garly called La Fata Morgana*. The philosophical reader will find its causes and operations learnedly accounted for in Kircher, Minasi, and other authors. I shall only give a description of its appearance, from one that was an eye-witness. Father Angelucci is the first that mentions it with any degree of accuracy, in the fol. lowing terms:

"On the 15th of August, 1643, as I stood at my window, I was surprised with a most wonderful, delectable vision. The sea that washes the Sicilian shore swelled up, and became, for ten miles in length, like a chain of dark mountains; while the waters near our Calabrian coast grew quite smooth, and in an instant appeared as one clear polished mirror, reclining against the aforesaid ridge. On this glass was depicted, in chiara scuro, a string of several thousands of pilasters, all equal in altitude, distance, and degree of light and shade. In a moment they lost half their height, and bent into arcades, like Roman aqueducts. A long cornice was next formed on the top, and above it rose castles innumerable, all per. fectly alike. These soon split into towers, which were shortly after lost in colonnades, then windows, and at last ended in pines, cypresses, and other trees, even and similar. This was the Fata Morgana, which, for twenty-six years, I had thought a mere fable."

To produce this pleasing deception, many circumstances must concur, which are not known to exist in any other situation. The spectator must stand with his back to the east, in some elevated place behind the city, that he may command a view of the whole bay; beyond which the mountains of Messina rise like a wall, and darken the back-ground of the picture. The winds must be hushed, the surface quite smoothed, the tide at its height, and the waters pressed up by currents to great elevation in the middle of the channel. All these events coinciding, as soon as the sun sur. mounts the eastern hills behind Reggio, and rises high enough to form an angle of forty-five degrees on the water before the city,every object existing or moving at Reggio will be repeated a thousand-fold upon this marine looking-glass; which, by its tremulous motion, is, as it were, cut into facets. Each image will pass ra.

The name is probably derived from an opinion, that the whole spectacle is produced by a fairy, or a magician. The populace are delighted whenever the vision appears, and run about the streets shouting for joy,-calling every body out to partake of the glorious sight.

pidly off in succession, as the day advances, and the stream carries down the wave on which it appeared.

Thus the parts of this moving picture will vanish in the twink. ling of an eye. Sometimes the air is at that time so impregnated with vapours, and undisturbed by winds, as to reflect objects in a kind of aërial screen, rising about thirty feet above the level of the In cloudy, heavy weather, they are drawn on the surface of the water, bordered with fine prismatical colours.

sea.

[Nicholson's Journ. 4to. vol. ii. Swinburne.

SECTION III.

Singular Instance of atmospherical Refraction, by which the Coast of Picardy, with its more prominent Objects, was brought apparently close to that of Hastings.

By William Latham, Esq. F.R.S. and A.S.

JULY 26, about five o'clock in the afternoon, while sitting ia my dining-room at this place, Hastings, which is situated on the Parade, close to the sea shore, nearly fronting the south, my attention was excited by a great number of people running down to the sea side. On inquiring the reason, I was informed that the coast of France was plainly to be distinguished with the naked eye. I immediately went down to the shore, and was surprised to find that, even without the assistance of a telescope, I could very plainly see the cliffs on the opposite coast; which, at the nearest part, are between forty and fifty miles distant, and are not to be discerned, from that low situation, by the aid of the best glasses. They ap peared to be only a few miles off, and seemed to extend for some leagues along the coast. I pursued my walk along the shore to the eastward, close to the water's edge, conversing with the sailors and fishermen on the subject. At first they could not be persuaded of the reality of the appearance; but they soon became so the roughly convinced, by the cliffs gradually appearing more elevated, and approaching nearer, as it were, that they pointed out, and named to me, the different places they had been accustomed to visit; such as, the Bay, the Old Head or Man, the Windmill, &c. at Boulogne; St. Vallery, and other places on the coast of Picardy; which they afterwards confirmed, when they viewed them through their telescopes. Their observations were, that the places

appeared as near as if they were sailing, at a small distance, into the harbours.

Having indulged my curiosity on the shore for near an hour, during which the cliffs appeared to be at some times more bright and near, at others more faint and at a greater distance, but never out of sight, I went on the eastern cliff or hill, which is of a very considerable height, when a most beautiful scene presented itself to my view; for I could at once see Dengeness, Dover cliffs, and the French coast, all along from Calais, Boulogne, &c. to St. Vallery; and, as some of the fishermen affirmed, as far to the westward even as Dieppe. By the telescope, the French fishing-boats were plainly to be seen at anchor; and the different colours of the land on the heights, with the buildings, were perfectly discernible. This curious phenomenon continued in the highest splendour till past eight o'clock, though a black cloud totally obscured the face of the sun for some time, when it gradually vanished. I was assured, from every inquiry I could make, that so remarkable an instance of atmospherical refraction had never been witnessed by the oldest inhabitant of Hastings, nor by any of the numerous visitors come to the great annual fair. The day was extremely hot. I had no barometer with me, but suppose the mercury must have been high, as that and the three preceding days were remarkably fine and clear. To the best of my recollection, it was high water at Hastings about two o'clock P. M. Not a breath of wind was stir. ring the whole of the day; but the small pennons at the mast-heads of the fishing-boats in the harbour were in the morning at all points of the compass. I was, a few days afterwards, at Winchelsea, and at several places along the coast, where I was informed the above phenomenon had been equally visible. When I was on the eastern hill, the cape of land called Dengeness, which extends nearly two miles into the sea, and is about sixteen miles distant from Hastings, in a right line, appeared as if quite close to it; as did the fishingboats and other vessels, which were sailing between the two places; they were likewise magnified to a great degree *.

[Phil. Trans. 1798.

In this case, as in that of the desert between Alexandria and Cairo, adverted to in section i. the refractive power of the atmosphere was proba bly produced by a diminution of the density of its lower stratum, in consequence of the increase of heat communicated to it by the rays of the sun

SECTION IV.

On Refractions and double Refractions in the Atmosphere.

We have endeavoured, in the first section, to present the reader with a clear outline of the chief principles upon which the atmo. spherical deceptions we have thus far noticed are founded. There is, however, another cause which has of late years been brought forward as of the utmost importance in their production; and particularly in the production of that double refraction which is the principal source of multiplied rainbows, parhelions, and paraseli. nites, or mock-suns and mock-moons, glories, and the singular refracting power of the Iceland crystal. This cause is the tendency of plates or strata of different thickness both to vary the re. fraction, and to alter the intensity or the order of the colours. In the Philosophical Transactions we meet with two excellent articles upon this subject: one by Mr. Huddart, and the other by Dr. Wol. laston, but of too abstruse a character for introduction into the present work. We shall, however, in order to render the subject more

powerfully reflected from the surface of the earth. In another article in the same volume of the Philosophical Transactions, Professor Vince observes, that he remarked a similar apparent approximation of the French coast to that of Ramsgate, in the summer of 1798. Of two ships," says he," which in different parts were equally sunk below the horizon, the inverted image of one would but just begin to appear, while that of the other would represent nearly the whole of the ship. But this I observed in general, that as the ship gradually descended below the horizon, more of the image gradually appeared, and it ascended; and the contrary when the ships were ascending. On the horizon, in different parts, one ship would have a complete inverted image; another would have only a partial image; and a third would have no image at all. The images were in general extremely well defined ; and frequently appeared as clear and sharp as the ships themselves, and of the same magnitude. Of the ships on this side of the horizon, no phænomena of this kind appeared. There was no fog on our coast; and the ships in the Downs, and the South Foreland, exhibited no uncommon appearances. The usual refraction at the same time was uncommonly great; for the tide was high, and at the very edge of the water I could see the cliffs at Calais a very considerable height above the horizon; whereas they are frequently not to be seen in clear weather from the high lands about the place. The French coast also appeared, both ways, to a much greater distance than I ever observed it at any other time, particu larly towards the east, on which part also the unusual refraction was the strongest.

easily comprehensible, take leave prior to our giving a few examples of these singular phænomena, to copy Dr. Young's very neat abridgment of the last paper, and to introduce it with a few valuable observations of his own.

The atmospherical phænomena, says he, of rainbows and halos present us with examples of the spontaneous separation of colours by refraction. The rainbow is universally attributed to the refraction and reflection of the sun's rays, in the minute drops of falling rain or dew; and the halos, usually appearing in frosty atmo. spheres, are in all probability produced by the refraction of small triangular or hexagonal crystals of snow. It is only necessary, for the formation of a rainbow, that the sun should shine on a dense cloud, or a shower of rain, in a proper situation; or even on a number of minute drops of water, scattered by a brush or by a syringe, so that the light may reach the eye after having undergone a certain angular deviation, by means of various refractions and reflec. tions; and the drops so situated must necessarily be found somewhere in a conical surface, of which the eye is the vertex, and must present the appearance of an arch. The light, which is reflected by the external surface of a sphere, is scattered almost equally in all directions, setting aside the difference arising from the greater efficacy of oblique reflection; but when it first enters the drop, and is there re. flected by its posterior surface, its deviation never exceeds a certain angle, which depends on the degree of refrangibility, and is, therefore, different for light of different colours: and the density of the light being the greatest at the angle of greatest deviation, the appearance of a luminous arch is produced by the rays of each colour at its appropriate distance. The rays which never enter the drops produce no other effect, than to cause a brightness, or haziness round the sun, where the reflection is the most oblique: those which are once reflected within the drop exhibit the common inter. nal or primary rainbow, at the distance of about forty-one degrees from the point opposite to the sun; those which are twice reflected, the external or secondary rainbow, of fifty-two degrees; and if the effect of the light, three times reflected, were sufficiently powerful, it would appear at the distance of about forty-two degrees from the sun. The colours of both rainbows encroach consi derably on each other; for each point of the sun may be considered