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6 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 hand 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 follow 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 wingmanship—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