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.