18 THE FLIGHT OF BIRDS 



air cannot so easily escape. When the moving 

 surface is a plane, the air is unconfined and the 

 force is dissipated. The reduction of the drift 

 (i.e. of the resistance to the forward movement of 

 the surface) may be illustrated by the passage of 

 a boat, built on good lines, through the water. The 

 resistance of the water to the bow is balanced, or 

 nearly so, by the shove given to the stern by the 

 water closing behind it, so that there is little, except 

 friction, to retard the boat. In the same way the air 

 closes upon the hinder part of the curved surfaces 

 of an aeroplane ; there is no region of " dead air " 

 behind it. The matter is well explained in a little 

 book on Model Flying Machines (pp. 19 and 20) 

 by Mr. W. G. Aston. There is a region of " dead 

 air " behind a flat surface when it is given an upward 

 incline and is driven forward horizontally. The 

 amount of " dead air " is the measure of the amount 

 of resistance. 



Area of Supporting Surface. 



We can hardly leave the subject of gliding without 

 touching on this important question : What area 

 of supporting surface is required for, say, one pound 

 weight ? Obviously no hard and fast rule can be 

 laid down. Since it is the front part of the plane 

 on which the wind mainly impinges, the back part 

 is of less importance ; indeed it may be quite 

 superfluous, a useless encumbrance, if the breadth 

 from front to back is excessive. Besides this, as 

 I shall show, we cannot frame a formula that will 

 apply equally to big aeroplanes and small, to the 

 big bird and the small bird. The Aero Manual 



