Cone (Geometry a solid figure)
Collier's New Encyclopedia (1921)
in geometry, a solid figure described by the revolution of a rightangled triangle about one of the sides containing the right angle, which side remains fixed. If the fixed side be equal to the other side containing the right angle, the cone is called a right-angled cone; if it be less than the other side, an obtuseangled, and, if greater, an acute-angled cone. The axis of the cone is the fixed straight line about which the triangle revolves. The base of a cone is the circle described by that side containing the right angle which revolves. Similar cones are those which have their axes and the diameters of their bases proportionals. (Euclid.) In optics, a pencil of rays of light emanating from a point and diverging as they proceed on their course. In astronomy, a conical-shaped shadow projected by a planet on the other side from that on which it is illuminated by the sun. In geology, a conical mound or hill produced by the showering down around the orifice of eruption of scoriæ, dust, and the various other materials ejected. In zoölogy (1) the English name of any shell of the large tropical molluscous genus Conus. The name also of any animal of that genus. (2) Pl. (cones), the English name of the Conidæ, a family of Gasteropodous mollusks. See CONE-SHELLS. In botany, a kind of anthocarpous or collective fruit, called also Strobilus, shaped somewhat like a mathematical cone, and consisting of an ament, the carpella of which are (scale-like) spread open, and bear naked seeds.
What Has Changed in the Last Century
The geometry in Collier's 1921 entry is Euclid's own, and it is still correct: a cone is generated by revolving a right triangle, and its volume is a third of the enclosing cylinder. What has shifted is emphasis rather than fact. Modern courses drop the Euclidean labels "right-angled, obtuse-angled and acute-angled cone" (they came from the ancient way of cutting conic sections) and instead treat the cone as any base joined to an apex, including oblique cones, and as the infinite double-napped surface of coordinate geometry, from which ellipse, parabola and hyperbola are all sliced. The entry's other senses have aged more: the tropical Conus shells listed under zoology are now known to hunt with venomous harpoons, and one of their toxins was developed into the pain drug ziconotide, approved in 2004.
Written for this edition, 2026.