Sometime in the late first century BCE, the Roman architect Marcus Vitruvius Pollio described the oldest known steam engine: the aeolipile. Devices of this type were described again in the following century by Hero (or Heron) of Alexandria, who gave more detail about their construction. Aeolipiles were hollow metal tanks with angled vents, filled with water and mounted on a pivot over a fire. When the water inside the tank boiled, the jets of steam hissing out through the vents would make the whole tank spin.
While the later Hero is remembered more often than the earlier Vitruvius in connection with these ancient gadgets, Hero himself refers to even earlier work on them by another Alexandrian, Ctesibius. No writings by Ctesibius have survived, and although later writers attributed several inventions to him, they do not mention the aeolipile as one of them, so the actual inventor may have been even more ancient.
Hero’s explanations imply that aeolipiles were temple showpieces whose rotation astounded but did nothing useful. There are no records or remains to suggest that any form of steam power was ever applied practically in ancient times. But Vitruvius had a different notion of what aeolipiles were good for. He referred to the aeolipile almost as an ancestor of the particle accelerator: “a scientific invention” which could be used to “discover a divine truth lurking in the laws of the heavens.”
Vitruvius was still a classical writer. The book in which he mentions the aeolipile also expounds the theory of architectural proportion, based on the human form, whose illustration by Leonardo da Vinci would become the Renaissance icon of classical humanism: Vitruvian Man. Vitruvius had no idea of the enormous practical potential of steam engines. He mentions the aeolipile in a chapter on weather. The truths he learned from the spinning tank of steam were about wind, not heat and power.
The aeolipile was a toy. It could barely turn itself, much less produce the labor power of a single slave. A far greater advance in power technology than the aeolipile was the medieval invention of the collar harness, which let horses replace oxen as draft animals. The uselessness of the aeolipile is an excellent example of the vastly under-appreciated role of materials in technological development. Whoever invented the aeolipile was a brilliant scientist, who must at least partially have understood deep principles of force and reaction, and then made them work in a real device; but it would be two thousand years before vessels could be forged that would hold enough pressure to let steam power change the world.
Nevertheless the aeolipile did indeed demonstrate a divine truth lurking in natural law. It may not have shown just how much power a machine could deliver, but it showed that a machine could have power to move. It hinted at the future power of artificial heat engines to do work beyond the limits of the human body.