confined to small areas or local chimneys,
as pointed out by Sir H. Maxim and others.

7th.--Once altitude is gained, progress may be made in any direction by
gliding downward by gravity.

The bird's flying apparatus and skill are as yet infinitely superior
to those of man, but there are indications that within a few years the
latter may evolve more accurately proportioned apparatus and obtain
absolute control over it.

It is hoped, therefore, that if there be found no radical error in the
above computations, they will carry the conviction that soaring flight
is not inaccessible to man, as it promises great economies of motive
power in favorable localities of rising winds.

The writer will be grateful to experts who may point out any mistake
committed in data or calculations, and will furnish additional
information to any aviator who may wish to attempt the feat of soaring.

CHAPTER XXI. FLYING MACHINES VS. BALLOONS.

While wonderful success has attended the development of the dirigible
(steerable) balloon the most ardent advocates of this form of aerial
navigation admit that it has serious drawbacks. Some of these may be
described as follows:

Expense and Other Items.

Great Initial Expense.--The modern dirigible balloon costs a fortune.
The Zeppelin, for instance, costs more than $100,000 (these are official
figures).

Expense of Inflation.--Gas evaporates rapidly, and a balloon must
be re-inflated, or partially re-inflated, every time it is used. The
Zeppelin holds 460,000 cubic feet of gas which, even at $1 per thousand,
would cost $460.

Difficulty of Obtaining Gas.--If a balloon suddenly becomes deflated, by
accident or atmospheric conditions, far from a source of gas supply, it
is practically worthless. Gas must be piped to it, or the balloon carted
to the gas house--an expensive proceeding in either event.

Lack of Speed and Control.

Lack of Speed.--Under the most favorable conditions the maximum speed
of a balloon is 30 miles an hour. Its great bulk makes the high speed
attained by flying machines impossible.

Difficulty of Control.--While the modern dirigible balloon is readily
handled in calm or light winds, its bulk makes it difficult to control
in heavy winds.

The Element of Danger.--Numerous balloons have been destroyed by
lightning and similar causes. One of the largest of the Zeppelins was
thus lost at Stuttgart in 1908.

Some Balloon Performances.

It is only a matter of fai