Lighter-than-air craft consist of three distinct types: Airships, whichare by far the most important, Free Balloons, and Kite Balloons, whichare attached to the ground or to a ship by a cable. They derive theirappellation from the fact that when charged with hydrogen, or someother form of gas, they are lighter than the air which they displace.Of these three types the free balloon is by far the oldest and thesimplest, but it is entirely at the mercy of the wind and otherelements, and cannot be controlled for direction, but must driftwhithersoever the wind or air currents take it. On the other hand, theairship, being provided with engines to propel it through the air, andwith rudders and elevators to control it for direction and height, canbe steered in whatever direction is desired, and voyages can be madefrom one place to another--always provided that the force of the windis not sufficiently strong to overcome the power of the engines. Theairship is, therefore, nothing else than a dirigible balloon, for theengines and other weights connected with the structure are supported inthe 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 highlytechnical dissertation on aerostatics, although it is an intricatescience which must be thoroughly grasped by anyone who wishes topossess a full knowledge of airships and the various problems whichoccur 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 bedescribed as briefly and as simply as is possible.
The term "lift" will appear many times in the following pages, and itis necessary to underst