s to the trees, it was evident that there
could be no progressive motion.

Rollo's mother had been standing all this time at the step of the door,
watching the progress of the experiment; and, when she saw the
expression of satisfaction upon Rollo's countenance, while he was
standing, with his eyes shut, holding the end of the string, she wanted
to come and take hold of it herself, so as to see what sort of a
sensation the vibratory motion of the string produced.

So she came out through the gate, and asked Mr. Holiday to wait a moment
while she went to where Rollo was standing, and took hold of the string.
But he said that it would not be necessary for her to go there, as she
could take hold of his end of the line just as well, and let Rollo
strike the other end.

They accordingly performed the experiment in that way, and Rollo's
mother could feel the vibrations very distinctly.

"One thing you must observe," said Mr. Holiday; "and that is, that the
vibrations pass along from one end of the line to the other very quick
indeed. We feel them at one end almost at the same instant that the
other end is struck."

"_Exactly_ at the same instant, sir," said Rollo.

"No," replied his father, "not exactly at the same instant, though it is
very nearly the same."

"I did not see any difference," said Rollo.

"No," replied his father, "you cannot perceive any difference in so
short a string, but if we had a string, or a wire, a mile long, I
presume that we should find that it would require a sensible period of
time to transmit the vibrations from one end to the other."

"What do you mean by a sensible period of time, father?" asked Rollo.

"Why, a length of time that you could perceive," said his father; "just
as it was with the man cutting wood. We could see that some time elapsed
between the striking of the blow, and our hearing the sound."

"Yes," said Rollo, "just as long as it took him to lift up his axe."

"That is not certain," replied his father, "because the sound that we
heard might have belonged to a blow made before. That is, it might be
that, when he had struck one blow, he had time to raise his axe and
strike another, and then raise his axe again, before the sound of the
first blow came to us."

"Yes, sir," said Rollo, "I understand."

Mr. Holiday then told Rollo that he might unfasten the string from the
trees, and wind it up again into a ball, and bring it in. Then he and
Rollo's mother went into t