"Yes, of course. Because the things that float are lighter than the things that sink."

"Not exactly. That log there is too heavy for you to lift, while you can carry a bullet between your thumb and finger. The log is many hundred times heavier than a bullet, but the log will float while the bullet will sink always."

"That's so," said Tom, "and I don't know what does make some things float and other things sink."

"Did you ever set a teacup in the water and see it float?"

"Yes, many a time."

"But if you fill it with water it will sink, won't it?"

"Yes, of course."

"Well, now I can explain the thing to you, I think. If a thing is heavier,--the whole thing I mean, than the amount of water it displaces,--that is, if it is heavier than exactly its own bulk of water, it will sink; but if it is lighter than its own bulk of water it will float."

"Oh, yes, I see."

"Now a bullet weighs a good deal more than its own bulk of water, and so it sinks. A log weighs less than its own bulk of water, and so it floats. An empty teacup weighs less than a solid body of water equal to it in size, and it therefore floats. If you fill it with water, however, you increase its weight without adding anything to the amount of water it displaces,--or rather, as you let water into all the hollow s.p.a.ce, you lessen by that much the amount of water it must displace in sinking without taking away anything from its weight, and so it sinks; or, if you break the teacup you lessen the amount of water it must displace without lessening its weight, and so it sinks in that case, too. Do you understand that?"

"Yes, I think I do," said Tom; "but I don't exactly see how it applies to the drift-pile."

"I'll explain that presently. I want to make it plain first that the ability of a thing to float depends not on its weight, but on its weight as compared with that of a like bulk of water. This comparative or relative weight is called _specific gravity_, and in measuring the specific gravity of substances water is taken as the standard usually, though sometimes gold is used for that purpose. Now to come to the drift-pile. When the water rises say two or three feet, it will be above the level of the lower logs, and these would float away, if they were free, because their specific gravity is less than that of water. But there is twenty feet of other timber on top of them, and its weight must be added to theirs. The water displaced is exactly equal to their bulk, while the weight is many hundred times greater than theirs. Do you understand?"

"Yes, I think I do. You mean that the water must come high enough to pretty nearly cover the whole drift-pile before any of it can float."

"Yes. The pile must be considered as a whole, and it won't float until there is water enough to float the whole. The bottom logs can't float while those above them are clear out of water, if their weight rests on the bottom logs, as it does in the drift-pile. You see when you put anything into the water, it sinks until it has displaced a bulk of water equal to its own weight, and then stops sinking. In other words, that part of the floating thing which goes under the water is exactly the size of a body of water equal in weight to the whole thing. If a log floats with just half of itself above water, you know that the log weighs exactly the same as half its own bulk of water, or, in other words, that its specific gravity is just half that of water. Water two inches deep won't float a great saw-log, because a great saw-log weighs more than the amount of water it takes to cover its lower part two or three inches deep; and water two or three feet deep won't float a drift-pile twenty feet high, because such a drift-pile weighs a good deal more than a body of water two or three feet deep, of its own length and width. But even if the water were to rise to the top of the hammock, the pile wouldn't float away. It would float, of course, and some of the wood near its edges would be carried away, but the main pile would remain here, because it is all tangled together and can't go away except in one great ma.s.s. It is so firmly lodged against the trees as to prevent that, and as a freshet big enough to cover, or nearly cover it, would bring down a great quant.i.ty of new drift and deposit it here, the pile would grow bigger rather than smaller. But the river won't get very high at this season, or at any rate it won't rise to anywhere near the top of the hammock, as I have already explained to you, because it is evidently only the biggest freshets that ever come near the top, and the biggest freshets never come in the fall, but always in the spring. It isn't rising fast enough either. It isn't rising nearly so fast now as it was before it got over the bank."

"Why, how do you know that, Sam? You haven't been to look."

"No, but I know it, nevertheless, simply because I know that water, left to itself, will find its level."

"I don't see what that's got to do with it," said Tom.

"Perhaps not, but it has something to do with it for all that," replied Sam; "and I can make you see how, too."

He paused, to think the matter over and determine how to present it to Tom's comprehension.

"You see," he then resumed, "that the river inside its banks is about four hundred yards wide. When it rises above the banks, however, it spreads out over the level ground, and becomes, in some places, many miles wide, averaging a mile at least in width. Now there is only a certain amount of water coming into the river every hour. The rain has stopped, but the soil is full of water, and so there is about as much running into the river now as there was while the rain lasted. But the surface of the stream is now many times greater than it was, and as water finds its level, all that comes into the river spreads out over its whole surface, and of course doesn't raise its level nearly so much as the same quant.i.ty did while the stream was still within its banks. Do you understand now?"

"What a great big brother you are, Sam, anyhow!" was all the reply Tom made.

CHAPTER XVI.

WHERE IS JOE?

It was now getting late, and Sam knew that it was not well for him to talk longer. He felt so much better, however, that he knew he would continue to talk in spite of himself unless the whole party should go to sleep at once. Joe had not been in the drift cavern for more than two hours, and Sam, observing his prolonged absence, said:

"Tom, I'm afraid some of us have hurt poor Joe's feelings. Go and look at your water-mark, and while you are out, find the poor fellow and find out what's the matter with him. He's a good boy, and has done his part faithfully ever since we started. I can't bear to think of him moping."

Tom went out and examined his stake, which showed that the water was not more than an inch or two over the bank, and was not rising very rapidly now; but he could see nothing of Joe anywhere. He went to the look-out, but the boy was not there, and a diligent search through the drift-pile, showed that he was nowhere in the neighborhood of the fortress. Tom was now fairly alarmed, and returning, was about to report the facts to Sam, when little Judie, in a whisper, informed him that the big brother was asleep. As his fever had risen somewhat, Judie rightly thought it better not to disturb him, as he certainly could not aid in any way in finding Joe.

"I must just think," Tom said to himself, "as Sam does, and then I can do all there is to be done. Now I know Joe isn't anywhere in the hammock, because I knew every place he could squeeze himself into, and I've looked in every one of them. It's no use then to waste time looking there any more. He must have left here, either accidentally or on purpose. He couldn't have slipped off the drift and drowned, because he can swim pretty well and would have swam out in a minute. There is no other way in which he can have left here by accident, unless an Indian has killed him on the drift-pile somewhere, and if that were so I would have found his body. He must have run away on purpose."

But just as Tom reached this point in his thinking he remembered the earnestness with which poor Joe had begged him to bear witness in any and every event that he was not "a runaway n.i.g.g.e.r." And this reminded Tom of all the queer ways he had noticed in Joe of late. The boy must have had a premonition, he thought, that something was going to happen to him. Only two theories remained. One was that Joe had gone crazy under his long exile from civilized life and had madly put an end to himself by jumping into the river; and the other that, persisting in his belief in the instability of the drift-pile, he had gone to the upper bank for safety and had fallen asleep there. In that event he must be found, lest an Indian should discover him in the morning and put him to death. Tom went ash.o.r.e after explaining his purpose to Judie, so that she might not be alarmed at his absence, and literally spent the entire night in hunting for the black boy. Joe was nowhere to be found, and when daylight came, Tom saw that a further search was of no use whatever, and he therefore returned sadly to the drift cavern. The water was now going down again, and the bank was free from it, but the sand in the root fortress was still too wet to sit or lie upon, and so Tom made no immediate preparation for their return.

Sam's fever was very slight that morning, and his first question was about Joe. Tom told him of his night's search, and Sam's deduction from all the facts was that the poor boy had committed suicide, had been killed by an Indian and thrown into the river, or had fallen in accidentally and drowned.

"He would never have left us in any case," said Sam, "and even had he been less faithful, he would have been afraid to run away, not knowing where to run or how to take care of himself in the woods."

They were too much grieved for Joe's loss, to relish their breakfast, and that meal was dispatched very quickly. Tom watched the falling of the water all day, and at night reported that the river was well inside its banks again.

CHAPTER XVII.

A FAMINE.

The river having gone down until no water remained on the sandy bank, Tom reported the fact and added,

"Now let's move back again to the root-fortress. It's a safer place than this, by a good deal, if it isn't quite so big or quite so comfortable."

"No, we mustn't go back yet," said Judie, who had visited the fortress before Tom had, "because the sand in there is as wet as can be, and I can't let my big sick brother lie on it."

"There, Tom," said Sam, "my doctor forbids my return yet awhile, and a sick man always must obey the doctor you know. Besides, Judie is right.

It won't do for any of us to lie on wet sand; we must wait till it dries; but that won't be very long if the river continues to go down."

Accordingly they spent one more night in the drift cavern. Early the next morning Judie went to the fortress, and returning said, playing doctor.

"Now, then, Mr. Hardwicke, the floor of your lower house is quite dry, and I think it will be safe to move back again. Will you have your breakfast first, or will you wait until you get back home again before eating anything?"

"Oh, let's wait, by all means, and eat breakfast in the dear old root-fortress," said Tom, and as Sam made no objection, it was so arranged.

By nine o'clock the moss carpet was laid in the root-fortress and the little party was back in its old quarters again. The vacant corner which had been Joe's, reminded them sadly of his disappearance. Poor fellow!

they had learned to love him almost as a brother, and they could not think of him now without tears. When three people sit down with a silent grief, their conversation is very apt to be lively, or, if they cannot quite accomplish that, they are sure to talk only of indifferent matters, and so it was in the present case. Judie was the first to break the silence which had fallen upon all.

"Tom," she said playfully, "I'm afraid you're not a good provider. Here we are, hungry as wolves, and you haven't brought us a mite of anything to eat. You've moved everything but the provisions, and you've forgotten them entirely."

Master Tom admitted the grievousness of his fault and returned at once to the drift cavern after the forgotten provision pack. The bread, as they all knew, was long ago exhausted, but plenty of meat remained, and this Tom presently brought. When he opened the pack a disagreeable odor spread itself at once over the little room.

"Phew! what's that?" said Tom, and putting his nose to the meat, he looked up in blank consternation, saying:

"The meat is spoiled, Sam! What on earth shall we do?"