first letter, we are at liberty to imagine a time when there was much more land than there is at present, and when all the ucears were comparatively shallow. A. J. JUKES-BROWNE.

Galls,

BEFORE rushing into arguments on this subject, it appears to me that more good might be done by entering into investigations of the physiological and morphological problems involved. A gall fly of a particular species inserts an egg in a certain position on a certain plant (oak, for instance). Another gall-fy different species inserts its egg almost in the same position n the same plant. But the results are totally dissimilar. An abnormal growth is set up, from irritation, in either case; but the nature of this growth is quite different. The initial irritation is set up by the presence of the egg, and in most gall-insects the that is to say, it increases vastly in size before the lava is batched. The irritation is continued by the larva, and the gall is produced, varying in form in accordance with the species of gall-fly that deposited the egg. But I want to know in what consists the difference in the active initation that causes

great a divergence in the results? I am not aware that this has ever been answered. But I am quite sure it could be answered on purely physiological grounds if carefully studied. The answer would not in the least detract from the importance of the point as regards natural selection; but it might very waterally modify speculative theories based on results only, without a precise knowledge of the agencies that produced those Jesults R. MCLACHLAN.

Lewisham, November 29.

ALTHOUGH I see no need of a better explanation than Prof. Komanes's (NATURE, November 28, p. 80) of the difficulty which galls seem at first sight to present for natural selection, yeti teg leave to say some words of further elucidation.

When 11 was said by Darwin ("Origin of Species," chap vi: If it could be proved that any part of the structure of any one species had been formed for the exclusive goal of another species, it would annihilate my theory, for such could not have been produced through natural selection," he evidently meant only species living without organic connection with each other, viz. his own example of the rattlenake. The argument does by no means apply to organisms living in a relation of symbiosis, as is the case with gall-bearing 1 lange and "he 'arvæ inhabiting the galls. Such associations form, as it were, one cou found organism. Natural selection evidently

for the life of me understand how the theory of natural selection can be seriously assailed by investigations into the formation of galls by insects. Gall-formation has always appeared to me to be a pathological, that is a perverted physiological process, and to be due to the action of some animal irritant upon normal vegetable tissues during their period of active growth. These formations are therefore, to my mind, fairly on a par with the globular nests produced by the larvæ of the Estrus, or bot-fly, in the hides of oxen; or to the inflammatory foci in the tissues of the kidneys, due to the translation of Bacilli, in the case of ulcerative endocarditis. Other examples bearing on the subject will doubtless occur to your readers. instances we have certain changes in the cellular or protoplasmic tissue-elements of the host, brought about by the growth and development of a foreigner in their midst; and natural selection, in so far as it operates in such cases, seems to have sided mostly with the stranger, and to be to his advantage alone. That the host under these circumstances performs actions "which, if not self-sacrificing," are at least "disinterested," must be admitted; but it is the self-sacrifice of coercion and disinterestedness under compulsion. W. AINSLIE HOLLIS.

Brighton, December 1.

Luminous Night Clouds.

In all such

THE many inquiries and appeals regarding observations of luminous night clouds which have recently appeared in the columns of NATURE, and the growing importance of the subject, will justify me, perhaps, in sending to you, for publication in that journal, the following item, so long after the event it describes took place.

About the middle of November 1887, between eight and nine in the evening, as I was walking homewards from my day's work, I noticed what appeared to me to be the arch of a rainbow very low above the western horizon, and of a snow-white colour. A bank of clouds was rapidly approaching from the west, which, at the time of the first appearance of the arch, covered nearly half the sky, the eastern half being clear. The arch appeared to move eastwards, with and in the midst of the clouds, for it continually rose above the horizon, and, in the course of about half an hour, had approached the zenith.

At this time I called cut several people to witness the phenomenon, which certainly presented a most extraordinary appearance. The arch appeared to be uniformly of about 3 or 4 in width, and extended north-north-east and southsouth-west across the whole sky. The latter was about wholly overcast with the clouds at this time, except the arch, which

that the ect will not damage the other symbiont in such a degree as seriously to impair its existence. Some disinterested" experditure of energy and of organic substance is not excloded by natural selection, but may be promoted, if of advantage to the other partner. Thus the production of galls will scarcely do any serious injury to an oak, and even if such were sometimes the case, there would be no comparison to the damage worked, for instance, by Trichinæ, on the organism of man and animals, which hosts, nevertheless, in Ensequence of the stimulus caused by the parasite, afford the stance for capsules protecting the worms, just as plants protice manifold structures beneficial to the gall-insects. If Trichine would attack a species of mammals as frequently as, for instance, leaf-cutting ants attack some tropical plants, then those bosts would be forced either to develop, by survival of the hillest, some protection against their invasion, or they would succumb to the enemy and die out.

Analogous examples might be multiplied of both plants and animals, and it is especially to be remembered, as alluded to by Prof Romanes, that the chemical activities of parasites, including the elaboration of ferments affecting the saps and tissues of the bost, are as much under the guidance of natural selection as are their morphological variations. D. WETTERHAN. Freiburg, Badenia, November 30.

WITH all due deference to your able correspondents Dr. St. George Mivart and Prof. G. J. Romanes, I cannot

Tarein's thorough acquaintance with these important structures is teeny bis elaborate discussion in Animals and Plants under Domestica

cape trui funded vol. p. 272). It is particularly to be noted that Darwin insists on the accordance of galls, for instance, on roses, with structures arising through bud-variation.

with a weird splendour four or five times exceeding that of the brightest moonlight.

While at the zenith, the stars shone through the entire width of the arch with apparently more than ordinary brightness; but as the arch approached towards and receded from that point, the width of the transparency was observed to diminish rapidly with the distance, until at fo° or 15° on either side the stars were invisible through it.

The phenomenon appeared to be a division in the cloud stratum, the opposite walls of which were pretty clearly defined; and there appeared to be absolutely nothing between these opposite cloud walls but the purest air and the white light of the arch. I remember also that the wall or border of cloud on either side of the arch was slowly revolving upon an axis parallel with the arch; just as is often seen in the front bank of clouds of an approaching storm. But I do not remember the direction of the rotation, or whether both borders rotated in the same or in opposite directions.

The arch moved towards the east at about the same pace that it approached from the west, and with apparently the same width and direction of extension. There was no moonlight at the time, and only a gentle breeze was blowing. The weather preceding the phenomenon was fine for several weeks; but a few days afterwards, or on November 19, there was a sudden and extraordinary fall of the temperature, accompanied by some snow and very high wind.

I have thought that possibly this phenomenon might throw some light on the subject of luminous clouds, and that this tolerably accurate description of it may therefore be of interest to the students of that subject. I may add, however, that the luminosity of the arch did not appear to proceed directly from the clouds themselves, but from the clear space between the

clouds; although, according to the best of my recollection, luminous filaments seemed to extend from the clouds for a short distance into the span of the arch. EVAN MCLENNAN. Brooklyn, Iowa, U.S.A., November 22.

Electrical Figures.

I RECENTLY noticed a pretty form of electrical discharge, which has probably been described before, but was new to me. Perhaps one of your readers will be able to refer us to an account of it.

The poles of a Voss machine are put very near together: a plate of ebonite inch thick is placed between them. As the machine works, a succession of delicate ramified discharges run over both surfaces of the plate: they are bright green, and each crooked line is discontinuous-a series of dashes, as if stitched out in silk, now above and now below the surface. Winchester College, December 6.

W. B. CROFT.

NEW DOUBLE STARS.

THE HE highest quality of seeing, as of acting or of thinking, needs initiative. A mental impulse is the spring of discovery, even by a purely visual process. The mind prompts the eye, interprets what it suggests, bodies out its semi-disclosures. So that to perceive what has never been perceived before is, in a sort of way, an act of invention. It thus happens that an accurate is not always an original observer. Novelties, as such, are almost inaccessible to many persons with exquisite powers of vision for whatever is already known to be within its range.

The late Baron Dembowski was an example of a firstrate observer but slightly endowed for detection; Mr. Burnham, on the other hand, is a born discoverer. The accidents of his career have turned his attention almost exclusively to double stars; and his glance seems to have a compulsive power of turning simple into compound objects by long and intent looking. His Chicago thousand of new pairs are famous; he bids fair to accumulate an equally imposing array at Lick. Nor does he neglect the old in the search for the new. The more exciting is not permitted to exclude what is in many respects the more useful occupation.

Progress in double-star astronomy is absolutely de: pendent upon remeasurements of the relative positions and distances of known pairs. We can otherwise learn nothing as to the nature of their connection. Inquiries about them can, by this means alone, be pushed through the three successive stages leading up towards complete knowledge. In the first place, it has to be decided whether the stars shift their places perceptibly with reference one to the other. If they are "fixed," but with a common proper motion, then they may safely be set down as physically coupled, although centuries may elapse before the character of their mutual revolutions becomes apparent. In the next place, the nature of relative motions, where they exist, has to be ascertained. Should they prove to be rectilinear, that fact alone overthrows the possibility of any real connection between the stars. Each pursues its way independently of the other. Finally, in the interesting cases in which curvilinear motion shows itself, persistent micrometrical measures are required to determine the shape and period of the orbit traced out.

Yet the majority of these objects receive little or no attention. This is in part due to their great numbers. About 12,000 double stars-using the term in the widest sense-are now known; nearly 5000 are in really close conjunction-so close, in some 1400 instances, as to render the chances of accidental juxtaposition all but evanescent. Only between fifty and sixty stellar orbits have, however, as yet been computed, and many of them from most inadequate data. The truth is, that this branch of work wants organizing. It is too vast and too important to be abandoned to the capricious incursions of

irresponsible amateurs, whose industry is often wasted 'r being misapplied. There ought, nevertheless, to be litt difficulty in distributing the observational resources avai able as advantageously as possible by the intervention. some recognized authority, a central repository being a the same time constituted whence computers could obtar on demand the materials needed for the investigation & particular systems. The tasks of stellar astronomy are so multitudinous as imperatively to demand combination f their effectual treatment.

Discovery, meanwhile, must advance as it can. It far from desirable that it should remain stationar Although our acquaintance among double stars is alread embarrassingly large, we cannot refuse to extend it. Every addition to it, indeed, is, for a variety of reasons to be welcomed.

Information on the general subject of stellar com positeness can only be gained by continually widening the area of research. The comparative frequency of Occurrence can thus only be estimated. Struve found one in forty of 120,000 stars examined by him down to 1 to be compound; but the proportion was naturally higher for the brighter stars, as being in general much nearer the earth, and consequently of more facile optical separat.on Every twenty-fifth star in Piazzi's Catalogue, every eleventh in Flamsteed's, proved accordingly to have a companion within less than 32". But the process of dividing stars has since made such strides as to show that the real pre ponderance of single over double ones must be much smaller than these numbers indicate. Perhaps, indeed, no star can be called absolutely single. Between a sma companion sun and a large planet in its self-luminous stage it is not easy to establish a distinction. The str we know best may not always have been, in its "surpassing glory," so undeniably solitary as it now is. Jupiter, if t ever shone with anything like stellar lustre, would have constituted with it a fine unequal pair such as are plentifully exemplified in our catalogues.

The distribution of double stars is characterized by a somewhat irregular condensation towards the Milky Way They abound in Cygnus and Lyra, are scanty in Cas siopeia and Cepheus; while Struve met with rich regions where lucid stars are few, in Auriga, Telescopium, and Lynx Burnham, however, could detect no marked local pre ferences among his numerous pairs. Sir John Herschui was struck with the paucity of close doubles in the southern hemisphere; but no searching scrutiny has yet been carried out there with modern instruments.

The curious tendency of stars already in close associa tion to split up still further when sufficiently powerto means are brought to bear upon them, has been strongly accentuated by Mr. Burnham's investigations. Primaries with double satellites, such as Rigel, or satellites with double primaries, such as έ and B Scorpii, swarm on his lists. A fresh instance of the former kind is Piscin ( 100), registered by Struve as somewhat widely double. but found to be triple last autumn with the Lick twelveinch achromatic. The satellite of Struve's companion, a an interval of less than one second from it, is of the eleventh magnitude. The bright stars are estimated by Burnham as of sixth and eighth, but were photometrically determined at Harvard as of 54 and 64 magnitudes and Webb thought that the chief of the pair occasionally rose to the fourth rank of lustre. A presumption is th afforded that both fluctuate in light. Their spectrum, like that of most variable double stars, is of the Sirian type and their real fellowship is made manifest by a community of proper motion. We have here, then, a genuine ternar system.

Aldebaran is the centre of a mixed group. A smal star at 30" detected by Mr. Burnham at Chicago on October 31, 1877, was described by him as making with the rudds bright star, a pair resembling Mars and his outer satellite (Astr. Nach., No. 2189). A drift together through spac

geodic masses, in which they have formed by gradual Frystallization from the mother-liquor. They differed, bowever, of course, in being on the outer surface of the spherules."

It is evident from Prof. Houston's paper that this peculiar form of hail was unknown to him, and, as it must iso have been unknown to many who have propounded theones as to the formation of hail which will not account for it. I think that a service may be rendered to meteorogy by the reproduction of three of the exquisite lithographs of this form of hail given in Prof. Abich's paper, Leber krystallinischen Hagel im Thrialethischen gerge," published at Tiflis in 1871. The hailstones represented in Figs. 1-3 all fell on June 9 (21), 1867, at Bjeloi Kirutsch, a village about twenty miles south-west of Tiflis,

FI. 3.

and 12.425 feet above sea-level (lat. 41° 33' N., long. 44° 30 E

Theories of the formation of hail are almost innumerable. I was reading a pamphlet not long since which contained summaries of, I think, twenty-three theories. Some like Prof. Schwedoff's, that hailstones come from interplanetary space (Brit. Ass. Report, Southampton, 1882, p. 458)-are very droll; but the subject is a very difficult one, and one upon which I do not know of a single good treatise in our language. Possibly, the reproduction of these figures may induce someone to prepare an exhaustive memoir. I could place a large amount of historical and theoretical material at the disposal of any competent person who would undertake the preparation of such a work, it being quite impossible for me to do it myself. G. J. SYMONS.

NOTES.

AT a largely attended meeting in Edinburgh on Tuesday, Dec. 3. Sir Douglas Maclagan in the chair, it was resolved that Mr. Geo. Reid, R.S.A., should be commissioned to paint a portrait of Prof. P. G. Tait, to be placed permanently in the rooms of

the Royal Society of Edinburgh. A committee was appointed to carry out the resolution, including, among others, Mr. John Murray (Challenger Expedition), Convener; Mr. Gillies Smith, Hon. Treasurer; Lord President Inglis, Lord Kingsburgh, Lord Maclaren, Sir William Thomson, Sir Arthur Mitchell, Prof. Robertson Smith, Prof. Chiene, Dr. Alexander Buchan, Mr. Robert Cox, and Mr. William Peddie. It was proposed that an etched engraving of the portrait be prepared for distribution among the subscribers, the plate to be destroyed after the required number of copies have been thrown off. It was further resolved that all the Fellows of the Royal Society of Edinburgh, the Professor's old pupils, and others, be afforded an opportunity of taking part in this public recognitition of Prof. Tait's eminent services to science.

ITALY, France, and the United States of America were represented in the elections to foreign membership of the Royal Society on Thursday last. Prof. Stanislao Cannizzaro, of Rome, was elected on the ground of his researches on molecular and atomic weights; Prof. Chauveau, of Paris, for his researches on the mechanism of the circulation, animal heat, nutrition, and the pathology of infectious diseases; and Prof. Rowland, of Baltimore, for his determination in absolute measure of the magnetic susceptibilities of iron, nickel, and cobalt; for his accurate measurements of fundamental physical constants; for the experimental proof of the electro-magnetic effect of electric convection ; for the theory and construction of curved diffraction-gratings of very great dispersive power; and for the effectual aid which he has given to the progress of physics in America and other

countries.

ADMIRAL MOUCHEZ and MM. Janssen and Perrotin, head astronomers of the Observatories of Paris, Meudon, and Nice, were raised, in November, to the grade of Officer of the Order of the Rose of Brazil, and MM. Frassenet, Paul, and Prosper Henry, admitted to knighthood in the same order. The Paris Correspondent of the Daily News says that the diplomas securing to them these distinctions were the last official documents signed by Dom Pedro. He asked his secretary to add a personal compliment to each of the astronomers with whom he was personally acquainted.

SOME time ago we announced that a Physical Society was about to be formed in Liverpool. This has now been done, and we are glad to learn that the new Society begins its career under most favourable conditions. The meeting at which it was constituted was well attended, and displayed much interest in the scheme. Nearly ninety names were at once handed in to the secretary, Mr. T. Tarleton, for membership. Prof. Oliver Lodge, F. R. S., was appointed President. The next meeting will be held in the Physics Theatre, University College, Liverpool, on Monday, the 16th inst., at 8 o'clock, when the President will deliver his inaugural address.

DR. JOHN G. MCKENDRICK, F. R. S., Professor of Physiology in the University of Glasgow, has been elected President of the Philosophical Society of Glasgow.

PROF. LESQUEREUX, the eminent American bryologist and paleontologist, died in his house at Columbus, Ohio, on October 25, at the age of nearly eighty-nine years.

WE regret to learn from a memoir that has been sent to us by Prof. Barboza du Bocage, that Señor José Augusto de Souza died recently at Lisbon, where he was Curator of the Zoological Department in the Museum. He was the author of some useful memoirs on African birds, and is best known for his Catalogue of the Accipitres, Columbæ, and Galline in the Lisbon Museum.

THE fifth of the series of "One Man" Photographic Exhibitions at the Camera Club will be open for private and press