To ascend, then, we must discard ballast: and we cannot ascend beyond
a certain limit on account of the limit of allowable pressure on the
envelope fabric. To again descend, we must discharge some of the gas
which gives us lifting power. Every change of altitude thus involves
a loss either of gas or of ballast. Our vertical field of control
may then be represented by a series of oscillations of gradually
decreasing magnitude until finally all power to ascend is gone. And
even this situation, serious as it is, is made worse by the gradual but
steady leakage of gas through the envelope fabric. Here, in a word,
is the whole problem of altitude regulation. Air has no surface of
equilibrium like water. Some device supplementary to ballast and the
safety valve is absolutely necessary for practicable flight in any
balloon not staked to the ground.
A writer of romance has equipped his aeronautic heroes with a complete
gas-generating plant so that all losses might be made up; and in
addition, heating arrangements were provided so that when the gas
supply had been partially expended its lifting power could be augmented
by warming it so as to decrease its density below even the normal.
There might be something to say in favor of this latter device, if used
in connection with a collapsible gas envelope.
Methods of mechanically varying the size of the balloon, so as by
compressing the gas to cause descent and by giving it more room to
increase its lifting power and produce ascent, have been at least
suggested. The idea of a vacuum balloon, in which a rigid hollow shell
would be exhausted of its contents by a continually working pump, may
appear commendable. Such a balloon would have maximum lifting power for
its size; but the weight of any rigid shell would be considerable, and
the pressure tending to rupture it would be about 100 times that in
ordinary gas balloons.
It has been proposed to carry stored gas at high pressure (perhaps
in the liquefied condition) as a supplementary method of prolonging
the voyage while facilitating vertical movements: but hydrogen gas
at a pressure of a ton to the square inch in steel cylinders would
give an ultimate lifting power of only about one-tenth the weight of
the cylinders which contain it. These cylinders might be regarded as
somewhat better than ordinary ballast: but to throw them away, with
their gas charge, as ballast, would seem too tragic. Liquefied gas
might possibly appear rather more desirable, but would be altogether
too expensive.
[Illustration: SCREW PROPELLER FOR ALTITUDE CONTROL]
If a screw propeller can be used on a steamship, a dirigible balloon,
or an aeroplane to produce forward motion, there is no reason why it
could not also be used to produce upward motion in any balloon; and the
propeller with its operating machinery would be a substitute for twice
its equivalent in ballast, since it could produce motion either upward
or downward. Weight for weight, however, the propeller and engine give
only (in one computed case) about half the lifting power of hydrogen.
If we are to use the screw for ascent, we might well use a helicopter,
heavier than air, rather than a balloon.