xxviii Trans. Acad. Set. of St. Louis. 



interest in heavier than air flying machines was revived. Penaud con- 

 structed the first toy aeroplane with the propeller in the rear and 

 driven by a rubber band. This apparatus flew for an appreciable time, 

 utilizing motive energy which it carried with it, and this property dif- 

 ferentiates very sharply the experiment of Penaud from those of his 

 predecessors in which was realized only a fall more or less retarded by 

 the resistance of the air. 



The German Lilienthal followed Penaud, and from 1891 to 1896 

 studied the equilibrium, manoeuvering and landing of gliders, falling 

 to his death on his two thousandth flight, August 9, 1896. In this 

 country the French engineer Chanute and the American Langley had 

 meanwhile been experimenting and developing the laws of aerody- 

 namics, Langley's work going as far back as 1887 and continuing to 

 his unsuccessful attempts at flight in 1903. In 1891 he published the 

 results of his researches and definitely stated that it was possible to 

 construct machines which would give such velocity to inclined sur- 

 faces that bodies indefinitely heavier than air could be sustained 

 upon it, and moved through it with great speed. For sixteen years, 

 Langley continued his efforts to attain mechanical fiight, and this long 

 period of fruitful scientific achievement closed with failure due pri- 

 marily to lack of funds. Langley died February 27, 1906, about two 

 years before the Wright brothers astonished the world by their feats 

 in sustained flying in 1908. 



Turning then to the mechanical principles underlying flying. Dr. 

 James discussed the distribution and action of forces on an aeroplane 

 in uniform horizontal flight in still air. If a uniform horizontal wind 

 is blowing, whatever be its velocity, the machine behaves exactly as 

 in still air except that its velocity relative to the ground is the result- 

 ant of its velocity in still air and the velocity of the wind. The effect 

 is exactly as if the machine flew in a volume of still air enclosed in 

 a great spherical envelope while at the same time this envelope is 

 carried along by the wind. 



If the velocity of the aeroplane in still air is greater than that of 

 the wind the machine may move in any desired direction relative to 

 the ground, while if it is less it will be forcibly carried in the direc- 

 tion of the wind. 



The mechanical theory of the behavior of the aeroplane in flight is 

 built on the hypothesis that "the normal thrust on the sustaining 

 plane is proportional to its area, the square of its velocity and to the 

 sine of the angle of attack." By mathematical deductions follows the 

 interesting result that the power expended is inversely proportional to 

 the velocity and directly proportional to the angle of attack, and the 

 advantages of a small angle of attack are evident. In this discussion 

 the passive resistances have been neglected and the power necessary 

 to drive the machine against these increases with the velocity so that 

 there is a minimum angle of attack and a maximum speed for a given 

 machine and motor. 



Passing now to the question of stability, there are three types of 

 oscillations of an aeroplane which must be guarded against; rolling. 



