Why the rebound of falling water never equals its descent
The rebound is never as swift as the fall; submerged air rises like wind from a bellows and lifts the surface water.
A full page of mirror-writing continues an argument about water that has fallen and rebounds from the bottom. Citing a proposition from a 'seventh book', Leonardo states that a rebound can never be as swift as the descent that caused it, so the rebound never equals its antecedent. He then adds a second cause of the upward motion: air dragged under with the falling water, 'clothed in water', bursts upward with fury like wind pressed from a bellows, carrying the surface water up far higher than its own nature would require.
On this page
The rebound is never as swift as the descent
Leonardo invokes a rule that the motion of a rebound will never be as swift as the descent of the thing that rebounded, so the rebound can never equal its antecedent. He applies it to water leaving the bottom almost with the velocity of the fall that produced it.
Submerged air rises like wind from a bellows
A second velocity augments the upward motion: the air that was driven under by the falling water, now clothed in water, rises with fury and leaps within its element in the manner of wind pressed from a bellows. It drags the water nearest the surface up with it, making it leap far higher than its nature required.