e a - f b - g c. The line of equal natural obliquity will not be of equal curvature in any part of its length, but will become so much the more or less curved as it draws nearer to or farther from the center of the world. The line of natural obliquity is that which in every degree of its length approaches one degree toward the center of the world. The line of mathematical obliquity is straight, and in every part of its length gains one degree of nearness to the line of equality [the horizontal]. A thing dragged toward the summit of the natural obliquity will always be of equal resistance to its mover. And the thing dragged to the summit of the accidental obliquity will, at every degree of its height, acquire various powers in the resistance of its friction.

The thing that descends along the natural obliquity will, at every degree of descent, acquire power of impetus. But the thing that descends along the accidental obliquity will in one part of the motion acquire impetus, and in the other lose it and come to rest, and in place of its stopping it turns back.