But another thought had occurred to Sally. "If that's so," she inquired dubiously, "what's the use of all the rest of those letters and numbers. They must be there for _something_." "They may be just a 'blind,' and mean nothing at all," answered Doris. "You see they'd have to fill up the spaces somehow, or else, if I'm right, they'd have more than one vacant square. And one was all they wanted. So they filled up the rest with a lot of letters and figures just to puzzle any one that got hold of it. But there's something else I've thought of about it. You notice that the two outside lines of squares that lead up to the empty squares are just numbers,--not letters at all. Now I've added each line together and find that the sum of each side is exactly _twenty-one_. Why wouldn't it be possible that it means the sides of this empty square are twenty-one-something--in length. It can't possibly mean twenty-one _feet_ because the whole cave is only about nine feet square. It must mean twenty-one inches." Sally was quite overcome with amazement at this elaborate system of reasoning it out. "You certainly are a wonder!" she exclaimed. "I never would have thought of it in the world." "Why, it was simple," declared Doris, "for just as soon as I'd hit upon that first idea, the rest all followed like clockwork. But now, if all this is right, and the treasure lies somewhere under the vacant square, our business is to find it." Suddenly an awful thought occurred to Sally. "But how are you going to know _which_ corner that square is in? It might be any of the four, mightn't it?" For a moment Doris was stumped. How, indeed, were they going to tell? Then one solution dawned on her. "Wouldn't they have been most likely to consider the square of the floor as it faces you, coming in at the door, to be the way that corresponds to the plan on the paper? In that case, the extreme right-hand corner from the door, for the space of twenty-one inches, is the spot." It certainly seemed the most logical conclusion. They rushed over to the spot and examined it, robbing Genevieve of her candle in order to have the most light on the dark corner. It exhibited, however, no signs of anything the least unusual about it. The rough planks of the flooring joined quite closely to those of the wall, and there was no evidence of its having ever been used as a place of concealment. At this discouraging revelation, their faces fell. "Let's examine the other corners," suggested Doris. "Perhaps we're not right about this being the one." The others, however, revealed no difference in their appearance, and the girls restored her candle to Genevieve at the table, and stood gazing at each other in disconcerted silence.