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.