University of Virginia Library

2. CHAPTER II.
TESTIMONY OF THE ATMOSPHERE.—Concluded.

DURING a recent journey in Switzerland, at the close of a delightful summer's day, in the early part of July, I arrived at Interlachen, in company with a number of fellow-travellers. We had been sailing on the beautiful lake of Brienz, and some minutes before we reached our destination the sun had set, and the mountains had already cast their long shadows across the lake. Early in the afternoon the clouds had settled on the nearer hills, and we had been disappointed at not obtaining a view of the distant summits of the Bernese Oberland; but suddenly, as the boat neared the shore, the magnificent peak of the Jungfrau appeared from behind the veil of clouds, clothed in her white mantle of everlasting snow, and bathed with a flood of rosy light. The effect thus heightened by the contrast was grand beyond description, and as beautiful as it was grand. It seemed like a vision of the Heavenly Kingdom,—as if the glory of God had rested on the mountain. The scene completely filled the soul, and the heart overflowed with gratitude for the blessing it enjoyed. It was felt to be one of the

great privileges of a lifetime, and his would have been a dull understanding, and a duller heart, which did not recognize the Giver in the gift. The view so riveted the attention that we hardly noticed our arrival, and as we walked to the hotel we watched the successive shades of crimson and purple as they flitted up the mountain, until the last blended in the gray of the twilight.

It may not be permitted to many to behold the Jungfrau blushing before her retiring lord, but all have witnessed the same effect on even a grander scale, when the white clouds, piled up on the western horizon like vast mountain chains, become, at evening, resplendent with the rays of the setting sun; and many have watched their varying tints of gold and purple, until at last their ghostly forms vanished in the dusk of the evening, and the stars came out to take up with their measured twinkling the silent song of praise. Perhaps, also, there may be some who, after anxious watching through the night, have felt their hearts strengthened and their hopes revived when the blush of morning reassured them of their Father's providence, and all nature smiled in the floods of returning light.

All these glorious visions, all this beauty, and all the pure emotions of our hearts which they excite, we owe, my friends, to the skill with which the physical qualities of the atmosphere have been adjusted to the wants of our physical and moral natures, and they all thus become the silent witnesses not only of the wisdom, but also of the goodness of our God.

We have already, in the first lecture, discussed some of the adaptations of the physical condition of the atmosphere to the purposes which it subserves on the globe, and I wish this evening to develop still further the same subject, by considering a few additional examples; and first I will ask your attention to those evidences of design which are to be found in the relations of the atmosphere to light and heat. Here, however, I am met by a difficulty. In order to explain fully these relations it would be necessary to develop from first principles the sciences of optics and thermotics, and to do this in a popular manner would require several lectures. These sciences furnish some of the most wonderful evidences of design which are to be found in nature and I have no doubt will be given their appropriate place in this series of lectures. Without, therefore, attempting any detailed explanations, I will merely bring before you a few facts, drawn from these departments of knowledge, which illustrate the adaptations of the atmosphere to its appointed ends.

The atmosphere, although very much more pervious to light than any kind of solid or liquid matter, is far from being perfectly transparent. Indeed, the reverse is sufficiently evident from our daily experience. Every one has noticed that distant objects appear less distinct in proportion as they are removed, their colors become fainter, the contrast between light and shade less marked, and that they seem as if covered with a pale blue veil. This effect, always noticed on distant mountains, is owing to a partial absorption of the light while passing through

the atmosphere; for, were the passage of the rays wholly unimpeded, all objects, although reduced in size in proportion to their distance, would appear equally distinct, and their colors equally brilliant.

The transparency of the atmosphere differs very greatly under different circumstances, but it has been estimated that, under the most favorable conditions, at least thirty per cent. of all the light coming from the heavenly bodies is absorbed before reaching the surface of the earth, and in our latitude, at this season, even when the sun is on the meridian and the sky clear, fully one-half of his rays are thus spent. Do not suppose, however, that all the light so expended is lost. Quite the contrary, for every particle of the atmosphere, illuminated by the sunbeam, becomes itself a new centre of emission, radiating the light in every direction.

This diffusion of the sun's rays is the cause of that wonderful effect which we term daylight. I say wonderful effect, for, although so familiar, it is one of the most remarkable results of skilful adaptation and infinite wisdom. The very daylight which streams in at the open windows of our houses, filling them with cheerfulness, and penetrating to their inmost recesses, which enlivens the whole landscape, and which bars and bolts cannot wholly exclude even from the prisoner's dungeon, is another evidence of the adaptation of the atmosphere to the constitution of man. Indeed, the atmosphere is as much an essential condition of our seeing as of our breathing, and the immeasurable pleasure which we derive from our sense of vision depends upon its

adaptation to the organization of the eye. Were it not for the diffusive effect of the atmosphere on the sun's light, the contrast between light and shadows would be so greatly increased that, while objects directly illuminated by the sun would shine so brilliantly as to dazzle the eyes, all surrounding objects would be in darkness, and the interior of our dwellings would be as dark as night. Our eyes, as little fitted to such conditions as our lungs, would be blinded by the sudden alternations, and distinct vision would be impossible. This is not a matter of theory for similar effects are observed on the summits of lofty mountains, where the air is much rarer than at the sea level. On the top of Mont Blanc the sky has a blackish hue, and the stars are seen at midday; the glare of the direct light is insupportable to the eye, and even the reflection from the snow blisters the unprotected skin, while at the same time the contrast between light and shade, unnaturally increased, gives to all near objects a peculiar and ghastly aspect. We have here, it is true, a very great diminution in the density of the air; but when you reflect upon what delicate contrasts of light and shade the beauty of a landscape depends,—the clearness of the foreground, the gray of the middle distance, and the tender purple of the distant hills all blending into one harmonious whole,—you can appreciate how slight a change would disturb the result, and deprive the sense of beauty of its purest enjoyment.

I have thus far spoken only of the influence of the atmosphere in softening the intensity of the rays of

light, and in diffusing their action; but the atmosphere has also, under certain conditions, the power of decomposing the sun's rays, and thus producing, not only those displays of gorgeous tints which we witness in the sunset clouds, but also the pure blue which colors the dome of heaven.

In regard to the precise means which are employed by nature to produce these results, scientific men are not agreed. It has been proved that the blue color of the sky is seen by reflected light, and it is probable that the color is caused by repeated reflections of the sun's rays from the surfaces of the innumerable small water-bubbles which are constantly floating in the atmosphere. You have all noticed the blue color of the soap-bubble shortly before it breaks. This color is caused by the action of the very thin film of water in decomposing the light reflected from its surface, and it is supposed to be an action of the same sort, only very much increased by repeated reflections, which gives to the sky its azure hue.

While the blue color of the sky appears to result from changes in the white light of the sun caused by reflection, it is equally probable that the sunset tints arise from changes in the same white light caused by an unequal absorption of its different colored rays during their transmission through the atmosphere. Here, again, the vapor in the air is supposed to be the active agent; and the theory is, that the tints are produced while the vapor is condensing into clouds,—a change which naturally occurs at sunset. But this is a mere theory, and

our whole knowledge on these subjects is very imperfect.

So far, however, as our present argument is concerned, it is not essential that we should understand exactly how these glorious results are obtained. It is enough that we are constantly enjoying their beauty, and that we know they are owing to the peculiar constitution of the atmosphere. When future discoveries shall bring to light the methods, at present secret, by which nature gilds the sunset clouds and covers our beautiful dwelling-place with its canopy of blue, we shall unquestionably find fresh evidences of God's wisdom; but even now, when ignorant, perhaps, of these hidden causes, we have that which is far more excellent, the most conclusive evidence of His goodness and love. Our Father has not only adapted the atmosphere to the wants of our bodies, and made it conducive to our physical enjoyment, but He has also made it the scene of the highest beauty,—a beauty which satisfies the longings of our souls and calls forth their noblest and purest aspirations. Man, sinful as he is, cannot look up into the pure blue of heaven without a sense of reproach, and the feeling that it is a fit emblem of the kingdom of purity and peace. And when the setting sun lights up the evening altar in the West, who can repress the rising prayer of devotion, and hesitate to believe, with the child, that his Heavenly Father is smiling behind the clouds? There is a depth to the beauty of nature which man cannot fathom. Poetry cannot describe it, and the highest art only displays

its weakness when it attempts to copy it. The savage feels that it is immeasurably above him, and worships it. The artist seeks to attain it, but the more he strives, the more it surpasses his power, and he dies disappointed, unless, happily, he finds that the perfect ideal has been realized only in Christ, and thus through nature is led up to nature's God. Yes! the beauty of nature is in the Infinite Presence it conceals, and, unconsciously though it may be, it is the spirit, not the matter, which the artist loves.

Such are some of the evidences of design which we discover in the relations of the atmosphere to light. Let us now examine some of its relations to heat, which we shall find not less instructive. It was formerly supposed that the rays of heat, although accompanying the luminous rays in the sunbeam, were essentially different from those of light. But it is now almost universally believed that the rays of heat differ from those of light only, at most, as one color differs from another, and that even the same rays, which, falling on the retina of the eye, excite the sensation of light, when falling on the nerves of feeling may excite heat. But what, you may ask, is the difference between the different colors? The subject is somewhat abstruse, but if you will follow me attentively for a few minutes I will try to make it intelligible.

Every one who has dropped a stone into the water of a still lake has noticed the system of waves which, with its ever-increasing circles, spreads in every direction from the stone; but all may not

know that when two stones are struck together in the air a similar system of aerial waves spreads, in ever-widening spheres, through the atmosphere, and that it is these waves breaking on the tympanum of our ears, like the waves of water on a sand-beach which produce the sensation we call sound. Two stones thus struck together give rise to waves of unequal size, following one another at irregular intervals; and such waves produce an unpleasant sensation on our auditory nerves, which we call noise. But if, instead of striking together two stones, we set in vibration the string of a piano-forte or the reed of an organ-pipe, we excite a system of waves, all of equal size, and succeeding one another with perfect regularity, and these breaking on the ear produce by their regular beats what we call a musical note. If the waves follow one another with such rapidity that one hundred and twenty-eight break on the tympanum every second, the note has a fixed pitch, called in music "C natural.'' If the waves come faster than this, the pitch is higher, and if less rapidly the pitch is lower. What we are all familiar with as a pitch of a musical note depends, then, on the rapidity with which the waves of sound strike the ear, and may evidently be measured by the number of waves breaking on the tympanum in a second.

Our ears are so constituted that they can hear a musical note only when within certain fixed limits of pitch, differing to a slight extent with different individuals. The deepest bass note, which can be heard, as such, by a good ear, is produced by about

eight waves in a second. If the waves strike less rapidly than this, they are perceived as distinct beats, and beginning at this note the musical scale ascends to a note caused by twenty-four thousand waves a second, which is the highest note perceptible by human sense. The range of a piano generally extends from a note produced by sixteen waves in a second, to one caused by one thousand and twenty-four waves in a second, as is shown by the accompanying table.

DIMENSIONS OF WAVES OF SOUND.

               

Notes	Length of waves in feet.	Number of waves striking the ear in one second.
C-3	70	16
C-2	35	32
C-1	17.5	64
C1	8.75	128
C2	4.375	256
C3	2.178	512
C4	1.093	1,024

     

Name of Note,	C1	D1	E1	F1	G1	A1	B1	C2
Number of Waves,	128	144	160	170 2/3	192	213 1/3	240	256
Ratio of each number to that of Note C,}	1	9/8	5/4	4/3	3/2	5/3	15/8	2

Sounds of the highest pitch, like the cry of some insects, become disagreeable, and by some persons cannot even be distinguished. It is quite possible to produce a sound, which, though painfully shrill to one person, shall be entirely unheard by another.

Professor Tyndall, in his very interesting work on the glaciers of the Alps, relates an instructive anecdote of this sort, which I give in his own language.

"I once crossed a Swiss mountain in company with a friend; a donkey was in advance of us, and the dull tramp of the animal was plainly heard by my companion; but to me this sound was almost masked by the shrill chirruping of innumerable insects, which thronged the adjacent grass; my friend heard nothing of this, it lay quite beyond his range of hearing.''

There may, therefore, be innumerable sounds in nature to which our ears are perfectly deaf, although they are the sweetest melody to more refined senses. Nay, more, the very air around us may be resounding with the hallelujahs of the heavenly host, when our dull ears hear nothing but the feeble accents of our broken prayers.

We have been studying, my friends, the nature of sound, in order to comprehend more readily the nature of light and heat, for the phenomena included under these names are produced, like the phenomena of sound, by waves; not, however, by waves in the air, but by waves in a medium which is as much more subtile than air as air is more subtile than water,—indeed, a medium so exceedingly thin that it eludes all our powers of chemical analysis; but which, as we assume, pervades all space, and this, too, whether the space be filled or not, at the same time, by other forms of matter. We call this medium "ether,'' and through it the waves of

light speed with an inconceivable rapidity. Sound travels 1,100 feet in a second, but a wave of light spans 187,000 miles in the same time, and starting from the sun on its journey of unnumbered years, to Sirius or Arcturus, leaves the whole solar system behind in a single hour.

Yet great as is the difference of velocity, the analogy between sound on the one side, and light or heat on the other, is complete. Every luminiferous body, like this candle-flame, excites in the tenuous ether a system of waves, which spread, in ever-enlarging spheres, with the immense velocity just described; and it is these little billows which, passing through the humors of the eye, and breaking on the retina, produce the sensation we call light, or, falling on the skin, excite the less delicate nerves of feeling, and cause the sensation of heat.

Moreover, the difference between colors is of precisely the same kind as the difference between notes. Red, yellow, green, blue, violet, etc., are names we give to sensations caused by waves of ether breaking at regular intervals on the retina. Color corresponds to pitch, and—what may seem to you incredible—we are able to calculate from actual measurements the number of waves of ether which must break on the retina in a second in order to produce the sensation of a given color. Here are some of the numbers, and, extravagant as they appear, they are the sober results of science, and have been as accurately determined as the magnitudes and distances of astronomy.

               

Colors.	Number of waves in an inch.[*]	Number of waves in a second.
Red	39,000	447 million million.
Orange	42,000	506	''	''
Yellow	44,000	535	''	''
Green	47,000	577	''	''
Blue	51,000	622	''	''
Indigo	54,000	658	''	''
Violet	57,000	699	''	''

It is actually true, that when we are receiving the sensation of red there are no less than 477 million millions of ether waves breaking on the retina of our eyes every second. And more than this, we have measured the length of these waves, and we know that the length of a wave of red light from crest to crest is 1/39000 of an inch. By examining the table you will also discover that the sensation of red, as compared with other colors, results from the smallest number of waves, and that these waves are comparatively large. On the other hand, the sensation of violet is caused by the largest number of waves, which, however, are proportionally small in size. The red light, therefore, corresponds to low,

and the violet to high notes of music, and between these extremes there exists every gradation of pitch which is here manifested in color.

Waves of all the dimensions given in the table, together with waves of every possible length between certain extremes,—which are far wider than those indicated above,—move together in the sunbeam, and their combined impression produces the sensation of white light. We have a very simple way of analyzing the sunbeam and separating its different color-producing waves. The method consists in passing the sunbeam through a glass prism. The prism has the power of bending the beam from its rectilinear direction; but it does not change the direction of the motion of all the waves to the same extent. The longer waves, which give the sensation of red, are bent from their course much less than the shorter waves, which produce the sensation of violet, while waves of an intermediate length take a course between the two. Hence, after emerging from the prism the directions of the different waves diverge, and if we receive the beam of light thus analyzed, on a screen, the various color-producing waves strike the screen at different points of a continuous line. A more or less narrow band on the screen will thus be illuminated with lights of different colors in the following order—Red, Orange, Yellow, Green, Blue, Indigo, Violet—and this beautiful phenomenon is familiar to almost every one under the name of the solar spectrum.

Here, where we have the whole scale of colors spread out before us, the analogy of light to sound

becomes still more evident. As there are persons who cannot hear the shrill sound of some insects, so there are many who cannot see certain colors of the spectrum, and as there are unquestionably innumerable sounds in nature which are inaudible to our ears, so there are innumerable waves in the ether which are powerless to produce the sensation of light. Moreover, singular as it may seem, we have more palpable evidence of the existence of these non-luminiferous waves than we can obtain in the case of sound. There are waves in the ether far smaller, and undulating far more rapidly, than those which produce violet light; so small that they do not even jar the nerves of the retina, but which, nevertheless, breaking on the prepared plate of the photographer, leave there an impression which, developed by his skill, becomes a beautiful copy of nature or of art. On the other hand, there are waves in this same ether so large that the delicate retina cannot vibrate in unison with their rough beats, but which, nevertheless, breaking on the surface of the skin, disturb the coarser nerves of feeling, and produce the glow of heat. Most of the waves which impress the optic nerve will also affect the nerves of feeling; but the reverse is not true, for many of the waves which produce the sensation of heat are far too large, and undulate too slowly, to set in vibration the retina of the eye.

I hope that I have been able to make clear two points,—first, that light and heat are forms of motion; second, that the differences in the phenomena which have been referred to these two agents are

simply different sensations or different effects [*] produced by the same wave-motion. It would be highly interesting, in this connection, to examine the wonderfully delicate adjustments and to follow out the peculiarly intricate motions which concur to produce the phenomena of light and heat; for they are in themselves most striking illustrations of the wisdom of the Creator. But this would lead us too far from our proposed plan, and I must content myself with the few facts already given, which were necessary to illustrate the relations of the atmosphere to the thermal conditions of our globe.

From the principles stated, it is evident that the atmosphere must act in diffusing heat, just as we have seen that it acts in diffusing light. Indeed, this effect is one of the thousand conditions on which the existence of organic life depends. Were it not for the influence of the atmosphere, the greatest extremes of temperature would be produced by the alternation of day and night, and even were the

density of the atmosphere reduced only one-half, the variation would be so great as to render the existence of the higher forms of organic life impossible, except, perhaps, in the more favored regions of the earth.

But not only does the atmosphere diffuse the heat of the sun's rays, it also acts, and even more effectually, in retaining on the surface the heat which the earth is constantly receiving from that great central luminary. The atmosphere has been compared to a mantle, and the comparison is just; for, like a huge cloak, it envelops the earth in its folds, and protects it from the chill of the celestial spaces through which we are rushing with such frightful velocity. In order to understand how a thin and transparent medium like air can thus act to keep the earth warm, we must recur to some of the facts established above.

As the ether-waves, breaking on the eye more or less rapidly, produce the different sensations of color, so when breaking on the skin they occasion analogous differences in the sensation of heat, which, although not so accurately distinguished, because the sense is less delicate, nevertheless are as real as the difference between a low and sweet musical note, and one that is high and shrill. There are waves of heat which break upon our nerves of feeling like the shrill cry of the cricket on the ear, and seem to penetrate to the very brain, while there are others which fall like the low tones of an organ, diffusing throughout the system a genial glow. Such, for example, is the difference between the heat from a hard-coal fire and that from a steam radiator. The waves of the

first sort, from their small size and rapid motion, can readily pass through glass and other transparent media, when the large waves with their slow motion are in a great measure stopped.

Now it is found that the sunbeam is chiefly made up of waves of the first class, which are therefore able readily to penetrate the atmosphere and warm the surface of the earth. The earth thus warmed becomes itself a hot body, surrounded by an intensely cold space, and, like any other hot body, tends to lose its heat by radiation. But the waves of heat which the earth [*] sets in motion are of the second class, long and slow undulations, and these are in great measure arrested by the atmosphere; indeed, as experiments have proved, they are chiefly absorbed by the lower strata, [*] in which we live and move.

Thus it is that the atmosphere keeps us warm; and if you desire further proof of the correctness of these experimental deductions, ascend any high mountain, and, as the thickness of the aerial covering above you is diminished by the elevation, you will find that the chill increases, vegetation slowly disappears, and before long you will reach a region of eternal snow and ice. It is true that there are other causes acting to lower the temperature at high

elevations, but the one just noticed is by far the most important, as well as the primary cause. The effect of the atmosphere is precisely similar to that of the glass panes of a hot-house. The glass, like the atmosphere, allows the rapidly undulating waves of the sun to pass, [*] but almost entirely arrests the large and slowly undulating billows which radiate from the vegetation within. They are each, in fact, a trap to catch the sunbeam.

The atmosphere not only thus acts in diffusing the sun's rays, and retaining the heat which they bring to us, but it also subserves an equally important end in distributing their genial warmth over the whole surface of the earth, thus moderating the climate of the temperate zone, and mitigating the intense heat of the tropics. Air, like all gases, is expanded by heat, and thus rendered specifically lighter, and on this simple principle all our methods of warming and ventilating are based. When now it is remembered that the atmosphere under the tropics must become more intensely heated by the vertical rays of the sun than it is in the temperate zones, the result will be obvious. The heated air rises, and the cold air rushes in from the North and South to take its place. Thus, two general currents are excited in the aerial ocean of each hemisphere, one on the surface of the earth, tending towards the

equator, and another, higher in the atmosphere, tending towards the poles. These currents, however, do not blow due North or South; for many causes combine to turn them from their primitive directions.

In the first place, the rotation of the globe on its axis imparts to objects on the surface a motion from West to East, varying in velocity from nothing, at the poles, to the speed of a cannon-ball, at the equator. In consequence of this, a mass of air moving towards the equator is constantly arriving at a point on the surface of the earth, which is moving towards the East more rapidly than the point it has just left; and as, in virtue of the law of inertia, the moving mass cannot accommodate itself instantaneously to the increased velocity, it is left a little behind,—that is, a little to the West, at every step. Hence, the lower or polar currents bend more and more towards the West as they approach the equator, acquiring in the northern hemisphere a south-westerly, and in the southern hemisphere a north-westerly direction; and the currents of the two hemispheres, meeting at the equator, combine to produce the great trade-wind, which, in the Pacific Ocean, blows constantly from the East to the West, and would blow regularly in this direction all round the globe if the continents did not intervene to disturb its course at various points.

The effect of the earth's rotation on the current of warm air which flows from the equator in the upper atmosphere, must evidently be the reverse of that just described, bending it constantly to the

East, and giving to it in the northern hemisphere a north-westerly, and in the southern hemisphere a south-easterly, direction. But the upper and lower currents do not long retain this relative position; for, as the first comes northward, it gradually sinks, and, long before reaching this latitude, touches the surface of the earth. Then, of course, it comes in collision with the current from the North, and here a strife for the mastery ensues. Sometimes the one and sometimes the other prevails, and this alternating ascendency is one of the chief causes which render the winds of temperate climates so irregular.

Again, the unequal heating effect of the sun's rays on the earth, as compared with the sea, combined with the irregular distribution of land and water over the surface of the globe, tends to complicate still further the motion of the aerial currents. For reasons which will hereafter appear, the land is more quickly heated by the sun's rays than the sea when under the same conditions, and, on the other hand, as soon as the sun is withdrawn, it cools more rapidly. Hence, on an island in a warm climate we generally have, during the daytime, a current of heated air rising from the surface of the earth, and a current of cooler air flowing in on all sides from the ocean to take its place, while after sunset the land soon cools below the temperature of the surrounding ocean, and the current is reversed. Thus is produced the daily alternation of land and sea breezes, so familiar to every one who has visited the tropics, where the phenomena are most strongly marked.

Quite a similar reciprocal action between the continents and the great ocean is caused by the alternation of seasons, and of this the monsoons of the Indian Ocean are a remarkable illustration. This mediterranean ocean, shut off from the influence of the general trade-winds by the great continental masses which surround it, has a system of aerial currents, peculiar to itself, blowing six months of the year in one direction and six months in the other. These are set in motion by the unequal heating of the continents of Asia and Africa during the extreme seasons. In the months of December, January, and February, the part of Africa south of the equator is exposed to the vertical rays of a summer's sun, while the countries of southern Asia are feeling the comparative cold of their winter. The natural consequence is, that a stream of cold air rushes across the Indian Ocean to feed the intensely-heated current which is rising over the burning plains of Africa, and produces a strong north-easterly breeze, which is the winter monsoon of India. When, however, the sun comes north of the equator, all these conditions are reversed. The ocean air now rushes to the more heated plains of India, and the summer monsoon sets in, which blows from the south-west, the change from one to the other being always attended by variable winds and furious storms. Lastly, the position of mountain chains and the configuration of the continents, which break and turn the winds, or open to them a freer channel, have an important influence in determining the direction of the aerial currents on the earth.

But we have not time for further details; they are given in all works on physical geography, [*] and the student of natural theology will find that subject rich in illustrations of God's wisdom and power. We have already become sufficiently acquainted with the general plan to understand how the atmosphere acts in equalizing the climate of the globe. The aerial currents which come to us from the South bring with them the heat of the tropics, and distribute it over the temperate zone. As they blow from the south-west, they naturally exert the greatest heating power on the western coasts of the continents, and this is one great cause of the well-known fact that the climate of western Europe is so much milder than our own, and the climate of California and Oregon so much warmer than that of the corresponding latitudes on the eastern coast of Asia. Moreover, the sea-breezes on islands and along seacoasts, the monsoons of the Indian Ocean, and other local currents, all combine, as our theory shows, to produce the same general result, cooling such regions of the earth as from any cause have become overheated, and transferring the warmth to places where it is more needed. Just as the heat of burning fuel is diffused over a whole building from the furnace by the currents of air it sets in motion, so the sun's heat is diffused over the earth from the tropics by the great terrestrial currents we have so briefly described. Indeed, as already stated, in all our methods of heating, we merely apply, on

a small scale, the same general principles which are at work around us in the atmosphere.

But, although the heat of the sun might set in motion these aerial currents, they would have but little effect in warming our northern climate, were it not that the air has been endowed with a certain capacity for holding heat. All substances possess this capacity to a greater or less degree, but the differences between them are very large. Thus the amount of heat required to warm a pound of water is ten times greater than would be required to raise the temperature of a pound of iron, and thirty times greater than would be required to raise the temperature of a pound of mercury to an equal extent. Hence, under the same conditions, a pound of water may be said to contain ten times as much heat as a pound of iron, and thirty times as much as a pound of mercury; or, again, in other words, the capacity of water for heat is ten times greater than that of iron, and thirty times greater than that of mercury. The capacity of air for heat is, weight for weight, about twice as great as that of iron, and although only one-fifth as great as the capacity of water, it is yet greater than that of most other substances. The point, however, to which I wish to direct your attention, is the fact that this capacity is exactly adjusted to the office which the air has been appointed to fill. Were the capacity of the air less, the hot air from the tropics would bring to us proportionally less heat; were it greater, the reverse would be the case; and in either event, the distribution of temperature on the earth would be changed. To what extent

such a change would affect the general welfare of man, it is impossible to determine; but when we consider how far the history of man has been influenced by climate, it will appear that the present distribution of the human race—the existence, for example, of a large and influential city in this place— may be said to depend on the adjustment of the capacity of the atmosphere for heat; and yet it depends no less on ten thousand other conditions, many of them far more important than this. How truly, then, it may be said, that even here on earth we live in "a city which hath foundations, whose builder and maker is God''!

Such are a few of the more obvious marks of design, which may be discovered by studying the relations of the atmosphere to light and heat. I might here close one division of my subject; but I should fail to give you an adequate idea of the wonderful play of physical forces in the atmosphere, were I to leave out of view that mighty agent which charges the artillery of heaven and feeds the flaming torches in the northern sky. It is true that the atmospheric relations of electricity are very imperfectly understood, and the important ends which it undoubtedly subserves in the economy of nature almost entirely unknown. We cannot, therefore, expect them to furnish us with many additional illustrations of the Divine attributes; but since electrical phenomena play so conspicuous a part in the atmosphere, and must have been included in its plan, they certainly should not be overlooked if we

would gain a general idea, however imperfect, of the whole design.

Of all the assumed agents of nature there is hardly one which is so little understood, and yet has been so carefully studied, as electricity. To the uneducated it affords the convenient explanation of most obscure phenomena, while with men of science it is the object of much laborious investigation and careful theorizing. The study of its phenomena has been fruitful in the discovery of facts; but it has as yet led to but few general principles, and has furnished only a meagre explanation of those grand displays of nature in which it seems to be such an important agent.

In regard to the nature of electricity, we are entirely ignorant. The phenomena of light and heat [*] admit, to say the least, of an intelligible explanation, and can be referred to a dynamical origin; but in the case of electricity we are obliged to be content with collecting facts, and must await the further progress of science to reveal the now hidden cause. I am well aware that electricity has been regarded as a very rare and subtile "fluid,'' and that this theory has not only afforded a plausible explanation of most of the phenomena of statical electricity, but also that the numerical results based upon it have been most remarkably verified by experiment. Yet nevertheless, although the theory may still be used as a convenient frame in which to exhibit the facts, there are but few investigators of the present day

who would claim for it more than a very partial foundation in truth, and most would reject it altogether as utterly untenable.

The fundamental facts of electricity were known to the ancients, and are familiar to every one. If a stick of sealing-wax or a glass tube be rubbed with a warm silk handkerchief, it becomes, as we say electrified, and in this condition has the power of attracting pieces of paper or any light particles of matter. When the scientific men of the last century came to examine these phenomena more carefully, they found that the handkerchief was also electrified, and thrown into a state differing from that of the glass in the one case, and that of the resin in the other, very much as the north pole of a magnet differs from its south pole. They found, also, that the resin was electrified oppositely to the glass, and they hence concluded that there were two kinds of electricity, which they distinguished by the names resinous and vitreous, or positive and negative. They also discovered that this agent could readily be drawn off from electrified bodies by the metals, but only with difficulty, if at all, by such materials as india-rubber, glass, resin, or silk, and they were hence led to divide substances into conductors and non-conductors of electricity. A good conductor, when insulated by non-conductors, was found to retain for a short time the electricity it had received from the electrified glass or resin, although the charge was soon dissipated by the surrounding air, especially when moist. By bringing in the aid of machinery, and thus increasing the surface of friction,

it was found possible to enhance very greatly the effects obtained with a glass tube; and this was the origin of the electrical machine. This familiar instrument is merely a mechanical contrivance for rubbing together glass and silk, with two insulated metallic conductors for receiving the two kinds of electricity thus generated. If the hand or a metallic knob was brought near the prime conductor of the machine when highly electrified, it was found that a luminous discharge followed, which was termed an electrical spark; and it was found possible by means of a glass vessel, coated inside and outside with some metallic leaf, called a Leyden jar, to accumulate the two electricities in such large quantities that, when allowed to flow together, the discharge was capable of producing violent mechanical action, similar to that of lightning, although on a vastly reduced scale. It was also discovered that electricity passes readily through the greatly rarefied atmosphere in the receiver of an air-pump, causing a luminous effect similar to the aurora borealis. Lastly, it was observed that electricity readily escapes into the atmosphere from a pointed conductor, and, conversely, that a heavy charge can be silently and harmlessly drawn from an electrified body by holding near it the point of a needle. By attaching a pointed conductor to a boy's kite, Franklin succeeded in drawing an electric spark from a thunder-cloud, and having thus established the identity between atmospheric and frictional electricity, he erected the pointed rod, which protects our dwellings against the lightning's stroke.

More recently it has been discovered that friction is by no means the only source of electricity, and it seems probable that no change, either chemical or physical, takes place in nature without some manifestation of this agent. It was at first supposed that there were several kinds of electricity, which were named thermo-electricity, magneto-electricity, voltaic electricity, and animal electricity, according to the nature of the process in which the electrical action was developed; but it is now universally conceded that all are only different manifestations of the same agent, and most investigators believe that electricity will in time be shown to be a form of molecular motion analogous to that which produces the phenomena of light and heat, although it has not as yet been found possible to frame a comprehensive and intelligible theory based upon this hypothesis. Again, it has been found that friction is a far more general source of electricity than was at first believed. In fact, electrical phenomena appear to be a constant result of friction, whatever may be the nature of the substances rubbed. Thus it is developed by blowing air over glass, and the hydro-electric machine, one of the most effective means of generating electricity we possess, owes its surprising energy to the friction of globules of water against the sides of the vent-cock of a steam-boiler. [*]

When, now, we consider that the air is always

rubbing over the surface of the earth, at times with great rapidity, we shall not be surprised to learn that both bodies are constantly in an electrified condition, the earth being generally charged negatively, and the atmosphere positively. Even in fair weather it is always possible to detect the presence of free electricity in the atmosphere; and during a storm, when clouds filled with drops of water are hurried over the surface, grinding against the hills and the trees, or against each other, the atmosphere becomes a vast hydro-electric machine, whose sparks are the lightning, and the noise of whose discharges the thunder. Again, the various chemical and physical changes which are going on around us,—such as the vital processes of animals and plants, the combustion of fuel, volcanic action, the evaporation of water,—all undoubtedly add to the electrical excitement of the atmosphere, and more or less modify the result. It is not important for us, however, to study the action of each one of these causes; for we have, probably, in the friction of moist air driven by the winds, the chief source of atmospheric electricity; and when we consider the amount of friction which must attend the rapid motion of storm-clouds, or of a tornado through the atmosphere, the wonder is, not that an occasional thunderbolt should kindle a conflagration, or even cause a death, but that every storm does not lay waste the earth along its fiery track. Moreover, when we appreciate the vastness of the scale on which the electrical machine of nature is constructed, the thunder-storm ceases to surprise us, and only calls our attention to those beneficent provisions by which we and our race are saved by a constant miracle from the fate of the cities of the plain. That the atmospheric electricity was designed to subserve many important and beneficent ends, the whole analogy of nature compels us to believe; but while our present ignorance conceals them from our sight, we may still discover evidence of God's goodness and wisdom in those simple provisions by which the atmosphere is preserved from violent or frequent electrical excitements, and its charge drawn down harmlessly to the earth.

Since the atmosphere is, at best, a very poor conductor, the electricity developed by the processes just considered tends to accumulate; and under peculiar conditions the clouds may become so highly charged, that at length the pent-up power acquires sufficient force to break through all barriers, and the lightning dashes to the earth, crashing, rending, and burning on its way. To guard his roof from its destructive action, man erects the lightning-rod, whose bristling points quietly drain the clouds, or, failing to do this, receive the charge, and bear it harmlessly to the earth. But ages before Franklin pointed the first rod to the storm, the Merciful Parent of mankind had surrounded the dwellings of his children with a protection far more effectual than this; for, since the creation of organic life, every pointed leaf, every twig, and every blade of grass

has been silently disarming the clouds of their destructive weapon. It is difficult to improve upon nature, and man constantly finds that in his best inventions he has been anticipated from eternity by an Inventor greater than himself. So, not long after Franklin had discovered the efficacy of metallic points in dissipating charges of electricity, and had applied the principle in constructing the lightning-rod, it was found that a common blade of grass, sharpened by nature's exquisite workmanship, was three times as effectual to the end in view as the finest cambric needle, and a single twig far more efficient than the metallic point of the best-constructed rod. When, now, you reflect how many thousands of these vegetable points every large tree directs to the sky, and consider what must be the efficacy of a single forest with its innumerable twigs, or of a single meadow with its countless blades of grass; when you remember that these are only subsidiary to those vast lightning-conductors the mountain-chains, whose craggy summits pierce the clouds themselves; and still further, when you learn that rain-drops and snow-flakes also have been made good conductors, so that during storms a natural bridge for the lightning reaches across from the clouds to the earth, you will see how abundant the protection is, and with what care Providence has guarded us from the destructive agent. It is only under unusual circumstances, when electricity is developed more rapidly than it can be dissipated through these numberless channels, that a violent discharge takes place, and if then it tears, burns, or kills, it also reveals the Merciful Hand which constantly spares. Moreover, through this servant of his pleasure, God is constantly educating and elevating his creatures. In the wild coruscations of the lightning, and in the reverberating roll of the thunder, Nature exhibits one of her grandest aspects, and when, through the cold, dry air of the polar region the electric charges shoot down to the earth in tremulous flashes, we see her lighting up those grand displays of northern fire which enliven the long night of the arctic winter, or in this more favored climate excite the admiration of all.[*]

I must here conclude this very imperfect sketch of the physical adaptations of the atmosphere to the ends it subserves on the earth. We studied in the first place its aeriform condition, and found that its density not only formed an essential part of the scheme of organic nature, but also was closely related to the dimensions of the solar system. In this Lecture we have studied the relations of the atmosphere to light, heat, and electricity; and although we have been able only to glance at some of the more prominent features in these wonderful displays of creative energy, we have found, wherever we turned, abundant illustrations of the wisdom, power, and goodness of our God. I trust that you have been impressed by the vastness, the complexity, and yet

the simplicity and harmony of the whole design, for these are the chief points which I have endeavored to set forth. But oh how imperfect any conception which I can give you must be! This atmosphere is sustained in the proper working of all its parts only by the exact balancing of a thousand conditions. Attempt to make yourself acquainted with these conditions, and, disregarding those which you recognize at once as surpassing human intelligence, study only such as are thoroughly understood and universally admitted to have been primary conditions in the plan of nature before the atmosphere could exist as it is. This is not an impossible task. It would require years of study and it would lead you into every department of physical science, but the result would well repay your labor. You would find it easy to follow out any one line of the conditions, until it became lost in the obscurity of the unknown; but to form an adequate conception of the simultaneous working of all the conditions in their varied bearings, or even of two or three of them, you would soon discover to be a hopeless task. The complication of this wonderful machinery so far transcends man's insight, that to understand its combined action is simply impossible. But although thus made keenly sensible of the limits of human thought, you would be filled with gratitude for the high privilege enjoyed of studying the divine mechanism, even though you understood its workings only obscurely and in part.

Paley has compared the mechanism of nature to a watch, and, so far as the argument for design is

concerned, the analogy is perfect. We must never forget, however, that there is an essential difference between the scheme of nature and the most complicated human mechanism. I have seen a carpet-loom weaving a pattern composed of twelve different colors, and, as I watched the shuttles of various colored yarns which were selected by the hands of the machine with unerring certainty, and thrown through the warp, it seemed as if the very iron were endowed with intelligence, and the impression was one of wonder and bewilderment. To comprehend such complexity appeared impossible; but the more I studied the details of the machine, the more thoroughly I understood the mode of its action, until at last the wonder vanished; and although not ceasing to admire the skill of the inventor, I felt that I had comprehended the whole, and could even conceive of the mental process by which such a wonderful combination of means had been thought out and adjusted to produce the desired end. The artist was ingenious, but the machine was still human.

How different it is with the mechanism of nature! Here, also, it is true, the more we study, the more we understand the workmanship; but then we never reach the limit. The more our powers of thought and observation are developed, and the more our experience is enlarged, the more the field of possible knowledge expands before us. The larger our attainments, the less we seem to know.

We still recognize the unmistakable marks of intelligence

in the design, but it is no longer a fathomable intelligence; we feel that it is infinitely above us: in a word, we feel that it is God. Would that my feeble language might convey to you the full power of this impression; for until one has become conscious of the infinite beauty and skill with which the numberless parts of nature have been fashioned and adjusted, one cannot appreciate the force of the conviction which the impression gives. We may make ourselves familiar with the dimensions of Mont Blanc; we may read the most glowing descriptions of this "monarch of mountains,'' heightened by all the arts of eloquence or of poetry; we may cross the ocean and travel to the beautiful valley of Chamouni at its base; we may even climb its side, study its glaciers, and cross its fields of snow: but we can form no adequate conception of its grandeur, until, ascending one of the lofty mountain-peaks which surround it, we see its summit still towering above our heads, apparently higher than before. So it is in the study of nature. No description can convey an adequate conception of the impression which it leaves upon the mind. It is not until the student, after long study, has become thoroughly acquainted with some one portion, however limited, of its wonderful economy, that he begins to appreciate the perfection of its parts, the infinite skill with which all have been adjusted, and the true grandeur of the whole.

By most men these heights of knowledge are unattainable. Why, then, should we hesitate to receive the evidence of a philosopher like Newton, who,

after spending a long life in the investigation of nature, and with a success unparalleled in the history of science, uttered this memorable sentiment shortly before his death: "I do not know what I may appear to the world; but to myself I seem to have been only like a boy playing on the sea-shore, and devoting myself now and then to finding a smoother pebble or a prettier shell than ordinary, while the great ocean of truth lay all undiscovered before me.'' I know this sentiment has been so many times repeated as to seem trite, but, coming from whom it does, it cannot be too often quoted. It is the testimony of the foremost master of science to its greatest and sublimest truth.

We can all recognize the marks of design in nature, and when we add to this evidence of our senses the testimony of a man like Newton, who assures us that the more our powers are enlarged, and the wider our knowledge becomes, the grander and vaster the design will appear, until it surpasses all our powers of thought or imagination, we begin to feel the full depth of the truth I have been endeavoring to enforce. If our minds are incapable of comprehending the plan, who could have been equal to the design? "Whence, then, cometh wisdom, and where is the place of understanding, seeing it is hid from the eyes of all living, and kept close from the fowls of the air? * * * God understandeth the way thereof, and he knoweth the place thereof. For he looketh to the ends of the earth, and seeth under the whole heaven, to make the weight for the winds * * * and a way for the lightning of

the thunder. Then did he see it and declare it; he prepared it, yea, and searched it out. And unto man he said, Behold the fear of the Lord, that is wisdom, and to depart from evil is understanding.''