llow leaf traced by laying the leaf on the paper; _n o_, one of
the innumerable groups of curves at the lip of a paper Nautilus; _p_, a
spiral, traced on the paper round a Serpula; _q r_, the leaf of the
Alisma Plantago with its interior ribs, real size; _s t_, the side of a
bay-leaf; _u w_, of a salvia leaf; and it is to be carefully noted that
these last curves, being never intended by nature to be seen singly, are
more heavy and less agreeable than any of the others which would be seen
as independent lines. But all agree in their character of changeful
curvature, the mountain and glacier lines only excelling the rest in
delicacy and richness of transition.

Sec. XX. Why lines of this kind are beautiful, I endeavored to show in
the "Modern Painters;" but one point, there omitted, may be mentioned
here,--that almost all these lines are expressive of action of _force_
of some kind, while the circle is a line of limitation or support. In
leafage they mark the forces of its growth and expansion, but some among
the most beautiful of them are described by bodies variously in motion,
or subjected to force; as by projectiles in the air, by the particles of
water in a gentle current, by planets in motion in an orbit, by their
satellites, if the actual path of the satellite in space be considered
instead of its relation to the planet; by boats, or birds, turning in
the water or air, by clouds in various action upon the wind, by sails in
the curvatures they assume under its force, and by thousands of other
objects moving or bearing force. In the Alisma leaf, _q r_, the lines
through its body, which are of peculiar beauty, mark the different
expansions of its fibres, and are, I think, exactly the same as those
which would be traced by the currents of a river entering a lake of the
shape of the leaf, at the end where the stalk is, and passing out at its
point. Circular curves, on the contrary, are always, I think, curves of
limitation or support; that is to say, curves of perfect rest. The
cylindrical curve round the stem of a plant binds its fibres together;
while the _ascent_ of the stem is in lines of various curvature: so the
curve of the horizon and of the apparent heaven, of the rainbow, etc.:
and though the reader might imagine that the circular orbit of any
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