The upper end of the forward climbing shaft terminated in a small gun and observation cabin at the top of the ship, along which was a narrow walkingway extending from the forward cabin to a small turret near the tail of the ship, where provision had been made for fixing a machine gun. The main cabin, running along the keel of the ship, was an integral part of the hull, and because of this entirely rigid construction, which eliminated the necessity for cabins suspended below the hull, the O-220 was equipped with landing gear in the form of six, large, heavily tired wheels projecting below the bottom of the main cabin. In the extreme stern of the keel cabin a small scout monoplane was carried in such a way that it could be lowered through the bottom of the ship and launched while the O-220 was in flight. Eight air-cooled motors drove as many propellers, which were arranged in pairs upon either side of the ship and staggered in such a manner that the air from the forward propellers would not interfere with those behind. The engines, developing 5600 horsepower, were capable of driving the ship at a speed of 105 miles per hour. In the O-220 the ordinary axial wire, which passes the whole length of the ship through the center, consisted of a tubular shaft of Harbenite from which smaller tubular braces radiated, like the spokes of a wheel, to the tubular girders, to which the Harbenite plates of the outer envelope were welded. Owing to the extreme lightness of Harbenite, the total weight of the ship was 75 tons, while the total lift of its vacuum tanks was 225 tons. For purposes of maneuvering the ship and to facilitate landing, each of the vacuum tanks was equipped with a bank of eight air valves operated from the control cabin at the forward end of the keel; while six pumps, three in the starboard and three in the port engine corridors, were designed to expel the air from the tanks when it became necessary to renew the vacuum. Special rudders and elevators were also operated from the forward control cabin as well as from an auxiliary position aft in the port engine corridor, in the event that the control cabin steering gear should break down. In the main keel cabin were located the quarters for the officers and crew, gun and ammunition room, provision room, galley, additional gasoline and oil storage tanks, and water tanks, the latter so constructed that the contents of any of them might be emptied instantaneously in case of an emergency, while a proportion of the gasoline and oil tanks were slip tanks that might be slipped through the bottom of the ship in cases of extreme emergency when it was necessary instantaneously to reduce the weight of the load. This, then, briefly, was the great, rigid airship in which Jason Gridley and Tarzan of the Apes hoped to discover the north polar entrance to the inner world and rescue David Innes, Emperor of Pellucidar, from the dungeons of the Korsars. II