The Keystone That Holds the Arch

In the hill towns of central Italy, Roman aqueducts still stand after two thousand years. Engineers in Segovia, Spain, walk beneath arches built before the birth of Christ, marveling that they hold fast without a drop of mortar. The secret is the keystone — that single wedge-shaped block pressed into the crown of each arch. During construction, every stone along the curve leans on temporary wooden scaffolding.…