Far into the night the two men discussed plans for the project, for Tarzan was now thoroughly interested, and the next day they turned back toward the Urambi country and von Harben's mission, where they arrived on the fourth day and were greeted by Dr. von Harben and his son, Erich, as well as by the latter's wife, the beautiful Favonia of Castrum Mare. It is not my intention to weary you with a recital of the details of the organization and equipment of the Pellucidarian expedition, although that portion of it which relates to the search for and discovery of the native mine containing the remarkable metal now known as Harbenite, filled as it was with adventure and excitement, is well worth a volume by itself. While Tarzan and Erich von Harben were locating the mine and transporting the metal to the seacoast, Jason Gridley was in Friedrichshafen in consultation with the engineers of the company he had chosen to construct the specially designed airship in which the attempt was to be made to reach the inner world. Exhaustive tests were made of the samples of Harbenite brought to Friedrichshafen by Jason Gridley. Plans were drawn, and by the time the shipment of the ore arrived everything was in readiness to commence immediate construction, which was carried on secretly. And six months later, when the O-220, as it was officially known, was ready to take the air, it was generally considered to be nothing more than a new design of the ordinary type of rigid airship, destined to be used as a common carrier upon one of the already numerous commercial airways of Europe. The great cigar-shaped hull of the O-220 was 997 feet in length and 150 feet in diameter. The interior of the hull was divided into six large, air-tight compartments, three of which, running the full length of the ship, were above the medial line and three below. Inside the hull and running along each side of the ship, between the upper and lower vacuum tanks, were long corridors in which were located the engines, motors and pumps, in addition to supplies of gasoline and oil. The internal location of the engine room was made possible by the elimination of fire risk, which is an ever-present source of danger in airships which depend for their lifting power upon hydrogen gas, as well as to the absolutely fireproof construction of the O-220; every part of which, with the exception of a few cabin fittings and furniture, was of Harbenite, this metal being used throughout except for certain bushings and bearings in motors, generators and propellers. Connecting the port and starboard engine and fuel corridors were two transverse corridors, one forward and one aft, while bisecting these transverse corridors were two climbing shafts extending from the bottom of the ship to the top.