Now the clear, transparent air permits heat to be shot off, or rayed through it with great freedom. But it does not readily receive heat “by conveyance,” so long as it is still. If you put your hand into still air which is as cold as a cold metal knob, you do not know that the air is so chill as the metal, because it does not make your hand so cold. The heat is not conveyed away from your hand as quickly. When air is moving, instead of being still, the case is, however, altogether altered. A current of air, or wind, carries away heat from warm bodies very quickly as it blows over them. It does so because each fresh little particle of air which is pressed against them, receives its own share of the heat, and conveys it away, leaving fresh particles to come up in their turn, and do the same thing. A pint of boiling water in a metal pot placed in a strong wind having fifty degrees of heat, would lose all its excess of heat as soon again as it would if standing in still air having the same warmth. The old plan of cooling hot tea or broth by blowing it, is correct in principle, though not in accordance with good taste.
The laboratory of the living animal body has the supply of its fuel, and the capacity of its air-blasts, so arranged that just about as much heat is supplied through its internal furnace, as is lost from its surface by “raying off” and “conveyance,” when the surrounding air has a warmth of sixty degrees of the heat scale, and when its surface is somewhat protected by a light covering of clothing, to lessen the rapidity with which the heat is shot off and conveyed away. The heat is then produced as rapidly in the internal furnace, as it is thrown off from the outer surface, and the consequence is that the animal feels comfortably warm. It only feels uncomfortably hot, when more heat is produced in the furnace of the living laboratory than can be scattered through its surface. And it only feels uncomfortably cold when more heat is scattered from the surface than can be kept up through the burning of the inner furnace.
But in winter time the cold external air carries away heat much more quickly from the surface of living animals, than the warmer external air does in summer time. Here, then, is a little difficulty to be met, if the warmth of the body is to be kept precisely the same in both seasons. It is requisite that it should be always maintained at the same point, because that point is the one which is most suitable for the operations which are being carried on in the vessels and chambers of its laboratory. Nature has two distinct ways in which she insures this end.
In the first place, are you not aware that you get more hungry in winter than you do in summer time? All living animals have pretty much the same experience as yourself in this particular, and the reason is that nature intends, during the cold season, to have more fuel introduced into the supply-pipes of the body for the warming of its structures. The furnace of the laboratory gets quickened in a small degree; its slow fires are fanned into slightly increased activity, more fuel is burned, and so more heat is generated to meet the greater demand for it, dependent on the influence of the external cold.
But nature also thickens the clothing of animals during the cold season, and so affords increased obstruction, through which the escaping heat has to force its way. Have you not observed the sleek silky coat which the horse wears through the summer, and then noticed at the beginning of winter how this sleek coat is exchanged for a thick, fuzzy shag, that looks more like wool than hair? The warm winter coat economizes the heat produced in the furnace of the living body, and keeps it from being scattered to waste as quickly as it is through the sleek summer coat. This is nature’s other plan of meeting the difficulty brought about by the changing temperature of the air. Nearly all animals belonging to temperate and cold climates have this change of apparel provided for them in spring and autumn, but in some cases the change is rendered very striking in consequence of a summer garment of bright gay colors being replaced by a winter one of pure and spotless white. The fierce tyrant of the ice land himself, the polar bear, has a dingy yellow coat during the summer, but puts on furs as snowy as his own realms when once the summer sun has disappeared. These white winter furs are always warmer than dark ones. Birds which do not migrate to warmer regions of the earth in the cold season, have winter and summer suits of apparel, just in the same way as quadrupeds. In the winter a lining of the thick, soft white down is added beneath the outer feathers. There is one little bird which comes to England in the late autumn, driven there by the still greater cold further north, and which is familiarly known as having two remarkably different costumes for his English and his foreign residence. In England the snow-bunting appears with a white body and tail, but abroad and in summer time he is distinguished by a brilliant black tail and back, and a body and head of pure white.
Man follows the example which nature has set before him, in the matter of clothing. He prepares himself stout warm garments for winter time, and thin cool ones for the summer; and not only this; in the hottest regions of the earth, where there is most sunshine, he commonly goes nearly naked, while in the coldest regions, near the poles, he puts on the heaviest and warmest woolens and furs that he can procure. Now this is one reason why man has been apparently so uncared for by nature in the particular of clothing. The seeming indifference and carelessness is really consideration of the highest kind. All the different races of the lower animals have their own narrow tracts assigned them for their residence. In these tracts there is no very extreme diversity of temperature, and provision is therefore easily made to adapt their clothing to it just so far as is required. The human race, on the other hand, is intended to cover the entire earth, and to subdue it; to spread itself from the burning tropics to the frigid poles. The heat which has to be borne in the tropics, is as much greater than that which is experienced near the poles in winter time, as boiling water is hotter than ice. At the poles, one hundred degrees of frost often occur. In India, there are occasionally one hundred and thirty degrees of heat under the canvas of tents. It therefore becomes an affair of almost absolute necessity, that the skin of the widely scattered lords of creation should be as unencumbered as possible, and that warm clothing should have to be prepared and added as a covering whenever circumstances call for its use. The head only, of the human being, has a natural fur garment. This part of the body is covered with hair, because the most delicate portion of the entire frame, the brain, is contained within it. The skull is protected by hair, that the brain may not be hurt by too sudden a change from cold to heat, or from heat to cold.
There is another advantage attending upon the arrangement which has left human beings dependent upon an artificial supply of clothing, and which has ordained that they shall come into the world with naked skins. In consequence of this arrangement it is very easy to secure that amount of cleanliness which is necessary for the preservation of the health of such delicately framed creatures. The artificial clothes can be altogether changed at will, and they can be washed and aired, as they never could be if they were inseparably attached to the skin. Then, too, they can be removed from the skin in the early morning, or at convenient intervals, and its surface can be thoroughly cleansed and purified by bathing with water. Just think of the difference of going into a bath of refreshing water unencumbered by clothes, and of doing the same with thick, dabby garments clinging about you, and having to shake yourselves like great Newfoundland dogs when you come out; and also recall to mind the pleasure you experience every time you change soiled linen for clean, and you will become sensible of how much you owe to beneficent nature for having left you destitute of the feathers of the bird, or the fur of the bear. The extreme importance of making a fair use of this privilege has been already alluded to in its proper place.
But nature has effected yet another very bountiful provision for the comfort and safety of her tender charge, the living human animal. Even when only covered by very light clothing, it is possible human beings may be placed in air which is so warm, that heat is not carried off from their bodies so fast as it is produced in the interior furnace. In India, it sometimes happens that the air gets to be even hotter than the living body. All movement of the air, then, heats, rather than cools. Under such circumstances, nature adopts a very effectual course to prevent warmth from collecting more and more in the frame, until a disagreeable and injurious amount has been reached. Having first reduced the supply of fuel to the smallest limits consistent with keeping the fire going, by lessening the appetite, and by taking away the craving for heating food, and having given a hint to adopt such outer coverings for the body as are as little obstructive of the passage of heat as possible, the heat drenches the surface of the frame abundantly with moisture, which has the power to cool by its ready evaporation. Take a small piece of wet linen and lay it upon your forehead, or upon your arm, leaving it freely exposed to the air, and you will find, that as the moisture evaporates from the linen, your skin underneath will feel colder and colder. The heat of the skin is used up in converting the moisture of the linen into steam, exactly as the heat of a fire is used up in converting the water of a kettle into steam when this is made to boil. The steam flies away with the warmth of the skin very rapidly, and consequently the skin soon comes to feel cold. Now, when the body gets to be very warm, and the over-heated blood is rapidly pouring through the channels of its supply pipes, then the three millions of little holes or pores, which lie upon its surface, are opened, and floods of vapor and water are poured through them, producing just the same kind of effect as wet linen would do. This action is termed “perspiration,” or a “steaming through” the pores of the skin. The breathing blows up, or fans the slow furnace contained within the living animal frame, and so heats it above the surrounding air. The perspiration carries away portions of this heat when it has been raised too high, and so cools the heated body down. Some moisture also escapes as steam from the lungs and through the mouth in breathing, thus assisting the perspiring skin in its office of diminishing the excessive warmth of the body. You have often seen dogs, which have been heated by running, pant with opened mouths and outstretched tongues, the vapor steaming forth from their gaping throats. Dogs cool themselves in this way because they have very little perspiration passing through their skins. Their perspiration is really from their throats, rather than from their skins. Human beings sometimes lose, in hot weather, as much as five pints of water in twenty-four hours, by exhalation through the lungs and skin.
Give me now, good reader, your close attention for just a few minutes while I return to the notion with which we started on beginning the consideration of this subject, so that I may fit it into its right place, and leave it well packed away with the other notions that we have gained, while studying the value and uses of air, water, and food. Your body is a living laboratory, formed of an enormous quantity of little chambers and vessels. From a strong central force-pump, placed in the middle of that laboratory, liquefied food, or blood, is streamed out through branching supply-pipes to the several chambers, to carry to them the materials that have to be operated upon in their cavities for the production of animal power and warmth. The force-pump acts by repeated short strokes, but the liquefied food flows through chambers of the laboratory in continuous, even currents, because the supply-pipes are made of yielding and elastic substance, like India-rubber, and not of hard, stiff substance, like metal or wood. As the liquefied food gushes out from the force-pump, the elastic walls of the supply-pipes are stretched by the gush, but directly afterwards they shrink back again, as India-rubber would do, shut close a valve that prevents all return of the liquid into the force-pump, and so compel the liquid to run onwards in the other direction, through the pipes. Before the shrinking in of the pipes has altogether ended, the force-pump renews its stroke, and so the onward flow of the liquid never stays, although the pump has to make beat after beat. The liquefied food gushes out from the force-pump with a speed of about a foot in each second; but it has to supply such an enormous host of small chambers in the remote parts of the laboratory, that it does not flow through them with a speed greater than an inch in a minute. This, however, is no disadvantage, as it affords plenty of time for the full carrying out of all the intended changes in those chambers, whereby animal power and warmth are to be produced.
Remember, then, that as your heart beats in your chest, second after second, the red blood flushes through every crevice and every fibre of your living frame, just as it does through your cheek when it is crimsoned with a blush. Seventy or eighty times every minute, your beating heart pumps, and constantly, so long as you are alive, the flushing blood streams on everywhere. The blood, however, streams on in this continuous way, because its flow is not stopped, even when it has reached the remotest chambers and fibers. The trunks of the supply-pipes divide into branching twigs, which get very fine indeed where they are in connection with the working chambers of the laboratory, and which then lead on into return-pipes, that are gathered together into enlarging trunks, These, in their turn, are collected into main tubes which end in the cavity of the heart. At the extremity of these main trunks of the return-pipes, valves are so placed as to prevent the pumping action of the heart from forcing the blood back into them. Thus, as your heart pumps, swelling out and drawing in its walls, the blood flows into its cavity by the return-pipes, and is squeezed out therefrom through the supply-pipes. It always streams in one direction. It circulates through the living frame which it flushes; that is, it goes in an endless circle, now through the heart, now through the supply-pipes, now through the return-pipes, and now starts once again through the heart.