CARDIOID, a curve so named by G.F.M.M. Castillon (1708-1791), on account
of its heart-like form (Gr. [Greek: kardia], heart). It was
mathematically treated by Louis Carre in 1705 and Koersma in 1741. It is
a particular form of the limacon (q.v.) and is generated in the same
way. It may be regarded as an epicycloid in which the rolling and fixed
circles are equal in diameter, as the inverse of a parabola for its
focus, or as the caustic produced by the reflection at a spherical
surface of rays emanating from a point on the circumference. The polar
equation to the cardioid is r = a(1 + cos [theta]). There is symmetry
about the initial line and a cusp at the origin. The area is
{3/2}[pi]a^2, i.e. 1-1/2 times the area of the generating circle; the
length of the curve is 8a. (For a figure see LIMACON.)

CARDONA (perhaps the anc. _Udura_), a town of north-eastern Spain, in
the province of Barcelona; about 55 m. N.W. of Barcelona, on a hill
almost surrounded by the river Cardoner, a branch of the Llobregat. Pop.
(1900) 3855. Cardona is a picturesque and old-fashioned town, with
Moorish walls and citadel, and a 14th-century church. It is celebrated
for the extensive deposit of rock salt in its vicinity. The salt forms a
mountain mass about 300 ft. high and 3 m. in circumference, covered by a
thick bed of a reddish-brown clay, and apparently resting on a
yellowish-grey sandstone. It is generally more or less translucent, and
large masses of it are quite transparent. The hill is worked like a
mine; pieces cut from it are carved by artists in Cardona into images,
crucifixes and many articles of an ornamental kind.

CARDOON, _Cynara cardunculus_ (natural order Compositae), a perennial
plant from the south of Europe and Barbary, a near relation of the
artichoke. The edible part, called the _chard_, is composed of the
blanched and crisp stalks of the inner leaves. Cardoons are found to
prosper on light deep soils. The seed is sown annually about the middle
of May, in shallow trenches, like those for celery, and the plants are
thinned out to 10 or 12 in. from each other in the lines. In Scotland it
is preferable to sow the seed singly in small plots, placing them in a
mild temperature, and transplanting them into the trenches after they
have attained a height of 8 or 10 in. Water must be copiously supplied
in dry weather, both to prevent the formation of flower-stalks and to
increase the succulence of the leaves. In autum