Category Social Sciences

Bridges have been used since ancient times to span deep ravines, rivers and other stretches of water. Early bridges were made of wood which rotted easily and could not span great distances. In some parts of the world, people made bridges from wood and rope. They had walkways made of wooden slats and rope handrails. Bridges made of stone may last for centuries. However, stone is heavy to transport and long bridges need to have many sturdy supports.

During the Industrial Revolution, which began in the 18th century, engineers began to build bridges made from iron. These were much more durable than wooden bridges and could span longer distances than stone ones. The first iron bridge was built at Coalbrookdale, England, in 1779.

Modern bridges are built using steel and concrete. They may carry roads or railways over rivers, wide estuaries or high valleys, or above other roads and railways. There are a number of different kinds of bridge design available, each of which may be used in a modern bridge according to the type of crossing required.

A beam bridge is one of the simplest and oldest designs. The beam is supported at each end. The earliest bridges were tree trunks or stone slabs laid across a stream supported by the banks on either side. A clapper bridge is a type of Beam Bridge which is supported from beneath by several columns in the river bed or ground. The deck may be flat or made from a hollow girder (truss) containing the road or railway.

An arch bridge—a very strong type—has a deck supported on an arch fixed to the banks. Some arch bridges are too low for river traffic to pass underneath so a part of their span may be a bascule or lift bridge.

This can be raised in the middle like a drawbridge so that ships can pass through.

A cantilever is a beam or structure that is fixed at one end only. Cantilever bridges are built in two halves, with each beam cantilevered out from the bank and each half balanced on a support in the river. There may be a short central span where the two halves meet.

Suspension bridges can span long distances. The towers of the bridge are built first and long steel cables are suspended from the towers. Lengths of cable called hangers are fixed to the suspending cables. The deck of the bridge is lifted into position and attached to the hangers. Suspension bridges are ideal for long, high spans as they do not require a row of supporting columns that may interfere with river transport. One of the most famous examples of a suspension bridge is the Golden Gate Bridge (1280 metres), near San Francisco in the USA, which was completed in 1937.

A cable-stayed bridge has diagonal steel cables running from high supporting masts at either end connected to the deck of the bridge. The weight of the deck is supported by the masts.

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Even though aircraft now carry many of the world’s passengers, shipping is still a vital link between countries, particularly for carrying cargo. The biggest vessels are called bulk carriers. They include oil supertankers, some of which are more than 450 metres long. Container ships carry general cargo stored in large steel boxes stacked up like building blocks. These can be unloaded directly on to trucks.

The River Rhine rises in Switzerland and runs along the border between France and Germany, then on through Germany and the Netherlands, meeting the North Sea near Rotterdam. It is one of Europe’s most important industrial waterways. As well as barges carrying cargo, river boats take tourists along the river to see the vineyards and ancient castles on its banks.

Ships cross the oceans on fixed routes called shipping lanes. The world’s busiest shipping lanes link Europe and North America with the Middle East and East Asia. Ships go through the Suez and Panama Canals to shorten their journeys, although supertankers, being too large for the Suez Canal, still travel around the southern tip of Africa.

Modern cargo ships are much larger than vessels of the past, and big, efficient ports with docks (enclosed areas of water) are needed so that their cargoes can be loaded and unloaded as quickly as possible. Some ships take cargoes inland along large rivers and man-made waterways called canals.

Two major canals, cut through narrow necks of land, provide much shorter routes between ports. They are the 165-kilometre Suez Canal in Egypt, linking the Mediterranean Sea with the Red Sea, and the 82-kilometre Panama Canal, connecting the Atlantic and Pacific Oceans.

Canals often link natural waterways and provide a transport route across a continent. The Main-Danube Canal, for example, allows the movement of goods between Eastern and Western Europe. The United States and Canada have more than 41,000 kilometres of waterways linked to the St. Lawrence and Mississippi rivers and their tributaries. The St. Lawrence Seaway connects the Great Lakes, and the cities of Chicago, Detroit, Cleveland and Toronto among others, with the Atlantic Ocean.

Cargo on inland waterways in industrial countries, for example, the River Rhine in Germany, is usually carried by barges which are towed by tugs. Sometimes several barges are strapped together. Barges carry cargoes along the Rhine to and from the port of Rotterdam in the Netherlands, the world’s busiest port. Antwerp in Belgium is the largest inland port in the world. Even though it is 89 kilometres from the open sea, ships of all types load and unload cargoes there.

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Air travel has grown enormously since World War II. Until then, only the wealthy travelled by air. The development of the jet airliner in the 1950s made it possible for everyone to fly to destinations across the world.

The world’s busiest airport is O’Hare International near Chicago in the United States, with an average of one take-off or landing every 35 seconds and nearly 70 million passengers a year. Many of these flights are for people travelling within the United States – about 85 per cent of people travelling within the United States go by air. London’s Heathrow Airport handles more international traffic than any other airport with more than 55 million international passengers a year.

A large modern airport employs thousands of people. Air traffic controllers work in a control tower, directing all aircraft to and from runways and deciding when it is safe to take off or land. They have powerful radar equipment to keep watch over the whole airspace around the airport.

Baggage handlers load and unload suitcases from the aircraft. Once passengers have disembarked, ground crew prepares the aircraft to fly out again, and refuel it while firefighters stand by.

In the terminal, the passengers collect their baggage and go through customs, where officials check that they are not carrying drugs or goods which require import or export tax to be paid.

Airports also handle goods (air freight) that are required to be transported quickly. Warehouses store goods before loading and after arrival, when they are inspected by customs officials.

Security officers use X-ray equipment to check passengers for bombs, guns and other weapons. International passengers also have to pass through immigration where they show their passports and any visas that are required to enter the country. Officials often stamp the passport to show that passengers are entering the country legally. Airports also have lounges and restaurants where passengers can wait for their flights.

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Electricity is a type of energy that gives us heat and light and drives machines. To be useful, electricity must be made to flow in a current. In 1831 the British scientist Michael Faraday used a magnet to produce electricity. He moved a loop of wire over the magnet, causing an electric current to flow through the wire. This principle is used to generate electricity in power stations today. In thermal power stations, coal, oil or gas are burned to boil water, producing steam to drive a generator.

The steam from the boiling water rushes through pipes and turns a bladed wheel called a turbine. The turbine is connected to the generator, which consists of a huge magnet surrounded by copper wire. The turbine makes the magnet spin, thus producing an electric current in the wire.

The water can be heated by other means. The mineral uranium is the fuel used in a nuclear power station. Everything on Earth is made up of very tiny particles called atoms. Splitting the atoms that make up uranium produces a very intense heat for creating steam.

In a nuclear power station, energy is produced by creating a reaction in the nuclei (cores) of uranium atoms. Releasing energy by splitting atomic nuclei is called fission. Each nucleus contains particles called neutrons. Inside the reactor, these hit other nuclei, causing them to split and release more neutrons. This repeated process, called a chain reaction, produces immense amounts of heat energy. Water pumped around the reactor is heated.

Hydro-electric power stations use fast-flowing water to turn turbines. The water from rivers is stored in a reservoir behind a dam. The power station is located in front of the dam. Some of the water is allowed to rush out through pipes to make turbines spin and drive the generator.

SUPPLYING ELECTRICITY

The electricity is sent from the power station along thick wires called cables. They are supported above ground by tall pylons. The electric current is boosted by transformers along the way. The electricity goes to sub-stations from where cables carry it to houses, factories, shops and offices.

The cables from a power station are linked to form a country’s supply network or grid. This allows electricity to be sent to wherever it is needed. Electricity cannot be stored, so a constant supply flows through the cables and wires.

ALTERNATIVE POWER

Coal and oil-fired power stations cause pollution. Fossil fuels, once used up, cannot be replaced. Leaks of radioactivity from nuclear power stations are a potential hazard. So alternative methods for generating electricity are needed. Wind turbines on wind farms, solar power (in which solar panels store sunlight for conversion to electricity), tidal and wave power are all possibilities for the future.

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