the ratio of granule bead to the width of the bulb neck needed to be 1/12 or more but not greater than 1/2 the neck of the bulb. 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, 18 hourglasses from Barcelona were in the ship's inventory, after the trip being authorized by emperor Charles V. 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. A number of sandglasses could be fixed in a common frame, each with a different operating time, e.g. as in a four-way Italian sandglass likely from the 17th century, in the collections of the Science Museum, in South Kensington, London, which could measure intervals of quarter, half, three-quarters, and one hour (and which were also used in churches, for priests and ministers to measure lengths of sermons). Symbolic Use Unlike most other methods of measuring time, the hourglass concretely represents the present as being between the past and the future, and this has made it an enduring symbol of time itself. 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. 3 The former Metropolitan Borough of Greenwich in London used an hourglass on its coat of arms, symbolizing 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. WATER CLOCKS A water clock or clepsydra is any timepiece in which time is measured by the regulated flow of liquid into (inflow type) or out from (outflow type) a vessel where the amount is then measured. Water clocks, along with sundials and hourglasses, are likely to be the oldest time- measuring instruments. Where and when they were first invented is not known, and given their great antiquity it may never be. The bowl-shaped outflow is the simplest form of a water clock and is known to have existed in Babylon and in Egypt around the 16th century BC. Other regions of the world, including India and China, also have early evidence of water clocks, but the earliest dates are less certain. Some modern timepieces are called "water clocks" but work differently from the ancient ones. Their timekeeping is governed by a pendulum, but they use water for other purposes, such as providing the power needed to drive the clock by using a water wheel or something similar, or by having water in their displays. The Greeks and Romans advanced water clock design to include the inflow clepsydra with an early feedback system, gearing, and escapement mechanism, which were connected to fanciful automata and resulted in improved accuracy. Further advances were made in Byzantium, Syria and Mesopotamia, where increasingly accurate water clocks incorporated complex segmental and epicyclic gearing, water wheels, and programmability, advances which eventually made their way to Europe. Independently, the Chinese developed their own advanced water clocks, incorporating gears, escapement mechanisms, and water wheels, passing their ideas on to Korea and Japan. Some water clock designs were developed independently and some knowledge was transferred through the spread of trade. These early water clocks were calibrated with a sundial. While never reaching a level of accuracy comparable to today's standards of timekeeping, the water clock was the most accurate and commonly used timekeeping device for millennia, until it was replaced by more accurate pendulum clocks in 17th- century Europe. A water clock, also known as a clepsydra, uses a flow of water to measure time. If viscosity is neglected, the physical principle required to study such clocks is the Torricelli's law. There are two types of water clocks: inflow and outflow. In an outflow 4 water clock, a container is filled with water, and the water is drained slowly and evenly out of the container.