A hoarse-toned marine whistle, almost at Cobb's feet, answered these salutations, and also caused that gentleman to jump back with a startled expression. Drawing his hand from the whistle button, Lieutenant Sibley apologized for frightening him, saying: "It did not occur to me that I had others aboard than those who are accustomed to these vessels." The Tracer was a cigar-shaped vessel of two hundred feet in length by twenty beam, or middle diameter, and of nearly 1,000 tons displacement when submerged. With an outer shell of aluminum bronze and an inner shell of the finest steel, the vessel combined great strength with a minimum amount of metal in its construction. "Gentlemen, if you will follow me," said Lieutenant Sibley, "I will show you over the vessel." Descending the companion-way, the entrance to which could be closed by an air-tight door, the party proceeded about the vessel. Longitudinally and horizontally, from apex to apex of the cones, was a steel deck dividing the vessel into two equal parts. The first forty-five feet of each cone contained the tubes of compressed air and oxygen. There were in each end about 2,500 feet of five-inch steel tubes, one-half inch thick, containing over 4,500 cubic feet of air under a pressure of 1,500 pounds per square inch. This was sufficient, as Lieutenant Sibley explained, to sustain active life for the entire crew for two hours. "But we have other facilities," continued the Lieutenant, "by which the vitiated air is deprived of its carbonic acid, and then recharged with the lipthalene gas from the receivers and oxygen from the pipes, giving about eight hours of active life to the inmates of the vessel when totally deprived of air externally." The store-rooms, mess-rooms, and quarters of the men were visited. Small though these rooms were, they were made with every convenience, and given every useful contrivance which this great age of invention could produce. The Tracer was not a war vessel, but belonged to the Geographical Bureau, and was used in charting the Central Sea. Her complement was small: two engineers, two pilots, one electrician, cook, assistant cook, captain's boy, two helpers, and two officers. Everything was so admirably arranged, and machinery played such a wonderful part in the power required to handle the vessel, that a larger force was not only unnecessary, but would have been detrimental to a satisfactory working of the vessel. Cobb called attention to the steel partitions between the rooms, and asked why so much strength was required.