it,
Sam. You always are right whether I understand how things are or not."

"Let me explain it to you, then. Do you know why some things float and
others don't?"

"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 space,
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