48 
tlie down-strokes, wliicli but for this contrivance would be neutral- 
ized by the resistance of the up-stroke. In addition to this, the 
under-surface of the wing is more or less concave, whilst the upper 
surface is in a corresponding degree convex ; it is obvious, there- 
fore, that when the up-stroke is made the air will rush off and 
through the wing in all directions, but when the motion of tlie 
wing is reversed, the air will be gathered up in its hollow and the 
resistance immensely increased. We can thus easdy understand 
how it is that the depressing power of the up-stroke is as nothing 
compared with the elevating power of the down-stroke. 
The action of the wing which we have just described, would 
only enable the bird to ascend perpendicularly, and a separate 
motion would seem to be requii'ed to enable it to proceed in a 
forward direction — this, however, is not the case, for, by a won- 
derful contrivance, the same stroke which elevates the bird, gives 
it a forward motion also. The mode in which this is effected is 
thus described by the Duke of Argyle : — “ The air, which is 
struck and compressed in the hollow of the wing, being unable 
to escape through the wing, owing to the closing upward of the 
feathers against each other ; and being also unable to escape 
foi'wards, owing to the rigidity of the bones and of the quiUs in 
that direction, finds its easiest escape backwards. In passing 
backwards, it lifts by its force the elastic ends of the feathers ; 
and thus, whilst effecting this escape, in obedience to the law of 
action and reaction, it communicates, in its passage along the whole 
line of both wings, a corresj^onding push forwards to the body of 
the bird. By this elaborate mechanical contrivance, the same 
volume of air is made to perform the double duty of yielding 
pressure enough to sustain the bird’s wmight against the force of 
gravity, and also of communicating to it a forward impulse. The 
bird, therefore, has nothing to do but to repeat, with the requisite 
velocity and strength, its perpendicular blows upon the air, and by 
virtue of the structure of its wings, the same blow both sustains 
and propels it.”* 
The form of the wing varies very greatly, and determines the 
velocity and style of flight of its owner. Tliose birds, with very 
long and pointed wings, possess the greatest pow’ors of flight, as for 
instance, the sharp-winged merlin for speed, and the long-w'inged 
* “ Bcigii of Law,” 5th edition, pp. 138, 139. 
