Frank did so, and said, “Why, it is quite wet now.” “Yes,” said Uncle George, “it is covered all over with very small drops of water. Where did this water come from. It could not come _through_ the glass.” “It is like dew,” said Tom. “It is dew—_real dew_,” said Uncle George. “The water in the glass is much colder than the air around it. The film of air next the glass is cooled, and the ‘water-gas’ which this film of air contains is changed into water drops. “The earth is heated during the day by the sun, and the layer of air next to it becomes filled with water-gas. At night the earth gets cold. The water-gas, if the night is calm, comes out of the film of air next to the earth. It settles in the form of tiny drops on everything around. “When the earth gets very cold, the water freezes as it changes from gas to water, and instead of dew we have _frost_.” “Oh, that is why we have frost on the _inside_ of the window panes in winter,” said Frank. “That is so,” said his uncle. “The frost on your window pane is the water-gas of the warm room changed into particles of ice. But let us come back to our steam puff. We spoke about the long white streak of water-dust which the engine leaves behind it. Do you know of anything else like that outside?” “Oh yes,” said Tom, “the clouds far up in the sky are very like it.” “They are,” said Uncle George. “In fact, the clouds in the sky and the cloud behind the engine are just the same kind of thing. They are both made up of tiny particles of water. “We have learnt that the streak of cloud left by the steam-engine melts away quickly on a hot day, also that the puff is farther from the funnel on a hot day. This shows us that the warmer the air is, the more water can it take up and hold. We have also learnt that warm air is light and rises up.[2] “What happens when warm air, which holds much water-gas, rises up to the higher and colder parts of the sky?” “It gets cooled,” answered Frank. “Yes, and its water-gas gets cooled too. Then we can see it as great masses of water-dust. These masses we call _clouds_. If these masses of cloud get further cooled, the tiny water particles run together to form great drops—as they did on the cold knife. They are now too large and heavy to float in the air, so they fall to the earth as _rain_.” [Illustration: A Showery Day.] Questions and Exercises.