“Very well,” rejoined his father. “I believe you understand the distinction very well. Some other time, I will tell you more about both radiation and conduction. But now I see the tea is coming in.”
Rollo looked around, and saw a girl coming in, bringing a waiter with two tea-pots upon it, and a plate of toast covered with a bright cover, and some other things. She put them all upon the table, and then said to Mr. Holiday,—
“Your tea is ready, sir.”
So Rollo and his father and mother took their places at the tea-table to eat their supper. Nothing more was said on the subject, until, just before they rose from the table, Rollo happened to think that his father had said, at first, that a stove like the one in that room warmed the room both by radiation and conduction, and he asked his father how that was.
“In the first place,” replied his father, “it warms the room by radiation just like any other fire, as you observed by holding your hand before it. But then it also warms the air in the room by conduction, in this way; the heat of the fire heats the iron of the stove and of the pipe. The heat is conducted through the iron to the outside, and there some of it radiates, and the rest is conducted into the air which happens to touch it. Exactly as if you put your hand against the stove pipe, the heat is conducted through to your hand. So it is conducted to the air, which touches the pipe. This air becomes heated, and swells, and so grows light, and rises out of the way, and more cold air comes in to the stove pipe, and gets heated in the same way by conduction. So the air is continually coming up against the stove and pipe at all parts, and it gets heated by conduction, and then rises and floats away over the room, and thus helps warm the room. But in a common fireplace all the heat which is conducted off goes back among the bricks of the chimney, and does not help warm the room. If we had a light tuft of down or cotton, and were to put it near the pipe, perhaps it would be carried up to the top of the room, by the current of air which is heated by conduction of the pipe.”
“I wish I had a tuft to try it,” said Rollo. “It would be a very good experiment.”
Rollo had no tuft, but he had that evening an opportunity to witness the effect of the ascending current of heated air, in a way which he did not anticipate; for, after tea, they had the table out, for Rollo’s father to do some writing, while his mother sat in a rocking-chair near, reading. Rollo wanted to write, and so his father let him have a pen and ink. He said that he would write a little note to his cousin Lucy. It was a very small note, on a little piece of paper, and when he had written down one side, he undertook to dry what he had written by means of the radiation from the lamp, and accordingly held up his paper very near the flame of the lamp, upon one side. The writing, however, did not seem to dry very fast, and in changing the position of it, he held it over the flame, though so high that he thought that there was no danger of burning it. But the current of heated air which rises upward from a fire, even so small a fire as the flame of a lamp, is hotter, and rises higher, than most children would suppose. At any rate, before Rollo was aware of the danger, his note was on fire, and he was obliged to turn around suddenly, and throw his paper upon the hearth, to escape burning his fingers.
QUESTIONS.
In what two ways is heat conveyed from one body to another? Relate the circumstances which led Rollo and his father and mother to spend the night at a tavern. What sort of a stove did they find in the little parlor? What is radiation of heat? How did Mr. Holiday demonstrate it to Rollo? How did he prove that heat passes swiftly when it is radiated? How did Mr. Holiday explain and illustrate conduction? By which mode is the greatest quantity of heat conveyed? Which communicates heat most rapidly? What were some of the questions which Mr. Holiday put to Rollo to ascertain whether he understood the distinction? What incident occurred to Rollo in the evening?