Mr. Arthur Trevelyan discovered accidentally that a bar of iron, when
heated and placed with one end on a solid block of lead, in cooling
vibrates considerably, and produces sounds similar to those of an
Æolian harp. The same effect is produced by bars of copper, zinc,
brass, and bell-metal, when heated and placed on blocks of lead, tin,
or pewter. The bars were four inches long, one inch and a half wide,
and three-eighths of an inch thick.
The conditions essential to these experiments are, That two different
metals must be employed--the one soft and possessed of moderate
conducting powers, viz. lead or tin, the other hard; and it matters not
whether soft metal be employed for the bar or block, provided the soft
metal be cold and the hard metal heated.
That the surface of the block shall be uneven, for when rendered quite
smooth the vibration does not take place; but the bar cannot be too
smooth.
That no matter be interposed, else it will prevent vibration, with
the exception of a burnish of gold leaf, the thickness of which cannot
amount to the two-hundred-thousandth part of an inch.--_Transactions of
the Royal Society of Edinburgh._