most readable message without getting
caught at it. I recollected now how cautious he had been to hand me no
paper, and how openly and obviously he had dropped each specimen into my
book; because he knew someone was watching him and expecting him to slip
in a message. He had, as I could see now in the retrospect, been
conspicuously careful that nothing suspicious should pass from his hands
to mine.

[Illustration: Plate II]

Substitution ciphers are easy to solve, especially for those having some
experience. The method can be found in Edgar Allen Poe's "Gold Bug" and
in a host of its imitators. A Secret Service cipher man could have read
it in an hour. But I knew my friend's mind well enough to find a
short-cut. I knew just how he would go about devising such a cipher, in
fact, how ninety-nine persons out of a hundred with a scientific
education would do it.

If we begin adding horizontal arms to the middle stem, from top to
bottom and from left to right, the possible characters can be worked out
by the system shown on Plate III.

[Illustration: Plate III]

It is most logical to suppose that Benda would begin with the first sign
and substitute the letters of the alphabet in order. That would give us
the cipher code shown on Plate IV.

It was all very quick work, just as I had anticipated, once the key-idea
had occurred to me. The ease and speed of my method far exceeded that
of Poe's method, but, of course, was applicable only to this particular
case. Substituting letters for signs out of my diagram, I got the
following message:

AM PRISONER R PLANS CAPTURE OF N Y BY SEIZING POWER WATER AND
PHONES THEN WORLD CONQUEST S O S

[Illustration: Plate IV]

PART IV

_L'Envoi_

(By Peter Hagstrom, M.D.)

My solution of the message practically ends the story. Events followed
each other from then on like bullets from a machine-gun. A wild drive in
a taxicab brought me to the door of Mayor Anderson at ten o'clock that
night. I told him the story and showed him my photographs.

Following that I spent many hours telling my story to and consulting
with officers in the War Department. Next afternoon, photographic maps
of the Science Community and its environs, brought by airplanes during
the forenoon, were spread on desks before us. A colonel of marines and a
colonel of aviation sketched plans in notebooks. After dark I sat in a
transport plane with muffled exhaust and propellers, slipping through
the ai