[Illustration]
Provide a long, flat glass ruler or rod, as in the engraving, and
cement it with mastic to the edge of a drinking-glass fixed into a
wooden stand; support the other end of the rod very lightly on a
piece of cork, and strew its upper surface with sand; set the glass
in vibration by a bow, at a point opposite where the rod meets it,
and the motions will be communicated to the rod without any change in
their direction. If the apparatus be inverted, and sand be strewed on
the under side of the rod, the figures will be seen to correspond with
those produced on the upper surface.