Of the motion of the moving body.

Of the motion of the moving body that pours with continuous motion onto a moving site, or else when that which pours is itself moving.

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The motion of the liquid which pours through the bottom of a moving vessel will follow a straight line set at an oblique angle; and this obliquity will be of a greater or lesser inclination according as the motion of the vessel that generates it is of greater or lesser speed.

Of the motion made by the site that receives the thing poured from the vessel.

To receive upon a moving site the thing that pours from a motionless vessel amounts to the same as to move the pouring vessel of that thing over a motionless site.

But if the motion of the pouring vessel is equal to the motion of the site that receives the poured thing upon itself, then the motion of the thing that descends will be oblique and rectilinear, as shown above.

Of the motion of the arrow driven from the bow.

The arrow shot from the centre of the world toward the highest part of the elements will rise and descend along one and the same straight line, even though the elements are in revolving motion around the centre of the elements.

The heavy body that descends through the revolving elements always has its motion along the straightness of that line which extends from the beginning of the motion to the centre of the world.

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The 8 lines with the 8 divisions into which they are apportioned are meant to demonstrate a single line, and that one is straight, along which the weight that descends through the revolving elements passes through each of its 8 partitions; this line at the end returns to the same site from which it divided, and the motion of the heavy body has a double designation, that is, a rectilinear helical curvature.