The noble gases make up a group of chemical elements with similar properties. The six noble gases that occur are helium, argon, krypton and the radioactive radon. Oganesson is variously predicted to be a noble gas as well or to break the trend due to relativistic effects. For the first six periods of the periodic table, the noble gases are the members of group 18. Noble gases are highly unreactive except when under particular extreme conditions; the inertness of noble gases makes them suitable in applications where reactions are not wanted. For example, argon is used in incandescent lamps to prevent the hot tungsten filament from oxidizing; the properties of the noble gases can be well explained by modern theories of atomic structure: their outer shell of valence electrons is considered to be "full", giving them little tendency to participate in chemical reactions, it has been possible to prepare only a few hundred noble gas compounds. The melting and boiling points for a given noble gas are close together, differing by less than 10 °C.
Neon, argon and xenon are obtained from air in an air separation unit using the methods of liquefaction of gases and fractional distillation. Helium is sourced from natural gas fields that have high concentrations of helium in the natural gas, using cryogenic gas separation techniques, radon is isolated from the radioactive decay of dissolved radium, thorium, or uranium compounds. Noble gases have several important applications in industries such as lighting and space exploration. A helium-oxygen breathing gas is used by deep-sea divers at depths of seawater over 55 m. After the risks caused by the flammability of hydrogen became apparent, it was replaced with helium in blimps and balloons. Noble gas is translated from the German noun Edelgas, first used in 1898 by Hugo Erdmann to indicate their low level of reactivity; the name makes an analogy to the term "noble metals", which have low reactivity. The noble gases have been referred to as inert gases, but this label is deprecated as many noble gas compounds are now known.
Rare gases is another term, used, but this is inaccurate because argon forms a considerable part of the Earth's atmosphere due to decay of radioactive potassium-40. Pierre Janssen and Joseph Norman Lockyer discovered a new element on August 18, 1868 while looking at the chromosphere of the Sun, named it helium after the Greek word for the Sun, ἥλιος. No chemical analysis was possible at the time, but helium was found to be a noble gas. Before them, in 1784, the English chemist and physicist Henry Cavendish had discovered that air contains a small proportion of a substance less reactive than nitrogen. A century in 1895, Lord Rayleigh discovered that samples of nitrogen from the air were of a different density than nitrogen resulting from chemical reactions. Along with Scottish scientist William Ramsay at University College, Lord Rayleigh theorized that the nitrogen extracted from air was mixed with another gas, leading to an experiment that isolated a new element, from the Greek word ἀργός. With this discovery, they realized.
During his search for argon, Ramsay managed to isolate helium for the first time while heating cleveite, a mineral. In 1902, having accepted the evidence for the elements helium and argon, Dmitri Mendeleev included these noble gases as group 0 in his arrangement of the elements, which would become the periodic table. Ramsay continued his search for these gases using the method of fractional distillation to separate liquid air into several components. In 1898, he discovered the elements krypton and xenon, named them after the Greek words κρυπτός, νέος, ξένος, respectively. Radon was first identified in 1898 by Friedrich Ernst Dorn, was named radium emanation, but was not considered a noble gas until 1904 when its characteristics were found to be similar to those of other noble gases. Rayleigh and Ramsay received the 1904 Nobel Prizes in Physics and in Chemistry for their discovery of the noble gases; the discovery of the noble gases aided in the development of a general understanding of atomic structure.
In 1895, French chemist Henri Moissan attempted to form a reaction between fluorine, the most electronegative element, argon, one of the noble gases, but failed. Scientists were unable to prepare compounds of argon until the end of the 20th century, but these attempts helped to develop new theories of atomic structure. Learning from these experiments, Danish physicist Niels Bohr proposed in 1913 that the electrons in atoms are arranged in shells surrounding the nucleus, that for all noble gases except helium the outermost shell always contains eight electrons. In 1916, Gilbert N. Lewis formulated the octet rule, which conc
A breathing gas is a mixture of gaseous chemical elements and compounds used for respiration. Air is the most common, only natural, breathing gas, but other mixtures of gases, or pure gases, are used in breathing equipment and enclosed habitats such as scuba equipment, surface supplied diving equipment, recompression chambers, space suits, medical life support and first aid equipment, high-altitude mountaineering and anaesthetic machines. Oxygen is the essential component for any breathing gas, at a partial pressure of between 0.16 and 1.60 bar at the ambient pressure. The oxygen is the only metabolically active component unless the gas is an anaesthetic mixture; some of the oxygen in the breathing gas is consumed by the metabolic processes, the inert components are unchanged, serve to dilute the oxygen to an appropriate concentration, are therefore known as diluent gases. Most breathing gases therefore are one or more inert gases. Other breathing gases have been developed to improve on the performance of ordinary air by reducing the risk of decompression sickness, reducing the duration of decompression stops, reducing nitrogen narcosis or allowing safer deep diving.
A safe breathing gas for hyperbaric use has three essential features: It must contain sufficient oxygen to support life and work rate of the breather. It must not contain harmful contaminants. Carbon monoxide and carbon dioxide are common poisons. There are many other possibilities, it must not become toxic. Oxygen and nitrogen are examples of gases; the techniques used to fill diving cylinders with gases other than air are called gas blending. Breathing gases for use at ambient pressures below normal atmospheric pressure are air enriched with oxygen to provide sufficient oxygen to maintain life and consciousness, or to allow higher levels of exertion than would be possible using air, it is common to provide the additional oxygen as a pure gas added to the breathing air at inhalation, or though a life-support system. These common diving breathing gases are used: Air is a mixture of 21% oxygen, 78% nitrogen, 1% other trace gases argon. Being cheap and simple to use, it is the most common diving gas.
As its nitrogen component causes nitrogen narcosis, it is considered to have a safe depth limit of about 40 metres for most divers, although the maximum operating depth of air is 66.2 metres. Pure oxygen is used to speed the shallow decompression stops at the end of a military, commercial, or technical dive. Risk of acute oxygen toxicity increases at pressures greater than 6 metres sea water, it was much used in frogmen's rebreathers, is still used by attack swimmers. Nitrox is a mixture of oxygen and air, refers to mixtures which are more than 21% oxygen, it can be used as a tool to accelerate in-water decompression stops or to decrease the risk of decompression sickness and thus prolong a dive. Trimix is a mixture of oxygen and helium and is used at depth in technical diving and commercial diving instead of air to reduce nitrogen narcosis and to avoid the dangers of oxygen toxicity. Heliox is a mixture of oxygen and helium and is used in the deep phase of a commercial deep dive to eliminate nitrogen narcosis.
Heliair is a form of trimix, blended from helium and air without using pure oxygen. It always has a 21:79 ratio of oxygen to nitrogen. Hydreliox is a mixture of oxygen and hydrogen and is used for dives below 130 metres in commercial diving. Hydrox, a gas mixture of hydrogen and oxygen, is used as a breathing gas in deep diving. Neox is a mixture of neon sometimes employed in deep commercial diving, it is used due to its cost. DCS symptoms produced by neon have a poor reputation, being reported to be more severe than those produced by an equivalent dive-table and mix with helium. Breathing gases for diving are classified by oxygen fraction; the boundaries set by authorities may differ as the effects vary with concentration and between people, are not predictable. Normoxic where the oxygen content does not differ from that of air and allows continuous safe use at atmospheric pressure. Hyperoxic, or oxygen enriched where the oxygen content exceeds atmospheric levels to a level where there is some measurable physiological effect over long term use, sometimes requiring special procedures for handling due to increased fire hazard.
The associated risks are oxygen toxicity at depth and fire in the breathing apparatus. Hypoxic where the oxygen content is less than that of air to the extent that there is a significant risk of measurable physiological effect over the short term; the immediate risk is hypoxic incapacitation at or near the surface. Breathing gases for diving are mixed from a small number of component gases which provide special characteristics to the mixture which are not available from atmospheric air. Oxygen must be present in every breathing gas; this is. The human body cannot store oxygen for use as it does with food. If the body is deprived of oxygen for more than a few minutes and death result; the tissues an
Integrated Authority File
The Integrated Authority File or GND is an international authority file for the organisation of personal names, subject headings and corporate bodies from catalogues. It is used for documentation in libraries and also by archives and museums; the GND is managed by the German National Library in cooperation with various regional library networks in German-speaking Europe and other partners. The GND falls under the Creative Commons Zero licence; the GND specification provides a hierarchy of high-level entities and sub-classes, useful in library classification, an approach to unambiguous identification of single elements. It comprises an ontology intended for knowledge representation in the semantic web, available in the RDF format; the Integrated Authority File became operational in April 2012 and integrates the content of the following authority files, which have since been discontinued: Name Authority File Corporate Bodies Authority File Subject Headings Authority File Uniform Title File of the Deutsches Musikarchiv At the time of its introduction on 5 April 2012, the GND held 9,493,860 files, including 2,650,000 personalised names.
There are seven main types of GND entities: LIBRIS Virtual International Authority File Information pages about the GND from the German National Library Search via OGND Bereitstellung des ersten GND-Grundbestandes DNB, 19 April 2012 From Authority Control to Linked Authority Data Presentation given by Reinhold Heuvelmann to the ALA MARC Formats Interest Group, June 2012
Natural gas is a occurring hydrocarbon gas mixture consisting of methane, but including varying amounts of other higher alkanes, sometimes a small percentage of carbon dioxide, hydrogen sulfide, or helium. It is formed when layers of decomposing plant and animal matter are exposed to intense heat and pressure under the surface of the Earth over millions of years; the energy that the plants obtained from the sun is stored in the form of chemical bonds in the gas. Natural gas is a occurring hydrocarbon used as a source of energy for heating and electricity generation, it is used as a fuel for vehicles and as a chemical feedstock in the manufacture of plastics and other commercially important organic chemicals. Natural gas is called a non-renewable resource. Natural gas is found in deep underground rock formations or associated with other hydrocarbon reservoirs in coal beds and as methane clathrates. Petroleum is another fossil fuel found in close proximity to and with natural gas. Most natural gas was created over time by two mechanisms: thermogenic.
Biogenic gas is created by methanogenic organisms in marshes, bogs and shallow sediments. Deeper in the earth, at greater temperature and pressure, thermogenic gas is created from buried organic material. In petroleum production gas is burnt as flare gas; the World Bank estimates that over 150 cubic kilometers of natural gas are flared or vented annually. Before natural gas can be used as a fuel, but not all, must be processed to remove impurities, including water, to meet the specifications of marketable natural gas; the by-products of this processing include: ethane, butanes and higher molecular weight hydrocarbons, hydrogen sulfide, carbon dioxide, water vapor, sometimes helium and nitrogen. Natural gas is informally referred to as "gas" when compared to other energy sources such as oil or coal. However, it is not to be confused with gasoline in North America, where the term gasoline is shortened in colloquial usage to gas. Natural gas was discovered accidentally in ancient China, as it resulted from the drilling for brines.
Natural gas was first used by the Chinese in about 500 BCE. They discovered a way to transport gas seeping from the ground in crude pipelines of bamboo to where it was used to boil salt water to extract the salt, in the Ziliujing District of Sichuan; the discovery and identification of natural gas in the Americas happened in 1626. In 1821, William Hart dug the first natural gas well at Fredonia, New York, United States, which led to the formation of the Fredonia Gas Light Company; the state of Philadelphia created the first municipally owned natural gas distribution venture in 1836. By 2009, 66 000 km³ had been used out of the total 850 000 km³ of estimated remaining recoverable reserves of natural gas. Based on an estimated 2015 world consumption rate of about 3400 km³ of gas per year, the total estimated remaining economically recoverable reserves of natural gas would last 250 years at current consumption rates. An annual increase in usage of 2–3% could result in recoverable reserves lasting less as few as 80 to 100 years.
In the 19th century, natural gas was obtained as a by-product of producing oil, since the small, light gas carbon chains came out of solution as the extracted fluids underwent pressure reduction from the reservoir to the surface, similar to uncapping a soft drink bottle where the carbon dioxide effervesces. Unwanted natural gas was a disposal problem in the active oil fields. If there was not a market for natural gas near the wellhead it was prohibitively expensive to pipe to the end user. In the 19th century and early 20th century, unwanted gas was burned off at oil fields. Today, unwanted gas associated with oil extraction is returned to the reservoir with'injection' wells while awaiting a possible future market or to repressurize the formation, which can enhance extraction rates from other wells. In regions with a high natural gas demand, pipelines are constructed when it is economically feasible to transport gas from a wellsite to an end consumer. In addition to transporting gas via pipelines for use in power generation, other end uses for natural gas include export as liquefied natural gas or conversion of natural gas into other liquid products via gas to liquids technologies.
GTL technologies can convert natural gas into liquids products such as diesel or jet fuel. A variety of GTL technologies have been developed, including Fischer–Tropsch, methanol to gasoline and syngas to gasoline plus. F–T produces a synthetic crude that can be further refined into finished products, while MTG can produce synthetic gasoline from natural gas. STG+ can produce drop-in gasoline, jet fuel and aromatic chemicals directly from natural gas via a single-loop process. In 2011, Royal Dutch Shell's 140,000 barrels per day F–T plant went into operation in Qatar. Natural gas can be "associated", or "non-associated", is found in coal beds, it sometimes contains a significant amount of ethane, propane and pentane—heavier hydrocarbons removed for commercial use prior to the methane being sold as a consumer fuel or chemical plant feedstock. Non-hydrocarbons such as carbon dioxide, nitrogen and hydrogen sulfide must be removed before the natural gas can be transported. Natural gas extracted from oil wells is called casinghead gas (whether or not produced up the a
Arc welding is a welding process, used to join metal to metal by using electricity to create enough heat to melt metal, the melted metals when cool result in a binding of the metals. It is a type of welding that uses a welding power supply to create an electric arc between a metal stick and the base material to melt the metals at the point of contact. Arc welders can use either direct or alternating current, consumable or non-consumable electrodes; the welding area is protected by some type of shielding gas, vapor, or slag. Arc welding processes may be manual, semi-automatic, or automated. First developed in the late part of the 19th century, arc welding became commercially important in shipbuilding during the Second World War. Today it remains an important process for the fabrication of steel vehicles. To supply the electrical energy necessary for arc welding processes, a number of different power supplies can be used; the most common classification is constant current power supplies and constant voltage power supplies.
In arc welding, the voltage is directly related to the length of the arc, the current is related to the amount of heat input. Constant current power supplies are most used for manual welding processes such as gas tungsten arc welding and shielded metal arc welding, because they maintain a constant current as the voltage varies; this is important because in manual welding, it can be difficult to hold the electrode steady, as a result, the arc length and thus voltage tend to fluctuate. Constant voltage power supplies hold the voltage constant and vary the current, as a result, are most used for automated welding processes such as gas metal arc welding, flux cored arc welding, submerged arc welding. In these processes, arc length is kept constant, since any fluctuation in the distance between the wire and the base material is rectified by a large change in current. For example, if the wire and the base material get too close, the current will increase, which in turn causes the heat to increase and the tip of the wire to melt, returning it to its original separation distance.
The direction of current used in arc welding plays an important role in welding. Consumable electrode processes such as shielded metal arc welding and gas metal arc welding use direct current, but the electrode can be charged either positively or negatively. In general, the positively charged anode will have a greater heat concentration. "Note that for stick welding in general, DC+ polarity is most used. It produces a good bead profile with a higher level of penetration. DC- polarity results in less penetration and a higher electrode melt-off rate, it is sometimes used, for example, on thin sheet metal in an attempt to prevent burn-through." "With few exceptions, electrode-positive results in deeper penetration. Electrode-negative results in faster melt-off of the electrode and, faster deposition rate." Non-consumable electrode processes, such as gas tungsten arc welding, can use either type of direct current, as well as alternating current. With direct current however, because the electrode only creates the arc and does not provide filler material, a positively charged electrode causes shallow welds, while a negatively charged electrode makes deeper welds.
Alternating current moves between these two, resulting in medium-penetration welds. One disadvantage of AC, the fact that the arc must be re-ignited after every zero crossing, has been addressed with the invention of special power units that produce a square wave pattern instead of the normal sine wave, eliminating low-voltage time after the zero crossings and minimizing the effects of the problem. Duty cycle is a welding equipment specification which defines the number of minutes, within a 10-minute period, during which a given arc welder can safely be used. For example, an 80 A welder with a 60% duty cycle must be "rested" for at least 4 minutes after 6 minutes of continuous welding. Failure to observe duty cycle limitations could damage the welder. Commercial- or professional-grade welders have a 100% duty cycle. One of the most common types of arc welding is shielded metal arc welding, known as manual metal arc welding or stick welding. An electric current is used to strike an arc between the base material and a consumable electrode rod or stick.
The electrode rod is made of a material, compatible with the base material being welded and is covered with a flux that gives off vapors that serve as a shielding gas and provide a layer of slag, both of which protect the weld area from atmospheric contamination. The electrode core itself acts as filler material; the process is versatile, requiring little operator training and inexpensive equipment. However, weld times are rather slow, since the consumable electrodes must be replaced and because slag, the residue from the flux, must be chipped away after welding. Furthermore, the process is limited to welding ferrous materials, though specialty electrodes have made possible the welding of cast iron, aluminium and other metals; the versatility of the method makes it popular in a number of applications including repair work and construction. Gas metal arc welding called MIG, is a semi-automatic or automatic welding process with a continuously fed consumable wire acting as both electrode and filler metal, along with an inert or semi-inert shielding gas flowed around the wire to protect the weld site from contamination.
Constant voltage, direct current power source is most us
Sodium benzoate is a substance which has the chemical formula C6H5COONa. It is a used food preservative, with an E number of E211, it exists in this form when dissolved in water. It can be produced by reacting sodium hydroxide with benzoic acid. Sodium benzoate is produced by the neutralization of benzoic acid, itself produced commercially by partial oxidation of toluene with oxygen. Sodium benzoate occurs along with benzoic acid and its esters, in many foods. Fruits and vegetables can be rich sources berries such as cranberry and bilberry. Other sources include seafood, such as prawns, dairy products like milk and yogurt. Sodium benzoate is a preservative, with the E number E211, it is most used in acidic foods such as salad dressings, carbonated drinks and fruit juices, pickles and frogurt toppings. It is used as a preservative in medicines and cosmetics. Under these conditions it is converted into benzoic acid, bacteriostatic and fungistatic. Benzoic acid is not used directly due to its poor water solubility.
Concentration as a food preservative is limited by the FDA in the U. S. to 0.1% by weight. Sodium benzoate is allowed as an animal food additive at up to 0.1%, according to AFCO's official publication. Sodium benzoate has been replaced by potassium sorbate in the majority of soft drinks in the United Kingdom. Sodium benzoate is used as a treatment for urea cycle disorders due to its ability to bind amino acids; this leads to excretion of a decrease in ammonia levels. Recent research shows. Total Positive and Negative Syndrome Scale scores dropped by 21% compared to placebo. Sodium benzoate, along with phenylbutyrate, is used to treat hyperammonemia. Sodium benzoate is used in fireworks as a fuel in whistle mix, a powder that emits a whistling noise when compressed into a tube and ignited; the mechanism starts with the absorption of benzoic acid into the cell. If the intracellular pH falls to 5 or lower, the anaerobic fermentation of glucose through phosphofructokinase decreases which inhibits the growth and survival of microorganisms that cause food spoilage.
In the United States, sodium benzoate is designated as recognized as safe by the Food and Drug Administration. The International Programme on Chemical Safety found no adverse effects in humans at doses of 647–825 mg/kg of body weight per day. Cats have a lower tolerance against benzoic acid and its salts than rats and mice; the human body clears sodium benzoate by combining it with glycine to form hippuric acid, excreted. The metabolic pathway for this begins with the conversion of benzoate by butyrate-CoA ligase into an intermediate product, benzoyl-CoA, metabolized by glycine N-acyltransferase into hippuric acid. In combination with ascorbic acid, sodium benzoate and potassium benzoate may form benzene; when tested by the FDA, most beverages that contained both ascorbic acid and benzoate had benzene levels that were below those considered dangerous for consumption by the World Health Organization. Most of the beverages that tested higher have been reformulated and subsequently tested below the safety limit.
Heat and shelf life can increase the rate at which benzene is formed. Research published in 2007 for the UK's Food Standards Agency suggests that certain artificial colors, when paired with sodium benzoate, may be linked to hyperactive behavior; the results were inconsistent regarding sodium benzoate, so the FSA recommended further study. The Food Standards Agency concluded that the observed increases in hyperactive behavior, if real, were more to be linked to the artificial colors than to sodium benzoate; the report's author, Jim Stevenson from Southampton University, said: "The results suggest that consumption of certain mixtures of artificial food colours and sodium benzoate preservative are associated with increases in hyperactive behaviour in children.... Many other influences are at work but this at least is one a child can avoid." British Pharmacopoeia European Pharmacopoeia Food Chemicals Codex Japanese Pharmacopoeia United States Pharmacopeia Acceptable daily intake List of investigational antipsychotics Potassium benzoate International Programme on Chemical Safety - Benzoic Acid and Sodium Benzoate report Kubota K, Ishizaki T. "Dose-dependent pharmacokinetics of benzoic acid following oral administration of sodium benzoate to humans".
Eur. J. Clin. Pharmacol. 41: 363–8. Doi:10.1007/BF00314969. PMID 1804654. Although the maximum rate of biotransformation of benzoic acid to hippuric acid varied between 17.2 and 28.8 mg.kg-1.h-1 among the six individuals, the mean value was close to that provided by daily maximum dose recommended in the treatment of hyperammonaemia in patients with inborn errors of ureagenesis Safety data for sodium benzoate The Ketchup Conundrum
A tank is an armoured fighting vehicle designed for front-line combat, with heavy firepower, strong armour, tracks and a powerful engine providing good battlefield manoeuvrability. They are a key part of combined arms combat. Modern tanks are versatile mobile land weapon system platforms, mounting a large-calibre cannon in a rotating gun turret, supplemented by mounted machine guns or other weapons, such as ATGMs, or rockets, they combine this with heavy vehicle armour which provides protection for the crew, the vehicle's weapons, its propulsion systems, operational mobility, due to its use of tracks rather than wheels, which allows the tank to move over rugged terrain and adverse conditions such as mud, be positioned on the battlefield in advantageous locations. These features enable the tank to perform well in a variety of intense combat situations both offensively with fire from their powerful tank gun, defensively due to their near invulnerability to common firearms and good resistance to heavier weapons, all while maintaining the mobility needed to exploit changing tactical situations.
Integrating tanks into modern military forces spawned a new era of combat, armoured warfare. There are classes of tanks, some being larger and heavily armoured, with high calibre guns, while others smaller armoured, equipped with a smaller calibre, lighter gun; these smaller tanks move over terrain with speed and agility and can perform a reconnaissance role in addition to engaging enemy targets. The smaller faster tank would not engage in battle with a larger armoured tank, except during a surprise flanking manoeuvre; the modern tank is the result of a century of development from the first primitive armoured vehicles, due to improvements in technology such as the internal combustion engine, which allowed the rapid movement of heavy armoured vehicles. As a result of these advances, tanks underwent tremendous shifts in capability in the years since their first appearance. Tanks in World War I were developed separately and by Great Britain and France as a means to break the deadlock of trench warfare on the Western Front.
The first British prototype, nicknamed Little Willie, was constructed at William Foster & Co. in Lincoln, England in 1915, with leading roles played by Major Walter Gordon Wilson who designed the gearbox and hull, by William Tritton of William Foster and Co. who designed the track plates. This was a prototype of a new design that would become the British Army's Mark I tank, the first tank used in combat in September 1916 during the Battle of the Somme; the name "tank" was adopted by the British during the early stages of their development, as a security measure to conceal their purpose. While the British and French built thousands of tanks in World War I, Germany was unconvinced of the tank's potential, built only twenty. Tanks of the interwar period evolved into the much larger and more powerful designs of World War II. Important new concepts of armoured warfare were developed. Less than two weeks Germany began their large-scale armoured campaigns that would become known as blitzkrieg – massed concentrations of tanks combined with motorised and mechanised infantry and air power designed to break through the enemy front and collapse enemy resistance.
The widespread introduction of high-explosive anti-tank warheads during the second half of World War II led to lightweight infantry-carried anti-tank weapons such as the Panzerfaust, which could destroy some types of tanks. Tanks in the Cold War were designed with these weapons in mind, led to improved armour types during the 1960s composite armour. Improved engines and suspensions allowed tanks of this period to grow larger. Aspects of gun technology changed as well, with advances in shell design and aiming technology. During the Cold War, the main battle tank concept became a key component of modern armies. In the 21st century, with the increasing role of asymmetrical warfare and the end of the Cold War, that contributed to the increase of cost-effective anti-tank rocket propelled grenades worldwide and its successors, the ability of tanks to operate independently has declined. Modern tanks are more organized into combined arms units which involve the support of infantry, who may accompany the tanks in infantry fighting vehicles, supported by reconnaissance or ground-attack aircraft.
The tank is the 20th century realization of an ancient concept: that of providing troops with mobile protection and firepower. The internal combustion engine, armour plate, continuous track were key innovations leading to the invention of the modern tank. Many sources imply that Leonardo da Vinci and H. G. Wells in some way "invented" the tank. Leonardo's late 15th century drawings of what some describe as a "tank" show a man-powered, wheeled vehicle with cannons all around it; however the human crew would not have enough power to move it over larger distance, usage of animals was problematic in a space so confined. In the 15th century, Jan Žižka built armoured wagons containing cannons and used them in several battles; the continuous "caterpillar" track arose from attempts to improve the mobility of wheeled vehicles by spreading their weight, reducing ground pressure, increasing their traction. Experiments can be traced back as far as the 17th century, by the late nineteenth they existed in various recognizable and practical forms in several countries.
It is frequen