Of liquid impetus, that is, of air and of water and other kinds, etcetera. In every degree of time impetus acquires degrees of diminution, and the prolongation of its being arises from the air or the water which, behind the moving body, closes in to fill the void that the body penetrating it leaves behind. This air is more powerful in striking and pressing with percussion behind the moving body than is the air set in front to resist, by its condensation, the penetration of that body. And it is the condensation of air that diminishes the fury of the said impetus in the said moving body. Why is the fish swifter in water than the bird in air, when it ought to be the contrary, water being heavier and denser than air, and the fish heavier and of smaller fins than the bird's wings? It is for this reason: the fish is not moved from its place by the swift currents of the waters, as the bird is by the fury of the winds in the air. And besides this, the fish is seen to run upward along the very fall of the most sudden cascades of water with the swiftest motion, like a thunderbolt among the thick clouds, which seems a marvelous thing. And this happens through the immense velocity of such a moving body, which so exceeds the motion of that water that the water remains motionless with respect to the motion of that fish. And the proportion of the said motions is as one to ten: the motion of the water has a velocity of one and that of the fish is ten, which ten exceeds one by nine. Therefore the fish, having a power of ten, is left with a power of nine, with which power it climbs that fall; with a power of ten, and the water takes one from it, so that nine remains. This happens because water in itself is denser than air and consequently heavier, wherefore it is swifter to fill the void that the fish leaves behind in the place from which it departed. And moreover the water that it strikes before itself does not condense as the air does before the bird; rather they make a wave which, with its motion, prepares and augments the motion of the fish, wherefore it becomes swifter than the bird, before which the air condenses.

Impetus is an impression of local motion, transmitted from the mover to the moving body and maintained by the air or the water as it moves to prevent a void. The impetus of a body within water differs from the impetus of a body within air; and these differences result from the variety of the said liquids, because air is condensable toward infinity and water is not. The impetus of water divides into two parts, being of two natures, of which one is simple and the other compound. The simple is entirely beneath the water; the other is compound, that is, between the air and the water, as is seen in boats. Simple impetus does not condense the water before the motion of the fish, but moves the water behind the motion of that fish with the same velocity that its mover has, and the wave of water set in front will never be swifter than its mover. But the motion of a boat, called compound motion because it partakes of both water and air, divides into three principal parts, because such motion is made in 3 aspects: that is, against the course of the river, along the direction of the water's current, and across, that is, along the breadth of the river.