SEC. xn.j OF NATURAL PHILOSOPHY. 227 is generated by the revolution of the arc FE, and is cut any where, as in r, by the line6, the annular part of the super J'cies generated by the revolution of the arc rE will be as the lineola Dd, the radius of the sphere PE remainiag the same; as Archimedes has de monstrated in his Book of the Sphere and Cylinder. And the force of this super ficies exerted in the direction of the lines PE or Pr situate all round in the conical superficies, will be as this annular superficies itself; that is as the lineola DC/, or, which is the same, as the rectangle under the given radius PE of the sphere and the lineola DC/ ; but that force, exerted in the direction of the line PS tending to the centre S, will be less in the ratio PI) to PE, and therefore will be as PD X DC/. Suppose now the line DF to be divided into innumerable little equal par ticles, each of which call DC/, and then the superficies FE will be divided into so many equal annuli, whose forces will be as the sum of all the rec tangles PD X DC/, that is, as