\Man"o*graph\, n. [Gr. ? thin, rare + -graph: cf. F.
manographe.] (Engin.)
An optical device for making an indicator diagram for
high-speed engines. It consists of a light-tight box or
camera having at one end a small convex mirror which reflects
a beam of light on to the ground glass or photographic plate
at the other end. The mirror is pivoted so that it can be
moved in one direction by a small plunger operated by an
elastic metal diaphragm which closes a tube connected with
the engine cylinder. It is also moved at right angles to this
direction by a reducing motion, called a reproducer, so as to
copy accurately on a smaller scale the motion of the engine
piston. The resultant of these two movements imparts to the
reflected beam of light a motion similar to that of the
pencil of the ordinary indicator, and this can be traced on
the sheet of ground glass, or photographed.