[Illustration]
The illustration shows the plan of a prison of nine cells all
communicating with one another by doorways. The eight prisoners have
their numbers on their backs, and any one of them is allowed to exercise
himself in whichever cell may happen to be vacant, subject to the rule
that at no time shall two prisoners be in the same cell. The merry
monarch in whose dominions the prison was situated offered them special
comforts one Christmas Eve if, without breaking that rule, they could so
place themselves that their numbers should form a magic square.
Now, prisoner No. 7 happened to know a good deal about magic squares, so
he worked out a scheme and naturally selected the method that was most
expeditious--that is, one involving the fewest possible moves from cell
to cell. But one man was a surly, obstinate fellow (quite unfit for the
society of his jovial companions), and he refused to move out of his
cell or take any part in the proceedings. But No. 7 was quite equal to
the emergency, and found that he could still do what was required in the
fewest possible moves without troubling the brute to leave his cell. The
puzzle is to show how he did it and, incidentally, to discover which
prisoner was so stupidly obstinate. Can you find the fellow?