杏吧原创

The Black Sea port of Feodosia, in what is now Ukraine, is a place of dark secrets. One of Europe鈥檚 oldest towns, it has been home to Greeks and Genoese, Tartars and Armenians. In the Middle Ages, it housed one of Europe鈥檚 biggest slave markets, selling Ukrainians to Egypt. The Black Death entered Europe when a ship docked there in W . But, perhaps most mysterious of all, the surroundingforests house strange pyramid-like piles of stones as big as houses. Mostarchaeologists say they are tombs but local people say the pyramids once supplied the town with water. A centu ry ago, Russian engineer Frederich Zibold argued that these bizarre structures were abandoned chambersforcondensingdew. Had the people of Crimea discovered 鈥� and lost 鈥� the art of catching water from the air?The idea has been dismissed for decades. Butthere are now new claims that Zibold was right after all.

GILBERT WHITE, the celebrated chronicler of the 18th-century English countryside, knew all about catching dew. There was a pond high above his home in Selborne that was replenished mainly by dew. It was, he wrote, 鈥渘ever above three feet deep in the middle, and not more than 30 feet in diameter, and contained perhaps not more than two or three hogsheads of water, yet it is never known to fail, though it affords drink to 300 or 400 sheep and for at least 20 herds of cattle besides鈥�.

At the time, the downs of south-east England were dotted with dew ponds, most of them dug to provide water for huge flocks of sheep. The ponds capture some rain, but were artfully constructed to condense moisture from the air. Beneath their clay bottoms, a layer of straw insulated the water, keeping it colder than the soil at night; and above the clay, a layer of stones ensured that the pond shed heat quickly, lowering the temperature further. The siting was clever, too. Many sat at the heads of valleys that channelled mists and moist air uphill, cooling it as it went. Once established, dew ponds effectively generated their own water from the air. Many still do.

Moisture in the air is the great untapped water resource. The atmosphere contains around 13,000 cubic kilometres of water vapour at any one time, six times as much as in all the world鈥檚 rivers. It is a tantalising prize, and numerous inventors have tried to catch the water by making rain, catching fog or triggering condensation.

Cloud seeding is today almost an established technology, with planes and rockets spraying silver iodide into clouds from China to Israel and Russia to the US, to turn the droplets into rainfall (see 鈥淜icking up a storm鈥�). More outlandish was the plan of French meteorologist Bernard Dubos, who in 1935 proposed building a chimney 600 metres high with a fountain at its base to create an updraft of humid air that would, he believed, saturate the air above and generate rain. And in the 1940s, South Africa鈥檚 chief meteorologist Theodore Schumann proposed erecting an electric fence 50 metres high along the top of Table Mountain to make rain by ionising the air. Neither ever happened.

Another idea is to trigger rain with sound. On a cool, still night, the air can be so saturated with moisture that even modest air movements, such as sound waves, could in theory produce raindrops. In the mountains of southern China, villagers have a tradition of yelling to stimulate rain. The louder they shout, they say, the more it rains.

Catching fog has a more established track record. On a ridge in the rainless Atacama desert in northern Chile, you can still see the tattered remains of large sheets of plastic mesh that once harvested water from the fogs that roll in from the Pacific Ocean. This is recent history. In the 1990s each 12-by-3-metre net was catching an average of 150 litres a day, filling taps in the nearby fishing village of Chungungo. The village abandoned the nets after the government installed a piped water supply. But the Canadian researcher behind them, Bob Schemenauer, is erecting similar systems for remote communities in other arid, foggy places, including Haiti, Oman, Guatemala and Eritrea. The number of such places is limited. But water vapour in the atmosphere is universal. So the ultimate goal is still to find a sure-fire way to harvest that moisture by cooling the air to make dew.

The last traditional English digger of dew ponds hung up his spade in the 1930s. But in other places, dew-catching skills have lingered. Few holidaymakers on the island of Lanzarote in the Canaries will have noticed, but farmers there surround their vines with a 鈥渟tone mulch鈥� of volcanic gravel that condenses night-time moisture to water the crops.

And then there is the strange case of the Crimean pyramids. Frederich Zibold, a Russian engineer and forester, stumbled across them in 1900 while clearing trees. After talking to local people, he reported that until the late 19th century, the piles of stone were the main source of water for Feodosia, which in its slave-trading heyday had a population of 80,000.

By Zibold鈥檚 time, the pyramids 鈥� typically 30 metres across and 10 metres high 鈥� had been abandoned. But he reported that each night the stones lost heat and cooled the sea breezes blowing through the loosely packed stones sufficiently to cause moisture to condense. In the past, this flow of water trickling from the stones had filled networks of pottery pipes and several dozen reservoirs that provided water for the town.

Today the pyramids are no more than rubble, but the remains of the pipes and reservoirs survive. The amount of water they produced has been a matter of conjecture and fitful experiment ever since. Measurements made in the 19th century, but uncovered only after Zibold鈥檚 discovery, put the output of each pyramid at between 5000 and 10,000 litres a day. When Zibold built his own small pyramid near the town in an attempt to prove his theory, he collected around 300 litres a day, which he considered a failure. Attempts in the 1920s and 1930s by the Belgian inventor Achille Knapen and French bioclimatologist Leon Chaptal produced similar returns. Knapen鈥檚 patented 鈥渁ir well鈥� still stands derelict on a hilltop in Provence.

Around the same time, German researcher Wolf Klaphake discovered Arabic texts describing water condensers in Palestine a thousand years ago. He built his own, apparently successful, prototype 鈥渁ir well鈥� in Croatia. It was essentially a building with labyrinthine walls and a chimney to draw warm, moist air from outside into the cool interior. After emigrating to Australia, he failed in an effort to persuade the authorities there to build dew-making machines on the waterless Nullabor Plain. After this, the thirst for making dew seemed to die. Even Zibold鈥檚 work was disowned. Archaeologists excavating the remains of the Crimean pyramids argued that they were tombs after all.

鈥淭he atmosphere contains around 13,000 cubic kilometres of water vapour at any one time, six times as much as in all the world鈥檚 rivers鈥�

But now Israeli researchers have reopened the debate. Avraham Trakhtman, a mathematician at Bar-Ilan University in Ramat Gan, and Boris Kogan of the Center of Ecological Systems and Technologies in Jerusalem claim that those who rejected Zibold鈥檚 idea were 鈥渢oo hasty鈥�. In the climatic conditions of the Crimea, the original pyramids were much more efficient than later experiments suggested. Zibold鈥檚 mistake was to make too small a replica. The original pyramids were big enough for the stones at their heart to stay cool long enough for condensation to continue long into the day. And the pyramids had a crater in the top, which Trakhtman believes created a chamber of solar-heated air that stimulated a draught through the pyramid, maximising condensation.

Some other aficionados of the art of capturing dew have yet to be convinced. But it remains the case that archaeologists have uncovered similar stone heaps across arid lands from north Africa to southern Siberia. And, besides, says Trakhtman, 鈥淲hy would Zibold have made all this up?鈥�

This is no idle speculation from the Israelis. Their efforts to find water in the desert provide one of the central narratives of the Israeli state. And with humidity in parts of the Negev desert approaching 100 per cent for up to 200 nights a year, dew is potentially a major source of water there. Any way to maximise dew production would be a boon for a country that sees its survival tied as much to water as land. Could the town that brought the Black Death to Europe one day have a more positive legacy?