"Can't you?" said his father. "Well, I must admit that you are rather too young to study pneumatics." "Pneumatics?" repeated Rollo. "Yes," said his father; "that is the name of this science." [Illustration: "Then it sailed slowly away."--Page 85.] "What, the science of air?" said Rollo. "Yes," said his father, "the science which treats of air, and of all other compressible and expansive fluids. But let me think. I must try to tell you something which Nathan can understand and be interested in. If I had a very light feather, I could let him perform an experiment." "Would a little down do?" said Rollo's mother. "Yes," replied his father, "that would be better than a feather." Mrs. Holiday then went and brought a little down, and handed it to Rollo's father. Now, there was a lamp upon the table, of a peculiar kind, called a study lamp. It had a glass tube, called a chimney, around the wick, and consequently around the flame itself, being round, like a ring. Rollo's father told Nathan to hold the down over the top of this glass chimney, and then to let it go. Nathan did so. The little tuft of down was wafted up into the air, quite high above the lamp, and then it sailed slowly away, and fell down upon the table. "I know what makes it rise," said Rollo. "It is the heat. The heat makes it rise." "Do you think so?" said his father. "Then take the down, and lay it gently upon the hearth, before the fire, as near as you can." Rollo did so. He had to take his hand away very quick, for it was quite hot there. The little tuft remained quietly upon the hearth where he placed it. "There," said his father, "is not that a hotter place than it was over the lamp?" "Yes, sir," said Rollo. "Then, if it was heat that made it rise, why does not it rise now?" Rollo could not tell. "I will tell you how it was," said his father. "Heat makes air more expansive. When air is heated, it swells; when it is cool, it shrinks again. Now, if it swells, it becomes lighter, and so it is buoyed up by the heavier air around it; just as wood at the bottom of the sea would be buoyed up, and would rise to the surface of the water. Now, the heat of the lamp heats the air that is in the glass chimney, and swells it. This makes it lighter; and so the air around it, which is heavier, buoys it up, and it carries up the feather with it." "No, the down, father," said Nathan. "Yes, the down," said his father. "Then it seems to me, after all," said Rollo, "that it is the heat which makes it rise." "Yes," said his father, "it does, indirectly. It expands the air; that makes it lighter; then the heavy air around it buoys it up, and, when it goes up, it carries up the down. So that it is not strictly correct to say, that the heat carries it up. The heat sets in operation a train of causes and effects, the last of which results in carrying up the feather.