It extended itself to the length of 50 or 60 feet, and formed undulations to the west of the circle of perpetual apparition, north-west of Sagitarius, and then rolled itself into the shape of a ring incomplete on the northern side.

In the 21 year of the Christian era, in the sixth moon, two stones fell at Kiu-lou. Since the time of Hoeï-té, eleven instances of the fall of such stones have been observed, all accompanied by light and a noise resembling thunder.

310. Tenth moon, day keng-tseu, a star of fire fell with noise in the northwest region; it was sought for, and its fragments were sent to the Emperor at Phing-yang.

333. A star fell 6 leagues northeast of Ye; this was at first of a blackish red. A yellow cloud spread like a curtain several hundred feet. A noise like that of thunder was heard. It was on fire when it reached the earth; the dust rose into the air; the laborers who saw it fall, sought for it; the earth was still very warm. They saw a stone at least a foot through, blackish and very light, which resounded on being struck, like the instrument called king.

1057. At Hoang-lieï, in Corea, first moon, a stone fell with a great noise like thunder. This stone being sent to court, the president of the court of rites said that a stone had fallen in the time of Thsin, and that the same phenomenon had been observed from time to time under the dynasties of Tsin and of Thang; thus it was not a thing without precedent, nor could it be considered a bad omen.

1516. Twelfth moon, twenty-fifth day. At Chunking-fou, in the province of Sse-tchhouan, neither wind nor cloud preceding, a peal of thunder was heard, and six stones fell, the heaviest weighing from 15 to 17 pounds. The smallest weighed from 10 ounces to a pound.

To complete these interesting records we shall add a number of instances of the fall of stones which have been observed each century in China, from the 7th century before Christ to the 16th after Christ.

MASSES OF IRON TO WHICH A METEORIC ORIGIN MAY BE

ATTRIBUTED.

WE shall now give, from M. Chladni, a list of those masses of native iron which have been found upon the surface of the earth, and which are considered by some philosophers as meteoric, and as having fallen in very remote ages.

"These masses of iron which are probably meteoric, are distinguished by the presence of nickel, by their texture, by their malleability, and by their isolated situation. Some of these masses are spongy or cellular; the cavities being full of a stony substance resembling peridote. Among these are to be classed,

The stone found by Pallas, in Siberia, whose meteoric origin was known to the Tartars.

One found between Eibenstock and Johanngeorgenstadt.

One preserved in the imperial cabinet of Vienna, brought perhaps from Norway.

A small piece, weighing 4 pounds, now at Gotha.

Other masses are solid. In this case the iron is in rhomboids or octaedrons, coraposed of layers or parallel leaves.

The only known fall of this sort took place at Agram, 1751.

Similar substances have been found :

On the right bank of the Senegal. Compagnon, Forster, Golberry

At the Cape of Good Hope. Van Marum and De Dankelmann.

At Mexico, in different places. Sonneschmidt de Humboldt. See also Gazeta de Mexico, T. I. T. V.

In Brazil, province of Bahia. Wollaston and Mornay.

In the jurisdiction of Saint-Jago del Estero. Rubin de Celis.

At Elbogen, in Bohemia. Ann. de Gilbert, T. XLII. T. XLIV.

Near Red river. The stone was sent from New-Orleans to New-York. American Mineralogical Journal, Vol. Colonel Gibbs analyzed it and

found nickel.

(Similar masses are to be found in other parts of the same country ac cording to the New-York Minerva, 1824.)

Near Bitbourg, not far from Trèves. (This piece weighs nearly 3300 pounds and contains nickel. The analysis, as made by Colonel Gibbs, may be found in the American Mineralogical Journal, Vol. I.)

Near Brahin, in Poland. (These pieces, according to M. Laugier's analysis, contain nickel and a little cobalt.)

In the republic of Colombia, eastern Cordilleras of the Andes. Boussingault and Mariano de Rivero, Ann. de Chimie, T. XXV.

At some distance from the northern shore of Baffin's Bay, in a place named Sowallik. Here are two pieces, one apparently solid, the other stony and mingled with portions of iron, which the Esquimaux use for knives. Captain Ross.

We ought, perhaps, to refer to this class a great mass about 40 feet in height, found in the eastern part of Asia, not far from the source of Yellow River. The Mongols, who call it Khadasutfilao or Polar Rock, say that its fall was preceded by a meteor of fire. Abel-Rémusat.

There are masses of a problematic origin. Among these are

One at Aix-la-Chapelle containing arsenic. Ann. de Gilbert, T. XLVIII. One found in the Milanese. Ann. de Gilbert, T. L.

That fonnd at Groskamsdorf, which cantains, according to Klaproth, a little lead and copper.

(It appears that this has been melted, and that the pieces preserved at Fryburg and Dresden are melted steel, which has been substituted for the fragments of the original substance.)"

IV. MIRAGE.

MIRAGE OBSERVED IN EGYPT.

It often happens when we look at remote objects under certain circumstances, that these objects give several images erect, oblique, or inverted, and always more or less altered in their outlines. It is the appearance of these images without any visible reflector to produce them, which constitutes the phenomenon of Mirage.

We will first give a description of this phenomenon as it presents itself in the plains of Egypt.

The ground of lower Egypt forms one vast plain which is entirely overflowed by the Nile at the time of the inundation. Along the banks of the river, and for a great distance towards the deserts, both east and west, we see, at long intervals, little eminences, crowned with buildings or villages. Generally the air is calm and very pure. At sun-rise remote objects are seen with perfect distinctness; a vast horizon meets the eye, which, though uniform, is by no means monotonous; but as the heat of the day increases, and the earth is parched by the sun, the lower strata of the atmosphere partake of the high temperature of the ground, numerous currents are produced with more or less regularity, and from these proceed a tremulous motion in the air, very perceptible to the eye; remote objects

lose their distinctness of outline, and seem to separate and unite again continually. But this phenomenon, observable also in our climate during the heat of summer, is not the mirage. If the wind does not blow, and if the strata of air, reposing upon the plain remains perfectly at rest while heated by coming in contact with the earth, then the phenomenon of mirage is developed in all its magnificence. The observer from afar distinguishes still the direct image of the eminences, of the villages, and of all elevated objects; but below these he sees their image reversed, and consequently loses sight of the ground on which they are placed.

Thus all elevated objects appear as if in the midst of an immense lake, and the heavens, which look as if reflected from the surface of still water, complete the illusion. On advancing, the heated soil is seen where the observer supposed this image of the heavens or some other object to be situated, and, further on, a similar picture presents itself on every side. This phenomenon was frequently remarked during the expedition of the French army to Egypt. It was at once a new spectacle, and a cruel illusion, to the soldiers. When they saw from a distance the images of all these objects thus reversed, they entertained no doubt that they saw them reflected from the surface of a lake. Exhausted by forced marches, under an intense heat, and in an atmosphere charged with sand, they ran to the bank, but this bank fled before them; it was the heated air of the plain which caused this appearance of water, this reflection of the sky and of all the elevated objects around them. The learned men who accompanied the army, shared the illusion for a moment; but Monge immediately discovered the cause of the phenomenon and explained all its circumstances. It is, as we shall see, a peculiar case of refraction.

EXPLANATION OF MIRAGE.

Let A B (Fig. 1.) represent the horizontal surface of the earth powerfully heated by the sun's rays; experience proves that the lower strata of the air have a density which increases as you ascend; at a certain height this density becomes nearly constant; afterwards it decreases, according to the ordinary laws by which the constitution of the atmosphere is governed. This being premised, suppose an elevated point H, and let us see how the light from it must be modified, in order to meet the eye placed at P; it is evident in the first place that the eye will see a direct image of the point H, by the rays PH; these rays, it is true, will not come in absolutely straight lines; since, between P and H, the air does not preserve absolutely the same density; but they will experience only very slight inflexions, the effect of which will be to produce some irregularity in the outline of the image.

But among the rays which the point H sends out in all directions, will be found some, which will take the direction H I K L M N O P, and which consequently will give, in the direction P O Z, an inverted image of the

Figure 1,

M

B

object, like a reflection from a mirror. The ray, for instance, H I, passing obliquely into the stratum C', of a less refracting power than the stratum C, must be refracted, from the normal or perpendicular in the denser stratum. For the same reason it would depart from the normal in passing from the stratum C' into the stratum C", and still more in passing from this into the next stratum. Thus, as the obliquity increases continually, it may happen that the ray, not being able to pass from the medium in which it is, into the medium of a less refracting power to which it is directed, will at length necessarily be reflected; and, continuing its course towards the eye, will reach it in the direction M N O P. The eye will thus see the point H in the direction P O Z, and in a position nearly symmetrical with the point H, when referred to the plane M V, on which the reflection is supposed to take place.

The course of the ray is here traced in a broken line; but as the density increases by insensible degrees from the surface, we may suppose that the ray also deviates by insensible degrees, describing a curved rather than a broken line.

Such is the principle on which the mirage depends, as explained by Monge on the very ground where the phenomenon occurs; the explanation was published in the Memoirs of the Institute of Egypt.

The following experiment does not exactly represent this phenomenon, but it may serve to make the theory intelligible.