\He"li*o*stat\, n. [Helio- + Gr. ? placed, standing,
fr. ? to place, stand: cf. F. h['e]liostate.]
An instrument consisting of a mirror moved by clockwork, by
which a sunbeam is made apparently stationary, by being
steadily directed to one spot during the whole of its diurnal
period; also, a geodetic heliotrope.