parts or bodies of a denser medium than its general texture and constitution: in which case the fluent ray, if it do not enter the denser medium in a direct or perpendicular line, will be either reflected, or refracted, or both; and the object surveyed through it assume a new, and not unfrequently a grotesque appearance.

There are various causes that produce such irregularities in the tenor of the atmospheric fluid; of which, perhaps, the most common is the descent of rain, whose globules, when opposed to the sun or the moon, at their rising or setting, in a clear sky, are well known to exhibit the phenomenon of the rain-bow: a phenomenon which depends upon the very principle now adverted to; and proceeds, indeed, from a double reflection and refraction; or, in other words, from the globule which produces the arch being converted into a double mirror, and a double prism. In the formation of this beautiful meteor, it is necessary to observe, that the ray which issues from the centre of the sun, and does not immediately, or perpendicularly, pass through the centre of the opposed globule of rain, must, upon the common principles of dioptrics, in conse. quence of its entering a transparent body of a different medium from the atmosphere itself, in a certain degree, be bent, deflected, or refracted from the right line in which it was proceeding; and hence, instead of passing out at the posterior part of the globule, immediately opposite to that at which it entered, it will be driven towards another limb, or marginal portion of the globule, and form an angular line co-equal to the obliquity with which it deviates from a right line on its entrance into the globule; just as a stake, or the oar of a boat, plunged obliquely into a river, appears to be broken, or deflected, from the point at which it enters the water. At this point, the refracted ray, instead of passing out of the glo. bule, suffers another deflection, but from a very different cause: for the ray of light having been thrown across a certain portion of the posterior chamber of the globule of rain, without permeating it, all behind its passage becomes necessarily a dark shade, while the globule itself forms an anterior and polished surface to it; whence a regular mirror is produced, and the ray is now reflected or thrown back from it, in the same manner as an incident ray light, or image, is reflected or thrown back from a looking-glass, or a deep and clear stream of water; both of which, like the globule thus situated, consist of nothing more than a dark shadow with a

of

polished surface: the obliquity of its path, in the present instance, being precisely similar to that which it has previously suffered from refraction; the angular line of reflection being always co.equal with the angular line of incidence.

It is hence obvious, that the

ray, or fascicle of parallel rays, which entered obliquely below the centre of the globule, opposed to the centre of the sun, must be reflected obliquely above it; and as the same process necessarily takes place, but in an inversed order, with the antagonist ray, or fascicle of parallel rays that entered with the same degree of obliquity above it, it is also obvious that, from this double refracting and reflecting power of an individual globule of rain, situated as above described, an angle of light must be formed, from their an tagonism alone, exhibiting the different colours of which they con sist in a definite order, according to the degree of their refrangibi. lity that the spread, or hypotheneuse, of the angle must depend upon the diameter of the globule which produces it; and that its point being softened or obtunded to the eye by the distance through which it is beheld, agreeably to an observation of our poet in v. 375 of Good's Lucretius, the angle must be converted into an arch. And, hence, a beautiful and variegated rain.bow must necessarily result from a few rays of light acted upon by a single globule of rain, situated as above, from the fact alone of its possessing the power of a binary mirror or prism.

But a globule of rain is not the only substance in the atmosphere capable, at times, of producing the same effect; nor, since we are told that the mirage usually occurs when the sky is peculiarly tran quil and serene, could it be the cause of this last equally curious phenomenon. Our time, however, has not been lost in thus hastily investigating the theory of the iris; for the same principles will apply to the meteor before us. We are informed, not only that the mirage is chiefly to be noticed when the sky is clear and unclouded, but in the morning, and principally upon the coasts or banks of a large river. The mirage beheld by M. Crantz was on the shore of the Kookoernan islands near the Cape of Good Hope; it has often been traced at the back of the Isle of Wight; but the quarter in which, perhaps, it most frequently makes its appearance, is the Faro of Messina in Italy. In all these places, when the weather is perfectly calm, and, consequently, the sea almost with. out motion, the atmosphere, more especially in a dry and hot sea

son, imbibes a considerable portion of the water upon which its lower stratum presses; and hence, in the night-time, becomes condensed and hazy. As the morning rises, however, and the sunbeams resume their vigour, the atmosphere once more rarefies, and re-acquires its transparency. If it rarefy equably, and homogeneously, every object beheld through it must necessarily be exhibited in its real proportion and figure: but it happens, occasionally, that in some parts of its texture, it seems to be more closely inter. woven than in others; and hence in its general expansion, veins, or striæ, like those often discovered in glass, make their appear. ance, of different densities and diameters. In this case, every stria, like every globule of rain, in consequence of the variation of its density from the common density of the atmosphere, becomes a refracting or a reflecting body; in other words, a prism, or a mirror, or both. If, then, a single globule of rain, properly disposed, be able to produce a phenomenon so marvellous as that of the rainbow, what phenomena may we not expect, what variation, inversion, contorsion, and grotesque and monstrous representation of images, beheld through a column of the atmosphere, intersected by so many aerial prisms of different densities, and mirrors of dif. ferent surfaces, in which the catheti may be innumerable, and for ever varying? We may hence, moreover, readily trace the cause of an occasional duplication of images in the atmosphere, of a parhelion, and paraselene, or double sun, and double moon, from the reflection of these luminaries in an opposite part of the heavens, when they are a little above the horizon; as also of the very curi. ous mirage remarked by M. Monge, in the hot and sandy desert between Alexandria and Cairo; in which, from an inverted image of the cerulean sky intermixed with the ground scenery, the neighbouring villages appeared to be surrounded with the most beautiful sheeting of water, and to exist, like islands, in its liquid expanse, tantalizing the eye by an unfaithful representation of what was earnestly desired.

The mirage has not been suffered to lie neglected by the poets. It is to the aërial phantoms exhibited by this meteor, that Milton alludes, in the following verses :

As when, to warn proud cities, war appears
Waged in the troubled sky, and armies rush

To battle in the clouds; before each van

Prick forth the airy knights, and couch their spears,
Till thickest legions close; with feats of arms

From either side of heaven the welkin burns.

[Good's Lucretius, vol. ii. p. 25.

SECTION 11.

Fata Morgana, or Optical Appearances of Figures in the Sea and Air, in the Faro of Messina.

As when a shepherd of the Hebrid Isles
Placed far amid the melancholy main,
(Whether it be lone fancy him beguiles,
Or that aerial beings sometimes deign
To stand, embodied, to our senses plain)
Sees on the naked hill, or valley low,
The whilst in ocean Phoebus dips his wain,

A vast assembly moving to and fro;

Then all at once in air dissolves the wond'rous shew.

THOMSON.

Various philosophical writers and travellers, and among them our English travellers Brydone and Swinburne, make mention of a very striking phenomenon which occasionally appears in the Straits of Messina, and is known by the name of Fata Morgana, or, as some render it, the castles of the Fairy Morgana. The accounts differ from each other, as well with respect to the appear. ances, as the concomitant circumstances which are supposed to be necessary for producing them. How far the effects themselves may be subject to variation; or to what extent the imagination of the narrators, who speak of the exhibition as calculated to produce astonishment, may be subject to irregularitity, would admit of discussion; but the general certainty of the events is matter of universal notoriety, and admits of no doubt. I have not had the good fortune to meet with any of the authors who treat on this subject expressly from their own knowledge and observation, till lately that the Dissertation of Minasi was lent me by the Right Honourable Sir Joseph Banks, Bart, &c. In this treatise the facts are related with much simplicity and precision, and the philosophical reasoning of the author is kept distinct from the narrative. I have therefore chosen to collect the present account from this author.

His first chapter contains a description of the phenomenon. "When the rising sun shines from that point whence its incident ray forms an angle of about forty-five degrees 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, 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 there appear in the water, as in a catoptric theatre, various multiplied objects; that is to say, 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, armies of men on foot and horseback, and many other strange images, in their natural colours and proper actions, passing rapidly in succession along the surface of the sea, during the whole of the short period of time while the above-mentioned causes remain.

"But if, in addition to the circumstances before described, the atmosphere be highly impregnated with vapour, and dense exhalations, not previously dispersed by the action of the wind or waves, or rarefied by the sun, it then happens that in this vapour, as in a curtain extended along the channel to the height of about thirty palms, and nearly down to the sea, the observer will behold the scene of the same objects not only reflected from the surface of the sea, but likewise in the air, though not so distinct or well defined as the former objects from the sea.

"Lastly, if the air be slightly hazy and opake, and at the same time dewy and adapted to form the iris, then the above-mentioned objects will appear only at the surface of the sea, as in the first case, but all vividly coloured or fringed with red, green, blue, and other prismatic colours."

The author therefore distinguishes three sorts of Fata Morgana : that is to say, the first at the surface of the sea, which he calls the Marine Morgana; the second in the air, called the Aërial Morgana : and the third only at the surface of the sea, which he calls the Morgana fringed with prismatic colours.

In a note in this chapter P. Minasi enquires into the etymology of Morgana. After various remarks, he thinks the opinion of those who derive this word, which is so foreign to the Roman idiom, from μwpor tristis and yaviw lætitia afficio, is not far from the truth; considering the great exultation and joy this appearance