Wheel

In its primitive form, a wheel is a circular block of a hard and durable material at whose center has been bored a circular hole through which is placed an axle bearing about which the wheel rotates when a moment is applied by gravity or torque to the wheel about its axis, thereby making together one of the six simple machines. When placed vertically under a load-bearing platform or case, the wheel turning on the horizontal axle makes it possible to transport heavy loads; when placed horizontally, the wheel turning on its vertical axle makes it possible to control the spinning motion used to shape materials (e.g. a potter's wheel); when mounted on a column connected to a rudder or a chassis mounted on other wheels, one can control the direction of a vessel or vehicle (e.g. a ship's wheel or steering wheel); when connected to a crank or engine, a wheel can store, release, or transmit energy (e.g. the flywheel).

TricycleAntique
Three spoked wheels on an antique tricycle
Roue primitive
The earliest wheels were made of a solid piece of wood

Etymology

The English word wheel comes from the Old English word hweol, hweogol, from Proto-Germanic *hwehwlan, *hwegwlan, from Proto-Indo-European *kwekwlo-,[1] an extended form of the root *kwel- "to revolve, move around". Cognates within Indo-European include Icelandic hjól "wheel, tyre", Greek κύκλος kúklos, and Sanskrit chakra, the latter two both meaning "circle" or "wheel".[2]

History

The invention of the wheel falls into the late Neolithic, and may be seen in conjunction with other technological advances that gave rise to the early Bronze Age. This implies the passage of several wheel-less millennia even after the invention of agriculture and of pottery, during the Aceramic Neolithic.

Ur chariot
A depiction of an onager-drawn cart on the Sumerian "battle standard of Ur" (c. 2500 BCE)

The Halaf culture of 6500–5100 BCE is sometimes credited with the earliest depiction of a wheeled vehicle, but this is doubtful as there is no evidence of Halafians using either wheeled vehicles or even pottery wheels.[3] Precursors of wheels, known as "tournettes" or "slow wheels", were known in the Middle East by the 5th millennium BCE. One of the earliest examples was discovered at Tepe Pardis, Iran, and dated to 5200–4700 BCE. These were made of stone or clay and secured to the ground with a peg in the center, but required significant effort to turn. True potter's wheels, which are freely-spinning and have a wheel and axle mechanism, were developed in Mesopotamia (Iraq) by 4200–4000 BCE.[4] The oldest surviving example, which was found in Ur (modern day Iraq), dates to approximately 3100 BCE.[5]

Evidence of wheeled vehicles appeared by the late 4th millennium BCE. Depictions of wheeled wagons found on clay tablet pictographs at the Eanna district of Uruk, in the Sumerian civilization of Mesopotamia, are dated between 3699–3500 BCE.[6] In the second half of the 4th millennium BCE, evidence of wheeled vehicles appeared near-simultaneously in the Northern (Maykop culture) and South Caucasus (Early Kurgan culture) and Eastern Europe (Cucuteni-Trypillian culture). Depictions of a wheeled vehicle appeared between 3500–3350 BCE in the Bronocice clay pot excavated in a Funnelbeaker culture settlement in southern Poland.[7] In nearby Olszanica, a 2.2 m wide door were constructed for wagon entry; this barn was 40 m long with 3 doors.[8] Surviving evidence of a wheel-axle combination, from Stare Gmajne near Ljubljana in Slovenia (Ljubljana Marshes Wooden Wheel), is dated within two standard deviations to 3340–3030 BCE, the axle to 3360–3045 BCE.[9] Two types of early Neolithic European wheel and axle are known; a circumalpine type of wagon construction (the wheel and axle rotate together, as in Ljubljana Marshes Wheel), and that of the Baden culture in Hungary (axle does not rotate). They both are dated to c. 3200–3000 BCE.[10] Some historians believe that there was a diffusion of the wheeled vehicle from the Near East to Europe around the mid-4th millennium BCE.[11]

India - Kanchipuram - 023 - chariot unveiled for upcoming festival (2507526057)
Solid wheels on a heavy temple car, contrasted with the lighter wire-spoked wheels of the black roadster bicycle in the foreground

Early wheels were simple wooden disks with a hole for the axle. Some of the earliest wheels were made from horizontal slices of tree trunks. Because of the uneven structure of wood, a wheel made from a horizontal slice of a tree trunk will tend to be inferior to one made from rounded pieces of longitudinal boards.

The spoked wheel was invented more recently, and allowed the construction of lighter and swifter vehicles. The earliest known examples of wooden spoked wheels are in the context of the Andronovo culture, dating to c. 2000 BCE. Soon after this, horse cultures of the Caucasus region used horse-drawn spoked-wheel war chariots for the greater part of three centuries. They moved deep into the Greek peninsula where they joined with the existing Mediterranean peoples to give rise, eventually, to classical Greece after the breaking of Minoan dominance and consolidations led by pre-classical Sparta and Athens. Celtic chariots introduced an iron rim around the wheel in the 1st millennium BCE.

Twenty main wheels of Qatar Airways A380 (A7-APD) arr London Heathrow 10Sept2015 arp
Qatar Airways Airbus A380 arrives at London Heathrow Airport. It has twenty main undercarriage wheels (four + four on the wing, and six + six on the fuselage). (2015)

In China, the wheel was certainly present with the adoption of the chariot in c. 1200 BCE,[12] although Barbieri-Low[13] argues for earlier Chinese wheeled vehicles, c. 2000 BCE.

In Britain, a large wooden wheel, measuring about 1 m (3.3 ft) in diameter, was uncovered at the Must Farm site in East Anglia in 2016. The specimen, dating from 1,100–800 BCE, represents the most complete and earliest of its type found in Britain. The wheel's hub is also present. A horse's spine found nearby suggests the wheel may have been part of a horse-drawn cart. The wheel was found in a settlement built on stilts over wetland, indicating that the settlement had some sort of link to dry land.[14]

Remojadas Wheeled Figurine
A figurine featuring the New World's independently invented wheel

Although large-scale use of wheels did not occur in the Americas prior to European contact, numerous small wheeled artifacts, identified as children's toys, have been found in Mexican archeological sites, some dating to about 1500 BCE.[15] It is thought that the primary obstacle to large-scale development of the wheel in the Americas was the absence of domesticated large animals which could be used to pull wheeled carriages.[16] The closest relative of cattle present in Americas in pre-Columbian times, the American Bison, is difficult to domesticate and was never domesticated by Native Americans; several horse species existed until about 12,000 years ago, but ultimately became extinct.[17] The only large animal that was domesticated in the Western hemisphere, the llama, a pack animal but not physically suited to use as a draft animal to pull wheeled vehicles,[18] did not spread far beyond the Andes by the time of the arrival of Columbus.

Nubians from after about 400 BCE used wheels for spinning pottery and as water wheels.[19] It is thought that Nubian waterwheels may have been ox-driven.[20] It is also known that Nubians used horse-drawn chariots imported from Egypt.[21]

The wheel was barely used, with the exception of the Horn of Africa, in Sub-Saharan Africa well into the 19th century but this changed with the arrival of the Europeans.[22][23]

The spoked wheel was in continued use without major modification until the 1870s, when wire-spoked wheels and pneumatic tires were invented.[24] The wire spokes are under tension, not compression, making it possible for the wheel to be both stiff and light. Early radially-spoked wire wheels gave rise to tangentially-spoked wire wheels, which were widely used on cars into the late 20th century. Cast alloy wheels are now more commonly used; forged alloy wheels are used when weight is critical.

The invention of the wheel has also been important for technology in general, important applications including the water wheel, the cogwheel (see also antikythera mechanism), the spinning wheel, and the astrolabe or torquetum. More modern descendants of the wheel include the propeller, the jet engine, the flywheel (gyroscope) and the turbine.

Ljubljana (8629147956)

Ljubljana Marshes Wheel, from around 3150 BCE (the oldest known wooden wheel in the world).

Tekerlek

Twentieth-century solid wheel made of wooden boards, bound with a metal wheel rim

Greek chariot

Spoked wheels on the ancient Etruscan Monteleone chariot, 2nd quarter of the 6th century BCE

Spoked wheel from Arokalja

Spoked wheel with bronze sheeting from Árokalja, from around 1000 BCE.

Speichenkreuzung

Radially- (left) and tangentially- (right) wire-spoked wheels, both with pneumatic tires.

Mechanics and function

The low resistance to motion (compared to dragging) is explained as follows (refer to friction):

  • the normal force at the sliding interface is the same.
  • the sliding distance is reduced for a given distance of travel.
  • the coefficient of friction at the interface is usually lower.

Bearings are used to help reduce friction at the interface. In the simplest and oldest case the bearing is just a round hole through which the axle passes (a "plain bearing").

Example:

  • If a 100 kg object is dragged for 10 m along a surface with the coefficient of friction μ = 0.5, the normal force is 981 N and the work done (required energy) is (work=force x distance) 981 × 0.5 × 10 = 4905 joules.
  • Now give the object 4 wheels. The normal force between the 4 wheels and axles is the same (in total) 981 N. Assume, for wood, μ = 0.25, and say the wheel diameter is 1000 mm and axle diameter is 50 mm. So while the object still moves 10 m the sliding frictional surfaces only slide over each other a distance of 0.5 m. The work done is 981 × 0.25 × 0.5 = 123 joules; the work done has reduced to 1/40 of that of dragging.

Additional energy is lost from the wheel-to-road interface. This is termed rolling resistance which is predominantly a deformation loss. This energy is also lowered by the use of a wheel (in comparison to dragging) because the net force on the contact point between the road and the wheel is almost perpendicular to the ground, and hence, generates an almost zero net work. This depends on the nature of the ground, of the material of the wheel, its inflation in the case of a tire, the net torque exerted by the eventual engine, and many other factors.

A wheel can also offer advantages in traversing irregular surfaces if the wheel radius is sufficiently large compared to the irregularities.

The wheel alone is not a machine, but when attached to an axle in conjunction with bearing, it forms the wheel and axle, one of the simple machines. A driven wheel is an example of a wheel and axle. Wheels pre-date driven wheels by about 6000 years, themselves an evolution of using round logs as rollers to move a heavy load—a practice going back in pre-history so far that it has not been dated.

Construction

Rim

The rim is the "outer edge of a wheel, holding the tire."[25] It makes up the outer circular design of the wheel on which the inside edge of the tire is mounted on vehicles such as automobiles. For example, on a bicycle wheel the rim is a large hoop attached to the outer ends of the spokes of the wheel that holds the tire and tube.

In the 1st millennium BCE an iron rim was introduced around the wooden wheels of chariots.

Hub

The hub is the center of the wheel, and typically houses a bearing, and is where the spokes meet.

A hubless wheel (also known as a rim-rider or centerless wheel) is a type of wheel with no center hub. More specifically, the hub is actually almost as big as the wheel itself. The axle is hollow, following the wheel at very close tolerances.

Spokes

Wheel Iran
A spoked wheel on display at The National Museum of Iran, in Tehran. The wheel is dated to the late 2nd millennium BCE and was excavated at Choqa Zanbil.

A spoke is one of some number of rods radiating from the center of a wheel (the hub where the axle connects), connecting the hub with the round traction surface. The term originally referred to portions of a log which had been split lengthwise into four or six sections. The radial members of a wagon wheel were made by carving a spoke (from a log) into their finished shape. A spokeshave is a tool originally developed for this purpose. Eventually, the term spoke was more commonly applied to the finished product of the wheelwright's work, than to the materials used.

Wire

The rims of wire wheels (or "wire spoked wheels") are connected to their hubs by wire spokes. Although these wires are generally stiffer than a typical wire rope, they function mechanically the same as tensioned flexible wires, keeping the rim true while supporting applied loads.

Wire wheels are used on most bicycles and still used on many motorcycles. They were invented by aeronautical engineer George Cayley and first used in bicycles by James Starley. A process of assembling wire wheels is described as wheelbuilding.

1957 MG A Roadster
A 1957 MGA automobile with wire wheels

Tire/Tyre

Car tires
Stacked and standing car tires

A tire (in American English and Canadian English) or tyre (in some Commonwealth Nations such as UK, India, South Africa and Australia) is a ring-shaped covering that fits around a wheel rim to protect it and enable better vehicle performance by providing a flexible cushion that absorbs shock while keeping the wheel in close contact with the ground. The word itself may be derived from the word "tie," which refers to the outer steel ring part of a wooden cart wheel that ties the wood segments together (see Etymology below).

The fundamental materials of modern tires are synthetic rubber, natural rubber, fabric and wire, along with other compound chemicals. They consist of a tread and a body. The tread provides traction while the body ensures support. Before rubber was invented, the first versions of tires were simply bands of metal that fitted around wooden wheels to prevent wear and tear. Today, the vast majority of tires are pneumatic inflatable structures, comprising a doughnut-shaped body of cords and wires encased in rubber and generally filled with compressed air to form an inflatable cushion. Pneumatic tires are used on many types of vehicles, such as cars, bicycles, motorcycles, trucks, earthmovers, and aircraft.

Alternatives

While wheels are very widely used for ground transport, there are alternatives, some of which are suitable for terrain where wheels are ineffective. Alternative methods for ground transport without wheels include:

Symbolism

The wheel has also become a strong cultural and spiritual metaphor for a cycle or regular repetition (see chakra, reincarnation, Yin and Yang among others). As such and because of the difficult terrain, wheeled vehicles were forbidden in old Tibet. The wheel in ancient China is seen as a symbol of health and strength and utilized by some villages as a tool to predict future health and success. The diameter of the wheel is indicator of one's future health.

The winged wheel is a symbol of progress, seen in many contexts including the coat of arms of Panama, the logo of the Ohio State Highway Patrol and the State Railway of Thailand. The wheel is also the prominent figure on the flag of India. The wheel in this case represents law (dharma). It also appears in the flag of the Romani people, hinting to their nomadic history and their Indian origins.

The introduction of spoked (chariot) wheels in the Middle Bronze Age appears to have carried somewhat of a prestige. The sun cross appears to have a significance in Bronze Age religion, replacing the earlier concept of a Solar barge with the more 'modern' and technologically advanced solar chariot. The wheel was also a solar symbol for the Ancient Egyptians.[26]

See also

References

  1. ^ "wheel". Online Etymology Dictionary.
  2. ^ "American Heritage Dictionary Entry: wheel". Houghton Mifflin Harcourt Publishing Company.
  3. ^ V. Gordon Childe (1928). New Light on the Most Ancient East. p. 110.
  4. ^ D.T. Potts (2012). A Companion to the Archaeology of the Ancient Near East. p. 285.
  5. ^ Moorey, Peter Roger Stuart (1999) [1994]. Ancient Mesopotamian Materials and Industries: The Archaeological Evidence. Winona Lake, IN: Eisenbrauns. p. 146. ISBN 978-1-57506-042-2.
  6. ^ Attema, P. A. J.; Los-Weijns, Ma; Pers, N. D. Maring-Van der (December 2006). "Bronocice, Flintbek, Uruk, JEbel Aruda and Arslantepe: The Earliest Evidence Of Wheeled Vehicles In Europe And The Near East". Palaeohistoria. University of Groningen. 47/48: 10–28 (11).
  7. ^ Anthony, David A. (2007). The horse, the wheel, and language: how Bronze-Age riders from the Eurasian steppes shaped the modern world. Princeton, NJ: Princeton University Press. p. 67. ISBN 978-0-691-05887-0.
  8. ^ "35. Olszanica Longhouse 6: Why has it got wide doors?". 26 October 2018.
  9. ^ Velušček, A.; Čufar, K. and Zupančič, M. (2009) "Prazgodovinsko leseno kolo z osjo s kolišča Stare gmajne na Ljubljanskem barju", pp. 197–222 in A. Velušček (ed.). Koliščarska naselbina Stare gmajne in njen as. Ljubljansko barje v 2. polovici 4. tisočletja pr. Kr. Opera Instituti Archaeologici Sloveniae 16. Ljubljana.
  10. ^ Fowler, Chris; Harding, Jan and Hofmann, Daniela (eds.) (2015). The Oxford Handbook of Neolithic Europe. OUP Oxford. ISBN 0-19-166688-2. p. 109.
  11. ^ Attema, P. A. J.; Los-Weijns, Ma; Maring-Van der Pers, N. D. (December 2006). "Bronocice, Flintbek, Uruk, Jebel Aruda and Arslantepe: The Earliest Evidence of Wheeled Vehicles in Europe and the Near East". Palaeohistoria. University of Groningen. 47/48: 10-28 (19-20).
  12. ^ Dyer, Gwynne, War: the new edition, p. 159: Vintage Canada Edition, Randomhouse of Canada, Toronto, ON
  13. ^ Barbieri-Low, Anthony (February 2000) "Wheeled Vehicles in the Chinese Bronze Age (c. 2000–741 B.C.)", Sino-Platonic Papers
  14. ^ "Bronze Age wheel at 'British Pompeii' Must Farm an 'unprecedented find'". BBC. Retrieved 18 February 2016.
  15. ^ Ekholm, Gordon F (April 1946). "Wheeled Toys in Mexico". American Antiquity. 11 (4): 222–28. JSTOR 275722.
  16. ^ Diamond, Jared (1999). Guns, Germs, and Steel: The Fates of Human Societies. New York: Norton. p. 237. ISBN 978-0-393-31755-8.
  17. ^ Singer, Ben (May 2005). A brief history of the horse in America. Canadian Geographic Magazine. Archived from the original on 19 August 2014.
  18. ^ The Carriage Journal Vol 23 No 4 Spring 1986
  19. ^ "Crafts – Uncovering Treasures of Ancient Nubia". NYTimes.com. 27 February 1994.
  20. ^ "What the Nubians Ate". Discover Magazine.
  21. ^ Fage, J.D.; Oliver, Roland Anthony (1975). The Cambridge History of Africa. Cambridge University Press. ISBN 978-0-521-21592-3.
  22. ^ Chaves, Isaías; Engerman, Stanley L.; Robinson, James A. (2012). Reinventing the Wheel: The Economic Benefits of Wheeled Transportation in Early Colonial British West Africa (PDF). Weatherhead Center for International Affairs. p. 1. Archived from the original (PDF) on 6 January 2014. Retrieved 5 January 2014. One of the great technological puzzles of Sub-Saharan African economic history is that wheeled transportation was barely used prior to the colonial period. Instead, head porterage was the main method of transportation.
  23. ^ Law, Robin C. (1980). "Wheeled Transportation in Pre-Colonial West Africa". Africa. 50 (3): 249–62. doi:10.2307/1159117. JSTOR 1159117.
  24. ^ bookrags.com – Wheel and axle
  25. ^ Jewel, Elizabeth (2006). The Pocket Oxford Dictionary and Thesaurus. Oxford University Press. p. 722. ISBN 978-0-19-530715-3. Retrieved 4 January 2012.
  26. ^ Hall, Adelaide S. (2005). A Glossary of Important Symbols in Their Hebrew: Pagan and Christian Forms. p. 56. ISBN 978-1-59605-593-3.
Anti-lock braking system

An anti-lock braking system (ABS) is a safety anti-skid braking system used on aircraft and on land vehicles, such as cars, motorcycles, trucks, and buses. ABS operates by preventing the wheels from locking up during braking, thereby maintaining tractive contact with the road surface.

ABS is an automated system that uses the principles of threshold braking and cadence braking, techniques which were once practised by skillful drivers before ABSes were widespread. ABS operates at a much faster rate and more effectively than most drivers could manage. Although ABS generally offers improved vehicle control and decreases stopping distances on dry and some slippery surfaces, on loose gravel or snow-covered surfaces ABS may significantly increase braking distance, while still improving steering control. Since ABS was introduced in production vehicles, such systems have become increasingly sophisticated and effective. Modern versions may not only prevent wheel lock under braking, but may also alter the front-to-rear brake bias. This latter function, depending on its specific capabilities and implementation, is known variously as electronic brakeforce distribution, traction control system, emergency brake assist, or electronic stability control (ESC).

Bhavacakra

The bhavachakra (Sanskrit; Pāli: bhavachakka; Tibetan: srid pa'i 'khor lo) is a symbolic representation of saṃsāra (or cyclic existence). It is found on the outside walls of Tibetan Buddhist temples and monasteries in the Indo-Tibetan region, to help ordinary people understand Buddhist teachings.

Bicycle wheel

A bicycle wheel is a wheel, most commonly a wire wheel, designed for a bicycle. A pair is often called a wheelset, especially in the context of ready built "off the shelf" performance-oriented wheels.

Bicycle wheels are typically designed to fit into the frame and fork via dropouts, and hold bicycle tires.

Car layout

The layout of a car is often defined by the location of the engine and drive wheels.

Layouts can roughly be divided into three categories: front-wheel drive, rear-wheel drive and four-wheel drive. Many different combinations of engine location and driven wheels are found in practice, and the location of each is dependent on the application for which the car will be used.

Color wheel

A color wheel or color circle is an abstract illustrative organization of color hues around a circle, which shows the relationships between primary colors, secondary colors, tertiary colors etc.

Some sources use the terms color wheel and color circle interchangeably; however, one term or the other may be more prevalent in certain fields or certain versions as mentioned above. For instance, some reserve the term color wheel for mechanical rotating devices, such as color tops or filter wheels. Others classify various color wheels as color disc, color chart, and color scale varieties.As an illustrative model, artists typically use red, yellow, and blue primaries (RYB color model) arranged at three equally spaced points around their color wheel. Printers and others who use modern subtractive color methods and terminology use magenta, yellow, and cyan as subtractive primaries. Intermediate and interior points of color wheels and circles represent color mixtures. In a paint or subtractive color wheel, the "center of gravity" is usually (but not always) black, representing all colors of light being absorbed; in a color circle, on the other hand, the center is white or gray, indicating a mixture of different wavelengths of light (all wavelengths, or two complementary colors, for example).

The original color circle of Isaac Newton showed only the spectral hues and was provided to illustrate a rule for the color of mixtures of lights, that these could be approximately predicted from the center of gravity of the numbers of "rays" of each spectral color present (represented in his diagram by small circles). The divisions of Newton's circle are of unequal size, being based on the intervals of a Dorian musical scale. Most later color circles include the purples, however, between red and violet, and have equal-sized hue divisions. Color scientists and psychologists often use the additive primaries, red, green and blue; and often refer to their arrangement around a circle as a color circle as opposed to a color wheel.

Dharmachakra

The dharmachakra (Sanskrit; Pali: dhammacakka, "Wheel of Dharma") is a widespread symbol used in Indian religions such as Jainism, Hinduism and especially Buddhism. In Buddhism, the dharmachakra is widely used to represent the Dharma (Buddha's teaching and the universal moral order), Gautama Buddha himself and the walking of the path to enlightenment, since the time of Early Buddhism. The symbol is also sometimes connected to the Four Noble Truths, the Noble Eightfold Path and Dependent Origination.

Historically, the dharmachakra was often used as a decoration on Buddhist temples, statues and inscriptions, beginning with the earliest period of Indian Buddhism to the present. It remains a major symbol of the Buddhist religion today.

Drive shaft

A drive shaft, driveshaft, driving shaft, tailshaft (Australian English), propeller shaft (prop shaft), or Cardan shaft is a mechanical component for transmitting torque and rotation, usually used to connect other components of a drive train that cannot be connected directly because of distance or the need to allow for relative movement between them.

As torque carriers, drive shafts are subject to torsion and shear stress, equivalent to the difference between the input torque and the load. They must therefore be strong enough to bear the stress, while avoiding too much additional weight as that would in turn increase their inertia.

To allow for variations in the alignment and distance between the driving and driven components, drive shafts frequently incorporate one or more universal joints, jaw couplings, or rag joints, and sometimes a splined joint or prismatic joint.

Falun Gong

Falun Gong (UK: , US: ) or Falun Dafa (; Standard Mandarin Chinese: [fàlwə̌n tâfà]; literally, "Dharma Wheel Practice" or "Law Wheel Practice") is a Chinese religious spiritual practice that combines meditation and qigong exercises with a moral philosophy centered on the tenets of truthfulness, compassion, and forbearance (Chinese: 真、善、忍). The practice emphasizes morality and the cultivation of virtue, and identifies as a qigong practice of the Buddhist school, though its teachings also incorporate elements drawn from Taoist traditions. Through moral rectitude and the practice of meditation, practitioners of Falun Gong aspire to eliminate attachments, and ultimately to achieve spiritual enlightenment.

Falun Gong originated and was first taught publicly in northeastern China in 1992 by Li Hongzhi. It emerged toward the end of China's "qigong boom"—a period that saw a proliferation of similar practices of meditation, slow-moving energy exercises and regulated breathing. It differs from other qigong schools in its absence of fees or formal membership, lack of daily rituals of worship, its greater emphasis on morality, and the theological nature of its teachings. Western academics have described Falun Gong as a qigong discipline, a "spiritual movement", a "cultivation system" in the tradition of Chinese antiquity, or as a form of Chinese religion.

The practice initially enjoyed support from Chinese officialdom, but by the mid to late 1990s, the Communist Party and public security organizations increasingly viewed Falun Gong as a potential threat due to its size, independence from the state, and spiritual teachings. By 1999, government estimates placed the number of Falun Gong practitioners at 70 million. During that time, negative coverage of Falun Gong began to appear in the state-run press, and practitioners usually responded by picketing the source involved. Most of the time, the practitioners succeeded, but controversy and tension continued to build. The scale of protests grew until April 1999, when over 10,000 Falun Gong practitioners gathered near the central government compound in Beijing to request legal recognition and freedom from state interference. This demonstration is widely seen as catalyzing the persecution that followed.

On 20 July 1999, the Communist Party leadership initiated a nationwide crackdown and multifaceted propaganda campaign intended to eradicate the practice. It blocked Internet access to websites that mention Falun Gong, and in October 1999 it declared Falun Gong a "heretical organization" that threatened social stability. Falun Gong practitioners in China are reportedly subject to a wide range of human rights abuses: hundreds of thousands are estimated to have been imprisoned extrajudicially, and practitioners in detention are subject to forced labor, psychiatric abuse, torture, and other coercive methods of thought reform at the hands of Chinese authorities. As of 2009, human rights groups estimated that at least 2,000 Falun Gong practitioners had died as a result of abuse in custody. One observer reported that tens of thousands may have been killed to supply China's organ transplant industry (see Organ harvesting from Falun Gong practitioners in China). In the years since the persecution began, Falun Gong practitioners have become active in advocating for greater human rights in China.

Falun Gong founder Li Hongzhi has lived in New York City since 1996, and Falun Gong has a sizable global constituency. Inside China, estimates suggest that tens of millions continued to practice Falun Gong in spite of the persecution. Hundreds of thousands are estimated to practice Falun Gong outside China in over 70 countries worldwide.

Ferris wheel

A Ferris wheel (sometimes called in the case of the very tallest examples, giant wheel) is an amusement ride consisting of a rotating upright wheel with multiple passenger-carrying components (commonly referred to as passenger cars, cabins, tubs, capsules, gondolas, or pods) attached to the rim in such a way that as the wheel turns, they are kept upright, usually by gravity. Some of the largest modern Ferris wheels have cars mounted on the outside of the rim, with electric motors to independently rotate each car to keep it upright. These wheels are sometimes referred to as observation wheels and their cars referred to as capsules, however these alternative names are also used for wheels with conventional gravity-oriented cars.

The original Ferris Wheel was designed and constructed by George Washington Gale Ferris Jr. as a landmark for the 1893 World's Columbian Exposition in Chicago. The generic term Ferris wheel is now used in American English for all such structures, which have become the most common type of amusement ride at state fairs in the United States.The current tallest Ferris wheel is the 167.6-metre (550 ft) High Roller in Las Vegas, Nevada, which opened to the public in March 2014.

Four-wheel drive

Four-wheel drive, also called 4×4 ("four by four") or 4WD, refers to a two-axled vehicle drivetrain capable of providing torque to all of its wheels simultaneously. It may be full-time or on-demand, and is typically linked via a transfer case providing an additional output drive-shaft and, in many instances, additional gear ranges.

A four-wheeled vehicle with torque supplied to both axles is described as "all-wheel drive" (AWD). However, "four-wheel drive" typically refers to a set of specific components and functions, and intended off-road application, which generally complies with modern use of the terminology.

Front-engine, front-wheel-drive layout

In automotive design, an FF, or front-engine, front-wheel-drive (FWD) layout places both the internal combustion engine and driven roadwheels at the front of the vehicle.

Front-engine, rear-wheel-drive layout

In automotive design, an FR, or front-engine, rear-wheel-drive layout is one where the engine is located at the front of the vehicle and driven wheels are located at the rear. This was the traditional automobile layout for most of the 20th century. Modern designs commonly use the front-engine, front-wheel-drive layout (FF).

List of Wheel of Time characters

This article serves as an index of major characters in the fictional setting of Robert Jordan's The Wheel of Time series, with a description of their main roles or feats in the series. The Wheel of Time has 2782 distinct named characters.

London Eye

The London Eye is a cantilevered observation wheel on the South Bank of the River Thames in London. It is Europe's tallest cantilevered observation wheel, is the most popular paid tourist attraction in the United Kingdom with over 3.75 million visitors annually, and has made many appearances in popular culture.

The structure is 135 metres (443 ft) tall and the wheel has a diameter of 120 metres (394 ft). When it opened to the public in 2000 it was the world's tallest Ferris wheel. Its height was surpassed by the 525-foot (160 m) Star of Nanchang in 2006, the 165 metres (541 ft) Singapore Flyer in 2008, and the 550-foot tall (167.6 m) High Roller (Las Vegas) in 2014. Supported by an A-frame on one side only, unlike the taller Nanchang and Singapore wheels, the Eye is described by its operators as "the world's tallest cantilevered observation wheel".The London Eye used to offer the highest public viewing point in London until it was superseded by the 245-metre (804 ft) high observation deck on the 72nd floor of The Shard, which opened to the public on 1 February 2013.The London Eye adjoins the western end of Jubilee Gardens (previously the site of the former Dome of Discovery), on the South Bank of the River Thames between Westminster Bridge and Hungerford Bridge beside County Hall, in the London Borough of Lambeth.

Open-wheel car

An open-wheel car (formula car, or often single-seater car in British English) is a car with the wheels outside the car's main body, and usually having only one seat. Open-wheel cars contrast with street cars, sports cars, stock cars, and touring cars, which have their wheels below the body or inside fenders. Open-wheel cars are usually built specifically for road racing, frequently with a higher degree of technological sophistication than in other forms of motor sport. Open-wheel street cars, such as the Ariel Atom, are very scarce as they are often impractical for everyday use.

Prayer wheel

A prayer wheel is a cylindrical wheel (Tibetan: འཁོར་ལོ།, Wylie: khor lo) on a spindle made from metal, wood, stone, leather or coarse cotton. Traditionally, the mantra Om Mani Padme Hum is written in Newari language of Nepal, on the outside of the wheel. Also sometimes depicted are Dakinis, Protectors and very often the 8 auspicious symbols Ashtamangala. At the core of the cylinder is a "Life Tree" often made of wood or metal with certain mantras written on or wrapped around it. Many thousands (or in the case of larger prayer wheels, millions) of mantras are then wrapped around this life tree. The Mantra Om Mani Padme Hum is most commonly used, but other mantras may be used as well. According to the Tibetan Buddhist tradition based on the lineage texts regarding prayer wheels, spinning such a wheel will have much the same meritorious effect as orally reciting the prayers.

The Wheel of Time

The Wheel of Time is a series of high fantasy novels written by American author James Oliver Rigney Jr., under his pen name of Robert Jordan. Originally planned as a six-book series, The Wheel of Time spanned fourteen volumes, in addition to a prequel novel and two companion books. Jordan began writing the first volume, The Eye of the World, in 1984, and it was published in January 1990.Jordan died in 2007 while working on what was planned to be the twelfth and final volume in the series. He prepared extensive notes so another author could complete the book according to his wishes. Fellow fantasy author and long-time Wheel of Time fan Brandon Sanderson was brought in to complete the final book, but during the writing process it was decided that the book would be far too large to be published in one volume and would instead be published as three volumes: The Gathering Storm (2009), Towers of Midnight (2010), and A Memory of Light (2013).

The series draws on numerous elements of both European and Asian mythology, most notably the cyclical nature of time found in Buddhism and Hinduism, the metaphysical concepts of balance and duality, and a respect for nature found in Taoism. Additionally, its creation story has similarities to Christianity's "Creator" (Light) and Shai'tan, "The Dark One" (Shaitan is an Arabic word that, in Islamic contexts, is used as a name for the Devil). It was also partly inspired by Leo Tolstoy's War and Peace (1869).The Wheel of Time is notable for its length, detailed imaginary world, well-developed magic system, and large cast of characters. The eighth through fourteenth books each reached number one on the New York Times Best Seller list. After its completion, the series was nominated for a Hugo Award. According to Jordan's French publisher, as of 2017, the series has sold over 80 million copies worldwide, and is one of the best selling epic fantasy series since The Lord of the Rings. Its popularity has spawned an eponymous video game, roleplaying game, and soundtrack album. On April 20, 2017, it was announced that Sony Pictures will adapt the series for television and on October 2, 2018, Amazon ordered the series with Sony as a co-producer.

Water wheel

A water wheel is a machine for converting the energy of flowing or falling water into useful forms of power, often in a watermill. A water wheel consists of a wheel (usually constructed from wood or metal), with a number of blades or buckets arranged on the outside rim forming the driving surface.

Water wheels were still in commercial use well into the 20th century but they are no longer in common use. Uses included milling flour in gristmills, grinding wood into pulp for papermaking, hammering wrought iron, machining, ore crushing and pounding fiber for use in the manufacture of cloth.

Some water wheels are fed by water from a mill pond, which is formed when a flowing stream is dammed. A channel for the water flowing to or from a water wheel is called a mill race. The race bringing water from the mill pond to the water wheel is a headrace; the one carrying water after it has left the wheel is commonly referred to as a tailrace.In the mid to late 18th century John Smeaton's scientific investigation of the water wheel led to significant increases in efficiency supplying much needed power for the Industrial Revolution.Water wheels began being displaced by the smaller, less expensive and more efficient turbine, developed by Benoît Fourneyron, beginning with his first model in 1827. Turbines are capable of handling high heads, or elevations, that exceed the capability of practical-sized waterwheels.

The main difficulty of water wheels is their dependence on flowing water, which limits where they can be located. Modern hydroelectric dams can be viewed as the descendants of the water wheel, as they too take advantage of the movement of water downhill.

Wheel of Fortune (American game show)

Wheel of Fortune (often known simply as Wheel) is an American television game show created by Merv Griffin that debuted in 1975. The show features a competition in which contestants solve word puzzles, similar to those used in Hangman, to win cash and prizes determined by spinning a giant carnival wheel.

Wheel originally aired as a daytime series on NBC from January 6, 1975, to June 30, 1989. After some changes were made to its format, the daytime series moved to CBS from July 17, 1989, to January 11, 1991. It then returned to NBC from January 14, 1991, until it was cancelled on September 20, 1991. The popularity of the daytime series led to a nightly syndicated edition being developed, which premiered on September 19, 1983, and has aired continuously since.

The network version was originally hosted by Chuck Woolery and Susan Stafford, with Charlie O'Donnell as its announcer. O'Donnell left in 1980 and was replaced by Jack Clark. After Clark's death in 1988, M. G. Kelly took over briefly as announcer until O'Donnell returned in 1989. O'Donnell remained on the network version until its cancellation, and continued to announce on the syndicated show until his death in 2010, when Jim Thornton succeeded him. Woolery left in 1981, and was replaced by Pat Sajak. Sajak left the network version in January 1989 to host his own late-night talk show, and was replaced on that version by Rolf Benirschke. Bob Goen replaced Benirschke when the network show moved to CBS, then remained as host until the network show was canceled altogether. Stafford left in 1982, and was replaced by Vanna White, who remained on the network show for the rest of its run. The syndicated version has been hosted continuously by Sajak and White since its inception.

Wheel of Fortune ranks as the longest-running syndicated game show in the United States, with 7,000 episodes taped and aired as of May 10, 2019. TV Guide named it the "top-rated syndicated series" in a 2008 article, and in 2013, the magazine ranked it at No. 2 in its list of the 60 greatest game shows ever. The program has also come to gain a worldwide following with sixty international adaptations. The syndicated series' 36th season premiered on September 10, 2018, and Sajak became the longest-running host of any game show, surpassing Bob Barker, who hosted The Price Is Right from 1972 to 2007.

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