ATMOSPHERIC PHENOMENA—THUNDER AND LIGHTNING—AURORA BOREALIS—THE RAINBOW—MOCK SUNS AND MOCK-MOONS—HALOS—FATA MORGANA—REFLECTION AND REFRACTION—MIRAGE—SPECTRE OF THE BROCKEN.

There are a great number of interesting, and to inhabitants of these islands uncommon,—perhaps we might say fortunately uncommon,—phenomena, which overtake the traveller in other countries. We have referred to whirlwinds and tornados, and will now mention two phenomena connected with these storms. There is the water-spout, for instance, and sand-pillars in the desert, which are whirled up by these winds in spiral columns of water and sand respectively. The tiny whirlwind at cross-roads, which picks up straws and leaves, is the common appearance of whirling or crossing currents of air.

Waterspouts, when they are permitted to come near a ship at sea, or when they break upon land, which is seldom, are very destructive. The waterspout is begun generally by the agitation of the sea, and the cloud above drops to meet the water, which at last unites with it, and then the column of whirling liquid, tremendously disturbed at the base, advances with the prevailing wind. Its course is frequently changed, and ships within its influence would be speedily wrecked. The only way to save the vessel is to fire a cannon ball through the column and break it.

A waterspout once devastated a district in the Hartz mountains of Saxony. “A long tube of vapour descended to the earth, and several times was drawn upward again; but at last it reached the ground, and travelled along at the rate of four-and-a-half miles in eight minutes, destroying everything in its way.”

On another occasion at Carcassonne in 1826, “a reddish column was seen descending to the ground, and a young man was caught up by it and dashed against a rock.” His death was instantaneous.

The cause of these whirling winds is supposed to be in the action of vertical currents of air which ascend heated, and return rapidly as cold air. The “waterspouts,” etc., are quickly formed. The tornado is a monster whirlwind like a waterspout in form, and advances at a tremendous rate—eastward as a rule. It moves in leaps and bounds, passing over some portions of the ground and descending again. The current of air is directed to the centre; the cyclone, as mentioned, has a spiral or rotatory movement.

Thunder and lightning have been, to some extent, described under the head of Electricity, but some observations may also be introduced here, as storms of that nature appertain to meteorology distinctly.

Electricity is always present in the atmosphere, and arises from evaporation and condensation as well as from plants. As the air becomes moist, the intensity of the so-called “fluid” increases, and more in winter than in summer. Clear skies are positively electric, and when large, heavy clouds are perceived in process of formation in a sky up to that time clear, a storm is almost certain to follow. These “thunder clouds,” in which a quantity of electricity exists, attract or repel each other respectively. The cloud attracts the opposite kind of electricity to that within it; and when at last a tremendous amount has been stored up in the cloud and in the air, or in another cloud, the different kinds seek each other, and lightning is the result, accompanied by a reverberation and commotion of the air strata, called thunder.

Lightning most frequently darts from cloud to cloud, but often strikes the ground, whereon and in which are good conductors, such as wet trees, metals, running water, etc. The “electric fluid” assumes different forms—“forked,” “sheet,” and “globular.” The second is perhaps the most familiar to us, and the third kind is the least known of all. There are many well-authenticated instances on record in which lightning with the form and appearance of fireballs has entered or struck houses and ships.

“Fulgarites” are vitreous tubes formed in sandy soils by the lightning in search of subterranean water-courses, for running water is a great conductor of electricity.34 The fire-ball form of lightning has been known to enter a school-house where a number of children were, and to singe the garments of some, killing others. The ball passed out through a pane of glass, in which it bored a hole, breaking every other pane, however, in its transit. Another instance occurred in which the lightning ran about the floor of a room, and descending the stairs, exploded without doing any injury.

Lightning, like the electric current of the laboratory, will not always set fire even to inflammable objects. An electric spark can be passed through gunpowder without setting fire to it, and lightning will often shatter the object without firing it. Death by lightning is instantaneous, and in all probability quite painless; for we may argue from analogy, that as those who have been rendered insensible by lightning have had no remembrance of seeing the flash which strikes so instantaneously, nor of hearing thunder after it, it is instantaneous in its effects. Besides, the natural attitude is preserved, and the face is usually peaceful and limbs uncontorted after death by lightning.

There are some curious electrical phenomena, such as St. Elmo’s Fire, already noticed under Electricity; and in some parts of America, in very hot weather, such a light is perceived to issue from trees as the fire glides through the forest. Many instances are on record concerning the luminosity of pointed sticks, and even of the tails and manes of horses in certain conditions of the atmosphere, and of the universal power of electricity and its pervading influence in nature. The benefits conferred by thunderstorms in purifying the air, and in the production of ozone and nitric acid, are very great, and apart from the magnificent phenomena exhibited, are well worth our attention, though beyond our reach.

Terrestrial magnetism, however, is still more puzzling in its action than is electricity, and the study of the needle, its destination, inclination, and intensity, which are marked upon charts, just as are the weather reports of the Times, is an interesting one. These magnetic maps are termed the charts of Isoclinic and Isodynamic lines. The declination of the magnetic needle from the true north is its deviation from that point, and the “inclination” is its dip towards the horizon. The line of its direction being known as the magnetic meridian, its divergence from this line constitutes its declination. There are places where it does not deviate, and these, in direction north and south, are called lines of “no variation.” There are also places in the equatorial regions where the needle does not “dip.” The line connecting such places is termed the Magnetic Equator, and north or south of this the needle dips respectively to north or south in degrees coinciding with the distance from the equator.

The earth, then, acts as a magnet, and attracts the needle, but the magnetic poles are not identical with the terrestrial poles. The north magnetic pole was reached in 1831 by Sir James Ross, when the dip was only one minute less than 90°, and the south magnetic pole was very nearly reached also by him in 1840. The magnetic equator passes between these two points.

It is to magnetic atmospherical disturbance that the aurora is due. These northern (or southern) phenomena are extremely brilliant and diversified. In temperate regions the aurora does not present such grand forms as in the extreme north. There the spectacle is astonishingly beautiful. The sky at first clouds over, and mist is developed. Humboldt has eloquently described the aurora borealis, and the beautiful changes of light, the constant movement, flashes, etc., denoting a “magnetic” storm, as electrical discharges indicate an electric storm, although the area affected by the former is far more extensive than that of the latter, and there is no thunder accompanying the magnetic storm, with the production of which the electricity of the earth is unassociated. To the continuous flow of this electricity the aurora is due, and the flashes are only the electric current descending towards the earth. But the true reason of the phenomena may have to be yet discovered, for nothing absolutely certain is known as to the origin of the aurora.

Amongst the numerous effects of refraction and reflection of light the Rainbow is most common and the most beautiful. If we hold a chandelier “drop” in the sunlight, we shall see a brilliant representation of the rainbow on the wall or on the carpet. The three colours—red, yellow, and blue—mingle or shade away into seven—red, orange, green, blue, yellow, indigo, and violet. These colours are all found in the rainbow.

The colour of the atmosphere—the usual blue tint of the sky—arises from the blue rays of the spectrum being reflected more than the rest by the aerial particles, and the less vapour the bluer the sky, because the vapour gives it a whitish or misty tint. At sunset and sunrise the sky is red or yellow, like gold, or of crimson hue. This is because the sun’s rays have so much farther to come to us at sunrise or sunset, as you will readily perceive if you draw a line from the sun to the sides and then to the top of the arc of the heavens. The blue rays are thus lost in space, while red and yellow, which travel so much faster than blue, are transmitted to the eye, not giving the air time to absorb them.

If you go under water and look at the sun it will appear very fiery indeed, and we may likewise imagine that fiery crimson rays, which betoken atmospherical disturbance, very often are due to the moisture through which they are transmitted. Wet and storm frequently succeed a crimson sunset, which betokens much moisture in the air. The sun is similarly seen through the steam issuing from an engine, and the colours vary according to the density of the steam in its stages of condensation.

Vapour, we know, is invisible and transparent, but when it has been condensed into rain-drops, and the sun is shining, if we stand with our backs to the sun we see what we call the rainbow, because a ray of light entering the drop is reflected, and as all rays are not of equal refrangibility, the light, which is composed of three simple rays, is divided and reflected into those and the complementary colours. When the sun is at the horizon, the rainbow, to an observer on the earth (but not on a mountain), will appear to be a semi-circle. The higher the sun rises the lower is the centre of the rainbow. So we can never see rainbows at noon in summer because the sun is too high. A second rainbow is not uncommon, the second reflection producing the colours in a different order. The colours in the “original” range from violet to red; in the “copy” they extend from red to violet. “Rainbows” are often visible in the spray of waterfalls and fountains.

Halos are frequently observed surrounding the moon, and then we are apt to prognosticate rain.

is an old couplet referring to the appearance of the moon, and is supposed to foreshadow the weather by the size of the halo, which is caused, as we know, by the existence of vesicular vapour in the atmosphere.

Mock Suns, or parhelia, and mock-moons, or paraselenÆ, are continually observed in cold climates, where the tiny ice particles are so abundant in the air. These phenomena were recognized by the ancients, and halos round the sun can be observed by means of darkened glasses. We annex an illustration of a mock sun and moon seen on the continent of Europe. Readers of Mr. Whymper’s “Scrambles in the Alps” will remember the gorgeous, and to the guides mysterious, fog-bow or sun-bow seen as the survivors of the first and most fatal ascent of the Matterhorn in 1865 were tremblingly pursuing their descent over the upper rocks of that mountain.

The Mirage, or Fata Morgana, is a very curious but sufficiently common phenomena, and in the Asiatic and African plains it is frequently observed. When the weather is calm and the ground hot, the Egyptian landscape appears like a lake, and the houses look like islands in the midst of a widely-spreading expanse of water. This causes the mirage, which is the result of evaporation, while the different temperatures of the air strata cause an unequal reflection and refraction of light, which give rise to the mirage. Travellers are frequently deceived, but the camels will not quicken their usual pace until they scent water.

The Fata Morgana and the inverted images of ships seen at sea are not uncommon on European coasts. Between Sicily and Italy this effect is seen in the Sea of Reggio with fine effect. Palaces, towers, fertile plains, with cattle grazing on them, are seen, with many other terrestrial objects, upon the sea—the palaces of the Fairy Morgana. The inverted images of ships are frequently perceived as shown in the illustration (fig. 729), and many most extraordinary but perfectly authentic tales have been related concerning the reflection and refraction of persons and objects in the sky and on land, when no human beings nor any of the actual objects were within the range of vision.

It will be well to explain this phenomenon, and the diagrams will materially assist us in so doing, for the appearances are certainly startling when realized for the first time. The Spectre of the Brocken we see mimics our movements, and we can understand it. But when apparently solid buildings appear where no buildings have been erected,—when we see—as has been perceived—soldiers riding across a mountain by a path, or ledge, perfectly inaccessible to human beings even on foot, we hesitate, and think there is something uncanny in the sight. Let us now endeavour to explain the mirage.

Suppose that in the annexed diagram the space enclosed between the letters A, B, C, D, be a glass vessel full of water. The ship is below the horizon, the eye being situated at E—the glass vessel of course representing the atmosphere charged with moisture. The eye at E will perceive the top of the mast of the ship, S, and we may imagine a line drawn from E to S. Then put a (short focus) convex lens at a just above this (imaginary) line, and a concave one, b, just over it. Through the former an inverted ship will be seen, and an erect one through the latter at S' and S respectively. We now have the effect in the air just as reproduced in nature by the difference in temperature in air strata, which cause it to act like a concave lens when the density of the water diminishes towards the centre, and like a convex lens when it is increased.

This can be proved by heating the air (by hot irons) above the glass vessel filled with oil, and the effects will be just the same as through the lenses. Dr. Wollaston obtained the mirage by using a clear syrup,—about one-third of the vessel full,—and filling it with water. The gradual mingling of these fluids will produce the phenomenon. The illustration in the margin (fig. 731) shows us the rays proceeding from the ship’s hull, and refracted into the line reaching the eye, above the line proceeding from the mast, so the ship appears hull uppermost; the rays cross at x. But if they did not cross before they reach the eye, the image would appear as at s´ p´ in an erect position.

The Spectre of the Brocken arises from a different cause. Such appearances are only shadows,—projected on thin clouds or dense vapours at sunrise, or when the sun’s rays are directed horizontally,—for of course vertical rays will throw the shadow on the ground on to the zenith. Balloons are also reflected thus, and much interest has been caused by the appearance of a twin balloon, until the aerial voyagers have discovered the cheat by seeing the shadowy aeronaut imitating their actions, and the second balloon has been discovered to be an airy nothing.