"There are any number of formulae," I went on, "by means of which we may effect a transposition of letters, the substitution being variable or irregular. For instance, the 'Checkerboard,' invented by the Russian nihilists, and similar devices, most of which depend for secrecy upon single or double key-words. Perhaps the cleverest system in this group is the cypher called by the French, 'Le Chiffre Indéchiffrable.'"

"'The Undecypherable Cypher,'" commented Betty. "Sounds rather hopeless."

"Well, you can decide for yourself if there is any reasonable possibility of unravelling it, unless you are lucky enough to stumble on the key-word."

"Try me," she challenged.

"To begin with, you write down the twenty-six letters of the English alphabet in a horizontal line, indenting it the space of a single letter."

"Indenting?"

"You'll understand when you see the diagram I'm preparing."

"Oh, you're making a magic square!"

"Yes. Now you repeat the process twenty-five times, the only difference being that all these other lines begin at the left-hand margin, each with a different letter in their strict alphabetical order. Your diagram will then look like this. For the present I am putting it in skeleton form:"

ABCDEFG..........WXY
ABCDEFGH..........XYZ
BCDEFGHI..........YZA
CDEFGHIJ..........ZAB
DEFGHIJK..........ABC
EFGHIJKL..........BCD
FGHIJKLM..........CDE
.....................
.....................
WXYZABCD..........TUV
XYZABCDE..........UVW
YZABCDEF..........VWX