of us have seen, that of the parent birds teaching their young to fly, which demonstrates this proposition. Even with thousands of years' evolution and heredity, with adequate flying organs, the birdlings need instruction and experience.
Safety is the all-important requisite. It is indispensable to have a flying machine which shall be stable in the air, and to learn to master its management. Nothing but practise, practise, practise, will gain the latter, and upon this the school of Lilienthal and his followers is founded.
Otto Lilienthal was a German engineer of great originality and talent, who after making very valuable researches, assisted by his brother, published a book in 1889, 'Der Vogelflug als Grundlage der Fliegekunst,' which it is very desirable to have translated and published for the benefit of English investigators. Then, putting his theories to the test of practise, he built from 1891 to 1896 a number of aeroplane machines with which he diligently trained himself in gliding flight, using gravity for a motive power, by starting from hillsides. He grew exceedingly expert, and made, it is said, more than 2,000 flights, until one rueful day (August 9, 1896) he was upset and killed by a wind gust, probably in consequence of having allowed his apparatus to get out of order.
He was followed by Mr. Pilcher, an English marine engineer, who slightly improved the apparatus, but who, after making many hundred glides, was also upset and killed in October, 1899, through structural weakness of his machine.
The basis for the equilibrium of an apparatus gliding upon the air being that the center of gravity shall be on the same vertical line as the center of air pressure, both Lilienthal and Pilcher reestablished this condition by moving their bodily weight to the same extent that the center of pressure varied through the turmoils of the wind. The writer ventured to think this method erroneous, and proposed to reverse it by causing the surfaces themselves to alter their position, so as to bring the center of pressure back vertically over the center of gravity. He began experimentally with man-carrying gliding machines in June, 1896, and has since built six machines of five different types, with three of which several thousand glides have been effected without any accidents. The first was a Lilienthal machine, in order to test the known before passing to the unknown, and this was discarded some six weeks before Lilienthal's sad accident.
With three of the other machines favorable results were obtained. The best were with the 'two-surface' machine, equipped with an elastic rudder attachment designed by Mr. Herring, and this was described and figured in the 'Aeronautical Annual' for 1897.
Three years later Messrs. Wilbur and Orville Wright took up the problem afresh and have worked independently. These gentlemen
