We see the fact that confined air is a poor conductor of heat in a great many things. Some of them I will mention. You have sometimes seen double windows. It is the confined air between the outer and the inner windows that prevents the heat of the air in the room from spreading to the air out doors. When the window is single, the outside air cools the air in the room through the window in this way: The air in the room close to the window gives some of its heat to the glass, and, being thus cooled, it falls, and some more warm air comes to be cooled in like manner, and then falls, and so on continually. All this time the cold air on the outside keeps coming to get warmed by the glass, and as it is warmed it goes up, and more cold air comes to take its place. But all this is pretty much prevented where the windows are double, by the confined air between them.
A pear kept in snow.
There is a great deal of air in snow. This is the chief reason that snow is so apt to keep the ground from being frozen. It is the earth’s winter coat of confined air, for there is air mingled with its flakes as they are piled upon each other on the ground. Last spring I picked up a pear in my garden that was as fresh as it was when it fell upon the ground in the fall. It happened to lie in a spot where the snow lay all the winter, and was thus kept from freezing.
How furs keep in warmth.
Furs are commonly spoken of as if they had some warmth in them. This is a mistake. They are not warm of themselves. They only serve to keep in the heat that is made in the body, and they do this by the air that is mingled up with the fibres of the fur. This confined air is a poor conductor, and so the heat made in the body does not readily pass off through it into the air around. Fur is therefore to an animal, in this respect, what snow is to the ground, or what double windows are to a room; and the finer the fur is, the better does it keep the heat in, because the air is more confined among fine fibres than it is in coarse hair. And it is curious, that if an animal with thick fur is taken from the cold country where he belongs to a warm climate, and kept there, his fur gradually loses its fineness and thickness, and becomes like hair. This is because he does not need his thick, furry coat where the weather is warm.
You remember that I told you in Part First that inside of the covering with which every one of the buds on the trees is protected from the cold of winter there is a fine down. This, I told you, was the bud’s little blanket. You can understand, now, how this keeps it from being chilled by the wintry blasts. It is the air that is confined between the fibres of this downy blanket that does it.
Downy blankets of buds.
How straw protects trees from cold.
You remember, also, that I told you in Part First about tying straw around trees to protect them from the winter’s cold. Now you know that every stalk of straw is hollow, and so is full of air, and it is the air in all the stalks of the straw that makes it so good a coat for the trees. This coat protects them just in the same way, then, that an animal is protected by its furry coat, or the bud by its blanket of down.
Questions.—What is said of the spreading of heat? What is said about its going from one thing to another? How is ice melted? What is said about heat’s spreading from our bodies? Tell how fanning cools you. Why does blowing a hot thing help to cool it? Why does blowing upon cold fingers warm them? Explain what is meant when we say that some things are better conductors of heat than others. Give the illustrations. How does heat commonly spread in air? How would it be if the air could be kept still? Explain how double windows keep the heat in. What is said about snow? What about furs? Why does a fine fur keep the heat in better than a coarse one? How does taking an animal to a warm climate affect the fur? Tell about the blankets of the buds. Tell about covering trees with straw.