in the burner by an inferior mixture which succeeds it at a slower speed.

From the following experiment and considerations I am inclined to attribute the observed action of the disturbed flame almost entirely to direct influence of the sound upon the gas-jet, rather than to its effect upon the current of air passing through the conical cap of gauze that surrounds it. The current through the gauze is so slight that ascending smoke, slowly creeping round it, is not visibly drawn into its meshes. The sensitive action of the flame remains equally perfect when all but a very small aperture of the gauze is closely covered with thin sheet indiarubber. To determine if a naked jet, unsurrounded by wiregauze, would by itself produce a flame so sensitive, I easily obtained with a Ladd's tapering brass jet a flame of this description. Laying it upon its side with its point inclining downwards, and inserting this into a brass tube about in. wide and 15 in. long, also inclined, the flame at the lower end of this tube, when full gas was used, resembled a Bunsen-flame; but if the gas-supply is lowered, it becomes luminous; and at the lowest point at which it will continue to burn, the slight current in the tube appears to consist only of nearly pure coal-gas, and is of course (a useful point in the manipulation) quite inexplosive. A stamp, a cough, or other deep-pitched sound, as the exclamations Oh! and Ah! caused this flame to emerge from its hiding-place in the end of the tube into which it had retreated, and to rise in a tall tongue of light. It was not sensitive to notes of high pitch, to a hiss, nor to some of the acuter vowel-sounds of the voice, unless very strongly uttered; but a short groan or growl called it forth at once. The lower the speed of a jet the slower, possibly, may be the vibrations required to affect and sensibly to disturb its equilibrium. With a very perfect gas-meter the question might also be decided how much of the large additional gas-volume in the flame which occasionally reached a height of about 2 in., and which could easily be maintained permanently at a height of about I in. by continued stamping on a stone floor, is derived from the gas-jet itself, and how much from increased admixture with it of the surrounding air. As the jet is constantly being bent, as it leaves the fixed nozzle, into the shape of a corkscrew, or of some other wave-curve by the air-vibrations, it probably draws more air along with it, in the same way that a coarsely twisted rope in hair rope pumps raises more water than a smooth belt or a perfectly smooth and straight rope would do. Something of this kind, perhaps, may be supposed to take place; and contrary to the opinion which I at first entertained, above, of the cause of the sensitiveness at low gas-pressures of Barry's sensitive wire-gauze flame, it seems more probable that the flurry and depression of the flame produced by external sounds is the result of their action upon the gas-jet below, mixing the gas more thoroughly with air, and giving it explosive properties before it passes through the gauze. The gauze-flame must be regulated by lowering the gas-jet, until the brink of its stability and tendency to collapse and burn noisily on the gauze is nearly reached, in order to make this destruction of its equilibrium by external noises possible; and the explanation thus offered of the sensitiveness of the gauze-flame at lower gas-pressures than those used with other flames depends upon no assumption of mechanical actions of unusual delicacy, or indeed of any peculiar kinds of undulation taking place among the perforations of the gauze.

I have quite recently seen an instrument connected very closely with the acoustical properties of flames burning on wiregauze, showing how well instrument-makers have appreciated them, and how actively they are engaged in representing them in a convenient form. It resembles Geyer's sounding modification of Barry's sensitive flame so nearly, that but for its having received no such title from the maker, the source of its original invention might scarcely be considered doubtful; but it appears more probable, as will be seen from a description in NATURE (to be shortly again referred to) by Dr. A. K. Irvine (vol. x. p. 273), of Glasgow, ot identically the same instrument patented many years ago for a very different purpose, that the designer of this singing tube may also have been guided by a knowledge of that invention. Even allowing for the general knowledge of the acoustical properties of wire-gauze flames that has for a long time existed, the instrument shows signs of originality of design that cannot easily be accounted for without some such consideration. It consists of a brass stand with two sliding brackets, one of which supports, in a split cork, a glass tube tapered above to a point to mix a jet of gas with air. The other arm supports a brass tube five-and-a-half inches high, and about an inch and three-quarters wide, closed at the bottom with a disc of

gauze held there against a fixed rim in the tube by a wire ring. The position of the gauze close to the bottom of the tube and that of the tapering gas-jet under it, as well as the dimensions of the tube, are the counter-part of Mr. Geyer's experiments with Barry's sensitive flame, only differing in want of adjustibility of the relative positions of the tube and diaphragm of wire-gauze from his arrangement. The arrangement itself is, however, on the other hand, exactly that which Mr. Irvine patented, as will soon be seen, twelve years ago, for use in a new description of miner's safety lamp. The sound produced, when the flame is lighted on the wire-gauze inside the tube and the jet below it is fixel at a proper height, is, as might be anticipated from its high pitch, answering to the short length of the open tube, an excruciatingly piercing note.

I was not aware that the effect of heat alone in gauze-diaphragms to produce musical sounds in open tubes had been observed and investigated, as it is stated to have been by Prof. Barrett, so thoroughly by Prof. Rijke, of Leyden; and a perusal of that author's description of his experiments, and of his comments upon them, would undoubtedly be of exceeding interest. That the experiment has often been repeated since, and has been varied in many ways by those who were acquainted with it, is a conse quence that I was fully prepared to learn, from its great beauty, would follow very speedily upon the first publication of its dis

covery.

I have never examined sounding and sensitive flames with revolving mirrors; but the result could scarcely fail to prove very instructive. The indications of his own essays in pursuit of this method contained in Prof. Barrett's letter, both where I have been able to consult the original writings and drawings that he quotes, and where he offers us a short account of further results apparently more noticeable than those obtained before, of the appearance of a particularly active and impressionable sensitive flame affected by the vowel sounds, when viewed in a moving mirror, show that the characteristic comportment of these flames is eminently adapted for examination and discussion by such a mode of observation.

Similar experiments on the chirruping, whistling, trumpeting, and other sounding open flames, obtained by the collision of two jets, examined by Prof. Tyndall and Mr. Cottrell, here suggest themselves; but I must hasten to bring this long excursive letter to a close. I cannot, however, do so without expressing my obligation to Prof. Barrett for the valuable references and infor mation that he has been good enough to supply, and for the prompt and ingenuous manner in which he kindly rectified my oblivious association of his name with Mr. Barry's in certain recent observations of the sensitiveness of wire-gauze flames. The notices contained in a short space in his most interesting letter gave me a better acquaintance with the progress of this wide and curious subject, than repeated and anxious inquiries concerning it for several months previously in the scattered pages of many recent scientific journals had enabled me to acquire.

I must also add my acknowledgments to Mr. T. S. Wright and to Mr. A. K. Irvine for the interesting notes that they have furnished in NATURE (vol. x. p. 273, and p. 286) on the early use of wire-gauze flames to produce vociferously loud sounds in open tubes. That large iron tubes specially fitted inside with gauze-covered (or the so-called "smokeless") gas burners, to produce a mighty sound, should be preserved as working instra ments of a chemical laboratory in Edinburgh as long ago as the year 1842; and that as much as twelve years since a kind of safety-lamp for mines was patented by Mr. Irvine in this and other countries, sounding a loud alarm note when the lamp-flame lights the explosive mixture of fire-damp entering the bottom of the wick-tube through a wire-gauze disc placed there to cover it, are facts that need no comments to show that the surpassing power of such flames to excite and sustain musical sounds has long been known and used successfully. The excellent cha racter and performance of the instruments used in 1842, as described by Mr. Wright, makes it probable that frequent illus trations of the same kind mast already have preceded them. On the other hand, from the well-known scientific eminence of their possessor, Dr. David Boswell Reid, as a skilful director of large works of ventilation, it may also be presumed that they probably presented to his views novelty of some special kind, either of invention or of construction, or of both combined, the result of which was the production of several such superior instru ments. It may not be impossible from this consideration, at least if no evidence of considerably earlier origin could be produced, to fix the time, and perhaps the authorship by Dr. Reid himself, or by his brother the chemist, Dr. William Reid of

1

Edinburgh, in whose laboratory Mr. Wright practised with them, of the first use of smokeless coal-gas flames in acoustical experi. Pments as not long anterior to the date named by Mr. Wright as that of his practical experience of their use. But it must De borne in mind that of all highly inflammable and intensely Wheating gases next to hydrogen, the most easily procurable since the general extension of the use of coal-gas, is an explosive mix. ture of the latter gas with air; and the experiments of Sir H. Davy, in 1816, having demonstrated that such a mixture may be prepared safely underneath wire-gauze and may be safely burned above it, the use of the wire-gauze flame for laboratory heating purposes, and also to illustrate very suitably the chemical harmonicon, must have been a very early suggestion. Its unwieldy size and stentorian proportions for the latter purpose, however, have not impossibly led to its comparative abandonment and

J. WALLACE'S TABLE BUNSEN BURNER.

-Conical and spiral mixing-tube coiled inside the foot, terminating at the centre in a small chamber closed with wire-gauze at the top, at the foot of the flame-tube. b.-Conical wire-gauze cap, strengthened by three wires to support the gas-tube, to protect the gas from ignition, to keep off draughts, and to distribute the current of air to the gas (junctions all soldered). -Short flame-tube, closed at the bottom with wire-gauze to prevent the flame from flashing back when the gas is turned on or off. Whole height about 2 in. Height of flame, 1 in. or 2 in. Height of central bright flame, exactly in. disappearance from the scene of modern laboratory experiments, and to its general replacement, in coal-gas illustrations of the chemical harmonicon, by various modifications with different forms of jets, of the much more portable, convenient, and easily adaptable Bunsen-burner. Thus a long-recognised and important application of gauze-topped gas-burners in the student's scientific practice might have fallen into oblivion, or into disuse and comparative neglect, if contemporaneous experiments like those of Irvine, Barry, Govi, Geyer, Rijke, and it may safely be prophesied of many other active fellow-workers in the same field of discovery and research, did not revive the discussion, and continue to develop the observation of these flames with multiplied results that appear to be in perfect accordance with the principles, and to furnish the most beautifully effective illustrations possible of important properties of effluent gas-currents, which would perhaps otherwise escape detection. The laws of the flow of escaping gas-jets, their powers of producing ventilation and exhaustion, and, on the other hand, the means of providing for their escape with as little waste of their energy as possible, are questions of practical importance in so many useful industrial applications, that they amply deserve the increased measure of scientific attention which the beautiful succession of modern discoveries of sensitive and sounding flames has been very materially instrumental in attracting, and appears still further to be eminently capable of directing towards them. Newcastle-on-Tyne, Oct. 19

A. S. HERSCHEL

Insects and Colour in Flowers IN his second letter (NATURE, vol. xi. p. 28) Mr. Mott passes to the discussion of the general question whether beauty is an "object in nature." On that point my feeling is that our know

ledge is as yet far too limited for us to presume to declare with any confidence what is an object in nature. Still less should we venture to assert what is not an object, and least of all have we any right to affirm that beauty is not an object, when we see developed, beauty of form, of colour, of sculpture and marking, so constantly throughout the organic world, and by such a great variety of means. Sometimes beauty of colour undoubtedly exists when, so far as we can see, it confers no benefit whatever on its possessor. Mr. Darwin instances arterial blood and the autumnal tints of leaves. More frequently it is accompanied by some advantage, direct or indirect; and the question is whether in such cases it has been acquired through the operation of sexual or natural selection, more particularly whether in the case of flowers the selection has been effected through the agency of insects, which have favoured the most conspicuously coloured. It remains with Mr. Mott to show in what way the facts detailed in his original letter (I hope he will pardon me for taking him back to it) fail to harmonise with that doctrine. To my mind the fact that a cultivator, by carrying out a like selection, propagating from plants which bear the largest and brightest, double or showy sterile flowers, can produce like results, supports and corroborates the doctrine rather than militates against it. Nor can I see anything discordant in the fact that the colour of fruits has been acquired through the medium of an entirely different selecting agent.

One circumstance appears to me to present some difficulty; and, although it is in no way connected with Mr. Mott's letter, I should like to mention it in the hope that others may be able to supply a satisfactory explanation: it is the case of flowers that are coloured on the outside, but white within. Where such flowers from their position or form present to view principally their exterior, as Tulipa celsiana, this is an adaptation that can be readily understood; but some display mostly their interior, and it is then difficult to understand the acquirement of colour out. ide only. I would instance Simethis bicolor, Gypsophila cretica, Daphne jasminea, and several species of white-rayed Compositæ. Bellidiastrum michelii, for example, has frequently the inner surface of the ray florets quite white, and when the flower is open nothing else is seen; the colour on their outer surface only becomes visible when they close over the disc, as in dull and rainy weather. THOMAS COMBER

Newton-le-Willows, Nov. 16

I FIND that during my absence from England many applications have been made for plants of the Drosera and Pinguiculæ, and from the replies which have been sent on receipt of the plants they seem to have given satisfaction. Lately, however, in consequence of the weather, there has been some difficulty in obtaining D. intermedia, but before this is printed in your columns, all existing applications will be cleared off.

I wish to add, that in winter these plants can scarcely be expected to be as active as in spring and summer, and observers must wait patiently until spring before they may hope to obtain successful results from their observations: it cannot be necessary, I think, to feed carnivorous plants artificially during the winter, and a hot-house or conservatory cannot be absolutely necessary, as they have no such advantages in their native wilds.

Suicide of Scorpions

G. H. HOPKINS

THAT Scorpions do commit suicide, as described by your correspondent last week, is a well-known fact. My grandfather often related how he had seen these creatures, when surrounded by a circle of glowing embers, make for the inner side of their fiery prison, then deliberately move round the inside of the circle, and when arrived at the exact spot from which they started, turn back their tails and sting themselves to deh. Clyde Wharf, Nov. 16 M. L.

THE

ON MIRAGE*

HE name of "Mirage" is applied to certain illusory appearances due to excessive bending of the rays of light in their passage through the atmosphere. These appearances are by no means uniform.

Sometimes, especially in hot countries, the observer loses sight of the ground beyond a certain distance from his position, and sees in its stead, what looks like a sheet of water, either calm or with movements resembling waves; and if any distant objects are sufficiently lofty to be seen above this apparent lake, their images are seen beneath the objects themselves, inverted as if by reflection in this imaginary water. The dry and hot soil of Egypt is famous for the production of this form of the phenomenon. It is also mentioned as of frequent occurrence in the plains of Hungary, in the plain of La Crau in the South of France, and in the fen districts of England when dried up by the summer heat. It is also common in Australia. The Deputy Surveyor-General of South Australia once reported the existence of a large inland lake, which on further examination turned out to be nothing but a mirage.

Another class of appearances are known (especially among nautical men) under the name of looming. Distant objects are said to loom when they appear abnormally elevated above their true positions. This abnormal elevation not unfrequently brings into view objects which in ordinary circumstances are beyond the horizon. It is also frequently accompanied by an appearance of abnormal proximity (though this may perhaps be rather a subjective inference from the unusual elevation and clear visibility of the objects than a separate optical characteristic), and it is further accompanied in many, though not in all cases, by a vertical magnification, the heights of objects being many times magnified in comparison with their horizontal breadths, so as to produce an appearance resembling spires, pinnacles, columns, or basaltic cliffs. Some beautiful descriptions of these latter appearances, with illustrative plates, are given in Scoresby's "Greenland," the objects thus magnified being icebergs; and a very full and interesting account of the phenomena of mirage, as observed in high latitudes, will also be found in the Arctic Regions" of the same author.

It is usually across water that looming is observed; and as a surface of water stands naturally in contrast with a sandy desert or a surface of parched land, so also the optical effects produced are, in a manner, opposite. The inverted images which are often presented in looming are not beneath the object, as in the case of mirage on dry land, but above it, as is formed by reflection in the sky. The only examples that I have myself seen of mirage were of this kind. They were seen across sheets of calm water, the hills on the other side being seen with fictitious hills upside down resting on the tops of the real hills. In rare instances, two or even three of these images are seen one above another, vertically over the real object; but these multiple images are usually too small to be seen without the aid of a telescope-the objects whose images they are being so distant as to appear mere specks to the naked eye.

There is always more or less of change observable in the images formed by mirage, and the changes are greatest and most sudden when the images are most distorted, as compared with the true forms of the objects. The appearances also change with the height of the observer's eye. Looming is seen to the greatest advantage from an elevated position, such as the mast-head of a ship. The mirage of dry land is sometimes visible at any moderate height, but in other cases-especially in countries which are not very hot-the range of height from which it is visible is extremely limited. A very fine mirage, recently observed in the fen districts, was only A Paper read by Prof. J. D. Everett, M.A., D.C.L., before the Belfast

atural History and Philosophical Society.

seen when the observer was on the top of the marsh wall. But this case seems to have been peculiar. It was accompanied by the further peculiarity that a strong wind was blowing-the general rule being that mirage is only seen in calm weather. Observers of mirage on the sands of Morecambe Bay, and of the Devonshire coast, sta'e that it could frequently be only seen by stooping.

Mirage is seldom seen in winter. The hot shining of the sun seems to be an invariable antecedent; and this is true even of the polar regions, where Capt. Scoresby attributes the phenomenon to "the rapid evaporation which takes place in a hot sun from the surface of the sea, and the unequal density occasioned by partial condensations, when the moist air becomes chilled by passing

over considerable surfaces of ice."

Time will not allow me to do much in the way of quoting the very numerous records which exist. Scoresby's accounts alone would almost suffice to occupy the evening, and I would again refer to them as models of accurate observation and effective description. I will content myself with quoting nearly in full the account of a mirage observed at Hastings and neighbouring parts of the south coast of England in 1798, as given in the Philosophical Transactions for that year, the narrator being Mr. Latham, F.R.S. :

"On Wednesday last, July 26, about five o'clock in the afternoon, whilst I was sitting in my dining-room at this place (Hastings), which is situated upon the parade, close to the sea-shore, nearly fronting the south, my attention was excited by a great number of people running down to the sea-side. Upon inquiring the reason, I was informed that the coast of France was plainly to be distinguished with the naked eye. I immediately went down to the shore, and was surprised to find that, even without the assistance of a telescope, I could very plainly see the cliffs on the opposite coast, which at the nearest part are between forty and fifty miles distant, and are not to be discerned from that low situation by the aid of the best glasses. They appeared to be only a few miles off, and seemed to extend for some leagues along the coast. I pursued my walk along the shore to the eastward, close to the water's edge, conversing with the sailors and fishermen on the subject. They at first could not be persuaded of the reality of the appearance, but they soon became so thoroughly convinced, by the cliffs gradually appearing more elevated and approaching nearer, as it were, that they pointed out and named to me the different places they had been accustomed to visit, such as the Bay, the Old Head or Man, the Windmill, &c., at Boulogne, St. Valéry, and other places on the coast of Picardy, which they afterwards confirmed when they viewed them through their telescopes. Their observations were, that the places appeared as near as if they were sailing at a small distance into the harbours.

"Having indulged my curiosity upon the shore for near an hour, during which the cliffs appeared to be at some times more bright and near, at others more faint, and at a greater distance, but never out of sight, I went upon the eastern cliff, which is of a very considerable height, when a most beautiful scene presented itself to my view; for I could at once see Dungeness, Dover cliffs, and the French coast, all along from Calais, Boulogne, &c., to St. Valéry, and, as some of the fishermen affirmed, as far to the westward as Dieppe. By the telescope, the French fishing-boats were plainly to be seen at anchor, and the different colours of the land upon the heights, together with the buildings, were perfectly discernible. curious phenomenon continued in the highest splendour till past eight o'clock,... when it gradually vanished. The day was extremely hot,... not a breath of wind was stirring the whole of the day. . . . A few days afterwards I was at Winchelsea, and at several places along the coast, where I was informed the above phenomenon had been easily visible.

This

"I should also have observed that when I was upon the eastern hill, the cape of land called Dungeness, which extends nearly two miles into the sea, and is about sixteen miles distant from Hastings, in a right line, appeared as if quite close to it, as did the fishing-boats and other vessels which were sailing between the two places. They were likewise magnified to a great degree."

I have stated that the phenomena which constitute mirage are due to the bending of rays of light in the atmosphere, and I now proceed to point out the principles by which this bending is governed.

My esteemed colleague, Dr. James Thomson, has greatly contributed to the clearness of our knowledge, as regards the d sturbing effect of the atmosphere upon the direction of a ray of light. He has recently published an investigation, which, to say the least, is simpler and more satisfactory than any before given, of the precise law which determines the curved path of a ray through the air.

Referring you for the details to the last chapter but one of my own recently published edition of Deschanel's "Natural Philosophy," I will merely say that when a ray is passing through a portion of air which is not equally

* British Association Report, 1872, p. 41.

dense all round it, it is deflected towards the side on which the density is greatest; and that the sharpness of the curvature, as measured by the change of direction for a given length of the ray, is directly proportional to the rate at which the density varies along the normal. Strictly speaking, I ought, instead of "density," to have said "absolute index of refraction, diminished by unity;" but experiment has shown that the difference between these two statements, when there is no substance in question except air and aqueous vapour, is quite insignificant. Supposing the stratification of the air to be strictly horizontal, it follows that a ray tra velling vertically will not be bent at all, since there is no variation of density in the direction of its normal; and of all rays which traverse the same point, those which are horizontal will be bent the most, because the whole change of density is normal to them, and has a direct tendency to bend them downwards. For rays which are nearly horizontal, the curvature will be very nearly the same; and, as it is by such rays that we see the images which constitute mirage, the maximum bending of atmospheric rays is available for the explanation of the phenomena. In the average state of the atmosphere, the curvature of rays which are horizontal, or nearly so, is about one-fifth or one-sixth of the curvature of the earth's surface; though it is to be remarked, by way of caution, that the connection between these two curvatures is merely accidental; the curvature of the earth is not the cause, nor even a partial cause, of the curvature of rays.

of

Other things being equal, the curvature of rays should be greater in cold than in warm air, and greater with high than with low barometer; but these are not the principal modifying elements. The circumstance which it is most important to know, at any time, in order to predict the degree of curvature, is the rate at which the temperature changes with the height. The average change is a fall of about a degree Fahr. per foot of ascent. A fall of one fifty-third of a degree pr foot of ascent would make the air equally dense at all heights, and would cause rays to travel in absolutely straight lines. A more rapid fall than this would render the air aloft denser than that below, and would cause rays to bend up instead of down. The existence of denser, and therefore heavier air aloft, is obviously incompatible with stability of equilibrium; but unstable equilibrium may endure for a time, even under statical conditions; and when there is a powerful cause at work, tending to raise the temperature of the lower strata, it is quite conceivable that the lower air may be heated faster than it can get away (if I may be allowed a somewhat loose expression); so that, although there is a perpetual diffusion going on, the heated air ascending, and cooler air from above taking its place, there is, nevertheless, a difference of temperature perpetually maintained, exceeding one-fiftieth of a degree per foot. The circumstances under which the Egyptian form of mirage is observed are precisely such as are fitted to produce this state of things. A fierce sun scorching the parched