SPHEROID (Gr. c4aipa-etbis, like a

See also:

• SPHERE (Gr. vclsa?pa, a ball or globe)

sphere)  , a solid resembling, but not identical with, a sphere in shape . In geometry; the word is confined to the figures generated by an ellipse revolving about a diameter . If the axis of revolution be the major axis of the ellipse, the spheroid is " prolate "; if the minor axis, " oblate "; if any other, " universal." If the generating ellipse has for its equation x2/a2+y2/b2=1, and revolves about the major axis, i.e. the axis of x, the volume of the solid generated is Irab2, and its surface is 21r1 b2+(able) sin-le{, where e denotes the eccentricity . If the curve revolve about the minor axis, the volume is tra2b, and the surface is a{2a2+ (b2/e) log (1 +e) /(I —e) 1 . The figure of the earth is frequently referred to as an oblate spheroid; this, however, is hardly correct, for the geoid has three unequal axes . The Cartesian equation to a spheroid assumes the forms x2/a2+(y2+z2)/b2=1, for the prolate, and (x2+z2)/a2+y2/b2=1, for the oblate, the origin being the centre and the co-ordinate axes the axes of the original ellipse, x2/a?+y2/b2=1, and the line perpendicular to the plane containing them . In physics, the term " spheroidal state " is given to the following phenomenon . If drops of a liquid be placed on a highly heated surface, for example, the top of a stove, the liquid forms a number of tremulous globules which continually circulate internally . There is no visible boiling, although the globule diminishes slowly in size . The theory of the experiment is that the liquid is surrounded by an elastic envelope of its vapour which acts, as it were, as a cushion preventing actual contact of the drop with the plate . On the formation of a similar protective cushion of vapour depends the immunity of such experiments as plunging a hand into a bath of molten metal .