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?"

"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 whi