Alice groped forward in the darkness toward the barricade and, as she touched the logs, her bare arm touched Coquenil's bare arm.
Suddenly a faint sound broke the stillness and the detective started violently. He was in such a state of nervous tension that he would have started at the rustle of a leaf.
"Hark! What is that?"
It was a low humming sound that presently grew stronger, and then sang on steadily like a buzzing wheel.
"It's over here," said Coquenil, moving toward the door. "No, it's here!" He turned to the right and stood still, listening. "It's under the floor!" He bent down and listened again. "It's overhead! It's nowhere and—everywhere! What is it?"
As he moved about in perplexity it seemed to him that he felt a current of air. He put one hand in it, then the other hand, then he turned his face to it; there certainly was a current of air.
"Alice, come here!" he called. "Stand where I am! That's right. Now put out your hand! Do you feel anything?"
"I feel a draught," she answered.
"There's no doubt about it," he muttered, "but—how can there be a draught here?"
As he spoke the humming sound strengthened and with it the draught blew stronger.