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phænomena, Diodorus Siculus makes particular mention in the fiftieth section of his third book, and points out the regions of Africa situate between the Syrtes and Cyrene, as the theatre of their most extraordinary appearance: περι γαρ τινας καιρους, says he, και μαλιστα κατα τας νηνεμίας, συστασεις ορωνται κατα τον αερα, παντοιων ΖΩΩΝ ιδεας εμφαινουσαι· τουτων δ', αι μεν ηρεμουσιν, αι δε κίνησιν λαμβανουσι και ποτε μεν υποφευγουσι, ποτε δε διωκoυσι. “ Even in the serenest weather, there are sometimes seen in the air certain condensed exhalations, that represent the figures of all kinds of animals; occasionally, they seem to be motionless, and in perfect quietude; and occasionally to be flying; while immediately afterwards, they themselves appear to be the pursuers, and to make other other objects fly before them.” This phenomenon is, in reality, seldom observed, except in serene weather; and it should seem, upon every theory yet offered to account for it, from the ingenious explanation of our own poet to that of M. Monge in the Memoirs relative to Egypt, that such an atmosphere is nearly or altogether necessary to its existence. The illusion has been noticed as frequently by modern as by ancient observers; and M. Crantz, in his History of Greenland, Vol. I. 49. has given a pic. ture of it, not essentially differing from the above just quoted from Diodorus Siculus. It is not confined to any particular part of the globe, but generally makes its appearance on the coast ; the at: mosphere, as I bave already observed, being commonly clear and tranquil, and the phenomenon usually succeeded by a fall of rain. Our own sailors, from its more general appearance, call it a fog-bank ; by many writers, it is denominated fata Morgana, and by the French, mirage.
For this atmospheric delusion, various causes have been assigned ; and especially by Kircher, Scholt, and Gaspard Monge, who ac. companied Buonaparte in his Egyptian expedition, as one of the French Sçavans, and was a member of the Institute at Cairo: yet no explanation I have hitherto met with, has been given in satisi factory, or at least in popular language.
To illustrate it as clearly as may be, it is necessary, first of all, to call the reader's attention to the variable state of the atmosphere; which is commonly of an homogenous, or equable tenuity, and consequently suffers the rays of the sun to penetrate it without any obstruction or change; but at times it is irregular, and composed of 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 re. flected, 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 phenome. non 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 of 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
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 obli. quity 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 tranquil 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 Koukoernan 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 con. densed 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 homogene. ously, 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 phænomena 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
To battle in the clouds; before each van
(Good's Lucretius, vol.ii. p. 25.
Fala Morgana, or Optical Appearances of Figures in the Sea
and Air, in the Faro of Messina.
As when a shepherd of the Hebrid Isles
A vast assembly moving to and fro;
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 ac. counts 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 seasoning of the author is kept distinct from the narrative. I have therefore chosen to collect the present account from this author.