“No doubt they are working hard to solve the mystery. Unfortunately the best they can do is to guess. We hold the key. That is—I should correct that statement—we hold the knowledge, and they hold the keys.”
“The keys?” Harry wanted to know more.
The professor pointed to the three empty sockets in the great white stone above their heads. “These three missing stones are the keys. Until they are reset we cannot control the Spot. I had found two of them before I came through. I take it that both of you remember the blue one?”
“I think,” agreed Chick, “that neither of us is ever likely to forget it! Eh, Harry?”
The professor smiled. He was holding the light up to the snow-stone, at a spot that would have been the point of intersection had lines been drawn from the three missing gems, and the resulting triangle centred. He held his hand up to the substance. It was slightly rough at that point, as though it had been frozen.
Then he ran his fingers across the surrounding surface.
“Ah!” he exclaimed. “I thought so! That helps considerably. Chick—put your hand up here. What do you feel?”
“Rough,” said Chick, feeling the intersection point. “Slightly so, but cold and—and magnetic.”
“Now feel here.”
“Cool and magnetic, doctor; but smooth. What does it prove?”