He dashed down the stairs, but was late by only the fraction of a second, as a metal hand was just closing the huge door to the Graveyard of Genius. He fumbled at the secret combination, and as he was doing so Eva and Zita joined him.
The door swung open and they rushed through. But the place was deserted.
"They've carried your father through some secret passage," exclaimed Locke. "That would explain much that is strange that has happened about the house, too."
Just then Zita stepped forward with the plan in her hand. "See," she cried, "there is a secret passage marked on this."
Locke studied the plan for some time, but whoever had drawn it had carefully concealed both the exact location of the passage and the method by which it was reached. As he searched, however, an idea occurred to Locke.
"I'll rig a trap with a camera," he decided, finally.
A few minutes later he returned to the room with his special quick-shutter camera, a flash-bag, and a ball of light twine. Carefully he focused the camera on the wall where the plan showed the secret passage to be. Then he rigged up the flash-bag and connected the whole with the twine, which he strung all about the Graveyard of Genius, so that, should any part of the wall move, it would cause the twine to break which in turn would at the same time release the shutter of the camera and explode the powder of the flashlight. Thus, without any direct human agency, a photograph would be taken.
Next he attached wires and ran them to the library above, where he installed an annunciator, the needle of which would indicate when the trap was sprung and the picture taken. Fascinated, the two girls watched. Eva was almost fainting with grief at the terrible fate that had overtaken her father. Even in his sickness, at least she had had him. But now he was gone—to what she could only guess. Locke tried to console her as they paced the library above, even though he realized that such consolation was hollow.
It was perhaps half an hour later when suddenly the needle of the annunciator began to vibrate rapidly. All leaped to their feet and ran down the stairs to the Graveyard.
At once Locke rushed to the camera, put in a slide, and took out the plate-holder. Then they hurried up to his laboratory.