18 THE FLIGHT OF BIRDS 
air cannot so easily ‘cape. 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 
