Aviation has now advanced to the stage where a practical exposition of the subject is widely demanded. Many so-called "popular" books have been written, and contain much that attracts the attention of the average man, but little if anything that appeals to the more serious student of the subject. On the other hand, many valuable treatises have been written, but of so scientific and mathematical a nature that they are almost unintelligible to all but a few technical men; and in many cases it must be acknowledged that mathematics often lead to conclusions that are wholly at odds with the actual results of practice.

After an historical introduction in which the inestimable value of the work of Langley, Lilienthal and Chanute is pointed out, the design of aeroplanes is taken up. The theory of Aerodynamics is given as simply and completely as possible, and the fundamental principles are everywhere fully explained and emphasized. At the end of this section is given a complete example of the design of an aeroplane, which should prove of particular value to those actively engaged in aeroplane construction.

The monoplanes and biplanes in their various forms are then considered. Detailed descriptions of virtually all of the present successful types are given, supplemented by photographs and diagrams reproduced to the same scale, thus at once enabling a graphic comparison. Many of the types are changed from time to time and the data- is in many cases unreliable, but the author has spared neither time nor effort to render this section as exact as he was able to. Were the leading machines here described not to remain substantially the same for years to come, they should, nevertheless, prove of permanent value in that they represent distinct types with which concrete results were first obtained.

In the last part of the book, the leading types are compared and discussed, and from the results of actual practice conclusions are drawn, enabling the lines of probable future development to be pointed out. This section will prove of interest to almost every one, as it is the author's experience that the knowledge of this subject possessed by the average person is far greater than most writers suppose.

The numerous tragic and in many cases avoidable accidents constitute, probably, one of the greatest detriments to the progress of aviation. Their causes, and as far as possible with the meagre knowledge available, the means for their prevention, are considered in this section; and the fact that aviation is reasonably safe can unquestionably be concluded therefrom.

The closing chapter of the book deals with the "variable surface aeroplane", a development which the author believes to be the next great step forward in the rapid progress of aviation.

In this immortal "Rasselas," Dr. Samuel Johnson says, "instead of the tardy conveyance of ships and chariots, man might use the swifter migration of wings, the fields of air are open to knowledge, and only ignorance and idleness need crawl upon the ground." This fanciful prophecy has almost been realized in fact.

Over one thousand aeroplanes have successfully flown, covering an aggregate distance of at least 150,000 miles. The inscrutable Sphinx has seen the aeroplanes of to-day pass and re-pass, majestic in the exactness and ease of their flight. Chavez, in one of the most daring flights ever made, crossed over the chasms and snow-covered peaks of the Alps. Exploits, almost as thrilling, have been performed by a score of other aviators; the Pyrenees, the Irish Channel, and the Hudson River, are but a few of the scenes of well-executed achievements, and aeroplanes have been flown under weather conditions that, formerly, would have been considered prohibitive.

Throughout the past year aviators have exhibited consummate skill, as well as a courage that was often foolhardy, in mounting higher and higher, until finally Hoxsey had attained the wonderful altitude of 11,400 feet. The sight of these human birds, hovering beyond the clouds, like Pascal's famous point, "in equilibrium in the infinite," is truly an impressive one.

But in the active excitement of the present, the work of the early pioneers must not be lost sight of.

Langley, Lilienthal, and Chanute have contributed so largely and so well to the progress of aviation, that practical aeroplane designers of the present owe them a debt of gratitude that can hardly be repaid.

It is both interesting and appropriate to sum up the work done by these three great pioneers, and point out the effect their labors have had upon the highly successful efforts of the Wrights, Bleriot, Levavasseur, and their contemporaries.

It was in 1887 that Prof. Langley commenced his experiments in aerodynamics, the results of which led him to theoretical conclusions that are fundamental. Largely through the generosity of Mr. William Thaw, of Pittsburg, Prof. Langley was enabled to construct his famous "whirling table" at Allegheny, Pa. With the scientific thoroughness and exactness that had characterized his previous work in physics and astronomy, Langley set vigorously to work to investigate the problem of mechanical flight.

The "whirling table" consisted of a horizontal rotating arm, at the outer end of which were carried the surfaces, forms, and propellers that were to be tested. Almost all the results, of pressure, velocity, etc., were recorded automatically by means of ingenious electrical devices. The actual results of his experiments are referred to in full elsewhere in this work, but it may be pointed out that, unquestionably, his greatest contribution to the knowledge on this subject was his thoroughly scientific verification of the fact, that the old Newtonian theorem on the pressure of air, experienced by a surface inclined at small angles, gave results that were almost twenty times too small. In addition, Langley investigated the well-known constant K f and obtained a value nearer the correct one than any of his predecessors. He also determined fully the variation in position of the center of pressure, the analysis of the total pressure on a surface into a lifting force and a resisting one, the effect of "aspect ratio," and other equally important and valuable matters; but inasmuch as these experiments were made on flat surfaces, their results have had little application to the design of the present-day aeroplane. Langley considered the actual friction of the air negligible, and this is the only important characteristic of his work that is open to question.

Langley had an illustrious contemporary in Col. Kenard, the builder of the first successful dirigible balloon, the "La France." who experimented exhaustively on planes, propellers and shapes of "least resistance 77 in his laboratory near Paris, and whose results to-day are of immense value to designers of dirigible balloons. Maxim, Kress, Dines, Phillips, and Hargrave followed Langley, and contributed handsomely to the progress of aerodynamics, but it is in the character, and especially in the presentation, of his work that Langley stands out as the first and greatest pioneer.

In 1891, after the completion and publication of his "Experiments in Aerodynamics, 77 Langley actively began the construction of flying machines. At first he experimented with models driven by rubber bands, but he found the flights too short and erratic to give any practical results.

His first steam motor-driven ''model" aerodrome "No. 0," was then constructed, and was followed by "No. 1" and "No. 2" driven by compressed air and carbonic-acid gas motors. All of these failed because of the poor character of the motors. The next model, "No. 3," was built stronger and was more successful. The propellers were tested in the shop, being attached to a pendulum device. This pendulum, resting on knife edges, was prolonged above the points of support, and was counterbalanced to give indifferent equilibrium. The propellers were so mounted that the line of thrust passed through the center of gravity, and when power was applied, they lifted the pendulum, thus enabling the dead-lift power of the engines to become known.

The engines of "No. 3" lifted 30 per cent of their own weight. "No. 4" was then built and taken to the Potomac on a house-boat, to be extensively tested. Great difficulties were experienced in launching, and it was found that the upward pressure of the air deflected the wings, this minute difference causing the planes to act badly.

In 1894 and 1895 "No. 5" and "No. 6," stronger and better machines, were constructed.

Finally, on May 6th and November 28th, 1896, Langley's best model, driven by a 1 horse-power steam engine and weighing 27 pounds, was successfully flown several times; the best flight was over three-quarters of a mile long, and conclusively demonstrated the saneness and excellence of his work.