IELTS Academic Reading Practice Test 33

The first human beings to leave the ground in a flying machine did so not in an aeroplane but beneath a huge bag of heated air. The device was the creation of Joseph-Michel and Jacques-Étienne Montgolfier, two of the many children of a successful paper manufacturer in Annonay, a town in southern France. Their family business gave them access to paper and fabric, and their curiosity led them to experiment with lightweight bags held over fires. They noticed that such bags swelled and floated upwards, and in 1782 they began building larger versions. The brothers did not fully understand what they had discovered. They believed that burning certain materials released a special gas with the power to lift, and they experimented with fuels such as wet straw and chopped wool, which produced thick smoke.

In fact, smoke had little to do with it. When air is heated, it expands, so a given volume of hot air weighs less than the same volume of cooler air around it. A bag filled with hot air is therefore pushed upwards in the same way that a piece of wood is pushed upwards in water. The Montgolfiers' choice of fuel was based on a mistaken theory, but their balloons worked because the fires heated the air inside them.

On 4 June 1783 the brothers gave a public demonstration in Annonay in front of local dignitaries. Their unmanned balloon, made of fabric lined with paper, rose to a considerable height and came down about two kilometres away after roughly ten minutes in the air. News of the event travelled quickly to Paris, and the brothers were invited to repeat the experiment before the royal court. On 19 September, at the palace of Versailles, a balloon carried a sheep, a duck and a cockerel into the sky in front of King Louis XVI and Queen Marie Antoinette. The flight lasted around eight minutes, and all three animals landed safely, which reassured observers who had feared that the upper air might be unable to support life.

Two months later, on 21 November 1783, the first free flight with human passengers took off from the grounds of the Château de la Muette, on the western edge of Paris. The two men on board were Jean-François Pilâtre de Rozier, a young teacher of science, and the Marquis d'Arlandes, an army officer. They kept the balloon aloft by feeding straw into a fire burning in an iron basket beneath the opening of the envelope, and they drifted across the city for about twenty-five minutes before landing safely several kilometres away. The flight was not without danger: the fire caused parts of the fabric to smoulder, and the passengers had to deal with these burns while in the air.

The hot-air balloon's moment of glory was brief. Only ten days after Pilâtre de Rozier's flight, the physicist Jacques Charles and his assistant Nicolas-Louis Robert ascended from Paris in a balloon filled with hydrogen, a gas far lighter than heated air. Hydrogen balloons could stay aloft for hours without carrying a fire, and for the rest of the eighteenth and nineteenth centuries they dominated ballooning for science, exploration and entertainment. Pilâtre de Rozier himself died in 1785 while attempting to cross the English Channel in a balloon that combined a hydrogen chamber with a hot-air chamber, and he and his companion are often described as the first people to be killed in an air accident.

Hot-air ballooning was revived only in the mid-twentieth century, largely through the work of an American engineer, Ed Yost. Working for a company called Raven Industries under contracts with the United States Navy, Yost solved the problems that had held back the Montgolfiers' invention. He replaced the open fire with a burner fuelled by propane gas, carried in cylinders, and made the envelope from a lightweight, heat-resistant nylon fabric. On 22 October 1960 he made the first free flight in a balloon of this type from Bruning, Nebraska, staying in the air for well over an hour.

Modern balloons still rely on the same basic principle. The pilot heats the air inside the envelope with short bursts from the burner, and the balloon rises as the air becomes lighter. To descend, the pilot simply allows the air to cool, or pulls a cord that opens a vent at the top of the envelope so that hot air can escape.

Because the lift depends on the difference in temperature between the air inside and outside, a balloon can lift more on a cold morning than on a hot afternoon. A balloon cannot be steered in the ordinary sense, since it travels with the wind. Pilots can, however, change direction by climbing or descending to find air currents moving in different directions, and they usually fly soon after sunrise or before sunset, when winds tend to be light and steady.

Today hot-air ballooning is mainly a sport and a tourist attraction, and large festivals attract hundreds of balloons at a time. Yet the old rivalry between hot air and lighter gases has also been resolved in an unexpected way. In 1999 Bertrand Piccard and Brian Jones completed the first non-stop flight around the world by balloon in Breitling Orbiter 3, a craft that used both helium and hot air – a descendant, in principle, of the hybrid design in which Pilâtre de Rozier lost his life.

Anyone who walks through a tropical forest in Central or South America is likely to come across a remarkable procession: a column of ants, sometimes stretching for many metres, each carrying a fragment of leaf held above its head like a sail. Each fragment may weigh several times as much as the ant that carries it, and the traffic along a busy trail continues day and night. These are leafcutter ants, members of two related groups, or genera, known as Atta and Acromyrmex, which are found from the southern United States to northern Argentina. Their foraging can strip a large shrub of its leaves in a single night, and in many forests they remove more vegetation than any other plant-eating animal. Yet, surprisingly, the ants do not eat the leaves at all.

Instead, the leaves are used to grow food. Inside the nest, workers cut the fragments into smaller pieces, chew them into a pulp and add them to a spongy mass known as a fungus garden. A particular species of fungus grows through this material, digesting plant tissue that the ants themselves cannot break down. In return for this constant supply of food and protection, the fungus produces small swollen structures rich in sugars and fats, which the ants harvest to feed themselves and their larvae. The relationship is so close that the fungus is unknown in the wild, rather as many human crops can no longer survive without farmers. Studies of the evolutionary history of ants suggest that fungus farming began tens of millions of years ago, and that leafcutters are its most recent and most sophisticated practitioners.

Running such an enterprise requires an elaborate division of labour, and leafcutter workers differ in size far more than those of most ant species. In established colonies, biologists generally recognise four main groups. The smallest workers, often called minims, tend the fungus garden and care for the young. Slightly larger ants patrol the area around the nest. Medium-sized workers do most of the cutting and carrying, while the largest, sometimes called soldiers, defend the colony and help to clear obstacles from the foraging trails. Minims are also frequently seen riding on leaf fragments as they are carried home. One widely supported explanation is that these passengers guard the carriers against tiny parasitic flies, which try to lay their eggs on the ants, although researchers have suggested that the riders may perform other tasks as well.

The scale of a mature colony can be astonishing. A single nest of the genus Atta may contain several million workers, all of them the offspring of one queen, and the underground structure can include hundreds of chambers connected by tunnels and extending several metres below the surface. Researchers have mapped abandoned nests by pouring liquid cement into the entrances and then digging away the surrounding soil, revealing structures of extraordinary complexity, with separate chambers for fungus gardens and for waste. The fungus and the ants together consume large quantities of oxygen and produce carbon dioxide, and the architecture of the nest appears to help air circulate, preventing the gas from accumulating to harmful levels in the deeper chambers.

Like any farmer growing a single crop, the ants face the constant threat of disease. Their gardens can be attacked by a parasitic fungus called Escovopsis, which feeds on the cultivated fungus and can destroy an entire garden if left unchecked. The ants defend their crop in several ways. Workers inspect the garden continually and remove infected material, a process comparable to weeding. Many leafcutters also carry colonies of bacteria on their bodies, and these bacteria produce antibiotic substances that suppress the growth of the parasite. Waste material is taken to special chambers or dumps away from the garden, and the workers that handle it tend to remain there, which reduces the risk that infection will be carried back to the colony's food supply.

The way a new colony is founded shows how dependent the ants are on their crop. Before leaving her birthplace on her mating flight, a young queen stores a small pellet of fungus in a pouch inside her mouth. After mating, she sheds her wings, digs a chamber in the soil and spits out the pellet to start a new garden. She mates only during this single flight, storing enough sperm to fertilise all the eggs she will lay during a life that may last many years. At first she tends the garden herself until her first offspring mature. These first workers are all very small, and they immediately take over the care of the fungus. The odds are heavily stacked against her, however: the great majority of newly founded colonies fail within their first few months.

For people, leafcutter ants are a mixed blessing. In Latin America they are serious agricultural pests, capable of stripping crops and young plantation trees, and farmers spend large sums trying to control them. At the same time, by carrying huge amounts of plant material underground and mixing it with soil, the ants recycle nutrients and improve soil structure across large areas of forest. Scientists are also studying them for more practical reasons. The bacteria that protect the fungus gardens are of interest to researchers searching for new antibiotics, and the way the fungus breaks down tough plant material may offer insights to engineers trying to turn plant waste into fuel.

For most of the twentieth century, the centres of European cities were reshaped around the motor car. Squares became car parks, streets were widened, and pedestrians were directed into underpasses so that traffic could flow freely. In recent decades, however, a growing number of cities have begun to reverse this process, closing streets to vehicles and turning road space over to people on foot and on bicycles. Opponents warn that such schemes damage trade and inconvenience those who depend on cars. My own view is that car-free centres are usually worth pursuing, but that their success depends far less on the ban itself than on how it is designed and introduced.

The idea is not new. In 1962 Copenhagen closed its main shopping street, Strøget, to cars, initially as an experiment. Many local merchants predicted that their businesses would suffer, but the number of people using the street increased, and the closure was made permanent two years later. The Danish architect Jan Gehl spent years observing how people behaved in the street and in the city's other public spaces, and his research, which showed that people linger where streets are designed for them, has since influenced planners all over the world.

Perhaps the most thorough transformation has taken place in Pontevedra, a small city in Galicia in north-western Spain. After Miguel Anxo Fernández Lores became mayor in 1999, the city removed almost all traffic from its historic centre within a short period, and it later extended restrictions to surrounding districts. Parking on the street was largely prohibited, traffic lights were replaced with roundabouts and speed limits were lowered. The underlying principle was that streets should serve people on foot first, and that space occupied by parked vehicles could be better used by pedestrians, children and older residents. According to the city's own figures, motor traffic in the old centre has fallen by well over ninety per cent since the policy began.

Other cities have chosen less absolute methods. In 2017 the Belgian city of Ghent introduced a circulation plan that divided its centre into several sections around a central car-free zone. Cars are still permitted in each section, but a driver can no longer travel directly from one to another and must instead use a ring road around the centre. Residents can still reach their homes by car, while buses and emergency vehicles are allowed to pass between sections. Through-traffic, which had previously used the historic streets as a short cut, therefore largely disappeared. In the first year, the city reported a noticeable fall in car traffic at rush hour and a substantial rise in cycling.

Oslo provides a further lesson. When the Norwegian capital announced plans in 2015 to ban private cars from its central area, business owners objected strongly, and the proposal was modified. Instead of prohibiting cars outright, the city removed several hundred on-street parking spaces and replaced them with cycle lanes, benches and small parks. The effect on traffic was considerable, yet because nobody was formally forbidden to drive, the policy proved far less divisive than the original plan.

The strongest resistance to car-free schemes generally comes from retailers, who fear that their customers will go elsewhere. Research suggests that this fear is often exaggerated. A study of a shopping street in Bristol, in England, found that traders believed around forty per cent of their customers arrived by car, whereas the true proportion was little more than a fifth. A later survey of two shopping streets in Berlin produced a similar result: traders overestimated the proportion of customers who drove by a wide margin, and many shoppers lived within walking distance. Shopkeepers' concerns are not entirely without foundation, however. Businesses still need deliveries, disabled people may depend on cars, and residents of poorly served suburbs or rural areas may have no practical alternative.

There are other risks as well. Traffic removed from a centre does not always vanish; some of it may move to surrounding streets, whose residents may be less affluent and less able to protest. Barcelona's 'superblocks' illustrate both the potential and the difficulties. In each superblock, a group of nine city blocks is treated as a single unit, with through-traffic restricted to the roads around its edge, so that the inner streets can be used for play, seating and greenery. When the first full superblock was introduced in the Poblenou district in 2016, many residents objected, but opposition declined once people had experienced the changes. A further concern is that a centre made quieter and more attractive may also become more expensive, as rising demand pushes up rents and gradually displaces the small shops and long-standing residents that gave it its character.

The lesson I draw from these examples is that a car-free centre works best when it is part of a wider package. Good public transport, safe cycling routes and convenient arrangements for deliveries and for people with disabilities are essential if a ban is not simply to shift inconvenience onto those with the fewest choices. Trials, as in Copenhagen, allow people to judge a scheme by experience rather than by prediction. The question facing most cities is therefore no longer whether to reduce the dominance of cars in their centres, but how to do so fairly.