In treating of harmonic progression, the fact that the bisectors of an interior and corresponding exterior angle of a triangle divide the opposite side in the ratio of the other sides of the triangle has been used. This affords an interesting method of graphically explaining systems in involution. The sums of the natural numbers and of their cubes have been obtained graphically, and the sums of certain other series have been deduced therefrom.10Chapter XI deals with the general theory of regular polygons, and the calculation of the numerical value of n. The propositions in this chapter are very interesting.11. Chapter XII explains certain general principles, which have been made use of in the preceding chapters,-congruence, symmetry, and similarity of figures, concurrence of straight lines, and collinearity of points are touched upon.12. Chapters XIII and XIV deal with the conic sections and other interesting curves. As regards the circle, its harmonic