"But now, at last, I come to the notebook. I found it among some papers in my father's study desk, a while after his death, and I frankly confess I could make nothing of it whatever. It seemed to be filled with figures, added and subtracted, and, as my father had always been rather fond of dabbling with figures and mathematics, I put it down as being some quiet calculations of his own that had no bearing on anything concerning me. I laid it carefully away with his other papers, however, and there it has been, in an old trunk in the attic of the unused part all these years. When you spoke of a 'secret code,' however, it suddenly occurred to me that the notebook might be concerned in the matter. Here it is." She held it out to them and they crowded about her eagerly. But as she laid it open and they examined its pages, a disappointed look crept into Sally's eyes. [Illustration: "Why, there's nothing there but numbers"] "Why, there's nothing here but _numbers_!" she exclaimed, and it was even so. The first few lines were as follows: 56 + 14 - 63 + 43 + 34 + 54 + 64 + 43 + 16 - 52 + 66 + 52 + 15 + 23 - 66 + 24 - 15 + 44 + 43 - 43 + 64 + 43 + 24 + 15 - 61 + 53 - 36 + 24 + 14 - 51 + 15 + 53 + 54 + 43 + 52 + 43 + 43 + 15 - 16 + 66 + 52 + 36 + 52 + 15 + 43 + 23 - And all the rest were exactly like them in character. But Doris, who had been quietly examining it, with a copy of the code in her other hand, suddenly uttered a delighted cry: "I have it! At least, I _think_ I'm on the right track. Just examine this code a moment, Miss Camilla. If you notice, leaving out the line of figures at the top and right of the whole square, the rest is just the letters of the alphabet and the figures one to nine and another '0' that probably stands for 'naught.' There are six squares across and six squares down, and those numbers on the outside are just one to six, only all mixed up. Don't you see how it could be worked? Suppose one wanted to write the letter 't.' It could be indicated by the number '5' (meaning the square it comes under according to the top line of figures) and '1' (the number according to the side line). Then '51' would stand for letter 'T,' wouldn't it?" "Great!" interrupted Sally, enthusiastically, who had seen the method even quicker than Miss Camilla. "But suppose it worked the other way, reading the side line first? Then 'T' would be '15.'" "Of course, that's true," admitted Doris. "I suppose there must have been some understanding between those who invented this code about which line to read first. The only way we can discover it is to puzzle it out both ways, and see which makes sense. One will and the other won't." It all seemed as simple as rolling off a log, now that Doris had discovered the explanation. Even Miss Camilla was impressed with the value of the discovery. "But what is the meaning of these plus and minus signs?" she queried. "I suppose they stand for something."