truck.

All hard bodies, when struck return more or less of a sound; but
those which are destitute of elasticity, give no repetition of the
sound; the noise is at once produced and dies; while other bodies,
which are more elastic and capable of vibration, repeat the sounds
produced several times successively. These last are said to have a
tone; the others are not allowed to have any. If we wish to give
nonelastic bodies a tone, it will be necessary to make them continue
their sound, by repeating our blows quickly upon them. This will
effectually give them a tone; and an unmusical instrument has often
by this means a fine effect in concerts. The effects of a drum depend
upon this principle. Gold, silver, copper, and iron, which are
elastic metals, are sonorous; but lead, which possesses scarcely any
elasticity, produces little or no tone. Tin, which in itself has very
little more sound than lead, highly improves the tone of copper when
mixed with it. Bell metal is formed of ten parts of copper, and one
of tin. Each of these is ductile when separate, though tin is only so
in a small degree, yet they form when united a substance almost as
brittle as glass, and highly elastic. So curious is the power of tin
in this respect, that even the vapour of it, when in fusion, will
give brittleness to gold and silver, the most ductile of all metals.
Sonorous bodies may be divided into three classes; first, bells of
various figures and magnitudes: of these such as are formed of glass
have the most pure and elegant tones, glass being very elastic, and
its sound very powerful; secondly, pipes of wood or metal; thirdly,
strings formed either of metallic or animal substances. The sounds
given by strings are more grave or more acute according to the
thickness, length, and tension of the strings.

Air is universally allowed to be the ordinary medium of sound, or the
medium by which sounds are propagated from sonorous bodies, and
communicated to the ear. This may be shown by an experiment with the
air pump; also with the condenser.

But though air is the general vehicle of sound, yet sound will go
where no air can convey it; thus the scratching of a pin at the end
of a long piece of timber may be heard by an ear applied at the other
end, though it could not be heard at the same distance through the
air. On this account it is that sentinels are accustomed to lay their
ears to the ground, by which means they can often discover the
approach