The Flight of Birds and the Science of Winds
Flight can be known only through the winds, themselves derived from the motion of water
Under the heading 'Of flight,' Leonardo argues that the true science of birds' motion through the air must be built upon the science of the winds, which in turn he derives from the movements of water within itself. He notes that wind grows swift and dense where it crosses mountain ridges and becomes rarefied and slow beyond them, like water passing from a narrow channel into a wide basin. Three short notes then describe how a bird crosses from a slow to a swift current, beats obliquely against a headwind, and rights itself when a lateral gust threatens to overturn it. Small pen sketches of a bird in flight and of wind currents run down the right margin.
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Flight understood through winds and, in turn, through water
To give a true science of the motion of birds through the air, one must first give the science of the winds, which is demonstrated by means of the motions of water within itself. This sensory science, Leonardo says, becomes a ladder by which to reach the knowledge of flying creatures within the air and the wind.
Wind speeds and thins like water leaving a narrow channel
In passing the ridges of the mountains the wind becomes swift and dense; running beyond them it becomes rarefied and slow. Leonardo likens this to water that issues from a narrow channel into a wide expanse.
A bird crossing from a slow to a swift current of wind
When the bird passes from the slow to the swift course of the wind, it lets itself be carried along for some space by that course. It does so until it prepares itself with new supports, as promised later in the book.
Beating obliquely into a headwind
When the bird moves with impetus against the wind, it makes swift and long beats of its wings in an oblique motion. After such beats it draws itself in, remaining low and compact with all its limbs.
Righting itself against a lateral gust
A bird is overturned when, holding too little obliquity, it exposes its underside to a lateral gust. If it is about to be overturned it draws in the upper wing and immediately turns its body toward the earth; but if it draws in the lower wing instead, it is at once overturned by the wind.
Marginal sketches of a bird and wind currents
Down the right margin run small pen sketches of a bird in flight amid streaming lines that represent moving air. They illustrate the descriptions of how the bird meets and rides the wind.