"Look at the woodpile out there," said her uncle. "Where do you think
the weight of the wood would be the greater? On the ground or halfway to
the top?"

"On the ground, of course," answered Susie.

"Well, they found it was the same with the air. As they went up the
mountain the mercury in the tube fell."

"That showed that the weight on it was less, didn't it, uncle?" said
Frank. "I think that was a very wonderful discovery, don't you?"

"It was, indeed," replied Uncle Robert, "and that is how the first
barometer was made."

[Illustration: Barometer.]

"Is that what a barometer is?" asked Donald.

"Yes," was the reply, "simply a glass tube about thirty-three or
thirty-four inches long, closed at the top, and filled with mercury. It
is then placed in a small open cup, called the cistern, into which the
mercury flows until the air pressing on it there will let it fall no
farther."

"Does it always stay at the same height in the tube?" asked Donald.

"Oh, no," his uncle answered. "Some days the air is heavier than others,
and so presses harder on the mercury."