ised by the many irregular cracks in the masonry."

"If you're looking for a secret opening, why not come in the daytime
when you can see much better?"

"I've been here in the daytime and the door can't be detected--at least
not by the eye. I'm hoping to have better luck this time."

"I can't for the life of me see how," Susan began, but ended with a
startled gasp.

A portion of the massive wall was slowly moving backward.

"Just as I thought!" Penny chuckled in delight. "Now we know how Rap
Molberg escaped from the police the other night."

In fascination the girls watched the widening gap in the wall. Soon it
was large enough for an automobile to easily drive through into the
empty building.

"How did you open it?" Susan asked in awe.

"The beam of my spotlight struck a photo-electric cell which was
secreted near the eaves," Penny explained briefly. "You should have
read about it at the library."

"I wish I had now. It's almost uncanny."

"Let's drive in and have a look at the inside," Penny suggested
daringly.

"Won't it be dangerous?" Susan demurred.

"The place seems to be deserted. But probably it would be wiser if you
waited here and I went in alone."

"No, if you're going to risk it, so am I!"

"Then here goes," Penny said.

She drove the roadster through the opening into what appeared to be an
empty room. Curiously, the girls glanced about. Suddenly Susan
uttered a stifled scream.

"The door! It's closing!"

Already the opening had narrowed to a mere slit. It was too late to
retreat.

"Don't lose your nerve," Penny advised, although her own heart was
beating at a furious rate. "We'll find some way to open that door."

"Someone may have seen us drive in and closed it deliberately!"

"I don't think so, Susan. It must have closed automatically."

"Anyway, we're prisoners inside this horrible place! We'll starve to
death before anyone will suspect we're here!"

"I got you into this and I'll get you out," Penny announced firmly.
"There must be some button or lever that opens the door from the
inside."

Although the headlights of the roadster illuminated a portion of the
large room, many of the corners and crannies remained dark. Taking her
flashlight from the pocket of the car, Penny moved cautiously about
searching for some means of escape. Susan remained huddled in her
seat, too terrified to move.

Penny examined the door, but it would not budge when she threw her
we