IELTS Academic Reading Practice Test 453

Few machines have remained in use for more than two thousand years with almost no change to their design, but the water-lifting device known as the Archimedes screw is one of them. In its classic form it consists of a broad spiral surface wrapped around a central axle and enclosed in a tube. When the tube is tilted and the axle is turned, water at the lower end is carried upwards and pours out at the top. The Roman architect Vitruvius described the machine in the first century BC, and versions of it are still made today, pumping sewage, moving grain and generating small quantities of electricity.

The construction is simple. A flat strip of wood or metal, the blade, is wound in a helix around a solid shaft, rather as a thread winds around a bolt. The shaft is gripped at each end by bearings, which allow the whole screw to turn. A handle at the upper end lets an operator rotate the assembly.

Around the blade runs a hollow cylinder, or casing, open at both ends, which holds the water in place. Because the machine carries water rather than forcing it, the clearances between blade and casing need not be perfect: water escapes through small gaps, but the loss is tolerable. Ancient screws were usually built of timber, with blades formed from planks joined edge to edge, while modern ones are welded steel of considerable precision.

The screw is set at an angle of between about 20 and 45 degrees to the horizontal, with its lower end standing in the water to be raised. As it turns, each section of the blade scoops up water and holds it in a pocket formed between one turn of the helix and the next. Rotation carries these pockets progressively towards the top, where the water spills out. If the angle is too steep, water runs back faster than the screw can lift it, and efficiency falls. Turning the screw the other way does not pump water downwards: the pockets simply empty.

Tradition credits the invention to Archimedes of Syracuse, the Greek mathematician and engineer of the third century BC, whose name has been attached to the device ever since. Whether he invented it, or merely described and improved a machine already in use, is uncertain. Some historians have argued that screw pumps were employed in Egypt before his lifetime, and the Greek world had long experience of devices for raising water. What is not in doubt is that the screw was widely used in the ancient Mediterranean. In Egypt it lifted water from the Nile into irrigation channels; in the Roman empire it helped to drain mines.

The most impressive surviving evidence of that mining work comes from southern Spain. In the Rio Tinto region, where Roman engineers cut deep galleries through copper-bearing rock, screws and wheels were among the machines used to keep the workings dry, and the water they lifted was carried away along channels cut in the hillside. Ancient screws were made of wood, so few have survived intact, but the settings in which they stood can still be identified. A wall painting from Pompeii and the account given by Vitruvius allow archaeologists to reconstruct the machines in some detail.

Through the Middle Ages the screw remained in use across Europe, in mines, salt works and land drainage, chiefly because it demands neither precision manufacturing nor a supply of spare parts. A carpenter could build one. In the seventeenth century the machine attracted the attention of the new scientific academies, whose members tested the angle at which it worked best and argued over the theory behind it. With steam power, and later electricity, the applications multiplied, and the geometry of the screw began to appear in machinery that had nothing to do with water.

Today the largest numbers are found in sewage treatment, where the machine's ability to pass solid objects without clogging is valuable. Slowly turning screws lift rags and debris out of the incoming flow, and larger screws raise wastewater from one stage of treatment to the next. In agriculture the same geometry appears as the grain auger, a screw inside a tube that moves grain or animal feed upwards at a shallow angle. Very large screws serve in flood-defence schemes.

The reversal of the machine is its most surprising modern role. If water is allowed to run down through a screw connected to a generator, the screw acts as a turbine, extracting energy from the water and turning it into electricity. The principle suits sites with a low head of water, such as old mill weirs, irrigation canals and small river dams, where conventional turbines would be uneconomic. Because the gaps between blade and casing are wide and rotation is slow, fish can usually pass through unharmed, which matters where environmental rules are strict. A growing number of such installations now operate in Europe.

In efficiency the screw is modest by the standards of modern pumps, converting perhaps 60 to 80 per cent of the energy supplied into lifted water, but efficiency is rarely the deciding factor. What recommends it, above all, is its robustness. It tolerates sand, stones and rags that would destroy a centrifugal pump, it can run for decades with little attention, and it can be repaired in a village workshop. For all the sophistication of modern fluid engineering, a device that Archimedes would recognise remains the sensible choice in a great many situations.

Trust is usually defined by social scientists as a willingness to make oneself vulnerable to another person in the expectation that they will behave fairly. Such behaviour cannot be controlled or predicted with certainty. Between people who know one another well, trust rests on a long record of experience; between strangers it has to be created from very little: a tone of voice, a uniform, a line of print, a promise. Because almost every economic transaction depends on it to some degree, researchers in several disciplines have tried to establish where that initial confidence comes from.

The most influential starting point was a series of computer tournaments organised in 1980 by the political scientist Robert Axelrod. He invited specialists in game theory to submit strategies for a repeated version of the prisoner's dilemma. In this game two players each do better by cooperating than by both cheating, but do best of all by cheating on a partner who cooperates. The winning strategy, submitted by the mathematician Anatol Rapoport, was also the simplest: tit-for-tat, which cooperates on the first move and then copies whatever the other player did last. Its success rested on two features of the situation. The players met repeatedly, so a partner who was cheated could retaliate next time, and the strategy was quick to forgive, which prevented long feuds.

Experimental economists have since examined trust more directly, using a simple game. One player is given a sum of money and may send any part of it to a second player. The experimenter triples whatever is sent, and the second player then decides how much, if anything, to return. The sum sent is treated as a measure of trust and the sum returned as a measure of trustworthiness. In hundreds of such experiments, most senders have passed on roughly half their money and most receivers have returned something. Generosity in games of this family is also sensitive to being observed: in related experiments, when players are told that their decisions will remain anonymous even to the experimenter, the amounts they give tend to fall.

People evidently make rapid judgements about whether a stranger can be trusted. Psychologists have shown that observers watching a few seconds of silent video, or even a still photograph, agree remarkably on which faces look trustworthy, though how accurately those impressions predict real behaviour is disputed. Information passed on by other people does similar work. Gossip is often dismissed as idle, yet it supplies exactly what a stranger lacks: a record. Online marketplaces have turned this into a system, and studies of auction sites have found that sellers with a better feedback history attract more bidders and obtain higher prices than equally unknown competitors.

Where no record exists, people look for signals. In biology, a signal is reliable only if it is costly to produce, since a cheat would otherwise simply imitate it; the peacock's tail is the standard example. Much of human commerce follows the same logic. A money-back guarantee, a deposit held by an independent third party and an extended warranty that a manufacturer would rather not honour are all signals of this kind. Each is persuasive precisely because it would be expensive to fake. Even small gestures fit the pattern: negotiators often begin by conceding something trivial, showing that they intend to deal fairly.

Biology has also been used to explain trust in another way. In the mid-2000s a group of researchers reported that volunteers who inhaled oxytocin, a hormone involved in childbirth and in bonding between parents and infants, were more willing to entrust money to a stranger in a laboratory game. Attempts at replication have produced mixed results, however, and later work has suggested that the hormone may make people more sensitive to social cues in general rather than more trusting.

Culture and institutions supply a further explanation. Surveys that ask whether most people can be trusted find wide differences between countries, and these differences persist over time. Some societies appear to have what researchers call generalised trust, a readiness to deal fairly with strangers, while in others confidence is confined to family and known associates. Economists have argued that the difference is not merely cultural: where contracts are enforced cheaply and predictably by courts, strangers can do business without knowing one another, and repeated satisfactory dealings gradually produce the expectation of fair treatment.

What remains uncertain is whether trust between strangers can be manufactured quickly when it is needed. Teams assembled for a short project, or volunteers responding to a disaster, often seem to cooperate on very little acquaintance, a phenomenon researchers have labelled swift trust. The evidence for how it is produced is thin, however, and the label may describe the effect rather than explain it. It is also unclear whether online contact builds or erodes trust. On one point, at least, most of the literature agrees: trust is easier to sustain than to create, and much easier to destroy.

Offshore wind has become one of the fastest-growing sources of electricity in the world. The first commercial wind farm at sea, built off the Danish coast in 1991, was a modest experiment of eleven small turbines; the largest installations now contain several hundred machines, each with a capacity that the pioneers of the industry could not have imagined. For governments committed to reducing carbon emissions, the appeal is obvious, and planning rules have been rewritten to accommodate it. Yet the question of whether wind farms should be built at sea rather than on land is less straightforward than the impressive growth figures imply, and the answer differs sharply from one country to another.

The physical case for the sea is strong. Wind blows harder and more consistently over open water, because the surface is smooth and offers little friction, and the air is also less turbulent than over land, where buildings, trees and hills break up the flow. As a result, an offshore turbine of a given size will usually generate more electricity each year than the same machine onshore. In northern Europe, moreover, sea winds are strongest in winter, when demand for power is at its peak. Sea winds are also less variable from hour to hour, which makes them slightly easier to accommodate on the grid.

Against these advantages must be set the costs. Turbines at sea need foundations driven into the seabed, submarine cables to carry their output ashore, and specialised vessels to install them. Maintenance is awkward and expensive, since technicians must travel by boat or helicopter, and salt water corrodes steel. Offshore electricity has therefore generally cost more per unit than electricity generated on land, and the industry has depended on public support to develop. Costs fell sharply during the 2010s as turbines grew larger and manufacturing moved to mass production, but rising prices for steel, cables and ships have since shown that the downward trend cannot be assumed.

Much engineering effort has gone into the problem of supporting a turbine in deep water. Most existing projects use monopiles, single steel tubes hammered into the seabed, which are economical only where the water is relatively shallow; in deeper water engineers turn to steel lattice structures known as jackets, which spread the load across several legs. Beyond about sixty metres, fixed foundations become prohibitively heavy, and the industry is now moving towards floating platforms, moored to the seabed by chains and cables. Floating designs open vast areas of ocean to development, but they have not yet been proved at scale.

The environmental questions are harder to settle. Turbines kill some birds and bats, though the numbers recorded at most sites are small compared with other causes of death; the more serious worry is that birds and marine mammals may avoid the area altogether, effectively removing habitat. Pile-driving during construction produces underwater noise that can disturb harbour porpoises and fish for many kilometres, and the cables and foundations alter the seabed locally. The trouble is that these effects are difficult to measure against natural variation. The cumulative impact of hundreds of turbines across a sea basin is harder still, and scientists agree on the need for long-term monitoring but rarely on what the results mean.

There is a planning argument too, and it is often decisive in practice. Onshore wind is cheap, quick to build and easy to connect, but a single turbine on a ridge can be seen for twenty kilometres, and the objections of those who live near it can delay a project for years. Moving turbines out to sea removes the problem of visibility from most people's lives, since a farm ten kilometres offshore appears as a thin line on the horizon in clear weather. That is a genuine benefit, but it is a benefit to objectors rather than to the electricity system, and it should not be mistaken for an engineering advantage.

A further difficulty is shared by all wind power. Output depends on the weather, and although offshore farms are more productive than onshore ones, they also need back-up capacity or connections to distant regions so that lulls in one area can be covered from another. Offshore farms are usually built far from the centres of population they supply, which means long transmission lines and greater losses. Some analysts argue that a mixture of onshore wind, solar power, storage and international interconnectors may deliver the same reliability at lower cost, though such system-wide comparisons depend heavily on assumptions.

My own view is that the question is badly posed. Offshore wind is not inherently better or worse than onshore wind; it is better where the wind is strong, the water shallow and the coast crowded, and worse where the seabed is deep, the grid weak and the wildlife sensitive. What is needed is not a general preference for one or the other but a willingness to compare full costs, including transmission, back-up and ecological damage, site by site. Judged that way, offshore wind has an important part to play, particularly in the northern seas of Europe, but it is a part rather than the whole performance.