"Now, children, I am going to give you some writing to do, about the air."
"Writing?" said Rollo.
"Yes," said his mother. "I am going to explain to you something about the air, and then you must write down what I tell you."
"But I can't write," said Nathan.
"No," said his mother, "but you can tell Rollo what you would wish to say, and he will write it for you."
"Why, mother," said Rollo, "I don't think that that will be very good play."
"No," replied his mother, "I don't give it to you for play. It will be quite hard work. I hope you will take hold of it energetically, and do it well.
"First," said she, "I am going to perform some experiments for you, before I tell you what I want you to write."
By this time, she had cut off the downy part of several feathers, and had laid them together in a little heap. Then she took a fine thread, and tied this little tuft of down to the end of it. Then she took up the thread by the other end, and handed it to Rollo.
"There, Rollo," said she. "Now, do you remember what your father told you, the other day, about the effect of heat upon air?"