IELTS Academic Reading Practice Test 17

For most of history, moving heavy goods over land was slow and expensive. Roads were often little more than rutted tracks, and a horse pulling a cart could manage only a modest load. On calm water, by contrast, the same animal walking along a bank could tow a boat carrying many times that weight, because a floating hull meets far less resistance than wheels on uneven ground. Canals offered a way round these obstacles, but they presented engineers with a basic difficulty: water settles into a level surface, whereas the land it must cross rises and falls.

Some of the earliest solutions emerged in China, on the Grand Canal that carried grain from the fertile south to the political centres of the north. Where the canal changed level, boats were originally dragged up and down sloping ramps known as slipways, and heavily loaded vessels were frequently damaged. According to the official history of the Song dynasty, a transport administrator named Qiao Weiyue was troubled by the number of grain barges wrecked on one stretch of the canal, losses made worse because spilled cargo was being stolen.

In 984 he replaced a slipway with two gates set a short distance apart. The water between them could be raised or lowered, so that a boat could change level gently instead of being dragged. This arrangement, now called the pound lock, is the basis of almost every canal lock built since.

Medieval European rivers relied mainly on a cruder device, the flash lock. This was a single opening in a weir that could be unblocked to release a sudden surge of water, which carried boats travelling downstream through the gap. Flash locks wasted large quantities of water, endangered crews and caused frequent disputes with mill owners, who needed the water held back by weirs to turn their wheels. A key refinement was the mitre gate, whose two hinged leaves meet at an angle pointing upstream, so that the pressure of the water forces them more tightly shut. Leonardo da Vinci, an engineer to the rulers of Milan, drew such gates, and a lock built in the city in the 1490s used them. They gradually replaced gates that had to be lifted vertically, like a portcullis.

A pound lock is simply a watertight chamber with gates at each end. To raise a boat, the chamber is first emptied to the level of the lower canal and the boat moves in. Once the lower gates are closed behind it, small openings called paddles are opened to let water in from above, and the boat rises until the upper gates can be pushed open. Lowering a boat simply reverses the sequence. No pumps are required, since gravity does all the work. The drawback is that every passage sends a chamber-full of water downhill. A canal crossing high ground therefore needs a dependable supply of water at its highest section, known as the summit level, and finding that supply was often the hardest part of an engineer's task.

This challenge was met on a grand scale by the Canal du Midi in southern France, constructed between 1666 and 1681. Its creator, Pierre-Paul Riquet, was not a trained engineer but a wealthy collector of the salt tax. The canal runs for about 240 kilometres from Toulouse to the Mediterranean, and together with the River Garonne it linked the Atlantic and the Mediterranean without the long sea voyage around Spain.

Riquet's crucial insight was that streams in the Montagne Noire could be diverted to the summit and stored in a huge reservoir at Saint-Ferréol. Many of the lock chambers were given curved, oval walls, which resisted the pressure of the surrounding soil better than straight ones. Riquet invested much of his own fortune in the scheme, but he died in 1680, a few months before it opened.

In Britain, the canal age is usually dated from 1761, when a waterway built for the Duke of Bridgewater began carrying coal from his mines at Worsley into Manchester. Its engineer, James Brindley, avoided locks on the main line by keeping the channel at a constant height and carrying it across the River Irwell on an aqueduct. Within a short time the price of coal in Manchester had roughly halved. Across the Atlantic, the Erie Canal, completed in 1825, linked the Hudson River at Albany with Lake Erie at Buffalo. Its 363 miles and 83 locks cut the cost of moving a ton of freight between Buffalo and New York City from around one hundred dollars to roughly ten, helping New York to become the leading port in the United States.

Where the ground rose steeply, locks were built in a continuous series called a flight; at Caen Hill in Wiltshire, the Kennet and Avon Canal climbs through 29 locks in just over three kilometres. From the 1830s onwards, however, railways offered faster transport, and many canals gradually lost their traffic. In recent decades, many have been restored, often by volunteers, and they now carry pleasure boats rather than cargo. On many of them, boaters still work the locks themselves, turning the gear that raises the paddles with a detachable crank handle called a windlass. A holidaymaker passing through such a lock is relying on the same principle that Qiao Weiyue put to use more than a thousand years ago.

Along sheltered tropical and subtropical coastlines, where rivers meet the sea and fine mud accumulates, grows a distinctive kind of forest. Mangroves are not a single family of plants but a collection of several dozen tree and shrub species, from unrelated groups, that have independently evolved the ability to live in salt water. Satellite mapping by the Global Mangrove Watch initiative indicates that they covered roughly 147,000 square kilometres in 2020, spread across more than a hundred countries. Indonesia has a larger area than any other nation, while the Sundarbans, shared by India and Bangladesh, form the largest continuous mangrove forest on Earth. Although these forests occupy only a narrow fringe of land between the tides, their influence on people and wildlife is out of all proportion to their extent.

Living in seawater presents a plant with an obvious problem, since salt in high concentrations damages living tissue and makes it difficult for roots to absorb water. Different species deal with this in different ways. Some, such as the red mangrove, exclude most of the salt at the surface of their roots, which act as a kind of filter, allowing almost fresh water to pass into the plant. Others, including the black mangroves of the genus Avicennia, take up salt but then excrete it through specialised glands on their leaves, which can often be seen glistening with tiny crystals. A third strategy is to concentrate salt in older leaves that are then shed, removing it from the tree altogether.

The mud in which mangroves grow is waterlogged and contains almost no oxygen, so roots buried in it cannot breathe in the usual way. Many species have therefore developed roots that are exposed to the air. Avicennia sends up hundreds of pencil-like projections called pneumatophores, which stick out of the mud and take in air through small pores at low tide, while red mangroves stand on arching prop roots that both support the tree and absorb oxygen. Reproduction is equally specialised. In several species the seed germinates while it is still attached to the parent tree, producing a long, spear-shaped seedling known as a propagule. When it finally drops, it may lodge in the mud nearby or be carried away by the tide, remaining capable of growth for months until it reaches a suitable shore.

Beneath the water, the tangle of roots creates a sheltered habitat in which many commercially important fish, crabs and prawns spend their early lives, protected from larger predators. Fishing communities in many tropical countries depend directly on these nurseries. Mangroves also defend the coast itself. Their dense stems and roots slow the movement of water, reducing the height of waves and the force of storm surges, and they trap sediment that helps the shoreline keep pace with rising seas. The degree of protection depends on the width of the forest and the size of the wave, and a narrow belt of trees offers little defence against a large tsunami. Against the storms that strike tropical coasts far more often, however, intact forests have repeatedly been found to reduce damage to the settlements behind them.

In recent years, attention has turned to the role of mangroves in storing carbon. Because their waterlogged soils contain so little oxygen, dead roots and leaves decompose extremely slowly and accumulate over centuries. A study published in 2011 by Daniel Donato and colleagues, which measured carbon at 25 mangrove sites across the Indo-Pacific region, found that these forests held an average of more than 1,000 tonnes of carbon per hectare, most of it in the soil. This placed them among the most carbon-rich forests anywhere in the tropics. When mangroves are cleared and the soil is drained or dug out, much of this stored carbon returns to the atmosphere. Coastal ecosystems of this kind are now commonly described as 'blue carbon' habitats, and several governments have included their protection in national climate plans.

Despite these benefits, mangroves were cleared at an alarming rate in the late twentieth century. Large areas were converted into ponds for farming shrimp, particularly in Southeast Asia, while others gave way to rice fields, oil-palm plantations and coastal development. The pace of loss has since slowed considerably. According to Global Mangrove Watch, the net decline between 1996 and 2020 amounted to about 5,245 square kilometres, or 3.4 per cent of the 1996 area. In a few places, mangroves are actually advancing. In Florida, a study using satellite images taken between 1984 and 2011 found that mangrove cover had doubled at the northern edge of its range, a change linked not to higher average temperatures but to a fall in the number of extremely cold days.

Replanting has become a popular response to loss, but it is not always effective. A review of programmes in the Philippines published in 2008 reported that the long-term survival of planted mangroves was generally only 10–20 per cent. A common mistake was to plant seedlings on open mudflats or seagrass beds, where mangroves had never grown, simply because such land was easily available; the abandoned fishponds that had once been mangrove forest were often much harder to obtain. Many ecologists now argue that the most successful approach is to restore the conditions that allow mangroves to establish themselves naturally, above all the normal flow of the tides, and to involve the local communities that depend on the forest. Seen in this light, the future of mangroves depends less on the number of seedlings planted than on whether the right conditions are recreated in the right places.

Few ideas in psychology have travelled as far beyond the laboratory as the notion that willpower is a limited resource. According to this view, resisting temptation draws on a store of mental energy, much as exercise draws on the strength of a muscle; after one act of self-control the store is partly emptied, and the next act becomes harder. The idea is intuitively appealing, since most people have felt their resolve weaken at the end of a demanding day. It has also been popular with writers of self-help books, who advise readers to conserve their willpower for the decisions that matter most. Over the past decade, however, the evidence on which the idea rests has come under sustained attack, and the picture now emerging is both more complicated and, in some respects, more encouraging.

The theory, which became known as ego depletion, was developed by the social psychologist Roy Baumeister and his colleagues in the late 1990s. In a well-known experiment published in 1998, hungry volunteers were seated in front of freshly baked chocolate cookies and a bowl of radishes. Some were allowed to eat the cookies, while others were told to eat only the radishes and leave the cookies untouched. Afterwards, all were given a puzzle that, unknown to them, could not be solved. Those who had resisted the cookies gave up considerably sooner than those who had not been required to exercise restraint. In the following years, hundreds of studies appeared to confirm the pattern, and a review published in 2010, which combined the results of nearly two hundred experiments, concluded that the effect was real and of moderate size.

Some researchers went further, proposing that the resource in question was glucose, the sugar that supplies the brain with most of its energy. Studies reported that participants who drank lemonade sweetened with sugar after a demanding task performed better on a later test of self-control than those given a drink containing an artificial sweetener. Critics pointed out, however, that the brain's consumption of glucose barely changes during mental effort, and that the small quantities involved could not plausibly account for the results. Later experiments found that merely rinsing the mouth with a sugary solution, without swallowing it, seemed to improve persistence. This suggests that sugar may act as a signal that influences motivation, rather than as fuel that is burned by the brain.

More damaging still were attempts to reproduce the basic effect. In 2016, a project coordinated by Martin Hagger and Nikos Chatzisarantis, in which 23 laboratories followed an identical procedure agreed in advance, tested more than 2,000 participants and found an effect so close to zero that it could not be distinguished from chance. A second large project, led by Baumeister's long-standing collaborator Kathleen Vohs and published in 2021, involved 36 laboratories and more than 3,500 participants, and its main pre-registered analysis again failed to find convincing evidence of depletion. Other analysts had already argued that the earlier literature was inflated by publication bias, the tendency for studies with positive results to be published while those finding nothing remain unseen in researchers' files.

A different line of research suggested that what people believe about willpower may matter more than any physical reserve. In a study published in 2010, Veronika Job, Carol Dweck and Gregory Walton found that participants who regarded willpower as limited showed the usual decline in performance after a tiring task, whereas those who did not see it as limited showed no such decline. This finding, too, has not always been replicated, and it would be unwise to conclude that confidence alone can overcome fatigue. Nevertheless, it fits an alternative account in which mental effort is governed less by a fuel gauge than by the brain's continuous judgement about whether a task is worth pursuing. On this view, the feeling of depletion is genuine, but it reflects a shift in priorities – a growing desire to do something more rewarding – rather than an empty tank.

Perhaps the most practical insight comes from studies of self-control in everyday life. In one study conducted in Würzburg, Germany, 205 adults carried smartphones that prompted them seven times a day for a week to report any desires they were experiencing and whether they were trying to resist them. People who scored highly on a questionnaire measuring self-control experienced just as many desires as others, but they reported less inner conflict and attempted to resist temptation less often. Further research has linked high self-control to stronger beneficial habits which allow desirable behaviour to occur without a struggle. Successful self-controllers, it seems, are not people who win more battles against temptation but people who arrange their lives so that fewer battles take place.

This conclusion casts a famous earlier finding in a new light. In the marshmallow experiments conducted by Walter Mischel at Stanford University from the late 1960s, young children who waited longer for a second treat later tended to achieve more at school. When a team led by Tyler Watts repeated the study in 2018 with a larger and more varied sample, the association was considerably weaker, and much of it disappeared once family background was taken into account. For adults, the lesson is that self-control is less a fixed quantity to be rationed than a skill shaped by circumstances. Rather than relying on heroic resistance, people would do better to remove temptations from view, build routines that make good choices automatic, and treat tiredness as a reason to reconsider priorities rather than as proof that their resolve has run out.