Watanabe, perched between Sir Benjamin Baker and Sir John Fowler, is illustrating how cantilever beams support light central girders in cantilever bridges. Baker and Fowler represent the cantilevers, with their arms in tension and their sticks under compression. The bricks represent cantilever end piers, which are weighted with cast iron on the Forth Bridge.
First educated at Japan’s Imperial College of Engineering, (the precursor to the School of Engineering at the University of Tokyo), Watanabe went on to study Engineering Sciences at the University of Glasgow in Scotland – graduating with a BSc in 1886. His graduation came just a bit more than 30 years after Commodore Perry sailed into Tokyo Harbour in 1853, effectively putting an end to Japan’s 200-year isolation policy. Watanabe’s Scottish education was characteristic of the country’s aggressive push to modernize as quickly as possible during the Meiji Restoration.
After graduation, Watanabe got a job as a construction foreman on the Forth Bridge project, which was designed by Baker and Fowler to cross a 2.5-kilometer (1.55-mile) span of the Firth of Forth, an estuary of the river Forth, located about 9 miles west of Edinburgh.
Still in use today, the Forth Bridge is a UNESCO World Heritage Site and an iconic structure and symbol of Scotland. The bridge was completed and went into use in 1890, at which time it was the longest single cantilever bridge span in the world – a title it retained until 1917, when it was overtaken by the Quebec Bridge in Canada. The Forth Bridge remains the second-longest single cantilever span in the world to this day.
For another look into engineering history, check out this image of manual machining for NASA.