ow what you mean by that, exactly," said Marco.

"Why, the rule of floating bodies is this," rejoined Forester. "When any
substance, like a cake of ice, or a log of wood, or a boat, is floating
upon the water, a part of it being above the water and a part under the
water, if a man steps upon it, he makes it sink enough deeper to
submerge a part of the wood or ice as large as he is himself. If there
is just as much of the wood or ice above the water as is equal to the
bulk of the man, then the man, in stepping upon it, will sink it just to
the water's edge."

"But perhaps one man would be heavier than another man," said Marco.

"Yes," replied Forester; "but then he would be larger, and so, according
to the principle, he would make more wood sink before the equilibrium
was reached."

"What is _equilibrium_?" asked Marco.

"Equilibrium is an equality between two forces," replied Forester.

"I don't see what two forces there are," said Marco.

"There is the weight of the man pressing downwards," said Forester,
"for one, and the buoyant power of the water, that is, its upward
pressure, for the other. The weight of the man remains constantly the
same. But the upward pressure of the water increases in proportion as
the log sinks into it. For the deeper the log sinks into the water, the
more of it is submerged, and it is more acted upon and pressed upward by
the water. Now, as one of these forces remains constant, and the other
increases, they must at length come to be equal, that is, in
equilibrium; and then the log will not sink any farther. That's the
philosophy of it, Marco."

Marco did not reply, but sat looking at the barren and rocky shores of
the river, as the boat glided by them. Presently another log came into
view.

"There," said Forester, "look at that log, and see whether you think
that you could float upon it."

"Yes," said Marco, "I think I could."

"It depends," said Forester, "on the question whether the part of it
which is out of water is as big as you are."

"I think it is," said Marco.

"Yes," added Forester, "I have no doubt that it is."

"Only I should roll off," said Marco.

"True," replied Forester; "but the millmen, who work about the logs,
acquire astonishing dexterity in standing upon them. If there is only
enough of the log above water to equal their bulk, so that it has
buoyant power enough to float them, they will keep it steady with their
feet, and sail about upon it very safely