"If the glass was longer would the water stay in it just the same?"
asked Donald.

"Yes," was the reply. "If there was no air in the glass it would have to
be very many times as long as this glass is to hold the water that would
rise if it had a chance. But come, let us sit down on the steps again,
and I will tell you about it."

When they were settled he continued:

"Over two hundred and fifty years ago there lived a man named Galileo,
who learned a great many wonderful things by studying the stars and
doing just such things as we have been doing. It was he who made the
first thermometer. But there was one question that he could not answer.
He found that in a hollow glass tube, closed at one end, water would
rise thirty-four feet high, but no higher. He could not tell why. A pupil
of his thought he would try the same thing with the heaviest liquid
known----"

"That was mercury, wasn't it, uncle?" interrupted Donald.

"Yes; he used mercury, and found that it rose in the tube just thirty
inches. He knew that the mercury was thirteen and six-tenths times as
heavy as the water, so he felt sure that it was the pressure of the air
that made them both rise in the tube, for thirty-four feet is just
thirteen and six-tenths times thirty inches. But they wanted to see if
it was really the air, so they took the tube up on a high mountain."

"What difference would that make?" asked Susie.