Hence sound has in air a certain velocity which has been very
accurately measured, and found to be 1,093 feet per second when the
temperature of the air is at the freezing point of water. As the
temperature increases, the velocity of sound will increase a little more
than one foot for every Fahrenheit degree; so that at 60 deg. the velocity
is 1,125 feet per second. This is the velocity in air. In water the
velocity is about four times greater, in steel sixteen times, in
pine-wood about ten times.

CONSTITUTION OF A SINGLE SOUND-WAVE.

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