Like lightning, Irwin sprang to the camera and glanced in; then quickly reaching out his hand, his fingers touched a button, and the hoarse marine whistle of the Tracer thundered forth its warning; seizing the tiller-wheel, he threw it hard aport, and then, without pausing, pressed another button, and the large gongs of the ship pealed out their summons to its crew that danger was imminent. Even as the alarm sounded, came a shock, a shiver, a slight careening of the vessel, and as Irwin took his white face from the camera, the grateful exclamation: "Thank God! we are safe! Look! the monster passes by!" Into the camera Cobb again peered; the dark, black stern of a large freighter was passing to the southwest. Lieutenant Sibley and the crew of the Tracer were quickly huddled at the door of the pilot's room. "Lieutenant," said Irwin, with a salute, "I confess that we have had a very narrow escape from being run down by a heavy freighter. Explaining these instruments to Mr. Cobb, I failed to note the approach of the vessel." The alarm having subsided, the subject was fully discussed, and Mr. Irwin was exonerated by the Lieutenant. All parties then returned to their various occupations. Mr. Irwin then turned to Cobb and said: "It was very negligent of me not to carefully survey the field for approaching vessels. The Tracer carries but a single mast, and sits so low in the water, that these many merchant ships, with their sleepy crews, often fail to sight her until too late to make a proper clearing." Then returning to the subject upon which they had been speaking when Cobb's excited exclamation had burst forth, he continued: "I see that you have understood the object of the little dark box on the table. It is a camera-obscura. The single mast of the Tracer is of aluminum, strong, slight, and hollow, and rises to a height of twenty-eight feet. A lens at the top revolves by pushing this button; thus a perfect image of the surrounding water and all upon it is thrown on the white ground within the box. Sitting here and looking in the box, I note the proximity of objects and steer the vessel. The mast also serves to carry an arc light for night traveling, and our flag by day. Further, our air is drawn down through pipes in its interior; for, during heavy seas, we must have the air inlets far above the deck, which is constantly washed by the rollers." Some further conversation was indulged in, and then Cobb thanked Mr. Irwin for his kindness, excused himself, and was soon seated, with Lieutenant Sibley, in the latter's cozy cabin.