Steam is water in the gas phase, formed when water boils or evaporates. Steam is invisible. At lower pressures, such as in the upper atmosphere or at the top of high mountains, water boils at a lower temperature than the nominal 100 °C at standard pressure. If heated further it becomes superheated steam; the enthalpy of vaporization is the energy required to turn water into the gaseous form when it increases in volume by 1,700 times at standard temperature and pressure. Piston type steam engines played a central role to the Industrial Revolution and modern steam turbines are used to generate more than 80% of the world's electricity. If liquid water comes in contact with a hot surface or depressurizes below its vapor pressure, it can create a steam explosion. Steam is traditionally created by heating a boiler via burning coal and other fuels, but it is possible to create steam with solar energy. Water vapor that includes water droplets is described as wet steam; as wet steam is heated further, the droplets evaporate, at a high enough temperature all of the water evaporates and the system is in vapor–liquid equilibrium.
Superheated steam is steam at a temperature higher than its boiling point for the pressure, which only occurs where all liquid water has evaporated or has been removed from the system. Steam tables contain thermodynamic data for water/steam and are used by engineers and scientists in design and operation of equipment where thermodynamic cycles involving steam are used. Additionally, thermodynamic phase diagrams for water/steam, such as a temperature-entropy diagram or a Mollier diagram shown in this article, may be useful. Steam charts are used for analysing thermodynamic cycles. In agriculture, steam is used for soil sterilization to avoid the use of harmful chemical agents and increase soil health. Steam's capacity to transfer heat is used in the home: for cooking vegetables, steam cleaning of fabric and flooring, for heating buildings. In each case, water is heated in a boiler, the steam carries the energy to a target object. Steam is used in ironing clothes to add enough humidity with the heat to take wrinkles out and put intentional creases into the clothing.
As of 2000 around 90% of all electricity was generated using steam as the working fluid, nearly all by steam turbines. In electric generation, steam is condensed at the end of its expansion cycle, returned to the boiler for re-use. However, in cogeneration, steam is piped into buildings through a district heating system to provide heat energy after its use in the electric generation cycle; the world's biggest steam generation system is the New York City steam system, which pumps steam into 100,000 buildings in Manhattan from seven cogeneration plants. In other industrial applications steam is used for energy storage, introduced and extracted by heat transfer through pipes. Steam is a capacious reservoir for thermal energy because of water's high heat of vaporization. Fireless steam locomotives were steam locomotives that operated from a supply of steam stored on board in a large tank resembling a conventional locomotive's boiler; this tank was filled by process steam, as is available in many sorts of large factory, such as paper mills.
The locomotive's propulsion used connecting rods, as for a typical steam locomotive. These locomotives were used in places where there was a risk of fire from a boiler's firebox, but were used in factories that had a plentiful supply of steam to spare. Owing to its low molecular mass, steam is an effective lifting gas, providing 60% as much lift as helium and twice as much as hot air, it is not flammable, unlike hydrogen, is cheap and abundant, unlike helium. The required heat, leads to condensation problems and requires an insulated envelope; these factors have limited its use thus far to demonstration projects. Steam engines and steam turbines use the expansion of steam to drive a piston or turbine to perform mechanical work; the ability to return condensed steam as water-liquid to the boiler at high pressure with little expenditure of pumping power is important. Condensation of steam to water occurs at the low-pressure end of a steam turbine, since this maximizes the energy efficiency, but such wet-steam conditions must be limited to avoid excessive turbine blade erosion.
Engineers use an idealised thermodynamic cycle, the Rankine cycle, to model the behavior of steam engines. Steam turbines are used in the production of electricity. An autoclave, which uses steam under pressure, is used in microbiology laboratories and similar environments for sterilization. Steam dry steam, may be used for antimicrobial cleaning to the levels of sterilization. Steam is a non-toxic antimicrobial agent. Steam is used in piping for utility lines, it is used in jacketing and tracing of piping to maintain the uniform temperature in pipelines and vessels. Steam is used in the process of wood killing insects and increasing plasticity. Steam is used to accentuate drying in prefabricates. Care should be taken since concrete produces heat during hydration and additional heat from the steam could be detrimental to hardening reaction processes of the concrete. Used in cleaning of fibers and other materials, sometimes in preparation for painting. Steam is useful in melting hardened grease and oil resid
The grid plan, grid street plan, or gridiron plan is a type of city plan in which streets run at right angles to each other, forming a grid. The infrastructure cost for regular grid patterns is higher than for patterns with discontinuous streets. Costs for streets depend on four variables: street width, street length, block width and pavement width. Two inherent characteristics of the grid plan, frequent intersections and orthogonal geometry, facilitate pedestrian movement; the geometry helps with orientation and wayfinding and its frequent intersections with the choice and directness of route to desired destinations. In ancient Rome, the grid plan method of land measurement was called centuriation; the grid plan originated in multiple cultures. By 2600 BC, Mohenjo-daro and Harappa, major cities of the Indus Valley Civilization, were built with blocks divided by a grid of straight streets, running north–south and east–west; each block was subdivided by small lanes. The cities and monasteries of Gandhara, dating from the 1st millennium BC to the 11th century AD had grid-based designs.
Islamabad, the capital of Pakistan since 1959, was founded on the grid-plan of the nearby ruined city of Sirkap. A workers' village at Giza, housed a rotating labor force and was laid out in blocks of long galleries separated by streets in a formal grid. Many pyramid-cult cities used a common orientation: a north–south axis from the royal palace and an east–west axis from the temple, meeting at a central plaza where King and God merged and crossed. Hammurabi king of the Babylonian Empire in the 17th century BC, ordered the rebuilding of Babylon: constructing and restoring temples, city walls, public buildings, irrigation canals; the streets of Babylon were wide and straight, intersected at right angles, were paved with bricks and bitumen. The tradition of grid plans is continuous in China from the 15th century BC onward in the traditional urban planning of various ancient Chinese states. Guidelines put into written form in the Kaogongji during the Spring and Autumn period stated: "a capital city should be square on plan.
Three gates on each side of the perimeter lead into the nine main streets that crisscross the city and define its grid-pattern. And for its layout the city should have the Royal Court situated in the south, the Marketplace in the north, the Imperial Ancestral Temple in the east and the Altar to the Gods of Land and Grain in the west." Teotihuacan, near modern-day Mexico City, is the largest ancient grid-plan site in the Americas. The city's grid covered 21 square kilometres; the most well-known grid system is that spread through the colonies of the Roman Empire. The archetypal Roman Grid was introduced to Italy first by the Greeks, with such information transferred by way of trade and conquest. Although the idea of the grid was present in Hellenic societal and city planning, it was not pervasive prior to the 5th century BC. However, it gained primacy through the work of Hippodamus of Miletus, who planned and replanned many Greek cities in accordance with this form; the concept of a grid as the ideal method of town planning had become accepted by the time of Alexander the Great.
His conquests were a step in the propagation of the grid plan throughout colonies, some as far-flung as Taxila in Pakistan, that would be mirrored by the expansion of the Roman Empire. The Greek grid had its streets aligned in relation to the cardinal points and looked to take advantage of visual cues based on the hilly landscape typical of Greece and Asia Minor; this was best exemplified in Priene, in present-day western Turkey, where the orthogonal city grid was based on the cardinal points, on sloping terrain that struck views out towards a river and the city of Miletus. The Etruscan people, whose territories in Italy encompassed what would become Rome, founded what is now the city of Marzabotto at the end of the 6th century BC, its layout was based on Greek Ionic ideas, it was here that the main east–west and north–south axes of a town could first be seen in Italy. According to Stanislawski, the Romans did use grids until the time of the late Republic or early Empire, when they introduced centuriation, a system which they spread around the Mediterranean and into northern Europe on.
The military expansion of this period facilitated the adoption of the grid form as standard: the Romans established castra first as military centres. The Roman grid was similar in form to the Greek version of a grid, but allowed for practical considerations. For example, Roman castra were sited on flat land close to or on important nodes like river crossings or intersections of trade routes; the dimensions of the castra were standard, with each of its four walls having a length of 660 metres. Familiarity was the aim of such standardisation: soldiers could be stationed anywhere around the Empire, orientation would be easy within established towns if they had a standard layout; each would have the aforementioned decumanus maximus and cardo maximus at its heart, their intersection would form the forum, around which would be sited important public buildings. Indeed, such was the degree of similarity between towns that Higgins states that soldiers "would be housed at the same address as they moved from castra to castra".
Pompeii has been cited by both Laurence as the best preserved example of the Roman grid. Outside of the castra, large tra
Inverclyde. Together with the East Renfrewshire and Renfrewshire council areas, Inverclyde forms part of the historic county of Renfrewshire, which exists as a registration county and lieutenancy area - located in the west central Lowlands, it borders the North Ayrshire and Renfrewshire council areas, is otherwise surrounded by the Firth of Clyde. Inverclyde was one of nineteen districts within Strathclyde Region, from 1975 until 1996. Prior to 1975, Inverclyde was governed as part of the local government county of Renfrewshire, comprising the burghs of Greenock, Port Glasgow and Gourock, the former fifth district of the county, its landward area is bordered by the Kelly and South Routen burns to the south west, part of the River Gryfe and the Finlaystone Burn to the south-east. It is one of the smallest in terms of area and population out of the 32 Scottish unitary authorities. Along with the council areas clustered around Glasgow it is considered part of Greater Glasgow in some definitions, although it is physically separated from the city area by open countryside and does not share a border with the city.
The name derives from the extinct barony of Inverclyde conferred upon Sir John Burns of Wemyss Bay and his heirs. ‡ Taken from Inverclyde Ward 1 figure, minus Kilmacolm settlement population. Until Inverclyde was the only authority in the United Kingdom not to have named electoral wards: whilst the local authority reserved its right to name wards, it failed to supply any to the Local Government Boundary Commission; this was rectified in the 2006 review. Following the Council elections of 2017 the current composition of Inverclyde Council is: The election resulted in no overall control of the council; the council gained national notoriety in 2005 following harsh criticism from the Accounts Commission regarding poor leadership and accountability. In November 2005 the council was given a 6-month deadline to reorganise and improve further, following the resignation of the council chief in September and organisational changes in the wake of the original report. Following this criticism the Chief Executive of Inverclyde Council Robert Cleary stepped down and a new chief executive John Mundell was appointed.
The position of Chief Executive commands an annual salary of £112,000. There was criticism over the benefits the outgoing chief executive received on leaving—he was given a six figure severance payment and his pension will be £50,000 per annum. In June 2006, changes were still ongoing: Inverclyde Council altered its directorship structure by adding new corporate director positions and removing senior manager positions, it was expected that the £90,000 a year posts will be filled by new applicants, although existing Council workers were able to apply. There was some criticism with regards to the merging of council services; this was frowned upon as at the time the Director responsible for the two merged departments had an educational qualification, not a social work one. The 2007 council elections took place at the same time as the Scottish Parliament elections; the Liberal Democrats lost nine seats. The SNP and Conservatives both entered the council with five seats and one seat while an independent candidate won a place.
In 2014, one of the Labour councillors, Vaughan Jones, became an independent member after announcing support for Scottish independence. In the 2014 independence referendum, the "No" vote won in Inverclyde by just 86 votes and a margin of 0.2%. By either measure, this was the narrowest result of any of the 32 council areas. In the 2016 EU Referendum, Inverclyde posted a "Remain" vote of 64%. Inverclyde was one of nineteen local government districts in the Strathclyde region of Scotland, which existed between 1975 and 1996; the district was formed by the Local Government Act 1973 from part of the county of Renfrewshire. The boundaries remain the same as those of the modern council area; the remaining parts of the County of Renfrew where divided between two other districts: Eastwood and Renfrew District. In 1996 Inverclyde District was abolished under the provisions of the Local Government etc. Act 1994. Most of its area became the new Inverclyde council area, with the regions - such as Strathclyde - disappearing entirely.
Ardgowan Estate The Bogal Stone Cappielow Castle Levan Clyde Muirshiel Regional Park Greenock Cut Visitor Centre Custom House Quay and Museum Duchal House Finlaystone House Gourock Outdoor Pool Granny Kempock Stone Loch Thom Lunderston Bay McLean Museum and Art Gallery Newark Castle Waterfront Leisure Complex Inverclyde has twenty primary schools serving all areas of its settlements. These are: Aileymill Primary School, Greenock All Saints Primary School, Greenock Ardgowan Primary School, Greenock Gourock Primary School, Gourock Inverkip Primary School, Inverkip Kilmacolm Primary School, Kilmacolm/Port Glasgow King's Oak Primary School, Greenock Lady Alice Primary School, Greenock Moorfoot Primary School, Gourock Newark Primary School, Port Glasgow St. Andrew's Primary School, Greenock St. Francis' Primary School, Port Glasgow St. John's Primary School, Port Glasgow St. Joseph's Primary School, G
A harbor or harbour is a sheltered body of water where ships and barges can be docked. The term harbor is used interchangeably with port, a man-made facility built for loading and unloading vessels and dropping off and picking up passengers. Ports include one or more harbors. Alexandria Port in Egypt is an example of a port with two harbors. Harbors may be artificial. An artificial harbor can have deliberately constructed breakwaters, sea walls, or jettys or they can be constructed by dredging, which requires maintenance by further periodic dredging. An example of an artificial harbor is Long Beach Harbor, United States, an array of salt marshes and tidal flats too shallow for modern merchant ships before it was first dredged in the early 20th century. In contrast, a natural harbor is surrounded on several sides by prominences of land. Examples of natural harbors include Sydney Harbour and Trincomalee Harbour in Sri Lanka. Artificial harbors are built for use as ports; the oldest artificial harbor known is the Ancient Egyptian site at Wadi al-Jarf, on the Red Sea coast, at least 4500 years old.
The largest artificially created. Other large and busy artificial harbors include: Port of Houston, United States. Port of Rotterdam, Netherlands. A natural harbor is a landform where a part of a body of water is protected and deep enough to furnish anchorage. Many such harbors are rias. Natural harbors have long been of great strategic naval and economic importance, many great cities of the world are located on them. Having a protected harbor reduces or eliminates the need for breakwaters as it will result in calmer waves inside the harbor; some examples are: Port Hercules in Principality of Monaco. For harbors near the North and South Poles, being ice-free is an important advantage when it is year-round. Examples of these include: Hammerfest, Norway. Vardø, Norway. Although the world's busiest port is a hotly contested title, in 2006 the world's busiest harbor by cargo tonnage was the Port of Shanghai; the following are large natural harbors: Harbor Maintenance Finance and Funding Congressional Research Service "Harbor".
New International Encyclopedia. 1905
Emergency medical services in the United Kingdom
Emergency medical services in the United Kingdom provide emergency care to people with acute illness or injury and are predominantly provided free at the point of use by the four National Health Services of England, Scotland and Northern Ireland. Emergency care including ambulance and emergency department treatment is free to everyone, regardless of immigration or visitor status; the NHS commissions most emergency medical services through the 14 NHS organisations with ambulance responsibility across the UK. As with other emergency services, the public access emergency medical services through one of the valid emergency telephone numbers. In addition to ambulance services provided by NHS organisations, there are some private and volunteer emergency medical services arrangements in place in the UK, the use of private or volunteer ambulances at public events or large private sites, as part of community provision of services such as community first responders. Air ambulance services in the UK are not part of the NHS and are funded through charitable donations.
Paramedics are seconded from a local NHS ambulance service, with the exception of Great North Air Ambulance Service who employ their own paramedics. Doctors are provided by their home hospital and spend no more than 40% of their time with an air ambulance service. Public ambulance services across the UK are required by law to respond to four types of requests for care, which are: Emergency calls Doctor's urgent admission requests High dependency and urgent inter-hospital transfers Major incidentsAmbulance trusts and services may undertake non-urgent patient transport services on a commercial arrangement with their local hospital trusts or health boards, or in some cases on directly funded government contracts, although these contracts are fulfilled by private and voluntary providers; the National Health Service Act 1946 gave county and borough councils a statutory responsibility to provide an emergency ambulance service, although they could contract a voluntary ambulance service to provide this, with many contracting the British Red Cross, St John Ambulance or another local provider.
The last St John Division, to be so contracted is reputed to have been at Whittlesey in Cambridgeshire, where the two-bay ambulance garage can still be seen at the branch headquarters. The Regional Ambulance Officers’ Committee reported in 1979 that “There was considerable local variation in the quality of the service provided in relation to vehicles and equipment. Most Services were administered by Local Authorities through their Medical Officer of Health and his Ambulance Officer, a few were under the aegis of the Fire Service, whilst others relied upon agency methods for the provision of part or all of their services.” The 142 existing ambulance services were transferred by the National Health Service Reorganisation Act 1973 from local authority to central government control in 1974, consolidated into 53 services under regional or area health authorities. This led to the formation of predominantly county based ambulance services, which merged up and changed responsibilities until 2006, when there were 31 NHS ambulance trusts in England.
The June 2005 report "Taking healthcare to the Patient", authored by Peter Bradley, Chief Executive of the London Ambulance Service, for the Department of Health led to the merging of the 31 trusts into 13 organisations in England, plus one organisation each in Wales and Northern Ireland. Following further changes as part of the NHS foundation trust pathway, this has further reduced to 10 ambulance service trusts in England, plus the Isle of Wight which has its own provision. Following the passage of the Health and Social Care Act 2012, commissioning of the ambulance services in each area passed from central government control into the hands of regional clinical commissioning groups; the commissioners in each region are responsible for contracting with a suitable organisation to provide ambulance services within their geographical territory. The primary provider for each area is held by a public NHS body, of which there are 11 in England, 1 each in the other three countries. In England there are now ten NHS ambulance trusts, as well as an ambulance service on the Isle of Wight, run directly by Isle of Wight NHS Trust, with boundaries following those of the former regional government offices.
The ten trusts are: East Midlands Ambulance Service NHS Trust East of England Ambulance Service NHS Trust London Ambulance Service NHS Trust North East Ambulance Service NHS Foundation Trust North West Ambulance Service NHS Trust South Central Ambulance Service NHS Foundation Trust South East Coast Ambulance Service NHS Foundation Trust South Western Ambulance Service NHS Foundation Trust West Midlands Ambulance Service University NHS Foundation Trust Yorkshire Ambulance Service NHS TrustThe English ambulance trusts are represented by the Association of Ambulance Chief Executives, with the Scottish and Northern Irish providers all associate members. On the 14 November 2018 West Midlands Ambulance Service became the UK's first university-ambulance trust; the service was operated before reorganisation in 1974 by the St Andrews’ Ambulance Association under contract to the Secretary of State for Scotland. The Scottish Ambulance Service is a Special Health Board that provides ambulance services throughout whole of Scotland, on behalf of the Health and Social Care Directorates of the Scottish Government.
Due to the remote nature of many areas of Scotland compared to the other Home Nations, the Scottish Ambulance Service has Britain's only publi
The paddle steamer PS Comet was built in 1812 for Henry Bell and baths owner in Helensburgh, began a passenger service on 15 August 1812 on the River Clyde between Glasgow and Greenock, the first commercially successful steamboat service in Europe. Bell had become interested in steam propelled boats, corresponded with Robert Fulton and learnt from the Charlotte Dundas. In late 1811 he got John and Charles Wood of John Wood and Company, shipbuilders of Port Glasgow, to build a paddle steamer, named the Comet after the "Great Comet" of 1811; the 28 ton craft was 10 feet broad. It had two paddle wheels on each side, driven by engines rated at three horse power: at a date the twin paddlewheels were replaced by a single paddlewheel on each side; the two engines were made by John Robertson of Glasgow, the boiler by David Napier, Glasgow: a story has it that they were evolved from an experimental little steam engine which Bell installed to pump sea water into the Helensburgh Baths. The funnel was tall and thin, a yardarm allowed it to support a sail when there was a following wind.
A tiny cabin aft had a table. Comet was launched on 24 July 1812 with her trial run on 6 August from Port Glasgow to the Broomielaw in Glasgow, taking three and a half hours for the 20 miles; the same month Bell advertised in a local newspaper "The Greenock Advertiser": The Steamboat Comet Between Glasgow and Helensburgh for Passengers OnlyThe subscriber, having at much expense, fitted up a handsome vessel to ply upon the River Clyde from Glasgow, to sail by the power of air and steam, intends that the vessel shall leave the Broomielaw on Tuesdays and Saturdays about mid-day, or such hour thereafter as may answer from the state of the tide, to leave Greenock on Mondays and Fridays in the morning to suit the tide. On about 8 August Comet made the first commercial sailing from Glasgow for Bowling and Greenock, opening the era of the steamboat on the Clyde, more in Britain and Europe; the fare was "four shillings for the best cabin, three shillings for the second." The success of this service inspired competition, with services down the Firth of Clyde and the sea lochs to Largs, Rothesay and Inveraray within four years, the Comet was outclassed by newer steamers.
Bell tried a service on the Firth of Forth. Sir Walter Scott James Watt Bell had the Comet lengthened and re-engined, from September 1819 ran a service to Oban and Fort William, a trip which took four days. On 15 December 1820 the Comet was shipwrecked in strong currents at Craignish Point near Oban, with Bell on board. No lives were lost. One of the engines ended its working days in a Greenock brewery, is now in The Science Museum in London. Bell built another vessel, Comet II, but on 21 October 1825 she collided with the steamer Ayr off Kempock Point, Scotland; the Ayr, we learn, had a light out upon her bow. As the night, was clear, it is obvious that a bad look out had been kept up, most reprehensible neglect shown on both sides. At the moment the accident took place, those on the deck of the Comet were, it is said, engaged in dancing; the passengers who were below were in high spirits, amusing themselves telling and listening to diverting tales. The first stroke hit about the paddle of the Comet.
The Captain and passengers ran upon deck to see what was wrong. The moment this look place, the Ayr, instead of lending any assistance, gave her paddles a back stroke, turned round, went off to Greenock, leaving them to their fate. Comet II sank quickly, killing 62 of the estimated 80 passengers on board, including the son-in-law of John Anderson, a friend of Robert Burns. Drowned were married Captain Wemyss Erskine Sutherland of the 33rd Regiment and Sarah née Duff of Muirtown. After the loss of his second ship, Bell abandoned his work on steam navigation. A replica of the Comet made by shipyard apprentices now stands prominently in Port Glasgow. Clyde Pleasure Steamers Ian McCrorie, Pollock & Co. Ltd. Greenock, ISBN 1-869850-00-9 Significant Scots – Henry Bell A history of the growth of the steam-engine Greenock Telegraph Online RSA Treasure Trails – The Science Museum The Scotsman: "The passengers'precipitated into Eternity' when their steamship sank", by Iain Lundy Very old articles, Steam Navigation, including picture of the PS Comet Faded pages.com
Scotland is a country, part of the United Kingdom. Sharing a border with England to the southeast, Scotland is otherwise surrounded by the Atlantic Ocean to the north and west, by the North Sea to the northeast and by the Irish Sea to the south. In addition to the mainland, situated on the northern third of the island of Great Britain, Scotland has over 790 islands, including the Northern Isles and the Hebrides; the Kingdom of Scotland emerged as an independent sovereign state in the Early Middle Ages and continued to exist until 1707. By inheritance in 1603, James VI, King of Scots, became King of England and King of Ireland, thus forming a personal union of the three kingdoms. Scotland subsequently entered into a political union with the Kingdom of England on 1 May 1707 to create the new Kingdom of Great Britain; the union created a new Parliament of Great Britain, which succeeded both the Parliament of Scotland and the Parliament of England. In 1801, the Kingdom of Great Britain and Kingdom of Ireland enacted a political union to create a United Kingdom.
The majority of Ireland subsequently seceded from the UK in 1922. Within Scotland, the monarchy of the United Kingdom has continued to use a variety of styles and other royal symbols of statehood specific to the pre-union Kingdom of Scotland; the legal system within Scotland has remained separate from those of England and Wales and Northern Ireland. The continued existence of legal, educational and other institutions distinct from those in the remainder of the UK have all contributed to the continuation of Scottish culture and national identity since the 1707 union with England; the Scottish Parliament, a unicameral legislature comprising 129 members, was established in 1999 and has authority over those areas of domestic policy which have been devolved by the United Kingdom Parliament. The head of the Scottish Government, the executive of the devolved legislature, is the First Minister of Scotland. Scotland is represented in the UK House of Commons by 59 MPs and in the European Parliament by 6 MEPs.
Scotland is a member of the British–Irish Council, sends five members of the Scottish Parliament to the British–Irish Parliamentary Assembly. Scotland is divided into councils. Glasgow City is the largest subdivision in Scotland in terms of population, with Highland being the largest in terms of area. "Scotland" comes from the Latin name for the Gaels. From the ninth century, the meaning of Scotia shifted to designate Gaelic Scotland and by the eleventh century the name was being used to refer to the core territory of the Kingdom of Alba in what is now east-central Scotland; the use of the words Scots and Scotland to encompass most of what is now Scotland became common in the Late Middle Ages, as the Kingdom of Alba expanded and came to encompass various peoples of diverse origins. Repeated glaciations, which covered the entire land mass of modern Scotland, destroyed any traces of human habitation that may have existed before the Mesolithic period, it is believed the first post-glacial groups of hunter-gatherers arrived in Scotland around 12,800 years ago, as the ice sheet retreated after the last glaciation.
At the time, Scotland was covered in forests, had more bog-land, the main form of transport was by water. These settlers began building the first known permanent houses on Scottish soil around 9,500 years ago, the first villages around 6,000 years ago; the well-preserved village of Skara Brae on the mainland of Orkney dates from this period. Neolithic habitation and ritual sites are common and well preserved in the Northern Isles and Western Isles, where a lack of trees led to most structures being built of local stone. Evidence of sophisticated pre-Christian belief systems is demonstrated by sites such as the Callanish Stones on Lewis and the Maes Howe on Orkney, which were built in the third millennium BCE; the first written reference to Scotland was in 320 BC by Greek sailor Pytheas, who called the northern tip of Britain "Orcas", the source of the name of the Orkney islands. During the first millennium BCE, the society changed to a chiefdom model, as consolidation of settlement led to the concentration of wealth and underground stores of surplus food.
The first Roman incursion into Scotland occurred in 79 AD. After the Roman victory, Roman forts were set along the Gask Ridge close to the Highland line, but by three years after the battle, the Roman armies had withdrawn to the Southern Uplands; the Romans erected Hadrian's Wall in northern England and the Limes Britannicus became the northern border of the Roman Empire. The Roman influence on the southern part of the country was considerable, they introduced Christianity to Scotland. Beginning in the sixth century, the area, now Scotland was divided into three areas: Pictland, a patchwork of small lordships in central Scotland; these societies were based on the family unit and had sharp divisions in wealth, although the vast majority were poor and worked full-time in subsistence agriculture. The Picts kept slaves through the ninth century. Gaelic influence over Pictland and Northumbria was facilitated by the large number of Gaelic-speaking clerics working as missionaries. Operating in the sixth ce