IT ALWAYS RAINS
All are familiar with the answer given by the native of Skye to the irate tourist on that island, who, for the sixth day drenched, asked the question: “Does it always rain here?” “Na!” answered the workman, without at all understanding the joke; “feiles it snaas” (sometimes it snows). Yet, strange to say, the tourist’s question has been answered in the affirmative in every place where a cloud is overhead, visible or invisible.
Whenever a cloud is formed, it begins to rain; and the drops shower down in immense numbers, though most minute in size—“the playful fancies of the mighty sky.”
No doubt it is only in certain circumstances that these drops are attracted together so as to form large drops, which fall to the earth in genial showers to refresh the thirsty soil, or in a terrible deluge to cause great destruction. But when the temperatureand pressure are not suitable for the formation of what we commonly know as the rain, the fine drops fall into the air under the cloud, where they immediately evaporate from their dust free-surfaces, if the air is dry and warm. This is, in other words, the decay of clouds.
It is a curious fact that objects in a fog may not be wetted, when the number of water-particles is great. It seems that these water-particles all evaporate so quickly that even one’s hand or face is not sensible of being wetted. The particles are minutely small; and they may evaporate even before reaching the warm skin, by reason of the heated air over the skin.
There is a peculiarly warm sensation in the centre of a cumulus cloud, especially when it is not dense. A glow of heat seems to radiate from all points. Yet the face and hands are quite dry, and exposed objects are not wetted; but it is reallyalways raining. That is a curious discovery.
It is radiant heat that is the cause of the remarkable result. The rays of the sun, which strike the upper part of the cloud, not only heat that surface but also penetrate the cloud and fall on the surface of bodies within, generating heat there. These heated surfaces again radiate heat into the air attached to them. This warm air receives the fine raindrops in the cloud, and dissolves the moisture from the dust-particles before the moisture can reach the surfaces exposed. That a vast amount of radiant heat rushes through a cloud is clearly shown by exposing a thermometer with black bulbin vacuo. On some occasions, a thermometer would indicatefrom 40° to 50° above the temperature of the air, thus proving the surface to be quite dry.
These observations have been corroborated on Mount Pilatus, near Lucerne—1000 feet higher and more isolated than the Rigi. The summit was quite enveloped in cloud, and, though one might naturally conclude that the air was dense with moisture, yet the wooden seats, walls, and all exposed surfaces were quite dry. Strange to say, however, the thermometers hung up got wet rapidly, and the pins driven into the wooden post to support them rapidly became moist. A thermometer lying on a wooden seat stood at 60°, while one hung up read only 48°. This difference was caused by radiant heat.
It is well known that, when bodies are exposed to radiant heat, they are heated in proportion to their size; the smaller, then, may be moist, when the larger are dry by radiation. The effect of the sun’s penetrating heat through the cloud is to heat exposed objects above the temperature of the air; and if the objects are of any size they are considerably heated, and retain their heat more, while at the same time around them is a layer of warm air which is quite sufficient to force the water-vapour to leave the dust-particles in the fine rain.
Hence seats, walls, posts, &c., are quite dry, though they are in the middle of a cloud. They are large enough to throw off the moisture by the retained heat, or by the large amount of surrounding heat; whereas, small bodies, which are not heated to the same degree and cannot therefore retain their heat so easily, have not heat-power sufficient to withstand the moisture, and they become wetted. Hence, bythe radiant heat, the large exposed objects are dry in the cloud; whereas small objects are damp, and, in some cases, dripping with wet.
The fact is, then, that whenever a cloud overhangs,rain is falling, though it may not reach the earth on account of the dryness of the stratum of air below the cloud, and the heat of the air over the earth. So that on a summer day, with the gold-fringed, fleecy clouds sailing overhead, it is really raining; but the drops, being very small, evaporate long before reaching the earth. As Ariel sings at the end of “The Tempest” of Shakespeare, “The rain, it raineth every day.” It rains, but much of the melting of the clouds is reproduced by a wonderful circularity—the moisture evaporating, seizing other dust-particles, forming cloud-particles, falling again, and so onad infinitum, during the existing circumstances.
HAZE
What is haze? The dictionary says, “a fog.” Well, haze isnota fog. In a fog, the dust-particles in the air have been fully clothed with water-vapour; in a haze, the process of condensation has been arrested.
Cloudy condensation is changed to haze by the reduction of its humidity. Dr. Aitken invented a simple apparatus for testing the condensing power of dust, and observing if water-vapour condensed on the deposited dust in unsaturated air.
The dust from the air has first to be collected. This is done by placing a glass plate vertically, and in close contact with one of the panes of glass in the window, by means of a little india-rubber solution. The plate being thus rendered colder than the air in the room, the dust is deposited on it.
Construct a rectangular box, with a square bottom, 1½ inches a side and ¾ inch deep, and open at the top. Cover the top edge of the box with a thickness of india-rubber. Place the dusty plate—a square glass mirror, 4 inches a side—on the top of the india-rubber, and hold it down by spring catches, so as to make the box water-tight. The box has been provided with two pipes, one for taking in water and the other for taking away the overflow, with the bulb of a thermometer in the centre. Clean the dust carefully off one half of the mirror, so that one half of the glass covering the box is clean and the other half dusty. Pour cold water through the pipe into the box, so as to lower the temperature of the mirror, and carefully observe when condensation begins on the clean part and on the dusty part, taking a note of the difference of temperature. The condensation of the water-vapour will appear on the dust-particles before coming down to the natural dew-point temperature of the clean glass. And the difference between the two temperatures indicates the temperature above the dew-point at which the dust has condensed the water-vapour.
Magnesia dust has small affinity for water-vapour; accordingly, it condenses at almost exactly the same temperature as the glass. But gunpowder has great condensing power. All have noticed that the smokefrom exploded gunpowder is far more dense in damp than in dry weather. In the experiment it will be found that the dust from gunpowder smoke begins to show signs of condensing the vapour at a temperature of 9° Fahr. above the dew-point. In the case of sodium dust, the vapour is condensed from the air at a temperature of 30° above the dew-point.
Dust collected in a smoking-room shows a decidedly greater condensing power than that from the outer air.
We can now understand why the glass in picture frames and other places sometimes appears damp when the air is not saturated. When in winter the windows are not often cleaned, a damp deposit may be frequently seen on the glass. Any one can try the experiment. Clean one half of a dusty pane of glass in cold weather, and the clean part will remain undewed and clear, while the dusty part is damp to the eye and greasy to the touch.
These observations indicate that moisture is deposited on the dust-particles from air, which is not saturated, and that the condensation takes place while the air is comparatively dry,beforethe temperature is lowered to the dew-point. There is, then, no definite demarcation between what seems to us clear air and thick haze. The clearest air has some haze, and, as the humidity increases, the thickness of the air increases.
In all haze the temperature is above the dew-point. The dust-particles have only condensed a very small amount of the moisture so as to form haze, before the fuller condensation takes place at the dew-point.
At the Italian lakes, on many occasions when the air is damp and still, every stage of condensationmay be observed in close proximity, not separated by a hard and fast line, but when no one could determine where the clear air ended and the cloud began. Sometimes in the sky overhead a gradual change can be observed from perfect clearness to thick air, and then the cloud.
A thick haze may be occasioned by an increased number of dust-particles with little moisture, or of a diminished number of dust-particles with much moisture, above the point of saturation. The haze is cleared by this temperature rising, so as to allow the moisture to evaporate from the dust-particles.
Whenever the air is dry and hazy, much dust is found in it; as the dust decreases the haze also decreases. For example, Dr. Aitken, at Kingairloch, in one of the clearest districts of Argyleshire, on a clear July afternoon, counted 4000 dust-particles in a cubic inch of the air; whereas, two days before, in thick haze, he counted no fewer than 64,000 in the cubic inch. At Dumfries the number counted on a very hazy day in October increased twenty-fold over the number counted the day before, when it was clear.
All know that thick haze is usual in very sultry weather. The wavy, will-o’-the-wisp ripples near the horizon indicate its presence very plainly. During the intense heat there is generally much dust in the atmosphere; this dust, by the high temperature, attracts moisture from the apparently dry air, though above the saturation point. In all circumstances, then, the haze can be accounted for by the condensing power of the dust-particles in the atmosphere, at a higher temperature than that required for the formation of fogs, or mists, or rain.
HAZING EFFECTS OF ATMOSPHERIC DUST
The transparency of the atmosphere is very much destroyed by the impurities communicated to it while passing over the inhabited areas of the country. Dr. Aitken devoted eighteen months to compare the amount of dusty impurities in different masses of air, or of different airs brought in by winds from different directions.
He took Falkirk for his centre of observations. This town lies a little to the north of a line drawn between Edinburgh and Glasgow, and is nearly midway between them. If we draw a line due west from it, and another due north, we find that, in the north-west quadrant so enclosed, the population of that part of Scotland is extremely thin, the country over that area being chiefly mountainous. In all other directions, the conditions are quite different. In the north-east quadrant are the fairly well-populated areas of Aberdeenshire, Forfarshire, and the thickly populated county of Fife. In the south-east quadrant are situated Edinburgh and the well-populated districts of the south-east of Scotland. And in the south-west quadrant are Glasgow and the large manufacturing towns which surround it. The winds from three of these quadrants bring air polluted in its passage over populated areas, whereas the winds from the north-west come comparatively pure.
The general plan of estimating the amount ofhaze is to note the most distant hill that can be seen through the haze. The distance in miles of the farthest away hill visible is then called “the limit of visibility” of the air at the time. For the observations made at Falkirk, only three hills are available, one about four miles distant, the Ochils about fifteen miles distant, and Ben Ledi about twenty-five miles distant—all in the north-west quadrant. When the air is thick, only the near hill can be seen; then the Ochils become visible as the air clears; and at last Ben Ledi is seen, when the haze becomes still less. After Ben Ledi is visible, it then becomes necessary to estimate the amount of haze on it, in order to get the limit of visibility of the air at the time. Thus, if Ben Ledi be half-hazed, then the limit of visibility will be fifty miles. In this way all the estimates of haze have been reduced to one scale for comparison.
As the result of all the observations it was found that, as the dryness of the air increases, the limit of visibility also increases. A very marked difference in the transparency of the air was found with winds from the different directions. In the north-west quadrant the winds made the air very clear, whereas winds from all other directions made the air very much hazed. The winds in the other three areas are nearly ten times more hazed than those from the north-west quadrant. That is very striking.
The conclusion come to is that the air from densely inhabited districts is so polluted that it is fully nine times more hazed than the air that comes from the thinly inhabited districts; in other words, the atmosphere at Falkirk is about ten times thickerwhen the wind is east or south than it would be if there were no fires and no inhabitants.
It is interesting to notice that the limit varies considerably for the same wind at the same humidity. This is what might have been expected, because from the observations made by the dust-counter the number of particles varied greatly in winds from the same directions, but at different times. This depends upon the rise and fall of the wind, changes in the state of trade, season of the year, and other causes. During a strike, the dearth of coal will make a considerable diminution in the number of dust-particles in the air of large towns. With a north wind, the extreme limits of visibility are 120 to 200 miles; and with a north-west wind, from 70 to 250 miles. An east wind has as limits 4 to 50 miles, and a south-east wind 2 to 60 miles.
One interesting fact to be noticed, after wading through these tables, is this—that, as a general result, the transparency of the air increases about 3·7 times for any increase in dryness from 2° to 8° of wet-bulb depression. That is, the clearness of the air is inversely proportional to the relative humidity; or, put another way, if the air is four times drier it is about four times clearer.
THUNDER CLEARS THE AIR
The phrase “thunder clears the air” is familiar to all. It contains a very vital truth, yet even scientificmen did not know its full meaning until just the other day. It came by experience to people who had been for ages observing the weather; and it is one of the most pointed of the “weather-lore” expressions. Folks got to know, by a sort of rule-of-thumb, truths which scientifically they were unable to learn. And this is one.
Perhaps, therefore, we should respect a little more what is called “folk-lore,” or ordinary people’s sayings. Experience has taught men many wonderful things. In olden times they were keener natural observers. They had few books, but they had plenty of time. They studied the habits of animals and moods of nature, and they came wonderfully near to reaching the full truth, though they could not give a reason for it. The awe-inspiring in nature has especially riveted the attention of man.
And no appearance in nature joins more powerfully the elements of grandeur and awe than a heavy thunder-storm. When, suddenly, from the breast of a dark thunder-cloud a brilliant flash of light darts zigzag to the earth, followed by a loud crackling noise which softens in the distance into weaker volumes of sound, terror seizes the birds of the air and the cattle in the field. The man who is born to rule the storm rejoices in the powerful display; but kings have trembled at the sight.
Byron thus pictures a storm in the Alps:—
“Far alongFrom peak to peak, the rattling crags amongLeaps the live thunder! Not from one lone cloud,But every mountain now hath found a tongue,And Jura answers, through her misty shroud,Back to the joyous Alps, who call to her aloud!”
Franklin found that lightning is just a kind of electricity. No one can tell how it is produced; yet a flash has been photographed. When the flash is from one cloud to another there is sheet-lightning, which is beautiful but not dangerous; but, when the electricity passes from a cloud to the earth in a forked form, it is very dangerous indeed. The flash is instantaneous, but the sound of the thunder takes some time to travel. Roughly speaking, the sound takes five seconds or six beats of the pulse to the mile.
All are now taught at school that it is the oxygen in the air which is necessary to keep us in life. If mice are put into a glass jar of pure oxygen gas, they will at once dance with exhilarating joy. It occurred to Sir Benjamin Richardson, some time ago, that it would be interesting to continue some experiments with animals and oxygen. He put a number of mice into a jar of pure oxygen for a time; they breathed in the gas, and breathed out water-vapour and carbonic acid. After the mice had continued this for some time, he removed them by an arrangement. By chemical means he removed the water-vapour and carbonic acid from the mixed air in the vessel. When a blown-out taper was inserted, it at once burst into flame, showing that the remaining gas was oxygen.
Again, the mice were put into this vessel to breathe away. But, strange to say, the animals soon became drowsy; the smartness of the oxygen was gone. At last they died; there was nothing in the gas to keep them in life; yet, by the ordinary chemical tests, it was still oxygen. It had repeatedlypassed through the lungs of the mice, and during this passage there had been an action in the air-cells which absorbed the life-giving element of the gas. It is oxygen, so far as chemistry is concerned, but it has no life-giving power. It has beendevitalised.
But the startling discovery still remains. Sir Benjamin had previously fitted up the vessel with two short wires, opposite each other in the sides—part outside and part inside. Two wires are fastened to the outside knobs. These wires are attached to an electric machine, and a flash of electricity is made to pass between the inner points of the vessel. The wires are again removed; nothing strange is seen in the vessel. But, when living mice are put into the vessel, they dance as joyfully as if pure oxygen were in it. The oxygen in which the first mice died has now been quite refreshed by the electricity. The bad air has been cleared and made life-supporting by the electric discharge. It has been again vitalised.
Now, to apply this: before a thunder-storm, everything has been so still for days that the oxygen in the air has been to some extent robbed of its life-sustaining power. The air feels “close,” a feeling of drowsiness comes over all. But, after the air has been pierced by several flashes of lightning, the life-force in the air is restored. Your spirits revive; you feel restored; your breathing is far freer; your drowsiness is gone. Then there is a burst of heavenly music from the exhilarated birds. Thus a thunder-storm “clears the air.”
After the passage of lightning through the air ozone is produced—the gas that is produced after a flash of electricity. It is a kind of oxygen, withfine exciting effects on the body. If, then, the life-sustaining power of oxygen depends on a trace of ozone, and this is being made by lightning’s work, how pleased should we be at the occasional thunder-storm!
DISEASE-GERMS IN THE AIR
The gay motes that dance in the sunbeams are not all harmless. All are annoying to the tidy housekeeper; but some are dangerous. There are living particles that float in the air as the messengers of disease and death. Some, falling on fresh wounds, find there a suitable feeding-place; and, if not destroyed, generate the deadly influence. Others are drawn in with the breath; and, unless the lungs can withstand them, they seize hold and spread some sickness or disease. From stagnant pools, common sewers, and filthy rooms, disease-germs are constantly contaminating the air. Yet these can be counted.
The simplest method is that of Professor Frankland. It depends on this principle: a certain quantity of air is drawn through some cotton-wool; this wool seizes the organisms as the air passes through; these organisms are afterwards allowed to feed upon a suitable nutritive medium until they reach maturity; they are then counted easily.
About an inch from each end of a glass tube (5 inches long and 1 inch bore), the glass is pressedin during the process of blowing. Some cotton-wool is squeezed in to form a plug at the farther constricted part of the glass. The important plug is now inserted at the same open end, but is not allowed to go beyond the constricted part at its end. A piece of long lead tubing is attached to the former end by an india-rubber tube. The other end of the lead tubing is connected with an exhausting syringe. Sixty strokes of the 18 cubic inches syringe will draw 1080 cubic inches of the air to be examined through the plugs, the first retaining the organisms.
The impregnated plug is then put into a flask containing in solution some gelatine-peptone. The flask is made to revolve horizontally until an almost perfectly even film of gelatine and the organisms from the broken-up plug cover its inner surface.
The flask is allowed to remain for an hour in a cool place, and is then placed under a bell-jar, at a temperature of 70° Fahr. There it remains, to allow the germs to incubate, for four or five days. The surface of the flask having been previously divided into equal parts by ink lines, the counting is now commenced. If the average be taken for each segment, the number of the whole is easily ascertained. A simple arithmetical calculation then determines the number of organisms in a cubic foot, since the number is known for the 1080 cubic inches. That is the process for determining the number of living organisms in a fixed quantity of air.
No less than thirty colonies of organisms were counted in a cubic foot of air taken from the Golden Gallery of St. Paul’s Cathedral, London, and 140 from the air of the churchyard. An ordinary manwould breathe there thirty-six micro-organisms every minute.
Electricity has a powerful effect in destroying these organisms. Ozone is generated in the air by lightning, and it is detrimental to them. In fine ozoned Highland air scarcely a disease-germ can be detected. Open sea air contains about one germ in two cubic feet. It has been found that in Paris the average in summer is about 140 per cubic foot of air, but in bedrooms the number is double. During the twenty-four hours of the day the number of germs is highest about 6A.M., and lowest about mid-day.
Raindrops carry the germs to the ground. Hence the advantage of a thunder plout in a sanitary way. A cubic foot of rain has been found to contain 5500 organic dust-germs, besides 7,000,000,000 of inorganic dust-particles. In a dirty town the rain will bring down in a year, upon a square foot of surface, no less than 3,000,000 of bacteria, many of them being disease-bearing and death-bearing. No wonder, then, that scientific men are using every endeavour to protect the human frame, as well as the frame of the lower animals, from the baneful inroads of these floating nuclei of disease and death.
A CHANGE OF AIR
For weakness of body and fatigue of mind a very common and essentially serviceable recommendation is “a change of air.” Of course, the change of scenefrom coast to country, or from town to hillside, may help much the depressed in body or mind; and this is very commendable. But, strange to say, there is a healing virtue in breathing different air.
At first one is apt to think that air is the same all over, as he thinks water is—especially outside smoky towns; but both have varied qualities in different parts. You have only to be assured that in a cubic inch of bedroom air in the denser parts of a large town there are about 20,000,000 of dust-particles, and in the open air of a heathery mountain-side there are only some hundreds, to see that there is something after all on the face of it in the “old wives’ saw.”
Not that the dust-particles are all injurious; for most of them are inorganic, and many of the organic particles are quite wholesome; yet there is a change wrought, often very marked, in going from one place to another for different air.
Even in the country, especially in summer-time, one distinctly notices the great difference in the air of lowland and highland localities. The ten miles change from Strathmore to Glenisla shows a marked difference in the air. Below, it is close, weakening, enervating; above, it is exhilarating, invigorating, and strong.
So people must have a change—at least those who can afford it—for health must be seen to first of all, if one has means to do so. Oh! the blessing of good health! How many who enjoy it never think of the misery of its loss! In fact, health is the soul that animates all enjoyments of life; for without it those would soon be tasteless. A man starves at thebest-spread table, and is poor in the midst of the greatest treasures without health.
In these days half of our diseases come from the neglect of the body in the overwork of the brain. The wear and tear of labour and intellect go on without pause or self-pity. Men may live as long as their forefathers, but they suffer more from a thousand artificial anxieties and cares. The men of old fatigued only the muscles, we exhaust the finer strength of the nerves. Even more so now, then, do we require a change of air to soothe our overwrought nervous system.
And when that magic power, concealed from mortal view, works such wonders on the health, surely it is one’s duty to save up and have it, when it is within one’s means. For is not health the greatest of all possessions? What a rich colour clothes the countenance of the young after a month’s outing in the hill country! How fine and pure has the blood become! All stagnant humours, accumulated in winter town life, have been dispelled by the ozone-brightening charm. The weary looking office or shop man is now transfigured into a sprightly youth once more, ready with strongly recuperated power for another winter’s labours. The pale wife, who has been stifled for months in close-aired rooms, has now a healthy flush on her becoming countenance that speaks of gladly restored health. And all this has been brought about by a “change of air”!
For a thorough change to a town man, he should make for the Highlands. There he is never tired of walking, for the air which he breathes is full of ozone. This revivifying element in the air is produced bythe lightning-bursts from hill to hill. There is in the Highlands a continual rush of electricity, whether seen or not. Hence the air is very pure, free from organic germs, intensely exhilarating and buoyant.
Sportsmen are livingly aware of the recuperative power of the Highland air. Perhaps these city men do not benefit so much by the easy walking exercise on the grouse moors as in breathing the splendidly delight-inspiring air. What a change one feels there in a very few hours!
“A change of air” is an old wives’ adage. But much of the weather-lore of our forefathers was based on real scientific principles only now coming to light. Nature is ever true, but it requires patience to unravel her secrets. We therefore advocate an occasional “change of air” to improve the health—
“The chiefest good,Bestow’d by Heaven, but seldom understood.”
THE OLD MOON IN THE NEW MOON’S ARMS
After the sun’s broad beams have tired the sight, the moon with more sober light charms us to descry her beauty, as she shines sublimely in her virgin modesty. There is always a most fascinating freshness in the first sight of the new moon. The superstition of centuries adds to this charm. Why boys and girls like to turn over a coin in their pocket at this sight one cannot tell: yet it is done. Noyoung lady likes to look at the new moon through a pane of glass. And farmers are always confident of a change of weather with a new moon: at least in bad weather they earnestly hope for it.
But, banishing all superstition, we welcome the pale silver sickle in the heavens, once more appearing from the bosom of the azure. And no language can equal these beautiful words of the youthful Shelley:—
“Like the young moon,When on the sunlit limits of the nightHer white shell trembles amid crimson air,And while the sleeping tempest gathers might,Doth, as the herald of his coming, bearThe ghost of its dead mother, whose dim formBends in dark ether from her infant’s chair.”
That is a more charming way of putting the ordinary expression, “the old moon in the new moon’s arms.” We are regularly accustomed to the moonshine, but only occasionally is theearthshineon the moon so regulated that the shadowed part is visible. This is not seen at the appearance of every new moon. It depends upon the positions of the sun and moon, the state of the atmosphere, and the absence of heavy clouds. I never in my life saw the phenomenon so marvellously beautiful as on May 7th, 1894, at my manse in Strathmore. I took particular note of it, as some exceedingly curious things were connected with it.
At nine o’clock in the evening, the new moon issued from some clouds in the western heavens, the sun having set, about an hour before. The crescent was thin and silvery, and the outline of the shadowed part was just visible. The sky near the horizon wasclear and greenish-hued. As the night advanced the moon descended, and at ten o’clock she was approaching a purple stratum of clouds that stretched over the hills, while the position of the sun was only known a little to the east, by the back-thrown light upon the dim sky. Through the moisture-laden air the sun’s rays, reflected by the moon, threw a golden stream from the crescent moon, for the silvery shell became more golden-hued.
The horns of the moon now seemed to project, and the shadowed part became more distinct, though the circle appeared smaller. By means of a field-glass I noticed that this was extra lighted, with points here and there quite golden-tinged. The darker spots showed the deep caverns; the brighter points brought into relief the mountain peaks.
Why was the surface brighter than usual? I cannot go into detail about the phases of the moon; but, in a word, I may say that, while the sun can illuminate the side of the moon turned towards it, it is unable to throw any light on the shadow, seeing that there is no atmosphere around the moon to refract the light.
If we, in imagination, looked from the moon upon the earth, we should see the same phases as are now noticed in the moon; and when it is just before new moon on the earth, the earth will appear fully illuminated from the moon. We would also observe (from the moon) that the brightness of the illuminated part of the earth would vary from time to time, according to the changes in the earth’s atmosphere. More light would be reflected to the moon from the clouds in our atmosphere than from the bare earth orcloudless sea, since clouds reflect more light than either land or sea. Accordingly, we arrive at this curious fact—that the extra brightness of thedarkbody of the moon is mainly determined by the amount ofcloud in our atmosphere.
Accordingly, I concluded that there must be clouds to the west, though I could not see them, which reflected rays of light and faintly illuminated the shadowed part of the moon. It had become much colder, and I concluded that during the night the cloud-particles, if driven near by the wind, would condense into rain. And, assuredly, next morning I was gratified to find that rain had fallen in large quantities, substantiating the theory.
There is much pleasure in verifying such an interesting problem. The dark body of the moon being more than usually visible is one of our well-known and oldest indications of coming bad weather. And at once came to my memory the lines of Sir Patrick Spens, as he foreboded rain for his crossing the North Sea:—
“I saw the new moon late yestreenWi’ the auld moon in her arm;And if we gang to sea, master,I fear we’ll come to harm.”
This lunar indication, then, has a sound physical basis, showing that near the observer there are vast areas of clouds, which are reflecting light upon the moon at the time, before they condense into rain by the chilling of the air. According to the old Greek poet, Aratus: “If the new moon is ruddy, and you can trace the shadow of the complete circle, a storm is approaching.”
AN AUTUMN AFTERGLOW
A brilliant afterglow is welcomed for its surpassing beauty and a precursor of fine fixed weather.
A glorious sunset has always had a charm for the lover of nature’s beauties. The zenith spreads its canopy of sapphire, and not a breath creeps through the rosy air. A magnificent array of clouds of numberless shapes come smartly into view. Some, far off, are voyaging their sun-bright paths in silvery folds; others float in golden groups. Some masses are embroidered with burning crimson; others are like “islands all lovely in an emerald sea.” Over the glowing sky are splendid colourings. The flood of rosy light looks as if a great conflagration were below the horizon.
I remember witnessing an especially brilliant sunset last autumn on the high-road between Kirriemuir and Blairgowrie. The setting sun shone upon the back of certain long trailing clouds which were much nearer me than a range behind. The fringes of the front range were brilliantly golden, while the face of those behind was sparklingly bright. Then the sun disappeared over the western hills, and his place was full of spokes of living light.
Looking eastward, I observed on the horizon the base of the northern line of a beautiful rainbow—“the shepherd’s delight” for fine weather.
Soon in the west the light faded; but there cameout of the east a lovely flush, and the general sky was presently flamboyant with afterglow. The front set of clouds was darker except on the edges, the red being on the clouds behind; and the horizon in the east was particularly rich with dark red hues.
Gradually the eastern glow rose and reddened all the clouds, but the front clouds were still grey. The effect was very fine in contrast. The fleecy clouds overhead became transparently light red, as they stretched over to reach the silver-streaked west. The new moon was just appearing upright against a slightly less bright opening in the sky, betokening the firm hardness of autumn.
Soon the colouring melted away, and the peaceful reign of the later twilight possessed the land.
Now why that brilliancy of the east, when the west was colourless? Most of all you note the immense variety and wealth of reds. These are due to dust in the atmosphere. We are the more convinced of this by the very remarkable and beautiful sunsets which occurred after the tremendous eruption at Krakatoa, in the Straits of Sunda, thirty years ago. There was then ejected an enormous quantity of fine dust, which spread over the whole world’s atmosphere. So long as that vast amount of dust remained in the air did the sunsets and afterglows display an exceptional wealth of colouring. All observers were struck with the vividly brilliant red colours in all shades and tints.
The minute particles of dust in the atmosphere arrest the sun’s rays and scatter them in all directions; they are so small, however, that they cannot reflect and scatter all; their power is limited tothe scattering of the rays at the blue end of the spectrum, while the red rays pass on unarrested. The display of the colours of the blue end are found in numberless shades, from the full deep blue in the zenith to the greenish-blue near the horizon.
If there were no fine dust-particles in the upper strata, the sunset effect would be whiter; if there were no large dust-particles, there would be no colouring at all. If there were no dust-particles in the air at all, the light would simply pass through into space without revealing itself, and the moment the sun disappeared there would be total darkness. The very existence of our twilight depends on the dust in the air; and its length depends on the amount and extension upwards of the dust-particles.
But how have the particles been increased in size in the east? Because, as the sun was sinking, but before its rays failed to illumine the heavens, the temperature of the air began to fall. This cooling made the dust-particles seize the water-vapour to form haze-particles of a larger size. The particles in the east first lose the sun’s heat, and first become cool; and the rays of light are then best sifted, producing a more distinct and darker red. As the sun dipped lower, the particles overhead became a turn larger, and thereby better reflected the red rays. Accordingly, the roseate bands in the east spread over to the zenith, and passed over to the west, producing in a few minutes a universal transformation glow.
To produce the full effect often witnessed, there must be, besides the ordinary dust-particles, small crystals floating in the air, which increase the reflection from their surfaces and enhance the glow effects.In autumn, after sunset, the water-covered dust-particles become frozen and the red light streams with rare brilliancy, causing all reddish and coloured objects to glow with a rare brightness. Then the air glows with a strange light as of the northern dawn. From all this it is clear that, though the colouring of sunset is produced by the direct rays of the sun, the afterglow is produced by reflection, or, rather, radiation from the illuminated particles near the horizon.
The effect in autumn is a stream of red light, of varied tones, and rare brilliancy in all quarters, unseen during the warmer summer. We have to witness the sunsets at Ballachulish to be assured that Waller Paton really imitated nature in the characteristic bronze tints of his richly painted landscapes.
A WINTER FOREGLOW
Little attention has been paid to foreglows compared with afterglows, either with regard to their natural beauty or their weather forecasting. But either the ordinary red-cloud surroundings at sunrise, or the western foreglow at rarer intervals, betokens to the weather-prophet wet and gloomy weather. The farmer and the sailor do not like the sight, they depend so much on favourable weather conditions.
Of course, sunrise to the æsthetic observer has always its charms. The powerful king of day rejoices “as a bridegroom coming out of hischamber” as he steps upon the earth over the dewy mountain tops, bathing all in light, and spreading gladness and deep joy before him. The lessening cloud, the kindling azure, and the mountain’s brow illumined with golden streaks, mark his approach; he is encompassed with bright beams, as he throws his unutterable love upon the clouds, “the beauteous robes of heaven.” Aslant the dew-bright earth and coloured air he looks in boundless majesty abroad, touching the green leaves all a-tremble with gold light.
But glorious, and educating, and inspiring as is the sunrise in itself in many cases, there is occasionally something very remarkable that is connected with it. Rare is it, but how charming when witnessed, though till very recently it was all but unexplained. This is theforeglow.
It is in no respect so splendid as the afterglow succeeding sunset; but, because of its comparative rarity, its beauty is enhanced. I remember a foreglow most vividly which was seen at my manse, in Strathmore, in January 1893. My bedroom window looked due west; I slept with the blind up. On that morning I was struck, just after the darkness was fading away, with a slight colouring all along the western horizon. The skeleton branches of the trees stood out strongly against it. The colouring gradually increased, and the roseate hue stretched higher. The old well-known faces that I used to conjure up out of the thin blended boughs became more life-like, as the cheeks flushed. There was rare warmth on a winter morning to cheer a half-despairing soul, tired out with the long hours of oilreading, and pierced to the heart by the never-ceasing rimes; yet I could not understand it.
I went to the room opposite to watch the sunrise, for I had observed in the diary that the appearance of the sun would not be for a few minutes. There were streaks of light in the east above the horizon, but no colour was visible. That hectic flush—slight, yet well marked—which was deepening in the western heavens, had no counterpart in the east, except the colourless light which marked the wintry sun’s near approach. As soon as the sun’s rays shot up into the eastern clouds, and his orb appeared above the horizon, the western sky paled, the colour left it, as if ashamed of its assumed glory. A foreglow like that I have very rarely seen, and its existence was a puzzle to me till I studied Dr. Aitken’s explanation of the afterglows after sunset. I had never come across any description of a foreglow; and, of course, across no explanation of the curious phenomenon. The western heavens were coloured with fairly bright roseate hues, while the eastern horizon was only silvery bright before the sun rose; whereas, after the sun appeared and coloured the eastern hills and clouds, the western sky resumed its leaden grey and colourless appearance. Why was that? What is the explanation?
I have not space enough to repeat the explanation given already in the last chapter of the glorious phenomenon of the afterglow. But the explanation is similar. Before sunrise, the rays of the sun are reflected by dust-particles in the zenith to the western clouds. The colouring is intensified by the frozen water-vapour on these particles in the west.
One thing I carefully noted. Ere mid-day, snow began to fall, and for some days a severe snow-storm kept us indoors. Then, at any rate, the foreglow betokened a coming storm. It was, like a rainbow in a summer morning, a decided warning of the approaching wet weather.
THE RAINBOW
The poet Wordsworth rapturously exclaimed—
“My heart leaps up when I beholdA rainbow in the sky.”
And old and young have always been enchanted with the beautiful phenomenon. How glorious is the parti-coloured girdle which, on an April morning or September evening, is cast o’er mountain, tower, and town, or even mirrored in the ocean’s depths! No colours are so vividly bright as when this triumphal arch bespans a dark nimbus: then it unfolds them in due prismatic proportion, “running from the red to where the violet fades into the sky.”
A plain description of the formation of the rainbow is not very easily given, but a short sketch may be useful. Beautiful as is the ethereal bow, “born of the shower and colour’d by the sun,” yet the marvellous effect is more exquisitely intensified in its gorgeous display when the hand of science pointsout the path in which the sun’s rays, from above the western horizon, fall on the watery cloud, indicating fine weather—“the shepherd’s delight.”
One law of reflection is that, when a ray of light falls on a plane or spherical surface, it goes off at the same angle to the surface as it fell. One law of refraction is that, when a ray of light passes through one medium and enters a denser medium (as from air to water), it is bent back a little. By refraction you see the sun’s rays long after the sun has set; when the sun is just below the horizon, an observer, on the surface of the earth, will see it raised by an amount which is generally equal to its apparent diameter.
The rays of different colours are bent back (when passing through the water) at different rates, some slightly, others more, from the red to the violet end. The rainbow, then, is produced by refraction and reflection of the several coloured rays of sunlight in the drops of water which make up falling rain.
The sun is behind the observer, and its rays fall in a parallel direction upon the drops of rain before him. In each drop the light is dispersively refracted, and then reflected from the farther face of the drop; it travels back through the drop, and comes out with dispersing colours.
According to the height of the sun, or the slope of its rays, a higher or lower rainbow will be formed. And, strange, no two people can see the very same bow; in fact the rainbow, as seen by the one eye, is not formed by the same water-drops as the rainbow seen by the other eye.
When the primary bow is seen in most vividcolours on a dark cloud, a second arch, larger and fainter, is often seen. But the order of the colours is quite reversed. At a greater elevation, the sun’s ray enters the lower side of a drop of rain-water, is refracted, reflectedtwice, and then refracted again before being sent out to the observer’s eye. That is why the colours are reversed.
A one-coloured rainbowis a curious and rare phenomenon. It is a strange paradox, for the very idea of a rainbow brings up the seven colours—red, orange, yellow, green, blue, indigo, and violet. Yet Dr. Aitken tells us of a rainbow with one colour which he observed on Christmas Day, in 1888.
He was taking his walk on the high ground south of Falkirk. In the east he observed a strange pillar-like cloud, lit up with the light of the setting sun. Then the red pillar extended, curved over, and formed a perfect arch across the north-eastern sky. When fully developed, this rainbow was the most extraordinary one which he had ever seen. There was no colour in it but red. It consisted simply of a red arch, and even the red had a sameness about it.
Outside the rainbow there was part of a secondary bow. The Ochil Hills were north of his point of observation. These hills were covered with snow, and the setting sun was glowing with rosy light. Never had he seen such a depth of colour as was on them on this occasion. It was a deep, furnacy red. The sun’s light was shorn of all the rays of short-wave length on its passage through the atmosphere, and only the red rays reached the earth. The reason why the Ochils glowed with so deep ared was owing to their being overhung by a dense curtain of clouds, which screened off the light of the sky. The illumination was thus principally that of the direct softer light of the sun.
THE AURORA BOREALIS
He must be a very careless observer who has not been struck with the appearance of the streamers which occasionally light up the northern heavens, and which farmers consider to be indicators of strong wind or broken weather.
The time was when the phenomenon was considered to be supernatural and portentous, as the chroniclers of spectral battles, when “fierce, fiery warriors fought upon the clouds, in ranks and squadrons, and right form of war.” And even in the rural districts of Britain, the blood-coloured aurora, of October 24th, 1870, was considered to be the reflection of an enormous Prussian bonfire, fed by the beleaguered French capital.
In joyful spirit, the Shetlanders call the beautiful natural phenomenon, “Merry Dancers.” Burns associated their evanescence with the transitoriness of sensuous gratification:—“they flit ere you can point their place.” And Tennyson spoke of his cousin’s face lit up with the colour and light of love, “as I have seen the rosy red flushing in the northern night.”
Yet this phenomenon is to a great extent under the control of cosmical laws. One of the most difficultproblems of our day has been to disentangle the irregular webwork of auroræ, and bring them under a law of periodicity, which depends upon the fluctuations of the sun’s photosphere and the variations on the earth’s magnetism, and which have such an important influence upon the fluctuations of the weather.
The name “Aurora Borealis” was given to it by Gassendi in 1621. Afterwards, the old almanacs described it as the “Great Amazing Light in the North.” In the Lowlands of Scotland, the name it long went by, of “Lord Derwentwater’s Lights,” was given because it suddenly appeared on the night before the execution of the rebel lord. In Ceylon auroræ were called “Buddha Lights.”
The first symptom of an aurora borealis is commonly a low arch of pale, greenish-yellow light, placed at right angles to the magnetic meridian. Sometimes rays cover the whole sky, frequently showing tremulous motion from end to end; and sometimes they appear to hang from the sky like the fringes of a mantle. They are among the most capricious of natural phenomena, so full of individualities and vagaries. To the glitter of rapid movement they add the charm of vivid colouring. It is strongly asserted that auroræ are preceded by the same general phenomena as thunder-storms. This was borne out by Piazzi Smith (late Astronomer-Royal for Scotland), who observed that their monthly frequency varies inversely with that of thunder-storms—both being safety-valves for the discharge of surplus electricity.
Careful observers have, moreover, noticed aremarkable coincidence between the display of auroræ and the maxima of the sun’s spots and of the earth’s magnetic disturbances. Some have supposed that the light of the aurora is caused by clouds of meteoric dust, composed of iron, which is ignited by friction with the atmosphere. But there is this difficulty in the way, shooting stars are more frequent in the morning, while the reverse is the case with the aurora. The highest authorities have concluded, pretty uniformly, that auroræ are electric discharges through highly rarefied air, taking place in a magnetic field, and under the sway of the earth’s magnetic induction. They are not inappropriately called “Polar lightnings,” for when electricity misses the one channel it must traverse the other.
The natives of the Arctic regions of North America pretend to foretell wind by the rapidity of the motions of the streamers. When they spread over the whole sky, in a uniform sheet of light, fine weather ensues. Fitzroy believed that auroræ in northern latitudes indicated and accompanied stormy weather at a distance. The same idea is still current among many farmers and fishermen in Scotland.
Is there any audible accompaniment to the brilliant spectacle? The natives of some parts, with subtle hearing-power, speak of the “whizzing” sound which is often heard during auroral displays. Burns tells of their “hissing, eerie din,” as echoes of the far-off songs of the Valkyries. Perhaps the most striking incident which corroborates this opinion occurred during the Franco-Prussian War. Rolier, a practised aëronaut, left Paris in a balloon, on his mission of city defence, and fourteen hours afterwards landedin Norway. He had reached the height of two and a half miles. When descending, he passed through a peculiar cloud of sulphurous odour, which emitted flashed light and a slight scratching or rustling noise. On landing, he witnessed a splendid aurora borealis. He must, therefore, have passed through a cloud in which an electrical discharge of an auroral nature was proceeding, accompanied with an audible sound. There is, moreover, no improbability of such sounds being occasionally heard, since a somewhat similar phenomenon accompanies the brush discharge of the electric machinery, to which the aurora bears considerable resemblance.
Though no fixed conclusions are yet established about the causes of the brilliant auroral display, yet, as the results of laborious observations, we are assured that the stabler centre of our solar system holds in its powerful sway the several planets at their respective distances, supplying them all with their seasonable light and heat, vibrating sympathetic chords in all, and even controlling under certain—though to us still unknown—laws the electric streamers that flit, apparently lawlessly, in the distant earth’s atmosphere.
THE BLUE SKY
If we look at the sky overhead, when cloudless in the sunshine, we wonder what gives the air such a deep-blue colour. We are not looking, aschildren seem to do, into vacancy, away into the far unknown. And even, if that were the case, would not the space be quite colourless? What, then, produces the blueness?
Some of the very fine dust-particles, even when clothed with an exceedingly thin coating of water-vapour, are carried very high; and, looking through a vast accumulation of these, we find the effect of a deep-blue colour.
Why so? Because these particles are so small that they can only reflect the rays of the blue end of the spectrum; and the higher we ascend, the smaller are the particles and the deeper is the blue. But it is also because water, even in its very finest and purest form, is blue in colour. For long this was disputed. Even Sir Robert Christison concluded, after years of experimenting on Highland streams, that water was colourless.
Of course, he admitted that the water in the Indian and Pacific Oceans has frequent patches of red, brown, or white colour, from the myriads of animalcules suspended in the water. Ehrenberg found that it was vegetable matter which gave to the Red Sea its characteristic name. But these, and similar waters, are not pure.
It is to Dr. Aitken that the final discovery of the real colour of water is due. When on a visit to several towns on the shores of the Mediterranean, he set about making some very interesting experiments, which the reader will follow with pleasure.
It is a well-known fact that colour transmitted through different bodies differs considerably from colour reflected by them. In his first experiment hetook a long empty metal tube, open at one end, and closed at the other end by a clear-glass plate. This was let down vertically into the water, near to a fixed object, which appeared of most beautiful deep and delicate blue at a depth of 20 feet. Scientific men know that, if the colour of water is due to the light reflected by extremely small particles of matter suspended in the water, then the object looked at through it would have been illuminated with yellow (the complementary colour of blue). A blackened tube was then filled with water (which had a clear-glass plate fixed to the bottom), and white, red, yellow, and purple objects were sunk in the water, and these colours were found to change in the same way as if they were looked at through a piece of pale-blue glass. The white object appeared blue, the red darkened very rapidly as it sank, and soon lost its colour; at the depth of seven feet a very brilliant red was so darkened as to appear dark brick-red. The yellow object changed to green, and the purple to dark blue.
But, still further to satisfy himself that water is really blue in itself, even without any particles suspended in it, he tested the colour ofdistilledwater. He filled a darkened tube with this water (clear-glass plates being at the ends of the tube), and looked through it at a white surface. The effect was the same as before, the colour was blue, almost exactly of the same hue as a solution of Prussian blue.
This is corroborated by the fact that, the purer the water is in nature, the bluer is the tint when a large quantity is looked through. Some Highland lochs have crystal waters of the most extraordinary blue. Of course, some cling to the old idea that thisis accounted for by the reflected blue of the clear heavens above. No doubt, if the sky be deep blue, then this blue light, when reflected by the surface of the water, will enrich and deepen the hue. But the water itself isreallyblue.
At the same time, the dust-particles suspended in the water have a great effect in making the water appear more beautiful, brilliant, and varied in its colouring; because little or no light is reflected by the interior of a mass of water itself. If a dark metal vessel be filled with a weak solution of Prussian blue, the liquid will appear quite dark and void of colour. But throw in some fine white powder, and the liquid will at once become of a brilliant blue colour. This accounts for the change of depth and brilliancy of colour in the several shores of the Mediterranean.
When, then, you look at the face of a deep-blue lake on a summer evening—the heavens all aglow with the unrivalled display of colour from the zenith, stretching in lighter hues of glory to the horizon—though to you the calm water appears like a lake of molten metal glowing with sky-reflected light, so powerful and brilliant as entirely to overpower the light which is internally reflected, yet blue is the normal colour of the water:blueness is its inherent hue.
Looking upwards, we observe three distinct kinds of blue in the sky from the horizon to the zenith. All painters in water-colours know that. Newton thought that the colour of the sky was produced in the same way as the colours in thin plates, the order of succession of the colours gradually increasing in intensity.