025 Broom Bridge, Dublin, Ireland 53° 22 22.8 N, 6° 18 0 W Mathematical Equations As Urban Grafitti Mathematicians have a habit of thinking of things in the oddest of places: the Irish mathematician Sir William Rowan Hamilton came up with the theory of quaternions while out for a walk with his wife in 1843. Crossing the Broom Bridge in Dublin, Hamilton scratched the quaternion multiplication equation Equation 25-1) into the bridge's stonework using a knife. Equation 25-1. The equation of quaternion multiplication Hamilton's mathematical vandalism is no longer visible on the bridge, but the bridge itself is still standing and a plaque (see Figure 25-1) was erected by the Irish premier Eamon de Valera in honor of Hamilton. The bridge crosses the Royal Canal in a dubious area of Dublin; the best way to visit it is probably not to try to recreate Hamilton's stroll, but instead to take a number 120 bus or a local Western Commuter line train to the Broombridge stop. Figure 25-1. The plaque at Broom Bridge; courtesy of Robert Burke (robburke.net) 91