Collision of two streams of water
Two water courses cannot pass through each other and rebound, heaping the surface
Leonardo asks what happens when two lines of water cross in the middle of their courses: do they pass through or over one another, or rebound after the collision? He answers that they must bounce back, since it is impossible for two bodies to pass through each other. Struck together, the waters spread at the point of contact and try to part with equal distance from the centre of impact; the portion driven downward, unable to descend, heaps up the water above. A small crossed sketch at the upper right marks the two colliding lines.
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Two water streams rebound on collision
When two lines of water cross in the course of rivers, they do not pass one through or over the other but rebound backward. Struck together, they spread at the contact and try to part with equal distance from the centre of the collision. The downward part, unable to descend, raises the water above it.
Two bodies cannot pass through one another
The waters certainly bounce back, because it is impossible for two bodies to pass one through the other. This impenetrability drives the whole account of how the colliding streams redistribute.