“Then help me get things in shape to generate the gas,” the scientist said. “I want to give the new vapor the first real test in lifting power to-day. On the success of it depends the future of the ship.” Seeing there was no immediate danger of being carried to the centre of the earth, Washington resumed his labors. The professor, the boys, Bill and Tom were also hurrying matters to enable a test to be made before night. As will readily be seen, even by those not familiar with the construction of airships and submarines, the chief problem was to find some agent strong enough to lift from the earth a weight heavier than had ever before been put into an apparatus that was destined to traverse the clouds. For the _Flying Mermaid_ was not only an airship but an ocean voyager as well. It had to be made light enough to be lifted far above the earth, yet the very nature of it, necessitating it being made heavy enough to stand the buffeting of the waves and the pressure of water, was against its flying abilities. Professor Henderson realized this and knew that the chief concern would be to discover a gas or vapor with five times the lifting power of hydrogen, one of the lightest gases known, and one sometimes used to inflate balloons. After long study he had been partially successful, but he knew from experiments made that the gas he had so far been able to manufacture would not answer. What he wanted was some element that could be mixed with the gas, to neutralize the attraction of gravitation, or downward pull of the earth. While he was seeking this, and experimenting on many lines, the construction of the air-water ship went on. In general the outward construction was two cigar shaped hulls, one above the other. Aluminum, being the lightest and strongest metal that could be used for the purpose, formed the main part of both bodies. The upper hull was one hundred feet long and twenty feet in diameter at the widest part. It tapered to points at either end. It was attached to the lower hull by strong braces, at either end, while from the center there extended a pipe which connected with the lower section. This pipe was intended to convey the lifting gas to the part which corresponded to the bag of the balloon, save that it was of metal instead of silk, or rubber as is usual. There were two reasons for this. One was that it would not be liable to puncture, particularly in the proposed underground trip, and the other was that it did not have to be so large as a cloth bag would have had to be. It was also a permanent part of the ship, and on a voyage where part of the time the travelers would be in the air and part on the water, and when the change from one to the other would have to be made quickly, this was necessary. It would have taken too long to raise the ship in the air had a cloth bag been used to contain the gas.