By the end of the 13th century, Italian engineers were building large mechanical clocks inside towers, driven by slowly descending weights.

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This will involve some daring, with the aircraft spending all day climbing as its batteries are recharged and then descending slowly under power throughout the night to conserve energy.

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The Pope, who is turning 81 this month, looked frail as he arrived in Athens on Friday morning for the 24-hour visit, descending the steps of his plane slowly but unaided to be greeted by a Greek Air Force honour guard.

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Altitude sickness can be easily treated by descending immediately and allowing your body to acclimatise slowly.

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