152- ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1908. 



with equal surface areas, the biphme is much easier to construct, 

 especially when, as is the practice to-day, rigid surfaces are sought. 

 The biplanes are also more compact than the monoplanes, and permit 

 the use of an equal area of wing surface with only half the spread. 

 Furthermore, equilibrium is preserved much more easily in a biplane. 

 The underlying principles of these phenomena have not yet been 

 completely explained, I shall confine myself to recalling the ideas 

 of Chanute on the cellular forms (biplanes in compartments), ideas 

 apparently well founded. Chanute holds that an air current at high 

 speed confined by the walls or planes of the apparatus offers great 

 resistance to any lateral displacement. It is somewhat similar to the 

 action of the gyroscope, or rather the action is similar to that in a 

 hose from which water is rushing out at a great speed and which is 

 difficult to move. The same principles govern aeroplanes. 



Another detail : We have said that aeroplanes are sustained in the 

 air by means of plane surfaces. This is not absolutely accurate, since 

 the sustaining surfaces, viewed in section, show a slight curve. This 

 curve is the result of experiment. It was found that the best flights 

 were obtained when the wings cut the air squarely with their front 

 edges, and when the resistance of the air was used on surfaces inclined 

 gradually in greater degree. Air is so complex a medium and one 

 that we really understand so little that it is only by long and careful 

 experiment that the proper curve of the wings has been determined. 

 The Wright brothers and the Voisin brothers spent several years 

 determining this question. The Voisins experimented with a power- 

 ful electric fan, capable of generating a very swift current of air, in 

 front of which they placed linen surfaces mounted on frames with 

 various curves. They weighted these and then measured the reaction 

 of the air current on the surfaces. This method of experiment led 

 them to select the degree of curve adopted on all their machines, par- 

 ticularly those for Delagrange and Farman. 



Let us now examine the working of the Wright machine. This 

 machine is of the type whose balance is governed by the pilot. The 

 aviator has in his hands two levers. The left one controls the front 

 balancing planes, or horizontal rudder, and this lever is constantly 

 in motion. They are movements of very short amplitude (35 centi- 

 meters forward and 35 centimeters backward, a total amplitude of 

 only 70 centimeters), and that is sufficient to govern the pitching 

 tendencies of the machine. The operation of this requires an atten- 

 tion so close that the least slip would be fatal. It is similar to oper- 

 ating the handle bars of a bicycle, moving to the right or left to 

 retain the balance. The right-hand lever controls the vertical rudder 

 and the warping of the wings. If the machine leans to one side, 

 the operator increases the inclination of the wing on that side and 

 this rights his machine. The simultaneous movement of the rear 



