150 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1908. 



This solution of the problem is, a priori, better than that offered, 

 by the wing or screw machines. In fact, in applying the principle 

 of the inclined plane, instead of lifting directly the total weight of 

 the machine, it is necessary to bring into play forces proportional 

 only to one-eighth or one-fifth of this weight. 



I said this machine was in a way a compromise or combination. 

 We find in it, in fact, both an imitation of nature and an intervention 

 of artificial contrivances. 



If we observe the great soaring birds, such as the vulture and the 

 sea gull, we see them often remain motionless in the air, their wings 

 stretched, or even glide forward without flapping their wings. This 

 is true only under certain circumstances, and is what we call " le vol 

 a la voile" (sailing flight). It is really an optical illusion, for if 

 we substitute for our eyes a cinematograph or instruments of study, 

 such as those designed by Marey, we should be able, so to speak, to 

 dissect the flight of the bird, and we should find that the tips of 

 the wings are slightly moved from time to time. These movements 

 of the tips of the wings are supplanted in the machine by the screw 

 propeller. The propeller furnishes the same propulsion as that 

 secured by the wings of the bird. Consequently, a machine con- 

 structed in this manner, with a plane suitably inclined, and a sys- 

 tem of motor propulsion, should be able to lift itself in the air. But 

 to lift itself is not enough ; it should be capable of sustaining itself 

 and of being guided; and there occurs the question, would such a 

 machine equipped merely in this manner maintain itself in the air? 

 No, it would not, for the air is essentially mobile; the wind, even 

 when it seems most constant, is made up of pulsations, of layers of dif- 

 ferent speeds, pressures, and densities. So, considering merely its re- 

 sistance to the advance of the machine, since this resistance varies 

 in proportion to the density of the medium, the aeroplane, subjected 

 to these incessant fluctuations of the wind, will tend momentarily 

 to change its state of equilibrium; it will undergo various move- 

 ments, forward and backward, to the right and to the left, and will 

 doubtless capsize. Furthermore, the steadiest atmos])heric winds are 

 filled with counter currents. If there is one principal current flowing 

 in a definite direction, together with it are to be found accessory 

 currents, oblique winds, winds rising from eddies caused by obstruc- 

 tions of the ground, and the uneven slopes of the earth, trees, and 

 houses. These currents striking the large planes on the side would 

 tend to capsize the machine. 



There are three movements to be guarded against in an aeroplane : 

 pitching, rolling, and a tendency to veer unexpectedly. We must be 

 able to guide the machine at will. This question of longitudinal and 

 transverse equilibrium has been a source of trouble to our aviators 

 for a long time. And during the present year, 1908, aviators have, 



