f lightning followed by a low rumble of thunder
told of the nearer approach of the storm.

The Galveston observer took his watch from his pocket and counted the
seconds between the flash and the thunder.

"Fifty seconds!" he continued. "The front of the storm is still ten
miles away."

"Do you reckon five seconds to a mile between the lightning and the
thunder?" asked Anton.

"Yes," the observer replied, "light travels so fast that for something
as near as a lightning flash, you can reckon it as instantaneous, while
sound only travels at a little more than a thousand feet a second."

"But why does thunder make a noise?" asked Fred. "You told me the clouds
didn't bang together."

"They don't," the Forecaster answered. "Thunder is caused by the
electric discharge. You've heard Bob's big wireless outfit crackle, when
he sends out a spark, haven't you?"

"Sure," said Fred, "you can hardly hear yourself talk, when Bob's got
his wireless busy."

"And why does that crackle? Do you know, Bob?" he asked, turning to the
wireless expert.

"No," answered the boy.

"You've often heard the crackling of a near-by thunder compared to an
irregular volley of rifles, haven't you?"

"Yes."

"Naturally, because that's exactly what it is. A rifle shot is an
explosion caused by the firing of a powder, which, in turn, means the
expansion of the powder into gases, the force of that expansion driving
forward the bullet. Sound, as you know, is a series of air vibrations.
The explosion wave sets up a series of these vibrations, by compressing
the air in front of it.

"Lightning does the same thing. When a lightning flash breaks down the
resistance of the air, and passes through a channel of air, it heats the
air suddenly to a temperature of two or three thousand degrees, causing
a terrific expansion along the entire length of the flash and starting
an explosion wave. This compresses the air on all sides and sets sound
vibrations in action. As soon as the flash is discharged, the air rushes
back to fill the partial vacuum that the heating by electricity has
caused, adding force to the vibrations.

"That's why you hear the crackle of near-by thunder. You are near enough
to hear the explosions made by all the little side-branches of the
lightning flash--you can hear the same sometimes when you comb your hair
or rub a cat's fur--while the big crashes are due to your hearing, all
at once, the main wave of sound set in action by the f