10 Times Humanity Found the Answer (and Then Forgot)

Indirect Heating
A c. fourth-century Roman bath came complete with a hypocaust beneath the floor of its hot room to keep Roman toes toasty warm. CM Dixon/Print Collector/Getty Images

Heron was not alone taking delight in vaporous water. A Greek living in Egypt, he was part of the first-century Roman Empire, and the Romans knew their way around steam. For one thing, they used steam in their public baths — facilities so central to Roman society that some saw the bath as the symbol of Rome itself (the Baths of Caracalla covered more area than St. Paul's Cathedral and could handle 1,600 people at a go) [source: Suddath].

No mere watery tubs, various bath complexes featured outdoor sports areas, food stands, servants, dressing rooms, cold rooms, warm rooms and, of course, a steamy Caldarium. Making this sauna-like room possible was an innovative technology called the hypocaust — which also happened to be one of the first examples of indirect heat [source: PBS].

For most of human history (and prehistory), we were stuck with direct heat from fires, hearths and, later, stoves. Today, our homes use indirect heat, in which heat energy from a central source flows through the house via air, steam or water.

The Romans' hypocaust involved a brute-force version, which channeled fire-heated steam through passages beneath floors and inside walls, but it worked [source: PBS]. After Rome's collapse, Europe mostly returned to its pits and hearths for heat. Central heating would not re-enter the Western world in earnest until the late 18th and early 19th centuries, driven in part by the advances of the Industrial Revolution but slowed by ventilation issues, fire and explosion risks, arguments between architects and engineers, and the need to convince homeowners to convert from tried-and-true heating methods [source: Bruegmann].

Through their baths and other plumbing advances, the Romans spread hygiene throughout their empire — although, as we'll see in the next section, not as effectively as they might have hoped.