When the air is a little more cooled, its vapor is condensed and mist falls. When have we _rain_? When the air is still more cooled, its vapor is condensed and rain falls. When have we _snow_? When the air is cooled below thirty-two degrees, its vapor is condensed and frozen into snow. When have we _hail_? When the rain-drops, while falling, pass through a current of air cold enough to freeze them, hail is produced. Ice. What is ice? _Ice_ is frozen water. When does water freeze? Water freezes when its temperature is reduced to thirty-two degrees. Is water increased in bulk by freezing? Water is increased in bulk by freezing. Why do water-pipes often burst in cold weather? The water in them freezes, and as it increases in bulk, the pipes burst. Why are some springs never frozen? Because water, much above the freezing-point, is always flowing into them from the earth, and thus they are kept too warm to freeze. Why are some other springs frozen? Because the water flowing into them comes from near the surface of the earth, and is soon made cold enough to freeze. Does salt water freeze at the same temperature as fresh water? No; fresh water freezes at thirty-two degrees, but salt water requires a lower temperature depending upon the amount of salt it contains. How is this useful? The surface of the ocean is never frozen over except in very cold latitudes; hence, vessels may sail on it at all seasons of the year, and carry the products of one country to another; thus adding much to our comfort and our enjoyment. [Illustration: Horse.] [Illustration: Decoration.] CHAPTER VI. Sound. [Illustration: Illustrated Capital W] When the hands are brought quickly together, what is produced? They produce a shaking or vibration in the air. What is this vibration in the air called? It is called a wave in the air. What is this wave in the air like? It is like a wave in the water. When this wave in the air reaches the ear, what does it produce? It produces a sensation which is called _sound_. When a stone is dropped into the water, what does it produce? It produces waves in the water. How do these waves move? They move in every direction from the stone. Do they get smaller the farther they go? They do get smaller and smaller, until at last they disappear. Do the waves of sound in the air act in the same way as the waves in the water? They do act in the same way. What then becomes of the waves of sound? They get smaller and smaller the farther they go, until at last there is no wave left. When the wave in the air is large, what kind of sound is made? It makes a loud sound. As the wave gets smaller, does the sound grow fainter? Yes; the smaller the wave the fainter the sound, until at last both the wave and the sound die away. Where must we be to hear a loud sound?