Familiar Talks on Science: World-Building and Life; Earth, Air and Water.

Dr. Wells has shown that if we take the reading of two thermometers on a clear summer night, one of them lying on the grass and the other suspended two feet above it, we shall find that the one lying on the grass will read 8 or 10 degrees lower than the one suspended in the air. If the night is still there will be a cold stratum of air next to the earth, which will not tend to diffuse itself to a very great degree and dew will form. If, however, it is cloudy or the wind is blowing there is rarely any formation of dew. The reason in the former case, as we have explained, is that the radiated heat is held down to the earth in a measure, and in the latter case there is a constant change of air; so that in either case no part of it is allowed to cool down sufficiently to precipitate moisture.

It is a curious fact that often there will be a heavier dew under the blaze of a full moon on a clear night than at any other time. The moon has no screens about it of any kind to obstruct the free radiation of heat. It is supposed to be a dead cinder floating in space and not surrounded by an atmosphere, so that the sun's rays have full effect upon it during the time it is exposed to them, and at that time it becomes heated to a temperature of something like 750 degrees Fahrenheit. For half the month, say, the sun is shining continuously upon all or a part of it. In other words, the days and nights of the moon are about two weeks long. The moon does not revolve upon its own axis like the earth, therefore the same side or a portion of it is exposed to the sun for 14 days. During the time that the moon is in the earth's shadow it is supposed to fall to 187 degrees below zero, which is 219 degrees below the freezing point. When the moon is full and is heated up to over 700 degrees there is sufficient heat radiating from it to be felt sensibly upon the face of the earth, and it would be felt if it were not for the great envelope of atmosphere and its attendant cloud formations that surround the earth. There are but few days in summer when there is not a haze in the atmosphere, although we call the sky clear, which intensifies the light and gives everything a warmer tone. The heat coming from a full moon on a clear night is absorbed in causing the aqueous vapors that are partly condensed in the higher regions of the atmosphere, to be reabsorbed into transparent vapor. This clears away the heat screen in the atmosphere and allows radiation to go on more rapidly at the earth's surface, and thus cools it to a greater extent when the moon is shining brightly than when it is dark and in the shadow of the earth.

As we have already mentioned, the cold that is produced by radiation through the blades of grass and other radiating substances may be indicated by placing one thermometer on the ground and fixing another at some point in the air. Sometimes the difference is very marked, amounting to as much as 20 or 30 degrees. If under these conditions a cloud floats overhead, forming a heat screen, its presence will be readily noticed by a rise in the thermometer. Radiation into the upper regions of the atmosphere is checked, which causes a sudden rise in the temperature near the surface of the earth. By taking advantage of this principle of heat radiation from the earth's surface it is a very easy matter to protect tender vegetation from even quite a severe frost, if it occurs in the early fall, by a slight covering, such as thin paper. The paper will act as a heat screen and in a measure prevent the heat from radiating from the earth immediately under it. Frost – which of course is but frozen dew – at this season of the year will form on a still autumn night, although the atmosphere at some distance above the ground is some degrees above the freezing point. The reason for this will be obvious when we consider the facts that have been set forth concerning the power of radiation to produce cold.

It has been estimated by meteorologists that the amount of water condensed upon the surface of the earth in the form of dew amounts to as much as five inches, or about one-seventh of the whole amount of moisture that is evaporated into the air. It will thus be seen that dew performs an important part in supporting vegetation.

The same operation in nature's great workshop that forms the dews of summer creates the frosts of winter. The moisture in cold weather is condensed the same as in warm. When it is condensed at the surface of the earth we have the phenomenon of frost, but when condensed in the upper regions of the atmosphere we have that of snow.

Heat radiation from the earth goes on in winter, which is evidenced by the fact that a thick covering of snow is a great benefit to vegetation as a protection against the injurious effects of frost. The writer has seen flowers blooming abundantly at an altitude of 12,000 feet above the sea-level, protected only by the friendly shelter of a snowbank. In some cases the blooming flowers were in actual contact with the snow. By experiment it has been determined that the earth under a thick coating of snow is usually warmer by nine or ten degrees than the air immediately above the snow covering.

CHAPTER XV

HAILSTONES AND SNOW

A hailstone is a curious formation of snow and ice, and most of the large hailstones are conglomerate in their composition. They are usually composed of a center of frozen snow, packed tightly and incased in a rim of ice, and upon this rim are irregular crystalline formations jutting out in points at irregular distances. Frequently, however, we find them very symmetrically formed as to outline, and the snow centers are almost without exception round. Hailstones and hailstorms differ in different climates, but they are more pronounced in the torrid than in the temperate zone. Historians give accounts of hailstones of enormous size; the very large hailstones being undoubtedly aggregations of single stones that have been thrown together and congealed in the clouds during their fall to the earth.

It is recorded that on July 4, 1819, hailstones fell at Baconniere measuring fifteen inches in circumference, and very symmetrically formed, with beautiful outline. Hailstones in India are said to be very large – from five to twenty times larger than those in England or America – seldom less than walnuts and often as large as oranges and pumpkins. It is recorded that in 1826, during a hailstorm at Candeish, the stones perforated the roofs of houses like cannon shot, and that a single mass fell that required several days to melt, weighing over 100 pounds. It is further recorded that on May 8, 1832, a conglomerate mass of hailstones fell in Hungary a yard in length and nearly two feet in thickness. Still another instance is recorded of a hailstone having fallen in 1849 of nearly twenty feet in circumference. This hailstone is said to have fallen upon the estate of Mr. Moffat of Ord. We will only ask our readers to listen to one more hailstone story, in which it is related that during the reign of Tippoo, sultan, a hailstone fell as large as an elephant. Undoubtedly one of two things was true regarding this latter story; it was either a very large hailstone or a very small elephant. The historian fails to give the size of the elephant. There is no doubt, however, but that hailstones may adhere and form large masses owing to the violent agitation of the elements that always attends a hailstorm.

Hailstorms are almost universally attended by constant and heavy thunder and lightning, together with violent winds. They usually occur on a very hot day, and when the air is filled to saturation with moisture. When this is the case a column of air is very highly heated at some point, when it ascends with great force into the upper regions of the atmosphere to a greater altitude than is common in the case of ordinary thunderstorms. Here it meets with an intensely cold body of air, when it is suddenly condensed and readily frozen as soon as condensed, which not only forms hailstones, but sets free the energy that has been carried up in the moisture globules. This results in frequent electrical discharges, causing great waves of condensed and rarefied air, which, in the rarefied portions, produces still more intense cold; so that we have the conditions for a mighty struggle between the elements, which is intensified by a constant and terrific electric cannonade. Undoubtedly there are also whirlwinds in the cloud, similar to those that sometimes visit the earth, which would tend to gather up the hailstones and aggregate them into large masses. It is a mighty battle between the moisture-laden, superheated air, ascending from the surface of the earth, and the powers residing in the upper regions of cold. Nature is constantly struggling to find an equilibrium of her forces, and a hailstorm is only one of the little domestic flurries that take place when she is setting her house to rights. Hailstorms are usually confined to very narrow limits, and they can prevail on a grand scale only in hot climates, where we have the conditions for wide differences of temperature between the upper and lower regions of the atmosphere; and, also, where the conditions are favorable, for an enormous amount of absorption of moisture into the atmosphere.

When snow is formed in the atmosphere, the conditions are quite different from those of a hailstorm; it is usually in a lower plane of the atmosphere, and there is no violent commotion, as is the case with the latter. A volume of air laden with moisture comes in contact with a colder volume of air, when condensation takes place, as in the case of rain, except that the moisture is immediately frozen. In this case both volumes of air may be below the freezing point, but one is very much colder than the other. If the snow reaches the earth it will be because the air is below the freezing point all the way down. Snow is formed at all seasons of the year. We may have a snowstorm on a high mountain when we have extreme heat at sea-level.

In summer time of course the snow melts as soon as it falls into a stratum of air with a temperature above the freezing point, and continues its journey from that point as raindrops instead of snowflakes. In the formation of a snowflake Nature does some of her most beautiful work. A snowflake first forms with six ice spangles, radiating from a common center. Shorter ones form on these six spokes, standing at an angle of about sixty degrees, on each side of each spoke, of such length and arrangement as to form a symmetrical figure or flower. They do not always take the same form, but follow the same laws that govern the formation of ice crystals. The structure of a snowflake may be often found upon a window pane of a frosty morning. Here, however, the free arrangement of the parts of a snow crystal are interfered with by its contact with the window pane, but while floating gently in the air there is the utmost freedom for the play of nature's forces as they apply to the work of crystallization.

The difference in structure of snowflakes is chiefly due to the conditions under which they are formed. If the moisture is frozen too rapidly the molecular forces that are active in crystallization do not have time to carry out the work, in its completeness of detail, as it will where the freezing process, as well as the condensing process, goes on more slowly.

CHAPTER XVI

METEORS

Meteors are the tramps of interplanetary space. They sometimes try to steal a ride on the surface of the earth, but meet with certain destruction the moment they come within the aërial picket line of our world's defense against these wandering vagrants of the air. They have made many attempts to take this earth by storm, as it were, and many more will be made. They fire their missiles at us by the millions every year with a speed that is incredible, but thanks to the protecting influence of the great ocean of air that envelops our globe they become the victims of their own velocity.

Meteors or shooting stars are as old as the earth itself, and they are the material of which comets are made. Before it was determined what these meteors or shooting stars were, many theories were promulgated as to their origin. One was that they were masses of matter, large and small, projected by volcanic action from the face of the moon with such violence as to be brought within the attraction of the earth. Others supposed them to be the effect of certain phosphoric fluids that emanated from the earth and took fire in the upper regions of the atmosphere. This, however, was mere speculation and without any scientific basis of fact. Anyone who has been an observer of shooting stars will have learned that there are certain periods of the year when they are more numerous than at other times; notably in August and November. Then again there are longer periods of many years apart. By persistent observation it has been established that there are great numbers of schools or collections of cosmic matter that fly through interplanetary space, having definite orbits like the planets. Any one of these collections may be scattered through millions of miles in length. A comet is simply one of these wandering collections of meteoric stones having a nucleus or center where the particles are so condensed as to give it a reflecting surface something like the planets or the moon. This enables us to see the outline of the comet to the point where the fragments of matter become so scattered that they are no longer able to reflect sufficient light to reach our eyes. The fringe of a comet, however, may extend thousands or even millions of miles beyond the borders of luminosity.

There is scarcely a day or night in the year when more or less of these meteoric stones do not come within the region of our atmosphere, and when this happens the great velocity at which they travel is the means of their own destruction. They become intensely heated by friction against the atmosphere just as a bullet will when fired from a gun – only to a greater extent owing to the greater velocity. They disintegrate into dust which floats in the air for a time, when more or less of it is precipitated upon the surface of the earth. Disintegrated meteors, or star dust, as they are sometimes called, are often brought down by the rain or snow. Most of the shooting stars that we observe are very small, resembling fire-flies in the sky, but once in a while a very large one is seen moving across the face of the heavens, giving off brilliant scintillations that trail behind the meteor, making a luminous path that is visible for some seconds. These brilliant manifestations are due to one of two causes. Either there is a very large mass of incandescent matter or else they are so much nearer to us than in ordinary cases that they appear larger. It is more likely, however, that it is due to the former cause rather than the latter, from the fact of its apparently slow movement as compared with the smaller shooting stars. It has been determined by observation that the average meteor becomes visible at a point less than 100 miles above the earth's surface. It was found as far back as 1823 that out of 100 shooting stars twenty-two of them had an elevation of over twenty-four and less than forty miles; thirty-five, between forty and fifty miles; and thirteen between seventy and eighty miles. It was determined by Professor Herschel that out of sixty observations of shooting stars the average height of their first appearance was seventy-eight miles and their disappearance was at a point fifty-three miles above the earth.

It is a matter of history, however, that sometimes these meteoric stones descend to the surface of the earth before they are entirely disintegrated. A fine specimen of this kind is to be seen in the Smithsonian Institution. There are over forty specimens of these aërolites (air-stones) in the British Museum, labeled with the times and places of their fall. Instances of falling to the earth are so rare that there is little to fear from these wandering missiles of the air. We do not remember a case where life or property has suffered from the fall of a meteor.

This brings us to the consideration of the part which the great air envelope surrounding the earth plays as a protection against many outside influences. For instance, if it were not for the air, millions of these meteoric stones would be showered upon our earth every year and at certain times every day, which would render the earth untenable for human existence. We should be at the mercy of those wandering comets whose fringes strike our atmosphere more or less deeply at frequent intervals. It is not impossible that the earth may at some time pass directly through one, and yet there is little danger that in such a case there would be more than an unusual display of celestial fireworks.

From the facts that have been above stated it will be apparent to anyone that the number of these meteoric stones in the air is being constantly reduced by their constant collision with the atmosphere and consequent reduction to ashes or dust. Another conclusion is that the earth must be gradually, but imperceptibly perhaps, increasing in size on account of the constant settling upon its surface of meteoric dust.

CHAPTER XVII

THE SKY AND ITS COLOR

In the chapters on light in Vol. II. it will be stated that we see all objects by a reflected light, except those that are self-luminous, such as the sun or any other source of light. We see the moon and many of the planets entirely by reflection. There are myriads of smaller objects, too small to be seen as such, even under a microscope, that still have a power to reflect light that is sensible to our vision. The air surrounding the globe is literally filled with these microscopic light reflectors. They serve to give us a diffused light which enables us to see clearly all visible objects. We have all noticed the effect of a single electric arc light, situated at a distance from any other source of light, and how it casts extremely dark shadows and very high lights; so much so that it is difficult to see an object perfectly in this light, because the part of an object that is under the direct rays of the lamp is so highly illuminated that the shadow, by comparison, has the effect of simply a dark blot without form or shape. Many of you have noticed in a country village, where the streets are lighted with electric arc lamps, what a difference there is in the illuminating effect between a clear and a foggy night. When there is a fog, or when the clouds hang low down, we get a reflection from these which tends to diffuse and soften the powerful light rays that are sent out by these lamps. This effect is especially noticeable when the night is only moderately foggy. Each globule of moisture floating in the air becomes a reflector of light, and by myriads of reflections and counter reflections the light (which on a clear night is concentrated) is diffused over a large area, producing an illumination which for practical purposes is far superior to that produced on a clear night. When the latter condition prevails the rays of light are so intense on objects immediately surrounding the lamps that one is blinded; so that the places which are in shadow seem darker than they would be if there were no light at all. The only way to prevent this effect is to have the lights so close together that there will be cross lights, which tend to break up the intensity of the shadows. This principle of light diffusion is taken advantage of to produce an even illumination in stores that are lighted only on one or two sides. This is effected by a series of prisms or reflecting surfaces that are cast upon the panes of glass.

If now there were no atmosphere – or, to state it differently – if there were no floating substances in the atmosphere, the sun would produce an effect upon the earth similar to that of a single electric light. The lights would be extremely high, and the shadows extremely dense. To one looking off into space, the sky, instead of having the blue appearance that we see, would have the effect of looking into a deep, dark abyss without illumination.

Tyndall has shown us by a beautiful experiment that if there be in a glass tube a mixture of gases related to each other in a certain way chemically, they will combine into small globules or particles similar to moisture in the air. If now a beam of light is thrown upon this tube and a dark screen put behind it, we shall, in the beginning of the experiment, simply see the dark screen. As soon, however, as the molecules of the gases have combined in sufficient numbers to produce particles of sensible size we begin to have a reflection of light from them, the color of which is constantly changing as the combining particles grow in size. At a certain stage in its progress the color which the mixture of gases assumes is a beautiful azure blue, rivaling in purity the finest skies of Greece or southern Italy.

The sun is the great lamp that illuminates the world, while the atmosphere, which is filled with particles of various substances, becomes the shade of the lamp which diffuses and softens the light and gives it its color tones, whether of warmth or coldness. We could not well do without the reflected light of the sky. The poetry of life would be sadly marred. The beautiful effects of color and purity of tone would be wanting. We need to bathe in light as much as in water, and the character of the light is almost as important as the character of the water. Imagine a world with an atmosphere devoid of all substances that would in any way reflect light or give to it softness or color tone. Imagine a sun or a moon without visible rays – for without a reflecting atmosphere there would be none. Imagine a sky that was no sky at all, but only a dark void, with no protecting vault. Think of the shadows, so dark that you could see nothing in them. These would be some of the effects that would come from an atmosphere that had no sky substance in it. Imagine the world lighted by one great arc light. The reflex action upon the race living in such a light would be anything but desirable. The world would develop into an arc-light civilization – if one can imagine what that would be like; certainly one of intensely violent contrasts. Look on this picture and let us be thankful for the blue sky and golden sunsets.

"But," you ask, "why is the sky blue?"

In one of the chapters on the subject of light in Vol. II. the properties of soap bubbles are discussed. It is shown that when a film is stretched across the mouth of a tumbler held in a position so that the film is perpendicular, by the action of gravity (the moisture constantly falling to the lower part of the film) it will continually grow thinner, and horizontal bands of color will appear upon it, – first red, then followed by the other colors of the solar spectrum, ending with violet.

It is also stated that every color of light has a definite wave length. Where a band of blue color appears upon the film we know that its thickness is right for the wave length of that particular color which is reflected from the back of the film to the eye. If we could conceive the blue vault of the heavens to be half a sphere of a soap bubble, the color that the sky would appear to us (if the light could be thrown upon it from beneath) would be determined by the thickness of this film. If the film was 1-156,000 of an inch the sky would be red instead of blue. To reflect the other colors the film would have to grow thinner for each color, in the progression from red to violet. The color of the sky is determined by a light-reflection from minute globules of moisture floating in the air. If the sky is blue, then the globules must be of the right diameter to reflect that color. The various tints and colorings of the sky are determined by what is found in the atmosphere, and this is the reason why skies differ in coloring and tone in different sections of the globe. The finest skies are probably found in semi-tropical regions like southern Italy, Greece, and California.

In 1892 I visited Greece in the early part of June. In crossing the Adriatic, from Brindisi to Patras in Greece, the route was through the Ionian Islands that are grouped along the southwestern shore of Albania. The sky was without a cloud, and its beautiful blue color was reflected in the waters of the Adriatic, and I never shall forget the impression made upon my senses when we first came in sight of the mountains on the west coast of Albania. At this point they rise abruptly from the water and are colored with that peculiar azure haze, mixed with a shading of warmth, which is an effect that distance gives in the classic atmosphere of old Greece. The effect upon the beholder is to intoxicate the senses and to fill him with that deliciously poetic feeling that always comes when standing in the presence of the sublime in nature. It was not the mountains themselves that produced the effect, for I had seen grander than these; but it was the sky on the mountains. When we look at a distant mountain it seems to be partly hidden by a peculiar haze that is the color of the sky at that time; we are really looking at the mountain through a portion of the sky. While in Athens I took a trip to the top of Mount Pentelicus, which separates the plains of Athens on the south from those of Marathon on the north. From the summit of this mountain we have a most wonderful view of the archipelago of the Ægean Sea – a beautiful map of blue water and brown islands that melt together in the distance. At our feet lay the historic plains of Marathon, and in the distance rose the snow-capped peaks of Mount Olympus. It is doubtful if the world furnishes a more beautiful combination of ocean, island, continent, and sky than can be seen from Mount Pentelicus. Myriads of brown islands set in the bluest of water – graceful in outline and multiform in shape – jutting headlands and land-locked harbors – strong in color and outline in the immediate foreground, but gradually melting together in the distance, the brown becoming bluer and the blue a softer blue till the whole is lost on the horizon in a sky that shades back to the zenith in an ever-changing azure that for purity of tone baffles all description.