could not crowd the ball down."

"Could not a _very_ strong man?" said Nathan.

"No," replied his father, "the water cannot be sensibly compressed. But
now, if the tumbler contained only air, and if a ball were to be put in
at the top, just large enough to fit the tumbler exactly, and if a
strong man were to crowd it down with all his strength, he would,
perhaps, compress the air into half the space which it occupied before."

"Perhaps the tumbler would break," said Nathan.

"Yes," replied his father, "and the tumbler will answer only for a
supposition; but for a real experiment it would be best to have a
cylinder of iron."

"What is a cylinder?" said Nathan.

"An iron vessel, shaped like a tumbler, only as large at the bottom as
it is at the top, would be a cylinder. Now, if there was a cylinder of
iron, with the inside turned perfectly true, and a brass piston fitted
to it----"

"What is a piston?" said Nathan.

"A piston," said his father, "is a sort of stopper, exactly fitted to
the inside of a cylinder, so as to slide up and down. It is made to fit
perfectly, and then it is oiled, so as to go up and down without much
friction, that is, hard rubbing. There is a sort of stem coming up from
the middle of the piston, called the piston rod, which is to draw up the
piston, and to press it down by.

"Now," continued his father, "if a strong man had a cylinder like this,
with a piston fitted to it, and a strong handle across the top of the
piston rod, perhaps he might press the air into one half the space which
it occupied before. That is, if the cylinder was full of air when he put
the piston in, perhaps he could get the piston down half way to the
bottom. Then the air would be twice as _dense_ as it was before; that
is, there would be twice as much of it in the same space as there was
before. It would be twice as compact and heavy. This is called
_condensing_ air. The philosophers have ingenious instruments for
condensing air.

"If, however, a man condenses air in this way, by crowding down a
piston, he does not begin the condensation when the piston begins to
descend. The air is condensed a great deal before he begins. All the air
around us is condensed."

"How comes it condensed?" said Rollo.

"Why, you recollect that, when you bored a hole through the board in the
bottom of your dam, the water spouted out."

"No, father," said Rollo, "we pulled the plug out; Jonas bored the
hole."

"Well," said his