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Fery clearly explained, though not difficult to be either given or comprehended.
By far the greatest part of the light we possess is derived from the sun : but there are also numerous kinds of bodies that possess it in a latent or dormant state, and readily give it forth upon being properly excited, or under a proper elevation of tem. perature; and others that give it forth spontaneously, in the common temperament of the atmosphere. Of the former division are oils, bitumens, vinous spirits, carbon, phosphorus, and hydrogen gas. of the latter various vegetable and animal substances, on the commencement of putrefaction, which consists in a slow de. composition not wholly unlike combustion ; touch.wood, which is wood in a peculiar state of decay ; various species of boletus or agaric, and especially that which is denominated spunk; and a multiplicity of worms and insects in a state of perfect health, as pyrosmas, phloades, fire-balls, glow-worms, and elaters".
It was at one period, as we shall find in the ensuing section, a very general opinion among the learned, that the luminous appear. ances we refer to, were in every instance produced by phospho. rescent insects and worms; but the situations and periods of the year in which they are perhaps most frequently met with, together with a variety of other circumstances, strongly militate against such an idea ; and sufficiently prove, that the source of many of these meteors is to be sought for in the light exhaled by the decom. position of animal or vegetable materials, magnified, and deepened in hue by the haze or vapoury atmosphere of the moist and swampy low lands in which they are chiefly beheld ; and which, in consequence of their moisture and swampiness, are particularly favourable to the process of decomposition.
We may thus account for many of them, and particularly for those that evince no sensible heat during their illumination ; for the light exhaled or thrown off from the substances we now allude to, possesses no sensible heat whatever.
It not unfrequently happens, however, that a greater or less degree of heat, a proof of actual though slow combustion, has been evinced during the existence of these phænomena; as also that they have extended more widely than any local decomposition
See for a fuller account of tbese curious facts, Books II. and III. of the present work
would induce us to expect, and have even appeared to change their situation, and to dance about from place to place.
To explain meteors of this kind it is only necessary to observe, that the earth is perpetually exhaling a variety of inflammable gases and other materials, as hydrogen gas or inflammable air, phosphorus, carbonic acid gas, and occasionally sulphurous ta. pour, sometimes separately, and sometimes in a state of union : and that the most active of these, are particularly evaporating in the low stagnant marsh grounds where these phænomena chiefly make their appearance; and may at any time be collected with the greatest ease, by placing over the surface of the soil an inverted wine-glass or tumbler. Now although these gases will not infiame spontaneously, in the ordinary temperature of the atmosphere, they readily inflame from a great variety of natural causes to which they are perpetually exposed; and hence, in effect, those pune. rous fire-damps, in coal-mines, and other caverns of which we bave given a few tremendous examples in a former part of this work. Electricity may be a common cause of such inflammation; the heat generated during the decomposition of the animal or vege. table materials that may be locally decomposing, may be far more than sufficient for this purpose; for we know it to be sufficient to set hay-stacks on fire, when the grass has been put together in a state sufficiently damp to favour such decomposition. And it is not improbable, that some of these materials may catch the illumi. nation, as from a candle, from a body in the immediate vicinity that is in the act of spontaneous illumination.
Now the ball or general mass of inflammable vapour beiog once lighted or inflamed, from whatever cause, will continue to burn as long as its inflammable principle continues without being destroyed; and its combustible power may be more or less in proportion to its purity or state of concentration ; whence in some instances it may pour forth light with little or no sensible heat; in others, the heat combined with it may be sufficient to produce slow combustion like that of a dung-hill; and in others palpable and rapid flame. From the levity of the illumined or burning vapour, moreover, it must Decessarily change its place in various instances according to the current of air which it either finds, or by burning makes for itsell: and hence it must appear to move in various directions, upwards and downwards, to the right and to the left, from object to object,
in a constant dance before the spectator, according to the motion, because that operates upon it: while its colours and dimensions must vary according to the varying density of the fog, or haze, through which, in different places or situations, it is seen, or according to its actually increasing or diminishing and decaying bulk : hence, in our opinion, the usual origin of this class of singular meteors.
We hazard the opinion, however, with much diffidence, and shall leave it to our readers to adopt, or reject, as its merit may respectively strike them: and shall only further observe, before we proceed to offer a few examples of them, that while in all countries they are for the most part to be met in wet swampy lowlands, and stagnant marshes or morasses; they more usually occur, and with greater lustre, in hot climates, upon the approach of winter, when the sky, after wearing a fiery brightness, begins to be overcast, and the whole horizon to be wrapped as in a muddy cloud. Mists and vapours, says an intelligent writer, at this time continue to rise with peculiar density throughout all the regions under the line: the air which so lately before was clear and elastic, now becomes humid, obscure, and stifling: the fogs become so thick, that the light of the sun seems in a manner ex. cluded; nor would its presence be known, but for the intense and suffocating heat of its beams, which dart through the gloom, and, instead of dissipating, only serve to increase the mist. After this preparation, there follows an almost continual succession of thun. der, rain, and tempests. During this dreadful season, the streets of cities flow like rivers, and the whole country wears the appearance of an ocean. The inhabitants often make use of this oppor. tunity to lay in a stock of fresh water, for the rest of the year; as the same cause which pours down the deluge at one season, denies the kindly shower at another. The thunder which attends the fall of these rains, is much more terrible than that we are generally acquainted with. Among ourselves, the tlash is seen at some distance, and the noise shortly aiter ensues; our thunder generally rolls on one quarter of the sky, and one stroke pursnes another. But here the whole sky appears abruptly illuminated with unremitted flashes of lightuing; every part of the air seems productive of its own thunders; and every cloud producrs its own shock. The strokes come so thick, that the inbabitants can scarce TOL. IV.
mark the intervals; and all is one unremitted roar of elementary confusion. It should seem, however, that the lightning of those countries is not so fatal, or so dangerous, io a proportion to its energy, as that with us; since, in this case, the torrid zone would be uninhabitable.
When these terrors have ceased, with which, however, the na. tives are familiar, meteors of another kind begin to make their appearance. The intense beams of the sun, darting upon stagnant waters, that generally cover the surface of the country, raise sz. pours of various kinds. Floating bodies of fire, which assume different names, rather from their accidental forms, than from any real difference between them, are seen without surprise. The draco volans, or flying dragon; will-o'-the-wisp; the ignis fatuus, or war. dering fires of St. Helmo, or the mariner's light, are everywhere frequent; and of these we have numberless descriptions.
of the Ignis Fatuus, as observed in England.
By Sir Thomas Dereham, Bart. F.R.S. the Rev. William Derham, F.R.S.
It being the opinion of divers skilful naturalists, particularly Mr. Fr. Willoughby and Mr. Ray, that ignes fatui are only the shining of a great number of the male glow-worms in England, of of the pyraustæ in Italy, flying together, Mr. D. consulted his friend, Sir Thomas Dereham, about the phenomenon, being in. formed that those ignes fatui are common in all the Italian parts, But of the pyraustæ, or fire-flies, he says, he never observed any such effects, though there is an immense number of them in June and July. He also says, that these pyraustæ are called Lucciole, i. e, small lights, and that they are not the farfalls, as Mr. Ray thought, which are butterflies.
But Mr. D. has reason to think, that insects are not concereed in the ignes fatui, from the following observations; the first made by himself, and the others received from Italy, by the favour of Sir Thomas Dereham.
His own observation he made at a place in a valley between rocky bills, which he suspected might contain minerals, in some boggy ground near the bottom of those hills. Where, seeing one in a calm, dark night, with gentle approaches, he got up within two or three yards of it, and viewed it with all possible care. lle found it frisking about a dead thistle growing in the field, till a small motion of the air made it skip to another place, and thence to another, and another.
It is about fifty-five years since he saw this phenomenon, but he had as fresh and perfect an idea of it, as if it was but a few days. And as he took it then, so he is of the same opinion now, that it was a fired vapour.
The male glow-worms Mr. D. knows emit their shining light, as they fly; by which means they discover and woo the females; but he never observed them to fly together in so great numbers, as to make a light equal to an ignis fatuus. And he was so near, that had it been the shining of glow-worms, he must have seen it in little distinct spots of light; but it was one continuous body of light.
As to the comunication from Italy, it is observed that these lights are pretty common in all the territory of Bologna. In the plains they are very frequently observed; the country people call them cularsi, perhaps from some fancied similitude to those birds; and because they consider them as birds, the belly and other parts of which are resplendent like our shining flies. They are most frequent in watery and morassy ground, and there are some such places, where one may be almost sure of seeing them every night, if it be dark; some of them giving as much light as a lighted torch, and some no larger than the flame of a common candle. All of them have the same property in resembling, both in colour and light, a flame strong enough to reflect a lustre on neighbouring ob.. jects all around. They are continually in motion, but this motion is various and uncertain. Sometimes they rise up, at others they sink. Sometimes they disappear of a sudden, and appear again in an instant in some other place. Commonly they keep hovering about six feet from the ground. As they differ
As they differ in size, so also ia figure, spreading sometimes pretty wide, and then again contract. ing themselves. Sometimes breaking to all appearance into two, soon after meeting again into one body; sometimes floating like waves, and letting drop some parts like sparks out of a fire. And