thus
preventing in a good degree the interfering echoes. The wire should be
rather fine piano-wire, and it should be stretched so tightly as to give
out a low musical sound when plucked with the fingers. In a large hall
there should be twenty or more such wires.

RESONANCE.

When a tuning-fork is struck, and held out in the air, the vibrations
can be felt for a time by the fingers; but the sound is hardly audible
unless the fork be placed close to the ear. Let the stem of the fork
rest upon the table, a chair, or any solid body of considerable size,
and the sound is so much increased in loudness as to be heard in every
part of a large room. The reason appears to be, that in the first case
the vibrations are so slight that the air is not much affected. Most of
the force of the vibration is absorbed by the hand that holds it; but
when the stem rests upon a hard body of considerable extent, the
vibrations are given up to it, and every part of its surface is giving
off the vibrations to the air. In other words, it is a much larger body
that is now vibrating, and consequently the air is receiving the
amplified sound-waves.

If the stem of the fork had been made to rest upon a bit of rubber, the
sound would not only not have been re-enforced in such a way, but the
fork would very soon have been brought to rest; for India rubber
_absorbs_ sound vibrations, and converts them into heat vibrations, as
is proved by placing such a combination upon the face of a thermo-pile.

If one will but put his hand upon a table or a chair-back in any room
where a piano or an organ is being played, or where voices are singing,
especially in church, he cannot fail to feel the sound; and if he
notices carefully he will perceive that some sounds make such table or
seat to shake much more vigorously than others,--a genuine case of
sympathetic vibrations.

It is for this reason that special materials and shapes are given to
parts of musical instruments, so that they may respond to the various
vibrations of the strings or reeds. For instance, the piano has an
extensive thin board of spruce underneath all the strings, which is
called the sounding-board. This board takes up the vibrations of the
strings; but, unlike the rubber, gives them all out to the air, greatly
re-enforcing their strength, and changing somewhat their quality. But
the air itself may act in like manner. In almost any room or hall not
more than fifteen or twenty feet long, a