HELIOSTAT, in Optics, the name given by *Gravesande to an instrument devised by him for the purpose of
fixing, as it were, the solar rays during the whole time of observation—namely, by reflecting them in the same straight line by a mirror, to which a proper motion is given by means of clockwork. The original instrument is described in his Phys. Elementa Mathematica; but it has been greatly improved by Malus and others.