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.
Very soon after Jonas had driven out of the village, Mr. Holiday went on with his explanation. Rollo sat, as before, upon the front seat; but, by turning a little in his seat, he could hear what his father said very well.
“Let us see,” said his father; “where did we leave off?”
“Yes,” said his father. “We’ll take the case of the hay. Should you think, from what I told you, that hay would be a good or bad radiator?”
“Because,” replied Rollo, “it is not bright and polished.”
“Right,” said his father. “And should you think that it would be a good or a bad receiver of radiation?”
“Yes,” added his father, “because all those surfaces which are good radiators, are good receivers of radiation.”
“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?”
“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.”