TRANSFER OF HEAT IN SPACE.

We now turn our attention,” said Mr. Wilton, “to a new theme. In the vicissitudes of day and night and of summer and winter heat is transferred in time. We now are to look at the arrangements by which heat is transferred in space. But since the transfer of heat in space requires more or less of time, the means employed are such as suffice to accomplish both objects. Heat is treasured up and carried away to distant regions, and delivered up for use as occasion demands.

“In a previous lesson the inclination of the earth’s axis was spoken of. By this means the northern hemisphere of the earth is turned somewhat toward the sun during one half of the year, and receives a correspondingly larger portion of heat, while during the other half of the year the southern hemisphere is turned toward the sun and is warmed. This inclination of the earth’s axis to the plane of its orbit gives us the change of seasons.

“The change of seasons is manifestly designed for the welfare of man. Along with the genial warmth of summer, fruits and grains and the comforts of life are carried far toward the poles, into regions which otherwise would be desolate with perpetual frost. But these extremes need to be softened; otherwise, the violence of the changes would prove destructive rather than beneficent. The severity of these annual changes of temperature is ameliorated by some of the grandest movements and arrangements upon our globe. These arrangements we have in a very imperfect way already examined.

“But there are other inequalities of temperature besides those of day and night, summer and winter. Passing from the equator toward the poles, every degree of the earth’s surface passed over causes the sun to sink one degree from the zenith toward the horizon, and gives a corresponding lower temperature, till within the polar circles for a part of the year the sun is entirely hidden and winter reigns without a rival. The temperature of the sea differs from the temperature of the land; the sun comes nearer to one hemisphere than the other, and remains longer north of the equator than south. These and many other differences upon the earth give to different parts of the world every possible variety of temperature and climate. These differences of temperature upon sea and land, from zone to zone and from hemisphere to hemisphere, are equalized or ameliorated by many agencies, but chiefly by a transfer of heat in space, a transfer of heat from place to place.

“I do not need to tell you that while we in the northern hemisphere are enjoying the warmth of summer the southern hemisphere is enduring the severities of winter, and in turn, when winter comes to us, summer smiles upon the nations that live south of the equator. You also remember that the orbit of the earth is not an exact circle, but an ellipse, that is, what is sometimes called in common language a long circle. For this reason the earth is three millions of miles nearer the sun in one part of its orbit than when in another part. Can you tell us, Peter, at what season of year the earth is nearer the sun?”

“In midwinter, or about the first of January. I have always remembered it because it seemed so strange to me, when I learned it, that the sun should be nearest the earth at the coldest season of the year.”

“Yes, one is reminded by it of the humorous argument that the sun must emit cold instead of heat, because when we are at the point of the earth’s orbit which is nearest the sun it is winter, and the higher one ascends upon mountains toward the sun, the colder he finds it. But this nearness of the sun while south of the equator would naturally give the southern hemisphere a warmer summer than the northern. For this there is a beautiful compensation. The earth passes through her orbit more rapidly when nearer the sun, and that half of her orbit is also smaller, so that, as the result of this, the sun remains north of the equator about eight days longer than in the southern hemisphere. The sun is nearer while in the southern hemisphere, but the summer is shorter. That which the southern hemisphere gains in distance it loses in time, and that which the northern loses in distance it gains in time.

“The nearness of the sun while south of the equator, the shortness of the summer, and the corresponding distance of the sun and length of the winter would tend to give the southern hemisphere great extremes of heat and cold, a short and hot summer and a long and cold winter. For this also there is a most interesting compensation in the comparative amount of land and water north and south of the equator. Much more than one-half of the dry land lies in the northern hemisphere. This would tend to give the northern hemisphere extremes of heat and cold. South of the equator there is comparatively little land and much water, which tends to give the southern hemisphere evenness of temperature. The inequalities of the earth’s orbit and the earth’s motion in its orbit we find counterbalanced by the arrangement of land and water upon the earth’s surface.

“In connection with this we may notice still another compensation in the elevation of the lands by which the burning heat of the torrid zone and the rigors of the colder zones are more or less diminished. The greater the elevation of any region of country, the cooler must be its climate. Physical geographers like Baron von Humboldt and Guyot have made calculations which show that those grand divisions of the earth which lie in the hot regions of the earth are most elevated above the sea level. South America lies higher than North America, Asia is more elevated than Europe, and Africa is more elevated than Asia. The continents rise as they approach the equator and sink toward the sea level as they come nearer the poles. As these colder lands approach the water level their valleys sink beneath the sea, their coast lines become deeply indented with bays and gulfs, and lakes abound. Thus the warmer waters of the sea are interspersed among the cooler lands, and the temperature of the lands is raised. The very elevation of the continents and the configuration of the lands have a providential relation to the temperature and climate of the world. We cannot suppose that arrangements like these, so aptly fitted to the needs of man, came by chance. In the unmeasured ages past, while this earth was in preparation for man, God had the beneficent end in view; nay, in the very beginning, the whole plan and its beautiful completion was had clearly in mind. Millions of ages ago the great Creator tenderly considered the comfort and well-being of the human race, the latest born of his creatures, in these last ages.

“As a general statement, the torrid zone receives an excess of heat, while the frigid zones receive too little, and the temperate zones, lying between, receive, at different times and places, sometimes too little and sometimes too much. The providential arrangements for equalizing temperature are, then, chiefly arrangements for conveying heat from the overheated tropical regions and scattering it over the temperate and polar regions. First among these means we will notice the trade-winds, or, as for the sake of brevity they are often called, ‘the trades.’ Will you tell us, Samuel, how winds are caused?”

“The air is heated at some place and expands; it becomes lighter and rises, while the colder air around rushes in to fill its place.”

“You use the words which are commonly employed in explaining the origin of winds, and very likely your idea is right, but the language needs a little correction. The warm air does not rise of its own accord, so to speak, but is pressed upward. The warm air is expanded; it presses outward and upward; the same weight of warm air occupies more space than cold air; the warm air rises and overtops the surrounding air, and then flows off in order to reach the common level. The column of warm air is lighter than the cooler air, and cannot balance it; consequently, the cold air sinks down, pressing the warm air upward. In this manner an ascending current of warm air is formed, and also currents of cold air flowing from every direction toward the warm centre. These currents continue until the temperature of the air is equalized.

“The atmosphere is commonly believed to be forty-five or fifty miles in height, though some men have estimated its height as very much less than this, while others believe it to be six or seven hundred miles in height. Are we to suppose that the column of heated air reaches to the top of the atmosphere?”

“I think not,” answered Mr. Hume. “The rarefaction of the lower part of the column renders the whole column lighter than the air around, and the warm air, as we know by the movements of the clouds, after rising a little way, spreads off in every direction, forming upper currents corresponding to the currents below, but moving in the opposite direction.”“Only a few days ago,” remarked Peter, “I saw in the same part of the sky clouds moving in exactly opposite directions, and others which seemed to be standing still. I knew how one layer of clouds might be moving north and another layer moving south, but I did not understand why some should be standing still.”

“Do you imagine, Peter, that the upper and lower currents of air, moving in opposite directions, come sharply together, the one sliding against the other?”

“I think not,” said Peter.

“Supposing, then, as is certainly true, that a stratum of still air lies between the upper and lower winds, does not that explain how certain clouds might be standing still while the others were moving?”

“I might have thought of that myself.”

“But how does this carry heat from the warmer region to the colder regions around?” asked Ansel. “I see how the colder air coming in would cool the warm region, and how the warm ascending air would carry away the excess of heat, but how do the cooler regions get the advantage of this heat?”

“That is just what I was on the point of explaining. Do you remember what was said about the production of cold by expansion and of heat by compression?”

“I remember that if air be rarefied by removing pressure from it, its temperature falls: I think you said that a part of its sensible heat becomes latent; and if air be compressed, its temperature rises. I have seen experiments with the air pump and condenser to prove this.”

“That principle explains the transfer of heat by winds. If the heated air rose to the upper regions, and there radiated its heat, nothing would be gained; the heat would be simply radiated into space. But as the warm air rises pressure is more and more removed from it; it expands; its sensible heat becomes latent and is thus kept from radiation; its temperature falls, but not from loss of heat. This rarefied air forms the upper current flowing away from the heated centre. In due time this air must come to the surface of the earth again. Whenever this takes place the air is brought again under pressure; it is compressed, and its latent heat becomes again sensible. Heat is thus transferred from the warmer region to the colder in a latent condition, so that it cannot be lost. We must now apply this to the trade-winds. What are the trade-winds, Mr. Hume?”

“They are regular winds blowing from a little north and south of the tropics of Cancer and Capricorn south-west and north-west toward the equator.”

“These winds are called trade-winds,” continued Mr. Wilton, “on account of their great advantage to trade or commerce. The regular and steady sweep of these winds bears the merchantmen rapidly and safely on their way. The formation of ‘the trades’ is easily explained. By the intense heat of the sun under the equator the air is greatly expanded and rarefied; the heated air rises along the whole line of the equator; from both sides the cooler air presses in, is heated, and rises; thus steady winds are formed from the tropics, or a little beyond the tropics, toward the equator. If the earth had no rotation upon its axis, these winds would blow directly toward the equator, exactly south and north. The rotation of the earth gives the trade-winds their oblique, south-west and north-west direction. Suppose that a single particle of air at the tropic of Cancer starts upon its journey toward the equator. At its starting it has the same motion eastward as the surface of the earth at that place, that is, about nine hundred and fifty miles per hour. But as it moves on southward the degrees of longitude become longer and the motion of the earth’s surface becomes more rapid, till at the equator its motion is one thousand and forty miles per hour. But the particle of air we are watching is not fastened to the earth’s surface, and as the earth moves more rapidly the nearer we come to the equator, the particle of air falls behind, that is, the air moves southward and eastward, but the earth moves eastward more rapidly than the air, so that the air falls behind and seems to be moving westward. The result is that the air upon the earth’s surface moves south-west. That which takes place with a single particle takes place with the whole body of the air, and that which takes place north of the equator takes place south of it also, producing north-west winds. On reaching the equator the winds from the north and the south meet and stop, forming the equatorial calms, and mingling together, they rise into the higher regions. In rising, the air bears away heat from the torrid zone, and this heat, rendered latent by the expansion of the air, is carried north and south by the upper currents as far as the limits of ‘the trades.’ In due time these upper currents descend and their latent becomes sensible heat, and is used in raising the temperature. Mr. Hume, can you suggest any method by which we can estimate the amount of heat which is carried north and south by the return trades?”

“I know of no method, except to estimate the amount of heat necessary to raise that flood of air which pours in from the temperate zones to the equatorial heat. That immense amount of heat must, nearly all of it, be carried away to the temperate regions.”

“This is the general explanation of the trade-winds. You must understand, however, that, in certain regions and under certain conditions, the trades are liable to interruption or change of direction. Desert regions within or near the tropics give rise to local winds which overpower the trades. In Southern Asia, while the sun is north of the equator, the land becomes so much hotter than the sea under the equator that the trade-wind is overpowered and reversed, forming a wind which blows to the north-east instead of the south-west. But this is only a beautiful flexure, so to speak, of a general arrangement for the greater advantage of a particular region. By this means the summer winds of Southern Asia come from the sea. Northern winds would have been dry. Prevailing northern winds would have made the whole of Southern Asia a desert; but the south-west monsoons come from the Indian Ocean laden with vapor, and render Southern Asia a very garden for fertility.

“The next great agency for equalizing temperature between the torrid and temperature zones is the formation and condensation of vapor. This comes in here, because it depends for its efficiency upon the agency of winds. More than once this method of conveying heat from place to place has been hinted at, but deferred till we came to the proper place to speak of winds.

“The trade-winds, passing over from a colder to a warmer climate, are constantly accumulating vapor. Under the equator the annual evaporation from the surface of the ocean is set down at fifteen feet, or half an inch daily. The formation of this vapor consumes heat which would boil more than eighty feet of ice water. The vapor thus formed is borne upward by the ascending current of heated air. On reaching the higher regions a portion of it is condensed and forms a belt of clouds around the earth. This belt of clouds along the equator is known as the ‘cloud-ring.’ This cloud-ring shields the belt of calms from the burning rays of the sun and sends down almost incessant rains. But does not that condensation which forms the cloud-ring set free latent heat, and thus intensify the great heat of the equator? Latent heat becomes sensible, but it is given out into the ascending current of air, and serves only to give it another lift till by expansion of the air it again becomes latent. The heat is simply transferred from the vapor to the air. The vapor which remains uncondensed is borne away on the wings of the return ‘trades’ to the south and to the north, and in due time is condensed and returns to the earth as rain; the heat which is given out by its condensation, wherever and whenever it is condensed, is given over as latent heat to the keeping of the air, and is passed back for use whenever the air descends to the earth.

“Vapor gathered from sea or land is everywhere exerting this equalizing influence upon temperature. Does the temperature rise in any place? Vapor is formed. Every moist body begins to give up its moisture, and the excess of heat is employed in turning this water into vapor. This is the method by which perspiration cools man or beast; whether it be insensible perspiration from the invisible pores of the skin, or perspiration standing in beady drops upon the face of the toiling laborer, vapor is formed and heat is carried away. Have you not noticed on close, muggy days when nothing dries, showing that very little vapor is forming, that perspiration seems to have no cooling effect? It oozes from the skin, but does not evaporate, and hence does not carry off the surplus heat. Animals like dogs and oxen, that do not become wet with perspiration, do not bear heat well; they soon pant and loll, attempting to get rid of the excessive heat through the moist breath and open mouth.

“The sum-total of heat transferred by this agency is too great for comprehension. Look at the Amazon rolling to the ocean a flood broad as an arm of the sea. That great river is brought from the Atlantic Ocean on the shoulders of the trade-wind. As the vapor is slowly lifted by the rise of the land from the sea level to the summits of the Andes, it is condensed, and falls as rain. Well is it for South America that the Andes were thrown up on the western coast, for the winds west of the mountains are dry as a pressed sponge, and the most of that narrow slope is barren and desolate. South America would be a desert if the Andes ran along the eastern coast. Look at the Mississippi, and the great rivers of Europe, and the matchless rivers of Southern Asia. All the rivers of the world represent only the wastage of the rain which falls upon the land after supplying the wants of the vegetable kingdom and keeping the lands moist. All this water is lifted into the air by heat, and every movement of vapor is a movement of heat. Every particle of vapor goes freighted with heat. Every cloud driven across the sky represents the transfer of heat, and every transfer is in the direction of equalization. Everywhere the tendency is to equilibrium. Nature has no processes for transferring heat from colder to warmer regions.

“We may form a conception of the amount of heat transferred by the agency of vapor by estimating the amount of heat-force required to evaporate the water which forms our rain-clouds and lift them into the upper regions. According to a calculation of Mr. Allen, late of Providence, to evaporate one-eighth of an inch of water daily from that belt of the surface of the earth lying within the tropics, and raise it five thousand feet high, requires 4,700,000,000 horse-power, or one hundred and thirty times the effective force of the whole human race, reckoning it at 250,000,000 able-bodied men. But the actual evaporation from the sea within the tropics is believed to be about half an inch daily—four times as great as Mr. Allen’s supposition.

“I see, however, that our time is nearly exhausted, and I wish before closing to revert to that more important theme upon which I spoke this forenoon. I do not know how the truths preached interested or affected you, nor do I now wish to have you tell me. I wish only to say that, as the sermon was preached at your request, I hope it proved applicable to you, and that you will give the truths presented earnest attention. Consider them well, and make your conclusions known this evening.”

The conclusion which the evening made known, you, reader, have already learned.