Without wind.

That bird will rise into flight most easily which takes the impetus of its motion with a little descent at the beginning of its flying. With the wind, that is, to gain ground against that wind. When the bird flies against the wind, it is necessary that the ground it gains against such a wind be made along an oblique line toward the earth, entering under the wind. And because its weight is more powerful than that quantity of wind which strikes it at unequal angles, it would willingly press it down toward the earth, were it not that the air it meets in front along the line of its motion is a much smaller amount than the air that is beneath it, touching it. That motion is the conqueror only of the air which resists least, and that air resists least which is of the smaller quantity. Therefore, from what has been said, we are certain that the bird will move against the air which resists least, which touches the shoulders of its wings, and not against that which touches the underside of the whole extent of the said wings. Rectilinear motion. But when such a bird wishes to rise on high, it will enter above the wind, and will make much use of the impetus it has acquired in its aforesaid descent; for with such acquired speed it will lower the tail and likewise the elbows of its wings, and will raise its shoulders on high. Then it will be above the wind; and this impetus, being constrained neither to end nor to be consumed, its being is at once necessitated to continue along that line where least air impedes the motion, which will be along the line where the wings show themselves edge-on to the air where they strike, that is, always along the line where the air that is met is divided by the shoulders set in the thickness of the wings, and never along the line of their breadth. Therefore, this done, such a bird rises on high without beating its wings, because the wind that passes beneath it raises it up as the wedge raises the weight; and by this reckoning it would make it turn back, were it not that in such a climb the already acquired impetus goes on slowing and being consumed. And after that impetus is consumed, the bird would be overturned by the wind that led it on high, if it did not at once lower the rudders of the shoulders of its wings and enter under the wind, and lower its tail. Then the motion, already ended, begins again; and, descending, it regains the already lost impetus, with which then anew, by reflected motion, it rises on high, until once more it loses the already acquired impetus. But if such motion were circular, then the bird will observe other rules which shall be defined in order in what follows.

Because it is as much to move the air against the bird, itself motionless, as to move the bird against the still air. Therefore we shall say, by the first proposition, that .....