“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.

There Locke procured a developing-bag and started to work.  Nervously and impatiently Eva and Zita watched him at his task.

At last the negative was ready and Locke drew it from the bag and held it to the light.

There, glaring out of the plate, was the devilish face of Balcom!

Eva and Zita both uttered a cry of astonishment and consternation.  Even Locke was amazed.  But the strongest feeling he had was anger as he turned to them.

“You two take this plan,” he exclaimed.  “It shows a den with an exit indicated.  Get some one to go with you; find the place and wait for me there.  I can find the secret entrance from the Graveyard from this negative—­and I’m going through it.”

Balcom, in the passageway between the Graveyard of Genius and the Automaton’s den, was livid with fury.  He realized that his picture had been taken, surmised that the secret passage would be found and that some assault on the den would be attempted.  But he had had no time to locate the camera, which Locke had hidden well, nor had he dared to search longer for it when he heard Locke bounding down the stairs from the library.