PRESERVATION AND DISTRIBUTION OF HEAT.
Another Lord's Day comes, and the members of the class are, as usual, all
in their places. They find the subject increasing in interest after
leaving the review of the laws and principles of heat.
"A week ago," said Mr. Wilton, "we looked at the chief sources of heat.
These are the sun, the internal heat of the earth, chemical action, in
which combustion is most important, electrical action, and mechanical
action, or 'living force.' The amount of heat furnished from these sources
is above all comprehension. The Creator seems bountiful even to
prodigality in supplying heat for the needs of the world and the uses of
man. But with all this largeness of supply the provision would prove
wholly inadequate if it were not prudently husbanded and all the avenues
of waste carefully closed. Men of ample incomes sometimes come to want
from too free expenditure. Their incomes are large, but their expenses are
larger. So it would prove in respect to heat if Nature were not as prudent
in saving as she is bountiful in providing. Will some one mention some of
the general methods by which the waste of heat is prevented?"
No one answered. Mr. Hume did not think it best to put himself forward in
answering questions, and therefore answered only when personally
addressed. The others were silent because they had nothing to say.
"I see that I shall have to suggest the answer. Ansel, what part of the
atmosphere is warmest?"
"The bottom, I suppose, for the higher a man goes up upon the lofty
mountains or in a balloon, the colder he finds the air."
"That is right; and we need to ascend only about three miles, even in the
tropics, to reach the region of perpetual snow, while in the polar regions
the line of perpetual freezing comes down to the sea level. What would be
the effect, Ansel, if the atmosphere were as warm, or warmer, at the top
than at the surface of the earth? How would that affect the rate of
radiation from the earth?"
"It must, of course, increase the radiation very much. With the
temperature twenty or fifty or seventy degrees below zero, the radiation
must be very little."
"By some means, then, the atmosphere is kept warm at the surface of the
earth and cold in the higher regions, and in this manner the radiation of
heat into open space is prevented. This is accomplished notwithstanding
that the top of the atmosphere is nearer the chief source of heat, the
sun. This would be no very easy problem if its solution were left to human
ingenuity. The explanation is very simple, however, when once suggested.
The atmosphere is diathermic, that is, it permits the luminous heat from
the sun to pass directly through it without heating the air, but the solid
earth stops the heat by absorption, and is warmed by it. The warm surface
of the earth imparts, in turn, its heat to the atmosphere resting upon it.
This warm air, being expanded by the heat received, becomes lighter than
the cold air around, and rises, or rather is forced, upward by the
greater weight of the colder air. But as it rises and the pressure of the
air is diminished it expands still further. By this expansion its sensible
heat becomes latent, that is, the heat is transmuted into force, and, as
force, is incapable of being radiated. In this manner radiation from the
upper surface of the atmosphere is greatly hindered and waste of heat is
in a good degree prevented.
"In respect to this heating of the atmosphere from the surface of the
earth, a layer of clouds sometimes forms a kind of second surface which
receives the sun's rays and warms the air above. A few years ago I saw a
balloon ascension in Providence, R. I. The day was bleak and chilly, and
the sky entirely covered with clouds. The aeronauts were expecting a
chilly voyage. The balloon shot like an arrow toward the zenith, and in
five minutes was completely hidden by the clouds. But to the surprise of
the voyagers of the sky, on passing through the clouds their thermometer
rose ten degrees. This, doubtless, must very often be the case. The air
above the clouds must often be warmer than that below.
"I think you all must have noticed illustrations of this principle on a
small scale. Have you not seen that snow and ice often melt around straws
and sticks, the snow or ice remaining still frozen at a little distance,
as if the sticks and straws were warm and had melted them? Have you not
seen a dark-colored board covered with ice, and the ice remain firm till
the sun shone upon it, and then the ice melt upon the under surface,
leaving the upper surface unaffected?"
"I have seen such things a great many times," said Peter, "and wondered
what the reason was."
"The reason is that ice is _diathermic_. Heat passes through the ice
without warming; but when the rays of heat fall upon the stick or stone or
board, the heat is absorbed, the dark body is heated and in turn warms and
melts the ice. In the same manner the atmosphere is warmed. The heat-rays
of the sun pass through the atmosphere and fall upon the surface of the
earth; the earth is warmed, and in turn warms the air resting upon it.
"The gases and watery vapor contained in the air also hinder the radiation
of heat from the earth. Pure atmospheric air is perfectly diathermic to
both luminous and dark heat, and vapors and gases are also diathermic to
luminous heat. But to dark heat some of the gases are almost impenetrable.
Ammonia stops dark heat almost completely. In a smaller degree watery
vapor does the same. Gases and vapors thus serve as blankets to keep the
earth warm. The heat of the sun, being luminous heat, penetrates the
atmosphere with its vapors and foreign gases, and falls upon the earth
almost without loss, but, being absorbed by the earth, it becomes dark
heat, and cannot be radiated back through the same gases and vapors. Vapor
serves thus as a valve: it admits the heat of the sun to the surface of
the earth, but prevents its escape. Prof. Youmans calls watery vapor the
barb of heat; it catches the heat of the sun and holds it fast.
"Who can sufficiently admire the simplicity of these arrangements for
preventing the radiation of heat into the stellar regions?--and their
efficiency is no less admirable than their simplicity. Arrangements like
these show that the Creator had a definite object in view, and that object
is benevolent. For the advantage and enjoyment of the inhabitants of this
world these arrangements were made.
"We ought at this point to look at those adjustments by which the earth
receives just the amount of heat needed to maintain the requisite
temperature. The importance of maintaining some certain average
temperature cannot be over-estimated. Every animal and plant has its own
_habitat_--that is, its natural dwelling-place or location--outside of
which it perishes or maintains a stunted and precarious life. The habitat
of animals and plants depends in a very great degree upon temperature.
What a panorama would be seen if we could fly like a bird from the equator
to the poles, and look down upon the ever-changing animal and vegetable
life as we pass! How the luxuriant vegetation and flaunting colors of the
tropics would shade off into the scantier vegetable life and more sober
hues of the temperate zones, and these in turn die out and disappear in
polar barrenness! We should see the lion and tiger give place to the bear
and the wolf, the elephant and camel to the ox and horse, and these to the
white bear and reindeer. This sublime panorama we see, in miniature, in
ascending lofty mountains in the tropics. Around the base of the mountain
flourish the rich and various productions of the torrid zone; a few
thousand feet of elevation bring us among the productions of the temperate
zones. The most valuable fruits and grains thrive. Then vegetation becomes
scanty and stunted, and at last disappears. The top of Mt. Washington,
6234 feet high, in latitude forty-four degrees, is as bare of trees and
plants and every form of vegetation as the north pole.
"The fitting temperature is almost as necessary to the animal tribes as to
vegetable life. Animals which are native to the tropics do not thrive in
colder countries, or if the difference of temperature be very great, they
perish. A change from a cold to a warm region is equally disastrous. Man
indeed transfers animals from their natural habitat by protecting them
from the extremes of temperature, but this is, of course, no exception to
the general principle of which I am speaking. A change of only a few
degrees in the mean annual temperature would render this earth a hard
place for even the human race to subsist. But the temperature of the earth
depends upon many a wise adjustment--how many, we cannot tell. Will you
tell us, Samuel, the first adjustment or arrangement upon which the
temperature of the earth depends?"
"It must depend chiefly I think upon the intensity of the sun's heat."
"Whether or not that be the chief adjustment by which the right
temperature is secured, it is at least a very important item. The
intensity of the sun's heat must, of course, be considered in connection
with its distance from the earth. The distance of the sun is no less
important than the power of his rays; indeed, in one sense, it is more
important, for if the intensity of the sun's heat were doubled, the
temperature of the earth would be increased only twofold; whereas, if the
earth were brought to one-half its present distance from the sun, the heat
would be increased four times. Heat being one of the radiant forces, its
intensity diminishes in proportion to the square of the distance through
which it acts. If the earth were 190,000,000 of miles from the sun instead
of 95,000,000, as it now is, the force of the sun's rays would be
diminished fourfold. The Creator has so fixed the distance of the earth
and sun, and the power of the sun's heat, as to give to this world a
temperature suited to its various inhabitants.
"The temperature of the earth has also some dependence upon our
atmosphere. Can you tell us, Ansel, how the temperature of the earth is
affected by the atmosphere?"
"You have already told us that the atmosphere is _diathermic_, allowing
the heat of the sun to fall upon the earth almost undiminished in force.
If the air were so constituted as to intercept the sun's rays, it is plain
that the earth would receive less heat."
"This adaptation of our atmosphere to transmit the sun's rays," said Mr.
Wilton, "is more subtle than it appears at first sight. It is not merely a
matter of depth and density, though those are important considerations,
nor is it merely a question of the elements of which the atmosphere is
composed. Simple gases are _diathermic_. The atmosphere is therefore made
up of two simple gases, oxygen and nitrogen, not chemically combined, but
mixed together. Compound gases intercept the passage of heat. Ammonia,
composed of hydrogen and nitrogen chemically united, almost wholly stops
it. Even ozone, which is nothing but oxygen in a changed or _allotropic_
state, is not _diathermic_. The _diathermic_ quality of the air depends,
then, not only upon the fact that it is composed of simple elements
mingled, but not chemically joined, but also upon the _state_, or
_condition_, of those simple elements.
"Another point deserves attention. Oxygen is an element having a wide
range of very strong and active affinities. It is ready to unite with
every known substance, fluorine excepted. What if some other equally
active element were mingled with oxygen to form the atmosphere? What if,
in place of nitrogen, vapor of sulphur were substituted? What if hydrogen
were put in the place of nitrogen? The two elements would combine in
sudden combustion or explosion, and the atmosphere itself would perish.
But nitrogen is a substance so sluggish and inert that it can be brought
into union with oxygen only by indirect processes. Because the air is
composed of one so inert element as nitrogen, the atmosphere is preserved,
and, what is almost as important, it is kept, as it now is, composed of
simple elements, and hence _diathermic_. If our atmosphere were a compound
gas, the world would perish with cold.
"The temperature of the earth depends also upon certain qualities of the
earth's surface. I should be glad to have Mr. Hume explain this."
"I suppose," answered Mr. Hume, "that you refer to the qualities of the
earth as an absorbent and conductor of heat. The earth must needs have the
capacity of receiving and retaining the heat which falls upon it from the
sun. If the earth's surface were polished and brilliant, the heat of the
sun would be reflected into space as from the surface of a mirror, and
very small advantage would the earth receive from the solar heat. A dark
soil absorbs heat more readily than a soil of lighter color, and a wet
soil, on account of the high specific heat of water, requires more heat to
raise its temperature than a dry soil. The mineral elements of the soil
and its compactness or porosity also help to make up its capacity for
receiving and retaining heat. The color and constitution of the soil
sometimes go far toward making the climate of a region. The conducting
qualities of the earth's crust in its profoundest depths also must be
taken into account. If the crust of the earth were composed of silver, or
any other substance of like conducting quality, and the interior of the
earth were molten rock, as it now is, the interior heat would be so
rapidly conducted to the surface that everything upon the earth would be
consumed."
"Upon so many circumstances wisely adjusted and nicely blended," said Mr.
Wilton, "does the temperature of the earth depend. The intensity of the
sun's heat, the dimensions of the earth's orbit, the constitution of our
atmosphere in the subtlest qualities and relations of its elements, and
the material, structure, and color of the earth's crust,--on all these and
many other things which I cannot stop to mention depends the temperature
needful for the well-being of the inhabitants of this globe. I beg your
pardon, Mr. Hume, but allow me to ask whether such a combination of
agencies and conditions, uniting to work out good for man, does not seem
to you quite superhuman and worthy of a wise and good Creator?"
"I cannot deny it, sir," he replied; "I am not prepared to make any
objections. There are many things painful to man in the vicissitudes of
heat and cold, and if I were to make a world, I suppose I should leave
them out, or perhaps make the world upon a very different plan. But I am
not prepared to affirm that any changes which I could make would be
improvements, though I have thought until recently that more of knowledge
and power, and perhaps more of chance, too, than of wisdom and goodness,
were displayed in the works of Nature. But I must confess my opinion has
been much modified."
"I think your change of mind is in the right direction, and I am glad that
it is so. We learn the secrets of Nature and appreciate her spirit much
better when we come as reverent questioners than when we come with
preconceived notions and a patronizing air. I can well understand your
feelings and state, for I myself have traveled over the same ground. My
eyes were once dazzled with the glories of science; I worshiped at the
shrine of natural laws. But I have learned that God is greater than
Nature, the Creator is mightier than the creation. Nature has no mind or
purpose apart from the plan and will of the supreme Architect and Ruler,
and this inner plan and purpose of Nature is seen only in the government
and discipline of our sinful race. I shall greatly rejoice for you and
with you if you shall go on to the same end which I have reached."
"I shall much rejoice if I reach some satisfactory and peaceful
conclusion."
"To understand the management of heat," said Mr. Wilton, "we must take
note of the differences and fluctuations of terrestrial temperature. The
sources of heat are constant. The sun sends out its flood of heat
uninterrupted and changeless for ever. The internal fires of the earth
give an even inward heat. Mechanical and chemical agencies are active
everywhere. These sources of heat do not fluctuate, flaming up and dying
away, yet temperature is the most variable of all inconstant things. In
passing from equator to pole we go from torrid to frigid, from everlasting
summer to everlasting winter. And not only this, but in the same region
the temperature never remains the same for even twenty-four hours. The
thermometer may pass from forty degrees above to thirty below zero in a
very few hours. We must first consider the agencies by which these
inequalities are produced. Ansel may mention the first of these."
"The shape of the earth," said Ansel.
"How does the form of the earth operate to produce inequality of
temperature?"
"The earth is a sphere, and the rays of the sun fall upon it in nearly
parallel lines. Upon the centre of the hemisphere which is turned toward
the sun the rays fall perpendicularly, the sun is directly over head,
while toward the edges of the hemisphere, on account of the curvature of
the earth's surface, the rays fall more and more slanting, as if the sun
were sinking toward the horizon."
"What is that inequality of temperature which is produced by the shape of
the earth?"
"The five zones," answered Peter.
"This subject is so well understood," said Mr. Wilton, "that I need not
spend time in explaining it. Every boy knows the difference between
setting his wet slate before the fire to dry so that the heat will fall
squarely and perpendicularly upon it and placing it edgewise to the fire.
Upon the torrid zone the sun shines perpendicularly, upon the temperate
zones obliquely, and upon the frigid zones still more obliquely, and
during a part of the year the sun is entirely hidden. In proportion as the
rays of heat fall obliquely, any given amount of heat is spread, so to
speak, over a larger surface, and the larger the space over which it is
spread, the feebler it becomes. What is another cause of inequality of
temperature?" No one answered. "Samuel, what is the cause of day and
night?"
"The turning of the earth upon its axis."
"And the rotation of the earth upon its axis," continued Mr. Wilton,
"brings not only an alternation of light and darkness, but also of heat
and cold. The heat of the sun is withdrawn along with the light. The heat
of the sun is not withdrawn from the earth, but one-half of the earth's
surface is constantly turned away from its influence. This must produce a
daily change of temperature. This diurnal fluctuation of temperature may
be very small or it may amount to seventy or eighty degrees. Samuel, what
is a third cause of unequal temperature?"
"The inclined position of the earth's axis and the revolution of the earth
around the sun cause the change of seasons."
"If it were not for this, the earth would still have her zones of seasons;
a part of the earth would have endless summer, a part endless spring, and
the rest unbroken winter, but the alternation of seasons at the same place
would be unknown. The axis of the earth is now inclined about twenty-three
degrees, twenty-seven minutes, twenty-three seconds to the plane of the
earth's orbit, and as this axis maintains constantly the same position,
being parallel in one part of the earth's orbit to its position in any
other part of its orbit, during one part of the year the north pole is
turned twenty-three and a half degrees toward the sun, while in the
opposite part of the year the south pole is in like manner brought into
the light and heat. This causes the sun to appear to move to and fro,
north and south, twenty-three degrees, twenty-seven minutes, and
twenty-three seconds from the equator in either direction. The tropics, or
turning-places, mark the limits of the sun's northern and southern
journey. Everywhere between the tropics the sun, at some period of the
year, passes through the zenith, that is, exactly overhead at noon. North
and south of the tropics the sun seems to rise higher in summer and to
sink lower in winter. In summer the sun at midday is about forty-seven
degrees nearer the zenith than in winter. Within the polar circles, which
are the same distance from the poles as the tropics from the equator, the
heat of the sun is entirely withdrawn during a portion of the year, and
during another portion of about equal length the sun does not set. The
extremes of temperature, caused by the inclination of the earth's axis and
its revolution around the sun, are very great. In the northern part of
Minnesota, the temperature rises in summer to one hundred degrees, and in
winter sinks to fifty degrees below zero, giving thus an alternation of
one hundred and fifty degrees.
"In this connection you may also remember that the sun is nearer the earth
in one part of its orbit than in another part. This difference amounts to
about 3,000,000 miles. The sun also remains eight days longer north of the
equator than south of it. Our summer, therefore, is eight days longer than
the summer of the southern hemispheres, and our winters are
correspondingly shorter. These differences tend, however, to balance each
other, for while the southern summer is shorter, the sun at that time is
nearer, and while our summer is longer, the sun is more distant. Peter,
you may explain to us the effect upon temperature caused by the division
of the earth's surface into land and water."
"I learned while studying physical geography that the temperature is more
even upon the sea than upon the land. But why, I do not know."
"The smooth surface of the sea reflects heat better than the rough land:
for this reason, a larger proportion of the heat which falls upon the sea
is not absorbed, but reflected and lost, so far as the temperature of this
world is concerned. Water is also a very poor conductor of heat, and has
withal a very high specific heat. For these reasons the sea receives and
parts with heat more slowly than the land, and its absorption or radiation
causes a smaller variation of temperature. The result is, therefore, that
the sea is cooler in summer and warmer in winter than the land, and the
average ocean temperature is lower than the mean continental temperature.
The land receives heat more readily and parts with it more rapidly; the
fluctuations of temperature must therefore be greater. Hence, the
interiors of the continents have much greater extremes of temperature than
the sea-board. But of the influence of water in equalizing temperature I
shall have occasion to speak again more at length, and will pass it by for
the present. What effect, Peter, has the unevenness of the earth's surface
upon temperature?"
"The higher we ascend upon mountains, the colder we find it."
"That is, Peter, the greater the elevation of any place or country above
the sea level, the lower the temperature. Almost the whole surface of the
earth is an alternation of mountain and hill, valley and plain. One
continent has a very much greater mean elevation than another. One region
or tract of country lies sloping toward the sun, another is inclined from
it. The effect in the one case is the same as if the sun were brought more
nearly overhead; in the other case, the sun is depressed toward the
horizon. It is all the same as if the region of country were brought
nearer the equator or removed farther from it. The effects of the
curvature of the earth are obviated or exaggerated. Do clouds tend to
produce inequalities of temperature?"
"I think they must do this," answered Samuel. "Clouds cover one portion of
the earth's surface and shut out the heat of the sun, while other portions
are well exposed to the sun's rays."
"That is right, Samuel. Does any one think of another cause of inequality
of temperature?"
There was a pause. Then Mr. Hume answered: "Considering the unmeasured
cycles of the past, the gradual cooling of the earth has brought a great
change of temperature."
"And this change," continued Mr. Wilton, "has been very important for the
welfare of the human race. At the present temperature of the earth, the
coal-beds, so necessary for the culture and progress of the race, could
hardly have been formed, and at the temperature of the carboniferous
periods, when the coal-beds were deposited, the human race could with
difficulty have survived. The high temperature required to prepare the
earth for man is now no longer needed, but would prove destructive. And
this great change of temperature was doubtless caused by the cooling of
the earth.
"The result of all these agencies--the shape of the earth, its daily and
yearly motions, the inclination of its axis, the eccentricity of its
orbit, the division of its surface into land and water, the varying
elevation of its surface, and the clouds and storms that hide the sun--is
that we have great extremes and rapid transitions of heat and cold, and
every variety of climate. These changes of temperature are often painful
and, unless guarded against, dangerous. Yet, taken as a whole, can one
doubt that variety of climate and change of temperature are of advantage
to man? What weariness and lassitude a changeless temperature would
bring! How the cooler air of the night comes as a tonic after the
relaxation of the heated noonday! Who can estimate the value of our
northern winters, not alone in building up a vigorous and nervous physical
frame, but in helping the culture of men and nurturing the domestic
virtues? We might almost say that her winter evenings have been the making
of New England. But periods of heat are needed for bringing fruit and
grain to ripeness. What variety and richness of productions for the use of
man the different zones furnish! The supply of man's wants would be
comparatively meagre if we had but one zone, even though we had our choice
of the zones. But every zone is necessary for the perfection of the
temperate zones. That we may have the warmth of summer in the temperate
zones we must have the torrid zone. That we may have the tonic cold of the
temperate zones we must needs have the severity of polar winters. I do not
mean that the Creator could not devise a world that should not have these
painful extremes, yet enjoy the advantages of the temperate regions. But
that would plainly require a world constituted upon principles very unlike
those which now prevail. With God this is doubtless possible, but the
mode is to us inconceivable. But we can easily see that by the present
arrangement of things God has secured many great advantages for man--how
many and how great, we can hardly understand--and the apparent
disadvantages we cannot positively affirm to be real evils. We can safely
declare that this world is well adapted to man's necessities. But these
inequalities of temperature are modified and softened by a most
comprehensive and beneficent system of agencies by which the extremes are
prevented from becoming destructive. In this system of compensating
agencies two great divine ideas are clearly developed, economy in the
expenditure of heat and benevolence toward man. Upon this subject we are
now prepared to enter."