"There," said Frank, smiling, "I told you so!"

"Then air only presses on things when there is something like the gla.s.s to hold it down. Is that so, uncle?" asked Donald.

"Let us see," was the reply.

[Ill.u.s.tration: Air Pressure. Experiment No. 2.]

Filling the gla.s.s with water, he placed a piece of paper over it, and quickly turned it upside down. Not a drop of water fell from the gla.s.s.

The paper, now beneath the water, stayed there as though glued.

"Uncle," said Frank, "is it truly the air that holds the paper on and keeps the water in the gla.s.s? If it presses that way everywhere, why don't we feel it?"

"It is because it presses equally in every direction," replied Uncle Robert. "Put your hand in this pail of water. Do you feel it pressing on your hand?"

"No," said Frank.

"Place it lower in the water. Does it feel any heavier now?"

"Not at all," answered Frank.

"But you know that the water is heavy. Lift the pail, Donald."

"It is heavy," said Donald, setting it down. "I don't see why Frank didn't feel a little of the weight of it when his hand was under all the water."

"It is this way," explained Uncle Robert. "The water pressed on his hand from below as much as from above, and the same on both sides. When you lifted it you felt its weight pressing downward only. Now it is just so with the air. It presses with such equal pressure that we do not realize its weight. It is only when it presses harder from one direction than from another that we feel it."

"That's when the wind blows, isn't it, uncle?" asked Donald.

"Yes, my boy," was the reply. "You can see how it is out among the trees now."

"But, uncle," said Donald, "how can the air be weighed if it presses the same in all directions? It was only when I lifted the whole pail of water that I felt how heavy it was. The air can't be weighed if it presses up just as much as it does down."

"But if in some way it could be shut off so that it would only press in one direction?"

"It might be," answered Donald, "but I don't see how."

Uncle Robert told Susie to put the gla.s.s in the water so that it would all be below the surface, and, without taking it from the water, to turn it upside down. She did so, and then began to lift it slowly out of the water.

"See," cried Susie, "the water comes with it. The gla.s.s is full. Could I lift it clear out that way?"

"Try it," said Uncle Robert, smiling.

But no; when the edge of the gla.s.s came out of the water in the pail, down went the water with a splash.

"I see how it is," said Frank, who had watched it closely. "There wasn't any air in the gla.s.s to keep the water out, as there was when we turned it over the cork, so the water stayed in it."

"But what made it come up out of the pail?" asked Donald. "There wasn't any air under it to press it up."

"Would the air pressing on the water around the gla.s.s make it do so, uncle?" asked Frank, placing the gla.s.s in the water and raising it as Susie had done. "It seems as if it might be that."

"That is what it is," replied his uncle. "The air pressing on the water in the pail forces it into the gla.s.s, where there is nothing to keep it from rising."

"If the gla.s.s was longer would the water stay in it just the same?"

asked Donald.

"Yes," was the reply. "If there was no air in the gla.s.s it would have to be very many times as long as this gla.s.s 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 gla.s.s 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.

"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."

[Ill.u.s.tration: Barometer.]

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

"Yes," was the reply, "simply a gla.s.s 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."