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engine, which is why the chimneys at the onset of the Industrial Revolution were shorter than those later on. As the machines got bigger and the steam engines got more powerful towards the mid-nineteenth century, so the chimneys soared to amazing heights. (I suspect that the updraft in a tall chimney is also enhanced by the stronger, more reliable winds that blow over the chimney top far above the ground. Tall chimneys may also be less affected by the temperature inversions that sometimes virtually stop smoke rising in certain atmospheric conditions.)
    The tallest mill chimney of all was the Port Dundas Townsend chimney in Glasgow, built in 1859, which at 138.4 metres (454 feet) was the tallest man-made structure in the world until outstripped by the Eiffel Tower in 1889. But there were many other mill chimneys in mid-Victorian times that towered well over 100 metres. As steam engines gave way to other kinds of power, however, mill chimneys gradually became redundant, and the tallest stacks belonged to steel smelters and power stations. Once the dark, smoky skies of industrial regions were filled with the mill chimneys that rose up like forests of gaunt, leafless trees across the landscape. Now only a few survive – haunting reminders of the origins of the modern urban world.

Why can’t you light a candle in a spaceship?
    (Physics, Oxford)
    Well, actually you can light a candle in a spaceship. It would just be a very, very foolish thing to do if it was a conventional wax candle. Out in space, of course, there is no air, so spaceships must create their own oxygen-containing internal atmosphere for the astronauts to breathe. If the candle burns oxygen faster than the spaceship’s systems can replenish it, the astronauts would soon die of oxygen starvation. Even if the ship’s oxygen supply could keep pace with candle’s oxygen consumption, it would dramatically shorten the mission.
    Worse still, if the spaceship’s atmosphere is oxygen-rich, lighting a candle could start an inferno. The dangers of this were tragically illustrated in 1967 when three astronauts in America’s Apollo 1 space capsule lost their lives. Earth’s atmosphere is about 21 per cent oxygen and 78 per cent nitrogen, but in the lower pressures encountered in space, an atmosphere this rich in nitrogen would have givenearly astronauts the bends. The bends, otherwise known as decompression sickness, plagues scuba divers and occurs when nitrogen bubbles form in the blood, which can lead to paralysis and even death. So the capsule was filled with pure oxygen. Unfortunately, this proved fatal, because without the nitrogen to slow reactions down, oxygen is highly combustible. So when a spark started a fire, it engulfed the entire capsule in a ball of fire in less than a minute.
    On later Apollo missions, the astronauts wore oxygen-filled spacesuits for lift-off, while the cabin was filled with a safer mix of 60 per cent oxygen and 40 per cent nitrogen. Only once the spacecraft was up in space and past the dangerous take-off stage was the nitrogen vented and the cabin atmosphere turned to pure oxygen, allowing the astronauts to take off their helmets. It was still a high-risk strategy, and even a brief electrical spark could have brought disaster, let alone a candle.
    Now space missions and orbiting space stations use a mixture of oxygen and nitrogen similar to earth’s atmosphere, and the spaceship’s atmosphere is kept pressurised at a similar pressure too, to avoid the problem of the bends. So lighting a candle would not be quite so instantly catastrophic, but it would still consume vital oxygen at a terrifying rate. Short space missions like the American space shuttles and the Russian Buran are brief enough for the oxygen to be supplied entirely from tanks of liquid oxygen.
    Space stations, however, must generate their own oxygen supply and recycle as much as possible. Blowers continually circulate air through the spaceship and into ducts where