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
PELLUCIDAR
Just before daybreak of a clear June morning, the O-220 moved slowly from its hangar under its own power. Fully loaded and equipped, it was to make its test flight under load conditions identical with those which would obtain when it set forth upon its long journey. The three lower tanks were still filled with air and she carried an excess of water ballast sufficient to overcome her equilibrium, so that while she moved lightly over the ground she moved with entire safety and could be maneuvered almost as handily as an automobile.
As she came into the open her pumps commenced to expel the air from the three lower tanks, and at the same time a portion of her excess water ballast was slowly discharged, and almost immediately the huge ship rose slowly and gracefully from the ground.
The entire personnel of the ship's company during the test flight was the same that had been selected for the expedition. Zuppner, who had been chosen as captain, had been in charge of the construction of the ship and had a considerable part in its designing. There were two mates, Von Horst and Dorf, who had been officers in the Imperial air forces, as also had the navigator, Lieutenant Hines. In addition to these there were twelve engineers and eight mechanics, a negro cook and two Filipino cabin-boys.
Tarzan was commander of the expedition, with Jason Gridley as his lieutenant, while the fighting men of the ship consisted of Muviro and nine of his Waziri warriors.
As the ship rose gracefully above the city, Zuppner, who was at the controls, could scarce restrain his enthusiasm.