"There," said his father. "Stop. That, you see, was a progressive motion."
"Yes, sir," replied Rollo.
"The whole string advanced along the road," added his father. "It made progress, and so it was a progressive motion. Now, fasten your end of the string, Rollo, to that tree directly behind you."
Rollo looked behind him, and saw that he was standing near a small maple-tree, which had been planted, a few years before, by the side of the road.
"Tie it right around the stem of the tree," said his father, "about as high as your shoulder."
Rollo fastened the string as his father had directed. Then his father fastened his end, in the same way, to another tree, which was growing near where he was standing.
"Now," said he, "there can be no more progressive motion, but there can be a vibratory one. Take hold of the string near where it is fastened to the tree."
Rollo took hold of it, as his father had directed, and then his father told him to shut his eyes. When his eyes were shut, so that he could not see, his father said that he was going to strike the string, at his end of it, with his pencil-case, and he asked Rollo to observe whether he could feel any motion.
Rollo held very still, while his father struck the string; and immediately afterwards he called out, "Yes, sir." Then his father struck the string again, several times, and every time Rollo could feel a distinct vibratory or quivering motion, which was transmitted very rapidly through the string, from one end to the other; although, as the string was fastened by both ends 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.