The Big Brother: A Story of Indian War

by George Cary Eggleston

"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 mass. 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 quantity 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 quantity did while the stream was still within its banks. Do you understand now?"