IELTS Academic Reading Practice Test 42

Of all the structures that engineers have devised for crossing water, the suspension bridge is the one that most obviously appears to defy gravity. Its roadway, or deck, seems to float between two towers, held up by slender cables that curve gracefully from one shore to the other. Yet the principle on which it depends is ancient and remarkably simple. Bridges made from twisted grasses and vines have been used for centuries in mountainous regions such as the Andes and the Himalayas, where travellers walked directly on the sagging ropes themselves. The modern suspension bridge differs from these in one essential respect: its deck is not formed by the cable but hangs beneath it, and it is kept level rather than following the cable's curve.

The engineering logic is elegant. The main cables are draped over the tops of tall towers and fixed at each end into massive blocks of concrete or rock known as anchorages. Vertical cables called hangers connect the main cables to the deck at regular intervals. When a vehicle crosses, its weight passes through the hangers into the main cables, which are placed in tension, and from there into the towers, which are squeezed downwards in compression, and finally into the anchorages and the ground. Because steel is exceptionally strong when it is stretched, a cable can carry an enormous load relative to its own weight. This is why suspension bridges can cross greater distances than any other kind of bridge.

The first crossing to combine all of these elements is generally credited to James Finley, a local official in western Pennsylvania, who built a bridge over Jacob's Creek in 1801. It was a modest structure, but it used chains of wrought iron and, crucially, a level timber deck that was stiffened so that it did not sway excessively under traffic. Finley patented his design in 1808, and within a few years builders working under licence had erected several similar bridges in the eastern United States.

In Britain, the form was developed on a far grander scale. Thomas Telford's bridge across the Menai Strait in north Wales, which opened in 1826, had a main span of 176 metres, the longest of its time. The naval authorities insisted that the bridge be high enough for tall sailing ships to pass beneath it, which ruled out any design that relied on supports standing in the fast-flowing water. Telford's answer was to hang the deck from wrought-iron chains carried over two masonry towers. The deck proved vulnerable to storms and had to be strengthened more than once, and the original iron chains were eventually replaced with steel between 1938 and 1940, but the bridge remains in daily use today.

The decisive change in materials came with the use of steel wire. Rather than forging chains from heavy links, engineers learned to spin cables on site from thousands of individual wires, a method that produced great strength without the weak points that a single faulty link could introduce. The Brooklyn Bridge in New York, designed by John Augustus Roebling and completed in 1883, was the first major suspension bridge to use steel wire, and its main span of 486 metres made it the longest in the world.

Its construction was notoriously hazardous. Roebling died following an accident before building work had properly begun, and his son Washington, who took over, was left partly paralysed by decompression sickness after working in the pressurised chambers used to dig the foundations. His wife, Emily Warren Roebling, became the essential link between her husband and the engineers on site.

In the early twentieth century, improved mathematical methods persuaded designers that long decks could be made far lighter and more slender than before. The limits of this confidence were exposed on 7 November 1940, when the Tacoma Narrows Bridge in Washington State fell into the water barely four months after it had opened. Its deck, which was unusually narrow and shallow for its length, had moved up and down so noticeably in moderate winds that workers nicknamed the bridge 'Galloping Gertie'. On the final morning, a steady wind of around 40 miles per hour set the deck twisting violently until a large section tore away. No people died, although a dog left in an abandoned car was lost.

The disaster transformed bridge engineering. Designers came to understand that a bridge deck interacts with moving air rather like an aircraft wing, and that wind can feed energy into a structure until its own motion becomes self-reinforcing. Since then, scale models of almost every major suspension bridge have been tested in wind tunnels before construction begins. Decks are now given either deep, open trusses that allow air to pass through them or smooth, streamlined box sections that encourage it to flow around them without setting up dangerous oscillations.

These advances made possible a series of record-breaking spans. The Akashi Kaikyō Bridge in Japan, opened in 1998, has a central span of 1,991 metres; while it was being built, the 1995 Kobe earthquake moved its towers apart and lengthened the planned span by about a metre. In 2022, Turkey opened the 1915 Çanakkale Bridge across the Dardanelles, whose main span of 2,023 metres was chosen to mark the centenary of the Turkish Republic in 2023. Even so, engineers do not believe that the ultimate limit has been reached. On very long spans, the weight of the cables themselves becomes the main constraint, and lighter materials such as carbon fibre may one day allow crossings that are longer still.

Human beings have only a limited capacity to repair themselves. A cut heals, a broken bone knits together, and the liver can regrow a substantial part of its mass after surgery, but a lost finger or a severed spinal cord is gone for good. The axolotl, a salamander native to the lakes around Mexico City, is far more fortunate. Many other animals can regenerate to some extent: lizards can regrow a lost tail, and certain fish can replace their fins and even damaged heart tissue. Few vertebrates, however, can match the axolotl's range. It can rebuild entire limbs, its tail, parts of its heart and brain, and even its spinal cord, and the replacements are generally faithful copies of the originals, complete with bones, muscles, nerves and blood vessels in the right places. Little wonder, then, that this unusual amphibian has become one of the most intensively studied animals in regenerative biology.

The axolotl is unusual in other ways as well. Most salamanders begin life as aquatic larvae with feathery external gills and later undergo metamorphosis, losing their gills and moving onto land. The axolotl rarely does so. Instead, it remains in its juvenile form throughout its life, keeping its gills and living permanently in water, even after it becomes capable of breeding, a condition that biologists call neoteny. Metamorphosis can be triggered artificially by giving the animal thyroid hormone, and some researchers have reported that regeneration becomes slower and less accurate after the transformation. This has led to speculation that the axolotl's lifelong juvenile state contributes to its extraordinary powers of repair.

Scientific interest in the animal dates back to 1864, when a shipment of 34 live axolotls arrived in Paris from Mexico. They had been intended for display at a zoo, but the zoologist Auguste Duméril began to experiment with them, partly in an attempt to settle the question of whether they were adults or larvae. In the course of this work he removed parts of several animals and observed, almost incidentally, that the missing tissue grew back. Axolotls bred easily in captivity, and descendants of the Paris animals were soon distributed to laboratories across Europe and beyond, establishing the species as a standard research subject.

When an axolotl loses a limb, the process of repair begins within hours. Skin cells migrate across the wound and form a thin covering called the wound epidermis, which, unlike the scar tissue that forms in mammals, does not seal the injury permanently. Beneath this layer, cells from the stump accumulate into a cone-shaped mass known as a blastema. Over the following weeks, the blastema grows, and its cells divide and organise themselves, eventually rebuilding the limb in roughly the same sequence in which it formed in the embryo. Nerves play an essential part in this process: if the nerves supplying a limb are cut before amputation, no blastema forms and regeneration fails.

For many years, biologists assumed that the cells of the blastema returned to a fully unspecialised state in which they could become any kind of tissue. In 2009, however, a team led by Elly Tanaka in Dresden overturned this assumption. Using axolotls that had been genetically modified so that selected tissues glowed green, the researchers were able to follow the fate of individual cell types during regeneration. They found that cells largely retain a memory of where they came from: muscle gives rise to new muscle, and cartilage to new cartilage. The blastema, in other words, is not a uniform population of all-purpose cells but a mixture of specialised ones, each with a restricted range of possible fates. The result has practical implications, since it suggests that persuading human cells to regenerate may not require returning them to an embryonic state, a step that carries a risk of uncontrolled growth.

Another significant discovery concerns the immune system. In 2013, researchers led by James Godwin, then working in Australia, showed that macrophages, immune cells that consume debris and help to coordinate the body's response to injury, are indispensable. When the team used a chemical treatment to remove these cells from axolotls before amputation, the animals failed to regenerate their limbs; instead, the wounds closed over with scar tissue, much as they would in a mammal. Once the macrophage population had recovered, however, a fresh amputation was followed by normal regrowth. The finding suggested that the early inflammatory response, often regarded simply as a source of damage, may set the conditions in which regeneration can take place.

Progress has long been hampered by the sheer size of the axolotl's genome. At about 32 billion base pairs, it is more than ten times as large as the human genome, largely because it contains vast stretches of repetitive DNA. The publication of the first complete assembly in 2018 gave researchers a far more reliable map for identifying the genes involved in regeneration. Ironically, while the axolotl thrives in laboratories around the world, it is critically endangered in the wild. Surveys of the canals of Xochimilco, its last natural stronghold, suggest that its numbers fell from around 6,000 per square kilometre in 1998 to fewer than 40 by the middle of the 2010s, as a result of urban expansion, water pollution and the introduction of non-native fish such as carp and tilapia.

Few ideas from psychology have travelled as far, or as fast, as the 'growth mindset'. The term was popularised by the Stanford psychologist Carol Dweck, whose book Mindset, published in 2006, argued that people hold one of two broad beliefs about their own abilities. Those with a fixed mindset regard intelligence and talent as qualities they are born with and can do little to alter; those with a growth mindset believe that abilities can be developed through effort, good strategies and help from others. Within a decade the concept had become a fixture of staffrooms, teacher-training courses and corporate seminars, and classroom walls were covered with posters urging pupils to add the word 'yet' to any admission of failure. Few educational theories have ever enjoyed such a reception, and it is fair to ask whether the evidence has kept pace with the enthusiasm.

The appeal of the idea rests partly on a series of elegant experiments. In one widely cited study published in 1998, Claudia Mueller and Dweck gave children in their fifth year of school a set of problems and then praised some of them for being clever and others for working hard. When the children were afterwards offered a choice of tasks, those praised for intelligence were more inclined to pick easy problems that would allow them to go on looking smart. After a subsequent failure, they also persisted less, and enjoyed the work less, than those praised for effort. The implication seemed obvious: the way adults talk to children can shape how those children respond to difficulty. It is hard to read such findings without feeling that they capture something real about how people learn.

The difficulty lies in moving from laboratory demonstrations to measurable improvements in achievement. In 2018, two meta-analyses by Victoria Sisk, Brooke Macnamara and their colleagues pooled the results of several hundred studies. They concluded that the overall relationship between students' mindsets and their academic results was weak, and that the average effect of interventions designed to encourage a growth mindset was small. There were hints that certain groups, such as students from poorer backgrounds or those at high risk of failing, gained more, but these conclusions rested on relatively few studies. A later analysis by Macnamara and Alexander Burgoyne, published in 2023, went further. It reported that a large proportion of intervention studies contained weaknesses in their design, and that the better-designed studies were less likely to find any benefit at all.

Defenders of the theory reply that a weak average is not the same as no effect, and that an average can conceal large differences between groups. Their strongest evidence comes from the National Study of Learning Mindsets, reported in the journal Nature in 2019 by David Yeager and a large team of collaborators. More than 12,000 ninth-grade students, drawn from a nationally representative sample of 65 public high schools in the United States, were randomly assigned either to a short online programme explaining that the brain can develop through challenge or to a comparison programme about brain functions. Among lower-achieving students, the intervention raised grades in core subjects by about a tenth of a grade point, and participants were also more likely to enrol in more demanding mathematics courses the following year. Importantly, the gains were largest in schools whose culture supported the message.

Rival teams of researchers have since reanalysed much the same body of evidence and reached different conclusions, which tells us something uncomfortable about the field. When effects are small, decisions about which studies to include, how to measure achievement and which groups of pupils to examine separately can determine whether a result appears meaningful or negligible. Neither side has been dishonest. Rather, the debate shows how easily a modest finding can be presented as either a triumph or a failure, depending on what one hoped to see.

In my view, both camps are partly right, and the dispute has been made worse by inflated expectations. A tenth of a grade point is modest, but for an intervention that costs very little and requires so little classroom time, it is far from trivial, particularly for the students who need help most. What is harder to defend is the way the idea was sold to schools as a near-universal remedy. A teacher who praises effort while setting unchallenging tasks, or who tells struggling pupils simply to try harder without showing them how, is unlikely to change anything. Dweck herself has warned against what she calls a 'false growth mindset', in which the vocabulary of the theory is adopted without its substance, and her warning deserves more attention than her slogans have received.

The broader lesson concerns the journey that educational research makes into classrooms. A finding that holds under carefully controlled conditions, among particular students, may shrink almost to nothing when it is applied everywhere by busy teachers with little specialist training. Rather than abandoning the growth mindset, schools would do better to treat it as one modest ingredient of good teaching, effective mainly when combined with demanding work, useful feedback and an environment in which mistakes are genuinely treated as part of learning. Whether that more cautious version of the idea will ever prove as popular as the original is doubtful; modest claims rarely inspire posters.