The air n...
If air enters into fire, its neighbouring superior element ..., it acquires [weight]; and if it enters into water, its in[ferior element] ...
If the air is ...
Only that quantity of air will become heavy which enters into the element superior to it.
And that part of the air which enters into the element inferior to it will be the one that gives weight.

Line of equality. - Perpendicular line. That line is the less oblique which is nearer to the perpendicular line, and the more oblique is that which is more remote from that perpendicular.

Air, in the element above it, takes weight into itself, and in the element below it gives weight to that element.
The part of an element does not weigh within its whole unless it moves.
The part of an element weighs within its element when it moves.
The part of an element that moves within its element becomes more or less dense according to the slowness or swiftness of its motion.
That part of the air which moves within the air will be the heavier the denser it becomes.
That part of an element that is without motion within its element will likewise be without density or weight.
And that part of the element will acquire the greater density and weight within its element the greater its motion.
An element within an element does not weigh unless it moves, and if it moves it weighs by as much weight as [would answer to] moving in like time through like space.
Air within air does not condense or weigh unless it moves, and the faster it moves the denser and heavier it becomes.
And the more it condenses the more it resists weights.
Air has the potential to make itself dense, but it does not condense unless it moves.
Air within air moves, and in moving condenses, and it becomes the denser the faster it is; and in condensing it acquires heaviness, and the heavier it becomes the greater is its percussion against its object. To move the object against motionless air is as much as to move the air against the object at rest.
The bird, therefore, beating its heavy wings upon the thin air, comes to condense it and make it resist its descent.
And if the air moves against the motionless wings, that air sustains the weight of the bird in the air.
When the power of the air's motion is equal to the power of the bird's descent, that bird will stay in the air without motion.
And if the air's motion is more powerful, it will overcome and lift the bird on high.
And if the power of the air's motion is less than the weight of the bird, that bird will come down; if the bi[rd con]denses the a[ir] ...

Air outside its sphere gives of its weight to more ...
Air does not resist unless it condenses. Air does not condense unless it is moved.
Air condenses when it moves within the bounds of the air struck by it and of that which strikes. The air that moves weighs along the line of its motion.
The bird will stay in the air without change of position even though the power of the wind be greater than the power of the weight of that bird; and this it does with a small and swift motion of the wings, rowing with them behind, in the flight of the wind, with greater speed than that of the wind itself, and it stays in the position of equality. The bird will stay in the air without change of wings or of position, even though the power of the air's motion be more power[ful] than the weight of that bird.
And it makes this resistance by its obliquity, and stays along an oblique line.
First. - That heavy body will become the lighter which occupies more air.
And that will become the heavier which occupies less air.
With one and the same wingspread, that bird will become the heavier in the air which keeps to the more obl[i]que line.