The Sahara is a desert located on the African continent. It is the largest hot desert in the world, the third largest desert overall after Antarctica and the Arctic, its area of 9,200,000 square kilometres is comparable to the area of the United States. The name'Sahara' is derived from a dialectal Arabic word for ṣaḥra; the desert comprises much of North Africa, excluding the fertile region on the Mediterranean Sea coast, the Atlas Mountains of the Maghreb, the Nile Valley in Egypt and Sudan. It stretches from the Red Sea in the east and the Mediterranean in the north to the Atlantic Ocean in the west, where the landscape changes from desert to coastal plains. To the south, it is bounded by the Sahel, a belt of semi-arid tropical savanna around the Niger River valley and the Sudan Region of Sub-Saharan Africa; the Sahara can be divided into several regions including: the western Sahara, the central Ahaggar Mountains, the Tibesti Mountains, the Aïr Mountains, the Ténéré desert, the Libyan Desert.
For several hundred thousand years, the Sahara has alternated between desert and savanna grassland in a 20,000 year cycle caused by the precession of the Earth's axis as it rotates around the Sun, which changes the location of the North African Monsoon. The area is next expected to become green in about 15,000 years; the Sahara covers large parts of Algeria, Egypt, Mali, Niger, Western Sahara and Tunisia. It covers 9 million square kilometres, amounting to 31% of Africa. If all areas with a mean annual precipitation of less than 250 mm were included, the Sahara would be 11 million square kilometres, it is one of three distinct physiographic provinces of the African massive physiographic division. The Sahara is rocky hamada. Wind or rare rainfall shape the desert features: sand dunes, dune fields, sand seas, stone plateaus, gravel plains, dry valleys, dry lakes, salt flats. Unusual landforms include the Richat Structure in Mauritania. Several dissected mountains, many volcanic, rise from the desert, including the Aïr Mountains, Ahaggar Mountains, Saharan Atlas, Tibesti Mountains, Adrar des Iforas, the Red Sea Hills.
The highest peak in the Sahara is Emi Koussi, a shield volcano in the Tibesti range of northern Chad. The central Sahara is hyperarid, with sparse vegetation; the northern and southern reaches of the desert, along with the highlands, have areas of sparse grassland and desert shrub, with trees and taller shrubs in wadis, where moisture collects. In the central, hyperarid region, there are many subdivisions of the great desert: Tanezrouft, the Ténéré, the Libyan Desert, the Eastern Desert, the Nubian Desert and others; these arid areas receive no rain for years. To the north, the Sahara skirts the Mediterranean Sea in Egypt and portions of Libya, but in Cyrenaica and the Maghreb, the Sahara borders the Mediterranean forest and scrub eco-regions of northern Africa, all of which have a Mediterranean climate characterized by hot summers and cool and rainy winters. According to the botanical criteria of Frank White and geographer Robert Capot-Rey, the northern limit of the Sahara corresponds to the northern limit of date palm cultivation and the southern limit of the range of esparto, a grass typical of the Mediterranean climate portion of the Maghreb and Iberia.
The northern limit corresponds to the 100 mm isohyet of annual precipitation. To the south, the Sahara is bounded by the Sahel, a belt of dry tropical savanna with a summer rainy season that extends across Africa from east to west; the southern limit of the Sahara is indicated botanically by the southern limit of Cornulaca monacantha, or northern limit of Cenchrus biflorus, a grass typical of the Sahel. According to climatic criteria, the southern limit of the Sahara corresponds to the 150 mm isohyet of annual precipitation. Important cities located in the Sahara include the capital of Mauritania; the Sahara is the world's largest low-latitude hot desert. It is located in the horse latitudes under the subtropical ridge, a significant belt of semi-permanent subtropical warm-core high pressure where the air from upper levels of the troposphere tends to sink towards the ground; this steady descending airflow causes a drying effect in the upper troposphere. The sinking air prevents evaporating water from rising, therefore prevents adiabatic cooling, which makes cloud formation difficult to nearly impossible.
The permanent dissolution of clouds allows thermal radiation. The stability of the atmosphere above the desert prevents any convective overturning, thus making rainfall non-existent; as a consequence, the weather tends to be sunny and stable with a minimal chance of rainfall. Subsiding, dry air masses associated with subtropical high-pressure systems are unfavorable for the development of convectional showers; the subtropical ridge is the predominant factor that explains the hot desert climate of this vast region. The descending airflow is the strongest and the most effective over the eastern part of the Great Desert, in the Libyan Desert: this is the
Software engineering is the systematic application of engineering approaches to the development of software. Software engineering is a direct sub-field of engineering and has an overlap with computer science and management science, it is considered a part of overall systems engineering. When the first digital computers appeared in the early 1940s, the instructions to make them operate were wired into the machine. Practitioners realized that this design was not flexible and came up with the "stored program architecture" or von Neumann architecture, thus the division between "hardware" and "software" began with abstraction being used to deal with the complexity of computing. Programming languages started to appear in the early 1950s and this was another major step in abstraction. Major languages such as Fortran, ALGOL, COBOL were released in the late 1950s to deal with scientific and business problems respectively. David Parnas introduced the key concept of modularity and information hiding in 1972 to help programmers deal with the ever-increasing complexity of software systems.
The origins of the term "software engineering" have been attributed to various sources. The term "software engineering" appeared in a list of services offered by companies in the June 1965 issue of COMPUTERS and AUTOMATION and was used more formally in the August 1966 issue of Communications of the ACM “letter to the ACM membership” by the ACM President Anthony A. Oettinger. Independently, Margaret Hamilton named the discipline "software engineering" during the Apollo missions to give what they were doing legitimacy. At the time there was perceived to be a "software crisis"; the 40th International Conference on Software Engineering celebrates 50 years of "Software Engineering" with the Plenary Sessions' keynotes of Frederick Brooks and Margaret Hamilton. In 1984, the Software Engineering Institute was established as a federally funded research and development center headquartered on the campus of Carnegie Mellon University in Pittsburgh, United States. Watts Humphrey founded the SEI Software Process Program, aimed at understanding and managing the software engineering process.
The Process Maturity Levels introduced would become the Capability Maturity Model Integration for Development, which has defined how the US Government evaluates the abilities of a software development team. Modern accepted best-practices for software engineering have been collected by the ISO/IEC JTC 1/SC 7 subcommittee and published as the Software Engineering Body of Knowledge. Notable definitions of software engineering include: "the systematic application of scientific and technological knowledge and experience to the design, implementation and documentation of software"—The Bureau of Labor Statistics—IEEE Systems and software engineering – Vocabulary "The application of a systematic, quantifiable approach to the development and maintenance of software"—IEEE Standard Glossary of Software Engineering Terminology "an engineering discipline, concerned with all aspects of software production"—Ian Sommerville "the establishment and use of sound engineering principles in order to economically obtain software, reliable and works efficiently on real machines"—Fritz Bauer "a branch of computer science that deals with the design and maintenance of complex computer programs"—Merriam-WebsterThe term has been used less formally: as the informal contemporary term for the broad range of activities that were called computer programming and systems analysis.
Requirements engineering is about the elicitation, analysis and validation of requirements for software. Software design is about the process of defining the architecture, components and other characteristics of a system or component; this is called Software architecture. Software development, the main activity of software construction: is the combination of programming, software testing, debugging. A Software development process: is the definition, assessment, management and improvement of the software life cycle process itself, it uses Software configuration management, about systematically controlling changes to the configuration, maintaining the integrity and traceability of the configuration and code throughout the system life cycle. Modern processes use software versioning. Software testing: is an empirical, technical investigation conducted to provide stakeholders with information about the quality of the product or service under test, with different approaches such as unit testing and integration testing.
It is one aspect software quality. Software maintenance: refers to the activities required to provide cost-effective support after shipping the software product. Knowledge of computer programming is a prerequisite for becoming a software engineer. In 2004 the IEEE Computer Society produced the SWEBOK, published as ISO/IEC Technical Report 1979:2004, describing the body of knowledge that they recommend to be mastered by a graduate software engineer with four years of experience. Many softw
Revenue stamps of the Isle of Man refer to the adhesive revenue or fiscal stamps which were issued by the British Crown dependency of the Isle of Man between 1889 and 1976. British key type revenue stamps with an appropriate inscription were issued on the island until 1966, when revenue stamps showing various scenes and symbols of the island began to be issued; the last set of stamps was issued in 1976. From around 1920 to the 1970s, hundreds of contribution stamps were issued for National Insurance and related schemes. Since the Isle of Man was politically separate from the United Kingdom, it had a different fiscal system and did not use British revenues. Revenues were needed to pay fees connected to transfers of property; the first series came out in 1889, they were British key types portraying Queen Victoria with the inscription ISLE OF MAN in the bottom tablet. From 1894 some stamps were surcharged with the same face value in black in order to make the denomination easier to see. Similar key types were used for many years, with stamps bearing the portraits of the following monarchs: Queen Victoria King Edward VII King George V King George VI Queen Elizabeth II Most of these stamps are hard to find, some of them command high prices for collectors.
In fact, the Isle of Man's rarest stamp is the King George V £5 from 1921. According to Barefoot, only seven used examples exist. A new set of revenue stamps was issued on 5 July 1966. Unlike the previous issues, these were not key types, but each value had a different colour and design: 6d – Peel Castle 1/- – Tynwald Hill 2/- – The Nunnery 2/6 – Bible in Manx 5/- – Lifeboat 10/- – HMS Victory £1 – William Christian £2 – Castle Rushen £5 – Queen Elizabeth II, a triskelion superimposed on a Celtic crossAlthough the Isle of Man switched to decimal currency in 1971 along with the United Kingdom, the first decimal series of revenues was not issued until 15 October 1974 as the previous issue remained in use; the new issue had four values: 5p – Old Grammar School, Castletown 10p – Tynwald Hill 25p – St German's Cathedral 50p – Lady Isabella, a waterwheelThese replaced the 1/-, 2/-, 5/- and 10/- values of the previous issue. Meanwhile, the pound values remained in use. With the exception of the £5 stamp of 1966, none of the above stamps bore an inscription indicating they were revenue stamps.
To prevent people from attempting to use them for postage, on 18 June 1975 the four decimal stamps issued in 1974 and two of the pound values from 1966 were issued overprinted REVENUE. On 1 November 1976, a new set inscribed, it consisted of seven values, which depicted heraldic symbols, ships or castles. This was the last set of Isle of Man revenues issued. Unlike the keytypes, the 1966–76 issues were sold to collectors in presentation packs or as singles in mint condition, therefore unused stamps are more found; however they are still harder to find in used condition. The Isle of Man issued contribution stamps for National Insurance and its predecessor schemes. In around 1920, British National Health Insurance stamps were overprinted for use on the island, these were replaced by overprints on British Health & Pensions Insurance stamps in around 1930. After National Insurance was established in 1948, British issues were once again overprinted. From 1969 onwards, a new design depicting the triskelion was issued, although overprints on British issues continued.
Hundreds of National Insurance stamps were issued until the 1970s, many of them are rare. Postage stamps and postal history of the Isle of Man Revenue stamps of Guernsey Revenue stamps of Jersey Revenue stamps of the United Kingdom Isle of Man Revenues – gallery of various Manx revenue stamps by I. B RedGuy's Fine Stamps