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🎧📕 IELTS 17 TEST 1 - part 1 @IELTS_Cambridge1

🎧📕 IELTS 17 TEST 2 - part 1 @IELTS_Cambridge1

once heard without understanding🥲 it's time to enjoy the music 🥰

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The accidental rainforest According to ecological theory, rainforests are supposed to develop slowly over millions of years. But now ecologists are being forced to reconsider their ideas When PeterOsbeck. a Swedish priest, stopped off at the mid-Atlantic island of Ascension in 1752 on his way home from China, he wrote of ‘a heap of ruinous rocks’ with a bare, white mountain in the middle. All it boasted was a couple of dozen species of plant, most of them ferns and some of them unique to the island. And so it might have remained. But in 1843 British plant collector Joseph Hooker made a brief call on his return from Antarctica. Surveying the bare earth, he concluded that the island had suffered some natural calamity that had denuded it of vegetation and triggered a decline in rainfall that was turning the place into a desert. The British Navy, which by then maintained a garrison on the island, was keen to improve the place and asked Hooker's advice. He suggested an ambitious scheme for planting trees and shrubs that would revive rainfall and stimulate a wider ecological recovery. And, perhaps lacking anything else to do, the sailors set to with a will. In 1845, a naval transport ship from Argentina delivered a batch of seedlings. In the following years, more than 200 species of plant arrived from South Africa, from England came 700 packets of seeds, including those of two species that especially liked the place: bamboo and prickly pear. With sailors planting several thousand trees a year, the bare white mountain was soon cloaked in green and renamed Green Mountain, and by the early twentieth century the mountain's slopes were covered with a variety of trees and shrubs from all over the world. Modern ecologists throw up their hands in horror at what they see as Hookers environmental anarchy. The exotic species wrecked the indigenous ecosystem, squeezing out the islands endemic plants. In fact. Hooker knew well enough what might happen. However, he saw greater benefit in improving rainfall and encouraging more prolific vegetation on the island. But there is a much deeper issue here than the relative benefits of sparse endemic species versus luxuriant imported ones. And as botanist David Wilkinson of Liverpool John Moores University in the UK pointed out after a recent visit to the island, it goes to the heart of some of the most dearly held tenets of ecology. Conservationists' understandable concern for the fate of Ascension’s handful of unique species has, he says, blinded them to something quite astonishing the fact that the introduced species have been a roaring success. Today's Green Mountain, says Wilkinson, is ‘a fully functioning man-made tropical cloud forest' that has grown from scratch from a ragbag of species collected more or less at random from all over the planet. But how could it have happened? Conventional ecological theory says that complex ecosystems such as cloud forests can emerge only through evolutionary processes in which each organism develops in concert with others to fill particular niches. Plants eo-evolve with their pollinators and seed dispersers, while microbes in the soil evolve to deal with the leaf litter. But that’s not what happened on Green Mountain. And the experience suggests that perhaps natural rainforests are constructed far more by chance than by evolution. Species, say some ecologists, don’t so much evolve to create ecosystems as make the best of what they have. ‘The Green Mountain system is a man-made system that has produced a tropical rainforest without any co-evolution between its constituent species,’ says Wilkinson. Not everyone agrees. Alan Gray, an ecologist at the University of Edinburgh in the UK. argues that the surviving endemic species on Green Mountain, though small in number, may still form the framework of the new' ecosystem. The new arrivals may just be an adornment, with little structural importance for the ecosystem.

But to Wilkinson this sounds like clutching at straws. And the idea of the instant formation of rainforests sounds increasingly plausible as research reveals that supposedly pristine tropical rainforests from the Amazon to south-east Asia may in places be little more titan the overgrown gardens of past rainforest civilisations. The most surprising thing of all is that no ecologists have thought to conduct proper research into this human-made rainforest ecosystem. A survey of the island’s flora conducted six years ago by the University of Edinburgh was concerned only with endemic species. They characterised everything else as a threat. And the Ascension authorities are currently turning Green Mountain into a national park where introduced species, at least the invasive ones, are earmarked for culling rather than conservation. Conservationists have understandable concerns, Wilkinson says. At least four endemic species have gone extinct on Ascension since the exotics started arriving. But in their urgency to protect endemics, ecologists are missing out on the study of a great enigma. ‘As you walk through the forest, you see lots of leaves that have had chunks taken out of them by various insects. There are caterpillars and beetles around.' says Wilkinson. ‘But where did they come from? Are they endemic or alien? If alien, did they come with the plant on which they feed or discover it on arrival?’ Such questions go to the heart of how- rainforests happen. The Green Mountain forest holds many secrets. And the irony is that the most artificial rainforest in the world could tell us more about rainforest ecology than any number of natural forests.

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Although it dates from several hundred years after the building of the pyramids, its sophistication suggests that the Egyptians might nave been developing ideas of flight for a long time. And other ancient civilisations certainly knew about kites; as early as 1250 BC, the Chinese were using them to deliver messages and dump flaming debris on their foes. The experiments might even have practical uses nowadays. There are plenty of places around the globe where people have no access to heavy machinery, but do know how to deal with wind, sailing and basic mechanical principles. Gharib has already been contacted by a civil engineer in Nicaragua, who wants to put up buildings with adobe roofs supported by concrete arches on a site that heavy equipment can't reach. His idea is to build the arcnes horizontally, then lift them into place using kites. 'We've given him some design hints,' says Gharib. We're just waiting for him to report back.' So whether they were actually used to build the pyramids or not, it seems that kites may make sensible construction tools in the 21 st century AD.

Pulling strings to build pyramids No one knows exactly how the pyramids were built. Marcus Chown reckons the answer could be 'hanging in the air'. The pyramids of Egypt were built more than three thousand years ago, and no one knows how. The conventional picture is that tens of thousands of slaves dragged stones on sledges. But there is no evidence to back this up. Now a Californian software consultant called Maureen Clemmons has suggested that kites might have been involved. While perusing a book on the monuments of Egypt, she noticed a hieroglyph that showed a row of men standing in odd postures. They were holding what looked like ropes that led, via some kina of mechanical system, to a giant bird in the sky. She wondered if perhaps the bird was actually a giant kite, and the men were using it to lift a heavy object. Intrigued, Clemmons contacted Morteza Gharib, aeronautics professor at the California Institute of Technology. He was fascinated by the idea. 'Coming from Iran, I have a keen interest in Middle Eastern science/ he says. He too was puzzled by the picture that had sparked Clemmons's interest. The object in the sky apparently had wings far too short and wide for a bird. The possibility certainly existed that it was a kite/ he says. And since he needed a summer project for his student Emilio Graff, investigating the possibility of using kites as heavy lifters seemed like a good idea. Gharib and Graff set themselves the task of raising a 4.5-metre stone column from horizontal to vertical, using no source of energy except the wind. Their initial calculations and scale-model wind-tunnel experiments convinced them they wouldn't need a strong wind to lift the 33.5-tonne column. Even a modest force, if sustained over a long time, would do. The key was to use a pulley system that would magnify the applied force. So they rigged up a tent-shaped scaffold directly above the tip of the horizontal column, with pulleys suspended from the scaffold's apex. The idea was that as one end of the column rose, the base would roll across the ground on a trolley. Earlier this year, the team put Clemmons's unlikely theory to the test, using a 40-square-metre rectangular nylon sail. The kite lifted the column clean off the ground. 'We were absolutely stunned,' Gharib says. The instant the sail opened into the wind, a huge force was generated and the column was raised to the vertical in a mere 40 seconds.' The wind was blowing at a gentle 16 to 20 kilometres an hour, little more than half what they thought would be needed. What they had failed to reckon with was what happened when the kite was opened. There was a huge initial force - five times larger than the steady state force,' Gharib says. This jerk meant that kites could lift huge weights, Gharib realised. Even a 300-tonne column could have been lifted to the vertical with 40 or so men and four or five sails. So Clemmons was right: the pyramid, builders could have used kites to lift massive stones into place. 'Whether they actually did is another matter,' Gharib says. There are no pictures showing the construction of the pyramids, so there is no way to tell what really happened. The evidence for using kites to move large stones is no better or worse than the evidence for the brute force method,' Gharib says. Indeed, the experiments have left many specialists unconvinced. The evidence for kitelifting is non-existent,' says Willeke Wendrich, an associate professor of Egyptology at the University of California, Los Angeles. Others feel there is more of a case for the theory. Harnessing the wind would not have been a problem for accomplished sailors like the Egyptians. And they are known to have used wooden pulleys, which could have been made strong enough to bear the weight of massive blocks of stone. In addition, there is some physical evidence that the ancient Egyptians were interested in flight. A wooden artefact found on the step pyramid at Saqqara looks uncannily like a modern glider.