This happens because the water is of itself thicker than the air and consequently heavier, and it is therefore swifter in filling the vacuum which the fish leaves behind it in the place whence it departs; and also the water which it strikes ahead is not compressed as is the air in front of the bird, but rather makes a wave that by its movement prepares the way and increases the movement of the fish; and therefore it is swifter than the bird which has to meet compressed air ahead.
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In what way a man ought to learn to swim. Of the way in which a man should rest upon the water. How a man ought to defend himself against the whirlpools or eddies of the waters which suck him to the bottom. How a man when sucked down to the bottom has to seek the reflex current which will cast him out of the depths. How he ought to propel himself with his arms. How he ought to swim on his back. How he can only remain underwater as long as he can hold his breath.
Submarine
How by an appliance many are able to remain for some time under water. How and why I do not describe my method of remaining under water for as long a time as I can remain without food; and this I do not publish or divulge on account of the evil nature of men who would practise assassinations at the bottom of the seas by breaking the ships in their lowest parts and sinking them together with the crews who are in them; although I will furnish particulars of others which are not dangerous, for above the surface of the water emerges the mouth of a tube by which they draw breath, supported upon wine skins or pieces of cork.
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The lines of movements made by birds as they rise are of two kinds, of which one is always spiral in the manner of a screw, and the other is rectilinear and curved.
That bird will rise on high which by means of a circular movement in the shape of a screw makes its reflex movement against the coming of the wind and against the flight of this wind, turning always upon its right or left side.
30
The bird which takes longer strokes with one wing than with the other will progress with a circular movement.
If the bird which does not beat its wings should not wish to descend rapidly to a depth, then after a certain amount of slanting descent it will set itself to rise by a reflex movement and to revolve in a circle mounting after the manner of the cranes when they break up the ordered lines of their flight and gather into a troop and proceed to raise themselves by many turns after the manner of a screw, and then, having gone back to their first line, they follow their first movement again which drops with a gentle descent, and then return again to a troop and moving in a circle raise themselves.
31
When the bird is flying to the east with the wind from the north and finds its left wing above the said wind it will be turned over, unless at the onset of the wind it puts its left wing under the wind and by some such movement plunges towards the north-east and under the wind.
32
Why the bird sustains itself upon the air
The air which is struck with greater swiftness by the movable thing is compressed to a greater degree within itself. . . .
The atmosphere is an element capable of being compressed within itself when it is struck by something moving at a greater rate of speed than that of its own velocity and it then forms a cloud within the rest of the air. . . .
When the bird finds itself within the wind it can sustain itself without flapping its wings, because the function which they have to perform against the air requires no motion. The motion of the air against the motionless wings sustains them, while the movement of the wings sustains them when the air is motionless.
33
The wind in passing the summits of mountains becomes swift and dense and as it blows beyond the mountains it becomes thin and slow, like water that issues from a narrow channel into the wide sea.
When the bird passes from a slow to a swift current of the wind it lets itself be carried by the wind until it has devised a new assistance for itself. . . .
When the bird moves with impetus against the wind it makes long quick beats with its wings with a slanting movement, and after thus beating its wings it remains for a while with all its members contracted and low. The bird will be overturned by the wind when in less slanting position it is so placed as to receive beneath it the percussion of any lateral wind. But if the bird that is struck laterally by the wind on the point of being overturned folds its upper wing it immediately goes back to the position of having its body turned towards the ground, while if it folds its lower wing it will immediately be turned upside down by the wind.
34
The wind exercises the same force on a bird as a wedge lifting a weight.
35
Nature has so provided that all large birds can stay at so great an elevation, that the wind which increases their flight may be of straight course and powerful. For if their flight were low among mountains where the wind goes round and is perpetually full of eddies and whirls and where they cannot find any spot of shelter in the fury of the wind compressed in the hollows of the mountains, nor so guide themselves with their great wings as to avoid being dashed upon the cliffs and the high rocks and trees, would not this sometimes be the cause of their destruction? Whereas at great altitudes whenever through some accident the wind turns in any way, the bird has always time to redirect its course and in safety adjust its flight which will always proceed entirely free. . . . Inasmuch as all beginnings of things are often the cause of great results, so we may see a small almost imperceptible movement of the rudder to have power to turn a ship of marvellous size and loaded with a very heavy cargo, and that, too, amid such a weight of water as presses on its every beam, and in the teeth of the impetuous winds which are enveloping its mighty sails. Therefore we may be certain in the case of those birds which can support themselves above the course of the winds without beating their wings, that a slight movement of wing or tail which will serve them to enter either below or above the wind, will suffice to prevent the fall of the said birds.
36
The birds which fly swiftly, keeping at the same distance above the ground beat their wings downwards and behind them; downwards to the extent needed to prevent the bird from descending, backwards according as it wishes to advance with greater speed.
The speed of the bird is checked by the opening and spreading out of its tail.
37
In all the changes which birds make in their directions they spread out their tail.
The spreading and lowering of the tail and the opening of the wings at the same time to their full extent arrests the swift movement of birds. When birds are descending near the ground and the head is below the tail, they lower the tail, which is spread wide open, and take short strokes with the wings; consequently the head is raised above the tail, and the speed is checked so that the bird can alight on the ground without a shock.
38
Many birds move their wings as swiftly when they raise them as when they let them fall; such are the magpies and birds like them.
There are some birds which are in the habit of moving their wings more swiftly when they lower them than when they raise them, and this is seen to be the case with doves and such birds. There are others which lower their wings more slowly than they raise them, and this is seen with crows and similar birds.
39
The kite and the other birds which beat their wings only a little go in search of the current of the wind; and when the wind is blowing at a height they may be seen at a great height, and if it is blowing low they remain low. When there is no wind in the air, then the kite beats its wings more often in its flight, in such a way that it rises on high and acquires an impetus; with which impetus, dropping then gradually it can travel for a great distance without beating its wings. And when it has descended it does the same over again and so continues in succession, and this descent without beating the wings serves it as a means of resting in the air after the previous fatigue of the beating of the wings.
All the birds which fly in spurts rise on high by beating their wings; and during their descent they proceed to rest, for while descending they do not beat their wings.
40
When the kite in descending turns itself right over and pierces the air head downwards, it is forced to bend the tail as far as it can in the opposite direction to that where it wants to go; and then bending the tail swiftly in the direction in which it wishes to turn the change in the bird’s course will correspond to the turn of the tail—like the rudder of a ship which when turned turns the ship but in the opposite direction.
41
A bird supporting itself upon the air against the movement of the winds has a power within itself that desires to descend, and there is another similar power in the wind that strikes it which desires to raise it up. And if these powers are equal so that one cannot conquer the other, the bird will not be able either to raise or lower itself, and consequently will remain steady in its position in the air.
42
Why the flight of birds when they migrate is made
against the approach of the wind
The flight of birds when they migrate is made against the movement of the wind not in order that their movement may be made more swiftly but because it is less fatiguing. And this is done with a slight beating of their wings whereby they enter the wind with a slanting movement from below and then place themselves slantwise upon the course of the wind. The wind enters under the slant of the bird like a wedge and raises it upwards during the time that the acquired impetus consumes itself, after which the bird descends again under the wind. . . . Then it repeats the above-mentioned reflex movement upon the wind until it has regained the elevation that it lost, and so it continues in succession.
Why birds seldom fly in the direction of
the current of the wind
It very seldom happens that the flight of the birds is made in the direction of the current of the wind, and this is due to the fact that this current envelops them and separates the feathers from the back and also chills the bared flesh. But the greatest drawback is that after the slantwise descent its movement cannot enter the wind and by its help be thrown upwards to its former elevation unless it turns backwards which would delay its journey.
The bird spreads out the feathers of its wings more and more as its flight becomes slower and this is according to the law—which says: That body will become lighter which acquired greater breadth.
43
The bird weighs less when it spreads itself out more, and conversely it weighs more when it draws itself together more tightly; and the butterflies make experiments of this in their descent.
44
When the bird desires to rise by beating its wings it raises its shoulders and beats the tips of the wings towards itself, and comes to condense the air which is interposed between the points of the wings and the breast of the bird, and the pressure from this air raises the bird.
When the bird desires to turn to the right or left side by beating its wings, it will beat lower with the wing on the side on which it wishes to turn, and thus the bird will twist its movement behind the impetus of the wing which moves most and it will make the reflex movement under the wing from the opposite side.
40
When a bird flies against the wind its progress should be made in a slanting line towards the earth, entering underneath the wind . . . but when this bird wishes to rise to a height it will enter above the wind, and it will retain enough of the impetus it has acquired in the descent we have spoken of, so that by means of the speed thus gained it will lower its tail and the elbows of its wings and it will raise its helm. It will then be above the wind . . . .
45
The butterfly and like insects all fly with four wings, those behind being smaller than those in front. Those in front form a partial covering to those behind.
All insects of this clan have power to rise with straight movement, for as they raise themselves these wings remain perforated with the front wings much higher than those behind. And this continues as long as the impulse which urges them upwards; and then as they lower their wings the larger become joined to the smaller, and so as they descend they again acquire fresh impulse.
46
[
With drawing of a dragonfly.
]
The pannicola flies with four wings, and when those in front are raised those behind are lowered.
But it is necessary for each pair to be sufficient of itself to sustain the whole weight.
When one pair is raised the other is lowered.
In order to see the flying with four wings go into the moats and you will see the black ‘pannicole’.
47