they were all seated, he commenced as
follows:--

"I told you yesterday, that air may be compressed by force, while water
cannot be. It has another property, which is in some respects the
reverse of this. It springs back into its original bulk, when the
pressure is removed."

"How?" said Nathan; "I don't exactly understand you."

"Why, you remember what I said about the experiment with the iron
cylinder and a piston to fit it."

"Yes, sir," said Rollo.

"What was the experiment?" said his father.

"Why, if a man were to press the piston down hard, he could crowd the
air all into the lower half of the cylinder."

"Yes," replied his father. "Now, the property I am going to tell you
about this evening is this--that, if the man lets go of the piston rod,
the air that is condensed into the bottom of the cylinder, will spring
up, and force the piston up again. This property is called _elasticity_.
It is sometimes called the _expansive force_ of the air. For it is a
force tending to expand the air, that is, to swell it out into its
original dimensions. This is another great difference between air and
water.

"Now, as all the air around us," continued Rollo's father, "is pressed
down very heavily, and is condensed a great deal, it is all the time
endeavoring to expand; and it would expand, were it not that the great
burden of the air above it keeps it condensed. But water is not
compressed, and has no tendency to expand. The water of Rollo's dam, for
instance, had all the weight of the atmosphere resting upon it, but it
did not compress it at all, and so it did not tend to expand.

"And now," said his father, "I cannot perform any experiment, to show
you that air tends strongly to expand or swell out into a great space,
while water does not; but I can make a supposition, which will
illustrate it. Suppose we had a large, but very thin, glass bottle,
filled with water, and put down upon the floor in the middle of this
room. Suppose, also, that we had another bottle, of the same size and
shape, filled with air, and we put that down upon the floor by the side
of the other; both bottles being stopped very tight. Now, if we could by
any means suddenly take away all the air from the room, so that there
should be nothing around the bottles, then the bottle of water would
remain just as it is, for the glass would have nothing to support but
the weight of the water, and it would be strong enough for that. But the
bottle of a