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.

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.66

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?

D2

בד

Fig. 28.

P

ERECT IMAGE ABOVE THE OBJECT.

Let D be a ship, seen in the horizon in its true position, by the direct rays n P, m P, coming to the eye at P, through the stratum of air of uniform density, lying between the eye and the ship. Let 1-2, 2-3, 3-4, &c., be parallel strata of the atmosphere, decreasing in density from 2 to 6; and n r and m s, rays of light, proceeding upwards from the top and bottom of the ship. As these rays at r and s pass from the first stratum into the second, which is rarer, they are bent downwards, or from the perpendicular, according to a well-known law of optics, (C. 706,) and this change in direction continually occurs as they pass successively into strata still more and more rare; until at last, as at x and y, they meet the next superior stratum so obliquely, that they are unable to enter it, and are then totally reflected from the lower surfaces of strata 4 and 5, at the points x and y.

The rays, on their return, are now refracted downwards, or towards the perpendicular, (C. 706,) in passing from the rarer into the denser media, and converge to the eye at P, which sees the vessel in the direction of the last refracted rays. The ship D is therefore beheld at D2 by the rays Pr2 n2, and Ps2 m2.

Explain from figure 28. the phenomenon of an erect image above the ohject.

454. In figure 28., the upper ray before reflection is the upper ray after reflection, and the image consequently appears erect; but if, as in figure 29., the rays cross each

other before they reach the eye at P, then the image will appear inverted, as is evident from the inspection of the figure.

Under peculiar circumstances it may happen, that of two sets of rays, one from the top and the other from the bottom of an object, some may cross each other before they meet the eye and some may not; and then both erect and inverted images will be seen at the same time.

455. MAGNIFIED IMAGES. The real object in figures 28. and 29., is seen through the horizontal strata, under the visual angle n P m. If n2 P m2, the angle under which the image is seen, is greater than n P m, the image will be magnified (C. 746) in the direction of its length; and if an increase of the lateral visual angle occurs at the same time, then the image will be likewise magni

Explain from figure 29. the phenomenon of an inverted image above the object.

When may both images be seen at the same time?

When may the image be magnified both vertically ana norizontally?