height, can be steered in whatever direction is desired, and
voyages can be made from one place to another--always provided
that the force of the wind is not sufficiently strong to overcome
the power of the engines. The airship is, therefore, nothing
else than a dirigible balloon, for the engines and other weights
connected with the structure are supported in the air by an
envelope or balloon, or a series of such chambers, according to
design, filled with hydrogen or gas of some other nature.

It is not proposed, in this book, to embark upon a lengthy and
highly technical dissertation on aerostatics, although it is an
intricate science which must be thoroughly grasped by anyone who
wishes to possess a full knowledge of airships and the various
problems which occur in their design. Certain technical
expressions and terms are, however, bound to occur, even in the
most rudimentary work on airships, and the main principles
underlying airship construction will be described as briefly and
as simply as is possible.

The term "lift" will appear many times in the following pages,
and it is necessary to understand what it really means. The
difference between the weight of air displaced and the weight of
gas in a balloon or airship is called the "gross lift." The
term "disposable," or "nett" lift, is obtained by deducting the
weight of the structure, cars, machinery and other fixed weights
from the gross lift. The resultant weight obtained by this
calculation determines the crew, ballast, fuel and other
necessities which can be carried by the balloon or airship.

The amount of air displaced by an airship can be accurately