OBTUSE SECTION OF A (TINE, a name given to the hyper bola, by ancient geometricians, because they considered it only in shill a cone, whose suction through the axis was an obtuse-angled triangle.
(WU I; It:, i11 painting,, a colour prepared from a species of earth, also termed ochre, composed Ed' fine, soft, smooth, and argillaceous particles, coherent, rough to the touch, and easily diffisable ill water; ochre is of various colours. Yellow is the most prevalent. but there are red, green, blue, and black ochres. The colouring matter of ochre is almost always oxide of iron. Bed chalk is much used for drawing; for this purpose, it should be free from grit, and not too hard. Ill order to free. it from grit, and render it better for use. it is sometimes pounded, washed, mixed with gum, and east into moulds of convenient shape and size. Under the Wane of reddlc, this substance is much used for the marking of sheep, and, when mixed with oil. for the painting of pales,
gates. and the wood-wErrk of out-buildings.
OC'fAl_40N, (from oK7c, eight, and Twiria, sides) a figure of eight sides, and consequently as many an tics, \Viten all the sides and all the angles are equal, the figure is called a regula• orttryon.
OCT :111E111:0N, 01. ( (Greek, dhrdeopog) in !geometry, one of the five regular bodies, consisting of eight equal and equilateral triangles.
The octahedron may be conceived as consisting of two quadrilateral pyramids put together at their hares.
Its solidity. therefore, is had by multiplying the quadran gular base of either by one-third of the perpendicular height of Erne of them, and then doubling, the product. The square of the side of an oetaliedron is it) a subduple ratio of the square of the diameter of the circumscribing sphere. See IZEGUI.Alt BODY.