material. Any receptacle having sufficient strength to withstand the atmospheric pressure upon a vacuum would have a weight far too great for a vacuum to lift.'
'Perhaps,' said Tarzan, 'and, again, perhaps not.'
'What do you mean?' inquired Gridley.
'What you have just said,' replied Tarzan, 'reminds me of something that a young friend of mine recently told me. Erich von Harben is something of a scientist and explorer himself, and the last time that I saw him he had just returned from a second expedition into the Wiramwazi Mountains, where he told me that he had discovered a lake-dwelling tribe using canoes made of a metal that was apparently as light as cork and stronger than steel. He brought some samples of the metal back with him, and at the time I last saw him he was conducting some experiments in a little laboratory he had rigged up at his father's mission.'
'Where is this man?' demanded Gridley.
'Dr. von Harben's mission is in the Urambi country,' replied the ape-man, 'about four marches west of where we now are.'
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 Harben-ite, ??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 Harben-ite 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.
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 walking-way 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 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 Mines. 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.
'The sweetest thing I ever saw!' he exclaimed. 'She responds to the lightest touch.'
'I am not surprised at that,' said Hines; 'I knew she'd do it. Why we've got twice the crew we need to handle her.'
