Physical properties.

Atomic.

A lead atom has 82 electrons, arranged in an electron configuration of [Xe]4f5d6s6p. The sum of lead's first and second ionization energies—the total energy required to remove the two 6p electrons—is close to that of tin, lead's upper neighbor in the carbon group. This is unusual; ionization energies generally fall going down a group, as an element's outer electrons become more distant from the nucleus, and more shielded by smaller orbitals. The similarity of ionization energies is caused by the lanthanide contraction—the decrease in element radii from lanthanum (atomic number 57) to lutetium (71), and the relatively small radii of the elements from hafnium (72) onwards. This is due to poor shielding of the nucleus by the lanthanide

Bulk.

Pure lead has a bright, silvery appearance with a hint of blue. It tarnishes on contact with moist air and takes on a dull appearance, the hue of which depends on the prevailing conditions. Characteristic properties of lead include high density, malleability, ductility, and high resistance to corrosion due to passivation. Lead's close-packed face-centered cubic structure and high atomic weight result in a density of 11.34 g/cm, which is greater than that of common metals such as iron (7.87 g/cm), copper (8.93 g/cm), and zinc (7.14 g/cm). This density is the origin of the idiom "to go over like a lead balloon". Some rarer metals are denser: tungsten and gold are both at 19.3 g/cm, and osmium—the densest metal known—has a density of 22.59 g/cm, almost twice

Isotopes.

Natural lead consists of four stable isotopes with mass numbers of 204, 206, 207, and 208, and traces of five short-lived radioisotopes. The high number of isotopes is consistent with lead's atomic number being even. Lead has a magic number of protons (82), for which the nuclear shell model accurately predicts an especially stable nucleus. Lead-208 has 126 neutrons, another magic number, which may explain why lead-208 is extraordinarily stable.  With its high atomic number, lead is the heaviest element whose natural isotopes are regarded as stable;

Chemistry.

Bulk lead exposed to moist air forms a protective layer of varying composition. Lead(II) carbonate is a common constituent; the sulfate or chloride may also be present in urban or maritime settings. This layer makes bulk lead effectively chemically inert in the air. Finely powdered lead, as with many metals, is pyrophoric, and burns with a bluish-white flame.  Fluorine reacts with lead at room temperature, forming lead(II) fluoride. The reaction with chlorine is similar but requires heating, as the resulting chloride layer diminishes the reactivity of the elements. Molten lead reacts with the chalcogens to give lead(II) chalcogenides.  Lead metal resists sulfuric and phosphoric acid but not hydrochloric or nitric acid; the outcome depends on insolubility and subsequent passivation of the product salt. Organic acids, such as acetic acid, dissolve lead in the presence of oxygen. Concentrated alkalis will dissolve lead and form plumbites.

Inorganic compounds.

Lead shows two main oxidation states: +4 and +2. The tetravalent state is common for the carbon group. The divalent state is rare for carbon and silicon, minor for germanium, important (but not prevailing) for tin, and is the more important of the two oxidation states for lead. This is attributable to relativistic effects, specifically the inert pair effect, which manifests itself when there is a large difference in electronegativity between lead and oxide, halide, or nitride anions, leading to a significant partial positive charge on lead. The result

Lead(II).

Lead(II) compounds are characteristic of the inorganic chemistry of lead. Even strong oxidizing agents like fluorine and chlorine react with lead to give only PbF and PbCl. Lead(II) ions are usually colorless in solution, and partially hydrolyze to form Pb(OH) and finally [Pb(OH)] (in which the hydroxyl ions act as bridging ligands), but are not reducing agents as tin(II) ions are. Techniques for identifying the presence of the Pb ion in water generally rely on the precipitation of lead(II) chloride using dilute hydrochloric acid. As the chloride salt is sparingly soluble in water, in very dilute solutions the precipitation of lead(II) sulfide is achieved by bubbling hydrogen sulfide through the solution.  Lead monoxide exists in two polymorphs, litharge α-PbO (red) and massicot β-PbO (yellow), the latter being stable only above around 488

Lead(IV).

Few inorganic lead(IV) compounds are known. They are only formed in highly oxidizing solutions and do not normally exist under standard conditions. Lead(II) oxide gives a mixed oxide on further oxidation, PbO. It is described as lead(II,IV) oxide, or structurally 2PbO·PbO, and is the best-known mixed valence lead compound. Lead dioxide is a strong oxidizing agent, capable

Other oxidation states.

Some lead compounds exist in formal oxidation states other than +4 or +2. Lead(III) may be obtained, as an intermediate between lead(II) and lead(IV), in larger organolead complexes; this oxidation state is not stable, as both the lead(III) ion and the larger complexes containing it are radicals. The same applies for lead(I), which can be found in such radical species.  Numerous mixed lead(II,IV) oxides are known. When PbO is heated in air, it becomes PbO at 293 °C, PbO at 351 °C, PbO at 374 °C, and finally PbO at 605 °C. A

Organolead.

Lead can form multiply-bonded chains, a property it shares with its lighter homologs in the carbon group. Its capacity to do so is much less because the Pb–Pb bond energy is over three and a half times lower than that of the C–C bond. With itself, lead can build metal–metal bonds of an order up to three. With carbon, lead forms organolead compounds similar to, but generally less stable than, typical organic compounds (due to the Pb–C bond being rather weak). This makes the organometallic chemistry of lead far less wide-ranging than that of tin. Lead predominantly forms organolead(IV) compounds, even

Origin and occurrence.

In space.

Lead's per-particle abundance in the Solar System is 0.121 ppb (parts per billion). This figure is two and a half times higher than that of platinum, eight times more than mercury, and seventeen times more than gold. The amount of lead in the universe is slowly increasing as most heavier atoms (all of which are unstable) gradually decay to lead. The abundance of lead in the Solar System since its formation 4.5 billion years ago has increased by about 0.75%. The solar system abundances table shows that lead, despite its relatively high atomic number, is more prevalent than most other elements with atomic numbers greater than 40.  Primordial lead—which comprises the isotopes lead-204, lead-206, lead-207, and lead-208—was mostly created as a result of repetitive neutron capture processes occurring in stars. The two main modes of capture are the s- and r-processes.  In

On Earth.

Lead is classified as a chalcophile under the Goldschmidt classification, meaning it is generally found combined with sulfur. It rarely occurs in its native, metallic form. Many lead minerals are relatively light and, over the course of the Earth's history, have remained in the crust instead of sinking deeper into the Earth's interior. This accounts for lead's relatively high crustal abundance of 14 ppm; it is the 38th most abundant element in the crust.  The main lead-bearing mineral is galena (PbS), which is mostly found with

Etymology.

The modern English word "lead" is of Germanic origin; it comes from the Middle English "leed" and Old English "lēad" (with the macron above the "e" signifying that the vowel sound of that letter is long). The Old English word is derived from the hypothetical reconstructed Proto-Germanic "*lauda-" ("lead"). According to linguistic theory, this word bore descendants in multiple Germanic languages of exactly the same meaning.  There

History.

Prehistory and early history.

Metallic lead beads dating back to 7000–6500 BCE have been found in Asia Minor and may represent the first example of metal smelting. At that time lead had few (if any) applications due to its softness and dull appearance. The major reason for the spread of lead production was its association with silver, which may be obtained by burning galena (a common lead mineral). The

Classical era.

Because silver was extensively used as a decorative material and an exchange medium, lead deposits came to be worked in Asia Minor from 3000 BCE; later, lead deposits were developed in the Aegean and Laurion. These three regions collectively dominated production of mined lead until c. 1200 BCE. Beginning circa 2000 BCE, the Phoenicians worked deposits in the Iberian peninsula; by 1600 BCE, lead mining existed in Cyprus, Greece, and Sardinia.  Rome's territorial expansion in Europe and across the Mediterranean, and its development of mining, led to it becoming the greatest producer of lead during the classical era, with an estimated annual output peaking at 80,000 tonnes. Like their predecessors, the Romans obtained lead mostly as a by-product of silver smelting. Lead mining occurred in Central Europe, Britain, the Balkans, Greece, Anatolia, and Hispania, the latter accounting for 40% of world production.  Lead tablets were commonly used as a material for letters. Lead coffins, cast in flat sand forms, with interchangeable motifs to suit the faith of the deceased were used in ancient Judea. Lead was used to make sling

Middle Ages and the Renaissance.

Lead mining in Western Europe declined after the fall of the Western Roman Empire, with Arabian Iberia being the only region having a significant output. The largest production of lead occurred in South and East Asia, especially China and India, where lead mining grew rapidly.  In Europe, lead production began to increase in the 11th and 12th centuries, when it was again used for roofing and piping. Starting in the 13th century, lead was used to create stained glass. In the European and Arabian traditions of alchemy, lead (symbol in the European tradition) was considered an impure base metal which, by the separation, purification and balancing of its constituent essences, could be transformed to pure and incorruptible gold. During the period, lead was used increasingly for adulterating wine. The use of such wine was forbidden for use in

Outside Europe and Asia.

In the New World, lead production was recorded soon after the arrival of European settlers. The earliest record dates to 1621 in the English Colony of Virginia, fourteen years after its foundation. In Australia, the

Industrial Revolution.

In the second half of the 18th century, Britain, and later continental Europe and the United States, experienced the Industrial Revolution. This was the first time during which lead production rates exceeded those of Rome. Britain was the leading producer, losing this status by the mid-19th century with the depletion of its mines and the development of lead mining in Germany, Spain, and the United States. By 1900, the United States was the leader in global lead production, and other non-European nations—Canada, Mexico, and Australia—had begun

Modern era.

Further evidence of the threat that lead posed to humans was discovered in the late 19th and early 20th centuries. Mechanisms of harm were better understood, lead blindness was documented, and the element was phased out of public use in the United States and Europe. The United Kingdom introduced mandatory factory inspections in 1878 and appointed the first Medical Inspector of Factories in 1898; as a result, a 25-fold decrease in lead poisoning incidents from 1900 to 1944 was reported. Most European countries banned lead paint—commonly used because of its opacity and water resistance—for interiors by 1930.  The last major human exposure to lead was the addition of tetraethyllead to gasoline as an antiknock agent, a practice that originated in the United States in 1921. It was phased out in the

Production.

As of 2014, production of lead is increasing worldwide due to its use in lead–acid batteries. There are two major categories of production: primary from mined ores, and secondary from scrap. In 2014, 4.58 million metric tons came from primary production and 5.64 million from secondary production. The top three producers of mined lead concentrate in that year were China, Australia, and the United States. The top three producers of refined lead were China, the United States, and India. According to the International Resource Panel's Metal Stocks in Society report of 2010, the total amount of lead in use, stockpiled, discarded, or dissipated into the environment, on a global basis, is 8 kg per capita. Much of this is in more developed countries (20–150 kg per capita) rather than less developed ones (1–4 kg per capita).  The primary and secondary lead production processes are similar. Some primary production plants now supplement their operations with scrap lead, and this trend is likely to increase in the future. Given adequate techniques, lead obtained via secondary processes is indistinguishable from lead obtained via primary processes. Scrap lead from the building trade is usually fairly clean and is re-melted without the need for smelting, though refining is sometimes needed. Secondary lead production is therefore cheaper, in terms of energy requirements, than is primary production, often by 50% or more.

Primary.

Most lead ores contain a low percentage of lead (rich ores have a typical content of 3–8%) which must be concentrated for extraction. During initial processing, ores typically undergo crushing, dense-medium separation, grinding, froth flotation, and drying. The resulting concentrate, which has a lead content of 30–80% by mass (regularly 50–60%), is then turned into (impure) lead metal.  There are two main ways of doing this: a two-stage process involving roasting followed by blast furnace extraction, carried out in separate vessels; or a direct process in which the extraction of the concentrate occurs in a single vessel. The latter has become the most common route, though the former is still significant.

Two-stage process.

First, the sulfide concentrate is roasted in air to oxidize the lead sulfide:  As the original concentrate was not pure lead sulfide, roasting yields not only the desired lead(II) oxide, but a mixture of oxides, sulfates, and silicates of lead and of the other metals contained in the ore. This impure lead oxide is reduced in a coke-fired blast furnace to the (again, impure) metal:  Impurities are mostly arsenic, antimony, bismuth, zinc, copper, silver, and gold. Typically they are removed in a series of pyrometallurgical processes. The melt is treated in a reverberatory furnace with air, steam, and sulfur, which oxidizes the impurities except for silver, gold, and bismuth.

Direct process.

In this process, lead bullion and slag is obtained directly from lead concentrates. The lead sulfide concentrate is melted in a furnace and oxidized, forming lead monoxide. Carbon (as coke or coal gas) is added to the molten charge along with fluxing agents. The lead monoxide is thereby reduced to metallic lead, in the

Alternatives.

Research on a cleaner, less energy-intensive lead extraction process continues; a major drawback is that either too much lead is lost as waste, or the alternatives result in a high sulfur content in the resulting lead metal. Hydrometallurgical extraction, in which anodes of impure lead are immersed into an electrolyte and pure lead is deposited onto a cathode, is a technique that may have potential, but is not currently economical except in cases where electricity is very cheap.

Secondary.

Smelting, which is an essential part of the primary production, is often skipped during secondary production. It is only performed when metallic lead has undergone significant oxidation. The process is similar to that of primary production in either a blast furnace or a rotary furnace, with the essential difference being the greater variability of yields: blast furnaces produce hard lead (10% antimony) while reverberatory and rotary kiln furnaces produced semisoft lead (3–4% antimony). The

Applications.

Contrary to popular belief, pencil leads in wooden pencils have never been made from lead. When the pencil originated as a wrapped graphite writing tool, the particular type of graphite used was named "plumbago" (literally, "act for lead" or "lead mockup").

Elemental form.

Lead metal has several useful mechanical properties, including high density, low melting point, ductility, and relative inertness. Many metals are superior to lead in some of these aspects but are generally less common and more difficult to extract from parent ores. Lead's toxicity has led to its phasing out for some uses.  Lead has been used for bullets since their invention in the Middle Ages. It is inexpensive; its low melting point means small arms ammunition and shotgun pellets can be cast with minimal technical equipment; and it is denser than other common metals, which allows for better retention of velocity. It remains the main

Compounds.

In addition to being the main application for lead metal, lead-acid batteries are also the main consumer of lead compounds. The energy storage/release reaction used in these devices involves lead sulfate and lead dioxide:  Other applications of lead compounds are very specialized and often fading. Lead-based coloring agents are used in ceramic glazes and glass, especially for red and yellow shades. While lead paints are phased out in Europe and

Biological effects.

Lead has no confirmed biological role, and there is no confirmed safe level of lead exposure. A 2009 Canadian–American study concluded that even at levels that are considered to pose little to no risk, lead may cause "adverse mental health outcomes". Its prevalence in the human body—at an adult average of 120 mg—is nevertheless exceeded only by zinc (2500 mg) and iron (4000 mg) among the heavy metals. Lead salts are very efficiently absorbed by the body. A small amount of lead (1%) is stored in bones; the rest is excreted in urine and feces within a few weeks of exposure. Only about a third of lead is excreted by a child. Continual exposure may result in the bioaccumulation of lead.

Toxicity.

Lead is a highly poisonous metal (whether inhaled or swallowed), affecting almost every organ and system in the human body. At airborne levels of 100 mg/m, it is immediately dangerous to life and health. Most ingested lead is absorbed into the bloodstream. The primary cause of its

Effects.

Lead can cause severe damage to the brain and kidneys and, ultimately, death. By mimicking calcium, lead can cross the blood–brain barrier. It degrades the myelin sheaths of neurons, reduces their numbers, interferes with neurotransmission routes, and decreases neuronal growth. In the human body, lead inhibits porphobilinogen synthase and ferrochelatase, preventing both porphobilinogen formation and the incorporation of iron into protoporphyrin IX, the final step in heme synthesis. This causes ineffective heme synthesis and microcytic anemia.  Symptoms of lead poisoning include nephropathy, colic-like abdominal pains, and possibly weakness in the fingers, wrists, or ankles. Small blood pressure increases,

Exposure sources.

Lead exposure is a global issue since lead mining and smelting, and battery manufacturing/disposal/recycling, are common in many countries. Lead enters the body via inhalation, ingestion, or skin absorption. Almost all inhaled lead is absorbed into the body; for ingestion, the rate is 20–70%, with children absorbing a higher percentage than adults.  Poisoning typically results from ingestion of food or water contaminated with lead, and less commonly after accidental ingestion of contaminated soil, dust, or lead-based paint. Seawater products can contain lead if affected by nearby industrial waters. Fruit and vegetables can be contaminated by high levels of lead in the soils they were grown in. Soil can be contaminated through particulate accumulation from lead in pipes, lead paint, and residual emissions from leaded gasoline.  The use of lead for water pipes is a problem in areas with

Treatment.

Treatment for lead poisoning normally involves the administration of dimercaprol and succimer. Acute cases may require the use of disodium calcium edetate, the calcium

Environmental effects.

The extraction, production, use, and disposal of lead and its products have caused significant contamination of the Earth's soils and waters. Atmospheric emissions of lead were at their peak during the Industrial Revolution, and the leaded gasoline period in the second half of the twentieth century. Lead releases originate from natural sources (i.e., concentration of the naturally occurring lead), industrial production, incineration and recycling, and mobilization of previously buried lead. Elevated concentrations of lead persist in soils and sediments in post-industrial and urban areas; industrial emissions, including those arising from coal burning, continue in many parts of the world, particularly in the developing countries.  Lead can accumulate in soils, especially those with a high organic content, where it remains for hundreds to thousands of years. Environmental lead

Restriction and remediation.

By the mid-1980s, there was significant decline in the use of lead in industry. In the United States, environmental regulations reduced or eliminated the use of lead in non-battery products, including gasoline, paints, solders, and water systems. Particulate control devices were installed in coal-fired power plants to capture lead emissions. In 1992, U.S. Congress required the Environmental Protection Agency to reduce the blood lead levels of the country's children. Lead use was further curtailed by the European Union's 2003 Restriction of Hazardous Substances Directive. A large drop in lead deposition occurred in the Netherlands after the 1993 national ban on use of lead shot for hunting and sport shooting: from 230 tonnes in 1990 to 47.5 tonnes in 1995.  In the United States, the permissible exposure limit for lead in the workplace, comprising metallic lead, inorganic lead compounds, and lead soaps, was set at 50 μg/m over an 8-hour workday, and the blood lead level limit at 5 μg per 100 g of blood in 2012. Lead may still be found in harmful quantities in stoneware, vinyl (such as that used for tubing and the insulation of electrical cords), and Chinese brass. Old houses may still contain lead paint. White lead paint has been withdrawn from sale in industrialized countries, but specialized uses of other pigments such as yellow lead chromate remain. Stripping old paint by sanding produces dust which can be inhaled. Lead abatement programs have been mandated by some authorities in properties where young children live.  Lead waste, depending on the jurisdiction and the nature of the waste, may be treated as household waste (in order to facilitate lead abatement activities), or potentially hazardous waste requiring specialized treatment or storage. Lead is released to the wildlife in shooting places and a number of lead management practices, such as stewardship of the environment and reduced public scrutiny, have been developed to counter the lead contamination. Lead migration can be enhanced in acidic soils; to counter that, it is advised soils be treated with lime to neutralize the soils and prevent leaching of lead.  Research has been conducted on how to remove lead from biosystems by biological means: Fish bones are being researched for their ability to bioremediate lead in contaminated soil. The fungus "Aspergillus versicolor" is effective at absorbing lead ions from industrial waste before being released to water bodies. Several bacteria have been researched for their ability to remove lead from the environment, including the sulfate-reducing bacteria "Desulfovibrio" and "Desulfotomaculum", both of which are highly effective in aqueous solutions.

Lead () is a chemical element with the symbol Pb (from the Latin "plumbum") and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cut, lead is silvery with a hint of blue; it tarnishes to a dull gray color when exposed to air. Lead has the highest atomic number of any stable element and three of its isotopes are endpoints of major nuclear decay chains of heavier elements.

铅是一个化学元素,化学符号:(,英语:Lead,),原子序数82。它是一个比大多数常见材料密度更高的重金属。铅是柔软并可锻铸的,并且也有相对低的熔点。当刚切割出来时,铅是银色略带蓝色的,暴露在空气中后,它会失去光泽成暗灰色。


Early life and education.

Pascal was born in Clermont-Ferrand, which is in France's Auvergne region. He lost his mother, Antoinette Begon, at the age of three. His father, Étienne Pascal (1588–1651), who also had an interest in science and mathematics, was a local judge and member of the "Noblesse de Robe". Pascal had two sisters, the younger Jacqueline and the elder Gilberte.  In 1631, five years after the death of his wife, Étienne Pascal moved with his children to Paris. The newly arrived family soon hired Louise Delfault, a maid who eventually became an instrumental member of the family. Étienne, who never remarried, decided that he alone would educate his children, for they all showed extraordinary intellectual ability, particularly his son Blaise. The young Pascal showed an amazing aptitude for mathematics and science.  Particularly of interest to Pascal was a work of Desargues on conic sections. Following Desargues' thinking, the 16-year-old Pascal produced, as a means of proof, a short treatise on what was called the "Mystic Hexagram", "Essai pour les coniques" ("Essay on Conics") and sent it—his first serious work of mathematics—to Père Mersenne in Paris; it is known still today as Pascal's theorem. It states that if a hexagon is inscribed in a circle (or conic) then the three intersection points of opposite sides lie on a line (called the Pascal line).  Pascal's work was so precocious that Descartes was convinced that Pascal's father had written it. When assured by Mersenne that it was, indeed, the product of the son and not the father, Descartes dismissed it with a sniff: "I do not find it strange that he has offered demonstrations about conics more appropriate than those of the ancients," adding, "but other matters related to this subject can be proposed that would scarcely occur to a 16-year-old child."  In France at that time offices and positions could be—and were—bought and sold. In 1631, Étienne sold his position as second president of the "Cour des Aides" for 65,665 livres. The money was invested in a government bond which provided, if not a lavish, then certainly a comfortable income which allowed the Pascal family to move to, and enjoy, Paris. But in 1638 Richelieu, desperate for money to carry on the Thirty Years' War, defaulted on the government's bonds. Suddenly Étienne Pascal's worth had dropped from nearly 66,000 livres to less than 7,300.  Like so many others, Étienne was eventually forced to flee Paris because of his opposition to the fiscal policies of Cardinal Richelieu, leaving his three children in the care of his neighbour Madame Sainctot, a great beauty with an infamous past who kept one of the most glittering and intellectual salons in all France. It was only when Jacqueline performed well in a children's play with Richelieu in attendance that Étienne was pardoned. In time, Étienne was back in good graces with the cardinal and in 1639 had been appointed the king's commissioner of taxes in the city of Rouen—a city whose tax records, thanks to uprisings, were in utter chaos.  In 1642, in an effort to ease his father's endless, exhausting calculations, and recalculations, of taxes owed and paid (into which work the young Pascal had been recruited), Pascal, not yet 19, constructed a mechanical calculator capable of addition and subtraction, called "Pascal's calculator" or the "Pascaline". Of the eight Pascalines known to have survived, four are held by the Musée des Arts et Métiers in Paris and one more by the Zwinger museum in Dresden, Germany, exhibit two of his original mechanical calculators. Although these machines are pioneering forerunners to a further 400 years of development of mechanical methods of calculation, and in a sense to the later field of computer engineering, the calculator failed to be a great commercial success. Partly because it was still quite cumbersome to use in practice, but probably primarily because it was extraordinarily expensive, the Pascaline became little more than a toy, and a status symbol, for the very rich both in France and elsewhere in Europe. Pascal continued to make improvements to his design through the next decade, and he refers to some 50 machines that were built to his design.

Contributions to mathematics.

Pascal continued to influence mathematics throughout his life. His "Traité du triangle arithmétique" ("Treatise on the Arithmetical Triangle") of 1654 described a convenient tabular presentation for binomial coefficients, now called Pascal's triangle. The triangle can also be represented: He defines the numbers in the triangle by recursion: Call the number in the ("m" + 1)th row and ("n" + 1)th column "t". Then "t" = "t" + "t", for "m" = 0, 1, 2,... and "n" = 0, 1, 2,... The boundary conditions are "t" = 0, "t" = 0 for "m" = 1, 2, 3,... and "n" = 1, 2, 3,... The generator "t" = 1. Pascal concludes with the proof,  In 1654, he proved "Pascal's identity" relating the sums of the "p"-th powers of the first "n" positive integers for "p" = 0, 1, 2,..., "k".  In 1654, prompted by his friend the Chevalier de Méré, he corresponded with Pierre de Fermat on the subject of gambling problems, and from that collaboration was born the mathematical theory of probabilities. The specific problem was that of two players who want to finish a game early and, given the current circumstances of the game, want to divide the stakes fairly, based on the chance each has of winning the game from that point. From this discussion, the notion of expected value was introduced. Pascal later (in the "Pensées") used a probabilistic argument, Pascal's Wager, to justify belief in God and a virtuous life. The work done by Fermat and Pascal into the calculus of probabilities laid important groundwork for Leibniz' formulation of the calculus.  After a religious experience in 1654, Pascal mostly gave up work in mathematics.

Philosophy of mathematics.

Pascal's major contribution to the philosophy of mathematics came with his "De l'Esprit géométrique" ("Of the Geometrical Spirit"), originally written as a preface to a geometry textbook for one of the famous ""Petites-Ecoles de Port-Royal" ("Little Schools of Port-Royal")". The work was unpublished until over a century after his death. Here, Pascal looked into the issue of discovering truths, arguing that the ideal of such a method would be to found all propositions on already established truths. At the same time, however, he claimed this was impossible because such established truths would require other truths to back them up—first principles, therefore, cannot be reached. Based on this, Pascal argued that the procedure used in geometry was as perfect as possible, with certain principles assumed and other propositions developed from them. Nevertheless, there was no way to know the assumed principles to be true.  Pascal also used "De l'Esprit géométrique" to develop a theory of definition. He distinguished between definitions which are conventional labels defined by the writer and definitions which are within the language and understood by everyone because they naturally designate their referent. The second type would be characteristic of the philosophy of essentialism. Pascal claimed that only definitions of the first type were important to science and mathematics, arguing that those fields should adopt the philosophy of formalism as formulated by Descartes.  In "De l'Art de persuader" ("On the Art of Persuasion"), Pascal looked deeper into geometry's axiomatic method, specifically the question of how people come to be convinced of the axioms upon which later conclusions are based. Pascal agreed with Montaigne that achieving certainty in these axioms and conclusions through human methods is impossible. He asserted that these principles can be grasped only through intuition, and that this fact underscored the necessity for submission to God in searching out truths.

Contributions to the physical sciences.

Pascal's work in the fields of the study of hydrodynamics and hydrostatics centered on the principles of hydraulic fluids. His inventions include the hydraulic press (using hydraulic pressure to multiply force) and the syringe. He proved that hydrostatic pressure depends not on the weight of the fluid but on the elevation difference. He demonstrated this principle by attaching a thin tube to a barrel full of water and filling the tube with water up to the level of the third floor of a building. This caused the barrel to leak, in what became known as Pascal's barrel experiment.  By 1647, Pascal had learned of Evangelista Torricelli's experimentation with barometers. Having replicated an experiment that involved placing a tube filled with mercury upside down in a bowl of mercury, Pascal questioned what force kept some mercury in the tube and what filled the space above the mercury in the tube. At the time, most scientists contended that, rather than a vacuum, some invisible matter was present. This was based on the Aristotelian notion that creation was a thing of substance, whether visible or invisible; and that this substance was forever in motion. Furthermore, "Everything that is in motion must be moved by something," Aristotle declared. Therefore, to the Aristotelian trained scientists of Pascal's time, a vacuum was an impossibility. How so? As proof it was pointed out:  Following more experimentation in this vein, in 1647 Pascal produced "Experiences nouvelles touchant le vide" ("New experiments with the vacuum"), which detailed basic rules describing to what degree various liquids could be supported by air pressure. It also provided reasons why it was indeed a vacuum above the column of liquid in a barometer tube. This work was followed by "Récit de la grande expérience de l'équilibre des liqueurs" ("Account of the great experiment on equilibrium in liquids") published in 1648.  The Torricellian vacuum found that air pressure is equal to the weight of 30 inches of mercury. If air has a finite weight, Earth's atmosphere must have a maximum height. Pascal reasoned that if true, air pressure on a high mountain must be less than at a lower altitude. He lived near the Puy de Dôme mountain, tall, but his health was poor so could not climb it. On 19 September 1648, after many months of Pascal's friendly but insistent prodding, Florin Périer, husband of Pascal's elder sister Gilberte, was finally able to carry out the fact-finding mission vital to Pascal's theory. The account, written by Périer, reads:  Pascal replicated the experiment in Paris by carrying a barometer up to the top of the bell tower at the church of Saint-Jacques-de-la-Boucherie, a height of about 50 metres. The mercury dropped two lines.  In the face of criticism that some invisible matter must exist in Pascal's empty space, Pascal, in his reply to Estienne Noel, gave one of the 17th century's major statements on the scientific method, which is a striking anticipation of the idea popularised by Karl Popper that scientific theories are characterised by their falsifiability: "In order to show that a hypothesis is evident, it does not suffice that all the phenomena follow from it; instead, if it leads to something contrary to a single one of the phenomena, that suffices to establish its falsity." His insistence on the existence of the vacuum also led to conflict with other prominent scientists, including Descartes.  Pascal introduced a primitive form of roulette and the roulette wheel in his search for a perpetual motion machine.

Adult life, religion, philosophy, and literature.

Religious conversion.

In the winter of 1646, Pascal's 58-year-old father broke his hip when he slipped and fell on an icy street of Rouen; given the man's age and the state of medicine in the 17th century, a broken hip could be a very serious condition, perhaps even fatal. Rouen was home to two of the finest doctors in France: Monsieur Doctor Deslandes and Monsieur Doctor de La Bouteillerie. The elder Pascal "would not let anyone other than these men attend him...It was a good choice, for the old man survived and was able to walk again..." But treatment and rehabilitation took three months, during which time La Bouteillerie and Deslandes had become regular visitors.  Both men were followers of Jean Guillebert, proponent of a splinter group from Catholic teaching known as Jansenism. This still fairly small sect was making surprising inroads into the French Catholic community at that time. It espoused rigorous Augustinism. Blaise spoke with the doctors frequently, and after their successful treatment of his father, borrowed from them works by Jansenist authors. In this period, Pascal experienced a sort of "first conversion" and began to write on theological subjects in the course of the following year.  Pascal fell away from this initial religious engagement and experienced a few years of what some biographers have called his "worldly period" (1648–54). His father died in 1651 and left his inheritance to Pascal and his sister Jacqueline, for whom Pascal acted as conservator. Jacqueline announced that she would soon become a postulant in the Jansenist convent of Port-Royal. Pascal was deeply affected and very sad, not because of her choice, but because of his chronic poor health; he needed her just as she had needed him.  By the end of October in 1651, a truce had been reached between brother and sister. In return for a healthy annual stipend, Jacqueline signed over her part of the inheritance to her brother. Gilberte had already been given her inheritance in the form of a dowry. In early January, Jacqueline left for Port-Royal. On that day, according to Gilberte concerning her brother, "He retired very sadly to his rooms without seeing Jacqueline, who was waiting in the little parlor..." In early June 1653, after what must have seemed like endless badgering from Jacqueline, Pascal formally signed over the whole of his sister's inheritance to Port-Royal, which, to him, "had begun to smell like a cult." With two-thirds of his father's estate now gone, the 29-year-old Pascal was now consigned to genteel poverty.  For a while, Pascal pursued the life of a bachelor. During visits to his sister at Port-Royal in 1654, he displayed contempt for affairs of the world but was not drawn to God.

Brush with death.

On 23 November 1654, between 10:30 and 12:30 at night, Pascal had an intense religious vision and immediately recorded the experience in a brief note to himself which began: "Fire. God of Abraham, God of Isaac, God of Jacob, not of the philosophers and the scholars..." and concluded by quoting Psalm 119:16: "I will not forget thy word. Amen." He seems to have carefully sewn this document into his coat and always transferred it when he changed clothes; a servant discovered it only by chance after his death. This piece is now known as the "Memorial". The story of a carriage accident as having led to the experience described in the "Memorial" is disputed by some scholars. His belief and religious commitment revitalized, Pascal visited the older of two convents at Port-Royal for a two-week retreat in January 1655. For the next four years, he regularly travelled between Port-Royal and Paris. It was at this point immediately after his conversion when he began writing his first major literary work on religion, the "Provincial Letters".

The "Provincial Letters".

Beginning in 1656–57, Pascal published his memorable attack on casuistry, a popular ethical method used by Catholic thinkers in the early modern period (especially the Jesuits, and in particular Antonio Escobar). Pascal denounced casuistry as the mere use of complex reasoning to justify moral laxity and all sorts of sins. The 18-letter series was published between 1656 and 1657 under the pseudonym Louis de Montalte and incensed Louis XIV. The king ordered that the book be shredded and burnt in 1660. In 1661, in the midsts of the formulary controversy, the Jansenist school at Port-Royal was condemned and closed down; those involved with the school had to sign a 1656 papal bull condemning the teachings of Jansen as heretical. The final letter from Pascal, in 1657, had defied Alexander VII himself. Even Pope Alexander, while publicly opposing them, nonetheless was persuaded by Pascal's arguments.  Aside from their religious influence, the "Provincial Letters" were popular as a literary work. Pascal's use of humor, mockery, and vicious satire in his arguments made the letters ripe for public consumption, and influenced the prose of later French writers like Voltaire and Jean-Jacques Rousseau.  It is in the "Provincial Letters" that Pascal made his oft-quoted apology for writing a long letter, as he had not had time to write a shorter one. From Letter XVI, as translated by Thomas M'Crie: 'Reverend fathers, my letters were not wont either to be so prolix, or to follow so closely on one another. Want of time must plead my excuse for both of these faults. The present letter is a very long one, simply because I had no leisure to make it shorter.'  Charles Perrault wrote of the "Letters": "Everything is there—purity of language, nobility of thought, solidity in reasoning, finesse in raillery, and throughout an "agrément" not to be found anywhere else."

The "Pensées".

Pascal's most influential theological work, referred to posthumously as the "Pensées" ("Thoughts"), was not completed before his death. It was to have been a sustained and coherent examination and defense of the Christian faith, with the original title "Apologie de la religion Chrétienne" ("Defense of the Christian Religion"). The first version of the numerous scraps of paper found after his death appeared in print as a book in 1669 titled "Pensées de M. Pascal sur la religion, et sur quelques autres sujets" ("Thoughts of M. Pascal on religion, and on some other subjects") and soon thereafter became a classic. One of the "Apologie"s main strategies was to use the contradictory philosophies of Pyrrhonism and Stoicism, personalized by Montaigne on one hand, and Epictetus on the other, in order to bring the unbeliever to such despair and confusion that he would embrace God.  Pascal's "Pensées" is widely considered to be a masterpiece, and a landmark in "French prose". When commenting on one particular section (Thought #72), Sainte-Beuve praised it as the finest pages in the French language. Will Durant hailed the Pensées as "the most eloquent book in French prose".

Last works and death.

T. S. Eliot described him during this phase of his life as "a man of the world among ascetics, and an ascetic among men of the world." Pascal's ascetic lifestyle derived from a belief that it was natural and necessary for a person to suffer. In 1659, Pascal fell seriously ill. During his last years, he frequently tried to reject the ministrations of his doctors, saying, "Sickness is the natural state of Christians."  Louis XIV suppressed the Jansenist movement at Port-Royal in 1661. In response, Pascal wrote one of his final works, "Écrit sur la signature du formulaire" ("Writ on the Signing of the Form"), exhorting the Jansenists not to give in. Later that year, his sister Jacqueline died, which convinced Pascal to cease his polemics on Jansenism. Pascal's last major achievement, returning to his mechanical genius, was inaugurating perhaps the first bus line, the carrosses à cinq sols, moving passengers within Paris in a carriage with many seats.  In 1662, Pascal's illness became more violent, and his emotional condition had severely worsened since his sister's death. Aware that his health was fading quickly, he sought a move to the hospital for incurable diseases, but his doctors declared that he was too unstable to be carried. In Paris on 18 August 1662, Pascal went into convulsions and received extreme unction. He died the next morning, his last words being "May God never abandon me," and was buried in the cemetery of Saint-Étienne-du-Mont.  An autopsy performed after his death revealed grave problems with his stomach and other organs of his abdomen, along with damage to his brain. Despite the autopsy, the cause of his poor health was never precisely determined, though speculation focuses on tuberculosis, stomach cancer, or a combination of the two. The headaches which afflicted Pascal are generally attributed to his brain lesion.

Legacy.

In honour of his scientific contributions, the name "Pascal" has been given to the SI unit of pressure, to a programming language, and Pascal's law (an important principle of hydrostatics), and as mentioned above, Pascal's triangle and Pascal's wager still bear his name.  Pascal's development of probability theory was his most influential contribution to mathematics. Originally applied to gambling, today it is extremely important in economics, especially in actuarial science. John Ross writes, "Probability theory and the discoveries following it changed the way we regard uncertainty, risk, decision-making, and an individual's and society's ability to influence the course of future events." However, Pascal and Fermat, though doing important early work in probability theory, did not develop the field very far. Christiaan Huygens, learning of the subject from the correspondence of Pascal and Fermat, wrote the first book on the subject. Later figures who continued the development of the theory include Abraham de Moivre and Pierre-Simon Laplace.  In literature, Pascal is regarded as one of the most important authors of the French Classical Period and is read today as one of the greatest masters of French prose. His use of satire and wit influenced later polemicists. The content of his literary work is best remembered for its strong opposition to the rationalism of René Descartes and simultaneous assertion that the main countervailing philosophy, empiricism, was also insufficient for determining major truths.  In France, prestigious annual awards, Blaise Pascal Chairs are given to outstanding international scientists to conduct their research in the Ile de France region. One of the Universities of Clermont-Ferrand, France – Université Blaise Pascal – is named after him. The University of Waterloo, Ontario, Canada, holds an annual math contest named in his honour.  Pascalian theology has grown out of his perspective that humans are, according to Wood, "born into a duplicitous world that shapes us into duplicitous subjects and so we find it easy to reject God continually and deceive ourselves about our own sinfulness".  The 1969 Eric Rohmer film "My Night at Maud's" is based on the work of Pascal. Roberto Rossellini directed a filmed biopic, "Blaise Pascal", which originally aired on Italian television in 1971. Pascal was a subject of the first edition of the 1984 BBC Two documentary, "Sea of Faith", presented by Don Cupitt.  In 2014, Nvidia announced its new Pascal microarchitecture, which is named for Pascal. The first graphics cards featuring Pascal were released in 2016.  The 2017 game "" has multiple characters named after famous philosophers; one of these is a sentient pacifistic machine named Pascal, who serves as a major supporting character. Pascal creates a village for machines to live peacefully with the androids they're at war with and acts as a parental figure for other machines trying to adapt to their newly-found individuality.  Établissement scolaire français Blaise-Pascal in Lubumbashi, Democratic Republic of the Congo is named after Pascal.

Blaise Pascal (,,, ; 19 June 1623 – 19 August 1662) was a French mathematician, physicist, inventor, writer and Catholic theologian. He was a child prodigy who was educated by his father, a tax collector in Rouen. Pascal's earliest work was in the natural and applied sciences, where he made important contributions to the study of fluids, and clarified the concepts of pressure and vacuum by generalising the work of Evangelista Torricelli. Pascal also wrote in defence of the scientific method.

布莱兹‧帕斯卡(,1623年-6月19日-1662年-8月19日),法国神学家、哲学家、数学家、物理学家、化学家、音乐家、教育家、气象学家。帕斯卡早期进行自然和应用科学的研究,对机械计算器的制造和流体的研究作出重要贡献,扩展托里切利的工作,澄清了压强和真空的概念。帕斯卡还有力地为科学方法辩护。数学上,帕斯卡促成了两个重要的新研究领域。他16岁写出一篇题为射影几何的论文,1654年开始与皮埃尔·德·费马通信,讨论概率论,深刻影响了现代经济学和社会科学的发展。


Motivation.

There is an inherent trade-off between size and speed (given that a larger resource implies greater physical distances) but also a tradeoff between expensive, premium technologies (such as SRAM) vs cheaper, easily mass-produced commodities (such as DRAM or hard disks).  The buffering provided by a cache benefits both latency and throughput (bandwidth):

Latency.

A larger resource incurs a significant latency for access e.g. it can take hundreds of clock cycles for a modern 4 GHz processor to reach DRAM. This is mitigated by reading in large chunks, in the hope that subsequent reads will be from nearby locations. Prediction or explicit prefetching might also guess where future reads will come from and make requests ahead of time; if done correctly the latency is bypassed altogether.

Throughput.

The use of a cache also allows for higher throughput from the underlying resource, by assembling multiple fine grain transfers into larger, more efficient requests. In the case of DRAM circuits, this might be served by having a wider data bus. For example, consider a program accessing bytes in a 32-bit address space, but being served by a 128-bit off-chip data bus; individual uncached byte accesses would allow only 1/16th of the total bandwidth to be used, and 80% of the data movement would be memory addresses instead of data itself. Reading larger chunks reduces the fraction of bandwidth required for transmitting address information.

Operation.

Hardware implements cache as a block of memory for temporary storage of data likely to be used again. Central processing units (CPUs) and hard disk drives (HDDs) frequently use a cache, as do web browsers and web servers.  A cache is made up of a pool of entries. Each entry has associated "data", which is a copy of the same data in some "backing store". Each entry also has a "tag", which specifies the identity of the data in the backing store of which the entry is a copy. Tagging allows simultaneous cache-oriented algorithms to function in multilayered fashion without differential relay interference.  When the cache client (a CPU, web browser, operating system) needs to access data presumed to exist in the backing store, it first checks the cache. If an entry can be found with a tag matching that of the desired data, the data in the entry is used instead. This situation is known as a cache hit. For example, a web browser program might check its local cache on disk to see if it has a local copy of the contents of a web page at a particular URL. In this example, the URL is the tag, and the content of the web page is the data. The percentage of accesses that result in cache hits is known as the hit rate or hit ratio of the cache.  The alternative situation, when the cache is checked and found not to contain any entry with the desired tag, is known as a cache miss. This requires a more expensive access of data from the backing store. Once the requested data is retrieved, it is typically copied into the cache, ready for the next access.  During a cache miss, some other previously existing cache entry is removed in order to make room for the newly retrieved data. The heuristic used to select the entry to replace is known as the replacement policy. One popular replacement policy, "least recently used" (LRU), replaces the oldest entry, the entry that was accessed less recently than any other entry (see cache algorithm). More efficient caching algorithms compute the use-hit frequency against the size of the stored contents, as well as the latencies and throughputs for both the cache and the backing store. This works well for larger amounts of data, longer latencies, and slower throughputs, such as that experienced with hard drives and networks, but is not efficient for use within a CPU cache.

Writing policies.

When a system writes data to cache, it must at some point write that data to the backing store as well. The timing of this write is controlled by what is known as the "write policy". There are two basic writing approaches:  A write-back cache is more complex to implement, since it needs to track which of its locations have been written over, and mark them as "dirty" for later writing to the backing store. The data in these locations are written back to the backing store only when they are evicted from the cache, an effect referred to as a "lazy write". For this reason, a read miss in a write-back cache (which requires a block to be replaced by another) will often require two memory accesses to service: one to write the replaced data from the cache back to the store, and then one to retrieve the needed data.  Other policies may also trigger data write-back. The client may make many changes to data in the cache, and then explicitly notify the cache to write back the data.  Since no data is returned to the requester on write operations, a decision needs to be made on write misses, whether or not data would be loaded into the cache. This is defined by these two approaches:  Both write-through and write-back policies can use either of these write-miss policies, but usually they are paired in this way:  Entities other than the cache may change the data in the backing store, in which case the copy in the cache may become out-of-date or "stale". Alternatively, when the client updates the data in the cache, copies of those data in other caches will become stale. Communication protocols between the cache managers which keep the data consistent are known as coherency protocols.

Examples of hardware caches.

CPU cache.

Small memories on or close to the CPU can operate faster than the much larger main memory. Most CPUs since the 1980s have used one or more caches, sometimes in cascaded levels; modern high-end embedded, desktop and server microprocessors may have as many as six types of cache (between levels and functions). Examples of caches with a specific function are the D-cache and I-cache and the translation lookaside buffer for the MMU.

GPU cache.

Earlier graphics processing units (GPUs) often had limited read-only texture caches, and introduced morton order swizzled textures to improve 2D cache coherency. Cache misses would drastically affect performance, e.g. if mipmapping was not used. Caching was important to leverage 32-bit (and wider) transfers for texture data that was often as little as 4 bits per pixel, indexed in complex patterns by arbitrary UV coordinates and perspective transformations in inverse texture mapping.  As GPUs advanced (especially with GPGPU compute shaders) they have developed progressively larger and increasingly general caches, including instruction caches for shaders, exhibiting increasingly common functionality with CPU caches. For example, GT200 architecture GPUs did not feature an L2 cache, while the Fermi GPU has 768 KB of last-level cache, the Kepler GPU has 1536 KB of last-level cache, and the Maxwell GPU has 2048 KB of last-level cache. These caches have grown to handle synchronisation primitives between threads and atomic operations, and interface with a CPU-style MMU.

DSPs.

Digital signal processors have similarly generalised over the years. Earlier designs used scratchpad memory fed by DMA, but modern DSPs such as Qualcomm Hexagon often include a very similar set of caches to a CPU (e.g. Modified Harvard architecture with shared L2, split L1 I-cache and D-cache).

Translation lookaside buffer.

A memory management unit (MMU) that fetches page table entries from main memory has a specialized cache, used for recording the results of virtual address to physical address translations. This specialized cache is called a translation lookaside buffer (TLB).

In-network cache.

Information-centric networking.

Information-centric networking (ICN) is an approach to evolve the Internet infrastructure away from a host-centric paradigm, based on perpetual connectivity and the end-to-end principle, to a network architecture in which the focal point is identified information (or content or data). Due to the inherent caching capability of the nodes in an ICN, it can be viewed as a loosely connected network of caches, which has unique requirements of caching policies. However, ubiquitous content caching introduces the challenge to content protection against unauthorized access, which requires extra care and solutions. Unlike proxy servers, in ICN the cache is a network-level solution. Therefore, it has rapidly changing cache states and higher request arrival rates; moreover, smaller cache sizes further impose a different kind of requirements on the content eviction policies. In particular, eviction policies for ICN should be fast and lightweight. Various cache replication and eviction schemes for different ICN architectures and applications have been proposed.

Policies.

Time aware least recently used (TLRU).

The Time aware Least Recently Used (TLRU) is a variant of LRU designed for the situation where the stored contents in cache have a valid life time. The algorithm is suitable in network cache applications, such as Information-centric networking (ICN), Content Delivery Networks (CDNs) and distributed networks in general. TLRU introduces a new term: TTU (Time to Use). TTU is a time stamp of a content/page which stipulates the usability time for the content based on the locality of the content and the content publisher announcement. Owing to this locality based time stamp, TTU provides more control to the local administrator to regulate in network storage. In the TLRU algorithm, when a piece of content arrives, a cache node calculates the local TTU value based on the TTU value assigned by the content publisher. The local TTU value is calculated by using a locally defined function. Once the local TTU value is calculated the replacement of content is performed on a subset of the total content stored in cache node. The TLRU ensures that less popular and small life content should be replaced with the incoming content.

Least frequent recently used (LFRU).

The Least Frequent Recently Used (LFRU) cache replacement scheme combines the benefits of LFU and LRU schemes. LFRU is suitable for ‘in network’ cache applications, such as Information-centric networking (ICN), Content Delivery Networks (CDNs) and distributed networks in general. In LFRU, the cache is divided into two partitions called privileged and unprivileged partitions. The privileged partition can be defined as a protected partition. If content is highly popular, it is pushed into the privileged partition. Replacement of the privileged partition is done as follows: LFRU evicts content from the unprivileged partition, pushes content from privileged partition to unprivileged partition, and finally inserts new content into the privileged partition. In the above procedure the LRU is used for the privileged partition and an approximated LFU (ALFU) scheme is used for the unprivileged partition, hence the abbreviation LFRU. The basic idea is to filter out the locally popular contents with ALFU scheme and push the popular contents to one of the privileged partition.

Software caches.

Disk cache.

While CPU caches are generally managed entirely by hardware, a variety of software manages other caches. The page cache in main memory, which is an example of disk cache, is managed by the operating system kernel.  While the disk buffer, which is an integrated part of the hard disk drive, is sometimes misleadingly referred to as "disk cache", its main functions are write sequencing and read prefetching. Repeated cache hits are relatively rare, due to the small size of the buffer in comparison to the drive's capacity. However, high-end disk controllers often have their own on-board cache of the hard disk drive's data blocks.  Finally, a fast local hard disk drive can also cache information held on even slower data storage devices, such as remote servers (web cache) or local tape drives or optical jukeboxes; such a scheme is the main concept of hierarchical storage management. Also, fast flash-based solid-state drives (SSDs) can be used as caches for slower rotational-media hard disk drives, working together as hybrid drives or solid-state hybrid drives (SSHDs).

Web cache.

Web browsers and web proxy servers employ web caches to store previous responses from web servers, such as web pages and images. Web caches reduce the amount of information that needs to be transmitted across the network, as information previously stored in the cache can often be re-used. This reduces bandwidth and processing requirements of the web server, and helps to improve responsiveness for users of the web.  Web browsers employ a built-in web cache, but some Internet service providers (ISPs) or organizations also use a caching proxy server, which is a web cache that is shared among all users of that network.  Another form of cache is P2P caching, where the files most sought for by peer-to-peer applications are stored in an ISP cache to accelerate P2P transfers. Similarly, decentralised equivalents exist, which allow communities to perform the same task for P2P traffic, for example, Corelli.

Memoization.

A cache can store data that is computed on demand rather than retrieved from a backing store. Memoization is an optimization technique that stores the results of resource-consuming function calls within a lookup table, allowing subsequent calls to reuse the stored results and avoid repeated computation. It is related to the dynamic programming algorithm design methodology, which can also be thought of as a means of caching.

Other caches.

The BIND DNS daemon caches a mapping of domain names to IP addresses, as does a resolver library.  Write-through operation is common when operating over unreliable networks (like an Ethernet LAN), because of the enormous complexity of the coherency protocol required between multiple write-back caches when communication is unreliable. For instance, web page caches and client-side network file system caches (like those in NFS or SMB) are typically read-only or write-through specifically to keep the network protocol simple and reliable.  Search engines also frequently make web pages they have indexed available from their cache. For example, Google provides a "Cached" link next to each search result. This can prove useful when web pages from a web server are temporarily or permanently inaccessible.  Another type of caching is storing computed results that will likely be needed again, or memoization. For example, ccache is a program that caches the output of the compilation, in order to speed up later compilation runs.  Database caching can substantially improve the throughput of database applications, for example in the processing of indexes, data dictionaries, and frequently used subsets of data.  A distributed cache uses networked hosts to provide scalability, reliability and performance to the application. The hosts can be co-located or spread over different geographical regions.

Buffer vs. cache.

The semantics of a "buffer" and a "cache" are not totally different; even so, there are fundamental differences in intent between the process of caching and the process of buffering.  Fundamentally, caching realizes a performance increase for transfers of data that is being repeatedly transferred. While a caching system may realize a performance increase upon the initial (typically write) transfer of a data item, this performance increase is due to buffering occurring within the caching system.  With read caches, a data item must have been fetched from its residing location at least once in order for subsequent reads of the data item to realize a performance increase by virtue of being able to be fetched from the cache's (faster) intermediate storage rather than the data's residing location. With write caches, a performance increase of writing a data item may be realized upon the first write of the data item by virtue of the data item immediately being stored in the cache's intermediate storage, deferring the transfer of the data item to its residing storage at a later stage or else occurring as a background process. Contrary to strict buffering, a caching process must adhere to a (potentially distributed) cache coherency protocol in order to maintain consistency between the cache's intermediate storage and the location where the data resides. Buffering, on the other hand,  With typical caching implementations, a data item that is read or written for the first time is effectively being buffered; and in the case of a write, mostly realizing a performance increase for the application from where the write originated. Additionally, the portion of a caching protocol where individual writes are deferred to a batch of writes is a form of buffering. The portion of a caching protocol where individual reads are deferred to a batch of reads is also a form of buffering, although this form may negatively impact the performance of at least the initial reads (even though it may positively impact the performance of the sum of the individual reads). In practice, caching almost always involves some form of buffering, while strict buffering does not involve caching.  A buffer is a temporary memory location that is traditionally used because CPU instructions cannot directly address data stored in peripheral devices. Thus, addressable memory is used as an intermediate stage. Additionally, such a buffer may be feasible when a large block of data is assembled or disassembled (as required by a storage device), or when data may be delivered in a different order than that in which it is produced. Also, a whole buffer of data is usually transferred sequentially (for example to hard disk), so buffering itself sometimes increases transfer performance or reduces the variation or jitter of the transfer's latency as opposed to caching where the intent is to reduce the latency. These benefits are present even if the buffered data are written to the buffer once and read from the buffer once.  A cache also increases transfer performance. A part of the increase similarly comes from the possibility that multiple small transfers will combine into one large block. But the main performance-gain occurs because there is a good chance that the same data will be read from cache multiple times, or that written data will soon be read. A cache's sole purpose is to reduce accesses to the underlying slower storage. Cache is also usually an abstraction layer that is designed to be invisible from the perspective of neighboring layers.

In computing, a cache (, or in Australian English) is a hardware or software component that stores data so that future requests for that data can be served faster; the data stored in a cache might be the result of an earlier computation or a copy of data stored elsewhere. A "cache hit" occurs when the requested data can be found in a cache, while a "cache miss" occurs when it cannot. Cache hits are served by reading data from the cache, which is faster than recomputing a result or reading from a slower data store; thus, the more requests that can be served from the cache, the faster the system performs.

高速缓存(英语:cache, )-{zh-cn:简称缓存,; zh-tw:,;}-原始意义是指存取速度比一般随机存取内存(RAM)快的一种RAM,通常它不像系统主内存那样使用DRAM技术,而使用昂贵但较快速的SRAM技术。


Geography.

Location.

Coburg lies about south of Erfurt and about north of Nuremberg on the river Itz. It is an urban district and is surrounded by the "Landkreis Coburg". Coburg lies at the foot of the Thuringian Highland. Coburg, Bavaria was part of West Germany until reunification in 1990, but on three sides it borders Thuringia which was East Germany. The border between Bavaria and Thuringia was also the inner German border.

Subdivisions.

Coburg is divided into 15 "Stadtteile":

History.

Coburg was first mentioned in a monastic document dated 1056, which marked the transfer of ownership to the Archbishop-Elector of Cologne, although there was a settlement at the site that predates it called "Trufalistat". The origin of the name "Coburg" is unclear; the first element may be "kuh", which would give a literal meaning of "cow borough".  "Coburg" initially referred to a property centred on the hill where Veste Coburg was later built. Its oldest remains date to the 12th or 13th century. In 1248, the castle came into possession of the House of Henneberg and in 1353 it passed to the House of Wettin with the marriage of Frederick III with Catherine of Henneberg and was initially regarded by them as a Saxon outpost within Franconia.  During the Diet of Augsburg in 1530 reformer Martin Luther spent six months at the castle (located at the southernmost point of the Saxon duchy) while his liege lord, John, Elector of Saxony, attended the Diet. Luther was forbidden to attend by the Elector, who feared that he would be imprisoned and burned as a heretic. While quartered at the castle Luther continued with his translation of the Bible into German.  In 1547, the princely residence was moved from the Veste to a former monastery, rebuilt as a Renaissance palace, the Ehrenburg.  In 1596, Coburg was raised to the status of capital of one of the dynasty's splintered Saxon-Thuringian territories, the newly created Duchy of Saxe-Coburg under the leadership of Duke John Casimir (ruled 1596–1633). From 1699 to 1826, it was one of the two capitals of the Duchy of Saxe-Coburg-Saalfeld, and from 1826–1918 it was a capital of the Duchy of Saxe-Coburg-Gotha. Ernest Frederick, the fourth Duke of Saxe-Coburg-Saalfeld, moved his capital from Saalfeld to Coburg in 1764. Coburg then became capital of the Duchy of Saxe-Coburg-Saalfeld and later of the Duchy of Saxe-Coburg and Gotha.  In the early 19th century, the town's medieval fortifications were demolished and replaced by parks. The duke also started the collection of copperplate engravings that is today part of the Veste Coburg museum. Under his son, Ernest, the "" with what is today the "Landestheater Coburg" was created. He also rebuilt the Ehrenburg in Gothic revival style.  In the mid-19th century, Duke Ernest II supported national and liberal ideas and Coburg hosted the first meeting of the German National Association, the founding of the'and the first'(national sports festival).  During the 19th century, dynastic marriages created ties with the royal families of Belgium, Bulgaria, Portugal and Britain. This turned the ducal family from the rulers of a fairly obscure backwater duchy into one playing an influential role in European politics. The era of political influence peaked with Leopold Frederick; born Prince of Saxe-Coburg-Saalfeld, becoming the King of Belgium in 1831 and Prince Albert of Saxe-Coburg and Gotha, born in Schloss Rosenau, marrying his first cousin, Queen Victoria in 1840. The marriage between Albert and Victoria established the present British royal house, which renamed itself Windsor during World War I. This marriage in turn led to a union with Germany's ruling dynasty, the Hohenzollerns, when the couple's eldest child, Victoria, married the future Kaiser Friedrich III.  After her marriage, Queen Victoria said of Coburg:  Due to the royal connections among the royal houses of Europe, Coburg was the site of many royal Ducal weddings and visits. Britain's Queen Victoria made six visits to Coburg during her 63-year reign. In 1894 the wedding of Ernest Louis, Grand Duke of Hesse and Princess Victoria Melita of Saxe-Coburg and Gotha brought together Queen Victoria, her son Edward (future Edward VII), her second son Alfred (Duke of Saxe-Coburg-Gotha), her daughter the German Dowager Empress Friedrich (Victoria), and many of her grandchildren, such as future Tsar Nicholas and Alexandra of Russia (Alix of Hesse), Kaiser Wilhelm II of Germany, and the future King George V of the United Kingdom.  In November 1918, the last Duke of Saxe-Coburg and Gotha, Charles Edward, abdicated. The "Freistaat Coburg" which now came into being had to decide whether to become part of Thuringia or Bavaria. In a November 1919 referendum, the locals voted to join Bavaria with an 88% majority. On 1 July 1920, Coburg joined Bavaria.  In 1929, Coburg was the first German town in which the Nazi Party won the absolute majority of the popular vote during municipal elections. In 1932, Coburg was the first German town to make Adolf Hitler an honorary citizen.  Coburg had Jewish citizens as early as the 14th century. In the 1870s they were granted permission to lease permanently the Church of St. Nicholas for conversion into a synagogue. In 1931 an unofficial boycott was imposed against Jewish businesses. In 1932 the municipal council abrogated the lease of St. Nicholas Church, and a year later the synagogue was closed down (it still remains standing). On 25 March 1933, 40 Jews in Coburg were arrested and tortured. They were not released until the affair became internationally known. On 9 November 1938, all Jewish men were interned and Jewish homes, shops, and the school were destroyed. Coburg's Jewish community numbered 68 in 1869, 210 (1.3% of the total population) in 1880, 316 (1.3%) in 1925, and 233 (0.9%) in 1933. Around 150 managed to leave by 1942, either emigrating from Germany or moving to other German cities. The rest were deported to Riga, Izbica, and Theresienstadt in three transports between November 1941 and September 1942. The memorial book of the German Federal Archives for the victims of the Nazi persecution of Jews in Germany lists in particular 63 Jewish inhabitants of Coburg, who were deported and mostly murdered. Coburg's Jewish community was not reestablished after the war.  After World War II, which Coburg survived largely undamaged, the town faced the challenge of integrating over 15,000 refugees. In addition, whilst the other Saxon-Thuringian principalities were incorporated into the German Democratic Republic, Bavarian Coburg became part of West Germany. As a result, the town spent the Cold War years lying right next to the Iron Curtain, surrounded by East German territory on three sides and cut off from much of its natural back country.  In 1946, Polish ambassador Oskar R. Lange alleged that Coburg was a base for the Western Allies to organize a Polish armed insurgency led by Władysław Anders against the Soviet-backed communists in Poland.

Demographics.

Over two-thirds of Coburg's population live in the core town of Coburg rather than in one of the "Stadtteile" merged with it in the 20th century. Some of those retain a largely rural character.

Religion.

Most residents of Coburg are members of the Evangelical Church (Lutheran). Other Christian communities are Baptists, Seventh-day Adventists, the ICF Movement, Jehovah's Witnesses, Catholics, Old Catholics and the New Apostolic Church, as well as The Church of Jesus Christ of Latter-day Saints. There are also three communities of Muslims. Coburg had a large Jewish community until the 1940s. Jews had lived there since the 14th century. The old synagogue was a former church. Today it is used by Old Catholics. Coburg became Protestant after the Reformation. All Catholics were persecuted. A new Catholic community was founded in the 19th century.

Economy.

In 1919 and Ernst Jüngling established the metal works "Max Brose & Co." to manufacture car parts. The company is still in operation today, as "Brose Fahrzeugteile".  In 1950, the "Haftpflicht-Unterstützungs-Kasse kraftfahrender Beamter Deutschlands a. G." (today ) relocated from Erfurt to Coburg. HUK is today the largest employer and largest payer of "" (local corporate tax) in Coburg.  "Kapp Werkzeugmaschinen" has been a manufacturer of machines since 1953, after taking over the production assets of COMAG (Coburger Maschinenbau GmbH).  Coburg has an above-average share of goods-producing employees. In 2013, out of 32,962 employees 10,421 worked in the manufacturing or construction sectors (31% vs. a national average of 24%), 4,853 in trade, transport and tourism, 10,381 in professional services and 7,230 in public and private services.  Hotels in Coburg counted over 61,000 overnight visitors in 2014 (of which around 53,000 were from Germany). They stayed for a total of almost 120,000 nights, or close to two nights on average.  In 2017, the GDP per inhabitant was €91,506 in Coburg, placing it 5th among the 96 urban and rural districts (Bavarian average: €46,698).

Twin towns – sister cities.

Coburg is twinned with:

Coat of arms.

Coburg's coat of arms, honouring the town's patron Saint Maurice, was granted in 1493. In 1934, the coat of arms was replaced with one depicting a sword with a swastika on the pommel; the original coat of arms was restored in 1945.

Attractions.

Coburg has the typical features of a former capital of a German princely state. There are numerous houses from the 16th, 17th and 18th centuries. The most important landmarks include:

Arts and culture.

Coburg is home to two major festivals: "" and "Johann Strauss Musiktage". Coburg is referred to as "Europe's Capital of Samba."  As a result of the large presence of the US Army prior to German re-unification, Americans and American culture are still present in Coburg and the surrounding area. This influence ranges from American-style pubs and restaurants to two sports clubs sponsoring baseball teams.

Trivia.

The National Hot Dog and Sausage Council asserts that Frankfurt is traditionally credited with originating the frankfurter. According to the Council, this claim is disputed and the hot dog was created in the late 17th century by Johann Georghehner, a butcher, living in Coburg.  Coburg was the first German town to elect a Nazi Mayor.  A popular local delicacy is the Coburger Bratwurst, a sausage (the official measure of which is denoted by the Marshall's staff held by the statue of the town's patron, Sankt Mauritius, located on the town hall and overlooking the square) roasted over a pine cone fire. The sausage is served in a Semmel (a small bread bun, a third the size of the sausage itself), and is highly popular with locals and tourists alike. According to rumours, the Coburger Bratwurst was first produced in 1530 on the occasion of a stay and boarding of Martin Luther and the Electoral Train.  Coburg Peak on Trinity Peninsula in Graham Land, Antarctica is named after the town, in connection with the Bulgarian royal house of Coburg (Saxe-Coburg-Gotha).

Infrastructure.

Transport.

Car.

Coburg can be reached by car via B 303 Schweinfurt-Coburg-Schirnding, B 4 Hamburg-Coburg-Nuremberg or motorway A 73 Suhl-Coburg-Nuremberg.

Railways.

Coburg has four train stations:  From the main station one can go to Lichtenfels, Bamberg, Forchheim, Erlangen, Fürth and Nuremberg, to Neustadt bei Coburg, Sonneberg, to Bad Rodach and to Kulmbach, Neuenmarkt-Wirsberg. Since December 2017, the Coburg station is served by Intercity Express high speed trains of the Munich-Nuremberg-Coburg-Erfurt-Berlin(-Hamburg) line (Nuremberg–Erfurt high-speed railway).

Airports.

Small planes can land on the two airfields:  Large airports nearby are in Frankfurt, Erfurt and Nuremberg.

Local public transport system.

The public transport system in Coburg is operated by SÜC ("Stadt- und Überlandwerke Coburg") with 9 bus lines. The OVF ("Omnibus Verkehr Franken") covers Coburg's surrounding countryside with an additional 11 bus lines.

Notable people.

Besides various royalty, other famous individuals associated with Coburg include Hans Berger (graduated), William Frishmuth (born), and Eduard Study (born).

Coburg () is a town located on the Itz river in the Upper Franconia region of Bavaria, Germany. Long part of one of the Thuringian states of the Wettin line, it joined Bavaria by popular vote only in 1920. Until the revolution of 1918, it was one of the capitals of the Duchy of Saxe-Coburg and Gotha and the Duchy of Saxe-Coburg-Saalfeld. Through successful dynastic policies, the ruling princely family married into several of the royal families of Europe, most notably in the person of Prince Albert, who married Queen Victoria in 1840. As a result of these close links with the royal houses of Europe in the late 19th and early 20th centuries, Coburg was frequently visited by the crowned heads of Europe and their families.

科堡(Coburg)是德国巴伐利亚州的无属县城市,位于巴伐利亚北部的边缘,隶属于上弗兰肯行政区,也是科堡郡的首府。它是大学和欧洲城市位于北巴伐利亚的一个中心。


Overview.

Computer aided engineering primarily uses Computer Aided Design (CAD) software, which are sometimes called CAE tools. CAE tools are being used, for example, to analyse the robustness and performance of components and assemblies. The term encompasses simulation, validation, and optimisation of products and manufacturing tools. In the future, CAE systems will be major providers of information to help support design teams in decision making. Computer-aided engineering is used in many fields such as automotive, aviation, space, and shipbuilding industries.  In regard to information networks, CAE systems are individually considered a single node on a total information network and each node may interact with other nodes on the network.  CAE systems can provide support to businesses. This is achieved by the use of reference architectures and their ability to place information views on the business process. Reference architecture is the basis from which information model, especially product and manufacturing models.  The term CAE has also been used by some in the past to describe the use of computer technology within engineering in a broader sense than just engineering analysis. It was in this context that the term was coined by Jason Lemon, founder of SDRC in the late 1970s. This definition is however better known today by the terms CAx and PLM.

CAE fields and phases.

CAE areas covered include:  In general, there are three phases in any computer-aided engineering task:  This cycle is iterated, often many times, either manually or with the use of commercial optimization software.

CAE in the automotive industry.

CAE tools are very widely used in the automotive industry. In fact, their use has enabled the automakers to reduce product development cost and time while improving the safety, comfort, and durability of the vehicles they produce. The predictive capability of CAE tools has progressed to the point where much of the design verification is now done using computer simulations (diagnosis) rather than physical prototype testing. CAE dependability is based upon all proper assumptions as inputs and must identify critical inputs (BJ). Even though there have been many advances in CAE, and it is widely used in the engineering field, physical testing is still a must. It is used for verification and model updating, to accurately define loads and boundary conditions and for final prototype sign-off.

The future of CAE in the product development process.

Even though CAE has built a strong reputation as a verification, troubleshooting and analysis tool, there is still a perception that sufficiently accurate results come rather late in the design cycle to really drive the design. This can be expected to become a problem as modern products become ever more complex. They include smart systems, which leads to an increased need for multi-physics analysis including controls, and contain new lightweight materials, to which engineers are often less familiar. CAE software companies and manufacturers are constantly looking for tools and process improvements to change this situation. On the software side, they are constantly looking to develop more powerful solvers, better use computer resources and include engineering knowledge in pre- and post-processing. On the process side, they try to achieve a better alignment between 3D CAE, 1D System Simulation and physical testing. This should increase modeling realism and calculation speed. On top of that, they try to better integrate CAE in the overall product lifecycle management. In this way, they can connect product design with product use, which is an absolute must for smart products. Such an enhanced engineering process is also referred to as predictive engineering analytics.

Computer-aided engineering (CAE) is the broad usage of computer software to aid in engineering analysis tasks. It includes,,, durability and optimization. It is included with computer-aided design (CAD) and computer-aided manufacturing (CAM) in the collective abbreviation "CAx".

计算机辅助工程(Computer Aided Engineering),主要用于模拟分析、验证和改善设计。在近年来电脑与3D CAD快速发展情况下,CAE应用比例越来越高,使用难度也越来越低,使用者数量大幅成长。与CAD相比,CAE使用者需要更多物理相关知识来设定条件与解释结果。至于计算过程所使用到的数学方法,如微分方程、有限元素法、有限体积法等全部交给电脑处理就可以。


Health.

A healthy diet may improve or maintain optimal health. In developed countries, affluence enables unconstrained caloric intake and possibly inappropriate food choices.  Health agencies recommend that people maintain a normal weight by limiting consumption of energy-dense foods and sugary drinks, eating plant-based food, limiting consumption of red and processed meat, and limiting alcohol intake.  The Dietary Guidelines for Americans is an evidence-based information source that policy makers and health professionals use to advise the general public about healthy nutrition.

Dietary choices.

Many people choose to forgo food from animal sources to varying degrees (e.g. flexitarianism, pescetarianism, vegetarianism, veganism) for health reasons, issues surrounding morality, or to reduce their personal impact on the environment, although some of the public assumptions about which diets have lower impacts are known to be incorrect. Raw foodism is another contemporary trend. These diets may require multivitamin supplements to meet ordinary nutritional needs.

Weight management.

A particular diet may be chosen to seek weight loss or weight gain. Changing a subject's dietary intake, or "going on a diet", can change the energy balance and increase or decrease the amount of fat stored by the body. The terms "healthy diet" and "diet for weight management" are often related, as the two promote healthy weight management.

Eating disorders.

An eating disorder is a mental disorder that interferes with normal food consumption. It is defined by abnormal eating habits and thoughts about food that may involve eating much more or much less than needed. Common eating disorders include anorexia nervosa, bulimia nervosa, and binge-eating disorder.

Religious and cultural dietary choices.

Some cultures and religions have restrictions concerning what foods are acceptable in their diet. For example, only Kosher foods are permitted by Judaism, and Halal foods by Islam. Although Buddhists are generally vegetarians, the practice varies and meat-eating may be permitted depending on the sects. In Hinduism, vegetarianism is the ideal. Jains are strictly vegetarian and consumption of roots is not permitted.

In nutrition, diet is the sum of food consumed by a person or other organism. The word diet often implies the use of specific intake of nutrition for health or weight-management reasons (with the two often being related). Although humans are omnivores, each culture and each person holds some food preferences or some food taboos. This may be due to personal tastes or ethical reasons. Individual dietary choices may be more or less healthy.

在营养学中的饮食是指人类或是其他生物摄取食物的总和,此一词语常用在为了健康或是体重管理而增加特别养份,或是特别调整的饮食。人是杂食动物,不过不同文化会对饮食有不同的喜好及禁忌,每个人对饮食的喜好也有不同。这些可能是因为个人的口味不同,也有可能是因为种族、习惯或是宗教的因素。个人的饮食选择可能是较健康的,也可能是较不健康的,