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