interesting to Rollo and to his mother;
but Nathan found it rather hard to understand them all, and he began to
be somewhat restless and uneasy. At length he said,--

"And now, father, haven't you almost done telling about the air?"

"Why, yes," said his father; "I have told you enough for this time; only
you must remember it all."

"I don't think I can remember it quite all," said Nathan.

"Well, then, remember the general principle, at any rate," said his
father, "which is this--that we live at the bottom of a vast ocean of
air, and that the lower portions of this air are pressed down by the
load of all the air above; that, being so pressed, the lower air is
condensed,--so that we live in the midst of air that is pressed down,
and condensed, by the load of all that is above it; and that,
consequently, whenever the air is taken away, even in part, from any
place, as you removed some of it from the china closet, the pressure
upon the air outside forces the air in through every opening it can
find."

"I think that is a little too much for me to remember," said Nathan.

Nathan's father and mother laughed on hearing this, though Nathan did
not know what they were laughing at. His father told him that he could
not expect him to remember all; and that, to pay him for his particular
attention, he would tell him a story.

So he took Nathan up in his lap, and told him a very curious story of a
boy, who went about the yard with a little dog upon one of his
shoulders, a cat upon the other, and a squirrel on his head. The
squirrel was tame.

QUESTIONS.

Why cannot experiments be performed upon the pressure of air, as
conveniently as upon the pressure of water? How did Rollo's
father contrive to remove a part of the air from the china
closet? Where did they expect that the air would be forced into
the closet? How were they to make this effect visible? Did the
experiment succeed? Suppose the key-hole had been stopped up;
where would the air have been forced in? Suppose all the
crevices had been closed. Is water compressible? Is air
compressible? What is the shape of a cylinder? What is a piston?
How might air be compressed by means of a cylinder and piston?
What was the general principle which Rollo's father stated, in
conclusion?

CHAPTER VI.

BALLOONING.

The next evening, Rollo and Nathan had another conversation with their
father, respecting air. When