"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."