Because the clouds prevent radiation of heat from the earth; hence, the earth and the objects near it, remain too warm to cause any dew. Why is there a heavy dew on a clear night? Because there is nothing then to prevent free radiation of heat from the earth; hence, it soon cools so as to cause a heavy dew. How may dew be prevented from forming on any object? By placing a cover over that object. How does a cover prevent the formation of dew? It prevents the object that is covered from giving off its heat, so that it does not get cool enough to cause a deposit of dew. Why is there but little dew under trees? Because trees prevent the free radiation of heat from the objects beneath them; hence, but little dew is formed under trees. Why do stone walls _sweat_? Because the walls cool the air in contact with them, so that its vapor is condensed into water, and deposited on the walls. Why does the sweating of stones indicate rain? Because it shows that the air contains a large amount of moisture. When is a vessel of water said to sweat? When drops of water collect on the outside of the vessel, it is said to sweat. Why does a pitcher of cold water sweat? Because the vapor in the air in contact with the cold pitcher is condensed into moisture, and deposited on the pitcher. Why does the pitcher sweat most before a rain? Because the air then contains a large quantity of moisture. Why will not the pitcher sweat in fine weather? Because the air contains only a small quantity of moisture in fine weather. Why does moisture sometimes collect on a glass window in a warm room? Because the glass cools the air, and causes its moisture to collect in small drops on the glass. What makes the glass cold enough to condense the vapor in the air? The cold air, outside of the room, in contact with the glass. Where does the moisture in the air of the warm room come from? It comes from the breath of persons, and from the steam of the tea-kettle. When the breath is blown on a cold object, like a piece of glass or metal, what is the result? The moisture in the breath is condensed by the cold object, and is collected in small drops of water on its surface. Frost. When the temperature of the air is reduced below thirty-two degrees, what becomes of the moisture collected on plants and other things? The moisture is frozen. What is this frozen moisture called? It is called _frost_. What then is frost? Frost is the frozen moisture on the walls and window panes, and the frozen dew on plants and other objects. On what objects do we find the most frost? The most frost is found on objects having the most dew. What is a heavy dew when frozen called? It is called a hoar-frost. When do we have a hoar-frost? We have heavy frosts when the air is full of moisture. Why does a heavy frost indicate rain or snow? Because it shows that the air contains much moisture.