"Yes, some one has. At least, it was in this safe last night"--here a wave of his hand indicated a small and rather old-fashioned strong box in the corner--"and it wasn't there when I arrived this morning. I reported the matter to the Secretary and he asked me to give you the details." "You are certain that the cipher was there last evening?" asked the chief. "Not the cipher itself--at least not a code-book as the term is generally understood," explained the Under Secretary. "That's one of the beauties of the Carruthers system. You don't have to lug a bulky book around with you all the time. A single slip of paper--a cigarette paper would answer excellently--will contain the data covering a man's individual code. The loss or theft of one of these would be inconvenient, but not fatal. The loss of the master key, which was in that safe, is irreparable. If it once gets out of the country it means that the decoding of our official messages is merely a question of time, no matter how often we switch the individual ciphers." "What was the size of the master key, as you call it?" "Merely a slip of government bond, about six inches long by some two inches deep." "Was it of such a nature that it could have been easily copied?" "Yes, but anything other than a careful tracing or a photographic copy would be valueless. The position of the letters and figures mean as much as the marks themselves. Whoever took it undoubtedly knows this and will endeavor to deliver the original--as a mark of good faith, if nothing else." "Was this the only copy in existence?" "There are two others--one in the possession of the Secretary, the other in the section which has charge of decoding messages. Both of these are safe, as I ascertained as soon as I discovered that my slip was missing." A few more questions failed to bring out anything more about the mystery beyond the fact that the Assistant Secretary was certain that he had locked the safe the evening before and he knew that he had found it locked when he arrived that morning. "All of which," as Taylor declared, "means but little. The safe is of the vintage of eighteen seventy, the old-fashioned kind where you can hear the tumblers drop clean across the room. Look!" and he pointed to the japanned front of the safe where a circular mark, some two inches in diameter, was visible close to the dial. "Yes, but what is it?" demanded the Secretary. "The proof that you locked the safe last night," Taylor responded. "Whoever abstracted the cipher key opened the safe with the aid of some instrument that enabled them clearly to detect the fall of the tumblers. Probably a stethoscope, such as physicians use for listening to a patient's heart. Perfectly simple when you know how--particularly with an old model like this." Finding that there was no further information available, Taylor and the chief left the department, the chief to return to headquarters, Taylor to endeavor to pick up the trail wherever he could.