A cement is a binder, a substance used for construction that sets and adheres to other materials to bind them together. Cement is used on its own, but rather to bind sand and gravel together. Cement mixed with fine aggregate produces mortar for masonry, or with sand and gravel, produces concrete. Cement is the most used material in existence and is only behind water as the planet's most-consumed resource. Cements used in construction are inorganic lime or calcium silicate based, can be characterized as either hydraulic or non-hydraulic, depending on the ability of the cement to set in the presence of water. Non-hydraulic cement does not set under water. Rather, it sets as it reacts with carbon dioxide in the air, it is resistant to attack by chemicals after setting. Hydraulic cements set and become adhesive due to a chemical reaction between the dry ingredients and water; the chemical reaction results in mineral hydrates that are not water-soluble and so are quite durable in water and safe from chemical attack.
This allows setting in wet conditions or under water and further protects the hardened material from chemical attack. The chemical process for hydraulic cement found by ancient Romans used volcanic ash with added lime; the word "cement" can be traced back to the Roman term opus caementicium, used to describe masonry resembling modern concrete, made from crushed rock with burnt lime as binder. The volcanic ash and pulverized brick supplements that were added to the burnt lime, to obtain a hydraulic binder, were referred to as cementum, cimentum, cäment, cement. In modern times, organic polymers are sometimes used as cements in concrete. Non-hydraulic cement, such as slaked lime, hardens by carbonation in the presence of carbon dioxide, present in the air. First calcium oxide is produced from calcium carbonate by calcination at temperatures above 825 °C for about 10 hours at atmospheric pressure: CaCO3 → CaO + CO2The calcium oxide is spent mixing it with water to make slaked lime: CaO + H2O → Ca2Once the excess water is evaporated, the carbonation starts: Ca2 + CO2 → CaCO3 + H2OThis reaction takes time, because the partial pressure of carbon dioxide in the air is low.
The carbonation reaction requires that the dry cement be exposed to air, so the slaked lime is a non-hydraulic cement and cannot be used under water. This process is called the lime cycle. Conversely, hydraulic cement hardens by hydration. Hydraulic cements are made of a mixture of silicates and oxides, the four main components being: Belite; the silicates are responsible for the cement's mechanical properties—the tricalcium aluminate and brownmillerite are essential for formation of the liquid phase during the kiln sintering. The chemistry of these reactions is not clear and is still the object of research; the earliest known occurrence of cement is from twelve million years ago. A deposit of cement was formed after an occurrence of oil shale located adjacent to a bed of limestone burned due to natural causes; these ancient deposits were investigated in the 1970s. Cement, chemically speaking, is a product that includes lime as the primary curing ingredient, but is far from the first material used for cementation.
The Babylonians and Assyrians used bitumen to bind together burnt alabaster slabs. In Egypt stone blocks were cemented together with a mortar made of sand and burnt gypsum, which contained calcium carbonate. Lime was used by the ancient Greeks. There is evidence that the Minoans of Crete used crushed potshards as an artificial pozzolan for hydraulic cement. Nobody knows who first discovered that a combination of hydrated non-hydraulic lime and a pozzolan produces a hydraulic mixture —but such concrete was used by the Ancient Macedonians, three centuries on a large scale by Roman engineers. There is... a kind of powder. It is found in the neighborhood of Baiae and in the country belonging to the towns round about Mt. Vesuvius; this substance when mixed with lime and rubble not only lends strength to buildings of other kinds, but when piers of it are constructed in the sea, they set hard under water. The Greeks used volcanic tuff from the island of Thera as their pozzolan and the Romans used crushed volcanic ash with lime.
This mixture could set under water. The material was called pozzolana from the town of Pozzuoli, west of Naples where volcanic ash was extracted. In the absence of pozzolanic ash, the Romans used powdered brick or pottery as a substitute and they may have used crushed tiles for this purpose before discovering natural sources near Rome; the huge dome of the Pantheon in Rome and the massive Baths of Caracalla are examples of ancient structures made from these concretes, many of which still stand. The vast system of Roman aqueducts made extensive use of hydraulic cement. Roman concrete was used on the outside of buildings; the normal technique was to use brick facing material as the formwork for an infill of mortar mixed with an aggregate of broken pieces of stone, potsherds, recycled chunks of concrete, or other building ru
Leather is a natural durable and flexible material created by tanning animal rawhides and skins. The most common raw material is cattle hide, it can be produced at manufacturing scales ranging from artisan to modern industrial scale. Leather is used to make a variety of articles, including footwear, automobile seats, bags, book bindings, fashion accessories, furniture, it is decorated by a wide range of techniques. The earliest record of leather artifacts dates back to 2200 BC; the leather manufacturing process is divided into three fundamental subprocesses: preparatory stages and crusting. A further subprocess, can be added into the leather process sequence, but not all leathers receive finishing; the preparatory stages are. Preparatory stages may include: soaking, liming, bating and pickling. Tanning is a process that stabilizes the proteins collagen, of the raw hide to increase the thermal and microbiological stability of the hides and skins, making it suitable for a wide variety of end applications.
The principal difference between raw and tanned hides is that raw hides dry out to form a hard, inflexible material that, when rewetted, will putrefy, while tanned material dries to a flexible form that does not become putrid when rewetted. Many tanning methods and materials exist; the typical process sees tanners load the hides into a drum and immerse them in a tank that contains the tanning "liquor". The hides soak while the drum rotates about its axis, the tanning liquor penetrates through the full thickness of the hide. Once the process achieves penetration, workers raise the liquor's pH in a process called basification, which fixes the tanning material to the leather; the more tanning material fixed, the higher the leather's hydrothermal stability and shrinkage temperature resistance. Crusting is a process that lubricates leather, it includes a coloring operation. Chemicals added during crusting must be fixed in place. Crusting culminates with a drying and softening operation, may include splitting, dyeing, whitening or other methods.
For some leathers, tanners apply a surface coating, called "finishing". Finishing operations can include oiling, buffing, polishing, glazing, or tumbling, among others. Leather can be oiled to improve its water resistance; this currying process after tanning supplements the natural oils remaining in the leather itself, which can be washed out through repeated exposure to water. Frequent oiling of leather, with mink oil, neatsfoot oil, or a similar material keeps it supple and improves its lifespan dramatically. Tanning processes differ in which chemicals are used in the tanning liquor; some common types include: Vegetable-tanned leather is tanned using tannins extracted from vegetable matter, such as tree bark prepared in bark mills. It is the oldest known method, it is supple and brown in color, with the exact shade depending on the mix of materials and the color of the skin. The color tan derives its name from the appearance of undyed vegetable-tanned leather. Vegetable-tanned leather is not stable in water.
This is a feature of oak-bark-tanned leather, exploited in traditional shoemaking. In hot water, it shrinks drastically and congeals, becoming rigid and brittle. Boiled leather is an example of this, where the leather has been hardened by being immersed in hot water, or in boiled wax or similar substances, it was used as armor after hardening, it has been used for book binding. Chrome-tanned leather, invented in 1858, is tanned using chromium other chromium salts, it is known as "wet blue" for the pale blue color of the undyed leather. The chrome tanning method takes one day to complete, making it best suited for large-scale industrial use; this is the most common method in modern use. It is more supple and pliable than vegetable-tanned leather and does not discolor or lose shape as drastically in water as vegetable-tanned. However, there are environmental concerns with this tanning method. Aldehyde-tanned leather is tanned using oxazolidine compounds, it is referred to as "wet white" due to its pale cream color.
It is the main type of "chrome-free" leather seen in shoes for infants and automobiles. Formaldehyde has been used for tanning in the past. Chamois leather is a form of aldehyde tanning that produces a porous and water-absorbent leather. Chamois leather is made using marine oils that oxidize to produce the aldehydes that tan the leather. Brain tanned leathers are made by a labor-intensive process that uses emulsified oils those of animal brains such as deer and buffalo, they are known for their exceptional washability. Alum leather is transformed using aluminium salts mixed with a variety of binders and protein sources, such as flour and egg yolk. Alum leather is not tanned. In general, leather is produced in the following grades: Top-grain leather includes the outer layer of the hide, known as the grain, which features finer, more densely packed fibers, resulting in strength and durability. Depending on thickness, it may contain some of the more fibrous under layer, known as the corium. Types of top-grain leather incl
Sugar is the generic name for sweet-tasting, soluble carbohydrates, many of which are used in food. The various types of sugar are derived from different sources. Simple sugars are called monosaccharides and include glucose and galactose. "Table sugar" or "granulated sugar" refers to a disaccharide of glucose and fructose. In the body, sucrose is hydrolysed into glucose. Sugars are found in the tissues of most plants, but sucrose is concentrated in sugarcane and sugar beet, making them ideal for efficient commercial extraction to make refined sugar. Sugarcane originated in tropical Indian subcontinent and Southeast Asia, is known of from before 6,000 BP, sugar beet was first described in writing by Olivier de Serres and originated in southwestern and Southeast Europe along the Atlantic coasts and the Mediterranean Sea, in North Africa, Macaronesia, to Western Asia. In 2016, the combined world production of those two crops was about two billion tonnes. Other disaccharides include lactose. Longer chains of sugar molecules are called polysaccharides.
Some other chemical substances, such as glycerol and sugar alcohols, may have a sweet taste, but are not classified as sugar. Sucrose is used in prepared foods, is sometimes added to commercially available beverages, may be used by people as a sweetener for foods and beverages; the average person consumes about 24 kilograms of sugar each year, or 33.1 kilograms in developed countries, equivalent to over 260 food calories per day. As sugar consumption grew in the latter part of the 20th century, researchers began to examine whether a diet high in sugar refined sugar, was damaging to human health. Excessive consumption of sugar has been implicated in the onset of obesity, cardiovascular disease and tooth decay. Numerous studies have tried to clarify those implications, but with varying results because of the difficulty of finding populations for use as controls that consume little or no sugar. In 2015, the World Health Organization recommended that adults and children reduce their intake of free sugars to less than 10%, encouraged a reduction to below 5%, of their total energy intake.
The etymology reflects the spread of the commodity. From Sanskrit शर्करा, meaning "ground or candied sugar," "grit, gravel", came Persian shakar, whence Arabic سكر, whence Medieval Latin succarum, whence 12th-century French sucre, whence the English word sugar. Italian zucchero, Spanish azúcar, Portuguese açúcar came directly from Arabic, the Spanish and Portuguese words retaining the Arabic definite article; the earliest Greek word attested is σάκχαρις. The English word jaggery, a coarse brown sugar made from date palm sap or sugarcane juice, has a similar etymological origin: Portuguese jágara from the Malayalam ചക്കരാ, itself from the Sanskrit शर्करा. Sugar has been produced in the Indian subcontinent since ancient times and its cultivation spread from there into modern-day Afghanistan through the Khyber Pass, it was not plentiful or cheap in early times, in most parts of the world, honey was more used for sweetening. People chewed raw sugarcane to extract its sweetness. Sugarcane was a native of Southeast Asia.
Different species seem to have originated from different locations with Saccharum barberi originating in India and S. edule and S. officinarum coming from New Guinea. One of the earliest historical references to sugarcane is in Chinese manuscripts dating to 8th century BCE, which state that the use of sugarcane originated in India. In the tradition of Indian medicine, the sugarcane is known by the name Ikṣu and the sugarcane juice is known as Phāṇita, its varieties and characterics are defined in nighaṇṭus such as the Bhāvaprakāśa. Sugar remained unimportant until the Indians discovered methods of turning sugarcane juice into granulated crystals that were easier to store and to transport. Crystallized sugar was discovered by the time of the Imperial Guptas, around the 5th century CE. In the local Indian language, these crystals were called khanda, the source of the word candy. Indian sailors, who carried clarified butter and sugar as supplies, introduced knowledge of sugar along the various trade routes they travelled.
Traveling Buddhist monks took sugar crystallization methods to China. During the reign of Harsha in North India, Indian envoys in Tang China taught methods of cultivating sugarcane after Emperor Taizong of Tang made known his interest in sugar. China established its first sugarcane plantations in the seventh century. Chinese documents confirm at least two missions to India, initiated in 647 CE, to obtain technology for sugar refining. In the Indian subcontinent, the Middle East and China, sugar became a staple of cooking and desserts. Nearchus, admiral of Alexander of Macedonia, knew of sugar during the year 325 B. C. because of his participation in the campaign of India led by Alexander. The Greek physician Pedanius Dioscorides in the 1st century CE described sugar in his medical treatise De Materia Medica, Pliny the Elder, a 1st-century CE Roman, described sugar in his Natural History: "Sugar is made in Arabia as well, but Indian sugar is better, it is a kind of honey found in cane, white as gum, it crunches between the teeth.
It comes in lumps the size of a hazelnut. Sugar is used only for medical purposes." Crusaders brought sugar back to Europe after their campaigns in the Hol
The Pan-American Highway is a network of roads stretching across the American continents and measuring about 30,000 kilometres in total length. Except for a rainforest break of 160 km, called the Darién Gap, the roads link all of the Pacific coastal countries of the Americas in a connected highway system. According to Guinness World Records, the Pan-American Highway is the world's longest "motorable road". However, because of the Darién Gap, it is not possible to cross between South America and Central America with conventional highway vehicles. Without an all-terrain vehicle, it is necessary to circumnavigate this terrestrial stretch by sea; the Pan-American Highway passes through many diverse climates and ecological types, from dense jungles, to arid deserts, to barren tundra, some of which are passable only during the dry season, in many regions driving is hazardous. The Pan-American Highway system is physically complete and extends in de facto terms from Prudhoe Bay, Alaska, in North America to the lower reaches of South America.
Several southern highway termini are claimed to exist, including the cities of Puerto Montt and Quellón in Chile and Ushuaia in Argentina. West and north of the Darién Gap, it is known as the Inter-American Highway through Central America and Mexico where it splits into several spurs leading to the Mexico–US border; the concept of an overland route from one tip of the Americas to the other was proposed at the First Pan-American Conference in 1889 as a railroad. The idea of building a highway emerged at the Fifth International Conference of American States in 1923; the first conference regarding construction of the highway occurred on October 5, 1925. On July 29, 1937, Bolivia, Colombia, Costa Rica, El Salvador, Honduras, Nicaragua, Peru and the United States signed the Convention on the Pan-American Highway, whereby they agreed to speedy construction, by all adequate means. In 1950, Mexico became the first Latin American country to complete its portion of the highway. In practice the concept of the Pan-American Highway is more publicly embraced in Latin American countries than in North America.
Much of the road system in Latin America is explicitly marked as Pan-American. The Northern Pan-American Highway travels through 9 countries: The Southern Pan-American Highway travels through 8 countries: Important spurs lead into 4 countries: The Alaska Highway through Alaska and British Columbia is considered a de facto northerly extension of the Pan-American Highway, as well as the Dalton Highway in Alaska. In Canada, no particular road has been designated as the Pan-American Highway; the National Highway System, which includes but is not limited to the Trans-Canada Highway, is the country's only official interprovincial highway system. However, there are several Canadian routes that are a natural extension of several key American highways that reach the Canada–US border. British Columbia Highway 97 and Highway 2 to Alberta both pick up where the southern end of the Alaska highway leaves off. Highway 97 becomes U. S. Route 97 at the Canada–US border. British Columbia Highway 99 provides an alternate route from Highway 97 just north of Cache Creek that runs through Whistler and Vancouver before ending at the Canada–US border at the north end of Interstate 5 in Washington State, the beginning of the official Pan-American route south of British Columbia.
Meanwhile, Alberta Highway 2 runs south and east to Alberta Highway 3 leading into Lethbridge south on Alberta Highway 4 to the Canada–US border where it becomes Interstate 15 in Montana, the first official stretch of the Pan-American Highway south of the Alberta route. In 1966, the Federal Highway Administration designated the entire Interstate Highway System part of the Pan-American Highway System, although this has never been reflected in any of the official interstate signage. Of the many freeways that make up this comprehensive system, there are several that stand out because of their north-south orientation and their links to the main Mexican route and its spurs as well as to key routes in Canada that link to the Alaska Highway; these include the following: I-5 Runs north from San Diego to Seattle links indirectly with British Columbia Highway 97 north of the Canada–US border. A technically direct link between the same interstate and the highway 97 system can be found near Weed, California.
US route 97 runs northeast north from this junction and becomes BC highway 97 at the border with Canada. I-15 links San Diego with Alberta Highway 2 that crosses into British Columbia and ends at the southern terminus of the Alaska Highway. Interstate 8 provides an east-west link from San Diego to Interstate 10 near Arizona; the latter continues to Tuscon and links with Interstate 19 that becomes a spur of the Pan-American highway through Mexico at the Nogales border crossing. Interstate 25 runs north from Interstate 10 at New Mexico to Interstate 90 in Wyoming; this route has no direct extension into Canada, but links indirectly to Interstate 15. Interstate 25 in Albuquerque, New Mexico, was named the Pan-American Freeway, as an extension of highway 45, the Mexican spur linking El Paso to the original route along highway 85 north of Mexico City; this portion of I-25 follows the historic Camino Real, thus serves a culturally significant portion of the Pan American system. Like I-15, the complete route of Interstate 25 is an official northerly continuation towards Alberta, where Highway 2 provides a direct but unofficial Canadian link to the Alaska Highway.
Interstate 35 is a northerly continuation of the
Vegetables are parts of plants that are consumed by humans or other animals as food. The original meaning is still used and is applied to plants collectively to refer to all edible plant matter, including the flowers, stems, leaves and seeds; the alternate definition of the term vegetable is applied somewhat arbitrarily by culinary and cultural tradition. It may exclude foods derived from some plants that are fruits and cereal grains, but include fruits from others such as tomatoes and courgettes and seeds such as pulses. Vegetables were collected from the wild by hunter-gatherers and entered cultivation in several parts of the world during the period 10,000 BC to 7,000 BC, when a new agricultural way of life developed. At first, plants which grew locally would have been cultivated, but as time went on, trade brought exotic crops from elsewhere to add to domestic types. Nowadays, most vegetables are grown all over the world as climate permits, crops may be cultivated in protected environments in less suitable locations.
China is the largest producer of vegetables and global trade in agricultural products allows consumers to purchase vegetables grown in faraway countries. The scale of production varies from subsistence farmers supplying the needs of their family for food, to agribusinesses with vast acreages of single-product crops. Depending on the type of vegetable concerned, harvesting the crop is followed by grading, storing and marketing. Vegetables can be eaten either raw or cooked and play an important role in human nutrition, being low in fat and carbohydrates, but high in vitamins and dietary fiber. Many nutritionists encourage people to consume plenty of fruit and vegetables, five or more portions a day being recommended; the word vegetable was first recorded in English in the early 15th century. It comes from Old French, was applied to all plants, it derives from Medieval Latin vegetabilis "growing, flourishing", a semantic change from a Late Latin meaning "to be enlivening, quickening". The meaning of "vegetable" as a "plant grown for food" was not established until the 18th century.
In 1767, the word was used to mean a "plant cultivated for food, an edible herb or root". The year 1955 saw the first use of the shortened, slang term "veggie"; as an adjective, the word vegetable is used in scientific and technical contexts with a different and much broader meaning, namely of "related to plants" in general, edible or not—as in vegetable matter, vegetable kingdom, vegetable origin, etc. The exact definition of "vegetable" may vary because of the many parts of a plant consumed as food worldwide—roots, leaves, flowers and seeds; the broadest definition is the word's use adjectivally to mean "matter of plant origin". More a vegetable may be defined as "any plant, part of, used for food", a secondary meaning being "the edible part of such a plant". A more precise definition is "any plant part consumed for food, not a fruit or seed, but including mature fruits that are eaten as part of a main meal". Falling outside these definitions are edible fungi and edible seaweed which, although not parts of plants, are treated as vegetables.
In the latter-mentioned definition of "vegetable", used in everyday language, the words "fruit" and "vegetable" are mutually exclusive. "Fruit" has a precise botanical meaning, being a part that developed from the ovary of a flowering plant. This is different from the word's culinary meaning. While peaches and oranges are "fruit" in both senses, many items called "vegetables", such as eggplants, bell peppers, tomatoes, are botanically fruits; the question of whether the tomato is a fruit or a vegetable found its way into the United States Supreme Court in 1893. The court ruled unanimously in Nix v. Hedden that a tomato is identified as, thus taxed as, a vegetable, for the purposes of the Tariff of 1883 on imported produce; the court did acknowledge, that, botanically speaking, a tomato is a fruit. Before the advent of agriculture, humans were hunter-gatherers, they foraged for edible fruit, stems, leaves and tubers, scavenged for dead animals and hunted living ones for food. Forest gardening in a tropical jungle clearing is thought to be the first example of agriculture.
Plant breeding through the selection of strains with desirable traits such as large fruit and vigorous growth soon followed. While the first evidence for the domestication of grasses such as wheat and barley has been found in the Fertile Crescent in the Middle East, it is that various peoples around the world started growing crops in the period 10,000 BC to 7,000 BC. Subsistence agriculture continues to this day, with many rural farmers in Africa, South America, elsewhere using their plots of land to produce enough food for their families, while any surplus produce is used for exchange for other goods. Throughout recorded history, the rich have been able to afford a varied diet including meat and fruit, but for poor people, meat was a luxury and the food they ate was dull comprising some staple product made from rice, barley, millet or maize; the addition of vegetable matter provided some variety to the diet. The staple diet of the Aztecs in Central America was maize and they cultivated tomatoes, beans, pumpkins, squashes and amaranth seeds to supplement their tortillas and porridge.
In Peru, the Incas subsisted on maize in the lowla
Animal husbandry is the branch of agriculture concerned with animals that are raised for meat, milk, eggs, or other products. It includes selective breeding and the raising of livestock. Husbandry has a long history, starting with the Neolithic revolution when animals were first domesticated, from around 13,000 BC onwards, antedating farming of the first crops. By the time of early civilisations such as ancient Egypt, sheep and pigs were being raised on farms. Major changes took place in the Columbian Exchange when Old World livestock were brought to the New World, in the British Agricultural Revolution of the 18th century, when livestock breeds like the Dishley Longhorn cattle and Lincoln Longwool sheep were improved by agriculturalists such as Robert Bakewell to yield more meat and wool. A wide range of other species such as horse, water buffalo, llama and guinea pig are used as livestock in some parts of the world. Insect farming, as well as aquaculture of fish and crustaceans, is widespread.
Modern animal husbandry relies on production systems adapted to the type of land available. Subsistence farming is being superseded by intensive animal farming in the more developed parts of the world, where for example beef cattle are kept in high density feedlots, thousands of chickens may be raised in broiler houses or batteries. On poorer soil such as in uplands, animals are kept more extensively, may be allowed to roam foraging for themselves. Most livestock are herbivores, except for chickens which are omnivores. Ruminants like cattle and sheep are adapted to feed on grass. Pigs and poultry cannot digest the cellulose in forage, require cereals and other high-energy foods; the domestication of livestock was driven by the need to have food on hand when hunting was unproductive. The desirable characteristics of a domestic animal are that it should be useful to man, should be able to thrive in his company, should breed and be easy to tend. Domestication was not a single event. Sheep and goats were the animals that accompanied the nomads in the Middle East, while cattle and pigs were associated with more settled communities.
The first wild animal to be domesticated was the dog. Half-wild dogs starting with young individuals, may have been tolerated as scavengers and killers of vermin, being pack hunters, were predisposed to become part of the human pack and join in the hunt. Prey animals, goats and cattle, were progressively domesticated early in the history of agriculture. Pigs were domesticated in Mesopotamia around 13,000 BC, sheep followed, some time between 11,000 and 9,000 BC. Cattle were domesticated from the wild aurochs in the areas of modern Turkey and Pakistan around 8,500 BC. A cow was a great advantage to a villager as she produced more milk than her calf needed, her strength could be put to use as a working animal, pulling a plough to increase production of crops, drawing a sledge, a cart, to bring the produce home from the field. Draught animals were first used about 4,000 BC in the Middle East, increasing agricultural production immeasurably. In southern Asia, the elephant was domesticated by 6,000 BC.
Fossilised chicken bones dated to 5040 BC have been found in northeastern China, far from where their wild ancestors lived in the jungles of tropical Asia, but archaeologists believe that the original purpose of domestication was for the sport of cockfighting. Meanwhile, in South America, the llama and the alpaca had been domesticated before 3,000 BC, as beasts of burden and for their wool. Neither was strong enough to pull a plough which limited the development of agriculture in the New World. Horses occur on the steppes of Central Asia, their domestication, around 3,000 BC in the Black Sea and Caspian Sea region, was as a source of meat. Around the same time, the wild ass was being tamed in Egypt. Camels were domesticated soon after this, with the Bactrian camel in Mongolia and the Arabian camel becoming beasts of burden. By 1000 BC, caravans of Arabian camels were linking India with the Mediterranean. In ancient Egypt, cattle were the most important livestock, sheep and pigs were kept; the Nile provided a plentiful source of fish.
Honey bees were domesticated from at least the Old Kingdom, providing both wax. In ancient Rome, all the livestock known in ancient Egypt were available. In addition, rabbits were domesticated for food by the first century BC. To help flush them out from their underground burrows, the polecat was domesticated as the ferret, its use described by Pliny the Elder. In northern Europe, agriculture including animal husbandry went into decline when the Roman empire collapsed; some aspects such as the herding of animals continued throughout the period. By the 11th century, the economy had recovered and the countryside was again productive; the Domesday Book recorded every parcel of land and every animal in Britain: "there was not one single hide, nor a yard of land, moreover... not an ox, nor a cow, nor a swine was there left, not set down in writ." For example, the royal manor of Earley in Berkshire, one of thousands of villages recorded in the book, had in 1086 "2 fisheries worth 7s and 6d and 20 acres of meadow.
Woodland for 70 pigs." Exploration and colonisat
Mean sea level is an average level of the surface of one or more of Earth's oceans from which heights such as elevation may be measured. MSL is a type of vertical datum – a standardised geodetic datum –, used, for example, as a chart datum in cartography and marine navigation, or, in aviation, as the standard sea level at which atmospheric pressure is measured to calibrate altitude and aircraft flight levels. A common and straightforward mean sea-level standard is the midpoint between a mean low and mean high tide at a particular location. Sea levels can be affected by many factors and are known to have varied over geological time scales; however 20th century and current millennium sea level rise is caused by global warming, careful measurement of variations in MSL can offer insights into ongoing climate change. The term above sea level refers to above mean sea level. Precise determination of a "mean sea level" is difficult to achieve because of the many factors that affect sea level. Instantaneous sea level varies quite a lot on several scales of space.
This is because the sea is in constant motion, affected by the tides, atmospheric pressure, local gravitational differences, salinity and so forth. The easiest way this may be calculated is by selecting a location and calculating the mean sea level at that point and use it as a datum. For example, a period of 19 years of hourly level observations may be averaged and used to determine the mean sea level at some measurement point. Still-water level or still-water sea level is the level of the sea with motions such as wind waves averaged out. MSL implies the SWL further averaged over a period of time such that changes due to, e.g. the tides have zero mean. Global MSL refers to a spatial average over the entire ocean. One measures the values of MSL in respect to the land. In the UK, the Ordnance Datum is the mean sea level measured at Newlyn in Cornwall between 1915 and 1921. Prior to 1921, the vertical datum was MSL at the Victoria Liverpool. Since the times of the Russian Empire, in Russia and other former its parts, now independent states, the sea level is measured from the zero level of Kronstadt Sea-Gauge.
In Hong Kong, "mPD" is a surveying term meaning "metres above Principal Datum" and refers to height of 1.230m below the average sea level. In France, the Marégraphe in Marseilles measures continuously the sea level since 1883 and offers the longest collapsed data about the sea level, it is used for main part of Africa as official sea level. As for Spain, the reference to measure heights below or above sea level is placed in Alicante. Elsewhere in Europe vertical elevation references are made to the Amsterdam Peil elevation, which dates back to the 1690s. Satellite altimeters have been making precise measurements of sea level since the launch of TOPEX/Poseidon in 1992. A joint mission of NASA and CNES, TOPEX/Poseidon was followed by Jason-1 in 2001 and the Ocean Surface Topography Mission on the Jason-2 satellite in 2008. Height above mean sea level is the elevation or altitude of an object, relative to the average sea level datum, it is used in aviation, where some heights are recorded and reported with respect to mean sea level, in the atmospheric sciences, land surveying.
An alternative is to base height measurements on an ellipsoid of the entire Earth, what systems such as GPS do. In aviation, the ellipsoid known as World Geodetic System 84 is used to define heights; the alternative is to use a geoid-based vertical datum such as NAVD88. When referring to geographic features such as mountains on a topographic map, variations in elevation are shown by contour lines; the elevation of a mountain denotes the highest point or summit and is illustrated as a small circle on a topographic map with the AMSL height shown in metres, feet or both. In the rare case that a location is below sea level, the elevation AMSL is negative. For one such case, see Amsterdam Airport Schiphol. To extend this definition far from the sea means comparing the local height of the mean sea surface with a "level" reference surface, or geodetic datum, called the geoid. In a state of rest or absence of external forces, the mean sea level would coincide with this geoid surface, being an equipotential surface of the Earth's gravitational field.
In reality, due to currents, air pressure variations and salinity variations, etc. this does not occur, not as a long-term average. The location-dependent, but persistent in time, separation between mean sea level and the geoid is referred to as ocean surface topography, it varies globally in a range of ± 2 m. Adjustments were made to sea-level measurements to take into account the effects of the 235 lunar month Metonic cycle and the 223-month eclipse cycle on the tides. Several terms are used to describe the changing relationships between sea level and dry land; when the term "relative" is used, it means change relative to a fixed point in the sediment pile. The term "eustatic" refers to global changes in sea level relative to a fixed point, such as the centre of the earth, for example as a result of melting ice-caps; the term "steric" refers to global changes in sea level due to thermal expansion and salinity variations. The term "isostatic" refers to changes in