One day when it was Donald's turn to go for the mail he found among
Uncle Robert's letters a small paper. On the wrapper he read "United
States Weather Report."

It had come. There was already quite a line of figures in each of their notebooks. Now they could see what this other record was like. As he left the post office he stopped to look at the old thermometer beside the door. Then he mounted Nell and rode down the village street and out into the pleasant country road.

Uncle Robert was waiting for him on the porch, and as Donald rode away to the barn, after giving him the mail, he heard him say:

"Here, Frank, is the Weather Report. Open it and look at it while I read my letters."

Donald took off the saddle and gave the horse her supper. Then he hurried back to see what Frank had found on the inside of the important-looking wrapper. It proved to be a map with queer, crooked lines all over it, but it did not look at all interesting.

"Here it says temperature," said Frank, pointing to a list of figures in the corner. "Perhaps this is what we want."

"I don't see any numbers there like mine," said Donald, taking his notebook from his pocket.

"Let me help you," said Uncle Robert, laying aside his letters and coming to where they sat on the steps.

They made room for him, and, as he took the map, he explained:

"This, you see, is a map of the United States. These dotted lines tell about the temperature. For instance, look at this one which is marked fifty degrees. At every place in the country that is touched by this line on the map the thermometer stood at fifty degrees at the time the map was made."

[Illustration: United States weather map.]

"See," said Susie, "how crooked the line is. Why isn't it straight, uncle?"

"Because," was the reply, "as I told you, it goes wherever the temperature was fifty degrees. You remember, the first day we had our thermometer, we found that there are many things which affect the temperature. At some places along this line there are prairies, at others forests, at others lakes, and here," pointing to the map, "there are high mountains. All of these things affect the temperature, and that, of course, changes the direction of the line."

"You say Chicago is the nearest station to us, uncle," said Frank, looking down the temperature column. "My record for that day is not so very different from the one given here for Chicago."

"Which shows that yours is probably as nearly correct as this is," said
Uncle Robert, with an encouraging smile.

"But I haven't one number in my book like that," said Susie, looking disappointed. "I don't see why."

[Illustration: Susie's notebook]

"I do," replied Uncle Robert. "You make your record at noon, and of course, it is warmer then. That is what your book says, does it not?"

"Yes," said Susie, "every number in my book is more than that one."

"That is right," was the reply, "for this record was made at eight o'clock in the morning, which is nearer Frank's hour than it is yours. So we would expect his to be nearer like this than yours, wouldn't we?"

"It isn't like mine either," said Donald.

"We may have one some time that will be more like yours," said Uncle Robert, "for these records are made at eight in the evening as well as in the morning."

"Uncle," said Frank, looking closely at the map, "here it says 'High,' and there it says 'Low.' What does that mean?"

"It means," said Uncle Robert, "that here there is a low barometer, and there the barometer is high."

"Barometer," said Donald. "What is a barometer, uncle? Is it like a thermometer?"

"Well, not exactly," was the reply. "With the thermometer, you know, we tell the temperature of the air, and with the barometer we tell how heavy it is."

"How heavy the air is!" exclaimed Susie. "How funny! Why, uncle, air doesn't weigh anything, does it?"

"More than you think, little girl," said Uncle Robert, smiling. "But perhaps we can prove whether it does or not. Frank, will you get a pail of water? Donald, see if you can find a cork some place; and Susie, run in and get a tumbler."

When all was ready Uncle Robert asked Frank to fill the pan with water, and Donald to put the cork into it.

[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 put over it? There was just as much air about it then, and more, too."

"Let go of the glass, uncle," said Frank, "and see what it will do."

Uncle Robert did so, and the glass instantly turned over, while a big bubble of air escaped through the water.

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

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

"Let us see," was the reply.

[Illustration: Air Pressure. Experiment No. 2.]

Filling the glass with water, he placed a piece of paper over it, and quickly turned it upside down. Not a drop of water fell from the glass. 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 glass? 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 glass 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 glass is full. Could I lift it clear out that way?"

"Try it," said Uncle Robert, smiling.

But no; when the edge of the glass 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 glass 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 glass make it do so, uncle?" asked Frank, placing the glass 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 glass, where there is nothing to keep it from rising."

"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.

"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."

"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."