§ PIGEONS. 



made to perform the double duty of yielding pressure enough to sustain the 

 bird's weight 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 truth of this explanation of the mechanical theory of flight may be 

 tested in various ways. Perhaps the simplest is an experiment which may be 

 very easily made. If we take in the band the stretched wing of a heron, which 

 has been dried in that position, and strike it quickly downwards in the air, we 

 shall find that it is very difficult indeed to maintain the perpendicular direction 

 of the stroke, requiring, in fact, much force to do so ; and that if we do not apply 

 this force, the hand is carried irresistibly forward, from the impetus in that 

 direction which the air communicates to the wing in its escape backwards from 

 the blow. 



"Another test is one of reasoning and observation. If the explanation now 

 given be correct, it must follow that since no bird can flap its wings in any 

 other direction than the vertical — i.e., perpendicular to its own axis (which is 

 ordinarily horizontal) — and as this motion has been shown to produce necessarily 

 a forward motion, no bird can ever fly backwards. Accordingly no bird ever does 

 so — no man ever saw a bird, even for an instant, fly tail foremost. A bird can, 

 of course, allow itself to fall backwards by merely slowing the action of its wings 

 so as to allow its weight to overcome their sustaining power; and this motion 

 may sometimes give the appearance of flying backwards — as when a swift drops 

 backwards from the eaves of a house, or when a humming-bird allows itself to 

 drop in like manner from out of the large tubular petals of a flower. But this 

 backward motion is due to the action of gravity, and not to the action of the 

 bird's wings. In short, it is falling, not flying backwards. Nay, more, if the 

 theory of flight here given be correct, it must equally foUow that even standing 

 still, which is the easiest of all things to other animals, must be very difficult, 

 if not altogether impossible, to a bird when flying. This, also, is true in fact. 

 To stand still in the air is not indeed impossible to a flying bird, for reasons to 

 be presently explained, but it is one of the most difficult feats of wingmanshvp — 

 a feat which many birds, not otherwise clumsy, can never perform at all, and 

 which is performed only by special exertion, and generally for a very short 

 time, by those birds whose structure enables them to be adepts in their glorious 

 art. 



"Another fact observable in reference to birds of easy and powerful flight, is, 

 that their wings are all sharply pointed at the end. 



" The motion of a bird's wing increases from its minimum at the shoulder- 

 joint to its maximum at the tip. The primary quills, which form the termination 

 of the wing, are those on which the chief burden of flight is cast. Each feather 

 has less and less weight to bear, and less and less force to exert, in proportion as 

 it lies nearer the body of the bird ; and there is nothing more beautiful in the 



