To illustrate the design principle behind Scotland’s Forth Bridge, engineer Sir Benjamin Baker offered a personal demonstration. Sir John Fowler (left) and Baker (right) each hold two wooden poles with outstretched arms, forming two diamond shapes. When construction foreman Kaichi Watanabe sits in the center, the diamonds are prevented from tipping inward because their outer ends are anchored.

It worked. The bridge, opened in 1890, held the record as the world’s longest single cantilever bridge span for 17 years.