he house, to see if breakfast was not almost
ready.

That morning, after they were all seated at the breakfast table, Rollo
said to his father that he did not exactly understand what sort of a
motion the vibratory motion of the air was, after all.

"No," said his father, "I suppose you do not. And, in fact, I do not
understand it very perfectly myself. I only know that the philosophers
say, that, when a man strikes a blow with an axe upon a log of wood, it
produces a little quivering motion of the air, which spreads all around,
darting off in every direction very swiftly. If a boy strikes a tin pail
with a drum-stick, it makes another kind of quivering or vibration,
which is different from that which is made by the axe; but I don't know
precisely how it differs. So, when the air is full of sounds, on a still
morning, it is full of these little vibrations, like a string which
trembles from end to end, though its ends are fastened so that it cannot
move away."

"Then the air is never at rest," said Rollo's mother.

"No; certainly not, when any sound is to be heard; and it is never
perfectly silent."

"There is one thing very extraordinary," said Mrs. Holiday.

"What is it?" asked Rollo's father.

"Why, that, when a great many sounds are made at the same time," she
replied,--"as, for example, when we are upon the top of a hill, on a
still morning, and hear a great many separate sounds, as a man cutting
wood, birds singing, a bell ringing, and perhaps a man shouting to his
oxen,--all those tremblings or vibrations, being in the air together, do
not interfere with one another."

"Yes," said Mr. Holiday, "it is very extraordinary indeed. They do not
seem to interfere at all. When there are too many sounds, or if there is
a wind with them, they do interfere; but, in a calm morning, like this,
when the air is at rest, you can hear a great many distant sounds very
distinctly."

"Yes," said Rollo, "and I mean to go up to the top of the rocks again
after breakfast, and listen."

QUESTIONS.

What time of the year was it when Rollo took this walk? How did
Rollo satisfy himself that there was no wind at all? How did his
father prove that there was a little wind? Is all motion of the
air wind? What two kinds of motion are mentioned? What sound did
they hear? What made Rollo think the sound was not made by the
man whom they saw cutting wood? How did his father explain this
phenomenon? Wh