Confronted with these new and strange shapes in the sky, some making stately journeys of hundreds of miles, others whirring hither and thither with the speed of the whirlwind, wonder quickly gives way to the all-absorbing question: _How do they fly?_ To answer fully and satisfactorily, it seems wise, for many readers, to recall in the succeeding chapters some principles doubtless long since forgotten. * * * * * As with every great advance in civilization, this expansion of the science of aeronautics has had its effect upon the language of the day. Terms formerly in use have become restricted in application, and other terms have been coined to convey ideas so entirely new as to find no suitable word existent in our language. It seems requisite, therefore, first to acquaint the reader with clear definitions of the more common terms that are used throughout this book. _Aeronautics_ is the word employed to designate the entire subject of aerial navigation. An _aeronaut_ is a person who sails, or commands, any form of aircraft, as distinguished from a passenger. _Aviation_ is limited to the subject of flying by machines which are not floated in the air by gas. An _aviator_ is an operator of such machine. [Illustration: A free balloon, with parachute.] Both aviators and aeronauts are often called _pilots_. A _balloon_ is essentially an envelope or bag filled with some gaseous substance which is lighter, bulk for bulk, than the air at the surface of the earth, and which serves to float the apparatus in the air. In its usual form it is spherical, with a car or basket suspended below it. It is a _captive balloon_ if it is attached to the ground by a cable, so that it may not rise above a certain level, nor float away in the wind. It is a _free balloon_ if not so attached or anchored, but is allowed to drift where the wind may carry it, rising and falling at the will of the pilot. [Illustration: A dirigible balloon.] A _dirigible balloon_, sometimes termed simply a dirigible, usually has its gas envelope elongated in form. It is fitted with motive power to propel it, and steering mechanism to guide it. It is distinctively the _airship_. _Aeroplanes_ are those forms of flying machines which depend for their support in the air upon the spread of surfaces which are variously called wings, sails, or planes. They are commonly driven by propellers actuated by motors. When not driven by power they are called _gliders_. [Illustration: A biplane glider.] Aeroplanes exist in several types: the _monoplane_, with one spread of surface; the _biplane_, with two spreads, one above the other; the _triplane_, with three spreads, or decks; the _multiplane_, with more than three. The _tetrahedral plane_ is a structure of many small cells set one upon another. _Ornithopter_ is the name given to a flying machine which is operated by flapping wings. [Illustration: A parachute descending.]