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?
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.