with thy mighty
attraction, and hold it down as firmly as thou wilt.'"

"O father!" exclaimed Rollo; Nathan and James laughed, and Rollo's
mother looked up from her work to listen to this strange apostrophe.

"It would seem," continued his father, in a pompous tone, as if still
addressing the earth--"it would seem, most mighty planet, a very easy
thing for thee to release this single stone, for a few minutes, from the
grasp with which thou holdest all things down upon thy surface. And by
it I shall gain much, while thou wilt lose nothing; for, if thou wilt
not willingly give up the stone, I must get three or four yoke of strong
oxen, and, by main force, pull it away."

"Is that what makes everything heavy?" said Rollo.

"Yes," said his father, answering now in his natural tone; "the
attraction of the earth is what makes everything heavy, and holds it
down."

"And could we move a monstrous great stone," said Rollo, "as light as a
feather?"

"No," said his father, "it would not move along quick and light, like a
feather. You could not move it quick. Suppose, for instance, you had two
boats, floating upon the water, of the same size; one made very light
indeed, of something very thin, like paper, and empty; and the other
made of wood, and loaded with iron as heavily as it would bear. Now,
they would both be supported upon the water, so that their weight would
be neutralized; and yet they would move very differently. You could push
the light one about easily, anywhere, but the heavy one would move very
slowly. You would not have to push very hard upon it, but you would have
to push _for some time_, to set it in motion; and then it would be hard
to stop it. This is called its _inertia_."

"Yes," said Rollo, "it would go harder against the bank."

"The reason is," continued his father, "that the heavy boat contains a
great many more particles of matter than the light one, and they have
all got to be put in motion. So it requires greater effort, or the same
effort must be continued a longer time.

"For instance, if we suppose that the light boat has one million of
particles of matter, the heavy one would have, perhaps, twenty millions.
Of course the effect of the pushing has to be divided among twenty times
as many particles, and of course will only carry them one twentieth part
as far; so that the bodies that are now large and heavy, would only move
slowly, though they would move _easily_, if the attraction of the e