In so simple a figure as the circle lies the key to the solution of a multitude of problems, each of which would demand various appliances; whereas the solution results of itself, as it were, as one of the infinite number of elegant properties of this figure.

Are we, for example, asked to construct a triangle, being given the base and vertical angle? The problem is indeterminate, i.e. it can be solved in an infinite number of ways. But the circle embraces them altogether as the geometrical locus of the vertices of triangles satisfying the given conditions. Again, suppose that two lines are to cut one another so that the rectangle under the segments of the one should be equal to the rectangle under the segments of the other; the solution of the problem from this point of view presents much difficulty.