That all forms of acknowledged beauty are composed exclusively of curves will, I believe, be at once allowed; but that which there will be need more especially to prove, is the subtilty and constancy of curvature in all natural forms whatsoever. I believe that, except in crystals, in certain mountain forms admitted for the sake of sublimity or contrast, (as in the slope of debris,) in rays of light, in the levels of calm water and alluvial land, and in some few organic developments, there are no lines nor surfaces of nature without curvature, though as we before saw in clouds, more especially in their under lines towards the horizon, and in vast and extended plains, right lines are often suggested which are not actual. Without these we could not be sensible of the value of the contrasting curves, and while, therefore, for the most part, the eye is fed in natural forms with a grace of curvature which no hand nor instrument can follow, other means are provided to give beauty to those surfaces which are admitted for contrast, as in water by its reflection of the gradations which it possesses not itself. In freshly-broken ground, which nature has not yet had time to model, in quarries and pits which are none of her cutting, in those convulsions and evidences of convulsion, of whose influence on ideal landscape I shall presently have occasion to speak, and generally in all ruin and disease, and interference of one order of being with another, (as in the cattle line of park trees,) the curves vanish, and violently opposed or broken and unmeaning lines take their place. Sec. 16. The beauty of gradation. What curvature is to lines, gradation is to shades and colors. It is _there_ infinity, and divides them into an infinite number of degrees. Absolutely, without gradation no natural surface can possibly be, except under circumstances of so rare conjunction as to amount to a lusus naturae; for we have seen that few surfaces are without curvature, and every curved surface must be gradated by the nature of light, which is most intense when it impinges at the highest angle, and for the gradation of the few plane surfaces that exist, means are provided in local color, aerial perspective, reflected lights, etc., from which it is but barely conceivable that they should ever escape. Hence for instances of the complete absence of gradation we must look to man's work, or to his disease and decrepitude. Compare the gradated colors of the rainbow with the stripes of a target, and the gradual concentration of the youthful blood in the cheek with an abrupt patch of rouge, or with the sharply drawn veining of old age. Sec. 17. How found in Nature.