Tom Tiddler's Island, стр. 35

“No,” Colin adjudged. “ ‘I am calling and testing’ would be enough for practical purposes. They do say that.”

“Then it looks—doesn’t it?—as if that formula was simply to make it difficult for an outsider to fake a message: mislead the receiving station, I mean. It sounds natural enough, and yet it helps to assure the sender that it’s a message from the right source. That’s a guess, merely.”

“Let it go at that,” Colin agreed, eager to get to something more definite.

“The next bit’s interesting. ‘I shall repeat this message at ten, eleven, and twelve o’clock.’ What do you make of that, Trent?”

Colin considered for a moment.

“By Jove! Of course! I was a fool not to spot that,” no admitted handsomely. “You mean that the receiving station had no transmitter? If it had, it could have asked to get a doubtful bit of the message repeated. Since it hadn’t, the sender repeated the message at fixed times, so that the receiver could fill in any gaps due to atmospherics and so forth in the first transmission. Is that it?”

“That’s how I read it,” Northfleet confirmed. “So it amounts to this: we’re dealing with a transmitting station close at hand—as I pointed out to you the other night—and it’s communicating with somebody who has no transmitter and can’t reply. Well, that’s always something learned.”

“Guessed seems nearer the mark,” Colin commented sceptically.

“Guessed, then, if you like,” Northfleet conceded without ado. “Now we come to the Morse cipher. Here’s a copy of it.’ ”

He spread the paper out on the table as he spoke, and Colin stared again at the jumble of letters which had puzzled him completely.

“Well, barring odds and ends like ‘hat’ and ‘cad’ and ‘lean’ there’s nothing in it that suggests any language under the sun to me,” Colin declared as he ran his eye up and down the page. “Why, you don’t know what language the original was in, even.”

“It isn’t Spanish, anyhow,” Northfleet affirmed.

“Why not?”

“Because Q is fairly common in Spanish, and there isn’t a Q in the whole cipher. There’s no K in Spanish, but there’s one in the message. It might be in a proper name, of course.”

“Pass that,” Colin conceded.

“The message en clair was in English,” Northfleet pointed out. “So the chances were that the cipher was an English message also. It might have been enciphered in various ways. There’s simple substitution, where you agree to use, say, M instead of A, S instead of B, and so forth. You can complicate it by a double substitution if you like—it’s called the ‘Beaufort cipher,’ then.”

“I’ve heard rumours of the sort,” said Colin sardonically. “We used that at school.”

“There’s an entirely different brand of cipher,” Northfleet went, on quite unperturbed. “It’s called the ‘transposition cipher In it the letters remain unchanged, but during the enciphering their order in the message is altered.”

“I follow you, Watson,” Colin acknowledged.

“Then follow on,” Northfleet continued. “The letter T occurs about once in every ten letters in an English message. That is, about 10 per cent, of the letters in the message are T’s. Suppose you find only one or two T’s in a longish message in cipher, then either the original message was not in English, or else it’s a substitution cipher and the T in it stands for some other letter like J or K, which doesn’t occur frequently in ordinary English.”

“Faint, but still pursuing,” Colin assured him. “You’ve got to prove the message was in English originally, though. How about that?”

“That wasn’t so difficult, though it was troublesome,” Northfleet explained. “There are 336 letters in the long cipher message. I counted the number of times each letter occurred in it—so many A’s, so many B’s, and so on. Then I took the first passage in English that came into my mind. It happened to be the bit in the Bible starting with: ‘Now there arose up a new king over Egypt which knew not Joseph,’ and so forth. I counted off 336 letters and tabulated their frequencies as before. Then I put the two sets of results side by side. The stars represent the number of times a letter occurs in the passage from Exodus; the dots represent the number of times the letter occurs in the cipher message.”

He put down another sheet of paper before Colin.

“It’s not a question of getting exact agreement, naturally,” Northfleet went on, pointing with his pipe-stem as he proceeded. “But even so, you can see that in nearly half the alphabet the frequencies run side by side almost exactly.

The general run of the likeness is all that really matters; and you can see at a glance that the similarity’s far closer than chance could make it. That’s enough to satisfy me that the message is an English one and that the letters haven’t been altered. In other words, these are the letters of the original message in English, but they’ve been jumbled up in accordance with some prearranged scheme.”

“That sounds all right,” Colin admitted.

“If the original message had been in German it would have had more Z’s in it,” Northfleet said, to reinforce his argument. “Same if it had been French. Spanish would have had more Q’s.”

Colin nodded.

“Admitted,” he said. “You’ve proved it’s English, all jumbled up. Now what about un-jumbling it? That looks as much of a job as getting the eggs back out of an omelette, to me.”

“Well, obviously there must have been some plan in the jumbling: a definite system of transposition applied, letter after letter. If you can hit on the plan, the thing’s solved.”

“Maybe,” Colin conceded; “but I’m not the lad for the work. I admit that straight off. I don’t so much as see how you’d begin.”

“Look at it this way,” Northfleet suggested. “In English there are a lot of digraphs—I mean a pair of letters expressing one sound, like TH, CH, CR, and SH.