Broom Bridge, Canal bridge in Cabra, Dublin, Ireland

Broom Bridge is a stone road bridge carrying Broombridge Road over the Royal Canal in Cabra, north Dublin. It is named after William Broome, a director of the Royal Canal company who lived nearby. A stone plaque on the northwest corner of its underside commemorates William Rowan Hamilton's quaternion formula of 16 October 1843; it was later moved higher, under the bridge railing, after vandalism.

On 16 October 1843 Sir William Rowan Hamilton, walking along the Royal Canal, wrote down the fundamental formula for quaternions here; the bridge is named after William Broome, a director of the Royal Canal company who lived nearby. A stone plaque on the northwest corner of the underside commemorates the event, later moved higher under the railing after vandalism. Hamilton's sons William Edwin and Archibald Henry called it the Quaternion Bridge.

The bridge is named after William Broome, a director of the Royal Canal company who lived nearby. Hamilton's sons William Edwin and Archibald Henry called it the Quaternion Bridge, and 16 October is nicknamed Broomsday, echoing the 16 June of Bloomsday. Thomas Pynchon mentions the bridge, as Brougham Bridge, in his novel Against the Day.

The bridge carries Broombridge Road over the Royal Canal in Cabra, north Dublin, and can be crossed on foot from either side. The commemorative plaque sits on the northwest corner of the underside, now higher up beneath the railing after earlier vandalism, so look up rather than at eye level. The canal towpath passes underneath.

The plaque's text calls Hamilton "immortal" and credits him with carving the formula "here on the stone"; it now sits higher, under the railing, after vandalism. Hamilton's sons William Edwin and Archibald Henry called the crossing the Quaternion Bridge, and Thomas Pynchon mentions it, as Brougham Bridge, in Against the Day.