[Illustration: Experiment No. 1.]

"There," said Donald, as the cork floated about on the pan of water.

"But I want the cork on the bottom of the pan," said Uncle Robert, "not
on the top of the water."

"It won't stay there," declared Donald, pushing it into the water again
and again with his finger. "It is too light. Corks always float."

"How can we make it go to the bottom?"

No one could tell. The children looked puzzled.

"Let us see what this will do," and, taking the glass from Susie's hand,
Uncle Robert turned it over the cork, pressed it down into the water as
far as it would go, and held it there. Looking through the glass, they
could see the cork lying on the bottom of the pan.

"Why, Uncle Robert!" exclaimed Susie, "what--how--"

"It's the glass that does it," declared Donald.

"But the glass doesn't touch the cork," objected his uncle.

"There's air in the glass," said Frank, who had been looking at it
quietly as the others talked. "That is what presses it down."

"If it's air," said Donald, "why didn't it go down before the glass was