The Second Book of General Ignorance Read Online Free PDF Page A

No! Don’t look at me, don’t look at me!’

How does a shark know you’re there?

    You don’t have to be bleeding for one to track you down.
    Sharks have an astonishingly powerful sense of smell. They can detect blood at a concentration of one part in 25 million, the equivalent of a single drop of blood in a 9,000-litre (2,000-gallon) tank of water.
    It’s the currents that determine the speed and direction of a smell’s dispersal in water, so sharks swim into the current. If you are bleeding, even slightly, a shark will know. If the current is running at a moderate 3½ kilometres per hour (about 2¼miles per hour), a shark 400 metres (a quarter of a mile) downstream will smell your blood in seven minutes. Sharks swim at nearly 40 kilometres per hour (25 miles per hour), so one could reach you in sixty seconds. Faster currents make things worse – even allowing for the fact that the shark has more to swim against. In a riptide of 26 kilometres per hour (16 miles per hour), a shark less than half a kilometre (a quarter of a mile) downstream would detect you in a minute and take less than two to reach you – giving you three minutes in total to escape.
    Sharks also see very well, but even a short-sighted shark with a bad head cold (not that it happens) would still be able to find you. Sharks have excellent hearing in the lower frequencies and can hear something thrashing about at a distance of half a kilometre (a third of a mile). So you could try being very quiet indeed.
    A blind, stone-deaf shark with no nose would still find you without breaking stride. Sharks’ heads are riddled with jelly-filled canals by the name of the ‘ampullae of Lorenzini’ after Stefano Lorenzini, the Italian doctor who first described them in 1678. We’ve only recently discovered what their purpose is: to register the faint electrical fields generated by all living bodies.
    So, as long as you’re not bleeding, not moving and your brain and heart aren’t working, you should be fine.
    And there’s some more good news – sort of. Californian oceanography professor Dr Jamie MacMahan has found that the standard view of a riptide is wrong – it doesn’t run out to sea but is circular, like a whirlpool. If you swim parallel to the shore, he says, there’s a 50 per cent chance you’ll be swept out into the ocean deeps. But, if you just tread water, there’s a 90 per cent chance of being returned to shore within three minutes – perhaps just in time to escape the shark.
    If a shark does find you, try turning it upside down and tickling its tummy. It will enter a reflex state known as ‘tonicimmobility’ and float motionless as if hypnotised. Killer whales exploit this by flipping sharks over on to their backs and holding them immobile in the water until they suffocate. You have about fifteen minutes before the shark gets wise to your ruse. Careful, though: not all species of shark react the same way. Tiger sharks, for example, respond best to a gentle massage around the eyes. According to shark expert Michael Rutzen, it’s just like tickling trout: ‘All you have to do is defend your own personal space and stay calm.’
    Having said all this, relax. Sharks almost never attack people. Figures from all twenty-two US coastal states, averaged over the last fifty years, show that you are seventy-six times more likely to be killed by a bolt of lightning than by a shark.

Does the Mediterranean have tides?

    Yes it does, despite what every tour guide tells you.
    Most of them are very small: just a few centimetres back and forth on average. This is because the Mediterranean is cut off from the Atlantic (and the huge effect of the pull of the moon on it) by the narrow Straits of Gibraltar.
    Right next door to the entrance to the Med, sea levels can change by around 80 centimetres (3 feet) but in the Gulf of Gabes off the coast of eastern Tunisia, the tidal elevation can be as much as 2.5 metres (8 feet) twice a day.
    This is because tides