THE FALSE AURORAS.

Inangahua Times, Volume IX, Issue 1385, 9 April 1884, Page 2

THE FALSE AURORAS.

Everybody remembers the extraordinary rosy sunset skies of the six or* seven mouths just passedaway. They 1 have, as most people will also call to mind, been common to the whole world. The fiery atmosphere of Australia and the cloud-laden sky of Great Britain have alike been rendered beautiful in the late evening by those unusual appearances. A writer in the Argus very happily termed them " false auroras," and that title has therefore been chosen for the heading of a few remarks upon the subject, collated from " Science Notes," written by Professor Mattieu Williams in the February number of the " Gentlemen's Magazine." Great discussiou was raised at the time the phenomena were seen here and in Australia, and the wise men of the Antipodes were divided into two fairly defined parties, holding different theories. The first was that the roseate and carmine hues, which prevaded the western portions of the heavens loug after the sun had disappeared from them, were due to the reflection of light from clouds of dust thrown up after the great volcanic convulsion on August 26 last, from the crater of Krakatoa, and then travelling through upper regions of our atmosphere. Mr Ellery, Government astronomer of Victoria, combated this theory, and may certainly be said to have thrown cold water on it, for, having examined the " aurora" by means of spectrum analysis, he said there was nothing to lead him to believe that there was anything more extraordinary than a very large amount of moisture existing at great heights. This was the second theory, to which most people who' opposed the first pinned their faith. Mattieu Williams disposes of Krakatoa very summarily. In the first place, ho says that besides the fact that the glowing, sunset skies lasted for too many months for such a theory to hold water, they also began very much too early. He had observed and commented on them (in the spring and early summer) long before the Straits of Sunda ever felt the least disturbance. This, of course, is perfectly sufficient to put Krakatoa out of court, but, in addition, he shows that from j what is known of volcanic dust, the quantity thrown up would be far too small to produce the pheuomena in question. The fine dust which the wind could carry away forms, perhaps, one-thousandth part of the whole matter ejected from a volcauo. Draw a map of the world on hu.h a ECalo that it should cover St. Paul's Churchyard, and the burning mountain would be represented by a pea. "The i extravagauco of only supposing that the one-thousandth pan of the oruptionfrom this large pea could proJucw startling effects throughout tho wholearea for five-months, without diminution, becomes evident enough." And yot another point. Kr;i kotoa's disagreeable vagaries occurred in the lasfc days of August. .On September 2 accounts were published of the extraordinary sunsets then prevailing in the West Indies. If Krakatoa's volcanic dust had anything to say in the matter, it had travelled in " a direct line, without any cyclonic deviation, at the rate of 2000 miles per day !" Upon this statement Mr Williams ir.akes no comment ; he appnreutly thinks it superfluous. But after having thus disposed of this deus ex machina of most ordinary observers, our author has by no means exhausted his repertory of theories. He first of all makes a few pertinent observations as to " the whereabouts of the twilight glow," which he says has extended far above the clouds— even above the highest cirrus streaks, and which he ascribes to the presence of " atmospheric matter of some kind I*1 '* suspended at very great elevations. This settled, he proceeds to ventilate the other theories — the meteroic and the cometic. "Cosmical dust" enters largely into the first, and that dear old scientific niaid-of-all-work, the comet, who has lost her way, and plunged headlong to fiery destruction in the bosom of the sun, appears to form the material for the second theory. We say "appears," for the savant, although he > seems very clever at knocking other people's card-houses down, does not construct a very solid building of his own. Up to December he seems to have heen very much in love with the meteoric theory, but afterwards some serious difficulties presented themselves to his ciitical miod, and he leaves us rather in doubt as to whether he continued to place very much confidence in it. "Cosmical" dust consist of particles of ferruginous matter whioh have fallen to the earth from somewhere beyond it. Mr Cowper Eaynard, who has made it his special study, inclines to the belief that when the sunsets occurred we were passing through a cloud of it. Others besides Mr Williams had, independently of one another, caught . at the same idea. After various cunning manoeuvres to catch "cosmical" dust — spt eading sheets of oily paper, strips coated with vasseline, and employing other '• dodges" — a deposit of magnetic oxide of iron was actually secured. A triumphant explanation of the perplexing sunset appearances seemed close at hand, for if it could be proved . that the earth had heen travelling through a regiou of space unsually rich in meteroic dust, the twilight phenomena are satisfactorily explained. But the observer, after December, began to discover objections to the "cosmical dust" theory. First of all, his meteroric particles 'lacked in sufficiency." Next, he thought that the granular particles of iron oxide were some four times too heavy and compact tosuit his ideas, which demanded somo kind of impalpable dus^flioatin _j ia thfe

atmosphere. If this could be— and such he thinks not impossible— Mr Mattieu Williams holds that we *' should have in the higher regions of our atmosphere iron vapour, which, condensing under very low pressure, might produce particles too small to be visible even with microscopic aid, and yet capable of producing the optical effects observed, by reflection, or dititraction of iight. The last theory aired by Mr Mattieu Williams — the cometic— goes rather largely into scientific terms, but may he made fairly plain in a few words. Supposing a comet to have come into direct collision with the sun, the heat generated by such 9 collision would probably increase the amount of radiation from the sun, ©f which increase the earth would receive its •share. The vaparous envelope which surrounds the sun would. most likely be thinned to a considerable extent, and we should catch glimpses of the photosphere, or actual luminous surface beneath. Now, the photosphere, there is reason to believe, is blue, and it is a very curious fact that the prevalence of these very remarkable sunsete wasproceded by startling accounts of blue and green suns seen in countries where the, air is exceptionally dry. And another matter which gives an air of probability to this last extraordinary theory is that, supposing increased thermal radiation to have taken place, the greatest difference would be in the upper regions of the air. If it has occurred, there will ba more water in the air, its upperregions will be made warmer, and offer greater resistance to radiation. Not only would the amount of water be doubled, but, instead of condensinginto ice crystals, it would condense in the form of liquid particles, which act vpon light so differently. This is Mr Mattieu Williams' reasoning, and as far as any evidence to the contrary goes, he has arrived at the same conclusion as Mr Ellery, who' ■worked in the directest and most prati•cal way. Both favour the " large amount of aqueous-vapour" theory. But here the English man of science leaves us m the lurch; He demolishes the volcanic dust theory,- and gives ,his own meteoric dust only a lukewarm support. Having' auggestsd that if certain events, had Happened between the king of o'lir solar system and a wandereV.jnv space, such developments might Reasonably be expected to take place as would produce a state of things wttloh" Wtould render the twilight 'fefbws possible, he asks the question, " Have we any fair reason for supposing that the sun may have teen lately subjected to any unusual heat generating coillision?" To that question he promises to devote his next " Science Notes." We must for the present be content with this "To be concluded in our next," and wait for further revelation^ concerning " False auroras." \