“Only,” said Rollo, “the snow doesn’t melt under it. It all turns to ice.”
“True,” said his father. “How thick was the hay in the place that you refer to? that is, how much was there, and how thick a covering did it make upon the ground?”
“Not very thick,” replied Rollo.
“An inch?” asked his father.
“O, yes, sir, more than an inch.” Here Rollo held his two hands at a little distance apart, in order to show his father how thick the covering of hay was. It might have been three or four inches, though it is not probable that Rollo’s recollection of it was very exact.
“Very well,” said his father; “we will call it three inches. Now, you must remember that the sun shines only upon the upper surface of the hay, and, if it is ever so good a receiver of radiation, it is only the upper surface that can be warmed by the direct rays. In order to melt the snow, the hay has not only got to receive the heat from the sun by radiation on the upper surface, but it has also got to convey it down through this space of three inches by conduction. Now a substance may be a very good radiator, and a very good receiver of radiation, while it is a very bad conductor. And this is the case exactly with hay. It receives radiation very well, and so the top gets warm under the rays of the sun; but it conducts very slowly, and so the bottom keeps cool, although the top is warm. So, you see, a single blade of hay, laid upon the snow under the rays of the sun, would make the snow melt the faster; but a layer of it three inches thick would keep it from melting. A layer of two feet thick would perhaps keep it from melting all summer.”
“Why, father,” said Rollo, “I should think it would make it warmer.”
“No,” said his father; “it would keep the warmth from getting to it, because it is a slow conductor. If any thing already warm is covered up in hay, it keeps it from getting cool; if any thing already cool is covered up, it keeps it from getting warm.”
“What things are good conductors?” asked Rollo’s mother.