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[pg 153] 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,[pg 154] 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[pg 155] 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[pg 156] 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[pg 157] 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.
[pg 158]“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[pg 159] 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?”
[pg 160]“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[pg 161] 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[pg 162] 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[pg 163] 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[pg 164] 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[pg 165] 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 [pg 166]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[pg 167] 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[pg 168] 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[pg 169] 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[pg 170] 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[pg 171] 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[pg 172] 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.”
[pg 173]“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[pg 174] 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.[pg 175] 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.”
