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THURSDAY, OCTOBER 5, 1871

OBSERVATIONS UPON MAGNETIC STORMS IN HIGHER LATITUDES

THE HE extension of the telegraph into the more northern latitude of the Shetland Islands, between 59° 51' and 60° 51′ 30′′ N., has afforded a much better opportunity of observing the frequency and variation of the magnetic

and auroral storms that have of late excited some attention and discussion in these pages.

Some of the earliest recorded observations upon the strength and direction of these atmospheric storms, date from the time when the extension of the telegraphic wires over England rendered the phenomenon visible by the disturbance of the magnetic needle placed in circuit with the wires, and to a certain extent rendered possible the mapping down of the position and direction of the magnetic storm over certain tracts of Great Britain.

On the 24th September, 1847, remarkable magnetic disturbances were observed in London, and the direction and deflection of the magnetic needle noted. The effects of this magnetic storm were carefully observed at Dawlish, Norwich, Derby, Birmingham, Rugby, Cambridge, Tonbridge, Wakefield, Edinburgh, and York. The magnetic disturbance appears to have commenced about 1h 5m P.M. on the 24th, and continued with variable intensity until 7h 30m A.M. on the 25th.

It may be interesting to give some of the galvanometer readings recorded as indicating the rapid oscillation and deflection of the galvanometer needle. In the period of time between 4h 17m P.M., and 5h 48m P.M. on the 24th, or in about one hour and a half, the direction of the current had changed no less than ten times, showing a maximum swing of the needle over an arc of 50°.

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During this magnetic storm, the variation of the dipping needle which was observed in London every 30", ranged between 69° 30' and 67° 50'.

In some cases these magnetic storms were so severe as to impede the working of the railway signals. On the 18th of October, 1841, a very intense magnetic disturbance was recorded, and amongst other curious facts mentioned is that of the detention of the 10.5 P.M. express train at Exeter sixteen minutes, as from the magnetic disturbance affecting the needles so powerfully, it was impossible to ascertain if the line was clear at Starcross. The superintendent at Exeter reported the next morning that some one was playing tricks with the instruments, and would not let them work.

VOL. IV.

It will be fresh in the memory of many of our readers that during the month of October last year, very remarkable and brilliant "aurora" were observed in London, chiefly of a deep blood-red colour, spreading from the zenith over a great portion of the heavens.

It is, however, in the more northern latitude of the Orkney and Shetland Islands that the grandeur of these wonderful electrical phenomena can be observed, and that reliable data can be obtained from which hereafter some practical result may be deduced.

general rule, appears to concentrate and emerge from As observed in Orkney and Shetland, the aurora, as a

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behind a dense mass of dark cloud lying low down in the horizon towards the north. The edge of this cloud-bank is serrated and jagged, as if the mass were electrically in a high state of tension. From behind this cloud-bank "dark" streamers will appear to start up high into the zenith, appearing as if attenuated portions of the edge of the cloud-bank had been dragged by some invisible power, these dark auroral rays being at the same time transparent as regards the power of transmitting the light of the stars, which shone through with undiminished splendour. At the same moment that these dark rays are emicant, brilliant green, violet, crimson, and white rays appear to stream upwards towards the zenith, but always with a less persistence of duration. These coloured scintillations change with greater rapidity than the black rays.

During the month of December of last year, some very vivid prismatic tints were observed from the Island of Eday. From careful observation it was then remarked that the red coloured rays appeared generally to be of a partially opaque nature, and it could be readily seen that the light of a star, when viewed through the red scintillation, was dimmed as compared with the brilliancy of the same star when observed through the scintillations of another colour.

In some of these displays, the most vivid and varied colouring was exhibited. These were noted down as visible to the eye at the same time, and as the colours were observed in contrast, the distinctiveness and brilliancy of the tint became the more decided. Black, pale yellow, strong yellow, white, violet, pale blue bright green, crimson shade fading into a reddish pink, pale orange, and a delicate sea-green tint. So far nothing approaching to the indigo hue has been noticed. With this exception, the entire prismatic colours and blending tints may be said to have been perfectly developed in the rapid electrical scintillations of the aurora. The colours fade away and change with astonishing rapidity, and this variation in tint will take place without apparently any great electrical disturbance in the special ray observed, beyond a slight flickering motion. In these regions, where the atmosphere is so perfectly still and at times calm, repeated observation has determined the existence of very appreciable sound to the ear, as an accompanying phenomenon (to the rapid rush of the auroral streams towards the zenith. The intensity of the sound emitted varies considerably. At times, it greatly resembles that of the rushing noise caused by the firing of a rocket into the air when reaching the ear from a distance. At other times it has a strong resemblance to the sound produced by the crackling of burning embers, but wanting in any very distinctive sharpness.

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In all these cases of auroral displays the inductive effects upon the telegraph wires are very strongly marked; currents of varying intensity and direction flowing unceasingly through these metallic circuits.

The result of observations made in Shetland during the months of September, October, November, and December last year, tend to show that these auroral disturbances attained their maximum effect upon the wires between 8h 30m and 9h 30m A M., and between 8h30m and 10h 30m P.M.; and such is the unstableness of these induced auroral currents, that frequently in five minutes the electromotive force will vary from very much less than that of a Daniell cell to a current of such intensity that a brilliant stream of light will flash across the points of the lightning conductors with sharp detonating reports, the electromotive force of which would be scarcely equalled by 500 Daniell cells.

THE LIGHT OF JUPITER'S SATELLITES Ueber die Helligkeitsverhältnisse der Jupiterstrabanten, von Dr. R. Engelmann, Observator der Sternwarte zu Leipzig. (Leipzig; London: Williams and Norgate. 1871.) F all the satellite systems which so essentially enrich when we our own

moon behind us, promises such a reward for investigation as that of the planet Jupiter. The remoter ones may be, and probably are, intrinsically of a more remarkable character, but they are, and ever will remain to a great extent, beyond our reach; while the attendants of the largest among the planets are numerous enough to interest by individual peculiarities, which their comparative proximity enables us to study with advantage. Yet it is readily observable that though ordinary telescopes of good quality would have done much towards elucidating their phenomena, very little progress has been made in the inquiry, especially in this country; and the work now before us is the first attempt to collect and to make serviceable the scattered observations which exist, of which we are sorry to remark how few are due to the astronomers of England.

In January last very curious electrical phenomena were observed at Lerwick through the day-time, in connection with the N.E. gales so prevalent at that period of the year. In Shetland these gales are almost without exception accompanied with very severe hail-storms. The day begins bright and fine, a clear sky, the barometer rapidly rising; low on the horizon may be observed dense and angry-looking clouds. One by one these clouds travel fast towards the zenith, when all at once a fearful gust of wind, accompanied with the most violent hail-storm, will apparently break out of the cloud, and continue for about fifteen minutes. The wind then subsides, and the day appears as fine as before. In half an hour's time a second cloud will have appeared, and there will be a repetition of the temporary tornado and hail-storm. The remarkable circumstance attending these successive storm clouds is that they appear to be a purely electrical phenomenon. The moment that the icy discharge takes place from the cloud with its accompanying "crack" of wind, an induced electrical current appears upon the wire, so strong that it attracts firmly down the armatures of the telegraph Morse apparatus. The moment, however, that the hail ceases, the current passes off, but with this result, that each successive cloud storm appears to induce a current flowing inan opposite direc-"albedo," or reflecting power of each surface, it is, of tion from the last, that is to say, the currents appear to be (using conventional language) positive and negative in their effects.

That these storms are " electrically excited" there is no disputing, and that they occur during the prevalence of the chief auroral displays is also a matter of observation, but so far their connection with aurora has not been sufficiently determined to permit any opinion to be expressed.

The recent successful completion of the telegraph circuit to Shetland, and the extensions immediately to be carried out one hundred miles farther north, will afford much greater facilities for auroral observation than has hitherto existed. It is also proposed to institute a careful ́spectroscopical examination of the coloured scintillations; and now that the Meteorological Society are about to establish an observation station in Shetland, there is every prospect of some valuable data being collected on this interesting subject, which may hereafter guide our meteorological students in arriving at some satisfactory conclusion regarding the laws of electrical storms and auroral induction. At present we are only able to record a few carefully observed facts.

The especial object of the eminent observer at Leipzig has been not the theory of the motions of these satellites, but simply their physical aspect in regard to the variable light which they have long been known to reflect, and to this investigation the author, notwithstanding constant engagement in important zone observatio is, has contributed far more than all who have preceded him. The instrument which he employed was the astrophotometer of Zöllner. In this ingenious contrivance, the light of the object to be examined is referred to that of one or more known comparison stars, by means of an artificial star produced by a petroleum flame, adjustable for brightness and colour by a Nicol prism, and a colorimeter," or revolving wheel of tinted rock-crystal. But in order to eliminate the effect of unequal areas, so as to ascertain, not merely the absolute amount of light reflected, but the

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All

course, necessary to obtain reliable measures of these
minute specks of light; and in order to decide the
interesting question whether or not their rotation
and revolution are, as with our own satellite, syn-
chronous, their anomalies, or orbital positions relative
to their primary, have to be taken into account.
this has been done with most praiseworthy care; the
whole is discussed and reduced with scrupulous and
exemplary attention to every possible source of accidental
error; and the result is given to the eye in several elabo-
rate diagrams. We shall merely specify some of the con-
clusions, which will be found of considerable interest to
astronomers. The absolute brightness was found by the
author, as it has been by all previous observers, very
variable; and from the irregularity and occasional rapidity
of its changes, it becomes impossible to decide, in the case
of the three interior satellites, whether the periods of
This, however,

rotation and revolution are identical.
appears to be decidedly the fact with the outermost.
Herschel I. had extended the inference to all of them;
but such a result could not now be accepted; and it seems
probable that the spots which must occasion these varia-
tions, and which have been repeatedly noticed when the

satellite has been on the disc of Jupiter, and by Dawes and Secchi even in other positions, may be of changeable character. At a mean II. is relatively the most, IV. the least luminous. As to their micrometrical measurement, every one who is acquainted with the telescopic aspect of these minute discs will readily comprehend its difficulty. It has, however, been attempted in various ways, but not by the double-image micrometer, which does not seem to have been used; the results, as may be expected, present considerable discrepancies, but the final values obtained by a combination of different methods in the hands of various observers are as follow :-I., 1"'081 ; II.,o"910; III., 1"537`; IV., 1"-282; or, in English miles, 2,498, 2,102, 3,551, 2,962, the solar parallax being taken as 8"-90. These values, all things considered, differ so little from those given by Lockyer (Guillemin's "Heavens") - namely, 2,440, 2,192, 3,759, 3,062-that we may consider ourselves possessed of a very fair approximation to their real magnitudes.

As to the "albedo" of their surfaces, I. shows no great varia. tion; it falls, according to Zöllner's estimate of the reflective power of terrestrial materials, between that of marl and white sandstone; II. has the greatest variations of albedo, which at a mean somewhat exceeds that of white sandstone; III., the variations of which are smaller and more regular, comes between marl and quartzose porphyry; IV., which varies least, equals that of moist arable land. It will probably be thought, however, that curious as these comparisons may be, the standards are much too uncertain to give any satisfactory result. As to colour, Dr. Engelmann, after citing the elder Herschel's estimates-I., white; II., white, bluish, and ash-coloured; III., white; IV., dusky, dingy, inclining to orange, reddish, and ruddy-specifies as the determination of other observers: I., yellowish; II., white or yellowish; III., intensely yellow with low powers; IV., in achromatics a distinct dusky blue. (These colourvalues at any rate afford no countenance to the common impression that Herschel had a bias for red tints.) To the writer, whether with two achromatics, or a nine-inch silvered mirror, this satellite has always appeared ruddy when its colour has formed the object of notice; in such discrepancies something may be instrumental, something subjective. It is pleasant to see here a very full appreciation of the laborious perseverance and honest accuracy of the labours of Schröter, to whose merit time seems to be doing tardy justice; no notice is taken, however, of the observations of Gruithuisen, who twice appears to have seen spots on III on the background of the sky; nor is reference made to the irregular shape of that satellite remarked by Secchi and his assistant; nor to the apparent discrepancy which has often been noticed between the magnitudes of the satellites and their shadows. Still, the treatise may be considered as very nearly an exhaustive one; and a most important and acceptable contribution to planetary astronomy. It may be added that it contains a very valuable determination of the telescopic magnitude of Jupiter, from the average of eleven observers; the result being, with the double-image micrometer 37" 609 for the equatorial, 35"236 for the polar diameter; with the wire micrometer, 38" 312 and 35" 914: the former values, which he seems to prefer, exhibiting a flattening

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OUR BOOK SHELF

Transactions of the Geological Society of Glasgow. Vol. III. Supplement. On the Carboniferous Fossils of the West of Scotland: their Vertical Range and Distribution. By John Young, Vice-President. With a General Catalogue of the Fossils and their Mode of Occurrence, and an Index to the Principal Localities. By James Armstrong, Honorary Secretary. (Glasgow, 1871.) THIS catalogue of fossils will doubtless be of great use not only to local geologists, but to others at a distance, who may desire to compare the treasures of English and Irish Carboniferous strata with what the equivalent beds in Scotland have yielded. So far as they go, the lists appear to be drawn up with considerable care, and Mr. Armstrong is to be congratulated upon the result of what must have been somewhat laborious work. But we are sure he will to be done before the Scottish Carboniferous flora and be the first to admit that much, very much, still remains fauna can be satisfactorily compared with those of other countries. We are constantly being reminded throughout this catalogue that not only in private collections, but also in public museums in the West of Scotland, there are numbers of specimens under almost every class waiting to be identified, amongst which there is every reason to believe that not a few are species new to science. This, it seems, is specially the case with the plants, the rich flora of the Carboniferous period being represented in the catalogue by only ninety species. But Mr. Carruthers, we are told, has several undescribed specimens in hand, of There are

which we shall, no doubt, hear by-and-by. The fishes, eighty-four species, under forty genera, named in the it would appear, also need looking after. catalogue; but a large number in various collections have never been correctly identified with described species, and Mr. Young expresses a hope, in which we cordially join, that Prof. Young will be induced to prepare a special catalogue of these and the Reptilia, of which only seven species are given by Mr. Armstrong. The other classes are represented as follows:-Foraminifera, 2 genera, 4 species; Hydrozoa, I g. 2 sp.; Zoophyta, 22 g. 59 sp. ; Echinodermata, 6 g. 15 sp.; Annelida, 4 g. 7 sp.; Crustacea, 19 g. 71 sp.; Insecta, 2 g. 2 sp.: Polyzoa, 11 g. 36 sp.; Brachiopoda, 15 g. 50 sp.; Lamellibranchiata, 28 g. 127 sp.; Pteropoda, Ig. I sp.; Gasteropoda, 15 g. 75 sp.; Cephalopoda, 6 g. 46 sp. From these numbers it will be seen that the collectors have not been idle, and, no doubt, Mr. Armstrong's catalogue, with its minute index to localities, will be the means of sending many to hunt in quarters which they have not already conditions under which the fossils are distributed, and not visited. Let us hope that they will note something of the content themselves simply by bringing away good bags full. Collectors cannot be too often reminded that it is of more importance, in the interests both of natural history and geology, to know one limited district thoroughly, than to go roving over half a country merely for the purshould mark out for himself some practicable area, and pose of picking up finely preserved specimens. Each make it his endeavour to search every bed, even the most unpromising, noting not only the fossils he meets with, but the character of the strata in which they occur. He should also observe what effect a change in the character of a bed has upon the fossils it may happen to contain; whether vidually gain in size or become dwarfed, and, should certhey increase or decrease in numbers, whether they inditain species disappear, what others, if any, are substituted for them. It is only by marking carefully such points as these that we can ever hope to acquire an adequate conception of the natural history of the old carboniferous lands and seas. Mr. Young is quite sensible of the shortcomings of the collectors in this matter, and gives them some seasonable advice, which it may be hoped they will take to heart. If collectors paid better heed to these

matters they would assuredly derive greater pleasure and profit from their pursuit, and do much more towards the progress of science. Mr. Young himself, however, notwithstanding the good advice he gives, is not always careful in drawing conclusions, geological evidence being sometimes quite overlooked. Thus, we find him stating that the coal-measures (meaning, of course, the whole series of strata above the Millstone Grit) are "evidently of land and fresh-water origin," because they have yielded no marine organisms, save in one thin local bed near the top of the series. The occurrence of this stratum with its marine remains, indicates, as he believes, the return for a short time of the sea, which had for a very long period "been completely shut out by barriers." Mr. Young is welcome to his belief. If every bed or series of beds in which no marine organisms occur must necessarily be of fresh-water origin, the lakes of old must have been something worth seeing. There are several points suggested by the catalogue that we should like to have taken up, but our space is exhausted, and we can only conclude by strongly recommending Mr. Armstrong's work to the notice of our geological readers.

LETTERS TO THE EDITOR

J. G.

The Editor does not hold himself responsible for opinions expressed by his Correspondents. No notice is taken of anonymous communications.]

On the Solution of a Certain Geometrical Problem A WRITER in the number of NATURE for September 21, Mr. R. A. Proctor, in the course of a letter on the state of geometrical knowledge in the university, alludes manifestly to the solution of a problem which I have adopted in my edition of Euclid. The matter is of small importance in itself, but nevertheless as some points of interest are incidentally involved, I request you to allow me the opportunity of offering a few remarks.

The problem is this: to describe a circle which shall pass through a given point and touch two given straight lines. Your correspondent considers that in giving a solution which depends on the sixth book of Euclid, instead of one which depends only on the third book, I exhibit signal geometrical weakness.

The problem, I need scarcely say, is very old; indeed, so old that a writer who had been long engaged in teaching could not pretend to solve it afresh, for he would certainly have in his memory one or more solutions which had become quite familiar to him. The solution by the aid of the third book is well known, for it occurs in several of the collections of geometrical exercises. The solution which I have adopted is also old, but seems not so well known. It is, I think, conspicuous for simplicity, elegance, and completeness. The demonstration is of the best and most impressive kind, requiring no laborious effort to understand and retain it, but being almost self-evident from the diagram. Even if the problem be treated as an isolated exercise, the solution which I have preferred will sustain a favourable comparison with that which more commonly occurs.

But the determining cause of my choice was the position which the solution occupies as one of a connected series. I have just before treated a similar problem by the third-book method, so that if the same method had been used for the present problem, there would have been only repetition without any substantial increase of knowledge; whereas by the course adopted the student is introduced to fresh and valuable matter. The principle of similarity and the notion of a centre of similitude are most instructively involved, and the student is prepared for a subsequent investigation, which is similar but more complex. To sum up, the third-book method would have constituted no advance in the subject, where the sixth-book method takes a step important in itself and in its consequences; and therefore, following the example of an eminent geometer, I adopted the latter method. I may perhaps venture on the strength of my own experience as to the utility of the solution, to recommend it to the attention of other teachers.

It is very important to bear in mind the distinction between what I may call absolute and relative merit which I have just exemplified. The solution of a single problem furnished by a candidate under examination, or by a contributor to a mathema

tical periodical, is very different from the investigation of one out of a chain of propositions in a mathematical treatise. In the former case there are no antecedent or subsequent conditions to regard; in the latter case we have to consider what agrees best with the whole scope of the work, with what is to follow as well as with what has gone before. A writer, after arranging a paragraph or a chapter in what seems the best manner, may find himself constrained at a subsequent stage to make changes which would have been unnecessary, perhaps even undesirable, if the earlier portion had stood alone. Then, if a reader opens the book at random and criticises a passage without any regard to the author's sense, the criticism may very naturally be quite inappropriate.

There is, however, a very important consideration of another kind which has been frequently disregarded, but which is pressed upon our notice by the interest at present felt in geometrical Let us determine the reason which leads us in some, or studies. in many, cases, to prefer a solution which involves only the third book of Euclid to a solution which depends on the sixth book; this, I apprehend, is merely a persuasion that Euclid's order is a natural order, so that in a well-arranged system the propositions of the third book ought to precede those of the sixth book. I am of this persuasion myself; I think that no scheme can be perfect, and, on the whole, I am well satisfied with Euclid's. But there are places where Euclid is strong, and there are places where Euclid is weak; and the position which he has assigned to the last three propositions of his third book, must rather be classed with the latter than with the former. His object, of course, must have been to lead up to his construction of a regular pentagon, and we cannot be surprised at the introduction of that remarkable process. But I have always envied the advantage in this respect to be claimed for the non-Euclidean systems, which transfer these propositions and place them after the doctrine of similar triangles; thus the long and rather artificial treatment which they receive from Euclid is superseded, and the propositions become almost intuitive. Hence, in fact, if we have recourse to the sixth book of Euclid when we might have accom. plished our end by the aid of the first thirty-four propositions of the third book, we may be fairly liable to the charge that we have not adopted the simplest and most natural method; but the last three propositions of the third book are quite different in kind from the others, and instead of using them, it may be really as simple and as natural in many cases to use the principle of similar triangles.

I shall be obliged to any person who may be skilled in practical geometry if he will state what he considers the best method of actually solving the problem, supposing that both circles are to be determined which satisfy the conditions. I assume that we have the aid of compasses and also of one of the ordinary contrivances for drawing parallel lines. This is a matter of some interest, though of course unconnected with the theoretical solution of the problem.

I should be glad to make some remarks on the general subject which led to the notice of the particular problem I have discussed, but at present I have not sufficient leisure. I must content myself with having shown that the course into which I am supposed to have drifted by geometrical incapacity, was adopted deliberately under the guidance of reasonable geometrical knowledge. I. TODHUNTER

St. John's College, Cambridge, Oct. 2

Structure of Fossil Cryptogams

IT was unfortunate that at the recent meeting of the British Association, Prof. Williamson's paper had to be discussed in a very hurried manner, and he is, no doubt, justified in taking care "that there shall be no misunderstanding as to the real point at issue." I do not think that he has brought it out very plainly in his paper in NATURE, and perhaps, as he mentions me as an opponent of his views, I may be allowed to state precisely in what respects I differ from him.

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First, as to matters of fact. Prof. Williamson speaks of the central structure of the stems of the extinct Lycopodiaceae as a "vascular medulla," by which he explains that he means a structure containing vessels," and that there shall be no misapprehension he adduces Nepenthes as possessing it; the instance is a well-known one, and leaves no room for doubt as to Prof. Williamson's meaning. Now from the examination of specimens, and of the drawings of them published by Mr. Carruthers (the accuracy of which I believe Prof. Williamson does not dispute) I am quite satisfied that the central structure consists wholly of

scalariform vessels, and that there is in fact nothing medullary or medulla-like about it.

Outside this central structure is what Mr. Carruthers terms the investing, and Prof. Williamson the vascular woody cylinder. I believe that Mr. Carruthers is right in looking upon this as belonging to the central axis, which is therefore composed of two parts. I find, which I did not sufficiently appreciate at the time, that Prof. McNab regards this investing cylinder as homologous with the cylinder of wood cells surrounding the central axis of fibro-vascular bundles which is met with in many recent Lycopodiaceae. From this I certainly dissent for two reasons; (1) because I think its equivalent is to be found in the central axis itself, and not outside it; (2) because it is not composed of wood cells but of scalariform vessels.

Secondly, as to opinions. The terms Exogen and Endogen, as is pretty well known, were founded upon a mistake. A great deal too much has been made of the difference implied by them; in fact, if we compare a one-year-old dicotyledonous shoot with a monocotyledonous stem, we find that it does not exist. If Prof. Williamson will look at the stem of the common artichoke, he will find it difficult to convince himself that he is examining an exogenous" plant at all.

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The imagined characters which were implied by these terms are, nevertheless, as everyone knows, correlated with others, which in the aggregate enable phanerogamic plants to be divided into two satisfactory groups; but this is certainly not equally the case with the groups into which Prof. Williamson would divide the vascular cryptogams. These groups, I think, most botanists will agree in considering in the highest degree unnatural, inasmuch as, assuming the vegetative distinction upon which they are founded to exist, it is a wholly artificial ground for classificatory purposes. Nor is it any argument that one vegetative character must be good because others are in use, since the simple answer is that these coincide with natural divisions, while Prof. Williamson's does not.

I shall not dispute Prof. Williamson's position that our living Lycopodiacea should be interpreted by the more complete extinct types. To do this, however, the extinct types must be thoroughly understood; when we are dealing with imperfect material, comparison with the more perfect but less highly developed existing plants is not only justifiable but necessary.

It is obvious that the great development of the stem in the Lycopodiacea of the Coal Measures was correlated with their arborescent habit. I am inclined to think with Prof. Williamson that the stem increased in thickness; it is certain that Lepidodendron was branched, and not improbably also Sigillaria. The branches as they were gradually developed must have been the cause of an increasing strain upon the stem; it seems to me more congruous with known laws of the response of structure to circumstances, to conclude that the stem was proportionately developed as the strain increased, than that the stem should have been produced once for all of its maximum thickness without reference to the crown of branches that was finally to surmount it. I am quite prepared therefore to admit that the investing cylin der may have increased by external additions, and probably did do so; this would of course imply the existence of a cambium layer outside it. There is some analogy for this in the recent Isoëtes, where we have a "slight woody mass which occupies the longitudinal axis of the stem, but encloses no pith." + Outside this we have a "bark-forming" cambium (which also adds, but more sparingly, to the wood mass); in Sigillaria and Lepidodendron we might have had a cambium not merely renewing the bark but adding to the central axis.

In whatever way the increase took place, it was, as I think, nothing more than an incident in the life history of a particular race of plants, nothing more than an adjustment to an arborescent habit dropped when the arborescent habit was lost, but showing a lingering ancestral tendency in Isoetes. Comparing a simple stemmed palm with Dracana, we have a parallel instance of the strengthening of the stem pari passu with the continued development of a system of branches; only in Dracana it is the circumferential part of the stem alone which developes.

If I am right in regarding a stem gradually developing in size as the necessary correlate of a large system of branches, Prof. Williamson's view practically amounts to the old division of plants into trees and herbs. I cannot see how it can afford any safe ground for a re-arrangement of the vascular cryptogams. W. T. THISELTON DYER

London, Sept. 26

Monthly Micro. Journ., 1869, p. 169. ↑ Hofmeister, Higher Cryptogamia, pp. 356, 361

The Solar Spectrum

MAY I venture to suggest that quite possibly something of value might be obtained by observing the sun during totality with a spectroscope of reasonable dispersive power (say four or five prisms) without a collimator, or even simply with one of the socalled meteor spectroscopes.

If the bright rays and rifts are really and simply (or even mainly) composed of the green-line-giving substance, they will give a well-defined green image; if they are formed by reflection (either at the sun or in our atmosphere) of ordinary sunlight, they would be so dispersed as to be invisible or nearly so, and if formed by the reflection of chromosphere light they would give several images, the red (C) and blue-green (F) being most conspicuous. C. A. YOUNG

Hanover, N.H., U.S., Sept. 13

Arrangements have already been made for carrying out a similar suggestion to this by the Eclipse Committee; and the corona will also be observed with an open slit.-Ed. N.

I

Eclipse Photography and the Spectroscope THE endeavour of the Eclipse Committee to secure some uniformity in the photographs from different stations next December does not appear to be duly appreciated, it being contended that immense "personality" shown in various photographers' manipulation must frustrate the good intention. submit that in this case the personality is greatly over-estimated; that a number of competent photographers taking the same subject would probably produce, under any ordinary circumstances, pictures bearing considerable resemblance; while by using like apparatus and giving exposure of the sime duration, we might safely predict a similarity of result amply sufficient for comparative purposes, and for the identification of structural peculiarity should it exist.

Among others there is a possible advantage to accrue from uniform work by the philosophers which I have not seen or heard noticed. Supposing the outer corona, rays, streamers, or any portion of the apparently luminous matter be terrestrial, is it unreasonable to expect that photographs, taken at stations more or less widely separated, will, when properly combined in the stereoscope, give clear ocular proof of the sublunary situation of such luminous matter? HENRY DAVIS

Phenomena of Contact

MR. STONE can safely be left to meet the arguments specially addressed to him in Prof. Newcomb's letter; but as the subject relates to the only point of importance touched on in Prof. Newcomb's criticism of my chapter on the sun's distance, I crave permission to meet his general argument.

I submit that he tries to prove too much.

He admits that the phenomenon of irradiation exists in the case of a disc. The sun's disc, then, must be to some extent enlarged, and the dark disc of Venus must be to some extent reduced by the effects of irradiation. Now this being so, what becomes of the cusps, when Venus is all but wholly on the sun's disc? Either the irradiation is diminished near the cusps or it is not. If it is diminished there must be distortion, because the disc of Venus is then not uniformly reduced: if the irradiation is not diminished a ligament must appear.

Let any one draw a large circle (say a foot in diameter) on paper, and a small one (say an inch in diameter) extending very slightly (say by the twentieth of an inch) beyond the boundary of the first; and let him blacken the smaller circle as well as all the space outside the larger one. He has then a space representing the disc of the sun with a very large Venus upon it near the time of internal contact. Now let him conceive the whole of this space (a sort of exaggerated crescent) slightly enlarged as by irradiation, the enlargement-fringe extending outside the boundary of the large disc and inside the boundary of the small black (incomplete) disc. He will find the conception of this enlargement exceedingly easy everywhere save near the cusps; but here there is a difficulty in determining how the fringe outside the larger disc is to be joined on to the fringe inside the smaller disc. If he can conceive these two fringes meeting in such sort as to leave the reduced outline of the small disc completely circular up to the very points in which it meets the enlarged outline of the large disc, he will have done what Prof. Newcomb's theory requires. But note, this must be done for the case when the fringe of enlargement is wider than the twentieth of an inch, by which the small disc overlaps the large one. When this is the

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