CHAPTER I. OPTICAL PHENOMENA.
It has been already mentioned that clouds are collections of aqueous globular particles suspended in the air, and when the rays of the sun strike upon a cloud, they are partly reflected and partly absorbed by the suspended globules. Where a great quantity of water is suspended, the rays of light are proportionately absorbed, and hence the sky, when surcharged with rain, is dark, and the day looks gloomy. When, on the contrary, the clouds are thin, they reflect nearly all the rays that fall upon them, and hence, in fine weather, the clouds look white, and abundance of light is diffused through the sky. Owing to the excessive minuteness of the watery particles contained in the clouds, the rays of light which fall upon them are only reflected or absorbed by their external surface, without entering them; but when the watery particles are collected in large drops, as they are when it rains, the light enters them and becoming divided into its primitive colours, it forms what we call a rainbow. This, however, can only be seen when the sun shines upon the drops of rain which fall behind the spectator, the coloured arch being a portion of a circle, whose centre is a point in the sky directly opposite the sun. Generally the arch is less than a semicircle, but its length is increased in proportion as the spectator is raised above the surface of the earth; and hence, when viewed from a very lofty situation, rainbows appear almost circular. Sometimes a secondary rainbow is perceived, but as it is produced by a double reflection, its tints are inverted, and fainter than the first; and occasionally, but rarely, a third bow may be faintly traced. Lunar rainbows are also observed occasionally, but the faintness of their colours renders them much less conspicuous than the rainbows of the sun.
Halos or rings are often seen surrounding both the sun and moon, and they are evidently caused by a very thin vapour diffused through the atmosphere, which occasions a deflection of light. It has been observed that these halos are never seen but when the atmosphere is surcharged with moisture; and hence it is a common popular saying, that a dense halo round the moon portends rain.
The parhelia consists of two or more mock suns, which are frequently seen in the Arctic regions, and are occasioned by the refraction of light from the snowy spiculae constantly floating in the air. The parhelia sometimes presents a gorgeous appearance of intersecting luminous arches studded with numerous images of the sun, and it is described as being one of the most splendid of all the atmospheric phenomena.
Another most remarkable optical deception, called the mirage, occurs in the atmosphere on the verge of the horizon in various countries, and especially on level plains, and the surface of the sea. The appearance presented is that of a double image of a given object, one of the images being in the natural position, but greatly elevated and enlarged, and the other inverted. It was frequently seen on the dry sandy plains of Egypt by the French army, during their campaign in that country, and it assumed the appearance of a lake, reflecting the shadows of objects within and around it, but apparently receding as the thirsty soldiers eagerly approached it. The delusions of the Fata Morgana, which are seen in the Bay of Reggio, in the Straits of Messina, in Sicily, belong to the same class of optical deceptions; though the people of the country, who are extremely ignorant, fancy that it is actually the city of the fairy Morgana that they see, and they run down to the sea coast, clapping their hands, and crying with every mark of exultation and joy, "Morgana! Morgana! la bella fata Morgana!" This curious phenomenon is not seen very frequently, but when it is the sun must form an angle of 45° with the sea, which must not be disturbed by either wind or current, and then "the spectator being placed on an eminence of the city, with his back to the sun, and his face to the sea, on a sudden sees appear in the water, as in a magic glass, various multiplied objects; as, for example, a numberless series of pilasters, arches, castles, well delineated regular columns, lofty towers, superb palaces with balconies and windows, extended alleys of trees, delightful plains, with herds and flocks, &c., all in their natural colours and proper action, passing rapidly in succession along the surface of the sea, during the whole of the short period of time that the above mentioned causes remain." Sometimes the same objects are also reflected in the air, but less distinctly; and again, on other occasions, if the air be slightly hazy, the objects will appear fringed with all the colours of the rainbow. These objects are, in fact, only those really existing in the Bay of Naples, but which could not possibly be seen in Sicily under any other circumstances. Similar delusions have been observed in various places, such as the submerged cities in the Lakes of Albano and Killarney; the spectre of the Brocken, and that seen on Cumberland Fells; the flying Dutchman, and other ships, which have been seen from time to time apparently sailing in the air and many other similar apparitions, down to the common appearance of the looming of a ship at sea, when the distant vessel often appears twice the size that she is in reality. A very remarkable instance of the mirage occurred in the year 1798, when the coast of France was distinctly seen from the beach at Hastings, appearing greatly raised above the sea for nearly an hour, and then sinking gradually till it disappeared.
This curious phenomenon is produced when the surface of the earth or sea becomes suddenly much more heated than the atmosphere. The earth first communicates its heat to the layer of air immediately above it, and which thus becomes less dense than the upper strata, and whenever the rays of light pass through a dense medium to one less dense, they become refracted, and turn back. Sir David Brewster illustrated this phenomenon by holding a heated iron over a mass of water, and as the heat descended, the density of the fluid gradually increased from the surface to the bottom. He then withdrew the heated iron, and substituted another, on which a quantity of ice was laid. This suddenly cooled the upper part of the air over the water, leaving the lower part warm; and till the whole became of the same degree of heat, the lower stratum of air which was next the water produced all the beautiful effects of refracting light from the objects around it, which are observed in the different forms of the mirage.
CHAPTER II. METEORS.
It is well known that inflammable air is much lighter than common air, and hence, as the inflammable air contained in the atmosphere rises naturally to its upper strata, it is there where meteors and other phenomena, occasioned by the combustion of inflammable air in the atmosphere, take place.
The aurora borealis appears to approach nearest to those single electrical discharges from the clouds called sheet or summer lightning, and, like them, it generally appears in bright flashes, which are sometimes attended by a slight crackling noise. It has been observed that the aurora is never seen but in cold latitudes where thunder and lightning are unknown, or at least very uncommon; and as thunder and lightning only occur when the electric equilibrium of the atmosphere is restored by means of aqueous vapours, it has been conjectured that the aurora borealis is occasioned by the restoration of the electrical equilibrium by the intervention of frozen particles, which being imperfect conductors become luminous whilst transmitting this electricity. The aurora borealis is most common on the edge of the Frozen Sea, or wherever there is a vast accumulation of ice; and in these situations it is generally seen forming a luminous arch, with a dark black cloud below it, and shooting out vertical columns of pale yellow light. On other occasions it has been seen in milder climates, but it is observed, that in proportion as the climate where it appears becomes warmer, its appearance is more rare, and it becomes more like the common sheet lightning. On some occasions, however, very brilliant exhibitions of the aurora borealis have been seen even in the neighbourhood of London; and on the 12th of October, 1833, it appeared with unusual splendour at Hull. On this occasion there was a broad, irregular, semicircular belt of vivid white light stretching across the northern hemisphere of the heavens. It was exactly like a rainbow in shape, but of a pure white light, which was so extremely brilliant as to obscure the lustre of the fixed stars, and to give them a bluish tint, while the cloud beneath the arch appeared intensely black. It was first seen about half past eight in the evening, and soon after its first appearance, a number of brilliant rays began to shoot up from different parts of the arch, sometimes broad, sometimes narrow, sometimes vanishing immediately, and sometimes remaining visible for a minute or more, and looking like gigantic pillars of flame. The most extraordinary appearance, however, was a rapid waving motion attended by frequent brisk flashes of fire, which passed rapidly through the whole length of the curve. These flashes were often divided into a number of square portions, separated by broad black lines. The exhibition was most brilliant about ten o'clock, but it continued till after two in the morning. In 1828, an aurora borealis of this kind appeared in London; and on the 7th of January, 1831, a remarkably beautiful one was seen at Woolwich, which appeared again the following night, but much fainter. Sometimes it does not appear as a distinct arch, but looks like the reflection of a large fire, and a few years ago an appearance of this kind in London occasioned the fire engines to be out nearly all the night searching for imaginary fires.
AURORA BOREALIS.
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In the southern hemisphere, what is called the aurora australis exactly resembles the light seen in the aurora of the north.
The hydrogen gas, which is formed by the decomposition of water in the atmosphere, being lighter than the other gases, rises above them, and, being very inflammable, appears easily to ignite; when it does so, it descends rapidly in the form of fireballs and falling stars, till it is extinguished by the aqueous vapours it meets with in the clouds, or near the earth. In some cases these meteors are very beautiful, and in the year 1833 a shower of them fell in America, which looked like a discharge of fire-works, and lasted about half-an-hour. In South America these luminous appearances are not uncommon; and they have been seen occasionally in almost every part of the world.
Mr. Angas, in his Savage Life and Scenes in New Zealand, describes a large and very brilliant meteor which passed swiftly along the sky from the westward, leaving a train of sparks surrounded by a black nimbus, like a wreath of curling smoke. He saw it burst and fall into a shower of blazing stars.
Meteors of this kind have been frequently seen in Europe, and some of the most extraordinary have even appeared in Great Britain. One of these, which was called the flaming sword, was first seen at Leeds on the 18th of May, in the year 1710. It was broad at one end and small at the other, and it moved with the broad end foremost. The light from this meteor was so sudden and bright, that the people who beheld it distinctly saw their own shadows reflected. The meteor in its course was seen in various parts of Yorkshire, Lancashire, Nottingham, and Derby; and it was singular that those who saw it fall, though they were many miles distant from each other, all fancied that it fell within a few yards of them. Nine years afterwards a blazing meteor was seen in every part of England. It first appeared like a long stream of light, but it gradually thickened at one end till it assumed a pearshape; after which it became large and round like a full moon. It was agreed by all spectators in London that the splendour of this meteor was little inferior to that of the sun. In-doors the candles did not give any light, and out of doors both the moon and stars appeared so pale as scarcely to be visible. Various other meteors have been recorded, but none equal in brilliancy to those which have been already mentioned.