History in 3D — Ancient Inventions You Can Print

The Spartans sent secret orders 2,500 years ago with a scytale. Wind a narrow strip of paper in a spiral round the tube, write your message along it, one letter per turn, then unwind it: the letters scatter into nonsense. Only someone with a tube of exactly the same width can wind it back up and read it. The flat faces of this hexagonal tube keep the lines of writing straight, and the strip lives inside it under the screw cap. Change the width before you print and your pair of tubes becomes your own unbreakable key.

In 132 AD the Chinese astronomer Zhang Heng built the first seismoscope: eight bronze dragons each held a ball, and a tremor made one drop its ball into the mouth of a bronze toad, clanging the alarm and showing the direction of the quake. This is the one-dragon version. A marble sits in a shallow cup on top of a tall, slim screw-in column; a door slam, heavy footsteps or a real tremor tips it off and it lands in the dish with a clatter. Place it on different floors and see which part of the house shakes most.