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полная версияChambers\'s Edinburgh Journal, No. 441

Various
Chambers's Edinburgh Journal, No. 441

Полная версия

ARAGO ON THE SUN

In the Annuaire of the Bureau des Longitudes, recently published in Paris, appears a paper by the distinguished astronomer Arago—'On the Observations which have made known the Physical Constitution of the Sun and of different Stars; and an Inquiry into the Conjectures of the Ancient Philosophers, and of the Positive Ideas of Modern Astronomers on the Place that the Sun ought to occupy among the Prodigious Number of Stars which stud the Firmament'—in which all that appertains to the subject is so ably condensed, as to afford material for a popular summary, which we purpose to convey in the present article. The eclipse of the sun of last July, by enabling observers to repeat former observations and test their accuracy, furnished some of the results which serve to complete the paper in question, and which may be considered as settled, owing to the improvements continually taking place in the construction of instruments. Although astronomy is the exactest of sciences, its problems are not yet all fully solved; and for the determination of some of these, observers have to wait for years—in certain instances, for a century or more, until all the circumstances combine for a favourable observation. From the days of the Epicurean philosopher, who, judging from appearances, declared the sun to be no more than a foot in diameter, to those of living calculators, who give to the orb a diameter of 883,000 miles, there has been a marvellous advance. In these dimensions, we have a sphere one million four hundred thousand times larger than the earth. 'Numbers so enormous,' says M. Arago, 'not being often employed in ordinary life, and giving us no very precise idea of the magnitudes which they imply, I recall here a remark that will convey a better understanding of the immensity of the solar volume. If we imagine the centre of the sun to coincide with that of the earth, its surface would not only reach the region in which the moon revolves, but would extend nearly as far again beyond.' By the transit of Venus in 1769, it was demonstrated that the sun is 95,000,000 miles from the earth; and yet, distant as it is, its physical constitution has been determined; and the history of the successive steps by which this proof has been arrived at, forms one of the most interesting chapters in the progress of science.

It was in 1611 that Fabricius, a Dutch astronomer, first observed spots on the eastern edge of the sun, which passed slowly across the disk to the western edge, and disappeared after a certain number of days. This phenomenon having been often noted subsequently, the conclusion drawn therefrom is, that the sun is a spherical body, having a movement of rotation about its centre, of which the duration is equal to twenty-five days and a half. These dark spots, irregular and variable, but well defined on their edge, are sometimes of considerable dimensions. Some have been seen whose size was five times that of the earth. They are generally surrounded by an aureola known as the penumbra, and sensibly less luminous than the other portions of the orb. From this penumbra, first observed by Galileo, many apparently singular deductions have been made: namely, 'The sun is a dark body, surrounded at a certain distance by an atmosphere which may be compared to that of the earth, when the latter is charged with a continuous stratum of opaque and reflecting clouds. To this first atmosphere succeeds a second, luminous in itself, called the photosphere. This photosphere, more or less remote from the inner cloudy atmosphere, would determine by its outline the visible limits of the orb. According to this hypothesis, there would he spots on the sun every time that there occurred in the two concentric atmospheres such corresponding clear spaces as would allow of our seeing the dark central body uncovered.'

This hypothesis is considered by the most competent judges to render a very satisfactory account of the facts. But it has not been universally adopted. Some writers of authority have lately represented the spots as scoriæ floating on a liquid surface, and ejected from solar volcanoes, of which the burning mountains of the earth convey but a feeble idea. Hence observations become necessary as to the nature of the incandescent matter of the sun; and when we remember the immense distance of that body, such an attempt may well appear to be one of temerity.

The progress of optical science, however, has given us the means of determining this apparently insoluble question. It is well known, that physicists are enabled at present to distinguish two kinds of light—natural light and polarised light. A ray of the former exhibits the same properties on any part of its form; not so the latter. A polarised ray is said to have sides, and the different sides have different properties, as demonstrated by many interesting phenomena. Strange as it may seem, these rays thus described as having sides, could pass through the eye of a needle by hundreds of thousands without disturbing each other. Availing themselves, therefore, of the assistance of polarised light, and an instrument named the polariscope, or polarising telescope, observers obtain a double image of the sun, both alike, and both white; but on reflecting this image on water, or a glass mirror, the rays become polarised; the two images are no longer alike or white, but are intensely coloured, while their form remains unchanged. If one is red, the other is green, or yellow and violet, always producing what are called the complementary colours. With this instrument, it becomes possible to tell the difference between natural and polarised light.

Another point for consideration is, that for a long time it was supposed, that the light emanating from any incandescent body always came to the eye as natural light, if in its passage it had not been reflected or refracted. But experiment by the polariscope shewed, that the ray departing from the surface at an angle sufficiently small was polarised; while at the same time, it was demonstrated that the light emitted by any gaseous body in flame—that of street-lamps, for instance—is always in the natural state, whatever be its angle of emission. From these remarks, some idea will be formed of the process necessary to prove whether the substance which renders the sun visible is solid, liquid, or gaseous. On looking at the sun in the polariscope, the image, as before observed, is seen to be purely white—a proof that the medium through which the luminous substance is made visible to us is gaseous. If it were liquid, the light would be coloured; and as regards solidity, that is out of the question—the rapid change of spots proves that the outer envelope of the sun is not solid. On whatever day of the year we examine, the light is always white. Thus, these experiments remove the theory out of the region of simple hypothesis, and give certainty to our conclusions respecting the photosphere.

Here an example occurs of the aids and confirmations which science may derive from apparently trivial circumstances. Complaint was made at a large warehouse in Paris, that the gas-fitters had thrown the light on the goods from the narrow, and not from the broad side of the flame. Experiments were instituted, which proved that the amount of light was the same whether emitted from the broad or narrow surface. It was shewn also, that a gaseous substance in flame appears more luminous when seen obliquely than perpendicular, which explains what are known as faculæ and lucules, being those parts of the solar disk that shew themselves brighter than other portions of the surface. These are due to the presence of clouds in the solar atmosphere; the inclined portions of the clouds appearing brightest to the spectator. The notion, that there were thousands on thousands of points distinguishing themselves from the rest by a greater accumulation of luminous matter, is thus disposed of.

Still, there remained something more to be determined. The existence of the photosphere being proved, the question arose—was there nothing beyond? or did it end abruptly? and this could only be determined at the period of a total eclipse, at the very moment when the obscuration of the sun being greatest, our atmosphere ceases to be illuminated. Hence the interest felt in an eclipse of the sun of late years.

In July 1842, at a total eclipse of the sun visible in several parts of the continent, the astronomers noticed, just as the sun was hidden by the moon, certain objects, in the form of rose-coloured protuberances, about two or three minutes high, astronomically speaking, projected from the surface of the moon. These appearances were variously explained: some supposed them to be lunar mountains; others saw in them effects of refraction or diffraction; but no precise explanation could be given; and mere guesses cannot be accepted as science. Others, again, thought them to be mountains in the sun, the summits stretching beyond the photosphere; but at the most moderate calculation, their height would have been about 60,000 miles—an elevation which, as is said, the solar attraction would render impossible. Another hypothesis was, that they were clouds floating in a solar, gaseous atmosphere.

M. Arago considers the last as the true explanation: it remained the great point to be proved. If it could be ascertained, that these red protuberances were not in actual contact with the moon, the demonstration would be complete. Speculation was busy, but nothing could be done in the way of verification until another eclipse took place. There was one in August 1850 total to the Sandwich Islands, at which, under direction of the French commandant at Tahiti, observations were made, the result being that the red prominences were seen to be separated by a fine line from the moon's circumference. Here was an important datum. It was confirmed by the observations of July 1851, by observers of different nations at different localities, who saw that the coloured peaks were detached from the moon; thus proving that they are not lunar mountains.

 

If it be further ascertained, that these luminous phenomena are not produced by the inflexion of rays passing over the asperities of the moon's disk, and that they have a real existence, then there will be a new atmosphere to add to those which already surround the sun; for clouds cannot support themselves in empty space.

We come next to that part of the subject which treats of the true place of the sun in the universe. In the year 448 b.c., Archelaüs, the last of the Ionian philosophers, without having made any measurements, taught that the sun was a star, but only somewhat larger than the others. Now, the nearest fixed star is 206,000 times further from us than the sun: 206,000 times 95,000,000 of miles—a sum beyond all our habits of thought. The light from the star Alpha of the Centaur is three years in its passage to the earth, travelling at the rate of 192,000 miles per second; and there are 86,400 seconds in a day, and 365 days in a year. Astounding facts! If the sun, therefore, were removed to the distance of a Centauri, its broad disk, which takes a considerable time in its majestic rising and setting above and below the horizon, would have no sensible dimensions, even in the most powerful telescopes; and its light would not exceed that of stars of the third magnitude—facts which throw the guess of Archelaüs into discredit. If our place in the material universe is thus made to appear very subordinate, we may remember, as M. Arago observes, that man owes the knowledge of it entirely to his own resources, and thereby has raised himself to the most eminent rank in the world of ideas. Indeed, astronomical investigations might not improperly excuse a little vanity on our part.'

Among the stars, Sirius is the brightest; but twenty thousand millions of such stars would be required to transmit to the earth a light equal to that of the sun. And if it were difficult to ascertain the nature and quality of the sun, it would appear to be still more so to determine these points with regard to the stars; for the reason, that the rays, coming from all parts of their disk, at once are intermingled, and of necessity produce white. This difficulty did not exist in similar investigations on the sun, because its disk is so large, that the rays from any one part of it may be examined while the others are excluded. Under these circumstances, further proof might seem to be hopeless; but advantage was taken of the fact, that there are certain stars which are sometimes light, sometimes dark, either from having a movement of rotation on their own axis, or because they are occasionally eclipsed by a non-luminous satellite revolving around them. It is clear, that while the light is waxing or waning, it comes from a part only of the star's disk; consequently, the neutralisation of rays, which takes place when they depart from the whole surface at once, cannot then occur; and from the observations on the portion of light thus transmitted, and which is found to remain white under all its phases, we are entitled to conclude, in M. Arago's words, that 'our sun is a star, and that its physical constitution is identical with that of the millions of stars strewn in the firmament.'

BARBARA'S SEA-SIDE EXCURSION

It certainly appeared a most improbable circumstance, that any event should occur worthy of being recorded, to vary the even tenor of life which Mr and Mrs Norman enjoyed in the holy state of matrimony. They were young folks—they had married from affection—and, moreover, their union had been a strictly prudent one; for their income was more than sufficient for all their unaspiring wants and tastes; and it was also a 'certainty,' a great good in these days of speculation and going ahead. Charles Norman held a government situation, with a small but yearly increasing salary; his residence was at Pentonville; and his domestic circle comprised, besides his good, meek helpmeet, two little children, and an only sister, some years Charles's junior: indeed, Bab Norman had not very long quitted the boarding-school. Bab and Charles were orphans, and had no near relatives in the world; therefore Bab came home to live with her dear brother and his wife until she had a home of her own—a contingency which people whispered need not be far off, if Miss Barbara Norman so inclined. This piece of gossip perhaps arose from the frequent visits of Mr Norman's chosen friend, Edward Leslie—a steady and excellent young man, who filled an appointment of great trust and confidence in an old-established commercial house. Edward Leslie was not distinguished for personal attractions or captivating manners; but he was an honest, manly, generous-hearted fellow, and sensitive enough to feel very keenly sometimes that the pretty spoiled little Barbara laughed at and snubbed him. Notwithstanding Bab's folly, however, it would have given her great pain had Edward Leslie courted another. He was patient and forbearing; and she fluttered and frisked about, determined to make the most of her liberty while it lasted. 'Of course she meant to marry some day,' she said with a demure smile, 'but it would take a long time to make up her mind.'

Charles quite doted on his pretty sister, and often could not find it in his heart to rebuke her, because she was motherless, and had only him and Cary to look to; and Cary's office was not to rebuke any one, much less her dear little sister-in-law. So Barbara was spoiled and humoured; while the children were kept in high order—a proper discipline being exercised in the nursery, as became a well-regulated and nicely-decorated house. Cary thought Bab a beauty, and so did Charles; the young lady herself was not at all backward in estimating her own charms; and it was a pity to see them so often obscured by affectation, for Bab had a kind heart and an affectionate disposition. One day when Charles returned home after business-hours were over, Bab flew towards him with an unusually animated countenance, holding an open letter in her hand, and exclaiming: 'Oh, dear Charles, read this! You'll let me go—wont you? I never was at the sea-side in my life, you know; and it will do me such a deal of good.'

Charles smiled, took the letter, and tapping his sister's dimpled rosy cheek, he said fondly: 'I don't think, Bab, that you want "doing good to" so far as health is concerned. The sea-air cannot improve these roses.'

'Well, well, Charles, never mind the roses—there's a dear. They only ask me to go for a fortnight, and I should so like it; it will be so nice to be with one's schoolmates at the sea. Bell and Lucy Combermere are such bathers, they say; and as for me, I do believe, Charles, I shall drown myself for love of the sea! Oh, you must let me go—do!'

There was no resisting this coaxing; so Charles said he 'would see about it, and talk the matter over with Caroline.'

'Cary thinks it will be delightful for me,' exclaimed Barbara: 'she's always a good-natured darling.' And Bab felt sure of going, if Charles talked the matter over with Cary; so she flew off in an ecstasy of joy, dancing and singing, and forthwith commenced preparations, by pulling off the faded pink ribbons which adorned her bonnet, and substituting gay bright new streamers.

The invitation in question came from Mrs Combermere, who, with her two unmarried daughters, were sojourning at a favourite watering-place—always crowded during the season—and where Mr Combermere, a rich citizen, could join his family every week, and inhale a breath of pure air. Charles did not particularly like the Combermeres. Mrs Combermere was a fussy woman, full of absurd pretension, and with a weakness for forming aristocratic acquaintance, which had more than once led her into extravagance, ending in disappointment and mortification. The Misses Combermere inherited their mamma's weakness; they were comely damsels, and expectant sharers of papa's wealth, who was 'very particular' on whom he bestowed his treasures. Bell and Lucy had been at school with Barbara Norman, and a strong friendship—a school friendship—had been struck up amongst the trio, whom the French dancing-master denominated 'the Graces.' And now Barbara had received an invitation to stay with them for a fortnight, a private postscript being inserted by Miss Bell, to the effect that 'Bab must be sure to come very smart, for there were most elegant people there, and such beaux!'

Bab went accordingly on Saturday, escorted by Mr Combermere, who always returned on the following Monday. Never before had Bab beheld so gay a scene; never till now had she looked on the glorious ocean; never had she promenaded to the sounds of such exhilarating music. Her pretty little head was quite bewildered, though in the midst of all her delight she wished for Charles and Cary, and the children; there was such delicious bathing for the tiny ones; such digging with their little spades in the golden sands! Innocent, happy gold-diggers they!

She found Mrs Combermere and the girls in the full swing of sea-side dissipation—quite open-house kept, free-and-easy manners, which at home would not have been tolerated. But it came only once a year, and they could afford it. Quite established as an intimate, was a tall young gentleman, with delicate moustache, who seemed to be on terms of friendly familiarity with half the aristocracy of the nation. Mrs Combermere whispered to Bab, that Mr Newton was a most 'patrician person,' of the 'highest connections;' they had met with him on the sands, where he had been of signal use in assisting Mrs Combermere over the shingles on a stormy day. He was so gentlemanly and agreeable, that they could not do otherwise than ask him in; he had remained to tea, and since then had been a regular visitor.

Mr Newton had been at first treated with great coolness by Mr Combermere; the latter gentleman did not like strangers, and always looked on a moustache with suspicion. But Mr Newton was so deferential, so unexceptionable in deportment, and prudent in his general sentiments, warmly advocating Mr Combermere's political opinions, that he had at last won the good opinion even of the father of the family. Besides, he paid no particular attention to the Misses Combermere: there was no danger of his making up to them—that was clear; and Mrs Combermere, mother-like, felt a little mortified and chagrined at such palpable indifference. But when pretty Bab Norman appeared, the case was different: her brunette complexion and sparkling dark eyes elicited marked admiration from the patrician Mr Newton; and he remarked in an off-hand way—sotto voce, as if to himself: 'By Jupiter! how like she is to dear Lady Mary Manvers.' Bab felt very much flattered by the comparison, and immediately began to like Mr Newton immensely; he was so distingué, so fascinating, so refined. Bab did not add, that he had singled her out as an especial object of attention, even when the fair dashing Misses Combermere challenged competition.

The fortnight passed swiftly away—too swiftly, alas! thought little Barbara Norman; for at the expiration of the term, Mrs Combermere did not ask her to prolong the visit, but suffered her to depart, again under the escort of Mr Combermere, without a word of regret at parting. Cruel Mrs Combermere! she wished to keep Mr Newton's society all to herself and her daughters! However, the young gentleman asked Barbara for permission to pay his respects to her when he returned to the metropolis; this had been accorded by Barbara, who, on her return to Pentonville, for the first time found that comfortable home 'insufferably dull and stupid.' Edward Leslie, too—how dull and stupid even he was, after the chattering perfumed loungers of the elysium she had just quitted! Yet Edward was never considered either dull or stupid by competent judges; but, quite the contrary—a sensible, well-informed, gentlemanly personage. But, then, he had no great friends, no patrician weaknesses; he knew nothing about racing, or betting, or opera-dancers, or slang in general. In short, he seemed flat and insipid to Bab, who had been compared to the beautiful Lady Mary Manvers by the soft and persuasive tongue of Lady Mary Manvers's dear friend. Yet, in her secret heart of hearts, Bab drew comparisons by no means disadvantageous to Edward Leslie. 'Yes,' thought Bab, 'I like Mr Newton best by the sea-side in summer-time, when harp-music floats on the balmy air; then I should always like him, if summer was all the year round. But for everyday life, for winter hours, for home, in short, I'm sure I like Edward Leslie best—I'm sure I love Edward Leslie;' and Bab blushed and hesitated, though she was quite alone. Cary listened good-naturedly to all Bab's descriptions of the happiness she had enjoyed; and Cary thought, from all Bab said, that Mr Newton must be at least some great lord in disguise. She felt quite nervous at the idea of his coming to such a humble house as theirs, when he talked of parks, and four-in-hands, and baronial halls, as things with which he was familiar, and regarded as matters of course. Cary hoped that Charles and Edward Leslie would be present when Mr Newton called, because they were fit to associate with royalty itself. Cary had a very humble opinion of herself—sweet, gentle soul! Charles often wished his dear sister Bab might closely resemble her. At length, Bell Combermere wrote to say, they were about returning to town; and Mr Newton declared he could not remain behind. Bab's heart fluttered and palpitated at each sound the knocker gave; and she was thankful that Cary's cousin, Miss Ward, was staying with them, to call attention off from herself.

 
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