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In 1816, and some other Connecticut clockmakers developed a way of mass-producing clocks by using. As a result, they were very inaccurate, with errors of perhaps an hour a day. Hairspring In 1675, Huygens and invented the , or the hairspring, designed to control the oscillating speed of the. Chinese business etiquette:: a guide to protocol, manners, and culture in the People's Republic of China.

Existing solo mechanisms that used were being adapted to take their driving power from falling weights. Tip calibration makes use of all relevant fossil taxa during clock calibration, rather than relying on only the oldest fossil of each clade. Wipe off with a clean, soft, dry towel. In a way Prime Dating for Ghosts is good metaphor for romance in video games. A major stimulus to improving the accuracy and reliability of clocks was the importance of precise time-keeping for navigation. Both the candle clock and the incense clock work on the same principle wherein the consumption of resources is more or less civil allowing reasonably precise and repeatable estimates of time passages. Retrieved 11 July 2015.

Reset the hands to the correct time and let the clock run without further correction for another week. Grace : You have a tattoo? According to , in 1198 during a fire at the abbey of St Edmundsbury now , the monks 'ran to the clock' to fetch water, indicating that their water clock had a reservoir large enough to help extinguish the occasional fire.

Cookies blocked - Other regions of the world, including and , also have early evidence of water clocks, but the earliest dates are less certain.

A clock is an instrument used to measure, keep, and indicate. The clock is one of the oldest human , meeting the need to measure intervals of time shorter than the natural units: the , the , and the. Devices operating on several physical processes have been used over the millennia. There is a range of duration timers, a well-known example being the. A major advance occurred with the invention of the , which made possible the first mechanical clocks around 1300 in , which kept time with oscillating timekeepers like. A silent instrument missing such a mechanism has traditionally been known as a timepiece. Spring-driven clocks appeared during the 15th century. During the 15th and 16th centuries, clockmaking flourished. The next development in accuracy occurred after 1656 with the invention of the. A major stimulus to improving the accuracy and reliability of clocks was the importance of precise time-keeping for navigation. The was patented in 1840. The development of in the 20th century led to clocks with no clockwork parts at all. The timekeeping element in every modern clock is a , a physical object that vibrates or at a particular. This object can be a , a , a , or the vibration of in as they emit. Clocks have different ways of displaying time, connected to its internal clockwork type: Analog clocks usually indicate time using angles. Digital clocks display a numeric representation of time. Two numeric display formats are commonly used on clocks: 24-hour notation and 12-hour notation. Most digital clocks use electronic mechanisms and , , or displays. For convenience, distance, telephony or , auditory clocks present the time as sounds. There are also clocks for the blind that have displays that can be read by using the sense of touch. Some of these are similar to normal analog displays, but are constructed so the hands can be felt without damaging them. The evolution of the technology of clocks continues today. The study of timekeeping is known as. Main article: The apparent position of the in the sky moves over the course of each day, reflecting the rotation of the. Shadows cast by stationary objects move correspondingly, so their positions can be used to indicate the time of day. A shows the time by displaying the position of a shadow on a usually flat surface, which has markings that correspond to the hours. Sundials can be horizontal, vertical, or in other orientations. Sundials were widely used in. With the knowledge of latitude, a well-constructed sundial can measure local with reasonable accuracy, within a minute or two. Sundials continued to be used to monitor the performance of clocks until the. The Jantar Mantar At Delhi and Jaipur are examples of sundials. They were built by Maharaja Jai Singh II. Devices that measure duration, elapsed time and intervals The flow of in an can be used to keep track of elapsed time. Many devices can be used to mark passage of time without respect to reference time time of day, minutes, etc. Examples of such duration timers are , and the. Both the candle clock and the incense clock work on the same principle wherein the consumption of resources is more or less constant allowing reasonably precise and repeatable estimates of time passages. In the hourglass, fine pouring through a tiny hole at a constant rate indicates an arbitrary, predetermined, passage of time. The resource is not consumed but re-used. Water A of 's Clock Tower, built in 11th century ,. It was driven by a large , , and mechanism. Water clocks, also known as clepsydrae sg: clepsydra , along with the sundials, are possibly the oldest time-measuring instruments, with the only exceptions being the vertical and the day counting. Given their great antiquity, where and when they first existed is not known and perhaps unknowable. The bowl-shaped outflow is the simplest form of a water clock and is known to have existed in and in around the 16th century BC. Other regions of the world, including and , also have early evidence of water clocks, but the earliest dates are less certain. Some authors, however, write about water clocks appearing as early as 4000 BC in these regions of the world. The and civilizations are credited for initially advancing water clock design to include complex , which was connected to fanciful and also resulted in improved accuracy. These advances were passed on through and times, eventually making their way back to. Some water clock designs were developed independently and some knowledge was transferred through the spread of trade. Instead, water clocks in ancient societies were used mainly for reasons. These early water clocks were calibrated with a. While never reaching the level of accuracy of a modern timepiece, the water clock was the most accurate and commonly used timekeeping device for millennia, until it was replaced by the more accurate in 17th-century Europe. Islamic civilization is credited with further advancing the accuracy of clocks with elaborate engineering. In the 13th century, , an engineer from Mesopotamia lived 1136—1206 who worked for king of Diyar-Bakr, , made numerous clocks of all shapes and sizes. A book on his work described 50 mechanical devices in 6 categories, including water clocks. The most reputed clocks included , Scribe and , all of which have been successfully reconstructed. As well as telling the time, these grand clocks were symbols of status, grandeur and wealth of the Urtuq State. See the for details. August 2016 The word horologia from the Greek ὥρα, hour, and λέγειν, to tell was used to describe early mechanical clocks, but the use of this word still used in several for all timekeepers conceals the true nature of the mechanisms. According to , in 1198 during a fire at the abbey of St Edmundsbury now , the monks 'ran to the clock' to fetch water, indicating that their water clock had a reservoir large enough to help extinguish the occasional fire. A water-powered cogwheel clock was created in in AD 725 by and. This is not considered an mechanism clock as it was unidirectional, the and 1020—1101 incorporated it into his monumental innovation of the astronomical clock-tower of in 1088. A mercury clock, described in the Libros del saber, a Spanish work from 1277 consisting of translations and paraphrases of Arabic works, is sometimes quoted as evidence for Muslim knowledge of a mechanical clock. A mercury-powered cogwheel clock was created by In Europe, between 1280 and 1320, there is an increase in the number of references to clocks and horologes in church records, and this probably indicates that a new type of clock mechanism had been devised. Existing clock mechanisms that used were being adapted to take their driving power from falling weights. This power was controlled by some form of oscillating mechanism, probably derived from existing bell-ringing or alarm devices. This controlled release of power—the —marks the beginning of the true mechanical clock, which differed from the previously mentioned cogwheel clocks. These mechanical clocks were intended for two main purposes: for signalling and notification e. The former purpose is administrative, the latter arises naturally given the scholarly interests in astronomy, science, astrology, and how these subjects integrated with the religious philosophy of the time. The was used both by astronomers and astrologers, and it was natural to apply a clockwork drive to the rotating plate to produce a working model of the solar system. Simple clocks intended mainly for notification were installed in towers, and did not always require faces or hands. They would have announced the or intervals between set times of prayer. Canonical hours varied in length as the times of sunrise and sunset shifted. The more sophisticated astronomical clocks would have had moving dials or hands, and would have shown the time in various time systems, including , canonical hours, and time as measured by astronomers at the time. Both styles of clock started acquiring extravagant features such as. In 1283, a large clock was installed at ; its location above the suggests that it was not a water clock. Over the next 30 years there are mentions of clocks at a number of ecclesiastical institutions in England, Italy, and France. In 1322, a , an expensive replacement for an earlier clock installed in 1273. This had a large 2 metre astronomical dial with automata and bells. The costs of the installation included the full-time employment of two for two years. They no longer exist, but detailed descriptions of their design and construction survive, and modern reproductions have been made. They illustrate how quickly the theory of the mechanical clock had been translated into practical constructions, and also that one of the many impulses to their development had been the desire of astronomers to investigate celestial phenomena. Wallingford's clock had a large astrolabe-type dial, showing the sun, the moon's age, phase, and node, a star map, and possibly the planets. In addition, it had a and an indicator of the state of the tide at. Bells rang every hour, the number of strokes indicating the time. Dondi's clock was a seven-sided construction, 1 metre high, with dials showing the time of day, including minutes, the motions of all the known planets, an automatic calendar of fixed and , and an eclipse prediction hand rotating once every 18 years. It is not known how accurate or reliable these clocks would have been. They were probably adjusted manually every day to compensate for errors caused by wear and imprecise manufacture. Water clocks are sometimes still used today, and can be examined in places such as ancient castles and museums. The , built in 1386, is considered to be the world's oldest surviving mechanical clock that strikes the hours. Spring-driven Spring driven Matthew Norman carriage clock with winding key Clockmakers developed their art in various ways. Building smaller clocks was a technical challenge, as was improving accuracy and reliability. Clocks could be impressive showpieces to demonstrate skilled craftsmanship, or less expensive, mass-produced items for domestic use. The escapement in particular was an important factor affecting the clock's accuracy, so many different mechanisms were tried. Spring-driven clocks appeared during the 15th century, although they are often erroneously credited to watchmaker or Henle, or Hele around 1511. The earliest existing spring driven clock is the chamber clock given to Phillip the Good, Duke of Burgundy, around 1430, now in the. Spring power presented clockmakers with a new problem: how to keep the clock running at a constant rate as the spring ran down. This resulted in the invention of the and the in the 15th century, and many other innovations, down to the invention of the modern going in 1760. Early clock dials did not indicate minutes and seconds. A clock with a dial indicating minutes was illustrated in a 1475 manuscript by Paulus Almanus, and some 15th-century clocks in indicated minutes and seconds. An early record of a seconds hand on a clock dates back to about 1560 on a clock now in the Fremersdorf collection. Some of the more basic table clocks have only one time-keeping hand, with the dial between the hour markers being divided into four equal parts making the clocks readable to the nearest 15 minutes. Other clocks were exhibitions of craftsmanship and skill, incorporating astronomical indicators and musical movements. The was invented in 1584 by , who also developed the. Bürgi's clocks were a great improvement in accuracy as they were correct to within a minute a day. These clocks helped the 16th-century astronomer to observe astronomical events with much greater precision than before. The next development in accuracy occurred after 1656 with the invention of the. He determined the mathematical formula that related pendulum length to time about 99. The first model clock was built in 1657 in , but it was in that the idea was taken up. The also known as the grandfather clock was created to house the pendulum and works by the English clockmaker William Clement in 1670 or 1671. It was also at this time that clock cases began to be made of wood and to utilize as well as hand-painted ceramics. In 1670, William Clement created the , an improvement over Huygens' crown escapement. Clement also introduced the pendulum suspension spring in 1671. The concentric minute hand was added to the clock by , a London clockmaker and others, and the second hand was first introduced. Hairspring In 1675, Huygens and invented the , or the hairspring, designed to control the oscillating speed of the. This crucial advance finally made accurate pocket watches possible. The great English clockmaker, , was one of the first to use this mechanism successfully in his , and he adopted the minute hand which, after a variety of designs were trialled, eventually stabilised into the modern-day configuration. The rack and snail striking mechanism for , was introduced during the 17th century and had distinct advantages over the 'countwheel' or 'locking plate' mechanism. During the 20th century there was a common misconception that invented striking. In fact, his invention was connected with a repeating mechanism employing the rack and snail. The , that chimes the number of hours or even minutes was invented by either Quare or Barlow in 1676. Marine chronometer A major stimulus to improving the accuracy and reliability of clocks was the importance of precise time-keeping for navigation. The position of a ship at sea could be determined with reasonable accuracy if a navigator could refer to a clock that lost or gained less than about 10 seconds per day. This clock could not contain a pendulum, which would be virtually useless on a rocking ship. In 1714, the British government offered large to the value of 20,000 pounds, for anyone who could determine longitude accurately. In 1735, Harrison built his first chronometer, which he steadily improved on over the next thirty years before submitting it for examination. The clock had many innovations, including the use of bearings to reduce friction, weighted balances to compensate for the ship's pitch and roll in the sea and the use of two different metals to reduce the problem of expansion from heat. The chronometer was tested in 1761 by Harrison's son and by the end of 10 weeks the clock was in error by less than 5 seconds. Mass production The British had predominated in watch manufacture for much of the 17th and 18th centuries, but maintained a system of production that was geared towards high quality products for the elite. Although there was an attempt to modernise clock manufacture with techniques and the application of duplicating tools and machinery by the British Watch Company in 1843, it was in the that this system took off. In 1816, and some other Connecticut clockmakers developed a way of mass-producing clocks by using. Main article: In 1815, published the powered by batteries. The electric clock's mainspring is wound either with an or with an and armature. In 1841, he first patented the pendulum. By the end of the nineteenth century, the advent of the dry cell battery made it feasible to use electric power in clocks. Spring or weight driven clocks that use electricity, either AC or DC , to rewind the spring or raise the weight of a mechanical clock would be classified as an. This classification would also apply to clocks that employ an electrical impulse to propel the pendulum. In electromechanical clocks the electricity serves no time keeping function. These types of clocks were made as individual timepieces but more commonly used in synchronized time installations in schools, businesses, factories, railroads and government facilities as a and. Electric clocks that are powered from the supply often use. The supply current alternates with a frequency of 50 in many countries, and 60 hertz in others. The of the motor rotates at a speed that is related to the alternation frequency. Appropriate gearing converts this rotation speed to the correct ones for the hands of the analog clock. The development of in the 20th century led to clocks with no clockwork parts at all. Time in these cases is measured in several ways, such as by the alternation of the AC supply, vibration of a , the behaviour of crystals, or the quantum vibrations of atoms. Electronic circuits divide these high-frequency oscillations to slower ones that drive the time display. Even mechanical clocks have since come to be largely powered by batteries, removing the need for winding. Quartz The properties of crystalline were discovered by and in 1880. The first crystal oscillator was invented in 1917 by after which, the first quartz crystal oscillator was built by in 1921. In 1927 the first was built by Warren Marrison and J. Horton at in Canada. The following decades saw the development of quartz clocks as precision time measurement devices in laboratory settings—the bulky and delicate counting electronics, built with , limited their practical use elsewhere. The National Bureau of Standards now based the time standard of the United States on quartz clocks from late 1929 until the 1960s, when it changed to atomic clocks. In 1969, produced the world's first quartz , the. Their inherent accuracy and low cost of production resulted in the subsequent proliferation of quartz clocks and watches. Atomic As of the 2010s, are the most accurate clocks in existence. They are considerably more accurate than as they can be accurate to within a few seconds over thousands of years. Atomic clocks were first theorized by in 1879. In the 1930s the development of created practical method for doing this. A prototype device was built in 1949 at the U. Although it was less accurate than existing , it served to demonstrate the concept. The first accurate atomic clock, a based on a certain transition of the atom, was built by in 1955 at the in the UK. Calibration of the caesium standard atomic clock was carried out by the use of the astronomical time scale ET. A chiming clock's mechanism. The invention of the mechanical clock in the 13th century initiated a change in timekeeping methods from processes, such as the motion of the 's shadow on a or the flow of liquid in a , to periodic processes, such as the swing of a or the vibration of a , which had the potential for more accuracy. All modern clocks use oscillation. Although the mechanisms they use vary, all oscillating clocks, mechanical, digital and atomic, work similarly and can be divided into analogous parts. They consist of an object that repeats the same motion over and over again, an , with a precisely constant time interval between each repetition, or 'beat'. Attached to the oscillator is a controller device, which sustains the oscillator's motion by replacing the energy it loses to , and converts its oscillations into a series of pulses. The pulses are then counted by some type of counter, and the number of counts is converted into convenient units, usually seconds, minutes, hours, etc. Finally some kind of indicator displays the result in human readable form. Power source Keys of various sizes for winding up mainsprings on clocks. Mechanical clocks must be wound periodically, usually by turning a knob or key or by pulling on the free end of the chain, to store in the weight or spring to keep the clock running. In clocks that use AC power, a small is often included to keep the clock running if it is unplugged temporarily from the wall or during a power outage. Battery powered analog wall clocks are available that operate over 15 years between battery changes. Oscillator The timekeeping element in every modern clock is a , a physical object that vibrates or repetitively at a precisely constant. As a result, they were very inaccurate, with errors of perhaps an hour a day. The advantage of a harmonic oscillator over other forms of oscillator is that it employs to vibrate at a precise natural or 'beat' dependent only on its physical characteristics, and resists vibrating at other rates. The possible precision achievable by a harmonic oscillator is measured by a parameter called its , or quality factor, which increases other things being equal with its resonant frequency. This is why there has been a long term trend toward higher frequency oscillators in clocks. Balance wheels and pendulums always include a means of adjusting the rate of the timepiece. Quartz timepieces sometimes include a rate screw that adjusts a for that purpose. Atomic clocks are , and their rate cannot be adjusted. Later versions without pendulums were triggered by a pulse from the master clock and certain sequences used to force rapid synchronization following a power failure. The counting may be done electronically, usually in clocks with digital displays, or, in analog clocks, the AC may drive a which rotates an exact fraction of a revolution for every cycle of the line voltage, and drives the gear train. Although changes in the line frequency due to load variations may cause the clock to temporarily gain or lose several seconds during the course of a day, the total number of cycles per 24 hours is maintained extremely accurately by the utility company, so that the clock keeps time accurately over long periods. Sometimes computers on a LAN get their time from a single local server which is maintained accurately. Controller This has the dual function of keeping the oscillator running by giving it 'pushes' to replace the energy lost to , and converting its vibrations into a series of pulses that serve to measure the time. A thin gas of atoms is released into the cavity where they are exposed to. A laser measures how many atoms have absorbed the microwaves, and an electronic control system called a tunes the microwave oscillator until it is at the frequency that causes the atoms to vibrate and absorb the microwaves. Then the microwave signal is divided by to become the. In mechanical clocks, the low of the balance wheel or pendulum oscillator made them very sensitive to the disturbing effect of the impulses of the escapement, so the escapement had a great effect on the accuracy of the clock, and many escapement designs were tried. The higher Q of resonators in electronic clocks makes them relatively insensitive to the disturbing effects of the drive power, so the driving oscillator circuit is a much less critical component. Counter chain This counts the pulses and adds them up to get traditional time units of , , , etc. It usually has a provision for setting the clock by manually entering the correct time into the counter. The gear train also has a second function; to transmit mechanical power from the power source to run the oscillator. There is a friction coupling called the 'cannon pinion' between the gears driving the hands and the rest of the clock, allowing the hands to be turned to set the time. Often pushbuttons on the case allow the hour and minute counters to be incremented and decremented to set the time. Indicator A with mechanical and sound producer striking on the 8th hour on the analog dial. This displays the count of seconds, minutes, hours, etc. Many clocks to this day are which strike the hour. The hours are indicated with an , which makes two revolutions in a day, while the minutes are indicated by a , which makes one revolution per hour. In mechanical clocks a gear train drives the hands; in electronic clocks the circuit produces pulses every second which drive a and gear train, which move the hands. A common misconception is that a digital clock is more accurate than an analog wall clock, but the indicator type is separate and apart from the accuracy of the timing source. The only other widely used clock face today is the , because of the use of in organizations and timetables. Before the modern clock face was standardized during the , many other face designs were used throughout the years, including dials divided into 6, 8, 10, and 24 hours. During the the French government tried to introduce a , as part of their decimal-based of measurement, but it didn't catch on. An Italian 6 hour clock was developed in the 18th century, presumably to save power a clock or watch striking 24 times uses more power. A simple 24 hour clock showing the approximate position of the sun. Another type of analog clock is the , which tracks the sun continuously, registering the time by the shadow position of its. Because the sun does not adjust to daylight saving time, users must add an hour during that time. Corrections must also be made for the , and for the difference between the longitudes of the sundial and of the central meridian of the that is being used i. Sundials use some or part of the 24 hour analog dial. There also exist clocks which use a digital display despite having an analog mechanism—these are commonly referred to as. Alternative systems have been proposed. Diagram of a mechanical digital display of a Digital clocks display a numeric representation of time. Most digital clocks use electronic mechanisms and , , or displays; many other display technologies are used as well , , etc. After a reset, battery change or power failure, these clocks without a backup or either start counting from 12:00, or stay at 12:00, often with blinking digits indicating that the time needs to be set. Some newer clocks will reset themselves based on radio or Internet that are tuned to national. Since the advent of digital clocks in the 1960s, the use of analog clocks has declined significantly. Some clocks, called '', have digital displays that work mechanically. The digits are painted on sheets of material which are mounted like the pages of a book. Once a minute, a page is turned over to reveal the next digit. These displays are usually easier to read in brightly lit conditions than LCDs or LEDs. Also, they do not go back to 12:00 after a power interruption. Flip clocks generally do not have electronic mechanisms. Usually, they are driven by -. Hybrid analog-digital Main article: Some clocks, usually digital ones, include an optical that shines a magnified image of the time display onto a screen or onto a surface such as an indoor ceiling or wall. The digits are large enough to be easily read, without using glasses, by persons with moderately imperfect vision, so the clocks are convenient for use in their bedrooms. Usually, the timekeeping circuitry has a battery as a backup source for an uninterrupted power supply to keep the clock on time, while the projection light only works when the unit is connected to an A. Completely battery-powered portable versions resembling are also available. Tactile Auditory and projection clocks can be used by people who are blind or have limited vision. There are also clocks for the blind that have displays that can be read by using the sense of touch. Some of these are similar to normal analog displays, but are constructed so the hands can be felt without damaging them. Another type is essentially digital, and uses devices that use a code such as to show the digits so that they can be felt with the fingertips. Multi-display Some clocks have several displays driven by a single mechanism, and some others have several completely separate mechanisms in a single case. Clocks in public places often have several faces visible from different directions, so that the clock can be read from anywhere in the vicinity; all the faces show the same time. Other clocks show the current time in several time-zones. Watches that are intended to be carried by travellers often have two displays, one for the local time and the other for the time at home, which is useful for making pre-arranged phone calls. Some have two displays, one showing and the other , as would be shown by a sundial. Some clocks have both analog and digital displays. Clocks with Braille displays usually also have conventional digits so they can be read by sighted people. An old clock in a restaurant in Clocks are in homes, offices and many other places; smaller ones are carried on the wrist or in a pocket; larger ones are in public places, e. A small clock is often shown in a corner of , and many. The primary purpose of a clock is to display the time. Clocks may also have the facility to make a loud alert signal at a specified time, typically to waken a sleeper at a preset time; they are referred to as alarm clocks. The alarm may start at a low volume and become louder, or have the facility to be switched off for a few minutes then resume. Alarm clocks with visible indicators are sometimes used to indicate to children too young to read the time that the time for sleep has finished; they are sometimes called training clocks. A clock mechanism may be used to control a device according to time, e. Such mechanisms are usually called. Clock mechanisms are also used to drive devices such as and , which have to turn at accurately controlled speeds to counteract the rotation of the Earth. Most depend on an internal signal at constant frequency to synchronize processing; this is referred to as a. A few research projects are developing CPUs based on. Some equipment, including computers, also maintains time and date for use as required; this is referred to as time-of-day clock, and is distinct from the system clock signal, although possibly based on counting its cycles. A UK government official gave a watch to mayor unaware of such a taboo which resulted in some professional embarrassment and a pursuant apology. It is undesirable to give someone a clock or depending on the region other timepiece as a gift. Traditional superstitions regard this as counting the seconds to the recipient's death. Time standards Main articles: and For some scientific work timing of the utmost accuracy is essential. It is also necessary to have a standard of the maximum accuracy against which working clocks can be calibrated. An ideal clock would give the time to unlimited accuracy, but this is not realisable. Many physical processes, in particular including some between atomic , occur at exceedingly stable frequency; counting cycles of such a process can give a very accurate and consistent time—clocks which work this way are usually called. Such clocks are typically large, very expensive, require a controlled environment, and are far more accurate than required for most purposes; they are typically used in a. Navigation Until advances in the late twentieth century, depended on the ability to measure and. Latitude can be determined through ; the measurement of requires accurate knowledge of time. This need was a major motivation for the development of accurate mechanical clocks. The in still fires an accurate signal to allow ships to check their. Many buildings near major ports used to have some still do a large mounted on a tower or mast arranged to drop at a pre-determined time, for the same purpose. While systems such as the GPS require unprecedentedly accurate knowledge of time, this is supplied by equipment on the satellites; vehicles no longer need timekeeping equipment. Bell System Technical Journal. Archived from PDF on November 10, 2014. Retrieved November 10, 2014. Medieval Technology and Social Change. The British Sundial Society. Retrieved 10 November 2014. North American Sundial Society. Retrieved 10 November 2014. New York, NY: Ballantine Books. IX, page 390, available at www. Edmundsbury: A Picture of Monastic and Social Life on the XIIth Century. London: Chatto and Windus. Translated and edited by L. God's Clockmaker: Richard of Wallingford and the Invention of Time. London: Hambledon and London 2005. Medieval Technology and Social Change. New York: Oxford Univ. The New Encyclopædia Britannica. 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