440. INSTANCES. On the first of August, 1798, Dr. Vince observed, at Ramsgate, a vessel in the distance, the topmast only being visible above the horizon, as at A, fig. 26. Two complete images of the vessel were seen at the same time in the air, the one at C erect, and the other below at B inverted: between them a distinct image of the sea appeared at D E. The two images were still visible when the real ship had passed entirely out of sight.

Fig. 26.

MIRAGE.

441. Similar phenomena were noticed by Capt. Scoresby in 1820, while navigating the arctic seas. In one instance he beheld from the mast-head eighteen sail of ships, at the distance of twelve miles; one appeared taller than its actual height, another shorter and above several of the rest, inverted images were seen. In 1822, he recognized his father's ship, the Fame, by an inverted image of the vessel in the air, though it was subsequently found to have been at that time thirty miles distant, and seventeen miles beyond the horizon.

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442. During the late Exploring Expedition, a singular instance of mirage was seen off Terra del Fuego, from the decks of the Vincennes and Peacock, and which is thus related. "On the 17th of February, 1839, we had an extraordinary degree of mirage or refraction of the Peacock, exhibiting three images, two of which were upright and one inverted. They were all extremely well defined. The temperature on deck was 54° Fah., that at the mast-head 62° Fah. A vessel, that was not in sight. from the Vincennes' deck, became visible, and the land was much distorted, both vertically and horizontally.

Relate the several cases of mirage, T 440-445,

On board the Peacock, similar appearances were observed of the Vincennes and Porpoise. There was, however, a greater difference between the mast-head temperature and that on deck, the thermometer standing at 62° Fah. at the mast-head, while on the deck it was but 50° Fah., being a difference of 12°; that on board the Vincennes differed only 8°."

443. Simpson, while exploring the coasts of the north polar seas, in the summer of 1837, beheld a remarkable display of the mirage. As he rowed over the tranquil ocean, he seemed to be traversing a valley; the waters apparently rising on either hand, like the sides of a mountain, and the huge icebergs upon their surface appearing ready to topple down upon him.

444. During the march of the French army over the sandy plains of Egypt, many instances of the mirage occurred. The villages, situated upon small eminences, were successively seen like so many islands in the midst of an extensive lake, and beneath each village appeared its inverted image. In the same direction, an image of the blue sky was seen, clothing the sand with its own bright hues, and causing the wilderness to appear like a rich and luxuriant country. So complete was the deception, that the troops hastened forward to refresh themselves amid these cool retreats; but, as they advanced, the illusion vanished, only to re-appear at the villages beyond.

445. This phenomenon is so common on the deserts of Asia and Africa, that the Koran calls every thing deceitful by the word serab, which signifies mirage. It remarks, for example, that "the actions of the incredulous are like the serab of the plain; he who is thirsty takes it for water, and finds it to be nothing."

446. While Baron Humboldt was at Cumana, he frequently saw the islands of Picuita and Boracha, apparently hanging in the air, and sometimes with inverted images; and at Mesa de Pavona, cows were beheld

Where does this phenomenon frequently occur?
What instances are given by Humboldt and Tschudi ?

seemingly suspended in the air, at the distance of 2,132 yards.

When Dr. Tschudi and his party were traversing a deep sandy plain, near the river Pasamayo in Peru, they beheld the figures of themselves, riding over their own heads, magnified to gigantic proportions.

447. FATA MORGANA. This name is given to an extraordinary optical phenomenon, which has been often seen in the straits of Messina, between the island of Sicily and the Italian coast. It has been described by many writers, and, though known for centuries, has but lately been considered as the effect of mirage. The following is the description by Antonio Minasi, which is regarded as the most correct.

"When the rising sun shines from a point, whence its incident ray forms an angle of about 45° on the sea of Reggio, and the bright surface of the water in the bay is not disturbed either by the wind or the current, a spectator placed on an eminence in the city of Reggio, with his back to the sun, and his face to the sea, suddenly beholds in the water numberless series of pilasters, arches, castles well delineated, regular columns, lofty towers, superb palaces, with balconies and windows, extended valleys of trees, delightful plains with herds and flocks, armies of men on foot and horseback, all passing rapidly in succession along the surface of the sea."

In a peculiar state of the atmosphere, when its dense vapors extend like a curtain over the waters, the same objects are not only reflected from the surface of the sea, but are likewise seen in the air, though not so distinct or well defined, and if the atmosphere is slightly hazy, the images seen upon the surface of the water are vividly colored or fringed with all the prismatic hues.

448. But a most extraordinary instance of the mirage occurred at Hastings, on the coast of Sussex, on the 26th of July, 1798. The cliffs of the French coast are fifty miles distant from this town, and in the usual state of the atmosphere, are below the horizon and completely

Describe the Fata Morgana.

hid from view; but on the day mentioned, at five o'clock P. M., they were seen extending to the right and left for several leagues, and apparently only a few miles off. As the narrator, Mr. Latham, walked along the shore, the sailors, who accompanied him, pointed out and named the different places on the opposite coast, which they were accustomed to visit. By the aid of a telescope, small vessels were plainly seen at anchor in the French harbors, and the buildings on the heights beyond were distinctly visible.

The Cape of Dungeness, which at the distance of 16 miles from Hastings, extends nearly two miles into the sea, appeared quite close to the town, and the fishing boats, that were sailing at the time between the two places, were magnified to a high degree. This curious phenomenon continued in its greatest beauty for more than three hours. The day was extremely hot, without a breath of wind.

449. A remarkable mirage of Dover Castle, was seen by Dr. Vince and another gentleman, on the 6th day of August, 1806, at Ramsgate.

The summits, v x wy, of the four turrets of the castle, (fig. 27.,) are usually seen beyond the hill A B, which lies between the castle and Ramsgate; but, on this day not only the turrets were visible, but the whole castle, m nr s, appeared as if it were on the side of the hill next to Ramsgate.

Relate the account of the mirage at Hastings, and of that at Ramsgate

Between the observers and the shore, from which the hill rises, there was about six miles of sea, and from thence to the top of the hill the distance was about the same. Their own height above the water was nearly seventy feet.

450. ORIGIN. The cause of mirage has been partially stated; but the subject demands a more complete explanation. The phenomena may be divided into three classes, viz. those produced by refraction, those produced by refraction and reflection conjointly, and those produced by reflection only.

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451. The image of Dover Castle was probably produced by refraction, simply; for the atmosphere gradually increasing in density from the lofty heights of the castle to the level of the sea, the rays of light proceeding from the edifice, reached the eyes of the spectators in a curved line, like those which emanate from a star, (Art. 438,) and the whole structure therefore appeared to the observers above its true position.

452. Phenomena, like those observed by Scoresby, are attributed to the combined influence of refraction and reflection. At such times, the stratum of air in contact with the sea is colder than that immediately above (Art. 442), and this likewise colder than the next superior stratum, and so on. Consequently, to a certain extent, the density of the atmosphere decreases with the distance from the ocean, and, under these circumstances, the rays of light from a ship may be so changed in direction, as they proceed through the air, that the observer will behold both erect and inverted images above the real object.

453. ERECT AND INVERTED IMAGES ABOVE THE OBJECT. The annexed figure will aid us in perceiving how erect images are caused.

What is said respecting the cause of mirage?

Into what classes may the phenomena be divided?

Explain the mirage of Dover Castle.

To what is attributed the phenomena of erect and inverted images above The object?

What is the state of the atmosphere as regards temperature at such times?