Hero (or Heron) of Alexandria worked in the great city of Roman Egypt. Scholars date him to the first century AD, partly because he describes an eclipse of the Moon that matches one in AD 62. Almost nothing is known about his life. What survives are his books: on air and water (Pneumatics), on automatic theaters (Automata), on measuring land (Metrica and Dioptra), on mirrors and on war machines. Many seem to be teaching books, written for students.
Key idea
The problem
Alexandria's teachers wanted to show that air is real stuff that takes up space and can push. Temples and wealthy owners also liked marvels: doors that opened by themselves, statues that poured wine, birds that sang. Hero's books show how to build both kinds of thing.
The spinning steam ball
Place a cauldron over a fire: a ball shall revolve on a pivot.
Water boils in a closed pot. The steam rises through a tube into a hollow ball that can turn. Two bent pipes stick out of the ball on opposite sides, pointing opposite ways. Steam jets out of each pipe, and the ball spins the other way.
Why it works
Story or history?
The aeolipile is often called the first steam engine, and people ask why the Romans did not start an industrial revolution. Be careful: Hero describes it as a demonstration. It did no useful work, and a spinning ball with open jets wastes almost all its steam. Historians give many reasons for the gap, from the cost of fuel and metalworking to the economics of the time. None is proven, so it is a question to think about, not a fact to learn.
Automatic doors and a coin machine
In other sections Hero shows how a fire on an altar could open temple doors by itself: heated air pushes water from one vessel into a bucket, the bucket gets heavy and pulls ropes that turn the door posts. When the fire dies, the air cools and the doors close. He also describes a vessel that pours out a little holy water when a coin is dropped in: the coin lands on a small pan, tips a lever and opens a valve until the coin slides off. It is often called the first vending machine.
In the Dioptra he describes an odometer for a cart: gears count the turns of a wheel and drop a pebble into a box every mile. Land surveyors could count pebbles instead of pacing.
Worked example: Hero's odometer
- One pebble means $400 \times 4 = 1600$ m.
- Nine pebbles: $9 \times 1600 = 14{,}400$ m, or 14.4 km.
Answer: 14,400 meters.
What he tried, and what we can't know
We don't know how many of Hero's devices he built himself, and which he copied from earlier inventors such as Ctesibius, the Alexandrian engineer who made pumps and water clocks a few centuries before him. Hero openly says he is building on others' work and adding his own. That is how most invention happens.
| When | What happened |
|---|---|
| 3rd century BC | Ctesibius in Alexandria builds pumps, water organs and water clocks |
| c. AD 10–70 | Hero's likely lifetime (the dates are uncertain) |
| AD 62 | An eclipse of the Moon that Hero describes in his Dioptra |
| 1575 | Hero's Pneumatics printed in a Latin translation, read by Renaissance engineers |
| 1851 | Bennet Woodcroft's English translation of the Pneumatics |
Worked example: How long ago?
- Both years are AD, so subtract.
- $2000 - 62 = 1938$ years.
Answer: 1,938 years.
What it led to
Renaissance engineers read Hero in print and built his devices again. His idea that air and steam can push was the seed of later work on air pumps and pressure. Today's jet engines and rockets turn on the same idea as his spinning ball.
Common mistake
Read more: Hero of Alexandria and Aeolipile.