COMETS

(From Marvels of the Heavens.)

By CAMILLE FLAMMARION.

This announcement of Trissontin's to Philaminte, who begins the parody on the fears caused by the appearance of comets, would not have been a parody four or five centuries ago. These tailed bodies, which suddenly come to light up the heavens, were for long regarded with terror, like so many warning signs of divine wrath. Men have always thought themselves much more important than they really are in the universal order; they have had the vanity to pretend that the whole creation was made for them, whilst in reality the whole creation does not suspect their existence. The Earth we inhabit is only one of the smallest worlds; and therefore it can scarcely be for it alone that all the wonders of the heavens, of which the immense majority remains hidden from it, were created. In this disposition of man to see in himself the centre and the end of everything, it was easy indeed to consider the steps of nature as unfolded in his favor; and if some unusual phenomenon presented itself, it was considered to be without doubt a warning from Heaven. If these illusions had had no other result than the amelioration of the more timorous of the community one would regret these ages of ignorance; but not only were these fancied warnings of no use, seeing that once the danger passed, man returned to his former state; but they also kept up among people imaginary terrors, and revived the fatal resolutions caused by the fear of the end of the world.

When one fancies the world is about to end,—and this has been believed for more than a thousand years,—no solicitude is felt in the work of improving this world; and, by the indifference or disdain into which one falls, periods of famine and general misery are induced which at certain times have overtaken our community. Why use the wealth of a world which is going to perish? Why work, be instructed, or rise in the progress of the sciences or arts? Much better to forget the world, and absorb one's self in the barren contemplation of an unknown life. It is thus that ages of ignorance weigh on man, and thrust him further and further into darkness, while Science makes known by its influence on the whole community, its great value, and the magnitude of its aim.

The history of a comet would be an instructive episode of the great history of the heavens. In it could be brought together the description of the progressive movement of human thought, as well as the astronomical theory of these extraordinary bodies. Let us take, for example, one of the most memorable and best-known comets, and give an outline of its successive passages near the Earth. Like the planetary worlds, Comets belong to the solar system, and are subject to the rule of the Star King. It is the universal law of gravitation which guides their path; solar attraction governs them, as it governs the movement of the planets and the small satellites. The chief point of difference between them and the planets is, that their orbits are very elongated; and, instead of being nearly circular, they take the elliptical form. In consequence of the nature of these orbits, the same comet may approach very near the Sun, and afterwards travel from it to immense distances. Thus, the period of the Comet of 1680 has been estimated at three thousand years. It approaches the Sun, so as to be nearer to it than our Moon is to us, whilst it recedes to a distance 853 times greater than the distance of the Earth from the Sun. On the 17th of December, 1680, it was at its perihelion—that is, at its greatest proximity to the Sun; it is now continuing its path beyond the Neptunian orbit. Its velocity varies according to its distance from the solar body. At its perihelion it travels thousands of leagues per minute; at its aphelion it does not pass over more than a few yards. Its proximity to the Sun in its passage near that body caused Newton to think that it received a heat twenty-eight thousand times greater than that we experience at the summer solstice; and that this heat being two thousand times greater than that of red-hot iron, an iron globe of the same dimensions would be fifty thousand years entirely losing its heat. Newton added that in the end comets will approach so near the Sun that they will not be able to escape the preponderance of its attraction, and that they will fall one after the other into this brilliant body, thus keeping up the heat which it perpetually pours out into space. Such is the deplorable end assigned to comets by the author of the "Principia," an end which makes De la Brétonne say to Rétif: "An immense comet, already larger than Jupiter, was again increased in its path by being blended with six other dying comets. Thus displaced from its ordinary route by these slight shocks, it did not pursue its true elliptical orbit; so that the unfortunate thing was precipitated into the devouring centre of the Sun." "It is said," added he, "that the poor comet, thus burned alive, sent forth dreadful cries!"

It will be interesting, then, in a double point of view, to follow a comet in its different passages in sight of the Earth. Let us take the most important in astronomical history—the one whose orbit has been calculated by Edmund Halley, and which was named after him. It was in 1682 that this comet appeared in its greatest brilliancy, accompanied with a tail which did not measure less than thirty-two millions of miles. By the observation of the path which it described in the heavens, and the time it occupied in describing it, this astronomer calculated its orbit, and recognized that the comet was the same as that which was admired in 1531 and 1607, and which ought to have reappeared in 1759. Never did scientific prediction excite a more lively interest. The comet returned at the appointed time; and on the 12th of March, 1759, reached its perihelion. Since the year 12 before the Christian era, it had presented itself twenty-four times to the Earth. It was principally from the astronomical annals of China that it was possible to follow it up to this period.

Its first memorable appearance in the history of France is that of 837, in the reign of Louis le Débonnaire. An anonymous writer of chronicles of that time, named "The Astronomer," gave the following details of this appearance, relative to the influence of the comet on the imperial imagination:

"During the holy days of the solemnization of Easter, a phenomenon ever fatal, and of gloomy foreboding, appeared in the heavens. As soon as the Emperor, who paid attention to these phenomena, received the first announcement of it, he gave himself no rest until he had called a certain learned man and myself before him. As soon as I arrived, he anxiously asked me what I thought of such a sign; I asked time of him, in order to consider the aspects of the stars, and to discover the truth by their means, promising to acquaint him on the morrow; but the Emperor, persuaded that I wished to gain time, which was true, in order not to be obliged to announce anything fatal to him, said to me: 'Go on the terrace of the palace and return at once to tell me what you have seen, for I did not see this star last evening, and you did not point it out to me; but I know that it is a comet; tell me what you think it announces to me.' Then scarcely allowing me time to say a word, he added: 'There is still another thing you keep back; it is that a change of reign and the death of a prince are announced by this sign.' And as I advanced the testimony of the prophet, who said: 'Fear not the signs of the heavens as the nations fear them,' the prince with his grand nature, and the wisdom which never forsook him, said, 'We must not only fear Him who has created both us and this star. But as this phenomenon may refer to us, let us acknowledge it as a warning from Heaven."

Louis le Débonnaire gave himself and his court to fasting and prayer, and built churches and monasteries. He died three years later, in 840, and historians have profited by this slight coincidence to prove that the appearance of the comet was a harbinger of death. The historian, Raoul Glader, added later: "These phenomena of the universe are never presented to man without surely announcing some wonderful and terrible event."

Halley's comet again appeared in April, 1066, at the moment when William the Conqueror invaded England. It was pretended that it had the greatest influence on the fate of the battle of Hastings, which delivered over the country to the Normans.

A contemporary poet, alluding probably to the English diadem with which William was crowned, had proclaimed in one place, "that the comet had been more favorable to William than nature had been to Caesar; the latter had no hair, but William had received some from the comet." A monk of Malmesbury apostrophized the comet in these terms: "Here thou art again, thou cause of the tears of many mothers! It is long since I have seen thee, but I see thee now, more terrible than ever; thou threatenest my country with complete ruin!"

In 1455, the same comet made a more memorable appearance still. The Turks and Christians were at war, the West and the East seemed armed from head to foot—on the point of annihilating each other. The crusade undertaken by Pope Calixtus III. against the invading Saracens, was waged with redoubled ardor on the sudden appearance of the star with the flaming tail. Mahomet II. took Constantinople by storm, and raised the siege of Belgrade. But the Pope having put aside both the curse of the comet, and the abominable designs of the Mussulmans, the Christians gained the battle, and vanquished their enemies in a bloody fight. The Angelus to the sound of bells dates from these ordinances of Calixtus III. referring to the comet.

In his poem on astronomy, Daru, of the French Academy, describes this episode in eloquent terms:

This ancient comet witnessed many revolutions in human history, at each of its appearances, even in its later ones, in 1682, 1759, 1835; it was also presented to the Earth under the most diverse aspects, passing through a great variety of forms, from the appearance of a curved sabre, as in 1456, to that of a misty head, as in its last visit. Moreover, this is not an exception to the general rule, for these mysterious stars have had the gift of exercising a power on the imagination which plunged it in ecstasy or trouble. Swords of fire, bloody crosses, flaming daggers, spears, dragons, fish, and other appearances of the same kind, were given to them in the middle ages and the Renaissance.

Comets like those of 1577 appear, moreover, to justify by their strange form the titles with which they are generally greeted. The most serious writers were not free from this terror. Thus, in a chapter on celestial monsters, the celebrated surgeon, Ambroise Paré, described the comet of 1528 under the most vivid and frightful colors: "This comet was so horrible and dreadful that it engendered such great terror to the people, that they died, some with fear, others with illness. It appeared to be of immense length, and of blood color; at its head was seen the figure of a curved arm, holding a large sword in the hand as if it wished to strike. At the point of the sword there were three stars, and on either side was seen a great number of hatchets, knives, and swords covered with blood, amongst which were numerous hideous human faces, with bristling beards and hair."

The imagination has good eyes when it exerts itself. The great and strange variety of cometary aspects is described with exactitude by Father Souciet in his Latin poem on comets. "Most of them," says he, "shine with fires interlaced like thick hair, and from this they have taken the name of comets. One draws after it the twisted folds of a long tail; another appears to have a white and bushy beard; this one throws a glimmer similar to that of a lamp burning during the night; that one, O Titan! represents thy resplendent face; and this other, O Phoebe! the form of thy nascent horns. There are some which bristle with twisted serpents. Shall I speak of those armies which have sometimes appeared in the air? of those clouds which follow as it were along a circle, or which resemble the head of Medusa? Have there not often been seen figures of men or savage animals?

"Often, in the gloom of night, lighted up by these sad fires, the horrible sound of arms is heard, the clashing of swords which meet in the clouds, the ether furiously resounding with fearful din which crush the people with terror. All comets have a melancholy light, but they have not all the same color. Some have a leaden color; others that of flame or brass. The fires of some have the redness of blood; others resemble the brightness of silver. Some again are azure; others have the dark and pale color of iron. These differences come from the diversity of the vapors which surround them, or from the different manner in which they receive the Sun's rays. Do you not see in our fires, that various kinds of wood produce different colors? Pines and firs give a flame mixed with thick smoke, and throw out little light. That which rises from sulphur and thick bitumen is bluish. Lighted straw gives out sparks of a reddish color. The large olive, laurel, ash of Parnassus, etc., trees which always retain their sap, throw a whitish light similar to that of a lamp. Thus, comets whose fires are formed of different materials, each take and preserve a color which is peculiar to them."

Instead of being a cause of fear and terror, the variety and variability of the aspect of comets ought rather to indicate to us the harmlessness of their nature.

THE TOTAL SOLAR ECLIPSE OF 1883

AN ASTRONOMERS VOYAGE TO FAIRY-LAND.

(From The Atlantic Monthly, May, 1890.)

By PROF. E.S. HOLDEN.

In 1883 calculations showed that a solar eclipse of unusually long duration (5 minutes, 20 seconds) would occur in the South Pacific Ocean. The track of the eclipse lay south of the equator, but north of Tahiti. There were in fact only two dots of coral islands on the charts in the line of totality, Caroline Island, and one hundred and fifty miles west Flint Island (longitude 150 west, latitude 10 south). Almost nothing was known of either of these minute points. The station of the party under my charge (sent out by the United States government under the direction of the National Academy of Sciences) was to be Caroline Islands.

Every inch of that island (seven miles long, a mile or so broad) is familiar now; but it is almost ludicrous to recollect with what anxiety we pored over the hydrographic charts and sailing instructions of the various nations, to find some information, however scanty, about the spot which was to be our home for nearly a month. All that was known was that this island had formerly been occupied as a guano station. There was a landing then.

After the personnel of the party had been decided on, there were the preparations for its subsistence to be looked out for. How to feed seventeen men for twenty-one days? Fortunately the provisions that we took, and the fresh fish caught for us by the natives, just sufficed to carry us through with comfort and with health.

In March of 1883 we sailed from New York, and about the same time a French expedition left Europe bound for the same spot. From New York to Panama, from Panama to Lima, were our first steps. Here we joined the United States steamship Hartford, Admiral Farragut's flagship, and the next day set sail for our destined port,—if a coral reef surrounded by a raging surf can be called a port. About the same time a party of French observers under Monsieur Janssen, of the Paris Academy of Sciences, left Panama in the Eclaireur.

It was an ocean race of four thousand miles due west. The station Caroline Islands was supposed to be more desirable than Flint Island. Admiral Wilkes's expedition had lain off the latter several days without being able to land on account of the tremendous surf, so that it was eminently desirable to "beat the Frenchman," as the sailors put it. With this end in view our party had secured (through a member of the National Academy in Washington) the verbal promise of the proper official of the Navy Department that the Hartford's orders should read "to burn coal as necessary." The last obstacle to success was thus removed. We were all prepared, and now the ship would take us speedily to our station.

Imagine our feelings the next day after leaving Callao, when the commanding officer of the Hartford opened his sealed orders. They read (dated Washington, in February), "To arrive at Caroline Islands (in April) with full coal-bunkers!"

Officialism could go no further. Here was an expedition sent on a slow-sailing ship directly through the regions of calms for four thousand miles. It was of no possible use to send the expedition at all unless it arrived in time. And here were our orders "to arrive with full coal-bunkers."

Fortunately we had unheard-of good-luck. The trade-wind blew for us as it did for the Ancient Mariner, and we sped along the parallel of 12° south at the rate of one hundred and fifty miles a day under sail, while the Eclaireur was steaming for thirty days a little nearer the equator in a dead calm. We arrived off the island just in time, with not a day to spare. It was a narrow escape, and a warning to all of us never to sail again under sealed orders unless we knew what was under the seal.

Here we were, then, lying off the island and scanning its sparse crown of cocoanut palms, looking for a French flag among their wavy tufts. There was none in sight. We were the winners in the long race. Directly a whale-boat was lowered, and rowed around the white fringe of tremendous surf that broke ceaselessly against the vertical wall of coral rock. There was just one narrow place where the waves rolled into a sort of cleft and did not break. Here was the "landing," then.

Landing was an acrobatic feat. In you went on the crest of a wave, pointing for the place where the blue seas did not break into white. An instant after, you were in the quiet water inside of the surf. Jump out everybody and hold the boat! Then it was pick up the various instruments, and carry them for a quarter of a mile to high-water mark and beyond, over the sharp points of the reef.

In one night we were fairly settled; in another the Hartford had sailed away, leaving us in our fairy paradise, where the corals and the fish were of all the brilliant hues of the rainbow, and where the whiteness of the sand, the emerald of the lagoon, and the turquoise of the ocean made a picture of color and form never to be forgotten.

But where are the Frenchmen? The next morning there is the Eclaireur lying a mile or so out, and there is a boat with the bo'sun—maître d'équipage—pulling towards the surf. I wade out to the brink. He halloes:

"Where is the landing, then?"

"Mais ici"—Right here,—I say.

"Yes, that's all very well for persons, but where do you land les bagages?"

"Mais ici" I say again, and he says, "Diable!"

But all the same he lands both persons and baggage in a neat, sailor-like way. In a couple of days our two parties of fifty persons had taken possession of this fairy isle. Observatories go up, telescopes, spectroscopes, photographic cameras are pointed and adjusted. The eventful day arrives. Everything is successful. Then comes the Hartford and takes us away, and a few days later comes the Eclaireur, and the Frenchmen are gone. The little island is left there, abandoned to the five natives who tend the sickly plantation of cocoa-palms, and live from year to year with no incident but the annual visit of "the blig" (Kanaka for brig), which brings their store of ship biscuit and molasses.

Think of their stupendous experience! For years and years they have lived like that in the marvellous, continuous charm of the silent island. The "blig" had come and gone away this year, and there will be no more disturbance and discord for a twelve-month longer.

Not so! for here comes a great warship out of the East under a press of canvas. What event is this? See! she clews up her light sails and fires an eleven-inch gun! One of those guns of Mobile Bay. Then swarms out the starboard watch, one hundred and sixty strong, and a fleet of boats brings ashore these pale astronomers with those useless tubes that they point at the sky every night. But there are useful things too; cooking-stoves, and lumber, and bricks.

What is all this? No sooner are these established than comes another ship and fires its gun! and another set of hardy sailormen pours out, and here is another party of madmen with tubes,—yes, and with cooking-stoves and lumber, too. Then comes the crowning, stupendous, and unspeakable event. The whole sun is hidden and the heavens are lighted up with pearly streamers! In the name of all the Polynesian gods, what is the meaning of all this?

And then in a few days all these are gone. All the madmen. They have taken away the useless tubes, but they have left their houses standing. Their splendid, priceless, precious cook-stoves are here. See! here is a frying-pan! here are empty tin cans! and a keg of nails! They must have forgotten all this, madmen as they are!

And the little island sinks back to its quiet and its calm. The lagoon lies placid like a mirror. The slow sea breaks eternally on the outer reef. The white clouds sail over day by day. The seabirds come back to their haunts,—the fierce man-of-war birds, the gentle, soft-eyed tern. But we, whose island home was thus invaded—are we the same? Was this a dream? Will it happen again next year? every year? What indeed was it that happened,—or in fact, did it happen at all? Is it not a dream, indeed?

If we left those peaceful Kanakas to their dream, we Americans have brought ours away with us. Who will forget it? Which of us does not wish to be in that peaceful fairyland once more? That is the personal longing. But we have all come back, each one with his note-books full; and in a few weeks the stimulus of accustomed habit has taken possession of us again. Right and wrong are again determined by "municipal sanctions." We have become useful citizens once more. Perhaps it is just as well. We should have been poor poets, and we do not forget. So ends the astronomer's voyage to fairyland.

HALOS—PARHELIA—THE SPECTRE OF THE BROCKEN, ETC

(From The Atmosphere.)

By CAMILLE FLAMMARION.

Treatises on meteorology have not, up to the present day, classified with sufficient regularity the divers optical phenomena of the air. Some of these phenomena have, however, been seen but rarely, and have not been sufficiently studied to admit of their classification. We have examined the common phenomenon of the rainbow and we have seen that it is due to the refraction and reflection of light on drops of water, and that it is seen upon the opposite side of the sky to the sun in day-time, or the moon at night. We are now about to consider an order of phenomena which are of rarer occurrence, but which have this property in common with the rainbow, viz., that they take place also upon the side of the sky opposite to the sun. These different optical effects are classed together under the name of anthelia (from Greek, opposite to, and Greek, the sun). The optical phenomena which occur on the same side as, or around the sun, such as halos, parhelia, etc., will be dealt with later on.

Before coming to the anthelia, properly so called, or to the colored rings which appear around a shadow, it is as well first to note the effects produced on the clouds and mists that are facing the sun when it rises or sets.

Upon high mountains, the shadow of the mountain is often seen thrown either upon the surface of the lower mists or upon the neighboring mountains, and projected opposite to the sun almost horizontally. I once saw the shadow of the Righi very distinctly traced upon Mount Pilate, which is situated to the west of the Righi, on the other side of the Lake of Lucerne. This phenomenon occurs a few minutes after sunrise, and the triangular form of Righi is delineated in a shape very easy to recognize.

The shadow of Mont Blanc is discerned more easily at sunset. MM. Bravais and Martins, in one of their scientific ascents, noticed it under specially favorable circumstances, the shadow being thrown upon the snow-covered mountains, and gradually rising in the atmosphere until it reached a height of 1°, still remaining quite visible. The air above the cone of the shadow was tinted with that rosy purple which is seen, in a fine sunset, coloring the lofty peaks. "Imagine," says Bravais, "the other mountains also projecting, at the same moment, their shadows into the atmosphere, the lower parts dark and slightly greenish, and above each of these shadows the rosy surface, with the deeper rose of the belt which separates it from them; add to this the regular contour of the cones of the shadow, principally at the upper edge, and lastly, the laws of perspective causing all these lines to converge the one to the other toward the very summit of the shadow of Mont Blanc; that is to say, to the point of the sky where the shadows of our own selves were; and even then one will have but a faint idea of the richness of the meteorological phenomenon displayed before our eyes for a few instants. It seemed as though an invisible being was seated upon a throne surrounded by fire, and that angels with glittering wings were kneeling before him in adoration."

Among the natural phenomena which now attract our attention, but fail to excite our surprise, there are some which possess the characteristics of a supernatural intervention. The names which they have received still bear witness to the terror which they once inspired; and even to-day, when science has stripped them of their marvellous origin, and explained the causes of their production, these phenomena have retained a part of their primitive importance, and are welcomed by the savant with as much interest as when they were attributed to divine agency. Out of a large and very diverse number, I will first select the Spectre of the Brocken.

The Brocken is the highest mountain in the picturesque Hartz chain, running through Hanover, being three hundred and thirty feet above the level of the sea.

One of the best descriptions of this phenomenon is given by the traveller Hane, who witnessed it on the 23d of May, 1797. After having ascended no less than thirty miles to the summit, he had the good fortune at last to contemplate the object of his curiosity. The sun rose at about four o'clock, the weather being fine, and the wind driving off to the west the transparent vapors which had not yet had time to be condensed into clouds. About a quarter-past four, Hane saw in this direction a human figure of enormous dimensions. A gust of wind nearly blowing off his hat at that moment, he raised his hand to secure it, and the colossal figure imitated his action. Hane, noticing this, at once made a stooping movement, and this was also reproduced by the spectre. He then called another person to him, and placing themselves in the very spot where the apparition was first seen, the pair kept their eyes fixed on the Achtermannshohe, but saw nothing. After a short interval, however, two colossal figures appeared, which repeated the gestures made by them, and then disappeared.

Some few years ago, in the summer of 1862, a French artist, M. Stroobant, witnessed and carefully sketched this phenomenon, which is drawn in full-page illustration, opposite p. 272. He had slept at the inn of the Brocken, and rising at two in the morning, he repaired to the plateau upon the summit in the company of a guide. They reached the highest point just as the first glimmer of the rising sun enabled them to distinguish clearly objects at a great distance. To use M. Stroobant's own words, "My guide, who had for some time appeared to be walking in search of something, suddenly led me to an elevation whence I had the singular privilege of contemplating for a few instants the magnificent effect of mirage, which is termed the Spectre of the Brocken. The appearance is most striking. A thick mist, which seemed to emerge from the clouds like an immense curtain, suddenly rose to the west of the mountain, a rainbow was formed, then certain indistinct shapes were delineated. First, the large tower of the inn was reproduced upon a gigantic scale; after that we saw our two selves in a more vague and less exact shape, and these shadows were in each instance surrounded by the colors of the rainbow, which served as a frame to this fairy picture. Some tourists who were staying at the inn had seen the sun rise from their windows, but no one had witnessed the magnificent spectacle which had taken place on the other side of the mountain."

Sometimes these spectres are surrounded by colored concentric arcs. Since the beginning of the present century, treatises on meteorology designate, under the name of the Ulloa circle, the pale external arch which surrounds the phenomenon, and this same circle has sometimes been called the "white rainbow." But it is not formed at the same angular distance as the rainbow, and, although pale, it often envelops a series of interior colored arcs.

Ulloa, being in company with six fellow-travellers upon the Pambamarca at daybreak one morning, observed that the summit of the mountain was entirely covered with thick clouds, and that the sun, when it rose, dissipated them, leaving only in their stead light vapors, which it was almost impossible to distinguish. Suddenly, in the opposite direction to where the sun was rising, "each of the travellers beheld, at about seventy feet from where he was standing, his own image reflected in the air as in a mirror. The image was in the centre of three rainbows of different colors, and surrounded at a certain distance by a fourth bow with only one color. The inside color of each bow was carnation or red, the next shade was violet, the third yellow, the fourth straw color, the last green. All these bows were perpendicular to the horizon; they moved in the direction of, and followed, the image of the person they enveloped as with a glory." The most remarkable point was that, although the seven spectators were standing in a group, each person only saw the phenomenon in regard to his own person, and was disposed to disbelieve that it was repeated in respect to his companions. The extent of the bows increased continually and in proportion to the height of the sun; at the same time their colors faded away, the spectre became paler and more indistinct, and finally the phenomenon disappeared altogether. At the first appearance the shape of the bows was oval, but toward the end they became quite circular. The same apparition was observed in the polar regions by Scoresby, and described by him. He states that the phenomenon appears whenever there is mist and at the same time shining sun. In the polar seas, whenever a rather thick mist rises over the ocean, an observer, placed on the mast, sees one or several circles upon the mist.

These circles are concentric, and their common centre is in the straight line joining the eye of the observer to the sun, and extended from the sun toward the mist. The number of circles varies from one to five; they are particularly numerous and well colored when the sun is very brilliant and the mist thick and low. On July 23, 1821, Scoresby saw four concentric circles around his head. The colors of the first and of the second were very well defined; those of the third, only visible at intervals, were very faint, and the fourth only showed a slight greenish tint.

The meteorologist Kaemtz has often observed the same fact in the Alps. Whenever this shadow was projected upon a cloud, his head appeared surrounded by a luminous aureola.

To what action of light is this phenomenon due? Bouguer is of opinion that it must be attributed to the passage of light through icy particles. Such, also, is the opinion of De Saussure, Scoresby, and other meteorologists.

In regard to the mountains, as we cannot assure ourselves directly of the fact by entering the clouds, we are reduced to conjecture. The aerostat traversing the clouds completely, and passing by the very point where the apparition is seen, affords one an opportunity of ascertaining the state of the cloud. This observation I have been able to make, and so to offer an explanation of the phenomenon.

As the balloon sails on, borne forward by the wind, its shadow travels either on the ground or on the clouds. This shadow is, as a rule, black, like all others; but it frequently happens that it appears alone on the surface of the ground, and thus appears luminous. Examining this shadow by the aid of a telescope, I have noticed that it is often composed of a dark nucleus and a penumbra of the shape of an aureola. This aureola, frequently very large in proportion to the diameter of the central nucleus, eclipses it to the naked eye, so that the whole shadow appears like a nebulous circle projected in yellow upon the green ground of the woods and meadows. I have noticed, too, that this luminous shadow is generally all the more strongly marked in proportion to the greater humidity of the surface of the ground.

Seen upon the clouds, this shadow sometimes presents a curious aspect. I have often, when the balloon emerged from the clouds into the clear sky, suddenly perceived, at twenty or thirty yards' distance, a second balloon distinctly delineated, and apparently of a grayish color, against the white ground of the clouds. This phenomenon manifests itself at the moment when the sun re-appears. The smallest details of the car can be made out clearly, and our gestures are strikingly reproduced by the shadow.

On April 15, 1868, at about half-past three in the afternoon, we emerged from a stratum of clouds, when the shadow of the balloon was seen by us, surrounded by colored concentric circles, of which the car formed the centre. It was very plainly visible upon a yellowish white ground. A first circle of pale blue encompassed this ground and the car in a kind of ring. Around this ring was a second of a deeper yellow, then a grayish red zone, and lastly as the exterior circumference, a fourth circle, violet in hue, and imperceptibly toning down into the gray tint of the clouds. The slightest details were clearly discernible—net, robes, and instruments. Every one of our gestures was instantaneously reproduced by the aerial spectres. The anthelion remained upon the clouds sufficiently distinct, and for a sufficiently long time, to permit of my taking a sketch in my journal and studying the physical condition of the clouds upon which it was produced. I was able to determine directly the circumstances of its production. Indeed, as this brilliant phenomenon occurred in the midst of the very clouds which I was traversing, it was easy for me to ascertain that these clouds were not formed of frozen particles. The thermometer marked 2° above zero. The hygrometer marked a maximum of humidity experienced, namely, seventy-seven at three thousand seven hundred and seventy feet, and the balloon was then at four thousand six hundred feet, where the humidity was only seventy-three. It is therefore certain that this is a phenomenon of the diffraction of light simply produced by the vesicles of the mist.

The name of diffraction is given to all the modifications which the luminous rays undergo when they come in contact with the surface of bodies. Light, under these circumstances, is subject to a sort of deviation, at the same time becoming decomposed, whence result those curious appearances in the shadows of objects which were observed for the first time by Grimaldi and Newton.

The most interesting phenomena of diffraction are those presented by gratings, as are technically denominated the systems of linear and very narrow openings situated parallel to one another and at very small intervals. A system of this kind may be realized by tracing with a diamond, for instance, on a pane of glass equidistant lines very close together. As the light would be able to pass in the interstices between the strokes, whereas it would be stopped in the points corresponding to those where the glass was not smooth, there is, in reality, an effect produced as if there were a series of openings very near to each other. A hundred strokes, about 1/25th of an inch in length, may thus be drawn without difficulty. The light is then decomposed in spectra, each overlapping the other. It is a phenomenon of this kind which is seen when we look into the light with the eye half closed; the eyelashes in this case, acting as a net-work or grating. These net-works may also be produced by reflection, and it is to this circumstance that are due the brilliant colors observed when a pencil of luminous rays is reflected on a metallic surface regularly striated.

To the phenomena of gratings must be attributed, too, the colors, often so brilliant, to be seen in mother-of-pearl. This substance is of a laminated structure; so much so, that in carving it the different folds are often cut in such a way as to form a regular net-work upon the surface. It is, again, to a phenomenon of this sort that are due the rainbow hues seen in the feathers of certain birds, and sometimes in spiders' webs. The latter, although very fine, are not simple, for they are composed of a large number of pieces joined together by a viscous substance, and thus constitute a kind of net-work.

If the sun is near the horizon, and the shadow of the observer falls upon the grass, upon a field of corn, or other surface covered with dew, there is visible an aureola, the light of which is especially bright about the head, but which diminishes from below the middle of the body. This light is due to the reflection of light by the moist stubble and the drops of dew. It is brighter about the head, because the blades that are near where the shadow of the head falls expose to it all that part of them which is lighted up, whereas those farther off expose not only the part which is lighted up, but other parts which are not, and this diminishes the brightness in proportion as their distance from the head increases. The phenomenon is seen whenever there is simultaneously mist and sun. This fact is easily verified upon a mountain. As soon as the shadow of a mountaineer is projected upon a mist, his head gives rise to a shadow surrounded by a luminous aureola.