them aside. She felt unusually restless.

Presently an electrical magazine attracted her attention. She glanced
over it carelessly until she came to a particular article which dealt
with photo-electric cells and the clever purposes for which they were
used.

"Why, these 'magic-eyes' are almost human," she commented in an
undertone to Susan. "They turn lights on and off, cook meals, and open
doors, when a beam of light strikes the cell----"

"I've heard of them before," Susan interrupted in a tone which clearly
implied that she was not in the least interested.

Penny took the hint and dropped the subject. But she became absorbed
in the article. When she closed the magazine a half hour later, her
face was flushed with excitement.

"Susan, let's get away from here," she proposed in a whisper. "I've
just had an inspiration!"

Grumbling a little at being forced to leave a fascinating story before
she had finished it, Susan followed her friend from the building.

"What about this inspiration of yours?" she demanded as they walked to
Penny's parked roadster.

"It's this way, Susan. I knew there was a logical explanation for the
mysterious disappearance of that car Rap Molberg was driving. Let's go
over to the Hamilton Factory this minute and test out my new theory."

"You may know what you're talking about, but I'm sure I don't, Penny
Nichols."

"That's because you wouldn't let me tell you about that article I was
reading," Penny laughed. "But I'll explain everything as we go along."

Without pausing to consider that it might not be safe to investigate
the abandoned manufacturing plant at such a late hour, the girls drove
directly into the hilly section of Belton City. Penny turned into the
familiar dead-end street and was relieved to find no sign of other
vehicles.

She halted her roadster at the very end of the pavement in such a
position that the bright headlights played upon the massive walls of
the Hamilton building.

"It must be located higher up," Penny murmured to herself.

"What is?" Susan demanded. "I don't see what you're about anyway."

Without answering, Penny directed the beam of her spotlight upon the
stone structure. Inch by inch she moved it systematically over the
high wall.

"Perhaps it's only a silly idea," she acknowledged at last, "but I
believe that somewhere in the wall there must be a secret door--one
mechanically operated. No doubt the outline of the opening is
disgu