Perhaps it was the uncertain light which made him fancy a queer expression flitted swiftly across Willis Chase’s eyes. But, glibly, laughingly, the guest made answer: “A pistol? Why, of course not! What’d be the sense in packing a gun here in the peaceful Berkshires? Thax, this burglar flurry has made you melodramatic. Good night, old man. Don’t snore too loudly over your sentry duty.” Vail departed for the study while Chase stuffed an armful of clothes into a handbag and made his way along the dark hall to Thaxton’s bedroom. At the stair-foot Vail all but stumbled over the collie. Then, refusing the dog’s eagerly mute plea to accompany him into the study, he whispered: “No, no, Mac! Lie down! Stay there on guard! _Stay_ there!” With a grunt of disappointment Macduff slumped down again at the foot of the stairs. Head between white paws, he lay looking wistfully after the departing man. The night wore on. Perhaps half an hour before the first dim gray tinged the sentinel black summit of old South Mountain to northwestward, the deathly silence of the sleeping house was broken by a low whistling cry--a sound not loud enough nor long enough to rouse any slumberer--scarce audible to human ears not tensely listening. Yet to the keen hearing of Macduff as he drowsed at the stair-foot the sound was vividly distinct. The collie reared himself excitedly to his feet. Then, remembering Thaxton Vail’s stern command to stay there on guard, the dog hesitated. Mute, statuelike, attentive, he stood, his teeth beginning to glint from up-curling lips, his hackles abristle. Macduff was listening now, listening with all that uncanny perception which lurks in the eardrums of a thoroughbred dog. He whined softly under his breath at what he heard. And he trembled to dash in the direction of the sound. But Vail’s mandate held him where he was. Presently a new sense allied itself to his hearing. His miraculously keen nostrils flashed to his brain the presence of an odor which would have been imperceptible to any human but which carried its own unmistakable meaning to the thoroughbred collie. Perhaps, too, there came to him, as sometimes to dogs, a strange perception that was neither sound nor smell nor sight--something no psychologist has ever explained, but which every close student of dogs can verify. The trembling changed to a shudder. Up went Macduff’s pointed muzzle, skyward. From his shaggy throat issued an unearthly wolf-howl. Again and again that weird scream rang through the house; banishing sleep and reëchoing in hideous cadences from every nook and corner and rafter. A hundredfold more compelling than any mere fanfare of barking, it shrieked an alarm to every slumbering brain. In through the open front doorway from the veranda rushed Thaxton Vail. “Mac!” he cried. “Shut up! What’s the matter?”