"But why do you say, of course? Why does not the dew--for frost is nothing but dew frozen as it forms--come upon the under side of the paper?" "How could the dew fall upon the under side?" "That is just the point which we need first of all to understand. Men commonly speak of dew as if it fell. I don't know but I have spoken of the falling of dew in this lesson. But dew does not fall at all. The vapor simply touches some cold object, and is condensed upon it. The vapor by its elasticity presses against the cold body, and the process of condensation continues until either the body is warmed by the heat set free so that its temperature rises above the dew point, or till the vapor is so far exhausted that the dew point falls below the existing temperature. Dew is formed upon the upper surface and not upon the under, because the upper surface is cool and the under surface is warmer. Beneath the paper spread over the tomato vine, the earth is radiating heat and the paper is radiating it back again. If the paper were not there, the heat would be radiated into space and not returned again. The vine would soon radiate away its little store of heat, its temperature would sink, below the dew point, and dew or frost would be deposited upon it. The under surfaces of objects are kept warm by the radiation from the earth. In the same manner clouds are wrapped around the earth and keep it warm by radiating back its radiant heat. Dew is not formed on cloudy nights, because they are warmer: the clouds throw back the heat which otherwise would be lost in open space." "I never knew before," said Peter, "that clouds were of any great use except to send down rain." "We shall see in the course of our lessons that clouds are of very great use in warming the earth in other ways, as well as by serving as blankets and radiating back the heat which otherwise would escape." "I wanted also to ask why dew falls--I mean, is formed--on grass and leaves of plants while stones are dry." "I will answer your question by asking another. Did you ever see barefoot boys running in the cold dew stop and stand upon a stone or rock to get their feet warm?" "Oh yes, sir; I have done it myself." "Why did you stand upon a rock?" "Because I had learned that the rocks would be warm." "I think that answers your question. The rocks and stones are warmer than the grass and the leaves. The blades of grass and the leaves are thin and pointed and rough, and have a very large radiating surface. They have but little heat, and that little they part with rapidly. The rocks and stones, on the other hand, are bulky, and contain a much larger store of heat, their radiating surface is comparatively small; only one side is exposed, the other being covered by the warm earth, from which they are drinking in heat almost as rapidly as they lose it. They therefore do not lose heat enough to sink their temperature to the dew point.