[Illustration]
Provide two discs of metal or glass, precisely of the same dimensions,
and a glass or metal rod; cement the two discs at their centres to
the two ends of the rod, as in the engraving, and strew their upper
surfaces with sand. Cause one of the discs, viz. the upper one, to
vibrate by a bow, and its vibration will be exactly imitated by the
lower disc, and the sand strewed over both will arrange itself in
precisely the same forms on both discs. But if, separately, they do not
agree in their tones, the figures on them will not correspond.