The

his various problems without always recurring to the mathematical point of view. Unfortunately, one word must be said regarding the typography. present reviewer has seldom read a book so badly corrected for the press. There are two pages of corrigenda; but a full statement of all the small misprints would with difficulty be contained in four or five pages more. If it is not c for o or e, it is u for n, or l for t, or b for h, or das for dass. This is the more to be regretted because-granted the author's point of view-the i's of the philosophy are quite carefully dotted.

BRITISH MINERALS.

A Handbook to a Collection of the Minerals of the British Islands in the Museum of Practical Geology. By F. W. Rudler, I.S.O. Pp. x+241. (London: H.M. Stationery Office, 1905.) Price 18.

INCE his retirement from the post he so long

SINCE

and efficiently held as curator of the Museum of Practical Geology, Mr Rudler has installed in that museum a collection illustrative of the modes of occurrence of British minerals. The museum has long possessed collections of British rocks, fossils, and ores, the last named arranged under the various metals which they contain. In the new collection, which is neatly arranged in twelve table-cases, the minerals found in each district are brought together; half the space is allotted to Cornwall and Devon, oneeighth to Scotland, Ireland, and the Isle of Man, and the remainder to the rest of England, the divisions being roughly according to the several mining districts, with a general group for the minerals of the Neozoic strata. The specimens, to the number of 1652, have mostly been selected from the Ludlam collection, which was bequeathed to the museum in 1880; though mostly small in size, they are of excellent quality. In addition to the name and locality attached to each specimen, there are many explanatory labels in the cases, and the present volume admirably serves the purpose of a guide to the collection.

The volume is by no means a tedious catalogue or descriptive list of all the individual specimens, but is rather an extremely readable and interesting account of the mode of occurrence and history of the more common British minerals, especially those which are of economic importance. Instead of long descriptions of the characters of species, much is said of their paragenetic relations, and many valuable suggestions are made as to their possible modes of origin. The book will therefore be found interesting and instructive not only to mineralogists, but also to geologists and miners; whilst quite apart from the collection, for which it is primarily intended, it will have a permanent value as a treatise. In this connection mention may be made of the numerous and extremely valuable references to original authorities consulted in the preparation of the work.

The mode of treatment is a novel one, and necessarily involves a certain amount of repetition, especially in the case of some of the more commonly

occurring minerals, such as quartz, calcite, galena. &c., which may be found in almost all the different districts; but this repetition is not tedious. As an example, the district of Cornwall and Devon may be taken, in which the main groups are as follows:cassiterite, minerals associated with cassiterite, copper sulphides and sulpho-ferrites, copper-bearing minerals of the gozzans, arsenates and phosphates of the copper-gozzans, ores of lead, zinc, antimony, &c., sulphides and sulpho-salts, ores of iron, &c., minerals of the rarer metals, the spars of the mineral veins, miscellaneous minerals.

Apart from a few minor misprints, the only point which calls for criticism is that undue importance seems to have been attached to many quite trivial and local names. As for the printing, there is certainly much room for improvement; the lines are so badly broken that it is surprising that the whole did not fall to pieces in the course of printing. L. J. S.

OUR BOOK SHELF.

Moths and Butterflies. By Mary C. Dickerson, Pp. xviii+344; with 200 photographs from life by the author. (Boston, U.S.A., and London: Ginn and Co., n.d.) Price 5s. net.

THIS is a prettily got-up book, intended for the training of classes in "nature-study," with reference to a considerable number of common and conspicuous North American butterflies and moths, the life-history of which is very fully described and illustrated. The concluding chapter, on collecting, keeping, and studying, recapitulates the points to be noted in practical observations on the insects themselves.

To English readers the book will be useful for the information it supplies about American forms, and also as indicating a similar method of study for British insects, but many of the species here noticed are much larger and more conspicuous than those likely to fall under our own observation, among them being several species of Papilio, and large Saturniidæ.

The figures, of which (including apparatus, &c.) there are 233 in all, are generally very good, though some are indistinct. The frontispiece, representing a Smerinthus at rest, and Fig. 17, on p. 147, representing a procession of the young caterpillars of Saturnia, may be specially noticed. But it looks odd to see a Smerinthus closely allied to our own S. ocellatus called "a most beautiful little moth " (p. 232); and, though we do not object to the use of appropriate English names, we are sorry to see on P. 231 a Sphinx allied to S. convolvuli called "the Humming-Bird Hawkmoth, a name by which the very different Macroglossa stellatarum has been known all the world over, ever since the commencement of the study of entomology.

We had expected to find some notice of the gipsy moth, the crusade against which has recently been given up in America in despair, but find only a passing reference. A few British species are noticed, such as Vanessa antiopa, called in America the mourning cloak, a translation of its German name; V. atalanta, Pieris rapae, &c.

A great deal of useful general information is given in the book, and it seems on the whole to be careful and accurate. One statement, however true in the abstract, ought not to have been made without quali fication or explanation in a popular book. On. p. 267 we read, "We are familiar with the fact that all living

creatures develop from eggs." Further comment is needless.

Although published in 1901 and mentioned in the Zoological Record for that year, this book has not previously been brought under our notice.

Second Stage Magnetism and Electricity. By Dr. R. Wallace Stewart. Second edition. Re-written and enlarged. Pp. viii+416. (London: W. B. Clive.) Price 35. 6d.

THIS book is primarily intended to serve the purposes of a candidate preparing for the second stage examination under the Board of Education (secondary branch). In reading it, we have by no means made our first acquaintance with Dr. Stewart, and the perusal has left us of our old opinion that, whether regarded as text-books intended to prepare a student for a particular examination or as a source of culture, the books prepared by the author can be very earnestly recommended. He is a lucid and accurate writer. He knows where to draw the line so that an elementary student shall not be repelled by the complication of a subject.

The present volume is brought up to date. The importance of the field-that is, the medium surrounding an electrified conductor or magnet--is insisted on; perhaps even their importance is emphasised too much. The tendency of modern thought amongst physicists is to restore to a conductor part, at any rate, of the position that it held in pre-Maxwellian days. The dielectric plays a most important part-that is a position, won for it by Maxwell, which it can never lose. the same time, one should not lose sight of the fact that there must be some mechanism at the ends of a line of induction, and to-day that mechanism is being studied under the name of electron. The electron is an essen

At

tial part of a conductor, and the complete phenomena of electricity are not fully accounted for without including it.

44

The volume is almost entirely re-written. It is not Surprising, therefore, that there are some unfortunate slips which have escaped the vigilance of the reader. As these are misleading, we will state that on the bottom of p. 33“ positive" and "negative" should be interchanged. The following phrase (p. 42) is very misleading:- The portions of those walls, which are, as it were, in the shadow of these objects, possess no induced charge." We think that the first thirty pages might be improved in any later edition. Considerable care has evidently been taken; yet in many cases confusion is introduced by the neglect of some tiny detail. Thus, in describing the attraction and repulsion of a pithball with subsequent re-attraction, if in the interval it comes in contact with an earth-connected body, the phrase that we have put in italics is omitted; and in several cases where a body is touched to earth it is not explicitly said whether the contact is to be broken before a succeeding operation is performed or not. Why is it "evident" (p. 16) that doubling a charge will double the force it exerts on another charge?

Memoria sobre el Eclipse Total de Sol del dia 30 de Agosto de 1905. By D. Antonio Tarazona. Pp. 125. (Madrid: Bailly-Bailliere E. Hijos, 1904.) THOSE who are familiar with the Spanish language and have made up their minds to go abroad and see the approaching total eclipse of the sun will find in this book a great amount of useful information relating to this interesting event. The work is issued from the Madrid Astronomical Observatory, the director, Francisco Iñiguez, having contributed a brief preface, and contains full particulars concerning the elements of this eclipse; in fact, it might be considered a treatise on the subject, so complete is the information. In

addition to a great many data which will be of special use to astronomers, there will be found a very full list of towns, in alphabetical order, at which totality occurs, with the times of the different phases of the eclipse. More generally useful perhaps will be found the maps at the end of the volume. These include a map of the world showing the position of the track from the commencement to the end of totality over the earth's surface. A second illustrates on a larger scale the Spanish portion of the track, with special lines showing the times of occurrence and duration of totality. The third, on a much larger scale (1: 1,000,000), indicates that part of Spain alone over which the shadow sweeps, and is very complete as regards names of places, railways, &c. Lastly, two star charts are added, one showing the position of the eclipsed sun among the stars, and the second a key map to this chart giving the designations of the stars and planets in this region.

Visitors to Spain will do well to supplement their literature by securing this volume, and thanks are due to the Madrid Observatory for producing so useful a book so far in advance of the event.

Naturalistische und religiöse Weltansicht. By Rudolf Otto. Pp. 296. [Tübingen: J. C. B. Mohr (Paul Siebeck), 1904.] Price 3 marks. No better book than this could be recommended to the young philosophical or theological student who wishes to obtain a clear and comprehensive view of the debatable ground where science, philosophy, and theology meet. The author is well read, a skilful debater, a vigorous writer; and as handbooks ought not to be unnecessarily multiplied, it is to be hoped that this one will be translated.

general, the book is not so strong on the constructive Like many other works in defence of religion in

as on the critical side. The author refers with

approval to the attitude of Kant when he solved certain contradictions or antimonies by a reference to the world of things in themselves. As this is precisely the point where Kant's philosophy is most seriously questioned, the argument probably suffers to that extent. But, on the other hand, the author fully realises the unity of the various phases of the one problem religion versus naturalism, and the harm which has been done by concentrating the attention on one phase (e.g. the question of miracles) as if it were the whole.

The work is valuable mainly for its survey of the most interesting biological theories of the last century, from Darwin, Häckel, Weismann, down to Wolff, Korschinsky, Driesch. The philosophical development of this last writer is sketched in an enlightening fashion. With regard to the general theory of development and 'descent," the author comes to the conclusion that with the confirmation of any such theory only something relatively external is given, a clue to creation, which does not so much solve its problems as group them afresh. The index at the end of the work gives an explanation of the more difficult terms employed by modern theorists. An Introduction to Projective Geometry and its Applications. By Dr. Arnold Emch. Pp. vii+267. (New York: Wiley and Sons; London: Chapman and Hall, Ltd., 1905.) Price 10s. 6d. THIS text-book of modern projective geometry forms an admirable introduction to the subject, and should be known to all who are interested in this branch of mathematics. The first chapter deals with the general properties of projective ranges and pencils and their products, including harmonic and perspective projection, and the projective properties of the circle. Then

follows an investigation of collineation in a plane, comprising perspective transformations, and the linear transformations of translation, rotation, and dilatation, with combinations of these. The intimate relation that exists between projective and descriptive geometry is shown. The third chapter gives the general theory of conics, the projective properties of the circle being extended to conics by perspective transformations. The next chapter deals with pencils and ranges of conics and their products, and especially with cubics, the latter being classified under the five standard types by the help of the Steinerian transformation. Throughout the book analytical and geometrical methods are employed side by side, some portions of the subject being better suited to the former treatment; moreover, the analysis affords excellent illustrations of modern analytical geometry. The main purpose of the author has been to develop the subject in regard to its practical applications in mechanics, and the last chapter is devoted to such examples. Thus we find problems in graphic statics, plane stresses, and in the stress ellipse of an elastic material, and there is an interesting account of various linkages by means of which linear and perspective transformations can be mechanically obtained. The book is excellently got up in every way, and the diagrams are quite perfect and may well serve as models of what such figures ought to be. The author is a very clever draughtsman, and his skill as a writer is equally pronounced.

LETTERS TO THE EDITOR.

[The Editor does not hold himself responsible for opinions expressed by his correspondents. Neither can he undertake to return, or to correspond with the writers of, rejected manuscripts intended for this or any other part of NATURE. No notice is taken of anonymous communications.]

Fictitious Problems in Mathematics.

In my younger days it was well recognised that such statements as perfectly smooth" and the like were mere conventional phrases for designating an ideal state of matter, which was assumed to exist for the purpose of simplifying the mathematical conditions as far as possible. Nobody can learn mathematics without working out a large number of problems and examples, and in order to make these sufficiently easy for the beginner, various fictitious hypotheses have to be introduced.

Similar objections would apply to the phrase "frictionless liquid "; but it would be impossible for anyone to learn hydrodynamics without first studying the mathematical theory of this fictitious form of matter. In fact, the introduction of viscosity leads to such formidable difficulties, that nobody has yet succeeded in solving such a simple problem as the motion due to a doublet situated at the centre of a sphere; and the solution, if it could be obtained, would throw much light on the mode of attacking more difficult problems. A. B. BASSET. May 28.

IN NATURE of May 18 the wording of a problem set near the beginning of my Rigid Dynamics" is rather adversely commented on. In the problem a man is described as walking along a perfectly rough board which rests on a smooth table, and the criticism is that the two suppositions are inconsistent; but this depends on what is meant by the words used, and perhaps I may be allowed to make an explanation.

When bodies are said to be perfectly rough, it is usually meant that they are so rough that the amount of friction necessary to prevent sliding in the given circumstances can certainly be called into play. In art. 156 of the treatise on dynamics, just after the laws of friction have been discussed, the words "perfectly rough are defined to have this meaning. The board in question has therefore no special peculiarity. All that is stated is that the

coefficient of friction between the man and the board exceeds a certain finite quantity.

The board rests on a smooth table, but the coefficient of friction now depends on both the board and the table. and this may be quite different from that between the man and the board. There is nothing amiss in supposing this coefficient to be zero. One way of effecting this experimentally would be to polish the table and remove all roughnesses from it. This was the plan indicated. Where, then, is the inconsistency?

By using the ordinary abbreviations of language, the wording of the question has been made concise, and thus attention was specially directed to the dynamical principle involved in the solution.

The problem has been understood by so many students in the sense above described, and worked without a single objection having been raised, that I think the meaning must be perfectly clear. Indeed, I cannot imagine what other meaning it could have. E. J. ROUTH. May 20.

On the Spontaneous Action of Radio-active Bodies on Gelatin Media.

IN the course of some experiments on the formation of unstable molecular aggregates, notably in phosphorescent bodies, I was led to try whether such dynamically unstable groupings could be produced by the action of radium upon certain organic substances. It will scarcely be necessary to enter here into an account of the many speculative experiments which I have at one time or another tried, but it will suffice if I describe, as briefly as possible, the experiment which, amongst others, has led to a very curious result, and that is the effect of radium chloride and radium bromide such as those

upon gelatin media, generally used for bacterial cultures.

An extract of meat of 1 lb. of beef to 1 litre of water, together with 1 per cent. of Witter peptone, I per cent. of sodium chloride, and 10 per cent. of gold labelled gelatin, was slowly heated in the usual way, sterilised, and then cooled. The gelatin culture medium thus prepared, and commonly known as bouillon, is acted upon by radium salts and some other slightly radio-active bodies in a most remarkable manner.

In one experiment the salt was placed in a small hermetically sealed tube, one end of which was drawn out to a fine point, SO that it could be easily broken. This was inserted in a test-tube containing the gelatin medium. The latter was stopped up with cotton wool in the usual way with such experiments, and then sterilised thirty minutes. Controls without radium at a temperature of about 130° C. under pressure for about were also at various times thus similarly sterilised.

When the gelatin had stood for some time and become settled, the fine end of the tube containing the radium salt was broken, from outside, without opening the test-tube, by means of a wire hook in a side tube.

The salt, which in this particular experiment consisted of 2 milligrams of radium bromide, was thus allowed to drop upon the surface of the gelatin.

After twenty-four hours or So in the case of the bromide, and about three or four days in that of the chloride, a peculiar culture-like growth appeared on the surface, and gradually made its way downwards, until after a fortnight, in some cases, it had grown fully a centimetre beneath the surface.

If the medium was sterilised several times before the radium was dropped on it, so that its colour was altered, probably by the inversion of the sugar, the growth was greatly retarded, and was confined chiefly to the surface.

It was found that plane polarised light, when transmitted through the tube at right angles to its axis, was rotated left-handedly in that part of the gelatin containing the growth, and in that part alone.

The controls showed no contamination whatever, and no rotation. The test-tubes were opened and microscopic slides examined under a twelfth power. They presented the appearance shown in Fig. 1. At first sight these seemed to be microbes, but as they did not give subcultures when inoculated in fresh media they could scarcely be bacteria. The progress of any of the subcultures after a month was extremely small, and certainly

too small for a bacterial growth. It was not at all obvious how bacteria could have remained in one set of tubes and not in the other, unless the radium salt itself acted as a shield, so to speak, for any spores which may originally have become mixed with the salt, perhaps during its manufacture, and when embedded in it could resist even the severe process of sterilisation to which it was submitted.

On heating the culture and re-sterilising the medium, the bacterial-like forms completely disappeared; but only temporarily, for after some days they were again visible when examined in a microscopic slide. Nay, more, they disappeared in the slides when these were exposed to diffused daylight for some hours, but reappeared again after a few days when kept in the dark. Thus it seems quite conclusive that whatever they may be, their presence is at any rate due to the spontaneous action of the radium salt upon the culture medium, and not alone to the influence of anything which previously existed therein.

When washed they are found to be soluble in warm water, and however much they may resemble microbes,

Prof. Sims Woodhead has very kindly opened some of the test-tubes and examined them from the bacteriological point of view. His observations fully confirm my own. He assures me that they are not bacteria, and suggests that they might possibly be crystals. They are, at any rate, not contaminations.

I have tried to identify them with many crystalline bodies, and the nearest approximation to this form appears to be that of the crystals of calcium carbonate, but these are many times larger, and, in fact, of a different order of magnitude altogether, being visible under comparatively low powers; and are, moreover, insoluble in water.

A careful and prolonged examination of their structure, behaviour, and development leaves little doubt in my mind that they are highly organised bodies, although not bacteria.

Unfortunately the quantity is so very minute that a hemical analysis of their composition is extremely difficult. The amount of salt in the first instance is so small, and the number of aggregates, or whatever they may be, thus produced perhaps still smaller.

The most effective method of studying their properties,

from the physicist's point of view, is that of long and, so far as possible, continual observation, a method similar to that which the astronomer is bound to adopt in his study of bodies over which he has not the control to deal with as he pleases.

From the accompanying photographs it will be observed that they are not all of the same size; they range from about 0.3 μ to the minutest specks; they are mostly, if not altogether, all of the same shape, and show distinct signs of growth; the larger ones appear to have sprung from smaller forms, and these in turn from still smaller ones, and they have all probably arisen in some way from the invisible particles of radium.

Fig. 2 distinctly shows the existence of nuclei in the larger and more highly developed forms, whilst Fig. 3 reveals, though indistinctly, what is their most remarkable property of all, and that is their subdivision when a certain size is reached. They do not grow beyond this size, but subdivide.

These photographs, together with the numerous results of eye observations, which indicate that a continuous growth and development take place, followed by segregation, leave little doubt that whilst on the one hand they cannot be said to be bacteria, they cannot be regarded as crystals either in the sense of being merely aggregates of symmetrically arranged groups of molecules, which crystals are supposed to be. The stoppage of growth at a particular stage of development is a clear indication of a continuous adjustment of internal to external relations, and thus suggests vitality.

They are clearly something more than mere aggregates in so far as they are not merely capable of growth, but also of subdivision, possibly of reproduction, and certainly of decay.

The subcultures do show, however slightly, some indicacation of growth after four or five weeks, although that growth is, I understand, too small for a bacterial subculture. Moreover, when examined in the polariscope they have not been found to yield the characteristic figures and changes of colour which crystals generally give.

Thus for these reasons I have been led to regard them as colloidal rather than as crystalline bodies, and probably more of the nature of " dynamical aggregates " than of" static aggregates," of which crystals are composed.

There appears to be a tendency amongst text-book writers to classify minute bodies which are not bacteria as crystals, but really without sufficient reason, and as these bodies cannot be identified with microbes, on the one hand, nor with crystals on the other, I have ventured, for convenience, in order to distinguish them from either of these, to give them a new name, Radiobes, which might, on the whole, be more appropriate as indicating their resemblance to microbes, as well as their distinct nature and origin.

Some slightly radio-active bodies appear also to produce these effects after many weeks.

A more detailed account of these experiments will be published shortly. This note merely contains some of the principal points so far observed.

I have to thank Mr. W. Mitchell, who sterilised the tubes, for the assistance he has rendered in these experiJOHN BUTLER BURKE. Cavendish Laboratory, Cambridge, May 10.

ments.

The Consolidation of the Earth. THERE are several points in Dr. See's last letter (NATURE, May 11) calling for remark from the geological point of view.

(1) The effect of (hydrostatic) pressure at depths tends not to liquefaction (as in the case of the ice of a glacier) but to promote crystallisation, the condition of the greatest density of mineral matter, as I showed years ago in my little work on metamorphism in discussing the relation of the crystalline to the vitreous states. It is here that the importance of "solid-liquid critical state" comes in.

(2) We have no right to assume the existence at any stage of the history of our planet of a mere molten ball radiating heat directly into cold space, since in that I pre-oceanic stage" it was surrounded by a non-conduct

66

ing mantle or jacket" of such enormous density and altitude as to contain (as its main constituents) (a) the greater part of the water of the present hydrosphere in the vapour state; (b) the CO, locked up in the limestones and other carbonates of the lithosphere, as well as that represented by the coal and the living vegetation of the globe; (c) the hydrocarbons possibly represented by Archæan graphite, together with (d) the halogens (if atomic evolution had reached that stage), including the Cl, of the 73 per cent. of the NaCl of the salts of the present ocean. It is conceivable that a vast convection system existed, as the outer zones of the primordial atmosphere underwent cooling with consequent condensation, and descended towards the molten globe; but there could scarcely be contact generally between such cooler portions and the heated molten mass. The conditions would be rather such as are partly illustrated by what a student of physics is familiar with as the " spheroidal state" of a liquid floating on a cushion of steam above a hot plate of metal. Under the enormous pressure prevailing at the surface of the globe in that pre-oceanic stage of its history great quantities of superheated steam and other gases must have been mechanically included, and in some cases, perhaps, occluded, in the hot crust in the inceptive stages of its development by congelation; and in such circumstances, as I suggested seventeen years ago, superheated water in traces would probably enter into the composition of such silicates as hornblende and mica, the two most characteristic of the minerals of the heavier metals of the Archæan gneisses and schists. A year or two later that hypothesis received demonstration from the splendid work of de Kroustchoff (see NATURE, vol. xliii. p. 545, also Bulletin de l'Académie des Sciences de St. Petersbourg, tome xiii., Über künstliche Hornblende," by K. von Chrustschoff). So, I take it, we can understand how such a crust could float on a magma of molten rock material, just as aircharged fragments of pumice or of charcoal float on water, yet sink quickly to the bottom under the exhausted receiver of an air-pump; or as even a coil of platinum foil (sp. gr. 21.5) can be made to float in water inside a good air pump, as it is pontooned by innumerable bubbles of distended atmospheric gases previously condensed upon its surface; or, again, as masses of lava slag of large dimensions are seen to float for a time upon the vast lake of liquid rock material in the crater of Kilauea. With tidal action in the magma greater when the moon was nearer the earth than at present, such a thin crust would easily undergo disruption, while portions of it would float off and be engulfed in the magma. This view, which I propounded some seventeen years ago, had been anticipated partly by Zöllner, and was adopted by the distinguished American geologist, Dr. A. C. Lawson, to explain the phenomena presented by the enormous inclusions of more basic rock masses in the gneiss of the Rainy Lake region, which excited great interest among our leading British geologists at the International Geological Congress in London in 1888, though it seems at the time to have been very imperfectly perceived by most of them. So far the evidence we have goes to support Dr. See's contention that the descent of such masses into the magma would be arrested long before they even approached the centre of the sphere: but one feels great difficulty in following his argument based on Laplace's law, for reasons given in my former letter (NATURE, May 4). By a slip I wrote, it appears, impossibility " for possibility in the top line of p. 8 in my last letter. Bishop's Stortford, May 17.

66

A. IRVING.

The Spirit-level as a Seismoscope.

A MISCONCEPTION seems to prevail among seismologists as to the behaviour of a spirit-level. A displacement of the bubble is regarded as conclusive evidence of the tilting of the instrument. It should be pointed out, however, that this is far from being the case. For a second cause, equally effective in producing displacement of the bubble, is a horizontal acceleration of the instrument in the direction of the tube. The position of the bubble should be taken as indicating, not the normal statical vertical, but

the dynamical residual vertical obtained by subtracting the acceleration of the instrument (as a vector) from that of gravity. (I disregard, in this statement, the slight lag due to viscosity.)

A couple of simple experiments, serving to emphasise this, may be suggested. A spirit-level is suspended in a horizontal position by two equal strings attached one to each end. In one case the strings hang vertically from two hooks; in the other case they are attached both to one hook. If the level is set swinging in the plane of the strings, then in the first case the bubble will be found to have an oscillatory movement relatively to the tube, the tube having linear acceleration but no tilting movement. In the second case the tube has both movements, but their effects exactly neutralise each other, and the bubble remains stationary in the tube. The expert waiter (may it be added?) who hurries about with plates of soup has a very effective empirical knowledge of this last case of compensation.

The motion of the bubble of a level has been brought forward as evidence in favour of the undulatory character of the disturbance producing the motion; but if the above suggestions are to be accepted, the motion might as reasonably be urged as evidence of a horizontal disturbance; the truth being that the instrument is sensitive to both disturbances, and is quite ineffective as a means of discriminating between them.

The evidence referred to is contained in the British Association report, 1902 (seismological committee report, P. 72). The view finds acceptance in some recent and authoritative works,' and seems, so far, to have passed unchallenged. G. T. BENNETT.

Emmanuel College, Cambridge.

taken on April 22 near the summit of Carnedd Llewelyn, N. Wales (3484 feet above sea-level). These delicate frost feathers appear gradually to grow outwards from the rock face on the windward side, and the delicacy of their form is, no doubt, modified in some degree with the varying rate of the wind and the temperature. I have found, in the same district, these "feathers " 9 inches from root to tip; those shown are about 6 inches long. They form a comparatively solid mass where they touch, but the tips keep distinct, and the whole mass is in reality very brittle. and easily breaks up into small pieces.

44 Highbury Park, N., May 16.

H. M. WARNER.

1 Dutton, "Earthquakes in the Light of the New Seismology," p. 137: Davison, "A Study of Recent Earthquakes," p. 280.