Like almost all of the engineering feats we've listed, the
Romans didn't invent the arch -- but they sure did perfect it.
Arches had been around for nearly two thousand years before the
Romans got a hold of them. What Roman engineers realized (quite
brilliantly, as it turned out) was that arches need not be
continuous; that is, they don't have to span a gap in one go.
Instead of trying to cross gaps in one great leap, they could be
broken up into several, smaller sections. Turning an arch into a
perfect semicircle wasn't necessary so long as each section had
struts underneath. That's where the segmental arch came in. This
new form of arch-building had two distinct advantages. First,
because the arches could be repeated rather than having a single
stretch across a gap, the potential distance for a bridge span
could be increased exponentially. Second, because less material
was required, segmental arch bridges were more amenable to the
flow of water underneath them. Instead of forcing water through
a single smal