and. Rollo's father then took up
the bar carefully, and passed it around, so that all who were seated at
the table could examine it closely.

"It is thickest on all the edges and corners," said Rollo.

"Yes," said his mother; "and the sand forms little black bristles,
pointing off in every direction."

They all examined it attentively, and observed the little black bristles
pointing out every way from the edges and corners at the ends.

"This shows you," said Rollo's father, "exactly how the magnetic power,
so far as its attractive force on other bodies is concerned, is
distributed. You see it resides in the two ends, and the two ends seem
to be exactly alike."

"Yes, sir," said Rollo, "exactly."

"They _seem_ to be so," continued his father; "but the fact is, the
magnetism of one end is very different from that of the other."

"I see that the cluster of sand is a little bigger at one end, than it
is at the other," said Rollo's mother. She was more observing than the
others, and had noticed a little difference, which had escaped the rest.

"That indicates only a difference in degree," said Rollo's father; "but
there is a difference in _kind_."

"What do you mean by that, father?" asked Rollo.

"Why, if the attractive powers at the two ends were both alike in their
nature, only one was stronger than the other, then the difference would
be in _degree_; but there is a difference in the nature of the magnetism
itself. In fact, the magnetisms of the two ends are of opposite natures
in some respects."

"Why, both ends attract the sand," said Rollo, "just alike."

"True," said his father; "they seem to attract the sand in precisely the
same way; and, looking at the bar, as I now hold it up," he added, "with
the sand adhering in the same way at the two ends, one would suppose
that they were both magnetic alike. But, in fact, there is a great
difference between them."

All the company looked upon the two ends of the bar, as Rollo's father
held it up, wondering how he would show that there was any difference
between them.

"Now, in the first place," he continued, "we must get the sand off of
the ends. Do you think you can get it off for me, Rollo?" said he.

Rollo took the bar very eagerly, and attempted to brush the sand back
upon the paper. He succeeded in brushing off a little of it; but the
greater portion remained. When he rubbed upon one side, it moved round
to the other; and he could not get it off.

"H