When the time comes for some historian of the far-distant future to survey critically the technical achievements of the nineteenth and twentieth centuries and to weigh the comparative economic importance of those achievements, it may be that the invention of the aeroplane flying-machine will be deemed to have been of less material value to the world than the discovery of Bessemer and open-hearth steel, or the perfection of the telegraph, or the introduction of new and more scientific methods in the management of our great industrial works. To us, however, the conquest of the air, to use a hackneyed phrase, is a technical triumph so dramatic and so amazing that it overshadows in importance every feat that the inventor has accomplished. If we are apt to lose our sense of proportion, it is not only because it was but yesterday that we learned the secret of the bird, but also because we have dreamed of flying long before we succeeded in ploughing the water in a dug-out canoe.

Although the romantic aspects of aviation have not been ignored in the following pages, it is the chief purpose of this book to explain as simply and accurately as possible the principles of dynamic flight and aeroplane construction, so that an intelligent reader will learn why a machine many times heavier than the air stays aloft for hours at a time and why it is constructed as it is. The limitations imposed by a popular book are such that it is impossible to discuss with anything like thoroughness such difficult matters as equilibrium and stability, the correct proportioning of supporting surfaces to weight and speed, and the resistance encountered in the air by planes in motion. Indeed, these questions are not definitely settled in the minds of technical men. Besides presenting an elementary account of a flying-machine's way in the air, it has been deemed advisable to discuss the screw and the internal-combustion motor as applied to the flying-machine. There can be little doubt that the propeller and the engine offer many a problem for solution before the aeroplane can compete successfully with other forms of locomotion, and a discussion of the driving mechanism of an aeroplane should, therefore, constitute an essential part of even a popular book on flying.

So marked have been the changes that have been made in the construction of well-known biplanes and monoplanes and so many are the new machines that appear almost from week to week that it is almost a hopeless task to present anything like a complete account of existing aeroplanes. Hence it has been deemed advisable to limit the descriptions of types to those machines which have been in a measure standardized.

In the preparation of this volume the author has been ably assisted by several friends to whom he wishes to make due acknowledgment. He is indebted to Mr. Carl Dienstbach for a critical reading of the entire manuscript before it passed to the press, and for many valuable suggestions; to Mr. C. Fitzhugh Talman, librarian of the United States Weather Bureau, for a painstaking revision of the chapter entitled " The New Science of the Air "; to Mr. H. A. Toulmin for information on the points at issue in the various suits brought by the Wright Brothers for the alleged infringement of their patents; to Francis W. Aymar, Professor of Law in the New York University Law School, for valuable aid in the preparation of the chapter on " The Law of the Air "; and to the Smithsonian Institution and the Wright Brothers for various photographs.

Acknowledgment is also made to Messrs. Harper & Brothers for permission to use material which appeared in an article written by the author and published in " Harper's Monthly Magazine".

New York, N. Y., January, 1911.