[mdiesel]

For those interested in reading further, the ideal (unloaded) shape of an arch isn't a semicircle, but a catenary.

It sounds so simple: so hangs the chain, stands the arch. Took until Hooke in the 17th century before that was written down, though there are earlier (15th century) examples in architecture.

The Romans were still working on the Greek ideology that the circle was the perfect shape. Not to belittle what they did, but the key advances were really in concrete and having an authoritarian empire giving unprecedented resources to public works.

[ahazred8ta]

"Ut pendet continuum flexile, sic stabit contiguum rigidum inversum -- As hangs a flexible cable, so inverted stand the touching pieces of an arch." (although they figured out at some point that this curve was close to being a parabola)