"To-morrow will be the fifteenth birthday of the Quaternions. They
started into life full-grown on the 16th October, 1843, as I was
walking with Lady Hamilton to Dublin, and came up to Brougham
Bridge--which my boys have since called Quaternion Bridge. I pulled
out a pocketbook which still exists, and made entry, on which at the
very moment I felt that it might be worth my while to expend the
labour of at least ten or fifteen years to come. But then it is fair
to say that this was because I felt a problem to have been at that
moment solved, an intellectual want relieved which had haunted me for
at least fifteen years before.

"But did the thought of establishing such a system, in which
geometrically opposite facts--namely, two lines (or areas) which are
opposite IN SPACE give ALWAYS a positive product--ever come into
anybody's head till I was led to it in October, 1843, by trying to
extend my old theory of algebraic couples, and of algebra as the
science of pure time? As to my regarding geometrical addition of
lines as equivalent to composition of motions (and as performed by
the same rules), that is indeed essential in my theory but not
peculiar to it; on the contrary, I am only one of many who have been
led to this view of addition."

Pilgrims in future ages will doubtless visit the spot commemorated by
the invention of Quaternions. Perhaps as they look at that by no
means graceful structure Quaternion Bridge, they will regret that the
hand of some Old Mortality had not been occasionally employed in
cutting the memorable inscription afresh. It is now irrecoverably
lost.