of the temperature
within its interior. The Doctor's plan of using hot air alone as the
floating power had been modified to the extent of dividing one-half of
the globe's interior into several compartments by thin sheets of
aluminum, and these were filled with hydrogen gas. The gas fell but
little short of the power necessary to float the ship, so that a slight
elevation of the temperature in the air chamber above that of the
external atmosphere was sufficient to float the vessel. When it was
desirable to descend, a trap being opened in the upper and lower parts
of the air chamber caused the hot air to rush out and the cold air in,
and the descent could be made rapidly or slowly, at the will of the
commander. By virtue of the zinc lining of the air chamber the
temperature would remain at a given point for many hours without the
consumption of a particle of fuel.

The Doctor and Will together had devised a most ingenious method of
heating the hot-air chamber instantly. By the use of a small air pump
hundreds of atmospheres could be compressed into a very strong aluminum
chest or cylinder. Beneath this cylinder were a number of burners that
heated the compressed air several hundred degrees. As we said before,
when they desired to descend, an upper and lower trap were opened, the
hot air rushed out above and the cold air in below, causing the globe to
descend with great rapidity. This descent could be arrested at any level
by closing the trap, and a certain amount of the air let off from the
hot-air chest, and any temperature desired could be attained at once.
All this could be done at an expense of oil that was ridiculously and
incredibly small. While they could by no means steer or guide this ship,
yet, if the Doctor's theory of air currents should prove to be
scientifically correct, then they were by no means entirely at the mercy
of any and every adverse gale. And, at the worst, when a favorable
current could not be found, they could descend to the earth and anchor
until a fair wind prevailed. One thing further should be explained. When
it became desirable to ascend suddenly or rapidly, the hot-air chest
was thrown completely open, and the vast chamber was instantly filled
with air at any temperature required. When this operation was from any
cause necessary, the upper trap was closed and all the lower apertures
opened. The hot air from the chest immediately mounted to the upper end
of the air chamber, and forced the