392 POPULAR SCIENCE MONTHLY. 



of lis 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 1S89, 'Der Vogelflug als Grundlage der 

 Fliegekunst, ' which it is very desirable to have translated and pub- 

 lished 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 hill- 

 sides. 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 re- 

 verse 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 ma- 

 chines 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 dis- 

 carded 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 de- 

 scribed 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 



