550 SCIENCE PROGRESS. 



I know of no other mechanism in which this difficulty of 

 longitudinal stability occurs except the old fashioned " Otto 

 bicycle " where the rider sat between two large wheels, and 

 the analogy suggests that the manipulation of a gliding 

 machine is probably about as difficult to learn as riding on 

 one ivheel. This problem of longitudinal stability of aero- 

 planes does not appear to hav^e been worked out mathe- 

 matically. Experience has shown that a single surface of 

 considerable fore and aft length is very unstable, and the 

 narrow planes which are best for lifting are much better, 

 but it is essential for stability that either a horizontal rudder 

 or tail should be attached, or two or more planes should be 

 placed one behind the other. 



It is clear that if a change of inclination shitted the 

 centre of pressure of a plane two feet wide through a 

 distance of two inches, the shift for two separate planes 

 each one foot wide w^ould be only one inch, and con- 

 sequently the combination would sail much more steadily, 

 a result which any one can verify by experimenting with 

 gliders. As regards the tail, the steadying action of birds' 

 tails is sufficiently known, but it may be worth while to call 

 attention to the fact that one of the most skilful of European 

 butterflies in orlidino: fliorht, is the lon^-tailed swallows-tail 

 [Papi/io /^odaiirnis). The practical necessity ot a tail in 

 experiments on artificial flight was discovered by Lilienthal. 



While birds are able to control their longitudinal 

 balance by slightly turning their wings Lilienthal and 

 Pilcher, experimenting with flxed wings, have been obliged 

 to move their weight about in the machine, for which pur- 

 pose they have supported themselves by their elbows only, 

 leaving their legs free for the violent exertion required in 

 tussling with a gusty wind. 



Mr. Octave Chanute's experiments have, we believe, 

 been directed towards obtaining a large machine that will 

 glide steadily by itself without the performance of these 

 gymnastic feats, and he considers that until this can be 

 done, but little good can result from experimenting with 

 motors. A number of successful glides have been made by 

 him, many of them of considerable length. The successes 



