"ripples" (as I have called them), seen in advance of a body moving uniformly through water; also a passage quoted by Russell from a paper of date, Nov. 16, 1829, by Poncelet and Lesbros,* where it seems this class of waves was first described.

Poncelet and Lesbros, after premising that the phenomenon is seen when the extremity of a fine rod or bar is lightly dipped in a flowing stream, give a description of the curved series of ripples (which first attracted my atten- | tion in the manner described in the preceding letter). Russell's quotation concludes with a statement from which I extract the following: - . . . . "on trouve que les rides sont imperceptibles quand la vîtesse est moyennement au dessous de 25c. per seconde."

Russell gives a diagram to illustrate this law. So far as I can see, the comparatively long waves following in rear of the moving body have not been described either by Poncelet and Lesbros or by Russell, nor are they shown in the plan contained in Russell's diagram. But the curve shown above the plan (obviously intended to represent the section of the water-surface by a vertical plane) gives these waves in the rear as well as the ripples in front, and proves that they had not escaped the attention of that very acute and careful observer. In respect to the curves of the ripple-ridges, Russell describes them as having the appearance of a group of confocal hyperbolas, which seems a more correct description than that of Poncelet and Lesbros, according to which they present the aspect of a series of parabolic curves. It is clear, however, from my dynamical theory that they cannot be accurate hyperbolas; and, as far as I am yet able to judge, Russell's diagram exhibiting them is a very good representation of their forms. Anticipating me in the geometrical determination of a limiting velocity, by observing the angle between the oblique terminal straight ridge-lines streaming out on the two sides, Russell estimates it at 8 inches (21 centimetres) per second.

Poncelet and Lesbros's estimate of 25 centimetres per second for the smallest velocity of solid relatively to fluid which gives ripples in front, and Russell's terminal velocity of 21 centimetres per second, are in remarkable harmony with my theory and observation which give 23 centimetres per second as the minimum velocity of propagation of wave or ripple in water.

Russell calls the ripples in front "forced," and the oblique straight waves streaming off at the sides "free" -appellations which might seem at first sight to be in thorough accordance with the facts of observation, as, for instance, the following very important observation of

his own :

“It is perhaps of importance to state that when, while these forced waves were being generated, I have suddenly withdrawn the disturbing point, the first wave immediately sprang back from the others (showing that it had been in a state of compression), and the ridges became parallel; and, moving on at the rate of 8 inches per second, disappeared in about 12 seconds." Nevertheless I maintain that the ripples of the various degrees of fineness seen in the different parts of the

• Memoirs of the French Institute, 1829.

The dynamical theory shows that the length from crest to crest depends on the corresponding component of the solid's velocity. For very fine ripples it is approximately proportional to the reciprocal of the square of this component velocity, and therefore to the square of the secant of the angle between the line of the solid's motion and the horizontal line perpendicular to the ridge of the ripple.

fringe are all properly "free" waves, because it follows from dynamical theory that the motion of every portion of fluid in a wave, and, therefore, of course, the velocity of propagation, is approximately the same as if it were part of an infinite series of straight-ridged parallel waves, provided that in the actual wave the radius of curvature of the ridge is a large multiple of the wave-length, and that there are several approximately equal waves preceding it and following it.

No indication of the dynamical theory contained in my communication to the Philosophical Magazine, and described in the preceding letter to Mr. Froude, appears either in the quotation from Poncelet and Lesbros, or in any other part of Mr. Scott Russell's report; but I find with pleasure my observation of a minimum velocity below which a body moving through water gives no ripples, anticipated and confirmed by Poncelet and Lesbros, and my experimental determination of the velocity of the oblique straight-ridged undulations limiting the series of ripples, anticipated and confirmed by Russell. W. T.

ALLBUTT ON THE OPHTHALMOSCOPE On the Use of the Ophthalmoscope in Diseases of the Nervous System and of the Kidneys; also in certain other General Disorders. By Thomas Clifford Allbutt, M.A., M.D., Cantab. &c. (London and New York: Macmillan and Co., 1871.)

THE advances that have been made in the knowledge of the diseases of the eye since the introduction of the ophthalmoscope are now very widely known, not alone in the medical profession but to the general public. This little instrument, essentially consisting of a mirror with a hole in the centre by which a ray of light can be thrown into the interior of the eye, lighting up its recesses, and enabling, with the aid of a common hand lens, almost every portion of it to be explored, may be said to have revolutionised the surgery of the eye. Many separate and distinct types of disease have been distinguished in conditions that were formerly grouped together under the general term of amaurosis, and the ophthalmic surgeon, no longer administering, as was too often formerly the case, his remedies in rash ignorance, is now able either to infuse well-grounded hope of recovery, or to spare his patient the annoyance of protracted treatment when treatment would be hopeless. For nearly twenty years the use of the ophthalmoscope has been, as was natural, almost entirely restricted to those who devoted themselves to the study of ophthalmic diseases. Like other mechanical aids employment requires practice, the opportunities for acto diagnosis, as the stethoscope and laryngoscope, its quiring a mastery over it were till recently rare, and its value in the practice of medicine was by no means generally recognised. Within the last few years, however, several excellent surgeons and physicians, amongst whom Mr. Hutchinson, Dr. Hughlings Jackson, Dr. John Ogle, and the author of the treatise before us may be especially mentioned, have gradually begun to recognise that the ophthalmoscope may be made available not only to determine the nature of any defect of vision of which the patient may complain, but as a means of reading within certain limits changes in the conditions of the system at large, and of the nervous system in particular.

The work of Dr. Allbutt is, however, the first treatise in English that is occupied exclusively with the ophthalmoscopic appearances presented in cases of cerebral disease, or in other words with the diagnosis of nervous affections by the ophthalmoscope. Abroad he has been preceded by M. Bouchut, whilst the volumes of the "Archiv für Ophthalmologie" are a mine of original memoirs written by the best ophthalmologists in Germany on the bearings of ophthalmoscopic observations on nervous affections. To these, of course, Dr. Allbutt makes frequent reference. In no instance, however, have we noticed a servile adherence to the opinions of others, the statements he quotes being always checked by his own observations, and every page bearing the stamp of very careful and sound investigation. It is impossible with the limited space here at disposal, and it would perhaps scarcely be interesting to many of our readers, to give what the work really deserves, a résumé and discussion of its successive chapters; but we may here perhaps point out one or two of the principal points of interest.

In speaking of the disc of the optic nerve, Dr. Allbutt expresses himself in favour of the view of Galezowski, who is fortified by the observations of Leber, to the effect that the vascularity of the disc is to a great extent independent of that of the retina, and rather forms a part of the vascular system of the brain. The importance of this principle in enabling deductions to be drawn respecting the occurrence of intercranial disease is obvious. Proceeding on this hypothesis, Dr. Allbutt points out the changes that are visible in a large number of different affections. He draws a strong line of distinction between ischemia and optic neuritis, conditions that have hitherto been almost invariably confounded by ophthalmic sur geons, but of which the former is produced by some cause, often of a mechanical nature, interfering with the return of the blood from the retina, whilst the latter is a

true inflammation of the nerve. The diagnosis of the two

in their earlier stages is very clearly and correctly laid down. At a later period both conditions pass into white atrophy, and it is not always then easy to pronounce which of the two has previously been present. His views, in regard to changes in the optic disc from intercranial disease, are clearly laid down in the following passage (pp. 129, 130):-"We find optic changes in connection with two kinds of intracranial disease in particular; the one tumour, the other meningitis. When we analyse the matter one degree further, we ascertain that, although the choked disc (ischæmia) and the inflamed nerve may

co-exist with either of these kinds of disease, that nevertheless the choked disc is far more commonly found in association with tumour and hydrocephalus than the inflamed nerve. The inflamed nerve, on the other hand, is very commonly found in association with meningitis, and of meningitis not of the surface, nor of parts near any supposed vasomotor centres, but with meningitis And with this we are disposed substantially to agree. Dr. Allbutt expresses himself in very doubtful terms in regard to the existence of tobacco amaurosis, and it certainly is extraordinary that, if really constituting an effect of the use of that leaf, it is not of more frequent occurrence amongst the Germans and Americans, who are much larger consumers than either the French or ourselves.

near the centre."

Our readers will see that Dr. Allbutt has, if not exactly opened up, at all events vigorously worked at, a new fiell of medical inves igation. This field promises when duly cultivated to yield very valuable fruit; and, we are sure, the conclusion at which every candid reader will arrive, after carefully perusing it, will be that no physician should consider he has fully examined any case of cerebral disease unless he has accurately investigated the appearances presented by the eye under the ophthalmoscope. It is not to be supposed that Dr. Allbutt has by any means exhausted the subject. Many difficulties lie in the path of the most diligent inquirer. In many instances conditions of disease are seen to be present, as to the nature of which only a guess can be formed, and respecting which from forgetfulness or lack of observation on the part of the patient no history can be obtained; whilst in a multitude of cases the disease is seen only at one stage of its progress, and the physician is unable to ascertain, owing to his losing sight of his patient, the ulterior changes that take place.

Lastly, in many cases the prejudice of friends (a point to be greatly regretted) prevents the examination of the eyes after death. The fragmentary character of many of the reports of cases collected by Dr. Allbutt in his appendix is painfully evident, and leaves many hiatuses to be filled up by future research. We may, however, in conclusion, thank Dr. Allbutt for having published a work which constitutes an important step in the advancement of medicine, and will certainly form a very valuable guide to the profession at large, nor my we omit to thank the publishers for the excellent manner in which the book has been issued from the press. H. POWER

OUR BOOK SHELF

Hardy Flowers: Descriptions of upwards of thirteen hundred of the most ornamental species, and directions for their arrangement, culture, &c. By W. Robinson, F.L.S. (London: F. Warne and Co., 1871.) MR. ROBINSON is a prolific writer, but his prolificacy (as Webster has it, if Dr. Ingleby and Dr. Latham will allow us the word) does not degenerate into mere book-making. utility both to the professional gardener and to the Like its predecessors, this volume is one of practical cultivator of flowers for their beauty. Much the greater part of the volume is occupied with a descriptive list of the most ornamental hardy flowers, with directions for their culture, suitable positions, &c.; but this is introgardening. That Mr. Robinson has the courage to attack duced by some practical hints on the general subject of some time-honoured gardening customs, will be seen from the following paragraph :-" No practice is more general, or more in accordance with ancient custom, than that of digging shrubbery borders, and there is none in the whole course of gardening more profitless or worse. When winter is once come, almost every gardener, although animated with the best intentions, simply prepares to make war upon the roots of everything in his shrubbery border. The generally accepted practice is to trim, and often to mutilate, the shrubs, and to dig all over the surface that must be full of feeding roots. Delicate halfrooted shrubs are often disturbed; herbaceous plants, if at all delicate and not easily recognised, are destroyed; bulbs are often displaced and injured; and a sparse depopulated aspect is given to the margins, while the only 'improvement' that is effected by the process is the annual darkening of the surface of the upturned earth." After

this we find some pertinent and useful hints on the best mode of grouping hardy perennials, and the art of managing the rock-garden, the wild-garden, water, and boggy ground; on the culture and propagation of early flowers, and other subjects dear to the dweller in the country. Compared with the art of gardening as practised twenty years ago, we are certainly now in an altogether new and improved epoch, and Mr. Robinson is one of the pioneers to whom we are mainly indebted for the introduction of a better and more rational style.

A. W. B.

Hints on Shore-Shooting: with a chapter on skinning and preserving Birds. By James Edmund Harting, F.L.S., &c. (Lorden: Van Voorst, 1871.)

A GOOD Sportsman, whether he knows it or not, must be more or less of a good naturalist, and this Mr. Harting is. His unpretending little book, therefore, certainly deserves mention here, and the more so since he has worthily won his spurs by making the group of birds most sought by the "shore-shooter" an especial subject of study. What he tells us is the result of his own observation, and is pleasantly told. What he does not tell us is whether "shore-shooting" has, with most people,-for we except him—any other raison d'être than the "fine-day-let's-goand-kill-something" impulse. If not, we really do not see that there is much difference in principle between Pagham and Hurlingham.

LETTERS TO THE EDITOR

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

An Universal Atmosphere

I HAVE much pleasure in replying to Mr. Browning's question in NATURE, vol. iv. p. 487, as it is one that legitimately strikes at the root of all my speculations, and which, if unanswerable, conveys an objection that must demolish the whole structure I have endeavoured to erect in my essay on the "Fuel of the Sun."

If I am right, the atmospheres of the sun, the moon, the planets, or of any other cosmical body of known mass and dimensions, may be calculated in units of the earth's total atmosphere by the simple formula reasoned out in Chap. iii. of the above-named work, ie., by multiplying the mass of the body (expressed in units of the earth's mass) by its own square root, thus

nearly of that of the earth's mean atmospheric pressure. Such an atmosphere would support a column of mercury six-tenths of an inch in height. Mr. Browning will recognise this as about equal to the best vacuum obtainable in an old-fashioned air-pump of average defectiveness.

Such is the theoretical pressure upon every part of the moon's surface, supposing the form of the moon to be a perfect spheroid of rotation with a perfectly smooth surface. But the moon is no such regular body. It presents far more irregularities in proportion to its size than would our earth if the ocean were

evaporated, and its depths laid bare so that our mountain heights should be measured from the ocean bottom. Under such conditions the bulk of even our atmosphere would occupy the ocean valleys, and very rare indeed wou'd be the remainder that reached the mountain tops and elevated ridges of the earth. On the moon with its filmy atmosphere of only six-tenths of an inch would be carried beyond the limits of observable refractive power mean pressure, the rarefaction on the high lands and mountains under the conditions assumed-viz., of a special atmosphere merging gradually into the universal interstellar medium.

The visible edge of the moon which effec's the occultation of a star must in almost every possible case be formed by the ridges and summits of the lunar mountains, in no case by the bottom of the lower valleys, for in looking horizontally across the moon's rotundity these valleys and even the maria must be foreshortened, and their lower depths walled out of the reach of our vision by the great lunar elevations. Thus the occultation of a star would occur without its previous plunging behind any outlying lunar atmospheric matter of appreciable density. We must not forget that Sir J. Herschel's calculation, which assigns one second of refraction to an atmosphere equal to of the density of the earth, is based on the theory of a limited atmosphere with a sharp and definite boundary suddenly terminating in a vacuum. But this rarefaction on the elevated portions of the moon atmospheric matter in the valleys, crater-pits, and maria. Here demands a compensating condensation or concentration of the calculated mean. the pressure on the moon's surface should considerably exceed This consideration suggests a very interesting question. Would such an atmosphere, say capable of supporting one inch of mercury, produce any observable effects? If I am right in regarding water as one of the constituents of the universal atmosphere, there are good reasons for supposing that it would.

The small share of water due to the moon would all be raised far above its low boiling point, early in the lunar day, by the heated lunar surface. There would be no sea, no clouds, no rain, no snow, but on the plains and in the valleys a formation of hoarfrost should occur at the lunar eventide, beginning just where the sun's rays become too oblique to maintain the temperature of the rapidly radiating lunar surface above the freezing-point.

In a note appended to Mr. Lockyer's translation of M. Guillemin's work on "The Heavens," the Rev. T. W. Webb thus corrects the author's rather positive statements concerning the total absence of a lunar atmosphere: "After all fair deductions on the score of imperfection of observation or precipitancy of inference, there are still residuary phenomena, such as, for instance, the extraordinary profusion of brilliant points which on rare occasions diversify the Mare Crisium, so difficult of interpretation, that we may judge it wisest to avoid too positive an opinion." Now the Mare Crisium is a great depression of the lunar surface close upon that edge of the moon which, to our vision, first receives and loses the solar illumination. If I am right, aqueous vapour should be suddenly forming there during the early crescent period after the new moon, and the hoar-frost should be as suddenly precipitated as this wide depression rolls towards the darkness after the full moon. In that chapter of the "Fuel of the Sun" which is devoted to the meteorology of the moon and Mercury, I have discussed some of the theoretical results of these conditions and the appearances they should present. I may here merely add that, as the temperature of any part of the moon's unmantled surface must directly and very rapidly vary with the incidence of solar radiation, all the undulating regions of the moon must at morning and evening have a very patchy tempe rature, the slopes towards the sun being hotter than our tropics, while the opposite side of the same hill receiving the solar rays with great obliquity, and radiating into space almost without impediment, must retain a freezing temperature, and thus the cryophorous phenomena, which Sir John Herschel describes as a possible result of the contrasted temperatures of the opposite sides of the moon, should be effected even by the shady lunar craters and contrasted hill-slopes.

On the highlands of the moon no appreciable amount of hoar-frost precipitation should take place on account of the absence of sufficient atmosphere; but on the deeper maria, wherever the conditions are the most favourable, the patchy temperature should produce patches of such precipitation. If any where visible, these should be seen on the Mare Crisium, on account of its proximity to the edge of the moon, for there the morning rays that strike most obliquely upon the cold slopes would be the most effectively reflected towards the earth. Not

having seen any original or detailed account of the phenomena to which Mr. Webb alludes, I am unable to say whether they fulfil these theoretical conditions, but I believe that something more may be learned by means of careful observations specially directed to the elucidation of the questions I have suggested. W. MATTIEU WILLIAMS

Woodside, Croydon, Oct. 23

Pendulum Autographs

IT may interest some of your readers to know that they can for themselves observe in the most accurate manner the motion of the compound pendulum described by Mr H. Airy* by merely attaching the ends of a fine thread to two points in the ceiling of a room, and suspending a leaden bullet by means of a second thread tied to the middle point of the former, so that the bullet may just escape the floor. Lay underneath a large sheet of white paper ruled with two dark lines at right angles to each other to correspond to the two axes of vibration. It is Mr. Airy's experiment with the hoop on an extended scale. The motion of the bullet, unimpeded by contact of pencil with paper, is graceful and accurate in the extreme.

Perhaps the most remarkable case is that in which the two points of suspension are taken about an inch apart, and the third about half an inch below them; the pendulum will now keep reversing its motion as uniformly as before, and apparently without any adequate cause, a matter of astonishment to the uninitiated spectator.

I believe the general equation to the path, including all the curves described, will be found to be √n cos-1 = m cos-1 where the particle starts from the point (a, b) and is attracted to the axes of X and Y by forces - ny and -mx respectively. Woolwich, Oct. 24 GEO. S. CARR

=

Exogenous Structures in Coal Plants

I CORDIALLY agree with your recommendation that discussion on the Exogenous Stems of the Coal Measures should cease for the present. It is evident that I shall not convince my two opponents, and they are as far as ever they were from convincing me. But I must request that in justice to me, you will allow me to enter a protest against the last paragraph of Prof. Dyer's article, in which he objects to my applying the term Protoplasmic to the cambium layer, and endeavours to show that I am two hundred years behind the age in my physiology. I cannot but think that Prof. Dyer, when he penned that paragraph, knew perfectly well in what sense I used that expression. I meant by it nothing more than is implied in the following sentence, taken from Prof. Balfour's "Manual of Botany," p. 43, which certainly does not belong to the age of Grew :

"External to the woody layers, and between them and the bark, there is a layer of mucilaginous semifluid matter, which is particularly copious in spring, and to which the name Cambium has been given. In this are afterwards found cells, called Cambium Cells, of a delicate texture, in which the protoplasm and primary utricle are conspicuous."

Fallowfield, Oct. 25

W. C. WILLIAMSON

**This correspondence must now close.—En.

Classification of Fruits

It seems from the numerous attempts that have been made that a philosophical classification of fruits is either unattainable or practically of very little value when attained. At any rate working botanists have, as a rule, discarded the majority of the carpological terms that are to be found in text-books as too cumbrous or too uncertain in their application. Among the latest attempts at simplification in the matter of the classification of fruits are those of my friends Prof. Dickson and Dr. M‘Nab (see NATURE, vol. iv. p. 475). Both of these are open to some criticism on matters of detail, but I can hardly expect you to accord me space to point out what I believe to be the merits or shortcomings of their respective schemes. I should also trespass too much on your courtesy and on the patience of your readers did I enter into any engthened explanation of the following scheme, in which I have adopted to some extent the nomenclature of Prof. Dickson * See NATURE, vol. iv pp. 310, 317.

I believe that the foregoing arrangement will include most of the varieties of fruits and seed-vessels, though, as in all similar cases, exceptional forms are not readily sorted into their proper place; the fruit of such Cassias as C. Fistula, generally called a lomentum, is a case in point. For general purposes the varieties enclosed in brackets may well be omitted, save in the case of so well known and constantly used a term as siliqua, which, despite Prof. Dickson's veto, I think is too useful practically to be lightly abandoned. MAXWELL T. MASTERS

tion.

The Berthon Dynamometer

ABSENCE from home, and many engagements, have prevented an earlier reply to "W. R.'s" letter in NATURE, October 5. In my previous communication I believe I gave the address of the inventor, to whom I thought reference might naturally be made; in order, however, to meet "W. R.'s" wish, I will explain the construction of the very simple but efficient instrument in quesIt is merely a V gauge, formed of two pieces of thin brass converging at a very acute angle, and graduated along one of the edges; the divisions being viewed through a lens held in the *The pericarp is here understood as including not only the ripened carpellary wall, cut also any adjunct to it which in process of development may be combined with it. In the same manner the pulp may be a production from the carpel or from the seed itself.