[131] Nothing is more easy than to account for these productions, which have no relation to any events, no more than comets, that may happen to follow them. The appearance of two suns has frequently happened in England, as well as in other places, and is only caused by the clouds being placed in such a situation as to reflect the image of that luminary; nocturnal fires, inflamed spears, fighting armies, were no more than what we call aurora borealis, northern lights, or inflamed vapours floating in the air; showers of stones, of ashes, or of fire, were no other than the effects of the eruptions of some volcano at a considerable distance. Showers of milk were only caused by some quality in the air condensing and giving a whitish colour to the water, etc.
CHAPTER XVI.
PHENOMENA OF METEORS, OPTIC DELUSIONS, SPECTRA, ETC.
The meteors known to the ancients were called [Greek: Lampdes Pithoi] Bolides, Faces, Globi, etc. from particular differences in their shape and appearance, and sometimes under the general term of comets. In the Philosophical Transactions, they are called, indiscriminately, fire-balls, or fiery meteors; and names of similar import have been applied to them in the different languages of Europe. The most material circumstances observed of such meteors may be brought under the following heads: 1. Their general appearance. 2. Their path. 3. Their shape or figure. 4. Their light and colour. 5. Their height. 6. The noise with which they are accompanied. 7. Their fire. 8. Duration, 9. Their velocity. Under these different heads meteors have been investigated by the scrutinizing of philosophy, and many superstitious notions, long entertained concerning them, entirely exploded. Meteoric phenomena, it has been demonstrated, all proceed from one common cause—irregularity in the density of the atmosphere. When the atmospheric fluid is homogenous and of equal density, the rays of light pass without obstruction or alteration in their shape or direction; but when they enter from a rarer into a denser medium, they are refracted or bent out of their course; and this with greater or less effect according to the different degrees of density in the media, or the deviation of the ray from the perpendicular. If the second medium be very dense in proportion, the ray will be both refracted and reflected; and the object from which it proceeds, will assume a variety of grotesque and extraordinary shapes, and it will sometimes appear as in a reflection from a concave mirror, dilated in size, and changed in situation.
The following striking effects are known to proceed from this simple cause.
The first is the mirage, seen in the desert of Africa. M. Monge, a member of the National Institute, accompanied the French army into Egypt. In the desert, between Alexandria and Cairo, the mirage of the blue sky was inverted, and so mingled with the sand below, as to impart to the desolate and arid wilderness an appearance of the most rich and beautiful country. They saw, in all directions, green islands, surrounded with extensive lakes of pure and transparent water. Nothing could be conceived more lovely and picturesque than this landscape. On the tranquil surface of the lakes, the trees and houses, with which the islands were covered, were strongly reflected with vivid hues, and the party hastened forward to enjoy the cool refreshments of shade and stream, which these populous villages preferred to them. When they arrived, the lake, on whose bosom they floated, the trees, among whose foliage they were embowered, and the people who stood on the shore inviting their approach, had all vanished, and nothing remained but an uniform and irksome desert of sand and sky, with a few naked huts and ragged shrubs. Had they not been undeceived by their nearer approach, there was not a man in the French army who would not have sworn, that the visionary trees and lakes had a real existence in the midst of the desert.
The same appearance precisely was observed by Dr. Clarke at Raschid, or Rosetta. The city seemed surrounded by a beautiful sheet of water, and so certain was his Greek interpreter, who was acquainted with the country, of this fact, that he was quite indignant at an Arab, who attempted to explain to him, that it was a mere optical delusion. At length, they reached Rosetta in about two hours, without meeting any water; and, on looking back on the sand they had just crossed, it seemed to them, as if they had just waded through a vast blue lake.
A similar deception takes place in northern climates. Cities, battlements, houses, and all the accompaniments of populous places, are seen in desolate regions, where life goes out, and where human foot has never trod. When approached they vanish, and nothing remains but a rugged rock, or a misshapen iceberg.
Captain Scoresby, in his voyage to the arctic regions, on the coast of East Greenland, constantly saw those visionary cities, and gives some highly curious plates of the appearances they presented. They resembled the real cities seen on the coast of Holland, where towers, and battlements, and spires, "bosomed high in tufted trees," rise on the level horizon, and are seen floating on the surface of the sea. Among the optic deceptions noticed by Captain Scoresby, was one of a very singular nature. His ship had been separated by the ice, from that of his father for some time; and he was looking for her every day, with great anxiety. At length, one evening, to his utter astonishment, he saw her suspended in the air in an inverted position, traced on the horizon in the clearest colours, and with the most distinct and perfect representation. He sailed in the direction in which he saw this visionary phenomenon, and actually found his father's vessel by its indication. He was divided from him by immense masses of icebergs, and at such a distance that it was quite impossible to have seen the ship in her actual situation, or seen her at all, if her spectrum, or image, had not been thus raised several degrees above the horizon into the sky, by this most extraordinary refraction, in the same manner as the sun is often seen, after he is known to have set, and actually sunk far below the line of direct vision.
The Fata Morgana are further illustrations of this optic delusion. This phenomenon is seen at the Pharo of Messina, in Sicily, under certain circumstances. The spectator must stand with his back to the east, on an elevated place behind the city, commanding a view of the bay, and having the mountains, like a wall, opposite to him, to darken the back ground of the picture; no wind must be abroad to ruffle the surface of the sea; and the waters must be pressed up by currents, as they sometimes are, to a considerable height in the middle of the strait, and present a slight convex surface. When all these circumstances occur, as soon as the sun rises over the heights of the Calabrian shore, and makes an angle of 45º with the horizon, all the objects on the shore at Reggio are transferred to the middle of the strait, and seen distinctly on the surface of the water, forming an immoveable landscape of rocks, trees, and houses, and a moveable one of men, horses, and cattle; these are formed into a thousand separate compartments, presenting most beautiful and ever varying pictures of animate and inanimate nature, on the swelling surface of the water, broken by the currents, present separate plates of convex mirrors to reflect them; they then as suddenly disappear, as the broad aquatic mirror of the current passes on.