AVIATION IN FRANCE JOUEDAIN. 147 



first brought to bear. The screw, and consequently the whole 

 machine, sustains from the air upon which it acts, a reaction equal to, 

 but in the direction opposite to, that which it exercises. If we have a 

 screw turning in a certain direction, the whole apparatus, upon the 

 stability of which we rely to force the turning of the screw against 

 the air, will tend to turn in the opposite direction as the result of a 

 reactive force equal to that exercised by the screw. To overcome this 

 tendency the attempt was made to increase the inertia of the machine 

 by attaching vertical planes or surfaces in such a manner as to retard 

 its rotative movement. 



But these large surfaced vertical planes are cumbersome and heavy. 

 With laudable persistency investigators have devised a second method 

 more advantageous than the first, which consists in the use of two 

 screws turning in opposite directions, so that the efl'ect of the reaction 

 of the air on one screw is neutralized by the reaction on the other. 

 Such an apparatus is capable only of lifting and sustaining itself in 

 the air; it can not move horizontally. For this movement it must be 

 supplied with a third screw, or propeller, mounted on a horizontal 

 axis. This forms still another complication, for the use of only two 

 sustaining screws is a minimum depending on the weight of the 

 machine. And since the size of the screws is limited by considera- 

 tions of strength their number must be increased, always in pairs, to 

 four, six, or eight. 



M. Cornu in 1907 succeeded in lifting two passengers vertically. 

 To obtain a horizontal movement M. Breguet attached to his machine 

 cloth planes inclined at an appropriate angle, and it was through the 

 reaction of the air from the vertical displacement on these planes that 

 this apparatus was designed to move forward. 



Let us now examine the third solution, that of the aeroplane, which, 

 as I have said, is a compromise or a combination of the first two 

 systems. 



In the aeroplane the principal parts are comprised in an inclined 

 surface, and it is this inclined surface gliding at a certain speed 

 into the wind that sustains the machine. The total reaction of the air 

 upon this surface resolves itself into two components — the resistance to 

 horizontal advancement and the vertical thrust. These two forces 

 are proportional to the square of the speed of propulsion and the 

 area of the plane surface. Thus, if a given speed is doubled, we bring 

 to bear on a given surface area, a force equal to the square of the force 

 at the initial speed. This is the reason for the efforts to attain a 

 greater speed, a speed which depends upon the power of the engine 

 and screw of motor-propelled machines. 



