Are heavy frosts usually followed by rain or snow? Yes; a heavy frost, on two successive mornings, is mostly followed by rain or snow within a few hours. Rain, Snow, and Hail. What is a _mist_? A mist is very small drops of water falling through the air. When these drops are larger, what is it called? It is called _rain_. What causes rain? Rain is caused by the sudden cooling of the air above the earth. How does cooling the air cause rain? The cold condenses the vapor in the air, so that its moisture is forced out in drops, which fall to the earth. What becomes of the rain falling upon the earth? A part of it remains on the surface of the earth, and is either absorbed again by the air, or is carried away to the ocean by the creeks and rivers. What becomes of the remaining part? It sinks into the earth, to furnish moisture to the roots of plants, and to supply the streams beneath the earth’s surface with water. How does rain purify the air? By washing away many things that would, if left, decay and make the air impure and unhealthy. When the air is cooled below thirty-two degrees, what becomes of the moisture in it? The moisture in it is frozen. What is this frozen moisture in the air called? It is called _snow_. What are the particles of snow called? They are called snow-flakes. Are these snow-flakes all alike? No; they vary very much in size and shape. What does Fig. 23 represent? It represents a few of the various and beautiful forms of the snow-flake or snow-crystals. [Illustration: Fig. 23.] At what temperature of the air does snow usually fall? At a temperature of about thirty-two degrees. Is there any reason for the remark that it is “_too cold to snow_”? Yes; for it seldom snows when the temperature of the air is much below thirty-two degrees. Of what use is snow? The snow is a warm covering for the earth. Why is snow a warm covering for the earth? Because the air in the snow makes it a poor conductor of heat, so that it prevents the warmth of the earth from passing off into the air. Snow is, therefore, a protection to the plants, and prevents their being frozen by the cold weather of winter. What is hail? Drops of rain, when frozen, are called _hail_. How is hail formed? The rain-drops are frozen as they fall through the air; hence, the hail-stones of winter are no larger than the rain-drops. When have we _dew_? When the air in contact with the earth or the plants is cooled, its vapor is condensed into dew. When have we _frost_? When the air in contact with the earth or the plants is cooled below thirty-two degrees, its vapor is condensed into dew, and the dew is frozen into frost. When have we _fog_? When the air near the earth is slightly cooled, its vapor is condensed into fog. When have we _clouds_? When the air higher above the earth is slightly cooled, its vapor is condensed into clouds. When have we _mist_?