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