ace 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 weighed,
and varies according to barometric pressure and the temperature; but
for the purposes of this example we may take it that under normal
conditions air weighs 75 lb. per 1,000 cubic feet. Therefore, if a
balloon of 1,000 cubic feet volume is charged with air, this air
contained will weigh 75 lb. It is then manifest that a balloon filled
with air would not lift, because the air is not displaced with a
lighter gas.

Hydrogen is the lightest gas known to science, and is used in airships
to displace the air and raise them from the ground. Hydrogen weighs
about one-fifteenth as much as air, and under normal conditions 1,000
cubic feet weighs 5 lb. Pursuing our analogy, if we fill our balloon
of 1,000 cubic feet with hydrogen we find the gross lift is as follows:

1,000 cubic feet of air weighs 75 lb.
1,000 cubic feet of hydrogen weighs 5 lb.