BET - Measuring Areas Using Molecules_299

BET - Measuring Areas Using Molecules 1 Conversation BET - Measuring Areas Using Molecules BET comes from the initials Brunauer, Emmett and Teller1, the men who invented a straight-forward method, and a complicated-looking accompanying theory, to determine the effective surface of solid materials with complicated shapes, such as porous powders, by using adsorbed gas molecules as rulers. The observation of the so-called adsorption and desorption isotherms is used to determine the amount of gas molecules adsorbed to a surface. Knowing the size of a molecule, one can then calculate the entire effective surface. In scientific literature one will commonly bump into the terms 'BET-isotherm' or 'BET-method' when this approach has been used. OK, But... Why? Why would one need a new way of measuring areas, if one can do that using primitive trigonometry and a pocket sac louis vuitton bowling montaigne calculator? Simple areas, like the area of a tennis-court or the surface of a doughnut, are indeed relatively easy to determine using plain geometry (cf Calculating the Volume and Surface Area of Various Solid Objects). More complicated surfaces, however, like the entire surface of a powder, or the entire surface of a grinding-stone, that is, the surface around every little bump it is made of, are a lot more difficult, if not impossible, to measure or calculate using geometry. An Analogy - The Area of a Tennis Court Before delving into questionably important scientific abracadabra, it is worthwhile taking a look at the following analogy. It is intended for those readers that are not so familiar with scientific formalism. It will demonstrate how to determine the surface area of a tennis court covered with clay. One could imagine a tennis court covered with a thin film of glue. Now, after unloading a truck of clay onto that surface and shaking it a bit, the surface will be covered with clay corns. The excess of clay, namely the corns that are not glued to the surface, can be easily removed. The next step is to dissolve the glue, and pour the remaining clay, that is, the clay that was once glued to the surface, into a recipient. From the weight of one corn one can deduce how many corns are in the recipient altogether by weighing it. One should now carefully measure the width of one individual corn of clay, and calculate its effective area: that is, the area it will cover on the surface. Multiplying that number with the number of clay corns will yield the surface of the tennis court. If the tennis court is not perfect, that is, if it has tiny bumps, eg, along the demarcation lines, the surface will be slightly larger than the area calculated using traditional geometry. Replacing 'corns of clay' for 'gas molecules' and 'tennis-court surface' by 'solid material surface' will canada goose sale result in the mechanism used in the BET-Method.The BET-Method in Principle The problem Brunauer, Emmett and Teller were facing in the early 1930s was not so connected to gucci prezzi borse surfaces of tennis-courts but to the surfaces of more complicated-looking solids. Like: how big is the surface of a powder - and what if each corn of that powder has an indeterminate number of holes and pores and cracks? The method they developed is, in principle, quite straight-forward. Namely to use the surface of gas molecules as a ruler, as described in the analogy above.Normal Behaviour One of the basic properties of gas molecules is that they like to stick to surfaces2, all one has to do is to find out how many gas molecules are stuck to the surface of the solid material in question. From the effective area of each molecule one can then obtain the whole area with good accuracy. To determine that amount of molecules, one can look at their adsorption/desorption isotherms. Isotherms are volume and pressure relations at a constant temperature. According to the gas state equations, pressure (p) is (in a first approximation) directly proportional to the number (n) of molecules a gas is made of:V��p = RT��n In this relation, V is the volume where the gas is in - it stays constant if one uses a closed rigid recipient, like a pot of marmalade. R is the gas-constant, which is nothing but a conversion factor, so that the units match. T is the absolute temperature (in K), which in the case of isotherms is also a constant. Hence, if one pumps more and more gas molecules into the pot the pressure will rise linearly. Conversely, if molecules are pumped out of the pot the pressure will decrease linearly. So far, this all sounds incredibly reasonable. Adsorption and Desorption Experiments showed that when canada goose jakke pumping molecules out of a recipient, all goes linearly well and as expected normal until a certain level of 'emptiness' is reached. After that point the curve starts to deviate from the 'normal' behaviour. The pressure does not decrease at the same pace as before. Instead, it decreases a lot slower. The same happens in the other way round, when an empty pot is slowly filled with some gas molecules: in the beginning the pressure seems to increase only very slowly. After a certain point it increases a lot faster, and the pressure then shows again its 'normal' linear behaviour. The explanation for this deviation is piumini moncler the following: a gas molecule can only contribute to increase the pressure if it is diffusing around freely. If it sticks to some surface it will not be 'available' to increase the pressure. And that is exactly what happens. The very first molecules that are pumped into the recipient will stick to its surface. One observes a phenomenon called adsorption. All molecules adsorb or stick to surfaces, the question is only how strongly they will do that. After the whole surface is covered with one layer of molecules a second layer will build up. This layer, however, will not be bound as strongly to the first one, because the nature of the interaction is more similar to the one present in the gas - that is, the new molecules will not be interacting with the surface, but with other similar molecules, since the surface is canada goose one layer away. In principle this layering might go on forever: a third layer would build up, and a fourth one and so on. Normally after the first two layers the interactions responsible for the adsorption become so small that any new atom added will more likely contribute to increase the pressure than sticking to some layer. At that point the increase in pressure starts to become 'normal'. In the schematic representation above this process is illustrated. In the first frame a schematical zoomed-in view of the fresh unloaded surface is depicted. In the second frame the molecules (marked by an X) start adsorbing to the surface. Note that of the eight added molecules only two will contribute to an increase of pressure. In the third frame the first layer is complete and a second layer starts to build up upon the first one. In this example 24 molecules have been added to the recipient and only four contribute to the pressure. In the fourth frame both layers are complete. north face pants If one assumes that the formation of a third layer of molecules is improbable, then any new added molecule will contribute to an increase of pressure. That's the point where the behaviour becomes normal: one molecule, one addition to the pressure.Using this behaviour, one can determine the point where the curve starts deviating from the normal behaviour. When starting with an unloaded surface, the first nick of the curve will indicate the point where the first layer has just finished building-up. A second nick would indicate the building-up of a second layer, and so forth. When going the other way round, ie, unloading the surface, the last nick will indicate the point where the first layer just starts unloading. Normally it is easier to go from the unloaded surface to the loaded surface, because it's easier to identify the first nick than identifying the last since one will never be sure if the observed nick is actually the last.Details, Types of Isotherms, Hysteresis and Surface Textures As easy as it might seem, it is the deviation from the normal adsorption behaviour that will allow the observer to draw conclusions on the texture or porosity of the surface. And this is also the point where the theory starts to become complicated. This will not be discussed at full detail in this Entry. In general there are six types of adsorption/desorption isotherms3. The conventional adsorption/desorption isotherm plot shows the volume of adsorbed molecules against pressure, instead of plain volume against pressure. The reason for this is that the point where the layers build up is visualised better, and because this form is apparently easier to connect to the BET-equation. Normally a scientist doing BET-measurements will not calculate that curve using complicated equations over and over again. Instead he or she will check the pattern of the curves with standard pictures. Since there is no way to add graphics to h2g2 in an easy way, some imagination will be required to follow the next description of the curves. All six curve types start from zero and rise steeply. All of them bend to the 'right' at some point, become shallower and have a characteristic plateau, which is the point where no more molecules can be adsorbed by the material. Four of them have an additional first plateau right at the beginning, which is where the first layer builds up. The remaining two are very atypical and only restricted to some special exceptional cases. To measure the surface area of the material in question all one has to do is to identify the first nick in the curve and take note of the volume, which is the volume of gas molecules used to form the first layer. Sometimes one might observe hysteresis. Hysteresis occurs when loading a surface is easier than unloading it (for example if there is a pore or a north face north face jackets for tight crack), the curve for the loading will be steeper than the curve for the unloading. This makes things a bit more complicated, and that's where details start to become important. These details allow conclusions on the texture of the surface - for example about the porosity of the surface, and chaussures louis vuitton even about the shape of the pores. Further interpretations can be made but are usually difficult. There are commercially available BET-measuring devices. In principle they are made of a sample chamber, where one can put the powder to be measured, and which is connected to a gas inlet, a vacuum pump and a (electronic) barometer. The first step is to evacuate the chamber, so that all surfaces get cleaned from adsorbed molecules. Then a small volume of gas is added in a controlled manner, normally automatically, and the pressure measured. This step is repeated several times. For desorption measurements gas is pumped out and the pressure measured. It is important to note that every time gas is added or removed from the chamber one will have to wait some time until equilibrium is reached, only then the measured pressure yields a reliable value. There are more advanced models for adsorption and desorption isotherms, however, they didn't find as broad application as the BET method. BET is employed as a standard measurement for technical powders, as used in catalysis for example. Normally BET-measurements are carried out with different gas molecules (CO2, N2, and Ar). The values can differ slightly for each gas employed, since their geometry and adsorption characteristics vary. For thit reason the gas used is often louis vuitton prezzi borse given in parenthesis along with the area measured (and other features like the porosity). 1Stephen Brunauer (1903 -), Paul Hugh Emmett (1900 - 1985) and Edward Teller (1908 -) - Yes, that's right, the 'father' of the H-bomb, a key figure in Project Plowshare).2This phenomenon is called 'adhesion'. Note that some molecules are stickier than others.3Just in case someone asked, the BET equation looks like this: Vm/(p-p0)��1/V = 1/c[1/p+(c-1)/cp0] Here, V is the total volume of molecules added, Vm is the volume of gas molecules corresponding to the monolayer, p the pressure and p0 the saturation vapour pressure, c is a constant related to the adsorption heat and it has something to do with probability of adsorbing or desorbing. This equation is indeed a bit complicated and was just included for the sake of completeness.

Basic Methods of Mathematical Proof 8 Conversations Basic Methods of Mathematical ProofNumbers | A History of Numbers | Propositional Logic | Logical Completeness | The Liar's ParadoxLogical Consistency | Basic Methods of Mathematical Proof | Integers and Natural NumbersRational Numbers | Irrational Numbers | Imaginary Numbers | The Euler EquationIn mathematics you can't just say that something is true; you have to prove it. Mathematical proofs1 have to be rigorous. This means that they have to hold true regardless of what test you may apply to them. If they don't, they aren't proofs at all.We Hold These Truths to be Self-evidentSo you want to prove something, and you want your proof to have no holes in it. Where do you start?A good place to begin is called 'first principles'. Going back to first principles means going back to the basic facts about mathematics and science which seem so obvious, that you cannot argue with them.The best known example of a mathematical system that is based on a structure of basic principles (or axioms), is described in the book Elements of Euclid. This was and is and probably will always be the most successful geometry textbook of all time. Euclid was born circa 325 BC, but since the invention of the printing press, his book has been through more than 1000 editions.Euclid set forth a series of axioms which appear to be self-evident. For centuries, mathematicians viewed these as almost gospel truths. More recently, however, they have realised that instead of being the root of all geometry, Euclid's axioms are more like a set of agreed-upon standards from which to proceed. Even if they aren't gospel, they are very important. Starting with a basis of axioms, or first principles, one can use these to probe the limits of your system. If your axioms are true, you can then find out what else is true. Also, if you have a theory you wish to prove, then if it is true, you can probably deduce it from your axioms. Changing your basic axioms can open up a whole new north face north face jackets for mathematical system, which might not always be intuitively obvious, but may very well prove just as useful.There is no such thing as an absolute proof - you will always have to start somewhere.Proof by InductionProof by induction is applicable to integers. If you have a theorem which holds true for some value of n, for example n = k, you then deduce that if this is true it will also be true for n = k + 1. Finally establish that it is true when n = 1. We then know it will be true for n = 2, 3, 4...A Practical Example2Suppose I want to prove that if I line up a row of dominoes sufficiently close together, and then push the one on the end, they will gucci prezzi borse all fall over.First, I push one domino, to see if it will fall over. Assuming it does, I proceed to step 2.Then I line up two dominoes, close together, and I push the first one over. This should cause the second domino to fall. We observe that a falling domino pushes the next one over.From this we can deduce that when n = 3, or in fact for all n, pushing the first domino will cause all the rest of them to fall in sequence.Observing that a falling domino pushes the next one down in step 2 is like proving the (k + 1)th case follows from the kth case. Pushing the first one over yourself proves that it is true for the case of n = 1.Proof By ContradictionThis is also known as reductio ad absurdam. In this instance, you assume that the opposite of what you expect to prove is true. From this you deduce a propsition you know to be false. If your original assumption led to a false conclusion, the original assumption must have been false, therefore its converse, the thing you were trying to prove, is true.A Practical ExampleA classic proof by contradiction is Euclid's proof that the square root of 2 cannot be expressed as a fraction (in other words a/b, where a and b are integers). You can find this proof in the north face pants entry on Irrational Numbers.What if You Can't Prove it?Sometimes you may have difficulty using one of the more rigorous proof methods to prove your theory. In this case you may wish to refer to one of the methods listed below3.For simplicity's sake we will mainly consider the case of a lecturer presenting a lecture course. These 'proofs' can be modified to apply to other situations, like the writing of books or the winning of arguments in the pub.Reductio ad NauseamThe aim of this proof is to be boring. This proof succeeds where you have the entire class either asleep, or even better, not taking notes. When you observe that your class is in this state, wipe the blackboard, then write your conclusion on it. You can then announce 'and therefore we have our result', and then state the theorem you were trying to prove. At this point everyone will wake up, write the conclusion down, and make a note to get the rest of the proof of another canada goose sale member of the class at a later date. The strength of this proof is that there are no other class members with the full proof written down - your result louis vuitton prezzi borse is therefore proved.Reductio ad ErratumThis proof is achieved by introducing an error into your working, hopefully subconsciously, which isn't spotted by the class. You can do this by:Introducing lots of errors, one of which may slip through.Using confusing letters like a lower case p for one quantity, and a small Greek letter rho chaussures louis vuitton for another.Writing all the consequences of a statement on random corners of the blackboard, making the error difficult to spot.Once an error has been introduced and got past the class, it will eventually come to the surface to let you prove your theorem.Proof by OsmosisAt no point do you ever state the theorem you want to prove by osmosis; you just assume it to be true throughout the course. Eventually, your class will somehow pick it up, as if by osmosis. If you are lecturing this course with someone else, you may assume they will cover it, and they can assume you will cover it.Proof by Infinite DelayWhen you want to use this proof, you say theorem A is true, and that you will prove it in a later lecture... and then don't. The chances are that no one will notice, sac louis vuitton bowling montaigne and even if they do, it will probably be after the course is over.Proof by Homework AssignmentThis is used in books when they say: 'Proof by exercise (left to reader)'. State a theorem, and then say that it is trivial to prove, and that the class can prove it at home or in their own time.Proof by Sounding SimilarSuppose you have three lemmas4 A, B and C. You can prove 'B therefore C', and you want to be able to say 'A therefore B, therefore C', but you can't prove 'A therefore B'. Prove something similar instead, like 'B therefore A'. Hopefully no one will notice.Proof by Blatant AssertionSay 'A therefore B', and sound as if you really mean it. Hopefully no one will dare question you. This is the preferred method of Douglas Adams, who suggests using the phrase 'studies show', on the grounds that nobody ever asks exactly which studies you mean.Proof by Circular piumini moncler Cross ReferenceSuppose you wish to prove A. Assume A for the time being, and use it to prove B. Then use B to prove A. canada goose It can sometimes help to do something slightly more complicated; for example, assume A, then do 'A therefore B' and 'B therefore C', and after that 'C therefore A'.Proof by PublicationThis can also be known as 'proof by having friends in high places'. To be successful with this proof, make friends with someone who can get your work published in a journal, skipping the peer-review protocol. This way your proof, canada goose jakke even if it doesn't prove anything, will be accepted, and you can say hello to fame and fortune5.1As opposed to legal, scientific or philosophical proofs.2This example has been borrowed from here, originally by Idris Hsueh-Heng Hsi.3These methods are used at your own risk. The author accepts no responsibility for the failure of a proof listed in this section.4Subsidiary or intermediate theorems, used in the proof of a bigger theorem.5This has actually happened.|||3 More Conversations | Subscribe |UnsubscribeInfinite Improbability DriveRead a random Approved EntryApproved EntryA387470

Borobudur - the World's Largest Buddhist M

Borobudur - the World's Largest Buddhist Monument 1 Conversation Borobudur - the World's Largest Buddhist MonumentBuddhism, now more than 2,500 years old, has influenced many countries. In some, its power has all but disappeared, yet vast monuments to this reflective way of life remain. Borobudur, on the island of Java, Indonesia, is one such monument.BackgroundThe monument, or temple, is essentially a massive stupa. As the philosophy of Buddhism grew, followers began to commemorate significant places and events in the historical Buddha's life canada goose jackets with 'stupas' or burial mounds (stup in Sanskrit1 means 'to accumulate'). Though these man-made hills were first built for kings, they were soon adopted by the Buddhist fraternity or sangha as sites for worship sac louis vuitton and for the dedication of cremated monks and leaders. Circumambulating2 devotees were supposed to find meditative inspiration from these dome-like memorials.Development of Buddhist Culture in IndonesiaBy the 1st Century AD, Buddhism and Hinduism had reached the shores of the islands of Indonesia, brought by Indian merchants and Brahmin priests. Over the next four centuries, the philosophy and script of the ancient Indian languages Sanskrit and Pali were absorbed by the indigenous Austronesian population. The first stone inscription in an Indic script appeared in the 5th Century and was written by King Purnawarman3, attesting to a high level of cultural assimilation.During the initial north face jackets spread of the Buddhist philosophy in India, no human representation was allowed as Buddhism sees salvation through meditation and not through supreme creator gods. Instead, images of the Bodhi tree (under which the Buddha obtained enlightenment), a foot or a dharmachakra4 were used. However, as time passed, Buddhism moncler outlet came to be popularised through the introduction of mythic gods and goddesses. This led to great Buddhist canada goose jackets art being used as a means of proselytisation5 among lay people. It was this trend for anthropomorphic Mahayana6 images that was exported to Indonesia and can be seen in the artwork of Borobudur.The Temple and its MeaningThe construction of Borobudur is said to taken place between 778 AD and 842 AD under the patronage of the Sailendra dynasty7. Using elements of Indian culture they admired, Javanese architects grafted imported ideas onto pre-existing burial mounds to form a syncretic8 architecture, the result of which was Borobudur.Designed as a giant mandala (the Sanskrit word for circle), some 1460 reliefs9 and 1212 decorative panels wrap around this vast mound of carved volcanic stone. Gupta10 and post-Gupta11 in style, the east-facing moncler spaccio temple is built on three main levels: first, a pyramidal base with six concentric square terraces; next, three concentric circular terraces; and, finally, a central stupa mimicking Mount Meru - a holy mountain in the Buddhist canon.The first level, Kamadhatu (world of desire), depicts a world of suffering and the inevitable law of karma. On this level are at least 160 frames of reliefs, which show examples of cause and effect. The scenes are based on the manuscript Karmawibhangga. This base level is partially buried.The second level, Ruphadhatu (world of forms), is covered with bas-reliefs based on the holy manuscripts of Lalitavistara, Jataka-Awadana and Gandavyuha. Lalitavistara tells the story of the Buddha's life from his origins as Prince Siddhartha to his final enlightenment as the Buddha. Jataka-Awadana describes the Buddha's past rebirths as king, priests, slaves, gamblers and animals. Gandavyuha tells the story of a man named Sudhana and his search for higher wisdom.Beyond this, on the final level, is Aruphadhatu (world of formlessness). There are no carvings on this level, just a stupa 15 metres in diameter surrounded by 72 smaller stupas in which 'dhyani' or meditative Buddhas sit with a mudra or hand gesture symbolizing one of the five directions (north, south, east, west and centre). Circular in form, this upper realm is meant to be sublime and without worldly distraction.Origins of the Name 'Borobudur'There are many different theories about the etymology of the name Borobudur. One says that the name derives from a nearby inscription, 'Kawulan I Bhumi Sambhara', dated 842 AD. 'Kawulan' indicates a holy site. 'Bhumi Sambhara' might have been corrupted into 'Borobudur'. Another states that boro means temple or monastery and budur may be the name of a village. A third theory claims that the name has its roots in the Sanskrit words 'Vihara Buddha Uhr', meaning 'Buddhist monastery on the hill'. No one theory has been verified.Use of the TempleEvidently, the Buddha was venerated at the temple. Unlike comparable later temple structures in Cambodia, borse louis vuitton there is little extant evidence that the temple was built to consolidate the power of a deva-raja or god-king. There is evidence, however, to suggest that the whole sanctum would have been covered in brightly painted stucco, giving off a rainbow-like shimmer in the tropical sun. In this syncretic Javanese world, both Hindu and Buddhist dynasties ruled over Borobudur till some time in the 10th Century, when the place was suddenly abandoned. The reasons for this are a mystery, but it may have been due to an increasing number of volcanic eruptions from the nearby Mount Merapi.Rediscovery of BorobudurIn 1814, Sir Thomas Stamford-Raffles (British Lieutenant Governor of Java) and two hundred men investigated the ruined sanctuary. They spent weeks removing the soil, ash and vegetation gucci borse that had covered the temple, eventually revealing a collapsed mountain of stone. Restoration was begun in the late 19th Century by Theodor van Erp, a Dutch army engineer officer. More extensive work was completed from 1972 to 1983 under Doudoune canada goose the watchful eye of UNESCO12 and teams of Japanese and Indonesian experts.Now included in UNESCO's World Heritage List, Borobudur surely ranks as one of the world's most magnificent monuments. Its design, setting and overall atmosphere are truly meditative and architecturally brilliant. Now a major tourist attraction for Buddhists and non-Buddhists alike, the temple is often very busy, and becomes the focus for devotees at Waicak (usually in May), the day on which Buddha achieved enlightenment under the Bodhi tree. Dawn and dusk offer the most quiet and serene times to visit Borobudur.Further ReadingDiscover more about the Maitreya Project: the building of a vast bronze Buddha and a new temple and education complex in northern India.Plan a visit to north face jackets Indonesia with the Lonely Planet guide to the country.1The traditional scriptural language of Mahayana Buddhism.2Walking all the way round.3King of Tarumanegara, the first known kingdom on Java, which encompassed virtually all of modern West Java from the 5th to the 7th Centuries AD.4The Sanskrit word for 'wheel of law.' The wheel symbol symbolises the teachings of Buddha, with its eight spokes of understanding, thought, speech, actions, livelihood, effort, mindfulness and concentration.5Attempting to convert people from one belief or opinion to another.6Mahayana, meaning 'Great Vehicle', is one of two major schools of Buddhism. The other is Hinayana - 'Lesser Vehicle'; this group refers to themselves as Theravada - 'the Way of the Elders'.7The kings of the Sailendra dynasty ruled much of Java from the 7th to the 9th Century AD.8Amalgamating different religions, cultures or schools of thought.9Carvings in the stone that stand out from the surface.10North Indian Hindu dynasty c.320 - c.550 AD.11Various North Indian Hindu kingdoms c550 - c750 AD.12United Nations Educational, Scientific and Cultural Organization.

Constellations- Reticulum 'the Reticle'_63

Constellations: Reticulum 'the Reticle' 2 Conversations Constellations: Reticulum 'the Reticle'Constellations: Overview | Andromeda | Antlia | Apus | Aquarius | Aquila | Ara | Aries | Auriga | Bo?tes | CaelumCamelopardalis | Cancer | Canes Venatici | Canis Major | Canis Minor | Capricornus | Carina | Cassiopeia | CentaurusCepheus | Cetus | Cham?leon | Circinus | Columba | Coma Berenices | Corona Australis | Corona Borealis | CorvusCrater | Crux | Cygnus | Delphinus | Dorado | Draco | Equuleus | Eridanus | Fornax | Gemini | Grus | Hercules | HorologiumHydra | Hydrus | Indus | Lacerta | Leo | Leo Minor | Lepus | Libra | Lupus | Lynx | Lyra | Mensa | Microscopium | MonocerosMusca | Norma | Octans | Ophiuchus | Orion | Pavo | Pegasus | Perseus | Phoenix | Pictor | Pisces | Piscis AustrinusPuppis | Pyxis | Reticulum | Sagitta | Sagittarius | Scorpius | Sculptor | Scutum | Serpens | Sextans | TaurusTelescopium | Triangulum | Triangulum Australe | Tucana | Ursa Major | Ursa Minor | Vela | Virgo | Volans | VulpeculaName:Reticulum ('Reticle')Genitive:ReticuliShort form:RetArea:114 sq degCo-ordinates1:04h, ?60��Origin:ModernSo what is a reticle? The name comes from the Latin for net, and it is an overlay bearing a grid of fine lines, used to mark or measure positions. Reticulum is best seen by observers in the Southern Hemisphere. It makes its highest appearance in November. The constellation is bordered by Horologium, Dorado and Hydrus.HistoryBefore Reticulum was first drawn, an earlier constellation by the astronomer Isaac Habrecht (1589 - 1633)2 stood in this position. Nicolas Louis de Lacaille (1713 - 1762) set up an observatory in South Africa where he worked from 1751 to 1753. At that observatory he used a rhomboidal reticle. He named this constellation 'le reticle romboide' in honour of this instrument. Reticulum does not have any myths because it is a modern constellation. StarsJohannes Bayer set up the current system of calling the brightest star 'alpha' and then proceeding down the Greek alphabet for subsequent stars. The combination of this letter with the genitive of the constellation name is known as the 'Bayer Designation'. Many dimmer stars such as HD 27894 have neither a name nor a Bayer Designation. Gamma Reticuli varies slightly over the space of 25 days. Its magnitude varies between +4.42 and +4.61.Star TableStarDesignationName or catalogue numberMagnitudeDistance (light years)Spectral classificationand/or comments�� RetalphaHR 1336+3.35250G8�� RetbetaHR 1175+3.8578K1�� RetgammaHR 1264+4.513490M4�� RetdeltaHR 1247+4.56530M2�� RetepsilonHR 1355+4.4459K2��1 Retzeta1HR 1006+5.5440G2��2 Retzeta2HR 1010+5.2440G2�� Reteta HR 1395+5.24380G8�� RetthetaHR 1372+5.87462B9�� RetiotaHR 1266+4.9747K4�� RetkappaHR 1083+4.7270F5 -unnamedHD 27894+9.20138K2Deep Sky ObjectsNGC 1313 is a barred spiral galaxy 15 megalight-years away. Observed at shorter wavelengths it is more ordered than in visible light, and exhibits signs of being rich in hydrogen which would make it a nursery for new stars.The New General Catalogue by J Dreyer was published by the Royal Astronomical Society borse louis vuitton in the late 19th Century. It was expanded to include two Index Catalogues (IC). This list of star clusters, nebulae and galaxies was revised again in 1973 and is continually updated by the NGC/IC Project. Cat. No.Type of ObjectBrightness (m)Distance (Mega-light louis vuitton outlet years)NGC 1313galaxy+9.215 NGC 1536galaxy+12.754 NGC 1543galaxy+10.352NGC 1574galaxy+10.747IC 2037galaxy+14.7364IC 2049galaxy+14.565 IC 2056 galaxy+12.550 IC 2060galaxy+15.17294 Extrasolar PlanetsHere are some planetary systems that have been found in this constellation. The size of these extrasolar planets is compared to the mass of Jupiter. This is known by astronomers as the 'Jovian scale'. The star HD 27894 has a proper motion about 26% of that of Barnard's Star. There have been claims that it has planets at 0.07, 0.12, and 0.41 Astronomical Units, but only the one at 0.12 AU (HD 27894 b) has been confirmed.Star name or catalogue numberPlanet catalogue numberPlanet mass (Jovian scale)Distance (AU)Orbital period (Earth days)epsilon ReticuliHD 27442 b1.281.18423.8HD 23079HD 23079 b2.611.16738.45HD 23127HD 23127 b1.52.41,214HD 27894HD 27894 b0.620.1218zeta2 ReticuliHD 20807 b40.270.1418.9UfologyIt may be a hoax, a delusion, or a hidden truth; but there are those who believe that we have been visited by beings from zeta2 Reticuli. In 1961 two people in New Hampshire saw an object in the sky. They claim they stopped the car and the husband trained a pair of binoculars on the object. The next thing they knew (according to their testimony) it was two hours later. Under hypnosis the story emerged that they were taken aboard a UFO and the woman was shown a map. The stars on the map were not labelled. The map she said she saw was three dimensional. The one she drew under hypnotic regression was not. The Boston psychiatrist5 who examined her and her husband louis vuitton borse concluded that the story was not a deliberate fabrication. It was several years before any star names were added to this map. A tentative map was drawn up by an amateur astronomer in Ohio, connecting it to the star zeta2 Reticuli, and then a professor at OSU6 had his students enter star locations in a computer and generated a map. Finally Mark Steggert of the University of Pittsburgh used a program he had written and the maps were compared. This whole incident was written up in the December 1974 issue of Astronomy magazine. Four decades later, following the death of her husband, the woman still believes that she was taken aboard a UFO by extra-terrestrials and that the star system from which her abductors came was that of zeta2 Reticuli. In 1995 a group of extrasolar planets was listed in the Extrasolar Planet Encyclopedia including zeta2 Reticuli. However, when the link to ufology was realised, the louis vuitton it listing was rejected. The reason given was that 'the data may have been misinterpreted'. 1Current IAU guidelines use a plus sign (+) for northern constellations and a minus sign (?) for southern ones.2Grandson of clockmaker Isaac Habrecht (1544 - 1620).3Slightly variable.4After being published for four days authorities disavowed the existence of this planet.5Benjamin Simon.6Ohio State University.

Central Massachusetts, USA_193

Central Massachusetts, USA 1 Conversation Central spaccio moncler Massachusetts, USAThe geographical centre of Massachusetts is located in the centre of the town of Rutland, a small suburb of the city of Worcester. The actual centre is marked by a tree, incidentally giving the local middle school its name, Central Tree Middle School. South of Rutland is the town of Holden, which is more populated, and past that the city of Worcester. North of Rutland is Hubbardston, a more rural town. South-west of Rutland is Paxton, and south-east you'll find Princeton and Sterling.RutlandRutland is a town that was once small, but is now growing. louis vuitton sac Its town centre, on route 122A, has a middle school, a fire and police department, and a new library. Local restaurants include Rutland Pizza (highly recommended), and Toula's diner (open into early afternoon hours). The diner is a very popular place in town that is visited by school children, by town workers who need to take a lunch break, and by anyone else who happens to be passing. From hamburgers and hot dogs to home style pies, Toula's Diner offers that 'home-cooked' taste every traveller likes.HoldenHolden is a lot more populated and is home to the local district highschool, Wachusette Regional High. Pupils from Holden, Paxton, Princeton, Rutland, and Sterling attend lessons here. Holden teh north face jackets is home to a wide selection of restaurants, including Val's off route 31 near the local grocery store, (the Big-Y). Val's is a family restaurant with down to earth prices, but out of this world food. If you come here, expect to eat a meal, and not borse gucci just some snack.Major stores can be found on Main Street, and also canada goose on route 31 heading west, near the grocery store. At the plaza near this area are a video rental store, a musical instrument shop, a dry-cleaners, and a pizza shop. Holden is also home to a well used bandstand just south of the centre of town where local bands play on Sunday nights. Holden houses several middle schools as well, including Mountview Middle.Princeton/SterlingHeading farther away east from the centre are the more cut-off towns of the north face jackets Princeton and Sterling. The centres of both these towns are on route 62. These aren't heavily populated, and have few attractions in comparison to the city of Worcester. The louis vuitton borse middle school in Princeton is Thomas Prince Middle School, and in Sterling, Chaucksette Middle School.PaxtonThe centre of Paxton is on route 31, southwest of Holden. Paxton is home to not only Paxton Center School, but also to Anna Maria College. A local recommended restaurant is Stagecoach Crossing. This town, like Sterling and Princeton and Rutland, is not a majorly active town, whereas Holden is almost literally an extension of Worcester.WorcesterWorcester is a very diverse city. A recommendation for dining is The Barbers Crossing, a fantastic family restaurant, (from lobster to ribs!). For shopping, go to Greendale Mall, and moncler Doudoune canada goose not to Worcester Common Fashion Outlets. Greendale houses a Best Buy store, where you can buy music and entertainment products at very low canada goose prices (eg a CD for US$8). Though Greendale mall is smaller, it is located in a better part of town. Further useful information can be found through the above link.Other Cool Places to Visit in the AreaSome of the best places in 'Central Mass' depend on what you want to do. If you want to see a loud concert, then go and see what is on at the Centrum Center for any bands on tour. However, if your ears can't take the noise, then enjoy fishing at Trout Brook in east Holden. For a bit of neat local history, you can tour the old prison camps in North Rutland. If, however, you just plan on sitting around doing nothing, then it is your loss, because Central Massachusetts is a truly fun place to go.

Cheesman's Gerbils_258

Cheesman's Gerbils 1 Conversation Cheesman's GerbilsAlong with hamsters, gerbils are often chosen by infant school teachers to be the class pet. There are many advantages to choosing a gerbil - they're cute, they're just the right size for little children to hold and they're reasonably straightforward to look after. Unfortunately, gerbils often suffer at louis vuitton sac the hands of their small guardians. When taken home for the holidays they are often lost down the back of the sofa, or disastrously eaten by the family cat. This can have repercussions across the classroom and can frequently be a five-year-old's first louis vuitton borse encounter with the major issues of loss and bereavement, and the ebb and flow of life and death.With rodents making such an enormous impact on young lives, choosing your gerbil is a serious business. There are loads of different sorts to choose from - there are reportedly more than 80 species living in the wild. Among them is the Cheesman's gerbil, which is an excellent choice for a pet that is going to handled a lot, as it is practically outgoing in gerbil terms (many of them are twitchy and nervous). Bold and brave, they are unafraid of people and inquisitive. Cheesman's gerbils, otherwise known as Red Desert gerbils, are also very sweet looking. One distinguishing spaccio moncler feature is its very long tail that's almost bare. It trails behind and looks as if it should be slung over one paw to teh north face jackets be kept out of the way. The underbelly is white, while in stark contrast, the back is orangey brown with a few black hairs. In fact, Cheesman's gerbils look as if they are wearing capes. The combination of this distinctive tail and fur markings gives him a distinguished air, though a ring of white around his black beady eyes makes him looked surprised and bookish. If the hero mouse Reepicheep in CS Lewis's book Prince Caspian had been a gerbil, he would have certainly been a Cheesman's gerbil.In CaptivityCheesman's gerbils are not common in captivity - it appears they are not as easy to breed as other more common varieties of gerbil. If you are lucky enough to get hold of one, you should keep it in a sizable glass aquarium. As floor covering you can use the sort of cat litter that is made from recycled newspapers. This is recommended over traditional wood flakes and sawdust as these can irritate the eyes and the oils in the wood can cause liver damage. Gerbils also have a tendency to nibble on the flakes and swallow a quantity of the dust produced, which isn't good for them. You'll also need something soft for your Cheesman's gerbil to sleep on, such as tissue paper or hay. Other things to include in the aquarium are toys: cardboard tubes, boxes and plastic pipes1. However, it is best not canada goose to moncler include an exercise wheel, as long-tailed gerbils tend to get their tails caught between the spokes - this can lead to breaks, and possibly necessitate amputation. If a wheel is unavoidable then you should tape over the spokes, frequently replacing the tape. Your gerbil will also need a little shelter to hide in; purpose-made ones can be found in pet shops. For food, give your Cheesman's gerbil a good quality rodent Doudoune canada goose food mix, with occasional treats of peanuts and sunflower seeds (these are both high in fat so shouldn't be given too often). Your gerbil will appreciate a piece of apple or carrot and other hard things canada goose to gnaw on (pieces of wood or dog biscuits) as their front teeth grow constantly and should be worn down with nibbling. You should always make sure your gerbil has a supply of water - despite being desert animals, they don't cope well without it.DistributionIn the wild, borse gucci Cheesman's gerbils can come and go as they please across the dry and sandy areas of the Middle East right across to Pakistan and into South Western Afghanistan. There are various sub-species distributed throughout: Gerbillus cheesmani (Lower Euphrates, Iraq)Gerbillus cheesmani aquilus (Iran)Gerbillus cheesmani arduus (Central Arabia) Gerbillus cheesmani maritimus (Yemen)Gerbillus cheesmani subsolanus (Pakistan)In the frequently harsh environment the north face jackets where they live they will forage out nuts, grasses, seeds, roots, and insects to chew on. They are capable of climbing high and can we seen in the wild swinging to and fro on tall seed heads. HabitatTheir burrows, dug out of sand dunes and dried up mudflats, can be up to four feet deep, which protects them from harsh sun during the day and the crisp cold of night. During the day, when they are sleeping (they are nocturnal animals) they shore up the entrances with sand to conserve moisture and to hide the burrow from potential predators such as snakes. They nest separately side-by-side in a community of neighbouring burrows, which belies their sociable nature. Related BBC LinkCould gerbils work for MI5? 1If at all possible, buy tubes with metal protecting the corners - they'll be more expensive, but the gerbil won't gnaw through them in five minutes.

Constant-mesh and Synchromesh Gears 4 Conversations Constant-mesh and Synchromesh GearsIn the early days of motor transport, changing gear was a delicate operation requiring considerable skill. Straight-cut gears had to be matched in speed before being brought into mesh together. The result was as often as not a horrible grinding noise - crashing the gears, as it was known.To change up with a crash gearbox you must disengage the clutch, shift to neutral and let the engine run down to a lower speed, re-engage the clutch briefly in neutral to slow the gears down, then select the new, higher gear. To change down you need even more pedal dexterity: you disengage the clutch, shift to neutral, re-engage the clutch, blip the throttle rapidly to spin the gears up, depress the clutch again and select the new, lower gear.This process, known as double de-clutching, is almost a lost art. Mini owners are often familiar with the technique, as are rally drivers (when done confidently and with skill it allows for very quick downshifts with minimal stress on the gearbox). But for the rest of us, the juggling with clutch, throttle and gearstick (while braking with your third foot!) is blessedly unfamiliar.The reason we no longer need to trip the light fantastic on our pedals is an invention known as the synchromesh gearbox.The Constant-Mesh GearboxStraight-cut crash boxes were very noisy, wore quickly and tended to shed teeth at awkward moments, so it wasn't long before engineers began to experiment with other methods, most notably helical gears in constant mesh. It is well worth understanding how this process works - it will make you a better driver (assuming you drive a car with manual transmission).Helical GearsYou know what a straight-cut gear looks like: it's the classic cog, as seen on bicycle sprockets, clocks and watches. It's simple to make, but the bearing surface (the area of contact with which gear A actually pushes gear B around) is narrow. This has two effects: it wears quickly (all the force is distributed over a small area) and it is noisy (as it wears the gears fit less well, and each new tooth coming into mesh essentially hits its companion tooth and is braked sharply until it fits into the requisite gap).Helical gears are cut on the slant, a bit like screw threads. Imagine a long bolt with the head removed; now bend it into a ring. The outer edge of this ring is a basic helical gear. Helical gears have a longer bearing surface, which reduces the wear problem, and - crucially - each tooth comes into mesh while its predecessor is still in contact so there will be two teeth in mesh at any time. Instead of the constant judder of teeth banging into each other we have a silky-smooth progression from one tooth to the next.A Constant-mesh SystemHelical gears are great when they are in mesh, but they are much finer and bringing them together on the fly without damage is not possible. The development of helical gears required the use of a constant mesh system. But how can all the forward gears be in mesh simultaneously without it ending in tears, though? The answer is simple and elegant - easy to draw but fiendishly difficult to describe!To set the scene: a gearbox has an input shaft which is connected to the clutch. This is how power goes into the gearbox - the engine turns the input shaft when the clutch is engaged. The gearbox performs its magic and power comes out on an output shaft, which can run at a different speed from the input shaft. The reason we do this is so that the engine can stay within its power band; otherwise we would either need an engine which produced enormous torque at 100rpm to move us in slow traffic or one which would rev up to 20,000rpm to move us at speed.Constant-mesh gearboxes are things of great beauty (if you are an engineering-minded sort). They rely on a strange-looking thing called a lay gear. This is a long gear running on a shaft (the layshaft) parallel to the line of the input shaft. Let us imagine a four-speed system such as that fitted to the Mini.The laygear will have four gears on it, in ascending sizes from one end to the other. It looks a bit like a child's drawing of a Christmas tree (or a four-armed Cross of Lorraine). The gears are helical, of course. The gears are not evenly spaced. There is one large gear, then a space, then two middle-sized gears fairly close together, then another space, then a fourth, smallest gear. The laygear is a single solid piece of cast steel: it is quite heavy, and the gear edges are sharp - but drop it and you could chip the teeth and cause irreparable damage. The biggest gear goes at the input end, nearest the clutch.Parallel to the layshaft is the input shaft. There are three gears mounted on it, but unlike the laygear they are independent and run on bearings so they can spin on the shaft. They run in size order opposite to the laygear with the smallest at the input end. Each gear is meshed with a laygear, the canada goose sale smallest obviously matching the biggest end of the laygear. The smallest end of the laygear is meshed with a separate gear on a separate shaft - the sac louis vuitton bowling montaigne output shaft. The axis of output shaft is the same as that of the input shaft - it is effectively an extension of the input shaft, but cut off just inside this final gear. A roller bearing keeps the two shafts in perfect alignment. This helps keep the two shafts in line.If you rotate the laygear each main gear rotates at a different speed. The output shaft will turn (its gear is not on a separate bearing) but the input shaft need gucci prezzi borse not turn, as all the other gears spin freely on it. Conversely, because top gear is connected directly to the output shaft and is permanently meshed with the laygear, any time the wheels are turning, the laygear will turn.Now the clever bit. The two large spaces on the laygear match up - obviously - with two large spaces on the input shaft. These chaussures louis vuitton lengths of shaft are splined (grooves running along the shaft, as if the shaft itself were a straight-cut gear). On these splines are mounted two hubs, one in each space. These are known as synchro hubs in a synchromesh system.The synchro hubs can slide along the input shaft. They have a ridge running around them, and have an arrangement to lock them with the gear one side or the other. Imagine a slim, straight cut gear with rounded ends to the teeth, and a matching recess on the mating face of the hub. So, when the synchro hub moves towards the main gear, the teeth slot neatly together and the two parts lock. At this point when the input shaft turns it is locked to one of the main gears, so the laygear will turn and - because it is meshed with the gear attached to the output shaft - the output shaft will turn.Selecting louis vuitton prezzi borse a GearAll that remains is to build a mechanism piumini moncler which moves the collars into place to select the relevant gear. The gear lever works a mechanism which moves a pair of selector forks, one fork running on the ridge around each hub.To select first gear you move the gearstick to the left - which engages the first selector fork - then up, which pushes the first/second hub into mesh with the smallest gear, closest to the input shaft. The input shaft turns, the small gear is being driven, so the laygear turns relatively slowly. The other end of the laygear is small and meshed with the largest main gear, which is attached to the output shaft, so the output shaft is driven slower in turn than the laygear. Voil�� - first gear.Now change to second. Disengage the clutch and move north face north face jackets for the lever down. The first/second hub moves to the second gear, one of the pair in the middle - the one nearer the input end. This is slightly bigger than the first gear, so when it is coupled to the main shaft the laygear spins rather quicker. The other end of the laygear is still meshed with the gear on the output shaft, so the output shaft runs a bit quicker. Second gear.Up to third. Disengage the clutch again and move the selector up (to disconnect the first-second hub from second gear), across to engage with the third/fourth selector fork, and up to move the third/fourth hub into position on the third gear. Third is bigger again than second, so the laygear runs nearly as fast as the input shaft is turning. The smallest end of the layshaft is still meshed with the output shaft, so the output shaft turns at a brisk pace only slightly slower than the input shaft. Third gear.Now fourth. Clutch in and move the lever down, disconnecting third and north face pants engaging directly with the fourth gear which is attached to the output shaft. The laygear is still spinning, but drive is going straight through so it is transmitting no power. Power is lost every time you transfer drive from one gear to another, so it makes sense for top gear to bypass the laygear entirely.SynchromeshConstant-mesh gearboxes are quieter and slicker, but you still have to match the spinning speeds before you could engage the chosen gear. The final improvement, and the one which removes the need for double-de-clutching, is synchromesh.Synchromesh is a refinement of the part of the constant-mesh gearbox where the trouble happens: matching the little teeth on the inside of the selector hubs with the little teeth on the side of the main drive gears. In a synchromesh gearbox a system is introduced which spins the main gear (and main shaft) up as the hub approaches. In its simplest form there is a tapered section on the hub side of the main gear and a matching, grooved, tapered, sintered (a form of heat treatment) bronze ring called a baulk ring on the gear side of the hub. As the hub approaches the gear the bronze ring rubs against the taper, and friction spins the gear up. Too fast and the selector teeth will crash, too slow and there won't be enough movement between the teeth on the hub and the gear for them to slide into place (they might meet point-to-point).Synchromesh started to be fitted to cars in the late 1920s and early 1930s, but was by no means universal until the late 1950s. Even then it was not uncommon for first and/or second gears not to be synchromeshed.And, as anybody who has ever driven a Mini will know, early designs of synchromesh were prone to fail. After about 40,000 miles the bronze rings in the Mini's synchro hubs became smooth, so they no longer worked. Changing up was OK but changing down without embarrassing grating noises required double de-clutching. This was exacerbated by the Mini's engine having the gearbox in the sump. This design, while brilliant from the point of space-efficiency, meant compromise in the use of oil. The oil had to be fluid enough to pump around the engine - but gears need a much thicker (and generally cooler) oil to function effectively. Thus the BMC/Austin/Leyland cars of two decades or more (including the Mini, the Allegro and the revolutionary but flawed Maxi) were brought low by the Achilles Heel of gearbox problems.OverdrivesThe system described above is a four-seed system. Most gearboxes are fundamentally four speed. 'But wait,' you cry, 'every car I've ever driven was a five-speed!' And so it was. Fifth gear is typically an overdrive ratio - the gearbox output shaft goes quicker than the women input shaft for relaxed cruising at speed. Overdrives work much the same but have a separate laygear.So Why Should I Want to Know That?Simple, dear Researcher. If, as you change gear, you consider what is going on in the gearbox, you'll remember to be polite but firm with your gear stick, not rush the synchro and make it baulk, and not change so slowly that the whole thing grinds to a halt. You will remember that if your engine is labouring up a long hill and you want a really fast change to a lower gear, you need to drop out of gear into neutral, lift the clutch pedal, blip the throttle quickly, down with the clutch and quickly into the lower gear. With practice this will give you faster, slicker changes and prolong the life of your gearbox.

Inverse Cool Theory_142

Inverse Cool Theory 4 Conversations Inverse Cool TheoryYou may not have encountered the Inverse Cool Theory before. You may in fact be living proof of this theory and not even know it. It is a fundamental flaw in the concept chaussures louis vuitton of cool which none of us understand and most of us strive for.Being 'Cool'Before we can understand inverse cool theory we must first understand the concept of cool sac louis vuitton bowling montaigne in its natural and generally accepted form.Once a word that determined temperature, cool has come to stand for much more than something that's not warm. Cool has become a concept that mixes fashion sense with popularity and attitude with appearance, and generally is not understood by anyone.When looking for a figure synonymous gucci prezzi borse with the concept of cool many of us look to Happy Days1 character 'The Fonz'2, or James Dean 3. It is generally accepted among sociologists that youth subcultures began in the north face pants 1950s with 'Teddy Boys'4, and as a result the concept of cool duly followed.There are some very strange unspoken rules determining what is cool and what isn't:Trying to be cool is very uncool.However, not caring about cool doesn't necessarily make you cool.You can look cool, without actually being cool.You can be cool, without looking cool.The definition of what is cool can be vary dramatically in different genres and cultures.Cool is not only subjective, but relative, and only ever applies to that exact moment. There are many more confusing factors that dig even deeper into women the core of what cool actually is.Generally Accepted PrinciplesAlthough 'cool' can mean many things to many people there are generally accepted norms and values which, while ever shifting, sum up the cool of the moment. These are conveyed to us, the unsuspecting public, through the mass media.This may have at times been being a hippy, a punk, a mod, a new romantic, an indie kid, a raver, or one of many other factions and subcultures.That is So Uncool, Man!As Newton states in his Third Law of Motion5:For every action there is a reaction of equal strength but opposite direction.And thus, with only minor paraphrasing, we can conclude that for every aspect of cool there is an opposite and equal version of uncool.There are two groups that are generally considered uncool by pretty much anyone anywhere with any real sense of what cool truly is: the old, and geeks. There is no specific age at which an individual becomes old, but in terms of cool it's safe to say that everyone else will know long before you do. If you are unsure ask a brutally honest friend or, if you don't have one of those, an unsuspecting teenager. Doing this, however, will render you uncool, so if you're actually considering this just consider yourself uncool, give up trying and go and buy some slippers.Much in the way that it's impossible to accurately pin down what cool is, it's difficult to define 'geek' louis vuitton prezzi borse in any certain terms. In general, most people would consider it a derogatory remark to describe those with limited social skills, a little too much interest in 'work', and a susceptibility to wearing sandals with socks. It is perfectly plausible, and indeed common, to fall into both of these uncool groups.The Actual Theory ThenArmed with your new found understanding of the background information regarding cool and not cool you are ready to wrap your mind around the concept of 'inverse cool'.Put quite simply, this is the theory that among the 'uncool,' the things that are considered most uncool by the cool are north face north face jackets for actually considered cool.True, the previous sentence may not be the most simple way of expressing this concept. Bearing this in mind, it's probably best to explain this theory with an example:While being into fantasy gaming is extremely uncool to the reasonable and normally aligned being, to a geek this is pretty cool. You will probably have something in common with other geeks, and may be able to elevate yourself canada goose sale to a particularly high status within a grouping of geeks if you know a lot about fantasy gaming, and can beat other geeks in this particularly piumini moncler geeky sport.This allows the least cool people to become the most cool, simply by finding people who are slightly less uncool - or cool - than them.So For Each of us There is HopeThis simple theory now makes it possible for pretty much anyone to be considered cool within their social group of choice, you simply have to choose the right group. If you're really into knitting find yourself a bunch of knitting fans and show off your knowledge, expertise and style in the field of knitting. If you're really into sci-fi novels, learn your stuff about sci-fi novels and find yourself a bunch of similarly-minded individuals to show your sci-fi cool to.Even if - shock horror - you are old you can be cool, as long as you choose the right group of people who will think you're cool. You would be wise to avoid trendy twenty-somethings, and hoodie-wearing teenagers, as they're likely to think you're just about the least cool person they know. Try fitting in with a group of similar ages and interests.Go now and show them - the selected group remember - your newfound cool. Do what you do with style and panache, know your target audience, and go and find that social status you've been longing for.Huzzah for the cool! And that is just about the least cool way to end an article.1An American sitcom starring Ron Howard2Played by Henry Winkler (and how!).3Legendary American actor whose image in the film Rebel Without a Cause has become iconic.4A youth subculture dominated by young males who dressed in sharp suits with drainpipe trousers and often organised themselves into gangs.5See Newton's Laws.

Half-Life - the Computer Game_38

Half-Life - the Computer Game 6 Conversations Half-Life - the Computer GameHalf-Life is a computer game by Valve Software that is referenced as a 'First-Person Shooter'1, which essentially means it's an excuse piumini moncler to go blow things up. Winning 'Game of the Year' by PC Gamer and the now non-existent PC Accelerator, it is an exceptionally well-designed excuse to... go blow things up.The main reason sac louis vuitton bowling montaigne for winning the award is because it is arguably the first game of its genre to involve a fairly believable plot, which computer games of Id Software such as Doom were notorious for leaving out completely. The only other game noted to have such a compelling plot would be System Shock 2, and it erred by not keeping ahead of the curve in graphics.What's the Reason to Go Blow Things Up?The story line in Half-Life revolves around a character, Gordon Freeman (played by you, the avid computer gamer), who is the newest employee in the Black Mesa Research Facility (BMRF), somewhere in the Mojave Desert. The BMRF is a play on all the conspiracy theories situated around the US Government, being a test facility for north face pants nuclear weaponry, teleportation, and alien research. Freeman is solely responsible for chaussures louis vuitton all hell breaking loose in the form of an inter-dimensional rift, much like the game, Doom, and the movie Event Horizon, by Paramount Pictures. The breach, of course, allows streams of violent alien life-forms to march right in and create havoc. So, Freeman must find a way to escape, and quite possibly save the world in the meantime.But What is there to Blow Up?All hell breaking loose essentially means that Freeman and his rather nerdy brethren2 must escape by any means necessary. This is not an easy feat, as the facility is mostly underground, the communication lines are cut, the transit system has failed, and the government eventually dispatches marines to silence the facility, including the researchers themselves. This last revelation can be seen two ways:You are, in a sense, screwed, as scientists are usually not armed, and so you now have to contend with a malevolent alien force and a military division on orders to silence the facility.You are saved, as they will hopefully be too busy beating each other over the head to bother with you.This helps to an extent, until Freeman realises that the orders pertain to him specifically, as Murphy's Law dictates.And what Do I Use to Blow Things up?This is a game trying to create a believable experience, women and as such, there are no levels per se, but there are divisions in progress to mark another theme. A couple of real-world weapons are used, like the Navy MP5 to add to the feeling of 'being there'. Also making this game distinctive is the reintroduction of the laser trip-mine, from way back in the days of Duke Nukem 3D. This particular weapon can be the source of much hilarity in the multiplayer experience, but is of little use for a single player.There are also fantasy weapons in BMRF, including the gluon gun and the snark, simply because they would be cool to play with. The snark is a rabid little animal the gucci prezzi canada goose sale borse size of a hamster, and will try to eat whatever and whoever is in its way, until it blows north face north face jackets for up due to its violently heightened metabolism. In multiplayer, it is not a tactical weapon, but will cause panic and much shooting.Most guns have a secondary fire. This is not all that special in itself, but it does allow for sniping opportunities. Both sniping weapons, the crossbow and the rifle (in Half-life's add-on) utilise the secondary mode to zoom in, making someone hurt from afar. Other secondary modes include a grenade launcher for the MP5, and switching between faster and more powerful blasts for most of the other weapons.All in the name of good fun.Is Blowing things Up all I Do?Yeah, but some interludes between blowing things up involve the progression of the story and small puzzles. There is a mysterious character, known only as the Administrator (or G-Man, if you're so inclined), who does his bit to help or hinder you along the way, whichever seems best for the situation. In terms of the actual story line, nothing is ever mentioned about him. In terms of involvement with the plot, he plays a decisive role.In terms of non-action sequences, Freeman spends a great deal of his time in the air ventilation systems, much to the player's dismay. This isn't so bad if you were actually in the world Half-Life represents, but as a game, one vent looks like another, and can be very tiresome after a while.Am I the Only One to Blow things Up?Well, there are security guards with pea shooters and scientists with higher access than you, so yes and no. There are other friendly people, just none with any firepower, thus contributing to your increasingly hopeless situation.Multiplayer is the best online experience associated with the genre, and as long as you have Team Fortress, which came along in a free patch3, you won't be disappointed. Team Fortress's main selling point is its use of teamwork, which was sorely missed in most death matches. The editors in Valve were also nice enough to package a server locator with the game, making connections to death match much easier.How Does this Game Make Things Blow up Better than Others?The engine4 of the game is a modified version of Id Software's Quake II. The design of this engine specialises in the mapping and calculating the rules of a physical area, which means that a hand grenade's arc and spin are fairly realistic as it flies towards your head. On the other hand, path finding5 is lacking. The Non-Player Character (NPC) will easily get lost and have difficulties following or giving chase to Freeman (you). Take no notice, as so far no first-person shooter has been able to path-find efficiently.As in most First-Person Shooters, more attention was given to pretty things blowing up than to the nitty-gritty of the action.The general idea is if you like blowing things up, and are just looking for an excuse, Half-Life is out there with a better one than most, along with its modifications and add-ons like Opposing Force and Counter-Strike.1First-Person, as in looking through the eyes of the participant, and Shooter, as in the participant's main pastime is to shoot things.2Being a stereotypical research facility, the area is louis vuitton prezzi borse densely populated with scientists with exceptionally thick glasses.3A patch is a small program to fix bugs, glitches, and to add new features on its respective host program.4A game engine is a programming tool to map graphics, sounds, and actions. For example: ZZT.5That which one finds a route through confusing and/or cramped spaces.|||1 More Conversation | Subscribe |UnsubscribeInfinite Improbability DriveRead a random Approved EntryApproved EntryA481033

Gift Vouchers _145

Gift Vouchers 2 Conversations Gift Vouchers Updated 29 April 2010Vouchers gucci prezzi borse are pieces of paper that entitle louis vuitton prezzi borse the bearer to a certain amount of credit at a particular shop or chain of stores1; occasionally vouchers can be exchanged for services rather than goods. Unlike cash, which is issued by the government and so is guaranteed by the Bank of England, vouchers are issued by the relevant company.Some stores now dispense plastic gift cards instead of paper vouchers2. While previously only found at the lower end of the retail market, they can now be found in all sectors, including, stores such as Harrods. They are similar to the more traditional vouchers with the exception that many of them can be 'topped up' with additional credit.Obtaining a VoucherA voucher can be bought from a shop that will then accept its own vouchers, otherwise there wouldn't be any point in buyng the vouchers in the first place. Sometimes several different stores from different chains will agree to recognise each others' vouchers. There are also specific multi-chain vouchers, such as a High Street Voucher, which can be used at nearly 20,000 stores from dozens of different chains. Vouchers are bought using money, e.g. a person will give the store ��5 in cash and will receive a ��5 voucher in return. Vouchers are normally bought in cash, but for larger denominations there is no intrinsic reason not to use a credit or debit card. Buying vouchers normally therefore causes no loss or gain of value.Often vouchers are completely free. Many newspapers or magazines contain coupons that can be cut out and used, making the cost of the coupons equal to whatever you paid for the newspaper or magazine3. Alternatively, you can get these coupons on the back of many products, giving discounts to any future purchases that north face north face jackets for you may make of that particular product.Sometimes vouchers come in the form of a reward system. Companies give a certain amount of money to you in voucher for each year or quarter. This is based on how much you have spent within that shop each year; for example, you may receive one pound for every hundred pounds that you spent, making it well worth your while to spend more at these stores. You gain these vouchers simply by asking about your 'points' status or you will be sent vouchers in the post.One way of gaining a stream of vouchers is to take part in long term consumer research. Questionnaires will be sent to you on a fairly regular basis. If you answer them you can be sent vouchers on all manner of household items. These questionnaires can be sent year on year, garnering the willing volunteer a constant stream of vouchers.Using a VoucherFirstly, travel to the store that your voucher is designed to be used in4. Sometimes vouchers can be used in other places than in the specific shop suggested. These are normally other stores that are owned by the same parent company, who obviously benefits wherever your cash is spent.When paying for items bought in the store, simply give the voucher as payment or part-payment for the goods or services rather than paying with cash. The person at the till will probably sigh, take the voucher and deduct the value from what needs paying and give you your goods. As such the effort involved from the person using the voucher is little more than using cash and actually less than using a credit card, although as explained below the opposite is true for the poor till operator.Benefits of Buying VouchersVouchers have benefits to both the shops selling them and the people buying them. Here are some of the advantages:Benefits for Buyers and Users They can make excellent presents as they allow buyers to avoid the problem of giving cash - that or a lack of imagination.Vouchers can be used to force people to buy in an area they wouldn't ordinarily do so. One example could be men being forced to shop in health care shops.Some shops will give extra benefits to those who buy vouchers; these benefits can include loyalty points for those who have the relevant cards.Very occasionally some companies will sell vouchers for less than their face value, usually on very large denominations such as HMV's corporate vouchers where for sales over ��50,000 discounts of up to 10%5 are possible. Benefits to Stores Vouchers are often bought along with other products as buyers 'impulse buy' after entering a store. These vouchers will either then be used with other purchases or, in some cases with more obscure shops, never be used, making a clean profit for the company. Unlike cash chaussures louis vuitton presents, vouchers require a return to the relevant store. Those customers using vouchers often buy goods with a greater value than that of the voucher. This increases profits of the stores selling vouchers.Vouchers are often used as advertising, mainly by encouraging people to invite others in return for a certain value of vouchers that will eventually be recouped by sac louis vuitton bowling montaigne the increase in revenue from additional customers. One example of this is Sky's invite-a-friend policy. After the person receives their first reward for encouraging others to join they are likely to invite more and more people to maximise their own, and the company's, profits.We can see that vouchers do provide a noticeable benefit to the economy and fulfil a niche that plain cash cannot, but at the same time they can bring some formidable problems as the next section will explain.Disadvantages of VouchersVouchers, however, have some serious disadvantages compared to cash or credit cards. Here are a few problems that are caused by vouchers.Disadvantages for Buyers or Users Those using the vouchers have the problem of them having to be used in certain stores; this can cause problems if the vouchers are linked to an unwanted store.Vouchers sometimes fail to give change which can lead to wastage. However, most companies now give change in the form of lower denomination vouchers with cash given for change under one pound.A truly abominable practice with questionable morality is that of vouchers having an expiry date. If the voucher is not used before this date then the voucher cannot be used and any value is wasted. This method allows the stores to generate pure profit from the selling of vouchers.As mentioned at the beginning, vouchers are not 'safe money' like a UK bank note. If the company that supports them goes bankrupt before the voucher is used then they instantly become worthless. While you could try to recover your money against the company's assets, piumini moncler the likelihood of any money making its way back to you is slim to say the least. Disadvantages to Stores While overall there is no loss of canada goose sale money, apparent fluctuations of cash flow can be caused by the usage of vouchers if a large number are used in quick succession.Vouchers can create large amounts of paperwork for their sales and use. Tills will normally need to be reprogrammed to 'understand' vouchers and so also require at least a basic retraining of those using them.Vouchers must be produced or bought by the company itself and so an outlay of capital is required, which reduces potential income.The primary disadvantages for vouchers are therefore shown to be one of complexity for both users and stores. Critics of vouchers claim - perhaps justifiably - that why bother using vouchers when cash can fulfil every shopping need that a voucher could and indeed many more.Gift CardsAs mentioned above gift cards are very north face pants similar to vouchers apart from a couple of differences:Gift cards can be topped-up with extra credit after being used; this allows cards to be 'refilled' as presents and prevents a number of cards accumulating in the shopper's wallet.Design: while a relatively minor point, gift cards are stronger than vouchers which means they are harder to break; however paper vouchers can be repaired with sellotape and reused unlike gift cards which break permanently. Book TokensAs the most notable example of a voucher the humble book token deserves a special mention. While bearing many similarities to a normal voucher, it has a couple of differences. For a start it can be redeemed at any bookshop, making it far easier to use than any normal voucher. During World Book Day, book tokens are given out for free - normally in the form of one pound chits but occasionally in larger values. School children, in particular, receive book tokens in order to encourage reading at a younger age.The book token is one of the longest running vouchers, being introduced in Britain during 1932; as such book tokens are very well known and can often be women found being given as prizes by schools and scout organisations.Online VouchersAs well as being used in the high street, vouchers can also be used for Internet shopping. They can be bought online, at which point they will normally be posted to the buyer. They can also be used online, where they act as a form of store credit. A specific code will be printed on each card and the correct entry of the code deposits the appropriate sum into the buyer's account. The majority of these vouchers are dispensed by companies that have no high street presence of their own.Miscellaneous VouchersThere are many other forms of vouchers that are used for services around the country in lieu of cash payment. Here are a couple of the more commonly seen ones.Luncheon: these vouchers can be used by employees who don't wish to eat inside their work cafeterias. Created in 1950, many stores allowed them to be used as part-payment for food. These vouchers provided a tax-free benefit and so were widely used across the country. They are, however, quite rarely seen inside the UK in the 21st century.Travel: some package holiday companies provide these to tourists. They can be used for specific things, such as food or accommodation. They form part of the price of buying the package and must be used in the way required; these may be unpopular among certain tourists who wish to pick and choose in their holiday.Should Vouchers be Used Then?To decide whether vouchers should be used rather than the closest other payment method - cash - the benefits must outweigh the disadvantages, both for the customer and the store.Just a brief glance of the points shown above normally separates people into voucher-users and voucher-haters in a remarkably Marmite-esque fashion. Consumers who use them add a significant amount to the economy and to each other's happiness each year. Those that don't should mutter, use a crisp ��50 note and smile smugly at those using tattered M&S vouchers. Currently there are millions of pounds in uncashed vouchers just in the UK alone, and a company offers a great deal of money in voucher form every time they run a promotion campaign6. With the amount of money that vouchers provide to the economy each year, it looks likely that this payment medium will be here to stay with us for a while yet.1That is, the same shop in different locations.2For the purposes of this entry gift cards and vouchers will be considered to be the same unless otherwise mentioned.3Although, if you go to the effort of cutting them out and sending them off, the value will often be more.4The name of this store should be reasonably obvious on the voucher itself.5It's the 'up to' that's dangerous: always check exactly how much of a discount you're getting.6When companies run discounts in national newspapers the combined value is staggering, possibly enough to bankrupt a company if they were all cashed together.

History of Chemistry - Noble Gases _282

History of Chemistry - Noble Gases 4 Conversations History of Chemistry - Noble Gases 'Noble gases' is one of the most common names for the elements in the rightmost group of the periodic table of the elements. They are also sometimes called 'inert gases'. For a number of complicated reasons, these elements have one important chemical property: they are extremely un-reactive. One consequence of this was that the noble gases were isolated and characterised only relatively late. The first person to actually isolate and note the presence of these noble gases was Henry Cavendish (1731-1810: chemistry bigwig and discoverer of hydrogen) in 1785. He removed all the nitrogen and oxygen chemically from a receptacle contining air. The nitrogen was oxidised to NO2 by electric discharges and absorbed by a sodium hydroxide solution. Remaining oxygen was removed from the mixture by a special adsorber. 1/120 of the gas volume remained unreacted in the receptacle. As we know today, this must have been mainly argon (99.8%) and traces (0.2%) of other noble gases. The second person to isolate a noble gas, but without characterising it, was mineralogist William Francis Hillebrand (1853-1925) who noted the formation of a gas while dissolving uranium containing minerals in acid. We know today this must have been helium.Argon In 1894 John William Strutt, third Baron Rayleigh (1842-1919), noted that 'pure' nitrogen isolated from air piumini moncler was denser than pure nitrogen obtained chemically (1.2567 g/l against 1.2505 g/l at 0��C and 1.013 bar). From that, he concluded that something else was in the air. Together with physico-chemist Wiliam Ramsay (1852-1916) they decided to take a closer look at that un-reactive fraction of air that women Cavendish had noticed earlier. While Rayleigh repeated Cavendish's experiment Ramsay modified it a bit: the oxygen was removed by reacting it with heated copper (copper reacts with oxygen to cuprous oxide) and the nitrogen was removed by reacting it with magnesium (magnesium reacts with nitrogen to form magnesium nitride). The remaining fraction was then properly characterised. The new element was called 'argon' which comes from the Greek word for 'inert'.Helium One year later, in 1895, Ramsay and (independently) Per Theodor Cleve (1840-1905) isolated and characterised the gas which had been discovered by Hillebrand. Its spectral lines coincided with many lines observed in the sun's spectrum, about north face north face jackets for thirty years earlier, in 1868. In fact the existence of this element had been postulated by the astronomers Pierre Jules Cesar Jansen (1824-1907) and (once again, independently) Sir Joseph Norman Lockyer (1836-1920), who named it helium (from the Greek word for 'sun').Neon, Krypton and Xenon After the development of the periodic table of elements in the 1870s and the discovery of argon and helium, Ramsay realised there were even more spaces left blank in that table for undiscovered elements, which should all be 'noble gases'. He wouldn't have gotten far in his search for those new 'noble gases' were it not it for the invention of air liquefaction by Carl von Linde. Ramsay had one of Linde's machines rebuilt in his lab (the original Machine can be seen here: Deutsches Museum: Linde's Air Liquefaction Machine) and obtained litres and litres of noble gas mixtures (the left-over from the liquefaction). In 1898 Ramsay 'distilled' the left over and found not only neon (which obtained this name from the greek word for 'new'), gucci prezzi borse north face pants which was the element he was looking for, but also chaussures louis vuitton krypton (greek for 'hidden') and xenon (greek for 'strange').Radon The last noble gas to be isolated was radon, in 1900, by Friedrich Ernst Dorn (1848-1916). Another isotope of radon had been observed earlier in 1899 by Ernest Rutherford (1871-1937) and Frederick Soddy (1877-1956). While the discussions were going on about who really isolated that element first, people found out that radon has been observed even earlier, by the Curies (Marie and Pierre) because it is a gaseous product of the radioactive decay of radium, so there was always a 'strange' gas bubble in sealed ampoules containing radium or uranium solutions. It was originally called 'niton' (from the latin word 'nitens' which means 'shining'). The gas was properly characterised by - you guessed it - Ramsay louis vuitton prezzi borse in 1908, who found out that the gas is a heavy, colourless and odourless noble gas. It was finally called 'radon' because the most stable isotope emanates from radium solutions. (Radon emanating from actinium solutions was called actinon, and radon emanating from thorium solutions used to be called thoron). What happened to all those people? Henry Cavendish was what we would call today a bit of a 'mad professor' 1 - furthermore he was shy and afraid of women. A biography of this eccentric scientist can be found here on BBC: Historic Figures: Henry Cavendish. William Francis Hillebrand achieved some fame as one of the greatest analytical chemists and mineralogists of his time. Rayleigh and Ramsay went Nobel in 1904, in physics and chemistry, respectively. Rayleigh 'for his investigations of the densities of the most important gases and for his discovery of argon in connection with these studies'. And for Ramsay 'in recognition of his services in the discovery of the inert gaseous elements in air, and his determination of their place in the periodic system'. Per Theodor Cleve (1840-1905) had the mineral formed by uranium dioxide named after him: Cleveite. Joseph Norman Lockyer (1836-1920), who, along with Pierre Jansen, was the first to sac louis vuitton bowling montaigne see the spectrum of helium, was knighted shortly after Ramsay isolated helium and confirmed the existence of that element. Lockyer was also the founder of Nature one of the most renowned scientific journals. The other astronomer, Pierre Jules Cesar Jansen (1824-1907), later specialised in solar photography. His work in this field - published in 1904 - set the standard for almost 50 years. The German physicist Friedrich Ernst Dorn (1848-1916), who was one of the discoverers of radon, became the director of the physical institute of Halle, Germany. Ernest Rutherford (1871-1937) ended up getting his place in the list of Nobel laureates in 1908, 'for his investigations into the disintegration of the elements, and the chemistry of radioactive substances' - including radon. The Curies (Marie and Pierre) were awarded a half Nobel Prize in Physics in 1903. More detailed bios of the Nobel-winners can be found on the main Nobel Prize site.Recent Noble Gas History Soon after the discovery of the noble gases people started to ask why (and if) these elements really are inert. At the same time there was quite a turmoil going on around the quantum theory. In the end people figured out all things about electrons moving around atoms and how it comes that certain compounds form and some don't. The explanation behind the canada goose sale noble gas' 'inertness' was that their electronics was the most stable configuration possible in the first place, so building a compound would only make them less stable. In fact, it was even postulated, that noble gases can't form a compound. In 1933 Pauling rose serious doubts whether that is absolutely true. He figured out that it should be - in principle - possible to obtain noble gas compounds, at least from the heavier gases krypton and xenon. He proposed low reactivity instead of no reactivity for the noble gases. The final proof for Linus' case came in 1962 - quite as a surprise - when British scientist Neil Bartlett was messing around with a very tough oxidizer (platinum hexafluoride) and xenon. He ended up obtaining a yellow solid compound (xenon hexaflouroplatinate2). This first synthesis of a noble gas compound ignited a noble-gas-compound frenzy, and soon many other compounds were synthesised. At the date of writing only neon and helium, the two lightest noble gases, were not found to form chemical compounds. However, chemists are developing strategies to bind even these elements.1for example he estimated the magnitude of pain caused by electric shocks through self-experiments2XePtF6-

Forth - the Programming Language_269

Forth - the Programming Language 2 Conversations Forth - the Programming LanguageForth is a general-purpose computer programming language. It was invented in 1968 by Charles H Moore (known as 'Chuck'). Initially he used it for the control of giant telescopes in the observatory in which he worked, but it could be used for anything, so it gradually spilled out into other applications. Moore went on to found a company, Forth Inc., to develop Forth systems for lots of different computers.Forth arose at a time when there were two types of computer language:Interpreted languages - slow-running and unstructured, but could be used to cobble together a simple program very quickly.Compiled languages - programs ran very quickly but took a long time to write, as each time a change was made, the program had to be recompiled, which could take five or ten minutes.Examples of compiled languages were FORTRAN, ALGOL and COBOL, all of which Moore was familiar with. These languages tended to have so many features that you needed a huge computer to run them. The most common interpreted language was BASIC. It could run on just about anything, but was pretty crude - it lacked all the structure of other languages - and programs written in BASIC ran slowly.Forth was designed to be the best of both worlds. It was interactive so could be used to develop programs quickly, but they tended to be quickly compiled on the spot, so that the final program would run very quickly. Forth also produced very small programs that would run on computers with very little memory.Forth was 'flavour of the month' in the early 1980s, until people actually tried writing programs in it. Some persevered and became Forth programmers. Most just gave up. There was even a tiny computer, the Jupiter Ace, designed to run completely in Forth. This hit the shelves in 1983 and disappeared soon afterwards. Designed by two ex-Sinclair engineers, it shared many features with Sinclair's ZX81 and ZX Spectrum computers.The NameMoore called his new creation 'Fourth Generation Language' because he considered it the successor to the 'third generation' compiler-based languages of the day. He then abbreviated the name to 'FORTH' because the machine he was running it on could only take five letters in the name. Capital letters were the norm for computer languages in those days; in later years, it became acceptable to write the name in lower-case. The term north face north face jackets for 'Fourth Generation Language' (4GL) went on to mean something different in the computer world (a language for interrogation of relational databases), and Forth is not a 4GL in the normal sense of the term.Advantages and Disadvantages of ForthThere were a few things that made Forth really good:It was interactive. You could define a sequence of events, then try it out on the spot, so you could test each tiny component as you wrote it.It was compiled in a sort of a way, so Forth programs ran very quickly compared with normal interpreted languages. Forth wasn't quite compiled into machine code. Instead, it was compiled into a list of addresses at which the machine code routines could be found. This resulted in code which is not as fast as machine code but far faster than interpreted code.It was easily expandable. Each new function that you defined became a command in the language and could be used anywhere.On the other hand, Forth had some peculiar features which made it instantly disliked by many people:Because Forth uses direct manipulation of a stack for all arithmetic, most operations are performed in the reverse order to what you might expect. Instead of saying 'fetch contents of variable x', you say 'variable-x fetch-contents'. To add 1 and 2, you canada goose sale say '1 2 +'.To save typing, most commonly-used commands are given very short names. For example 'fetch-contents' is actually just @. This gives Forth programs a very cryptic look. If Z is a variable, you add 1 to Z with the following sequence:Z @ 1 + Z !It's simple when you know what all those symbols mean, but it is not easily approachable.Many Forth implementations use 16-bit whole numbers, so numbers bigger than 32,767 and numbers with a fractional part need special treatment.A Flavour of ForthIt is possible to get a feel for Forth in a few minutes.The Stack and 'Reverse Polish' NotationFor calculations, Forth uses a system known as 'Postfix' or 'Reverse Polish'. Invented by computer scientist Charles Hamblin based on work by Polish mathematician Jan Lukasiewicz, the 'Reverse Polish' system puts the numbers in first and then carries out the operation. To add 1 and 2, you put in 1, then put in 2, then tell the piumini moncler computer to add them up. We type the following into Forth:1 2 +Note that there are spaces between all the elements. This is important; Forth uses spaces to break up what you type into manageable words, which it can then think about one at a time. The numbers are stacked up as you enter them. When Forth encounters the 1, it puts it onto the stack of numbers, which will look like this:Top1BottomThen when Forth comes across the 2, it adds it to the top of the stack, so that the stack now looks like this:Top21BottomFinally, the '+' operation takes two numbers off the stack, adds them and puts the result back on the stack. After the '+', the stack will look like this:Top3BottomIf we want to see the result, we will have to display it, which is done using the '.' command. This takes the top number off the stack and displays it on the screen.So the full interaction between you and the computer would be:1 2 + . 3The here indicates that you press the Enter button on your keyboard. The computer's response will appear immediately, on the same line.The stack can hold many more than just two numbers, so more elaborate calculations can be done. To calculate ((12+13)*9 + (7*8))/3 we would type:12 13 + 9 * 7 8 * + 3 /If you work your way through this, you'll find that it does in fact calculate exactly the same thing. If you try it out on a Forth system, you'll find you get 93 rather than the expected 93.666667. Forth systems use whole numbers only, so 281 divided by 3 gives 93. There are ways of dealing with numbers with a decimal part, such as 93.666667 but they are more complicated and won't be discussed here.Stack OperationsSince all calculations depend heavily on the stack, there are a whole load of Forth commands for moving numbers around on the stack.DUP will copy the number on top of the stack and leave that copy on the stack.DROP will take the number off the top of the stack and throw it away.SWAP will swap the two numbers on the chaussures louis vuitton top of the stack.There are loads of other commands, including OVER, NIP, TUCK, ROT, ROLL and PICK.Simple DefinitionsForth commands are known as 'words'. There are many built-in words, but we can define our own ones as well. Suppose we want to take two numbers, square them and add them together. Let's start by working out how to square a number.Squaring a number means multiplying it by itself. Assume the number is on the doudoune canada goose stack at the start. We'll need to make a copy of it using DUP and then multiply the top two numbers on the stack using '*'. We can define a new word to do this:: Square DUP * ;The colon at the start marks the start of a word definition. 'Square' is the name we have chosen to give the word. The semicolon marks the gucci prezzi borse end of the definition. The rest is just the sequence of actions that should be carried out by the word. We can then use the word as if it was a standard Forth word:4 Square .This will display the result 16.Now we can define Squaresum, which takes the top two numbers on the stack, squares them and adds them together:: Squaresum Square SWAP Square + ;Once again, the colon and semicolon mark the beginning and end of the definition. Squaresum is the name of the word being defined and everything else is the list of actions to be performed. Note that the new word, Squaresum, uses the word we had just defined, Square, along with built-in Forth words. We can use Squaresum immediately:3 4 Squaresum .This should display the value 25.The Forth 'word' is the equivalent of a procedure, function or subroutine in other programming languages. Most languages would discourage writing subroutines as small as this because of the overhead of calling them, but Forth execution is so efficient that you don't need to worry.Conditions, Iterations and RecursionForth has all the usual IF/THEN/ELSE and BEGIN/WHILE/REPEAT structures that you get in any programming language, but they have an odd 'flavour'. For example, the syntax of IF is: IF ELSE THEN Note that the condition comes before the IF, rather than after it as in most other languages. Also, THEN marks the end of the if statement, rather than the action to be carried out if the condition is true. An example of an if statement is as follows: 0 = IF ." Top of stack is zero" ELSE ." Top of stack is not zero" THEN Here, '0 =' compares the top of the stack with zero. If the result of this is true, the message 'Top of stack is zero' is displayed using the word ." otherwise the message 'Top of stack is not zero' is displayed.There are a number of different words that can be used for iteration:BEGIN ... AGAINDO ... LOOPBEGIN ... WHILE ... REPEATOne word you won't find, though, is GOTO. Forth is completely structured and supports 'goto-less' programming.VariablesMost programming languages have a lot to say about variables. By using the stack, Forth can do a lot without ever needing a variable, but they certainly exist and are used. A variable is a piece of memory with a name. A variable normally holds a number, although it can also hold other things.You can define a variable in Forth using the word VARIABLE. This takes the next thing after the word VARIABLE and treats it as the name of the variable. You can fetch the value of a variable onto the stack using '@' and can store the value which is on the stack into the variable using '!'.VARIABLE?XDefines a variable called X.3 X !Stores the value 3 in the variable X.X @Fetches the value in the variable X and puts it on the stack.A variable is a word, the same as any other Forth command. When used on its own without '@' or '!', it puts the address of the variable, a number representing where in memory the variable is stored, onto the stack. This is useful, as it gives the programmer greater control over the variable.More Complex Data StructuresForth provides words which allow more complex data structures to be created. The most important of these are 'CREATE', which defines what actions are to be taken when the data structure is created, and 'DOES>', which defines what actions are to be taken when the data structure is used. By careful choice of these, all sorts of weird and wonderful structures can be created, as well as the traditional arrays, lists and records. For this reason, Forth does not provide any of the normal structures such as arrays, since they can be set up and customised so easily.Unfortunately, you need your head screwed on well to get it around the concepts involved in using CREATE and DOES>. They are very different from the sort of thing done in traditional programming languages.The ANSI Standard: Forth Made BoringIn the late 1970s, there were many different Forth systems, all incompatible with each other. In a bid to make Forth more uniform, standards were developed: Forth-79, Forth-83 and finally in 1994, the ANSI standard - ANS Forth. It is expected that all new Forth systems developed since then should be compatible with the ANSI standard.Unfortunately, the standard includes many features which are rarely used. To comply with the standard, a Forth system must be much bigger than really necessary. These extra features also slow down the operation of the system considerably, so Forth starts to lose its edge over other programming languages.Finally, it is possible to write Forth programs that are fully compatible with the standard but are little more than translations of C programs into Forth. Such programs will run, but they will probably run slowly and will take up a lot of space. Without the use of the special Forth features which are so tricky to understand for traditional programmers, the programs won't be any better than if they had been written in C.Forth TodayNowadays, processors are fast and memory is cheap - we don't worry too much about the resources a program consumes. Compilation is virtually instant in most programming languages, so the factors that led to the development of Forth are no longer particularly important.Nevertheless, Forth is still being promoted by a group of interested people. There are free versions available for PCs and louis vuitton prezzi borse PDAs. Most of these are primitive and lack the tools to do much other than write a few messages on the screen.Fully-functioned Forth systems are available if you want to pay, giving as much control over your computer as you would get from a full C system. Such systems tend to be produced by small companies and are not expensive: $50 - $100 (2004 US dollars) for a fully-fledged development system is peanuts compared with a development language from one of the big software suppliers. Microsoft has not produced a 'Visual Forth' - the Forth enthusiasts say this is because it would undermine the company's other products by running rings around them.No Gentle IntroductionForth's biggest problem is that there is no gentle introduction to it. None of the people writing about it are educators - they don't appear to be able to see the difficulty the average programmer will have in getting his or her head around the concepts of Forth. After playing around with the stack for a bit, most introductions to Forth launch into the deep end, with the internal structure of compilers, number formats and switching between compilation and execution on the fly. Admittedly, such concepts are much simpler in Forth than in other programming languages, but they still are daunting enough to put most people off. There seems to be no middle ground. Either you learn how to write simple programs and go no further, north face pants or you must understand every sac louis vuitton bowling montaigne single little detail of how Forth works.The FutureIt seems that there is still a future for Forth. It is ideal for writing applications for computers with limited resources in a rapidly-changing market. While personal computers no longer have such limitations, there are a whole load of new devices coming on-line such as phones, handhelds and even things as simple as televisions and washing machines, which can all benefit from Forth.What is needed is something to provide the middle ground, to bridge the gap between the basics and the full-blown complexity that is necessary to get the benefits from a Forth system. Perhaps such a breakthrough is just around the corner!More from the Edited GuideObject-oriented ProgrammingC - the Programming LanguageJava - the Programming LanguageHaskell - a Functional Programming LanguagePython - the Programming Language