be used for the soles of
contemplated boots and shoes, which they soon hoped to turn out.

Every morning the yaks would leave the enclosure and start out on trips
to the feeding grounds, and sometimes Harry or George would follow them
and hunt for game. On one occasion, while Harry was on the opposite
hill, George saw the flash of Harry's gun, and almost immediately
thereafter heard the report. This was the first time the difference
between the flash and the noise attracted his attention.

"Will you tell me why I saw Harry's fire before the sound reached me?"

"Did you say 'sound' or 'noise'?"

George looked at the Professor quizzically. "Is there any difference
between sound and noise?"

"Technically, there is a difference, although in common practice one
word is used for the other without discriminating. Sound means a
succession of vibrations produced in their regular order, like music,
whereas noise is a disorganized vibration. For instance, falling water,
like our cataract here, is sound, but the report of George's gun was a
noise."

"I can see the difference. Would a wagon going rapidly over a pavement
be a noise or a sound?"

"It would be a noise if the pavement should be irregular, but if the
pavement is regular and the vibrations or beats are uniform, it is then
called a sound. But you wanted to know why you saw the shot before you
heard it. Simply because sound does not travel as fast as light. Sound
moves 1,040 feet in a second, and light over 186,000 miles a second,
which is about 850,000 times faster than sound."

"Do soft and light sounds travel at the same speed?"

"Theoretically, yes; but numerous experiments have been made, and many
of them go to show that a loud noise really travels faster than a soft
noise."

[Illustration: _Fig. 23. MEASURING SOUND PITCH_]

"What is the cause of that?"

"It is attributed to the belief that a loud noise causes greater wave
motions, although the sound waves may be the same lengths in both cases.
Or, it might be said that loud noises have greater strength."

"When we were going to New York in the cars, a train was coming toward
us, and the engineer on that train blew his whistle when he was off
quite a distance, and kept it up until long after he had passed us. I
noticed that when the whistle started the sound had a very low pitch,
which kept increasing to a higher and higher pitch until the train
passed; what was the cause of that?"

"As the sound wave