hear sound in all directions. A good speaker is heard not
only by those directly in front of him, but by those on the side, and
even behind him.

254. Velocity of Sound. The transmission of motion from particle to
particle does not occur instantaneously, but requires time. If the
distance is short, so that few air particles are involved, the time
required for transmission is very brief, and the sound is heard at
practically the instant it is made. Ordinarily we are not conscious
that it requires time for sound to travel from its source to our ears,
because the distance involved is too short. At other times we
recognize that there is a delay; for example, thunder reaches our ears
after the lightning which caused the thunder has completely
disappeared. If the storm is near, the interval of time between the
lightning and the thunder is brief, because the sound does not have
far to travel; if the storm is distant, the interval is much longer,
corresponding to the greater distance through which the sound travels.
Sound does not move instantaneously, but requires time for its
transmission. The report of a distant cannon is heard after the flash
and smoke are seen; the report of a near cannon is heard the instant
the flash is seen.

The speed with which sounds travels through the air, or its velocity,
was first measured by noting the interval (54.6 seconds) which elapsed
between the flash of a cannon and the sound of the report. The
distance of the cannon from the observer was measured and found to be
61,045 feet, and by dividing this distance by the number of seconds,
we find that the distance traveled by sound in one second is
approximately 1118 feet.

High notes and low notes, soft notes and shrill notes, all travel at
the same rate. If bass notes traveled faster or slower than soprano
notes, or if the delicate tones of the violin traveled faster or
slower than the tones of a drum, music would be practically
impossible, because at a distance from the source of sound the various
tones which should be in unison would be out of time--some arriving
late, some early.

255. Sound Waves. Practically everyone knows that a hammock hung
with long ropes swings or vibrates more slowly than one hung with
short ropes, and that a stone suspended by a long string swings more
slowly than one suspended by a short string. No two rocking chairs
vibrate in the same way unless they are exactly alike in shape, size,
and material. An object