There was a short silence, during which Ben poked aimlessly at the burning brands and the stranger looked critically around the camp. In a moment, with a complimentary remark regarding the _Louise_ and the _Bertha_, the intruder arose from the ground where he had been sitting and walked carelessly toward the machines. Ben followed him, watching every movement as if his life depended upon the scrutiny. The two machines stood quite close together, and as the stranger approached them Ben stepped a pace in advance and whirled about. The stranger started back with an exclamation of surprise. “We don’t permit strangers to inspect our aeroplanes,” Ben said. “Pardon me,” the other smiled, “I really didn’t mean any harm. It is quite natural that one should desire to inspect a beautiful machine.” The stranger kept pushing on, and at last brought his thin body into contact with the boy’s sturdy one. There was no doubt in the mind of the boy now that the fellow was there for mischief. He struck out swiftly from the shoulder, but the intruder dodged the blow neatly and, taking a package from the right-hand pocket of his coat, hurled it toward the aeroplanes. Ben’s clenched fist caught the other’s arm as the throw was released, and the missile, whatever it was, went wide of the mark. Ben saw the glitter of a shining surface in the firelight, and the next instant an explosion which seemed to shake the earth sounded in his ears. Without waiting to see the effect of the explosion, the stranger faced about and ran at full speed toward the spot where he had left his aeroplane. Ben followed him a few paces and then, deciding that it would be unsafe to leave the machines, turned back toward the camp-fire to see Jimmie and Carl come tumbling out of the shelter tent, rubbing their sleepy eyes. What Ben feared was that a second person had landed from the stranger’s machine before it had shown above the camp-fire. “What’s coming off here?” demanded Jimmie. “Gee!” exclaimed Carl, “I thought that was the crack of doom!” “Get down to the machines, quick, you boys!” Ben cried out. “There may be some one trying to work them an injury.” The two boys darted away, stopping only to secure electric flashlights, and were soon seen examining the aeroplanes. Ben waited a moment for some indications that the boys had met with a lurking enemy, and then started away in pursuit of the treacherous aviator. He was not in time, however, to stop the fellow before his machine launched into the air. As his aeroplane rose, Ben saw that he swung his face for an instant toward the camp. For only a moment the light of the fire shone on the face so turned back. Ben thought he had never seen a more villainous expression on any human countenance. The boy returned to the machines and joined his chums with an angry scowl on his face. He was angry at himself for having for a minute regarded the stranger in a friendly spirit.