explorer blog

Welcome: September 2005

Wednesday, September 21, 2005


I INTRODUCTION Athens, city in south-east Greece, capital and largest city of the country. Situated on the Attic plain on the Greek mainland, it is surrounded by mountains on three sides, the most important of which are Pلrnis, Pendéli, and Hymettus (Imittَs). Two minor streams, the Kifisَs River in the west and the Illisَs River in the east, flow through the city. With its port, Piraiévs, or Piraeus, which is located about 8 km (5 mi) to the south-west on the Saronic Gulf (an inlet of the Aegean Sea), it forms a unified metropolitan region. Athens dominates the economic, cultural, and political life of Greece.

II THE CONTEMPORARY CITY A large portion of Greece's industrial activity is concentrated in and around Athens. Manufactured goods include textiles, alcoholic beverages, soap, flour, chemicals, paper products, leather, and pottery. Publishing, banking, and tourism are also important to the economy. The city is also the hub of the national transport network.
At the heart of the modern city is Constitution Square, on or near which are found the Parliament Building and several museums. Although most of the city dates from after the mid-19th century, important works of antiquity remain. The most prominent and famous landmark is the Acropolis, a flat-topped hill on which stand the remains of the Parthenon and several other important buildings erected in the 5th century BC.
Among the city's institutions of higher education are the National Capodistrian University of Athens (1837), the National Technical University of Athens (1836), and schools of art, business, and agriculture. Athens has numerous museums, including the National Archaeological Museum, the Byzantine Museum, the Acropolis Museum, and the Benaki Museum, with their notable collections.

III HISTORY The Acropolis of Athens has been inhabited since Neolithic times. As early as 1400 BC it was fortified in the manner of Mycenae, Tiryns, and other late Bronze Age citadels. At that time and in the subsequent “dark age” (1200-900 BC) that followed the Dorian invasions, Athens was one of a number of petty states in Attica.

to be continued :

Sunday, September 11, 2005


Tornado (Latin, tonare,”to thunder”), in meteorology, violent whirling wind, characteristically accompanied by a funnel-shaped cloud extending down from a cumulonimbus cloud. Commonly known as a cyclone or twister, a tornado can be a few metres to about a kilometre wide where it touches the ground, with an average width of a few hundred metres. It can move over land for distances ranging from short hops to many kilometres, causing great damage wherever it descends. The funnel is made visible by the dust that is sucked up and by condensation of water droplets in the centre of the funnel. The same condensation process makes visible the generally weaker sea-going tornadoes, called waterspouts, that occur most frequently in tropical waters. Most tornadoes spin anticlockwise in the northern hemisphere and clockwise in the southern, but occasional tornadoes reverse this.
The exact mechanisms that cause a tornado to form are still not fully understood, but the funnels are always associated with violent motions in the atmosphere, including strong updraughts and the passage of fronts. They develop within low-pressure areas of high winds; the speed of the funnel winds themselves is often placed at more than 480 km/hr (300 mph), although speeds of more than 800 km/hr (500 mph) have been estimated for extremely strong storms. Damage to property hit by a tornado results both from these winds and from the extremely reduced pressure in the centre of the funnel, which causes structures to explode when they are not sufficiently ventilated to adjust rapidly to the pressure difference. The pressure reduction is in keeping with Bernoulli's principle, which states that pressure is reduced as velocity increases.
Tornadoes are most common and strongest in temperate latitudes. In the United States they tend to form most frequently in the early spring; the “tornado season” shifts towards later months with increasing latitude. The number of funnels observed each year can vary greatly in any given region.

Tuesday, September 06, 2005

Irrigation :

INTRODUCTION Irrigation, artificial watering of land to sustain plant growth. Irrigation is practised in all parts of the world where rainfall does not provide enough ground moisture. In dry areas irrigation must be maintained from the time a crop is planted. In areas of irregular rainfall, irrigation is used during dry spells to ensure harvests and to increase crop yields. The procedure has greatly expanded the amount of arable land and the production of food throughout the world. In 1800 about 8.1 million hectares (20 million acres) were under irrigation, a figure that rose to 41 million hectares (99 million acres) in 1900, to 105 million hectares (260 million acres) in 1950, and to more than 222 million hectares (550 million acres) today. Irrigated land represents about 15 per cent of all land under cultivation but often produces over twice the yield of non-irrigated fields. Irrigation can, however, waterlog soil, or increase a soil's salinity (salt level) to the point where crops are damaged or destroyed. This problem is now jeopardizing about one-third of the world's irrigated land.

HISTORY Earliest records date the first use of irrigation to Egyptians along the River Nile about 5000 BC. By 2100 BC elaborate systems were in use, one of them a 19-km (12-mi) channel that diverted Nile floodwaters to Lake Moeris. The Sumerians relied heavily on irrigation to water fields in southern Mesopotamia (now southern Iraq) as early as 2400 BC, and the Chinese had irrigation techniques by 2200 BC. The Peruvians also built sophisticated systems before the time of Christ, and at the same time early Native Americans had more than 101,000 hectares (250,000 acres) of irrigated land in the Salt River valley of Arizona.
Among the early devices for lifting water from streams to higher-lying fields was the Egyptian shadoof, which is a bucket set on one end of a counterweighted pole. The Archimedes' screw, used for the same purpose, is a cylinder containing a wide-threaded screw turned by hand. The cylinder was set on an incline with the lower end in the stream, and as the screw was turned it lifted water to a higher level. The Persian wheel, still in use in India today, is a partly submerged vertical wheel with buckets attached to the rim. As the wheel is turned by draught animals rotating a geared horizontal wheel, the buckets are filled and emptied into a trough above that carries the water to crop-sown fields.
A method far less burdensome than lifting water was that of building permanent dams further upstream, whereby water could be raised to the desired level. The water was then allowed to flow by gravity through canals to lower-lying areas, where it was let out over gently sloping fields. This method had been practised on a large scale by early civilizations, using simple earthwork structures. It is essentially the same principle as that of modern irrigation, using masonry dams or such enormous concrete structures as the Grand Coulee Dam in Washington.

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