“It must be ours,” said the latter, at last. “I will go with you and make sure. Peter? Yes, he will be all right till I get back. He is much better. I will be ready in a moment.” She returned in less than a minute, a hat perched carelessly on her head and a shawl around her shoulders. Her eyes burned bright in her thin face. No one spoke on the way back. Mrs. Haddon, her lips set and her eyes fixed straight ahead, said not a word, and the girls were too awed by her emotion to break the silence. Miss Walters met them in the hall, said a few words to Mrs. Haddon, then, seeing that the woman was keyed to the breaking point, led the way straight to the tower room. The girls ran up the ladder ahead of the two older women. The latter followed more slowly. Billie pushed open the little door and entered the room. Then she started, gasped, rubbed her hand across her eyes to make sure she was not dreaming. For the spot where the queer wooden machinery had stood was empty. The invention was gone; and the blue prints were gone, too! CHAPTER XXI—MORE MYSTERY Billie Bradley turned cold all over. To have brought Polly Haddon here—to have practically promised her a fortune—and then to find—nothing! “Billie! They’re gone!” said a voice at her elbow, and she turned sharply to find Laura and Vi peering inquisitively over her shoulder. “I know they’re gone,” she cried, almost sobbing in her rage and disappointment “Oh, girls, what, can we do? We can’t tell Mrs. Haddon——” “What’s this you can’t tell me?” asked Polly Haddon herself, and Billie looked at the woman miserably. “The model,” she said, her voice almost inaudible. “It was here yesterday, and now it’s gone.” “_Gone!_” cried Miss Walters sharply. “How can that be? Is it possible that somebody else is in the habit of visiting this tower?” But Mrs. Haddon pushed her aside. “Do you mean that the model is gone—again—after bringing me here?” she cried wildly. “Oh, you could not be so cruel, you could not!” The last word caught in a sob, and Miss Walters put an arm about her compassionately. “Listen to me a moment,” she said, in a gentle voice of authority. “If the girls are certain that the machinery and the blueprints were here as late as yesterday——” “Oh, we are, we are!” cried Billie eagerly. “Then whoever has taken them since could not have got very far away with them in this short time,” she went on reassuringly. “Your husband’s invention—if indeed it was his model the girls found here—must still be in this neighborhood, perhaps in this very building. Though who,” she added thoughtfully, “in this place could wish to steal such a thing is indeed a mystery.” “Oh, Miss Walters!” cried Billie eagerly, “I’m sure nobody here in the Hall has stolen the invention. Nobody would have any use for it, and besides, it isn’t a thing that could be hidden very easily.” Suddenly Laura had what she thought was a bright idea.