If a person stands at the distance of fifteen or twenty rods from a
cannon that is fired, he will first see the flash, then the cloud of
smoke that rushes from the cannon's mouth, then the ground will be felt
to tremble, and lastly the sound will reach his ear at the same time
that a strong puff of air will be felt. This puff of air is the
sound-wave itself, travelling at the rate of eleven hundred feet or more
per second. At the instant of explosion of the gunpowder, the air in
front of the cannon is very much compressed; and this compression at
once begins to move outwards in every direction, so as to be a kind of a
spherical shell of air constantly increasing in diameter; and, whenever
it reaches an ear, the sound is perceived. Whenever such a sound-wave
strikes upon a solid surface, as upon a cliff or a building, it is
turned back, and the reflected wave may be heard; in which case we call
it an echo. When a cannon is fired, we generally hear the sound
repeated, so that it apparently lasts for a second or more; but when, as
in the first case, we hear the sound of a pencil struck upon the table,
but a single short report is noticed, and this, as may be supposed,
consists of a single wave of condensed air.
[Illustration: FIG. 6.]
[Illustration: FIG. 7.]
Imagine a tuning-fork that is made to vibrate. Each of the prongs beats
the air in opposite directions at the same time. Look at the physical
condition of the air in front of one of these prongs. As the latter
strikes outwards, the air in front of it will be driven outwards,
condensed; and, on account of the elasticity of the air, the
condensation will at once start to travel outwards in every
direction,--a wave of denser air; but directly the prong recedes,
beating the air back in the contrary direction, which will obviously
rarefy the air on the first side. But the disturbance we call
rarefaction moves in air with the same velocity as a condensation. We
must therefore remember, that just behind the wave of condensation is
the wave of rarefaction, both travelling with the same velocity, and
therefore always maintaining the same relative position to each other.
Now, the fork vibrates a great many times in a second, and will
consequently generate as many of these waves, all of them constituted
alike, and having the same length; by length meaning the sum of the
thicknesses of the condensation and the rarefaction. Suppose a fork to
make one hundred vibrations per second: at the end of the second, the
wave generated by the vibration at the beginning of the second would
have travelled, say, eleven hundred feet; and evenly distributed between
the fork and the outer limit, would be ranged the intermediate waves
occupying the whole distance: that is to say, in eleven hundred feet
there would be one hundred sound-waves, each of them evidently being
eleven feet long. If the fork made eleven hundred vibrations per second,
each of these waves would be one foot long; for sound-waves of all
lengths travel in air with the same rapidity. Some late experiments seem
to show that the actual amplitude of motion of the air, when moved by
such a high sound as that from a small whistle, is less than the
millionth of an inch.