Science for the School and Family, Part I. Natural Philosophy

by Worthington Hooker

305. Fur, Hair, and Feathers.—Animals that live in cold climates are provided with suitable coverings for their protection. Quadrupeds, for example, are covered with fur, and birds have an abundance of downy feathers. These coverings have no warmth in themselves, though in common language we speak of them as being warm. They are simply non-conductors, and so prevent the heat which is made in the body of the animal from escaping as fast as it otherwise would. But why are they non-conductors? It is not because the substance of which they are made is a non-conductor, but because among their numberless fibres is partially confined that great non-conductor, air. Let the fur or down be condensed into a thin hard plate upon the animal, and it would prove of little service as a protection against cold. Down is much more abundant on the birds of cold climates than on those in warmer regions, because more air can be confined among the fibres of down than among those of common feathers. Quadrupeds that are natives of warm climates generally have hair instead of fur. When therefore the horse is taken to a cold climate he requires in winter the defense of a blanket; and the ox needs under the same circumstances to be better housed than he ordinarily is. As the elephant is a native of a climate positively hot, his hairs are scanty and coarse. Formerly there were elephants in the cold regions of Siberia, as has been ascertained by remains found there. But the elephant of Siberia had under its hair, close to the skin, a fine wool to protect it against the cold. Animals that live in cold climates have their coverings become finer in fibre in the cold season of the year, to give them the additional protection which they then need. And animals with a furry covering, if they are carried into a warm climate, have their fur become coarse, and approximate the condition of hair.

306. Clothing.—Man has no covering to guard him against cold, because he is capable of contriving clothing suitable to the various degrees of temperature to which he may be exposed. The object of clothing is not to make the body warm, but to keep it so. The heat of the body is generated continually within itself, and under all circumstances this heat is maintained quite uniformly at 98°. This, you see, is a much higher degree than the atmosphere ordinarily has. We are all the time, then, giving off heat to the air around us, except when the air gets up to 98°. We are comfortable only when we are giving off heat to a considerable amount, for the point of temperature which is most agreeable to us when we are at rest is 70° or a little less, that is nearly thirty degrees below the temperature of our bodies. When the temperature is below this we need extra clothing. In making choice of clothing for various degrees of temperature we practically apply the principles which I have developed. Those articles of clothing which can confine or entangle, as we may say, the largest quantity of air among their fibres are the best non-conductors, or, in common language, are the warmest. So, too, loose clothing is warmer than tight, on account of the amount of air between the clothing and the body. Thus a loose glove is much warmer than a tight one. The same general fact is exemplified in the coatings of straw which we put upon tender trees and shrubs in winter. It is the air that is confined in the tubes of the straw which makes these coverings so effective a defense. It is probably the air in the pores of the brick which makes it a poorer conductor than stone, as illustrated by the fact stated in § 300.

307. Cocoons.—Many insects pass through their pupa or transition state in cocoons. When this is done during warm weather, as in the case of the silk-worm, the cocoon is simple. But when the pupa state lasts through the winter special provisions are made in the arrangements of the cocoon to guard the insect against the cold. I will cite as an example the cocoon of one of our largest moths, the Cecropia. This cocoon, fastened to some shrub, keeps its inmate secure from the rigors of the winter by a very beautiful arrangement. The real cocoon is similar to that of the silk-worm; but it has a very dense air-tight outer covering, and the space between these two coverings of the pupa is filled with a loose substance, which has air, of course, mingled with its fibres, and acts, therefore, the part of a blanket for the insect.

308. Buds of Plants in Winter.—In the latter part of summer buds are formed on trees and shrubs, and these contain the germs of the branches, leaves, and flowers which are to come out the next year. These of course must be guarded against the cold of winter, and it is done very much as the pupa is guarded in the cocoon. Each bud you can see has a covering of scales which is air-tight, and inside of this there is a soft downy substance, the blanketing of the bud. In these coverings, which have been called by some one the "winter-cradles" of the buds, the infant vegetation of another year rocks back and forth in the wintry winds secure from the cold, till the warm sun of spring wakes its hidden life into activity.

309. Snow a Protection to Plants.—Snow is a good blanket to the earth, keeping its warmth from escaping into the cold air. This is because it contains mingled with its feathery crystals such a quantity of air. If snow come early, before the ground and the plants in it have become frozen, it will keep them from freezing through the winter, if it remain during all that time. It is curious to observe the peculiar arrangement of the snow in the arctic regions for the preservation of vegetation. First in the autumn come soft light snows covering up the grasses, and heaths, and willows. Then as winter advances there are laid on top of these the denser snows, making a compact, stout roof over the lighter snows in which the scanty but precious vegetation of those regions is imbedded. On top of this roof are deposited the snows of spring. As these melt the water runs off from the icy roof down the slopes, leaving untouched the plants underneath, which lie there alike secure from the rush of waters and from the nightly frosts until the season is sufficiently advanced to bring them out with safety from their concealment. Then the icy roof melts, and with it the light snows that have so long encircled the plants, and the sun wakes them from their long sleep to a new life.

310. Influence of the Conduction of Heat on Sensation.—If you place your hand upon fur hanging at the door of a fur-store it does not feel as cold as the wood from which it hangs, and the wood does not feel so cold as the iron bar of the shutter close by. Why is this, when these substances are exposed to the same atmosphere, and really have the same temperature? It is because the iron conducts the heat from your hand more readily than the wood, and the wood more readily than the fur. So the iron handle of a wooden pump feels colder than the pump, and the pump colder than the snow around it. For the same reason, in a cold room the rug or the carpet will not feel as cold as the poker and the hearth. If water has stood long enough in a room to be of the same temperature with the air of the room your hand will feel colder in the water than in the air, because the water is the better conductor. So much for the sensation of cold. On the other hand, when substances are so heated as to give us the sensation of heat, the conductors do this more than the non-conductors. As they receive heat readily they also readily impart it. For this reason, with a brisk fire the hearth-stone feels very hot, while the rug before the fire does not.

311. Radiation of Heat.—Every substance sends heat into space constantly in straight lines in every direction. These lines are radii, and hence the term radiation is applied to heat diffused in this way. It is very obvious in regard to the sun that it radiates heat in all directions. The same can be seen in the case of a heated iron ball. In whatever direction you hold your hand, above, below, or laterally, you feel the heat. And it makes no difference whether the ball be red-hot or not. That is, heat is radiated either with or without light. When a room is warmed by a furnace it is warmed altogether by convection; but when it is warmed by a fire, either in a fire-place or a stove, we have both convection and radiation. The heat which we receive from the sun comes altogether by radiation.

312. Connection between Heat and Light.—The heat and light of the sun pass together through transparent substances, as air, glass, water, etc., without heating them to any extent. Thus, when the heat is transmitted through a lens, § 272, the lens is little heated, that is, it lets almost all the heat pass through it. The air is heated by the sun, but not directly to any amount. It is heated indirectly in this way: the rays of the sun passing through the air heat the earth, and then the air receives a part of this heat from the earth, which is diffused through it by convection.

It is otherwise with heat that comes from a common fire. It does not seem to be so thoroughly united with the light, and therefore readily parts company with it, as we may say. While the heat and light of the sun go together through all transparent bodies the heat of a fire will not go with its light through all of them. So while the heat of the sun does not warm the glass through which it passes the heat of a fire will warm it, and therefore glass is an effectual screen against it. In some operations in the arts a mask of glass is sometimes worn to ward off the heat. The connection of light and heat will be farther noticed when I come to treat of light.

313. Relation between Radiation and Absorption.—All surfaces that radiate will absorb also equally well the heat that is radiated upon them. All rough and dark surfaces both absorb and radiate freely; but all light-colored and polished surfaces do both slowly. For this reason the black, rough tea-kettle is well fitted to heat water in; but it is not fitted to retain the heat in the water. On the other hand, the bright, polished tea-pot absorbs heat poorly, but retains it well.