For example, the " ungula is formed by cutting a right circular cylinder by means of an oblique plane through a diameter of the circular base . " And also the " " double ungula " formed from cylinders with axes at right angles ."
12.
VII . The mind can determine in many ways the ideas of things, which the understanding forms from other ideas : thus, for instance, in order to define the plane of an ellipse, it supposes a point adhering to a cord to be moved round two centres, or, again, it conceives an infinity of points, always in the same fixed relation to a given straight line, or a cone cut in an oblique plane, so that the angle of inclination is greater than the angle of the vertex of the cone, or in an infinity of other ways.
