In chapter 3 of "The Revolutions of the Heavenly Spheres" where Copernicus defines "how the Earth forms a single sphere with water", the Edward Rosen translation of the book states this.
Moreover, there is no difference between the earth's centers of gravity and magnitude. This can be established by the fact that from the ocean inward the curvature of the land does not mount steadily in a continuous rise.
Based on this. I'm believe that the center of magnitude is referring to the center of mass. And that the figure that he is referring to with a difference between the center of gravity and mass, would be a hollow Earth. Assuming that this is correct, how would a hollow Earth have land that would continuously rise with no changes in elevation from that.