The inscription on Brougham Bridge

But on the 16th day of October…your mother was walking with me along the Royal Canal, to which she had perhaps driven; and although she talked with me now and then, an undercurrent of thought was going on in my mind which gave at last a result, whereof it is not too much to say that I felt at once the importance. An electric circuit seemed to close; and a spark flashed forth the herald (as I foresaw immediately) of many long years to come of definitely directed thought and work by myself, if spared, and, at all events, on the part of others if I should even be allowed to live long enough distinctly to communicate the discovery. Nor could I resist the impulse—unphilosophical as it may have been—to cut with a knife on a stone of Brougham Bridge, as we passed it, the fundamental formula with the symbols i, j, k:

i ² = j ² = k ² = ijk = −1

which contains the Solution of the Problem, but, of course, the inscription has long since mouldered away.

—William Rowan Hamilton, on the discovery of quaternions