By passing into a warmer current of air the cloud is changed into a vapor, and may thus quickly disappear. Are clouds of different heights? Yes; some clouds touch the earth, while other clouds are high up in the air. Why are clouds higher on a fine day? Because there is not so much moisture in the air, and, therefore, it rises higher before its vapor is condensed into clouds. Why are clouds lower on a rainy day? Because there is so much moisture in the air, that it is condensed into clouds near the earth. Are the clouds high during a thunder-storm? No; during a thunder-storm the clouds are seldom more than one-third of a mile high, and very often are much nearer the surface of the earth. Is there any difference in the thickness of the clouds? Yes; some are only a few inches thick, while others are a mile or more in thickness. When the sun is about to rise, which of the seven colors of light is the first to appear? The _red_ color appears first. When the sun sets, which of the seven colors is the last to fade away? The _red_ color is the last to fade away. Why are the clouds red just before sunrise? Because they are tinged by the red ray of light, before the other rays appear. Why are the clouds red after sunset? Because they are tinged by the red ray of light, after the other rays have faded away. Dew. What is dew? Dew is the moisture that collects on plants and other objects, during the night. What causes dew upon any object? The air in contact with that object has its vapor condensed, so that the water is deposited on it in the form of dew. Does the same quantity of dew collect upon all objects? No; it collects much more upon some objects than upon others. Upon what objects does it collect the most? Upon those which radiate or give off heat the best. Why do objects that are good radiators, collect the most dew? Because they give off their heat rapidly after sunset, and before morning, become so cold that they condense the vapor in the air touching them, and cause the dew to be deposited on their surfaces. What objects are the best radiators of heat? Grass, wood, the leaves of plants, and, in fact, all things of a vegetable nature. What objects then collect the most dew? Plants collect the most dew. Of what use is the dew to plants? It supplies them with moisture, and answers in the place of rain. When do we have the heaviest dew? When the air is still and the sky is free from clouds. Why does wind prevent the deposit of dew? Because the air in contact with the grass and other objects, is removed so quickly that it does not get cool enough to deposit dew. Why are dews heaviest when the air is still? Because the air remains in contact with the grass and other objects, until it is cool enough to deposit the dew. Why is there but little dew on a cloudy night?