2. Continuation of the above Account, from the same, to the same, dated August 23, 1694. An intelligent person*, who lives near Harlech in Merionethshire, assured me the fire still continues there; that it is observed to come from a place called Morva.bychan in Caernarvonshire, about eight or nine miles off, over part of the sea. That cattle of all sorts, as sheep, goats, hogs, cows, and horses, still die apace; and that for certain, any great noise, as winding horns, drums, &c. repels it from any house, or barn, or stacks of hay: on account of which remedy, they have had few or no losses in that kind since Christmas. That it happened during this summer, at least, one night in a week, and that commonly either Saturday or Sunday; but that now of late it appears something oftener. The place whence it proceeds is both sandy and marshy. [Phil. Trans. 1693-4. It is not impossible that the winding of horns, drums and other noises here referred to may have been serviceable in destroying the flame; if, as there can be little doubt, the inflammable material were hydrogen gas; for whatever would tend to change and ventilate the gas, as all loud sounds must necessarily do, would speedily render it weaker, or in other words more freely combined with the uninflammable air of the atmosphere. [EDITOR. ON ATMOSPHERIC CHAP. XLVIII*. DECEPTIONS.-FATA MORGANA, MIRAGES, GLAMER OR LOOMING; HALOS; MULTIPLIED RAINBOWS; PARHELIONS, AND PARASELENITES, OR MOCK-SUNS, AND MOCK. MOONS; GLORIES; REFRACTION OF ICELAND CRYSTAL. SECTION I. Explanation of the principle of Atmospheric Deceptions. ALL these curious and interesting phænomena proceed from one common cause, irregularity in the tenuity of the atmospheric fluid. To enter however, very fully into their origin and distinctions, would lead us farther into the laws of optics than the nature of the present work would justify. One of the clearest and most concise explanations that has occurred to us, and at the same time most adapted to popular comprehension, is contained in a note to Mr. Good's Translation of Lucretius, book iv. v. 144, in which the poet enters upon a description of the mirage (or glamer as it is called in the Highlands) a distorted and fantastic representation of the scenery before us-a description which we regret that we have not space to copy. The note is as follows: These monstrous appearances in the atmosphere are not equally common to all countries, but depend in a great measure upon lo. cal causes and combinations. According to Pliny, the regions of Scythia within Imaus; and, according to Pomponius Mela, those of Mauritania behind mount Atlas, are peculiarly subject to them; and they are generally regarded by the barbarous inhabitants of such countries as spectres, or aerial demons. Of such grotesque * It might perhaps be expected, before we thus enter upon a new subject, that we should touch upon the phenomenon of Fairy Rings, or Circles; which have from a very high antiquity, been generally ascribed to the effect of lightning, or fiery meteors of some kind or other. More accurate attention, however, has proved them to be the production of a fungus, the agaricus orcades, and hence to fall within the range of the curiosities of Botany. [EDITOR. 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 κατά τας νηνεμίας, συστάσεις ὁρωνται κατά τον αέρα, παντοίων ΖΩΩΝ ιδέας εμφαίνουσαι· τούτων δ', αι μεν ηρεμούσιν, αι δε κίνησιν λαμβανουσι· και ποτε μεν ὑποφευγουσι, ποτε δε διωκουσι. "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 immedi ately 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 have 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 satis 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 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 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 retraction; 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 consist 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 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 |