-end rod and a wire of the same
fundamental note would not have the same _timbre_, or quality, owing to
the difference in the harmonics.

COLUMNS OF AIR.

In wind instruments we employ, instead of rods or wires, columns of air
as the vibrating medium. The note of the column depends on its length.
In the "penny whistle," flute, clarionet, and piccolo the length of the
column is altered by closing or opening apertures in the substance
encircling the column.

RESONANCE OF COLUMNS OF AIR.

Why does a tube closed at one end, such as the shank of a key, emit a
note when we blow across the open end? The act of blowing drives a thin
sheet of air against the edge of the tube and causes it to vibrate. The
vibrations are confused, some "pulses" occurring more frequently than
others. If we blew against the edge of a knife or a piece of wood, we
should hear nothing but a hiss. But when, as in the case which we are
considering, there is a partly-enclosed column of air close to the
pulses, this selects those pulses which correspond to its natural period
of vibration, and augments them to a sustained and very audible musical
sound.

[Illustration: FIG 136.--Showing how the harmonics of a "stopped" pipe
are formed.]

In Fig. 136, _1_ is a pipe, closed at the bottom and open at the top. A
tuning-fork of the same note as the pipe is struck and held over it so
that the prongs vibrate upwards and downwards. At the commencement of an
outward movement of the prongs the air in front of them is _compressed_.
This impulse, imparted to the air in the pipe, runs down the column,
strikes the bottom, and returns. Just as it reaches the top the prong is
beginning to move inwards, causing a _rarefaction_ of the air behind
it. This effect also travels down and back up the column of air in the
pipe, reaching the prong just as it arrives at the furthest point of the
inward motion. The process is repeated, and the column of air in the
pipe, striking on the surrounding atmosphere at regular intervals,
greatly increases the volume of sound. We must observe that if the
tuning-fork were of too high or too low a note for the column of air to
move in perfect sympathy with it, this increase of sound would not
result. Now, when we blow across the end, we present, as it were, a
number of vibrating tuning-forks to the pipe, which picks out those
air-pulses with which it sympathizes.

LENGTH AND TONE.

The rate of vibration is found to be inversely