The Sergeant of the Law was "full rich of excellence. Discreet he was,
and of great reverence." He was a very busy man, but, like many of us
to-day, "he seemed busier than he was." He was talking one evening of
prisons and prisoners, and at length made the following remarks: "And
that which I have been saying doth forsooth call to my mind that this
morn I bethought me of a riddle that I will now put forth." He then
produced a slip of vellum, on which was drawn the curious plan that is
now given. "Here," saith he, "be nine dungeons, with a prisoner in every
dungeon save one, which is empty. These prisoners be numbered in order,
7, 5, 6, 8, 2, 1, 4, 3, and I desire to know how they can, in as few
moves as possible, put themselves in the order 1, 2, 3, 4, 5, 6, 7, 8.
One prisoner may move at a time along the passage to the dungeon that
doth happen to be empty, but never, on pain of death, may two men be in
any dungeon at the same time. How may it be done?" If the reader makes a
rough plan on a sheet of paper and uses numbered counters, he will find
it an interesting pastime to arrange the prisoners in the fewest possible
moves. As there is never more than one vacant dungeon at a time to be
moved into, the moves may be recorded in this simple way: 3--2--1--6, and
so on.
[Illustration]