“For nothing could explain away the livid terror that had dropped over his face while he stood there sniffing the air. And nothing — no amount of blazing fire, or chatting on ordinary subjects — could make that camp exactly as it had been before. The shadow of an unknown horror, naked if unguessed, that had flashed for an instant in the face and gestures of the guide, had also communicated itself, vaguely and therefore more potently, to his companion.”