The next morning as they left the breakfast table Donald said:

"It's going to be warmer to-day."

"I think not," said Frank. "When I went to the barn it seemed quite cool."

"What do you think, Susie?" asked Uncle Robert.

"It was cool under the trees when I went to the spring for a pitcher of water," said Susie, "but it seemed rather warm in the sun. I think it is a lovely morning."

"What makes it warm?" asked Uncle Robert.

"Why, the sun," replied Donald, looking rather surprised at such a question.

"But does the sun make it warm in the winter?" asked Uncle Robert.

"The sun is nearer the earth in spring and summer," said Frank confidently.

"You are mistaken," said Uncle Robert. "The sun is farther from us in summer than it is in winter."

"But it's almost over our heads in summer," said Frank. "How can it be farther away?"

"The story of the warmth that the sun gives us is not told by distance," said Uncle Robert, "but by the length of the shadows at noon."

"How is that?" asked Donald.

"When is your shadow the longest?" asked Uncle Robert.

"In the evening," said Donald.

"In the morning," said Susie.

"When is your shadow the shortest?"

"At noon!" they all shouted.

"When is it coolest?"

"Morning," they replied together.

"When is it warmest?"

"Noon," said Susie quickly.

"Now you are wrong," said Frank. "It is often warmer at one or two o'clock."

"Frank is right," said Uncle Robert. "How can we tell just how warm it is at any time?"

"If we had a thermometer," said Donald, "that would tell, but we haven't."

"There's one at the post office," said Frank, "but I never saw any one look at it unless it was very cold or very hot."

"Perhaps we can find one nearer than the post office," said Uncle
Robert. "Susie, would you know one if you saw it?"

Susie shook her head.

"I would," said Donald.

"Well," said Uncle Robert, "please go to my room, and if you find a thermometer bring it to me."

Donald soon returned, and when Susie saw what he had in his hand she exclaimed:

"Is that a thermometer? I never saw anything like that at the post office."

"Well, I should think not," said Donald. "This isn't much like the old thing they have up there."

"What does it say?" asked Uncle Robert.

"Sixty-eight degrees above zero," said Frank, taking the thermometer in his hand.

[Illustration: Thermometer.]

"That isn't cold, is it, uncle?" asked Donald.

"That's just right for the house," said Uncle Robert. "How is it out of doors?"

"Let's take it out and see," said Frank.

Out on the porch they went and eagerly watched the thermometer.

"It's moving—it's going down!" cried Donald.

"I'll hang it on this nail," said Frank.

"When they looked again Donald said:

"It's fifty-six now."

"How much colder is it than it was in the house?" asked Uncle Robert.

"Twelve degrees," said Frank, counting up the column.

"Oh, let's take it in by the stove," said Susie, "and see how far it will go up."

"What makes you think it will go up by the stove?" asked Uncle Robert.

"Well," answered Susie, "if it goes down when it is cold I should think it would go up when it is warm."

Susie took the little instrument, and, going into the kitchen, held it close to the stove.

"Come," she called, "it is going up already. See!"

"How fast it moves!" said Donald. "Hold it close to the stove, Susie.
Maybe it will go to the very top."

"Let us put it in cold water," said Frank. "It won't hurt the thermometer, will it?"

"Not at all," was the reply. "Try it."

So they held it in the bucket of cold spring water.

"How fast it goes down now!" said Susie. "I wonder if it will go lower than it did out on the porch. It's down to forty-eight."

"Why does Jane set the kettle of cold water on the stove?" asked Uncle
Robert, pointing to it.

"To boil the water," answered Susie.

"What makes the water boil?"

"Why, the fire, of course."

"How long will the stove stay hot?"

"As long as there is fire in it."

"Longer than that," said Donald. "It doesn't grow cold the minute the fire is out."

"What becomes of all the heat?" asked Uncle Robert.

"Oh, it goes all round the room."

"Let's put the thermometer in the hot water," said Susie.

"Oh, see it go up!" said Donald. "It is one hundred and fifteen already."

"What is the difference in degrees between the cold and the hot water?" asked Uncle Robert.

"Sixty-seven degrees," said Frank.

"What makes the difference in degrees?"

"The difference in the heat," said Frank.

"If the water was boiling and the thermometer large enough," said Uncle
Robert, "it would go to two hundred and twelve."

"That would be ninety-seven degrees higher," said Frank.

"Wouldn't that be a big thermometer!" exclaimed Susie.

"Now put the thermometer on the floor," said Uncle Robert.

"It's seventy-two degrees now," said Donald in a few minutes.

"Let's put it on the broom," said Susie, "and hold it up to the ceiling."

"It's warmer up there," said Frank, looking at the little gray cylinder when they brought it down. "It is six degrees higher than it was on the floor."

"Why?" asked Uncle Robert.

"The heat must go up there," said Donald.

"It goes into the next room when the door is open," said Frank.

"Does it go outdoors?" asked Uncle Robert.

"Let's open the window and see," said Susie.

Frank opened the window, but, instead of feeling the warm air going out, he felt the cool air coming in.

"Uncle," asked Donald, "isn't the room full of air already?"

"Yes," answered Uncle Robert.

"Then I don't see how any more can come in at the window."

"Are you sure none goes out?"

"I could feel it coming in," said Frank.

"Jane," asked Uncle Robert, "have you a candle?"

"Here is one, sir," said Jane, taking a candlestick from beside the clock on the shelf.

Uncle Robert lighted it and held it near the window, just below the sill. The flame flickered as the air from the window struck it, and then turned straight into the room. He raised it just above the opening. Instantly the flame pointed toward the window, but it did not flicker as it had when held below the sill.

"The air must be going out up there," said Frank, "but it doesn't blow so strongly as the air coming in."

"The air that comes in is cooler than the air that goes out," said
Donald.

"What makes the water boil?" asked Uncle Robert, turning to the kettle on the stove, which had now begun to sing.

"Why, the heat, of course," said Donald.

"What raises the lid?" asked Uncle Robert.

"The kettle is too full," said Frank. "It is going to boil over."

"Why didn't the water run over when it was cold?" asked Uncle Robert.
"The kettle didn't seem full then."

"Somehow it seems to get more than full when it boils," said Donald.
"See, it is boiling over."

Just then Jane took a pan of apples out of the oven. Each one looked like a small volcano.

"What happens to the apples when they bake?" asked Uncle Robert.

"They just swell up so big their jackets won't hold them," said Donald, laughing.

"It is heat that makes the bread rise, isn't it?" asked Frank.

"Of course," said Susie. "Don't you know sometimes if the bread doesn't rise, mother says it is because it is too cold?"

"There is something besides heat that makes the bread rise," said Uncle
Robert.

"Yes," replied Susie, "the yeast; but it must be warm—I know it must."

"It seems as though everything is bigger when it is hot than when it is cold," said Frank. "And now I believe I understand something that happened not long ago."

"What was it?" asked Uncle Robert.

"Peter and I were driving to town," began Frank, "and the tire of one of the wagon wheels slipped right off. We managed to get to the blacksmith's shop, and he put the tire in the fire until it was hot. Then he put it on the wheel, but it was still loose. We couldn't have gone a step without its coming off again. He brought cold water and poured over it, and soon it was as tight as could be. I thought the water made the wood of the wheel swell up—you know water does that to the pails and tubs when they leak; but now I believe the fire made the tire larger, and then the cold water made it small again. That is just what happened."

[Illustration: The blacksmith shop.]

"But air can't grow bigger, can it?" asked Donald.

"If you can find an empty bottle, Donald," said Uncle Robert, "perhaps we can soon find out about it."

Uncle Robert took a piece of thin rubber out of his pocket and tied it tightly over the mouth of the bottle."

"By the way," he said, "is there anything in this bottle?

"No," said Susie, looking through the glass.

"Oh, yes," said Donald, "there is air in it."

"Well," replied Uncle Robert, "please get a pan of hot water, Frank."

Frank brought the water, and as Uncle Robert began to put the bottle into it they all exclaimed:

"Be careful; you'll break the bottle!"

"What will make it break?" asked Uncle Robert, pausing.

"Why, the hot water," said Susie.

"It always breaks glass if you put it in too quickly," said Donald.

[Illustration]

"Well, we'll warm it a little first," holding the bottle close to the water. "I think I can try it now."

As he spoke he lowered the bottle into the water, and the rubber tied over the neck began to bulge out.

"See!" cried Susie. "What makes it do that?"

"Try the cold now," said Uncle Robert. "Here, Donald, hold the bottle in this pail of cold water."

"The rubber is going down," said Donald in a moment. "It is going right into the bottle."

"Does the air in the bottle pull the rubber in with it?" asked Susie.

"But, Uncle Robert," said Donald, "what if wagon tires, apples, and air do swell up when they are hot? I don't see what all that has to do with the thermometer."

"I think I see," said Frank. "Why wouldn't this gray stuff in the thermometer get bigger when it's hot, if everything else does?"

"What is it that moves up and down in the thermometer?" asked Susie.

"It is mercury," answered Uncle Robert, "which is sometimes called quicksilver."

"It looks like silver," said Susie, examining it closely.

"Perhaps you can see this better," said Uncle Robert, taking a small bottle of mercury from his pocket and pouring a little into Donald's hand.

"How heavy it is!" exclaimed he, letting it roll about. "It feels just like lead."

"It is almost twice as heavy as lead," replied Uncle Robert.

"Put it in my hand, Donald," said Susie. "There, you've spilled it on the floor! Just see it run around!"

"Is it always soft like this?" asked Frank.

"No, it becomes hard when it is very, very cold."

"How cold, uncle?" asked Donald, looking at the thermometer.

"Thirty-nine or forty degrees below zero," was the reply. "In the coldest of countries alcohol thermometers are used. It must be much colder than that to freeze alcohol."

"Why is mercury used, uncle?" asked Frank.

"Because it takes a very great heat to make it boil." said Uncle Robert. "Then you have seen how quickly it shows a change of temperature. When it is warm we call it a high temperature, and when it is cold it is called a low temperature."

"That is because the mercury goes up when it is hot, and down when it is cold, isn't it?" said Donald. "I wonder how it would feel if it was forty degrees below zero. See, it is away down to there!"

"Do you remember that day last winter when Peter froze his ears driving to town?" asked Frank. "Well, it was twenty below that day at the post office. I saw it. But father is calling me; I must go."