First define the motion of the wind and then describe in what way the birds steer through it with only the simple balancing of the wings and tail; and this you will describe after the description of their anatomy.
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Of the bird’s movement
To speak of this subject you must in the first book explain the nature of the resistance of the air, in the second the anatomy of the bird and its wings, in the third the method of working the wings in their various movements, in the fourth the power of the wings and of the tail when the wings are not being moved and when the wind is favourable to serve as guide in various movements.
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What is impetus?
Impetus is a power transmitted from the mover to the movable thing, and maintained by the wave of the air within the air which this mover produces; and this arises from the vacuum which would be produced contrary to the natural law, if the air which is in front of it did not fill up the vacuum, so causing the air which is driven from its place by the aforesaid mover to flee away. And the air that goes before it would not fill up the place from which it is separated if it were not that another body of air filled up the place from which this air was separated. . . . And this movement would continue to infinity if the air were capable of being compressed to infinity.
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Impetus is the impression of local movement transmitted from the mover to the movable thing and maintained by the air or by the water as they move in order to prevent the vacuum.
The impetus of the movable thing within the water is different from the movable thing within the air, and these differences result from the dissimilarities of these elements, because air is condensable to infinity and water is not.
The impetus of the water is divided into two parts through its being of two natures, one simple and the other complex. The simple is entirely beneath the surface of the water, the complex is between the air and the water, as with boats.
The simple impetus does not compress the water in front of the movement of the fish, but moves the water backwards with the same speed that the mover has; and the wave of the water will never be swifter than its mover.
But the movement of the boat, called complex movement since it is shared by the water and the air, is divided into three chief parts because it is carried on in three directions, namely against the course of the river, in the direction of its current, and crosswise, that is across the breadth of the river.
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Every movement will retain its course or rather every body when moved will continue on its course as long as the power of the impulse is maintained therein.
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The impetus generated in still water has a different effect from that generated in still air. This is shown by the fact that water is never compressed in itself by any movement below its surface, as the air is within itself when struck by a moving object. And this we may see from the bubbles with which the water is encumbered from its surface to its bed, which cluster around where the water fills up the vacuum that the fish leaves behind it as it penetrates; and the movements of this water strike and drive the fish, because water only has weight within water when it has movement, and this is the primary cause of the increase of movement for its mover.
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Of the wind
The air moves like a river and carries the clouds with it; just as running water carries all the things that float upon it.
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The movement of the air against a fixed thing is as great as the movement of the moving thing against an air which does not move.
And it is the same with water which a similar circumstance has shown me to act in the same way as does the air, as with the sails of ships when accompanied by the lateral resistance of their helm.
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When the heavy substance descends in the air, and the air moves in a contrary direction in order to fill up continuously the space evacuated by this heavy substance, the movement of this air is a curve, because when it desires to rise by the shortest line it is prevented by the heavy substance which descends upon it, and so it is obliged to bend and then return above this heavy substance in order to fill up the vacuum that has been left by it. And if the air would not be compressed beneath the speed of the heavy substance, the birds would not be able to support themselves upon the air that is struck by them; but it is here necessary to affirm that the air is compressed beneath that which strikes it and it becomes rarefied above in order to fill up the void left by that which has struck it.
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Of the science of the weight proportioned to the
powers of their movers
The force of the mover ought always to be proportionate to the weight of the thing to be moved and to the resistance of the medium in which the weight moves. But one cannot deduce the law of this action unless one first gives the quantity of the condensation of the air when struck by any movable thing; and this condensation will be of greater or less density according to the greater or less speed of the movable thing pressing on it; as is shown in the flight of birds, for the sound that they make with their wings in beating the air is deeper or shriller according to whether the movement of the wings is slower or swifter.
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II. STRUCTURE OF BIRDS’ WINGS
First do the anatomy of the bird’s wing; and then of its feathers deprived of fluff and then with the fluff.
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You will study the anatomy of the wings of a bird together with the muscles of the breast which are the movers of these wings. And you do the same for man in order to show the possibility that there is in man to sustain himself amid the air by the flapping of wings.
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A plea for the undertaking that disposes of the objections.
You will perhaps say that the sinews and muscles of a bird are incomparably more powerful than those of a man, because all the strength of so many muscles and flesh of the breast goes to aid and increase the movement of the wings, while the bone in the breast is all in one piece and consequently affords the bird very great power, the wings being all covered with a network of thick sinews, and other very strong ligaments of cartilage and the skin being very thick with various muscles.
The reply to this is that such great strength is given as a reserve of power beyond what it ordinarily uses to support itself on its wings, since it is necessary for it whenever it may desire either to double or to treble its motion, in order to escape from its pursuer or follow its prey. Therefore in such a case it becomes necessary for it to put forth double or treble the amount of effort and in addition to this to carry through the air in its talons a weight corresponding to its own weight; so one sees a falcon carrying a duck and an eagle carrying a hare, which circumstance shows well enough where the excess of strength is spent; for they need but little force in order to sustain themselves and to balance themselves on their wings and flap them in the pathway of the wind, and to direct their course a slight movement of the wings is sufficient, and the movement will be slower in proportion as the bird is greater in size.
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The feathers that offer a feeble resistance are set beneath those that offer a strong resistance, and their extremities are turned towards the tail of the bird; because the air underneath flying things is thicker than it is above them, and thicker in front than it is behind; and the necessity of flight requires that these lateral extremities of the wings are not found by the stroke of the wind because they would immediately be spread out and separated, and would be instantly penetrated by the wind. Therefore, the resistances are so placed that the parts with a convex curve are turned towards the sky so that the more they are struck by the wind the more they lower themselves drawing closer to the lower resistances underneath them and so preventing the entry of the wind beneath the front. . . .
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The wing of the bird is always concave in its lower part extending from the elbow to the shoulder, and the rest is convex. In the concave part of the wing the air is whirled round, and in the convex it is pressed and condensed.
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The longest feathers of the wings will be flexible since they are not covered by other feathers from their centre to their tip.
The helms placed on the shoulders of wings are extremely necessary for they keep the bird poised and motionless in the air as it faces the course of the wind.
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With drawing.
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This helm is placed near where the feathers of the wings bend, and because of its strength it bends but little or not at all, being situated in a very strong place and supplied with powerful sinews and being of hard bone and covered with very strong feathers, protecting and supporting one another.
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Birds with short tails have very wide wings; which by their width take the place of the tail; and they make considerable use of the helms set on the shoulders when they wish to turn to any spot.
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Why the sinews beneath the birds’ wings are more powerful than those above. It is done for the movement. The shoulder, being the helm of the wing, is hollow below like a spoon; and being concave below, it is convex above. It is fashioned thus in order that the process of going up may be easy, and that of going down difficult and meeting with resistance; and it is especially adapted for going forward drawing itself back in the manner of a file.
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The helms formed on the shoulders of the wings of birds are provided by resourceful nature as a convenient means of deflecting the direct impetus, which often takes place during their headlong flight. For a bird finds it much more convenient to bend by direct force one of the smallest parts of the wings than the whole of them; and the reason why their feathers are made very small and very strong is that they may serve as cover for one another and in doing so arm and fortify each other with marvellous power. And these feathers are based in small and very strong bones, which are moved and bent over the massive joints of these wings by sinews.
The movement and position of these bones on the shoulders of the wings resemble that of the thumb in the human hand, which being surrounded by four sinews equidistant from one another at the base, makes therewith an infinite number of movements both circular and straight.
We may say the same of the rudder placed behind the movement of the ship, imitated from the tails of the birds; as to which experience teaches us how much more readily this small rudder is turned during the rapid movements of great ships than the whole ship itself. . . .
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Bats when they fly must of necessity have their wing completely covered with a membrane, because the creatures of the night whereon they feed seek to escape by means of confused revolutions and this confusion is enhanced by their various twists and turns. So the bats must sometimes follow their prey upside down, sometimes in a slanting position or in various other ways, which they could not do without causing their own destruction if their wings were of feathers that let the air pass through them.
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III. SWIMMING AND FLIGHT
When two forces strike against each other it is always the swiftest which leaps backwards. So it is with the hand of the swimmer when it strikes and presses upon the water and makes his body glide forward in a contrary movement; so it is also with the wing of the bird in the air.
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Swimming upon water teaches man how birds do upon the air.
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Swimming illustrates the method of flying and shows that the largest weight finds most resistance in the air.
Observe the goose’s foot: if it were always open or always closed the creature would not be able to make any kind of movement. While with the curve of the foot outwards it has more perception of the water in going forward than the foot would have as it is drawn back; this shows that the same weight the wider it is the slower is its movement. Observe the goose moving through the water, how as it moves its foot forward it closes it occupying but little water and consequently acquiring speed; and as it pushes it back it spreads it out and so makes itself slower, and then the part of the body that has contact with the air becomes swifter.
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Why is the fish in the water swifter than the bird in the air when it ought to be the contrary since the water is heavier and thicker than the air and the fish is heavier and has smaller wings than the bird? For this reason the fish is not moved from its place by the swift currents of the water as is the bird by the fury of the winds amid the air; also we may see the fish speeding upwards on the very course down which the water has fallen abruptly with very rapid movement like lightning amid incessant clouds, which seems a marvellous thing. And this is caused by the immense speed with which it moves which so exceeds the movement of the water as to cause it to seem motionless in comparison with the movement of the fish. . . .