The Double Dangerous Book for Boys. Conn Iggulden

that will look great. The trouble is that the paper tore really easily – and even though they were easy to fix, with strips of more paper and a dab of glue, we ended up cutting up a supermarket plastic bag and making a new tail and wings out of that instead. Fiddling and improving is the key here.

       THE WINGS

      We tried two wing shapes – curved and later a triangle. Both seemed to work. They should be taut, however. Leaving them to flop about just leads to rips.

      The first wings were formed as a big semicircle of 16in radius (40cm). We used a circular tray to trace it on tissue paper. We then cut out a smaller semicircle at the centre point, to keep the mechanism clear. When you are finished, it is a thing of beauty and enormously satisfying to use. Photograph it immediately, before you snap it in two.

      When you wind the elastic for the first time, it’s quite possible it will jam. The band pulls tight on the front assembly and friction is the enemy. The plastic bead on the outer side of the crank will help, but a drop of oil on every moving part is vital.

      The elastic is two bands doubled and doubled again. We tried various kinds, but thin ones seemed to provide better power than thick ones. Buy a box of them and put oil on the bands to ease friction.

      That’s it. Wind it up forty or fifty turns, find a high place and let it fly. Fetch it back, raise the tail a little and let it fly again. If there is a bad landing, be prepared to fix it on the spot with superglue and thread. The mechanism is a wonder to behold – it moves a little bit like the mandibles of an insect. Making a rear winder is easy enough, with a cork to help turn the wire and a groove to lock it, ready for take-off.

       FINAL THOUGHTS

      After half a dozen flights, our first ornithopter snapped almost entirely in half while we were wrestling with the crank. One of the wing spars broke at the same time. The amazing glue-and-thread combination restored it all well enough to flap again! No damage is so great that it means the end – though it will get heavier.

      One bead works better than two. Flapping speed is vital, so you might consider purchasing a range of rubber bands.

      Remember, the purpose of this is to build a flying machine, yes, but also to introduce you to the combination of balsa, thread and glue. Using the same set-up, you could get a small plastic propeller from a model shop and glue it to a hook instead of a crank. An epoxy glue would be more suited to holding the propeller in place. If you add a fixed wing made from either balsa sheeting or paper, it shouldn’t be too hard to make a small prop plane. Pictures of planes you have made can be sent to the authors care of the publishers – and are very welcome. Good luck.

       QUESTIONS ABOUT THE WORLD – PART ONE

      The first Dangerous Book asked and answered a number of questions: Why is a summer day longer than a winter day? Why is it hotter at the Equator? What is a vacuum? What is latitude and longitude? How do you tell the age of a tree? How do we measure the earth’s circumference? Why does a day have twenty-four hours? How far away are the stars? Why is the sky blue? Why can’t we see the other side of the moon? What causes the tides? How do ships sail against the wind? Where does cork come from? What causes the wind? and What is chalk? Could there possibly be any questions left unanswered? Well, yes.

       1. What is the tallest mountain on earth?

       2. Why does the earth have a magnetic field?

       3. Where are the hottest and the coldest places on earth?

       4. What was Pangaea?

       5. Is the earth slowing down?

       6. What are the longest rivers?

      1

WHAT IS THE TALLEST MOUNTAIN ON EARTH?

      Hawaii. Mauna Kea on the island of Hawaii is the tip of a much larger mountain. From the sea floor to that mountaintop is over 33,000 feet (10,203m). In comparison, Mount Everest on the Nepal/Tibet border is just over 29,000 feet (8,848m). Of course, at the bottom of Mauna Kea, pressure is crushing, so the mountain may never be climbed.

      If you measured all the peaks from the centre of the earth, the mountain known as Chimborazo in Ecuador is furthest out. It lies almost on the Equator. As an actual bottom-to-top measurement, though, Hawaii’s Mauna Kea is No. 1.

      (The deepest spot on earth is the Mariana Trench. Everest could be dropped into it.)

      2

WHY DOES THE EARTH HAVE A MAGNETIC FIELD?

      The earth’s magnetic field deflects charged particles coming off the sun, known as the ‘solar wind’. We suspect it is a key requirement for life to exist, so it’s something we look for in the solar system and further out.

      It is generated by the rotation of liquid metal at the earth’s core. The forces involved are almost beyond imagination, but that much molten iron/nickel moving at that speed produces a huge magnetic field. In comparison, Mars has a very weak magnetic field – and one problem of Mars colonisation would always be that lack of protection from solar particles.

      By measuring the strength of gravity at the surface (scientists jumping up and down, mostly), we are able to estimate the mass of the earth – it’s around 5.9 sextillion tonnes. Nothing on the surface is heavy enough to produce the gravity we can observe, which means the centre has to be a dense metal able to produce a magnetic field. Iron and nickel are the best candidates – both dense enough to explain gravity and a magnetic field.

      A number of planets in the solar system have magnetic fields. Yet when it comes to future exploration, our best bet might be the only moon that does – Ganymede, a moon of Jupiter. Dwarfed by the largest planet in the solar system, Ganymede is a respectable size – about two-thirds the size of Mars. Although its magnetic field is relatively weak, it should mean it has a liquid core, which suggests a source of heat to tap for future explorers.

      3

WHERE ARE THE HOTTEST AND THE COLDEST PLACES ON EARTH?

      The hottest air temperature ever recorded was in Death Valley, California, in June 1913, of 134°F (56°C). (There was actually a temperature of 136°F recorded in Libya in 1922, but that’s considered dubious due to problems with the instrumentation.) Either way, temperatures of over 110°F (43°C) are common in Death Valley each year from May to October, which makes it all the more extraordinary that long-distance races are run across it. The Badwater Ultramarathon is run across Death Valley in summer each year – 135 miles (217 km) from the lowest point in California to the foothills of Mount