When Rollo awoke the next morning, he found that the storm was over, and he was very glad to see that there was a prospect of a pleasant ride home. He heard a sound as of voices in the yard, under the window of the chamber where he slept. He looked out, and saw a large yard near the stables, with a pump upon one side of it. Before the pump was a large trough, nearly full of water. At one end of the trough there was a cow drinking. Jonas was coming from one of the stables leading a horse. He was going to let him drink. There was a boy at the pump, at work pumping water, to keep the trough full while the animals were drinking.

“I mean to go down there,” said Rollo to himself.

He accordingly finished dressing himself as soon as he could, and went down into the yard.

“Let me pump,” said Rollo to the boy.

“Well,” said the boy, “and I’ll go and be putting down the hay.”

So the boy left Rollo at the pump, and went off towards the stable.

“What does he mean by putting down the hay?” said Rollo to Jonas. For Jonas had by this time come to the trough, and his horse was beginning to drink.

“You pump too fast, Rollo,” said Jonas; “you make the trough run over.”

“Well,” said Rollo, “and does that do any hurt?”

“No,” replied Jonas, “only it exhausts your strength to no purpose, and by and by you’ll be tired, and can’t pump at all.”

“Then I’ll pump slower,” said Rollo.

So Rollo began to pump more slowly. Presently the cow went away, and two oxen came, one after the other, from the barn. The boy had let them out. Rollo went on talking with Jonas, pumping very slowly indeed, so that he did not supply water as fast as the animals drank it. The surface of the water in the trough began to subside, and then Jonas called upon Rollo to pump faster.

“O dear me, Jonas!” said Rollo; “you are very hard to be suited.”

“Yes,” replied Jonas, “I am. If a boy undertakes to do any thing, I want him to do it right,—and that is hard. There are two wrong ways to do a thing, and one right way. First you pump too fast, and then too slow; whereas I want you to pump just right.”

“And how should I pump?” said Rollo.

“Why, I want you to have the pumping and the drinking exactly in equilibrium.”

“In equilibrium?” repeated Rollo.

“Yes,” replied Jonas; “that is, you must supply water from the pump just as fast as the horse and the oxen drink it, and so keep it at the same level in the trough. Thus you’ll keep the supply and the consumption in equilibrium.”

“Well,” said Rollo. So he pumped a little faster, until he got the trough nearly full, and then he noticed the point on the inside of the trough where the surface of the water stood. When he found that the water was rising above that point, he relaxed his efforts a little until it sank again; and when he found that it was falling below, he pumped harder. Thus he contrived to keep the supply very nearly equal to the consumption.

It was pretty cold that morning, and, after Rollo had pumped for some time, his hands became cold. So he went into the house. He found his father and mother in the little parlor where they had been the evening before. There was a good fire in the stove, and Rollo held his hands up before it, and warmed them by the radiation. The breakfast-table was set, and before long a girl came in bringing in the breakfast. Among the other things was a little copper tea-kettle, with a heater to set it upon, in order to keep the water hot. The heater was made in this way: There was a sort of pan of planished tin, large enough for the tea-kettle to be set into it. In the middle of this pan was a round cavity, with a bottom and sides of sheet iron, and there was a piece of cast iron, about half an inch thick, made to fit into this cavity. This piece of cast iron was to be heated in the fire, and then put into its place with the tongs, and then the tea-kettle was to be put over it, in the pan, in such a manner that the bottom of the tea-kettle rested upon the top of the heater.

Rollo was very much interested in this apparatus. It was placed upon a corner of the table, near the waiter, and Rollo’s attention was first attracted to it by observing that the tea-kettle continued to boil a little on the table. He could not think what made it boil; and, as they were about sitting down to the table, his father began to explain it to him.

“The heater, you observe,” said he to Rollo, “furnishes heat to the tea-kettle by conduction, to supply the waste by radiation.”

“What, sir?” said Rollo. He did not understand his father very well.

“Why, the tea-kettle loses its heat by radiation principally, and the heater supplies it by conduction, and so one restores what is lost by the other.”

“I don’t understand it, exactly,” said Rollo.

“Well,” said his father, “I’ll explain it to you presently, alter we have taken our seats at the table.”

Accordingly, when they were all seated at the table, Rollo asked his father about the operation of the heater and the tea-kettle.

“Well,” replied his father, “I will explain it to you. It is a very good apparatus to illustrate the subject. The tea-kettle is a radiator, and the heater is a conductor, and so they show both modes of conveying heat.

“For,” continued his father, “as the sides of the tea-kettle do not come in contact with any thing but the air, they do not conduct away the heat much; for the air does not take heat easily by conduction. But the tea-kettle radiates heat continually. If you put your hand near it, you will feel the heat passing off into the air all around.”

Here Rollo, who was seated not very far from the tea-kettle, put out his hand towards it to feel the radiation.

“The tea-kettle would conduct away the heat very fast, if there was any thing touching it all around, which would take heat easily by conduction.”

“How fast?” said Rollo.

“Why, if you were to put your hands to it, clasping it all around, the heat would be conducted very fast into your hand, and you would be burned. Your hands would receive the heat readily by conduction, but the air does not.

“And the tea-kettle,” continued his father, “does not radiate the heat very fast, because it is a bad radiator. It is made so on purpose.”

“What do you mean by a bad radiator, father?” asked Rollo.

“Why, all white, and bright, and polished surfaces radiate very little,—and all dark, and dull, and rough surfaces radiate very fast. So the bright surfaces are called bad radiators, and those that are of dark color and dull, are called good radiators. A bright tea-pot, made of metal, is a bad radiator, and that makes a good tea-pot.”

Rollo laughed.

“Its being so bright prevents its radiating the heat of the water within it very fast away, and so the water keeps hot longer. But a stove pipe, which is of dark color, and not polished, is a good radiator. They make them so on purpose. A stove pipe made of some bright metal would look a great deal better, but it would not warm the room so well.”

“But, father,” said Rollo, “this copper tea-kettle isn’t white, nor very bright.”

“No,” said his father, “it is not very bright: but the surface is polished, you see, somewhat, and this prevents its radiating much. Still it radiates more of the heat than it would if it was of silver, and polished as bright as possible. But a common iron tea-kettle, black upon the outside, would radiate heat much faster than this one.

“There is another thing for you to understand and remember,” continued his father; “and that is, that those substances which radiate heat best, also receive heat by radiation the best.”

“I don’t know exactly what you mean by receiving heat by radiation, sir,” said Rollo.

“Suppose,” said his father, “that we were to cut out a square piece of sheet iron, such as the stove pipe is made of, as large as the palm of my hand, and also a piece of silver, like that which a tea-pot is sometimes made of, and have it polished as perfectly as possible. Suppose, then, that we were to carry both of these out, and lay them down in the sun. Or suppose we hold them up before the fire, so that they would receive the radiation from the fire. Now the iron plate would receive the rays of heat more rapidly than the other, and become hot the soonest.”

“Would it, sir?” said Rollo.

“Yes,” replied his father; “or, if you were to lay them down in the sun out of doors, the effect would be the same.”

“I wish we had such pieces to try it,” said Rollo.

“Did not you ever observe how cold bright brass andirons are, long after the fire has been built in the fireplace?”

“I have,” said Rollo’s mother.

“So have I, sir,” said Rollo.

“It is because they are polished, and so are neither good to radiate nor to receive radiation. There is another way by which the same principle is illustrated in the spring of the year, when the snow is melting away. Every little leaf, or stick, or insect, that happens to light upon the snow, soon sinks down.”

“Yes, sir,” said Rollo, “the bees do. Last spring, when I went out in the pasture on the crust one morning, I found a great many bees all down at the bottom of little holes in the snow. I couldn’t think how they happened to get into those little holes.”

“The holes were made by the heat of the sun,” said his father, “after the bee fell. The bee is of dark color, and his body receives the radiation of the sun much better than the snow, which is polished.”

“The snow polished?” said Rollo.

“Yes,” said his father. “The snow is composed of flakes, and the flakes of a great number of little needle-like crystals, and all these crystals are polished, so that they do not radiate well, nor receive radiation well. Therefore the heat of the sun has not much effect upon a pure snow bank. But the body of the bee is a good radiator, and also a good receiver of radiation. So it gets warm, and that melts the snow under it, and then it sinks down, so that after a time it gets down to the bottom of a little pit, which it has made itself.”

“That’s very curious,” said Rollo.

“Yes,” said his father; “and it is so with every little stick, or sprig, or dry leaf, which happens to fall upon the snow. You could see the effect more distinctly still, if you were to put a bright piece of tin, and a piece of sheet iron, like stove pipe of the same size, upon the snow in the sun. The iron would receive the radiation, and get warm, and would sink down in the snow, but the tin would not receive the radiation.”

“Wouldn’t it receive any at all?” asked Rollo.

“Yes,” replied his father, “it would receive some, I’ve no doubt, but perhaps not more than the snow itself would on each side of it; and so the snow that was all around it would melt away as fast as that which was under it, and thus would not make any pit or depression in the snow, as the black iron would.”

“But I have observed,” said Rollo’s mother, “that, in the spring of the year, wherever there are chips or shavings upon the snow, in the yard, it does not melt away under them, as fast as it does in other places.”

“Yes, father,” said Rollo, “and hay too. Last year, there was some hay left at the post where Jonas tied the horse; and, after the snow was all gone from the rest of the yard, I raked that hay away, and I found a great deal of ice under it.”

“Yes,” said his father, “I’ve no doubt of it. That depends upon another principle. But we shall not have time to talk about that now; I will tell you about it after we get into the carryall.”

In fact, they had by this time nearly finished their breakfast; and so, after a few minutes more, they rose from the table, and Rollo went out to tell Jonas that they should be ready to go whenever the horses were harnessed. In about fifteen minutes, Jonas drove up to the door, and they got into the carryall and set out for home.

QUESTIONS.

What did Rollo observe in the yard when he looked out the window the next morning? What were the difficulties in Rollo’s pumping? What apparatus attracted Rollo’s attention at the breakfast-table? Describe its construction. How did Mr. Holiday explain the operation of it? What sort of surfaces radiate fast? What sort of surfaces radiate slowly? What examples did Mr. Holiday mention to illustrate this? Describe the appearances on the snow in the spring of the year, which were mentioned by Rollo.