ir would fly all to pieces; for that would not rest quietly,
like the water, satisfied with the space which it already has, and only
pressing with its own weight upon the sides of the glass; but it would
immediately expand with so much force as to break the thin glass all to
pieces."

"Would it!" exclaimed Rollo and Nathan together. "And would it make a
loud noise?"

"Yes," replied their father, "I presume it would make a loud explosion;
that is, if the air in the room around it could by any means be all at
once and suddenly removed.

"And so you must remember," he continued, "that there are two very
remarkable differences between air and water. Air may be condensed by
pressure, and, as it exists all around us, is greatly condensed by the
pressure of the air above, and it may be compressed more. And air is
expansive, while water is not. Whenever the pressure upon it is removed,
it suddenly expands, or spreads out in all directions."

"O dear me!" said Nathan, with a sigh.

"What is the matter?" said his father.

"Why, I can't understand it very well."

"Can't you?" said his father. "Well, I must admit that you are rather
too young to study pneumatics."

"Pneumatics?" repeated Rollo.

"Yes," said his father; "that is the name of this science."

[Illustration: "Then it sailed slowly away."--Page 85.]

"What, the science of air?" said Rollo.

"Yes," said his father, "the science which treats of air, and of all
other compressible and expansive fluids. But let me think. I must try to
tell you something which Nathan can understand and be interested in. If
I had a very light feather, I could let him perform an experiment."

"Would a little down do?" said Rollo's mother.

"Yes," replied his father, "that would be better than a feather."

Mrs. Holiday then went and brought a little down, and handed it to
Rollo's father. Now, there was a lamp upon the table, of a peculiar
kind, called a study lamp. It had a glass tube, called a chimney, around
the wick, and consequently around the flame itself, being round, like a
ring.

Rollo's father told Nathan to hold the down over the top of this glass
chimney, and then to let it go.

Nathan did so. The little tuft of down was wafted up into the air, quite
high above the lamp, and then it sailed slowly away, and fell down upon
the table.

"I know what makes it rise," said Rollo. "It is the heat. The heat makes
it rise."

"Do you think so?" said his father. "Then take