I was cleaning out a drawer of junction-box spares when I thought about Hamilton and his penknife, scratching i\u00b2 = j\u00b2 = k\u00b2 = ijk = -1 into the parapet of Broom Bridge in 1843 because he hadn't any paper and didn't trust himself to hold it as far as Dublin. I followed the algebra about as far as the second line and then it got away from me, honestly. They're still in there doing the quiet work, though \u2014 every spacecraft and games console that needs to rotate something without tying itself in knots. The plaque on the wall is worn soft in places, but the formula's still legible.
Hamilton wrote it out for his son Archibald decades later — the walk along the canal with his wife, the talk going on beside him, and something like a spark closing a circuit. Not a plaque sentence at all; just a man remembering a particular afternoon and his wife not minding the pause.
Wendell, the carving itself is long gone — weather took it, and what's there now is a plaque from 1958. (The bridge is Broome with an e, though half the signs disagree.) Mathematicians still walk out from Dunsink every October to not-arrest him.