The Wings of Birds. 708 
: to the center, the level plane will no longer fall vertically but 
: move towards the edge bounding the sawdusted half. If we 
i extend the paper an inch over the edge opposite the saw- 
dust and snip it into the bamboo so that it will curve upwards, 
: we shall find on dropping the plane that its sideways motion will 
4 be augmented. If we paste a slip of paper to the front edge, 
allowing it to project one and one-half inches beneath, we shall 
‘ find the sideways motion to be further increased so as to be many 
2 times that of the fall. If we take the plane to the housetop when 
the air is calm and allow it to drop, it will describe a curve while 
itis being accelerated, but on uniform motion occurring it will 
slant downwards very gently and move laterally from six to eight 
times as fast as it falls vertically. This experiment is very deli- 
cate and the motion varies greatly, depending on the weight and 
humidity of the air and the construction of the plane. The 
cause of this behavior is obvious. The air in rushing along the 
under side of the plane to escape, drags the rough surface with it 
i to the front, slipping over the smooth part to the rear easily. 
f Then as it turns the curve it expands against it, augmenting the front 
, thrust. Where the air escaping toward the front meets the ledge, 
a Complete stoppage occurs, and the thrust against it is quite vio- 
: lent. The ledge in front of the roughened half of the surface, and 
: the rear curve, all tend to throw the plane edgeways in the same 
i direction while it falls. The ledge by directly stopping the mo- 
= tion of the rushing air ; the rough surface by frictional resistance, 
and the curve by presenting a base for the compressed air to 
~ Xpand against. The force required to produce these various 
results is put into the air by the descent of the plane upon it 
under the impulse of the gravity of its mass. The wing of a 
bird would act in the same way that the plane acts, and for the 
Same reason. The elastic feather tips give the curve. The con- 
Struction of the feather surfaces gives the smooth rear and rough > 
front part; and the front projection of the integument covering _ 
the bones forms the ledge. : 
+ 
it falls ? Obviously because the condensed air in contact with a: 
the surface moves with far higher velocity than the fall, carrying 
the plane with it. The inertia of the air resists sudden motion 
So that the particles are compressed before they begin to move, 
Why is it that the plane moves so much faster edgeways than — ~ 
