“What was it?”
“I don’t know, but something knocked us both out. You remember that the old man warned us not to advance another step—said it would mean instant death if we did.”
“Yes; but I thought the old duffer was bluffing.”
“So did I. I have since decided that he wasn’t.”
“You think he gave us the knock-out?”
“I do.”
“How could he?”
“Some way. He has some mysterious power, with the aid of which he guards the mouth of that cave.”
“And that power must be——”
“Electricity!”