moving body, or the curve described by a sling, was a curve of motion, he should observe that the circular character is given to the curve not by the motion, but by the confinement: the circle is the consequence not of the energy of the body, but of its being forbidden to leave the centre; and whenever the whirling or circular motion can be fully impressed on it we obtain instant balance and rest with respect to the centre of the circle. Hence the peculiar fitness of the circular curve as a sign of rest, and security of support, in arches; while the other curves, belonging especially to action, are to be used in the more active architectural features--the hand and foot (the capital and base), and in all minor ornaments; more freely in proportion to their independence of structural conditions. § XXI. We need not, however, hope to be able to imitate, in general work, any of the subtly combined curvatures of nature's highest designing: on the contrary, their extreme refinement renders them unfit for coarse service or material. Lines which are lovely in the pearly film of the Nautilus shell, are lost in the grey roughness of stone; and those which are sublime in the blue of far away hills, are weak in the substance of incumbent marble. Of all the graceful lines assembled on Plate VII., we shall do well to be content with two of the simplest. We shall take one mountain line (_e g_) and one leaf line (_u w_), or rather fragments of them, for we shall perhaps not want them all. I will mark off from _u w_ the little bit _x y_, and from _e g_ the piece _e f_; both which appear to me likely to be serviceable: and if hereafter we need the help of any abstract lines, we will see what we can do with these only.