"They drive a pin into the middle of each log, and then extend a rope along, fastening it to each pin. In this manner, the rope holds the logs together, and they form a long raft. When they catch the logs in booms, they afterwards form them into rafts, and so float them down the river to the mills, where they are to be sawed."

"Can men stand upon the rafts?" said Marco.

"Yes," replied Forester, "very well."

"They make a floor of boards, I suppose," said Marco.

"No," replied Forester; "they stand directly upon the logs."

"I should think the logs would sink under them," replied Marco, "or at least roll about."

"They sink a little," replied Forester; "just about as much as the bulk of the man who stands upon them."

"I don't know 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."

"I should like to try," said Marco.

"Perhaps we shall have an opportunity at some place on the river," said Forester.

Here Marco suddenly interrupted the conversation by pointing up the river to a column of smoke and steam which he saw rising beyond a point of land which was just before them.

"Here comes another steamboat," said he. "See, Forester."

"No," said Forester, "I believe that is a steam mill."

"A steam mill!" repeated Marco.

"Yes," replied Forester. "They have steam mills and tide mills to saw up the logs in this part of the river. Farther up, where there are waterfalls on the river, or on the streams which empty into it, they build mills which are carried by water. I presume that that is a steam mill."

At this moment, Marco's attention was diverted from the steam mill by a boat which came gliding into the field of view. There was one man in the boat rowing it. Another sat in the stern, with a pole in his hand. The pole had an iron hook in the end of it. A short distance before the boat was a log floating upon the water. The oarsman was rowing the boat towards the log. He brought it up to it in such a manner that the other man could strike his hook into it. When this was done, the oarsman began to pull the boat towards the shore, drawing the log with it.

By this time the whole group disappeared from Marco's view, behind a boat which was hanging on the quarter of the steamer. Marco, who wished to watch the whole proceeding, left Forester, and ran aft, in hopes that he could get another view of the men in the boat. He found, however, that the steamboat was proceeding so rapidly up the river, that he was fast losing sight of them; and then he concluded to go forward to the bows of the steamboat, thinking that, perhaps, there might be other logs coming down the river, with men after them in boats.

When he reached the bows, Marco found the deck encumbered with cables and anchors, and heavy boxes of freight, which made it difficult for him to find his way to a good place for a view. He finally reached a place where, by standing upon an anchor, he could look over the bulwarks, and get a view of the expanse of water before him. It was smooth, and its glassy surface was bright with the reflection of the rays of the morning sun.

Marco admired the beauty of the river and of its banks, but he could see no boats, or even logs coming. He saw some large sand banks before him, which had been left bare by the efflux of the tide. He wished that the steamboat would stop, and let him land upon one of them. He also looked down over the bows, and admired the graceful form and beautiful smoothness of the ripple, or rather wave, which was formed by the cutwater of the boat as it urged its way rapidly through the water. After gazing upon this for some time, Marco turned to go away in pursuit of Forester, when an occurrence took place, which being somewhat important in its consequences, the account of it must be deferred to the next chapter.

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