On Birds: The Labor of the Downstroke and Impetus
Why wings tire more downward, with a definition of impetus
Continuing "On birds," this page argues that a bird's wings always feel more effort on the downstroke than the upstroke, in proportion to how much more the bird weighs downward than in level motion, with a small bird sketched in the upper and lower right margins. Leonardo grounds this in the thirteenth proposition, that every heavy body weighs along the line of its motion in proportion to its speed. Between the two flight passages he inserts a formal Definition of Impetus, calling it a force created by motion and transmitted from the mover to the movable body, which keeps moving as long as the impetus has life.
On this page
Level flight: the wings labor more on the downstroke
In the inclination the wings take to move level with the earth, they must feel as much more effort downward than upward as the bird weighs more downward than in the motion of equality. This follows from the thirteenth proposition: every heavy body weighs along the line of its motion, more or less as that motion is faster or slower.
Definition of impetus
Impetus is a virtue (force) created by motion and transmitted from the mover to the movable body. That movable body has as much motion as the impetus has of life.
Ascending flight demands still more downstroke effort
In the bird that wishes to raise itself on high, the wings feel as much more effort downward than upward as the bird weighs more downward than upward.