"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."
"That would make it rise, wouldn't it?" asked Susie.
"Yes, dear."
"So, uncle," said Frank, taking up the Weather Report, "where it says
'High' here, it means that the air is heavier than where it says 'Low.'
Is that it?"
"That's right," replied Uncle Robert; "and when the barometer is low we know there will be a storm."
"Well"—and Donald stood up and stretched himself—"I wish I could see a barometer."
"You shall," said Uncle Robert "I will send for one. You may carry the letter to the post office to-morrow when you go for the mail."